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Pelvic floor muscles undergo a degree of stretch during vaginal delivery unparalleled elsewhere in the body, causing significant injury in the vagina (VA) and levator ani muscle (LA). Up to 20-30% of women over 20 years old and up to 50% over 50 may suffer from pelvic floor disorders. Challenges with experimental investigation of the biomechanics of childbirth, such as ethical and clinical restrictions in physical testing and observation of this complex phenomenon, have led to a shift towards computational modelling, which is heavily dependent on accurate material properties to predict realistic and accurate biomechanics. In particular, there is a lack of data on anisotropic and time-dependent mechanical properties of female LA and VA muscles. Thus, this work set out to investigate both the elastic and time-dependant nature of the LA and VA muscles. These two muscles undergo large strains during delivery and are frequently injured, leading to the development of pelvic floor disorders in many women. This study began with the development of a novel radial tissue stretcher (RTS), selection of ovine LA and VA muscles as suitable analogues for the corresponding human pelvic floor muscles based on literature review, followed by uniaxial and multiaxial testing of the LA and VA muscles of the ovine pelvic floor, and finally fitting a fibre-reinforced visco-hyperelastic constitutive model to the experimental data.

From the automotive industry to next-generation turbines, bearings are critical components in a wide range of machinery for varied applications. Their capacity to resist wear and fatigue during service is a critical factor in improving maintenance intervals and reduces the risk of premature or catastrophic failure. The work presented in this thesis primarily aims to elucidate the relationship between processing parameters of steels for bearings and mechanical and thermal performance. Particularly but not exclusively, the role of retained austenite in the microstructures of these steels is examined, in which there is a long-standing tenet that carbon content in austenite acts to stabilise it from phase transformation. The existence of retained austenite within a microstructure has complex implications on a bearing's performance characteristics, including dimensional stability, wear resistance in contaminated environments, and work hardening behaviour. A wide range of processing parameters were applied to SAE 52100 steel that were designed to promote carbon diffusion into austenite retained during quenching. Firstly, the industry standard quenching and tempering process was examined via dilatometry with in-situ synchrotron X-ray radiation in order to elucidate the phase transformation kinetics during processing, and provide fundamental insight into microscopic behaviour of martensite formation and tempering and generally serve as a reference point for the thesis. This was then compared to phase transformations during bainitic austempering, with particular emphasis on lattice strains in parent austenite and bainitic ferrite, and carbon segregation. Significant carbon loss in bainitic ferrite was observed due to the precipitation of nano-scale carbides and carbon partitioning into austenite. The thermal and mechanical performance of these microstructures was then tested again with in-situ synchrotron X-ray diffraction. It was observed that solid solution strengthening dominated the mechanical stability of austenite, but carbon content of austenite and strain relaxation of the bainitic ferrite matrix experienced during austempering increased the thermal stability of austenite. A higher carbon content in retained austenite did indeed increase its mechanical stability across a parameter space of like heat treatment types, but a tempered martensite matrix outperformed bainite matrices in all cases.

Fibre Reinforced Plastics (FRPs) are heavily utilised in high performance engineering applications due to their exceptional physical characteristics, including excellent specific strength, stiffness, fatigue loading performance, and corrosion resistance. Typical applications are aircraft primary and secondary structures, body panels in low-volume sports cars, and wind/tidal turbine blades. The rate of composite material uptake is expected to increase in future years, with some researchers anticipating increased utilisation in affordable solutions-over more typical high-cost products and services. With additional uptake, concerns have been raised regarding both financial and environmental costs, particularly in relation to waste disposal and recycling. Improved methodologies for the design and manufacture of composite materials are therefore required to prevent material waste and improve current end-of-life solutions. To contribute towards alleviating these concerns, this thesis addresses inefficiencies relating to the connections/joints in composite structures. More specifically, studies into the joining of FRPs and their reinforcement yarns have been conducted, where discrete investigations are presented for each. In this thesis, a detailed literature review is first provided, which gives background to all aspects considered in this work. An overview of manufacturing techniques, associated environmental concerns, and current waste disposal/recycling solutions is provided, followed by several more technical sections relevant to the experimental/modelling work conducted in this dissertation. Thereafter, focus is directed towards the development of a generalised modelling approach for the preliminary design of pin-loaded composite connections. Firstly, investigations were conducted into the performance of notched composite specimens, such that approximate characteristic curves could be generated for the prediction of failure loads in pin-loaded specimens. To form this empirical (CCA) method, notched/regular tensile and notched/regular compressive strength ratios were required for a spectrum of different combinations of stacking sequence and hole diameter. Layups were chosen where the mean of the magnitude of all ply-angles (APA)-relative to the loading direction-and corresponding variance value (VAR) could be correlated to these ratios. Regular tension, notched tension, regular compression, and notched compression specimens were manufactured and tested in accordance with various international test standards, in which a carbon fibre reinforced plastic (CFRP) prepreg system was evaluated. Results indicated that hole diameter had only marginal impact on notched strength ratios, whilst the stacking sequence-considered in terms of derived APA and VAR parameters-critically affected notched ratios. Utilising a least squares optimisation method, approximate curves for notched strength ratio versus APA were computed, and correspondingly scaled and shifted for each of the evaluated hole sizes. After performing a cubic-interpolated surface fit over these curves and adjusting the values, approximations for both tensile and compressive notched strength ratios could be obtained for any given combination of layup stacking sequence and hole diameter-for the specific material. Results in this section successfully identify trends which correlate stacking sequence and hole diameter to notched performance, whilst also introducing a novel empirical CCA method for estimating the characteristic curve. After proposing the CCA method for approximating characteristic curves, its integration within finite element models (FEMs) was considered. For all simulations, Python scripting was utilised to automatically generate and run analyses, based on a number of user (or optimiser) specified parameters. A detailed description of this FEM-CCA model, along with a mesh convergence study for elements around the specimen hole-edge/pin, is presented. A variation of fuzzy adaptive particle swarm algorithm (FA-PSO) was then considered as an optimisation approach for maximising the strength of the pin-loaded joint. Two optimisation tasks were performed on eight and twelve ply layups, in which layup symmetry was enforced and hole diameters were fixed. The resulting configurations outputted from the FA-PSO algorithm, along with several other non-optimised configurations, were then re-analysed/analysed, where the number of increments was increased to 400-improving predicted failure load precision. To verify the accuracy of the FEM-CCA modelling approach and whether the optimisation algorithm effectively obtained a high-performing solution, experimental tests-equivalent to the models-were performed. To conduct pin-bearing tests, a custom fixture was designed using topology optimisation software to minimise pin-displacement whilst allowing digital image correlation (DIC) to be utilised. During mechanical testing of various configurations, strain fields were correspondingly recorded throughout the load cycle. The Tsai-Wu failure index was observed to correlate most closely with model results, and strain fields produced from DIC were shown to be useful for observing damage progression. The aforementioned work presents a novel strategy for the preliminary design of pin-loaded composite panels, which could be readily extended to more complex specimen geometries and types of fixtures. Alongside the development of a preliminary design approach for mechanically fastened composite joints, an investigation into a novel pneumatic splicing method for creating connections between reinforcing fibres used in FRPs was conducted. Based on preliminary observations that altering the number of pulses fired along overlapped tows had significant influence on connection strength, dry-fibre connections were formed, where the number of pulses fired ranged from one to fifteen pulses per one-hundred-millimeter overlap. In addition to obtaining failure loads, linear stiffness values for the different specimen types were calculated using a series of nested loops in MATLAB 2019a software. For both linear stiffness and failure loads, increasing the number of pulses correlated asymptotically to increased performance. An assessment of the performance of spliced carbon fibre yarns as reinforcing materials in composites was then conducted, in which the best performing configuration from the fibre-only analysis was utilised. Unidirectional plates were manufactured via hand-laying and press curing, and tensile tested in accordance with international standards. To ensure consistent fibre volume fractions across the plate volume, two arrangements were proposed, in which the position of the spliced overlaps were staggered. To benchmark performance, analogous continuous-fibre unidirectional and chopped strand mat specimens were produced and tested. For all specimen configurations, infrared thermography was performed during testing, as a means of presenting the location/concentration of (heat) energy released during specimen failure. In addition to mechanical testing, scanning electron microscopy was performed, and density and fibre volume fractions were obtained for all types of specimen. The mechanical performances achieved by spliced specimens compared favourably with non-spliced configurations, and based on the obtained results, it is anticipated that better performance can be obtained after improvements to manufacturing processes. The results presented in this section indicate that yarns reconstituted via pneumatic splicing can carrying significant loads, whilst maintaining excellent tensile stiffness. It is therefore postulated that reconstituted yarns comprising of spliced reclaimed fibres have the potential to be utilised in manufacturing processes normally limited to continuous tows, such as 3D-printing and weaving. Overall, this thesis has proposed two discrete and novel approaches relating to joining composite structures and their reinforcing yarns, respectively. Firstly, an efficient modelling method has been proposed for pin-loaded laminate composites. The proposed approach was then successfully implemented within a meta-heuristic algorithm to optimise laminate stacking sequence for maximising joint strength. Secondly, a novel preliminary investigation has been conducted relating to the implementation of pneumatic splicing with tows of carbon fibre.

Fibre reinforced polymer composites are frequently replacing metals and alloys in structural applications for the aerospace, automotive and marine industries due to their high specific strength and stiffness and excellent corrosion resistance. However, a key limitation of composite laminates is their low resistance to out-of-plane loading and susceptibility to delamination. Improving the out-of-plane resistance to delamination in composites is achievable through the insertion of a z-direction reinforcement. There are several available techniques to achieve this, of which, through-thickness stitching is considered an effective, low cost option. Despite its popularity, limitations to the current stitch geometries employed for composites often result in significant geometrical defects and degradation of the in-plane properties. The aim of this research was to optimise the stitching geometry in order to effectively improve the impact resistance of textile composites without causing excessive damage to the in-plane tensile properties. This research details the employment and optimisation of a novel stitch type for the composite industry, the ISO-401 double-thread chain-stitch. Although this stitch type is used regularly throughout the textile industry, no research to date considers its suitability to composite reinforcement. Adjustment of the junction positon of the upper and lower stitching ISO-401 threads demonstrated that defects such as resin pocket size and fibre waviness can be reduced through optimisation. Thus, at high density stitching, which is reported as beneficial to improving the out-of-plane properties, the tensile properties are not significantly affected and therefore maintained. Under low velocity impact, stitching was found to significantly reduce the damage area through an arrest and bridging technique. This resulted in improved compressive strength after impact in stitched composites due to the smaller initiated damage size and further bridging effects of the stitches under loading. Considering the importance of the ISO-401 junction location, a final case study also revealed that the preform fabric characteristics can significantly affect the junction position under the same stitching conditions. Therefore, careful consideration of the sewing conditions and parameters must be taken to achieve the desired geometry. The work completed for this thesis demonstrates that ISO-401 shows good compatibility for the through-thickness reinforcement of textile composites and that it can address some of the limitations of other popular stitch types.

Metal matrix composites (MMCs) are composed of matrix, the parent material, and reinforcements that provide enhanced properties to individual components. Out of various MMCs, SiC particle reinforced aluminium (SiCp/Al) composite has outstanding mechanical properties such as high strength-to-weight ratio, stiffness, excellent wear resistance, ease of production and good corrosion resistance, hence, defined as an aerospace-grade. However, the existence of hard particulate SiC particles leads to machinability issues with conventional machining (CM) techniques. Conventional turning (CT) of SiCp/Al composite is typically characterised by low cutting parameters, thus, low material removal rates during the machining process. A hybrid machining process known as ultrasonically assisted turning (UAT) was introduced in the past to machining metals or alloys, in which, high-power, high-frequency vibration is superimposed on the cutting tool during the conventional machining process. This renowned machining technique results in a decrease in average cutting forces with surface quality improvement for modern alloys. Another hybrid machining process known as laser-assisted turning (LAT) was also introduced to machining metals and alloys, which utilises localised thermal softening before the cutting tool resulting in cutting force reduction at the cutting zone. This technique results in an improved material removal rate (MRR) with less tool damage. In this work, a new machining paradigm, called hybrid-hybrid assisted turning (laser-ultrasonic assisted turning, LUAT) is introduced for the processing of three types (217XG, 225XF and 225XE) of SiCp/AA2124 composites. In this technique, UAT is combined with the laser-assisted technique to gain combined advantages for machining metal matrix composite. To experiment, suitable ultrasonic transducers were designed and analysed first through FE simulation followed by experimental analysis on manufactured prototypes. Then, a selected transducer and laser-optic system were installed on a compact commercial lathe. Before the machining experiment, mechanical and thermal properties of the workpieces were analysed, with studies on the mechanism and characterisation of SiCp/Al composite with ultrasonic vibration during loading and heat-affected zone (HAZ) analysis. Acoustic softening was observed in all SiCp/Al workpieces during the uniaxial compression test with the main vibration applied in the loading direction, thus, the cutting force reduction mechanism was verified. SEM, EDX, micro-hardness, and FE numerical studies were performed in the HAZ to get an insight into the size of HAZ and metallurgical behaviour during laser exposure. It is found that HAZ showed hardened oxide and intermetallic phases that will adversely affect the cutting forces after cooled down. Moreover, it is numerically verified that thermal conduction decreases drastically by particle inclusions, faster conduction towards the particles with an area more parallel to the applied heated surface. Furthermore, reduction of Si element was observed in HAZ and the melt pool, verifying the particle sedimentation, if melting is present. UAT of the studied composites was analysed experimentally and numerically to demonstrate its advantages in terms of cutting forces reduction over various cutting conditions with the carbide tool. However, the evident surface roughness improvement was only visible with composite with the least particle content (217XG). LUAT studies follow with optimisation process through numerous trials that showed approximately 40% cutting force reduction compared to CT using the carbide tool. Surface integrity and roughness improvement were also minimal due to the high tool wear rate in CT, especially in the radial direction. LUAT performed with the PCD tool showed improvement in both cutting forces and surface roughness, showing improvement by an increase of laser power, although cutting forces were higher than that for LAT. It is mainly because the higher impact of ultrasonic vibration neglected the hardened phase but only increased the temperature at the cutting zone. 2D homogenous elastoplastic thermomechanically coupled FE models for the orthogonal turning process were developed for CT and UAT, followed by a 3D homogeneous FE model for the oblique turning process. The temperature profile was observed with moving localised surface heat by power and distance from the laser spot and the tool to predict the temperature of the workpiece nearest to the cutting zone. It is found that due to high thermal conductivity, the heat is quickly dissipated, leaving hardened oxide and intermetallic phases before the cutting tool. The LUAT was predicted in the model at various temperatures of the workpiece in terms of cutting forces. Therefore, it is not always suitable to use developed hybrid-hybrid turning for machining SiCp/Al composite, however, if optimised, it is beneficial in terms of providing thermal softening, expansion of feed grooves, and minimising workpiece damage from particles since particle sedimentation and hardened phases inhibit particle rolling or scratching on the tool-workpiece interface. A preferred setup for the SiCp/AA2124 composite for the used cutting parameter (V_c=30 m/min, f=0.14 mm/rev and a_p=0.15 mm) is to paint the surface in black after surface roughening to enhance absorptivity up to at least 0.3 and use the laser spot size in between 0.3 to 0.7 mm. The preferred energy density for the three workpieces: 217XG (SiC: <0.3 µm, 17 vol%), 225XF (SiC: <0.7 µm, 25 vol%) and 225XE (SiC: < 3 µm, 25 vol%), is 7.5, 6.7 and 8.2 kW/mm2, respectively.

Plant fibres composites (PFCs) have lately gained significant interest due to their environment friendly aspects and enhanced physical and mechanical performances. A subcategory of PFCs that uses thermoplastics has become widespread with the advent of the additive manufacturing industry. This work is focused on the investigation of the thermoformability of PFCs made using two techniques: hot-press thermoforming and 3D printing. The hot-press technique is widely used by researchers and industrials, whereas 3D printing is celebrated as a technique that can produce very complex shapes in a relatively facile manner. The main goal of the work here is to explore the ability of each technique to produce PFC objects without requiring elaborate setups. Another objective is to assess their integration, alongside the use of other assistive techniques and tools like a vacuum bagging and oven. These assessments have been aimed at achieving better manufacturing outcomes, such as a lower fabrication time. In the part related to the optimization of the hot-press technique, flax and polypropylene (flax/PP) laminates have been fabricated and examined, mechanically and aesthetically, for their sensitivity towards five fabrication parameters and two post-processing variables. The produced Flax/PP laminate has shown potential use as a replacement to its glass counterparts, due to their advantageous specific stiffness. Regarding the 3D printing technique, a fused filament fabrication (FFF) printer and Polylactic Acid (PLA) filament have been used to prototype auxetic cellular structures. The study begun by looking at the effect of the filament colour and the printer deposition angle on the mechanical performance of the prints. Afterward, auxetic cellular structures were printed and examined. Flat-shaped honeycombs Poisson's ratio (PR) and dome-shaped honeycombs indentation and drop impact properties have been investigated. Dome-shaped honeycombs, in particular, show an interesting softened post-buckling indentation, more accentuated with auxetic (i.e., negative Poisson's ratio) cellular configurations. PLA and wood-plastic composites filaments have been later successfully produced in the lab with printable characteristics using an off-the-shelf extruder. These filaments properties were found to rival their commercially available counterparts. PFCs fabrication and examination are yet to be established as a standardised industry, which is still impeded by the lack of open-access knowledge. Further developments are commented for the next big evolution in PFC laminate thermoforming technology.

Structural safety components of motor vehicles are designed to absorb the impact energy released during crash events in order to minimise the destructive consequences of unavoidable collisions. Due to the remarkable combination of price, density, and toughness, mineral-filled polymers have become the candidate material to manufacture automotive safety components. To predict their dynamic and inelastic mechanical response, experimentally and computationally expensive constitutive models have been implemented in finite element solvers, i.e. SAMP1 material model in the explicit finite element solver LS-DYNA. However, they struggled to be used in automotive applications due to their extensive experimental and numerical costs. The research project aims to develop an efficient and robust material model that is capable of simulating the mechanical behaviour of mineral-filled polymers when subjected to multi-axial loads at different strain-rates. To achieve this goal, the mechanical properties of a widely-used injection-moulded talc-filled Polypropylene compound were experimentally investigated and numerically simulated in LS-DYNA at quasi-static and dynamic strain-rates. In order to obtain accurate numerical simulations, reliable mechanical properties need to be extracted from material characterisation tests. The repeatability of uniaxial tensile tests was found to be improved when dog-bone shaped specimens were machined from injection-moulded plaques using the milling machine. Since no standardised methodology exists for non-fibre reinforced polymers, the repeatability of the in-plane shear test was also investigated. The modified Wyoming Iosipescu test exhibited the most reliable stress-strain results due to the consistent shear strain uniformity across the specimen during the experiment. A novel post-processing methodology was developed to accurately extract the stress-plastic strain curve that does not introduce numerical instability in the simulation of the uniaxial tensile test. A new definition of equivalent plastic strain was introduced in the constitutive equations to take into account the large volume dilatation that characterises mineral-filled polymers. Compared to SAMP1, the new constitutive model improves the numerical prediction of the material flexural behaviour by 20 % while reducing the simulation running time by 60%. When the bi-axial properties of the material are introduced in the constitutive equations using a reverse engineering methodology, the material model improves the numerical accuracy of the perforation test by 75 % for the quasi-static test and 65 % for the dynamic test. Since it reduces the simulation running costs by 60% for both tests, the new material model constitutes an efficient tool for CAE design engineers that want to simulate the dynamic and multi-axial response of structural components made of isotropic materials.

The research into artificial turf sport surfaces has seen significant growth over the last decade, linked to the proliferation of artificial turf surfaces in Europe for use in high participation sports. The latest third generation (3G) surfaces are typically comprised of multi-components exhibiting behaviours that are non-linear and rate dependant. Of particular importance is the vertical loading response, i.e. hardness or shock absorption, as it has been linked to both player performance and injury risk. Modelling sports surfaces can be of benefit to predict the loading response and allow for optimisation of geometry and materials in a virtual environment prior to changes in manufacture or construction. Thus, the work presented in this thesis is focussed upon the development of a numerical model to describe the behaviour of 3G artificial turf systems under vertical loading. The development of the numerical model required material stress-strain data to characterise the response to vertical loading. Material characterisation required the development of a novel methodology due to the limited loading rates of standard test devices. This methodology was based on the Advanced Artificial Athlete (AAA) vertical impact test with a specification developed to ensure valid stress-strain data was captured. Testing using this method, allowed for stress-strain data for the shockpad and carpet-infill layer to be collected at representative loading rates. This data, along with supporting stress relaxation data, provided the basis for material model calibration for each of these components. Material model calibration was a multi-stage process with the first calibration conducted by optimisation equations in a specialised material modelling software. A second manual optimisation, based upon initial results from a finite element (FE) simulation of the AAA FIFA test, allowed for refinement of the material model until a predefined set of accuracy criteria was met. Further simulations of AAA impacts from different drop heights were performed to validate the material models. Finite element models of two shockpads produced root mean square differences (RMSD) of < 5% from the experimental across AAA impacts at 25, 55 and 85 mm. The carpet-infill system modelled as a single part produced RMSD differences of ~8% however with the addition of a stiffer carpet backing added to the model, this was reduced to ~3% at the 55 mm drop height. Despite continued good agreement at the 25 and 85 mm drop heights (~5% RMSD), the energy absorption of the model was excessive (> 8%). Combining the models of shockpads with the carpet-infill system created a surface system model which was used to assess the predictive capability of a AAA impact. Results at 25 and 55 mm were good (<6%) but produced weaker agreement from 85 mm (< 10%). The work presented in this thesis supports the theory that FE modelling of 3G turf can assist in the design and optimisation of surfaces before physical construction. The methodology for experimental material characterisation and model calibration could be applied to different shockpads and carpet-infill systems. Further work should focus on the addition of the sand infill and the response to loading from successive AAA impacts.

Observing the physical effects of neutrons is logistically complicated due to their poor displacement efficiency, which requires irradiation durations extending from months to years to attain a relevant level of damage. Specimen activation leads to the requirement of a cooling off period to perform post irradiation examination outside of specialised facilities. Protons can produce defects with a similar structure and a density comparable to years of irradiation in a reactor in a few hours or days. Furthermore, at low energies their positive charge is repelled by nuclei, which reduces the probability of specimen activation and allows for off-site examination. The high dose rates and limited residual activity makes protons attractive as a surrogate for the study of irradiation damage. However, the limited penetration of protons, in the order of a few tens of microns, complicates studies probing standard mechanical properties beyond hardness measurement. In terms of the uniaxial tensile test the difficulty lies in the global nature of data acquisition, where the hardened surface properties are convoluted with that of the non-irradiated volume. The percentage of non-irradiated material in a 1 mm thick specimen irradiated with 3 MeV protons is upwards of 95%, which would lead to results heavily biased towards the non-irradiated properties. Therefore, the solution is to either employ a novel technique to directly record the surface properties or extract the desired layer. This project explored both solutions to the problem: 1) Directly probing the irradiated layer using a combination laboratory based X-ray diffraction (sin2ψ) & digital image correlation (DIC) and 2) Small scale tensile specimens were prepared using Xe+ plasma focused ion beam (PFIB) to increase specimen scale and attain a smallest representative volume. Both techniques were initially applied to non-irradiated SA508-4N for the purposes of validation. The combination of XRD and DIC accurately described both elastic and plastic regimes, that were recorded using the standardised technique on bulk specimens. Specimens prepared using PFIB exhibited a bulk representative yield stress, however specimen dimension and scale diminished the plastic response. Finite element analysis was applied to distinguish between the two limiting factors. It was demonstrated that the specimen geometry had a pronounced effect on the reduction of tensile strength and the reduced scale is thought to restrict strain hardening. The techniques were applied to SA508-4N irradiated with 3 MeV protons to 10 mdpa at the University of Manchester’s Dalton Cumbrian Facility. Both techniques recorded a positive yield shift in the same order estimated using indentation testing, with a reduction in strain hardening and an invariant tensile strength, which is consistent with literature reported behaviour. High resolution digital image correlation was applied to 316L in the non-irradiated state and irradiated to 100 mpda. A host of analysis techniques provided insight into deformation mechanisms just beyond the critical dose for dislocation channelling (100 mdpa). The work highlighted the differences in strain localisation and strain hardening behaviour of the two states and provided the opportunity to construct a flow curve based on the principal of equivalent pre-strain.

Dual-phase alpha plus beta Zr-Nb alloys have a higher strength and fracture toughness than single-phase alpha alloys and develop different crystallographic orientations (textures) during thermo-mechanical processing. The textures developed at manufacture are particularly important in determining the life-limiting in-reactor behaviour of nuclear components. Dual-phase Zr alloys tend to form a strong transverse (TD) texture of the basal pole, the origin of which is poorly understood and cannot be predicted by crystal plasticity texture evolution models. This is because the microstructure and texture evolution of these dual-phase alloys arises from complex interactions between the alpha (hexagonal-close-packed, hcp) and beta (body-centred-cubic, bcc) phases, during both deformation and phase transformation. The work presented here is an investigation of the texture evolution during high temperature rolling of an industrially used Zr-2.5Nb alloy, along with the hot-rolling and uniaxial compression of two model dual-phase Zr-Nb alloys (Zircaloy-4 with 2.5 wt.% Nb and Zircaloy-4 with 7 wt.% Nb). The aim was to determine the relative roles of plastic strain partitioning between phases, the activity of the different deformation modes and phase transformation on the final texture. The effect of temperature (700C to 825C), reduction ratio (50% to 87.5%) and strain rate, along with the influence of starting texture, was characterised using time-of-flight (TOF) neutron diffraction and EBSD techniques. The alpha transverse texture component, with prismatic alignment {11-20}, strengthens with greater rolling reduction at the higher temperatures, accompanied by a weakening of basal part orientations with 0002 in ND. Software reconstruction of EBSD orientation maps, using the Burgers relationship, shows how the strength of the transverse texture component varies across the material depending on the orientation of large prior-beta grains. A more detailed characterisation of the high temperature deformation and phase transformation behaviour was made on a hot-rolled Zircaloy-4 with 7 wt.% Nb alloy. Since a greater proportion of metastable beta-Zr is retained to room temperature, a snapshot of the material before beta to alpha phase transformation can be captured, distinguishing high temperature primary alpha grains from the nucleation and growth of secondary alpha variants. By analysing these structures in 3D, using a plasma focused ion beam (PFIB) and taking sequential EBSD slices, it was found that the degree of breakup is affected by the distribution of primary alpha laths within each beta-grain. Further analysis shows that the orientation of the primary alpha influences the breakup behaviour of the beta-grains. Softer alpha orientations, with 0002 in TD, are favoured through a slip compatibility with the beta-matrix. Harder alpha grain orientations develop a much higher misorientation, with a greater stored energy to undergo a dynamic transformation, during deformation. These findings suggest new ways in which the current models can be developed, to enable the successful prediction of hot-rolling textures in these alloys.

One of the main limitations of wrought magnesium alloys is their high yield asymmetry and anisotropy due to the intrinsic plastic anisotropy of the magnesium single crystal hexagonal close packed structure. This can lead to strong textures and twinning-dominated deformation. Rare earth additions to magnesium have been shown to reduce this yield asymmetry and anisotropy, though the high price of the rare earth metals and security of supply issues drives the need for alternative alloys that can replicate this e ect. Reducing the grain size of wrought alloys can achieve this to some extent, as can careful choice of precipitates in age-hardenable alloys which can preferentially strengthen weaker deformation systems such as twinning. All three approaches were explored in this thesis in an alloy design context. Magnesium Elektron UK set out an alloy criteria to full for this work with a focus on reduced yield asymmetry and anisotropy, and low cost. Two promising alloying systems were considered for this criteria, both with low rare-earth contents: extruded Mg-Nd-Gn-Zn-Zr (Elektron 21) and rolled Mg-Zn-Ca(-Ce). In addition, modelling methods were developed to design, from metallurgical principles, alloys that full the criteria by exploiting precipitation. This required a determination of the e ect of multiple precipitate types on mechanical asymmetry and anisotropy by extending Orowan strengthening theory to this case, and implementing the results in a polycrystalline crystal plasticity model. Through this modelling work, several extruded Mg-Sn-Zn(-Al-Ca-Na) alloys were designed, produced, and tested against the target properties. TEM observations revealed that ageing the Mg-Sn-Zn(-Al-Ca-Na) alloys at 150°C caused the di erent precipitate types to nucleate together, not independently as was the case when aged at 200°C. This co-precipitation caused a reduction in the predicted mechanical symmetry and isotropy, and strength. Na additions were shown to re ne the precipitate populations in these alloys but strengthen the texture through preferential growth of recrystallised grains during extrusion and solution treatment. The most effective method of reducing mechanical asymmetry and anisotropy was found in this work to be the texture weakening and solid solution strengthening produced by rare-earth additions, followed by grain-size reductions and producing precipitate populations that strengthened weaker deformation systems. Out of the alloys tested, TZA941+0.3Na fullled the mechanicalproperty criteria with a tensile yield stress in the extrusion direction of 360MPa and a minimum yield stress of 222MPa.

In the drive towards higher strength alloys, a diverse range of alloying elements is employed to enhance their strength and ductility. Limited solid solubility of these elements in steel leads to segregation during casting which affects the entire down-stream processing and eventually the mechanical properties of the finished product. The work presented in this PhD shows that segregation of alloying elements during casting, particularly aluminium, leads directly to banding in the final product. It has been demonstrated that no significant homogenisation is possible in this alloy within practical time constraints of the industrial thermo-mechanical process. A through-process model was developed to design a thermo-mechanical treatment aimed at reducing the effects of segregation on the formation of banding. A new polynomial function for calculating the local phase transformation temperature (Ae3) between the austenite + ferrite and the fully austenitic phase fields during heating and cooling of steel is presented. Material was produced both with and without banding and used to study the effect upon the mechanical properties. The banded steel variants show a significant reduction in tensile strength for a similar level of ductility compared to non-banded variants. In situ measurement under uniaxial loading using high-energy synchrotron diffraction allowed direct quantification of the impact of the mechanically induced transformation of metastable austenite on the work- hardening behaviour. The results reveal that the mechanically induced transformation of austenite does not begin until the onset of matrix yielding and the experimental evidence demonstrates that the austenite to martensite transformation increases the work-hardening rate of the ferrite phase and delays the onset of Stage-III hardening until the yield point of austenite. The increase in work-hardening rate (and thus work required) supports a driving force approach to transformation induced plasticity. The transformation work required leads to an increase in the macroscopic work-hardening rate after matrix yielding which offsets the decrease in the work-hardening rate in the ferrite and martensite phases up to the UTS. Steels with a high degree of banding do not show this extra contribution due to the more dominant anisotropic effect of martensite bands on the work-hardening of ferrite coupled to increased mechanical austenite stability as a result of increased carbon content. A list of revisions as requested by the examiners is produced on pages 18 and 19 of the thesis for examination. Abstract: In the drive towards higher strength alloys, a diverse range of alloying elements is employed to enhance their strength and ductility. Limited solid solubility of these elements in steel leads to segregation during casting which affects the entire down-stream processing and eventually the mechanical properties of the finished product. The work presented in this PhD shows that segregation of alloying elements during casting, particularly aluminium, leads directly to banding in the final product. It has been demonstrated that no significant homogenisation is possible in this alloy within practical time constraints of the industrial thermo-mechanical process. A through-process model was developed to design a thermo-mechanical treatment aimed at reducing the effects of segregation on the formation of banding. A new polynomial function for calculating the local phase transformation temperature (Ae3) between the austenite + ferrite and the fully austenitic phase fields during heating and cooling of steel is presented. Material was produced both with and without banding and used to study the effect upon the mechanical properties. The banded steel variants show a significant reduction in tensile strength for a similar level of ductility compared to non-banded variants. In situ measurement under uniaxial loading using high-energy synchrotron diffraction allowed direct quantification of the impact of the mechanically induced transformation of metastable austenite on the work- hardening behaviour. The results reveal that the mechanically induced transformation of austenite does not begin until the onset of matrix yielding and the experimental evidence demonstrates that the austenite to martensite transformation increases the work-hardening rate of the ferrite phase and delays the onset of Stage-III hardening until the yield point of austenite. The increase in work-hardening rate (and thus work required) supports a driving force approach to transformation induced plasticity. The transformation work required leads to an increase in the macroscopic work-hardening rate after matrix yielding which offsets the decrease in the work-hardening rate in the ferrite and martensite phases up to the UTS. Steels with a high degree of banding do not show this extra contribution due to the more dominant anisotropic effect of martensite bands on the work-hardening of ferrite coupled to increased mechanical austenite stability as a result of increased carbon content.

Butt fusion welding process is an extensively used method of joining for high density polyethylene (HDPE) pipe. With the increasing number of HDPE resin and pipe manufacturers and the diversity of industries utilising HDPE pipes, a wide range of different standards have evolved to specify the butt fusion welding parameters with inspection and testing methods, to maintain quality and structural integrity of welds. There is a lack of understanding and cohesion in these standards for the selection of welding parameters; effectiveness, accuracy, and selection of the test methods and; correlation of the mechanical properties to the micro and macro joint structure. The common standards (WIS 4-32-08, DVS 2207-1, ASTM F2620, and ISO 21307) for butt fusion welding were used to derive the six welding procedures. A total of 48 welds were produced using 180 mm outer diameter SDR 11 HDPE pipe manufactured from BorSafe™ HE3490-LS black bimodal PE100 resin. Three short term coupon mechanical tests were conducted. The waisted tensile test was able to differentiate the quality of welds using the energy to break parameter. The tensile impact test due to specimen geometry caused the failure to occur in the parent material. The guided side bend specimen geometry proved to be too ductile to be able to cause failures. A statistical t-test was used to analyse the results of the short term mechanical tests. The circumferential positon of the test specimen had no impact on their performance. Finite element analysis (FEA) study was conducted for the long term whole pipe tensile creep rupture (WPTCR) test to find the minimum length of pipe required for testing based on pipe geometry parameters of outer diameter and SDR. Macrographs of the weld beads supplemented with heat treatment were used to derive several weld bead parameters. The FEA modelling of the weld bead parameters identified the length to be a key parameter and provided insight into the relationship between the geometry of the weld beads and the stresses in the weld region. The realistic bead geometry digitised using the macrographs contributed a 30% increase in pipe wall stress due to the stress concentration effect of the notches formed between the weld beads and the pipe wall. The circumferential position of the weld bead had no impact on the pipe wall stresses in a similar manner to the results of the different mechanical tests. IV Nanoindentation (NI) and differential scanning calorimetry (DSC) techniques were used to study the weld microstructure and variation of mechanical properties across the weld at the resolutions of 100 and 50 microns, respectively. NI revealed signature 'twin-peaks and a valley' distribution of hardness and elastic modulus across the weld. The degrees of crystallinity obtained from DSC followed the NI pattern as crystallinity positively correlates with the material properties. Both techniques confirm annealing of the heat affected zone (HAZ) material towards the MZ from the parent material. The transmission light microscopy (TLM) was used to provide dimensions of the melt zone (MZ) which displays an hour glass figure widening to the size of the weld bead root length towards the pipe surfaces. Thermal FEA modelling was validated using both NI and TLM data to predict the HAZ size. The HAZ-parent boundary temperature was calculated to be 105 ⁰C. The 1st contribution of the study is to prove the existence of a positive correlation between the heat input calculated from FEA and the energy to break values obtained from the waisted tensile test. The 2nd contribution providing the minimum length of pipe for WPTCR based on the pipe dimensions. The 3rd contribution is the recommendation for the waisted tensile test with the test using the geometry designed to minimise deformation of the loading pin holes. The 4th contribution related the weld bead parameters to pipe wall stresses and the effect of notches as stress concentrators. The 5th contribution is a new method of visualising a welding procedure that can be used to not only compare the welding procedures but also predict the size of the MZ and the HAZ. The 6th contribution of the study is the proposal of new weld bead geometry that consist of the MZ bounded by the HAZ, for butt fusion welded joints of HDPE pipes.

Artificial surfaces are increasingly more common in a number of sports including football, rugby and hockey. Each specific sport has mechanical properties designed to suit the requirements of the sport which can be achieved through appropriate selection of surface specification, as well as the appropriate selection of footwear. In player-surface interactions, traction is a key system property that needs to be measured for comfort, performance and any potential injury risk. Many of the current industry tests used to measure traction are simplistic and have limitations when used in tests. The aim of the thesis was to make a contribution to knowledge with regard to the mobilisation of traction and apply this to the understanding of shoe-surface interactions. This was achieved by completing a number of objectives. These included reviewing current knowledge of player-surface interaction behaviour in relation to traction and obtaining relevant human boundary conditions for biofidelic mechanical test development. The mechanisms of traction were then investigated and the variables in the mobilisation of traction identified. The traction forces developed were quantified with appropriate measurement systems. Mechanical test equipment was then developed along with protocols to replicate the translational and rotational lower limb behaviour during sport specific behaviour. This included the standard FIFA rotational device being modified to include two sensors which record continuous data throughout a trial to allow for more than a peak torque value to be analysed. In addition, a piece of equipment to measure translational traction was developed and constructed to support the rotational traction device and help to understand the mobilisation of traction. The device pulled a tray containing a surface sample, with a shoe/plate placed on the sample. The horizontal force was measured, as well as the amount of stud penetration into the surface. It was also necessary to characterise the state of the surface and the effects that any changes may have on the traction that is mobilised. Testing completed involved repeated testing on both the rotational and translational to allow for comparison. Changes in the surface properties were made such as the number of fibres in a set area and the rubber infill density as well as shoe properties such as stud spacing, stud type and number of studs. In the results, the initial stiffness response of the surface was often focussed on as it was stated that this may be a better indicator of the mechanisms involved in the traction mobilised by subjects, compared to peak torque. This is due to actual foot rotation measured in subject testing being observed to be much smaller than the rotation/distance required to produce the peak force. The larger angles/displacements were also considered to help inform the mechanisms of traction. The final objective was to refine the mechanisms based on the experimental design. This all adds to the contribution of knowledge regarding the mobilisation of traction. A key outcome from the thesis is the effect the surface and shoe properties have on traction, therefore it is essential to state the specification when reporting results otherwise comparisons are not able to be made. The mechanism of traction has not previously been fully understood, with this thesis beginning to understand the details of how the change in surface or shoe properties affect how the surface reacts during shoe-surface interactions.

The majority of UK’s intermediate level radioactive waste is currently stored in 316L and 304L austenitic stainless steel containers in interim storage facilities for permanent disposal until a geological disposal facility has become available. The structural integrity of stainless steel canisters is required to persevere against environmental degradation for up to 500 years to assure a safe storage and disposal scheme. Hitherto existing severe localised corrosion observances on real waste storage containers after 10 years of exposure to an ambient atmosphere in an in-land warehouse in Culham at Oxfordshire, however, questioned the likelihood occurrence of stress corrosion cracking that may harm the canister’s functionality during long-term storage. The more corrosion resistant duplex stainless steel grade 2205, therefore, has been started to be manufactured as a replacement for the austenitic grades. Over decades, the threshold stress corrosion cracking temperature of austenitic stainless steels has been believed to be 50-60°C, but lab- and field-based research has shown that 304L and 316L may suffer from atmospheric stress corrosion cracking at ambient temperatures. Such an issue has not been reported to occur for the 2205 duplex steel, and its atmospheric stress corrosion cracking behaviour at low temperatures (40-50°C) has been sparsely studied which requires detailed investigations in this respect. Low temperature atmospheric stress corrosion cracking investigations on 2205 duplex stainless steel formed the framework of this PhD thesis with respect to the waste storage context. Long-term surface magnesium chloride deposition exposures at 50°C and 30% relative humidity for up to 15 months exhibited the occurrence of stress corrosion cracks, showing stress corrosion susceptibility of 2205 duplex stainless steel at 50°C.The amount of cold work increased the cracking susceptibility, with bending deformation being the most critical type of deformation mode among tensile and rolling type of cold work. The orientation of the microstructure deformation direction, i.e. whether the deformation occurred in transverse or rolling direction, played vital role in corrosion and cracking behaviour, as such that bending in transverse direction showed almost 3-times larger corrosion and stress corrosion cracking propensity. Welding simulation treatments by ageing processes at 750°C and 475°C exhibited substantial influences on the corrosion properties. It was shown that sensitisation ageing at 750°C can render the material enhanced susceptible to stress corrosion cracking at even low chloride deposition densities of ≤145 µm/cm². However, it could be shown that short-term heat treatments at 475°C can decrease corrosion and stress corrosion cracking susceptibility which may be used to improve the materials performance. Mechanistic understanding of stress corrosion cracking phenomena in light of a comprehensive microstructure characterisation was the main focus of this thesis.

Lower urinary tract disorders are significant prognostic indicators of institutionalisation and lower quality of life in the elderly and their incidence increases with age. Urodynamics, the gold standard in diagnosis, replicates symptoms to assess functionality through controlled filling and voiding of the bladder but its interpretation is subjective and may be inconclusive; often requiring further testing or leading to inappropriate treatment. Normal filling and voiding biomechanics of the bladder relate directly to the structural composition of the bladder wall. Alterations to tissue composition in aging and pathology have significant impacts on biomechanics but are yet to be fully described. The aim of this thesis was to gain insight into the individual microstructural components of the bladder wall and how they relate to the gross mechanical response. Additionally, representation of these observations in a mathematical model that can be used to improve our understanding of urodynamic data. This aim was achieved through a combination of in situ mechanical testing and the development of a microstructural constitutive model, which was then included within an overall micturition framework to simulate filling and voiding functions, and evaluated with clinical data. Coupled systems of multiphoton microscopy and uniaxial, biaxial and inflation testing were used to correlate extra cellular matrix interactions with the mechanical response of young and aged murine bladder. Wall-layer specific collagen fibre orientation, dispersion and recruitment were quantified and implemented into a novel microstructural constitutive model. The bladder was modelled as a nonlinear elastic, constrainedmixture planar membrane with contribution from smooth muscle and collagen fibres in the detrusor. Collagen recruitment in the detrusor was observed to occur at a finite stretch; correlated with a steep increase in stiffness of the tissue, while collagen of the lamina propria plays a capacitance role. Collagen recruitment was modelled using a triangular probability density function; quantified from sequential microscopy images and fitted to mechanical data. Increased collagen area fraction and changes in dominant fibre orientation were attributed to reduced compliance in aged bladder. This behaviour was captured by the model. The microstructural model was modified to an isotropic thin-walled spherical membrane for the filling phase of a micturition model framework, consisting of a bladder outlet relation and urethral resistance relation. A contractile smooth muscle element was included in the active response. In the first steps towards clinical application the model was applied to male and female 'normal' patient urodynamic data to observe quality of model fit and estimate baseline parameter values. The model simulated key filling and voiding features seen in normal male and female clinical data. Mechanobiological modelling combined with clinically relevant micturition modelling has the potential to quantify bladder dysfunction. Moreover, improved understanding of how the microstructure influences macroscopic mechanics will yield improved understanding of how changes to the bladder impair its functionality. We predict that modelling will become a clinically relevant tool in urodynamics; leading to new options for diagnosis and management of patients with bladder dysfunction.

3D woven composites, due to the presence of through-thickness fibre bridging, have the potential to improve damage tolerance and at the same time to reduce the manufacturing costs. However, the ability to withstand damage depends on weave architecture as well as the geometry of individual tows. A substantial amount of research has been performed to understand in-plane properties as well as the performance of 3D woven composites exposed to impact loads, but there is limited research on the damage tolerance and notch sensitivity of 3D weaves and no work is reported on the damage tolerance of 3D weaves with a weft binding pattern. In view of the recent interest in 3D woven composites, the influence of weft binder on the tensile, open hole tensile, impact resistance and subsequent residual compressive strength properties and failure mechanisms of 3D woven composites was investigated against equivalent UD cross-ply laminate. Four different 3D woven architectures; layer-to-layer, angle interlocked, twill angle interlock and modified angle interlock structures were produced under identical weaving conditions. All the above mentioned tests were performed in both the warp and weft directions on 3D woven and UD cross-ply laminates. Stress concentration and yarn waviness due to through-thickness reinforcement led to lower mechanical properties compared with the UD cross-ply laminate. However, improved in-plane and damage tolerance properties of 3D woven composites under tensile loads were achieved by modifying the weave architecture. The influence of the weave architecture and binder yarn orientation on the notch insensitivity and damage tolerance of 3D woven composites was less significant for compressive loads. Despite the lower undamaged compression strength of 3D woven structures, their residual compressive strength was found to be superior to their equivalent UD cross-ply laminates. The lower rate of strength reduction in the 3D woven fabrics laminates was attributed to a crack bridging mechanism, effectively inhibiting delamination propagation.

The research contained within this thesis formed part of an Engineering and Physical Sciences Research Council (EPSRC) funded project based at Loughborough University, which aimed to investigate the use of additive manufacturing (AM), and in particular sintering technologies, for the production of running footwear sole units. Laser sintering (LS) is an AM process which produces parts directly from a computer aided design (CAD) file by selectively fusing successive layers of powdered material using a CO2 laser. LS imparts significant advantages over traditional manufacturing techniques including extensive design freedom, the ability to manipulate the local properties of a single material part as well as economical manufacture of bespoke items due to the elimination of tooling. Modifying the mechanical properties and/or geometry of sole units has been shown to provide benefits in the areas of performance, injury risk reduction and comfort, especially when considering elite athletes on a subject specific basis. Given the attributes of LS outlined above, the technology offers significant potential to produce sole units offering high added-value compared to conventional counterparts which are limited by the constraints of traditional processing techniques such as injection moulding. However, the mechanical capacity of LS polymers in context of such application was unknown. Accordingly, this research investigated the suitability of a laser sintered elastomer (LSE) material, in view of key selected mechanical properties, for the manufacture of running shoe midsoles. The midsole is the primary functional component in the sole unit of a running shoe used for distance running on hard surfaces. Following a preliminary assessment of the selected LSE (TPE 210-S), a new dynamic test method was designed to assess the compressive, fatigue and time dependent recovery properties of midsole material specimens under loading conditions representative of in-service use. The method was successfully implemented on an electro-mechanical test apparatus (previously unreported upon in literature) and used firstly, to benchmark the aforementioned properties of a range of ethylene vinyl acetate (EVA) and polyurethane (PU) midsole foams representative of the range currently used in production, and secondly, to establish the same property set for TPE 210-S specimens produced across a range of laser powers (LP's). Initial cycle operating ranges in terms of key compressive properties were established for EVA and PU materials. All conventional variants showed considerable deterioration from these initial values over the 125,000 cycle test regime, but subsequently demonstrated partial recovery when left unloaded post-test. PU grades generally exhibited better fatigue performance and findings were consistent with those of previous studies. Whilst variation in LP facilitated linear variation in displacement and stiffness properties for TPE 210-S, all specimens yielded a stiffer and more elastic response than that of conventional foams at the outset; initial compressive operating ranges, whilst within close proximity, did not overlap. However, fatigue performance was found to be superior with only relatively small property changes occurring over the test regime regardless of LP. Furthermore, no signs of catastrophic specimen failure (e.g. cracking) were visually apparent. In this respect the material showed good suitability for midsole applications, but further work is required to address increasing the available compressive property range which fell outside the scope of this work.

Forty ovine femurs were harvested and allocated into four testing groups; Fast-Normal, Fast- Decalcified, Slow-Normal, Slow-Decalcified. Contralateral pairings were used within these groups for closer comparison. Dynamic testing apparatus was designed and built allowing rates of strain similar to road traffic accidents to be investigated. These strain rates were achieved by using a pneumatic actuator to apply the load. Slow rate loading was achieved by testing with a commercially available mechanical testing machine at a rate of strain similar to that created by walking. Bone quality was altered by ultrasonically assisted decalcification in EDTA. Levels of mineral dissolution equivalent to the loss of bone mineral density (BMD) of a 75 year old woman were targeted. Whole bone was used for these experiments to facilitate comparison with real fracture radiographs obtained from NHS database. Fracture patterns and degree of comminution were similar between experimental and patient data. Bone is often analysed as a simple beam (engineers bending theory). This method of stress analysis was compared with a method that recognised the change in cross section over the length of the bone. Accounting for this had a highly significant effect on the calculated flexural modulus (p<0.0005).The length to depth ratio of whole bone indicates that shear forces cannot be ignored. The effect of the contribution from shear force on the deflection was investigated. After accounting for deflections due to shear, calculated normal strains agreed with literature values. Deflection due to shear was found to make a significant contribution to the deflection The effect of storage (freezing) on the mechanical properties at high strain rate was evaluated: no significant differences were found for force and deflection at failure. The main body of testing gave the following results: Normal quality bone, rate compared showed significant differences for Ultimate Stress, Ultimate Strain, Yield Strain, Flexural Modulus and Toughness. Demineralising bone resulted in no statistically significant differences between the loading rates for the Stress at failure. Yield Strain, Ultimate Strain, Flexural Modulus and Toughness did show significant differences. The fast loading tests showed significant differences when comparing quality for Stress at failure but not at Yield. Significant differences were found when comparing toughness. Slow loading tests showed significant differences between bone qualities for Stress at failure in contralateral pairs. No significant differences were found for strain or toughness. These results indicate that bone of normal quality can withstand higher than normal stresses for short durations. This ability is lost in demineralised bone. The high loading rate tests revealed closely matched strains at failure for both bone qualities, lending support to the strain based failure theory for bone at traumatic strain rates.

High-performance fibres such as carbon, glass and kevlar are very promising for aerospace applications because of their high strength, stiffness, impact damage and excellent fatigue life. The high cost of the prepreg materials such as pre-impregnated fibre tape/tow and fabrics, and limitations of existing manufacturing processes are a big challenge for the aerospace industry to meet increasing performance demands. Their benefits can only be achieved by using low cost materials and manufacturing methods. In the past three or four decades, there have been substantial technological developments, which are governed by the new materials and their associated manufacturing techniques. The production of carbon fibre is slow and capital intensive, therefore, carbon manufactures produce higher tow counts (number of filaments) to increase production through-put in order to reduce its cost. In other words, 12k carbon tow is much cheaper than 6k or 3k carbon tow. In many applications finer tows are desirable. In this thesis, a fully automated laser feedback tow splitting line has been developed to split higher tow counts (12k spool) into smaller tow counts (split into 6k spools) in order to produce low cost material. The quality of the split tows has been evaluated by recording the data online during the splitting process. The recorded data was later analysed by statistical tools. A four axis modular gantry robotic system has been developed at the University of Manchester in order to deposit dry fibres in a completely flexible manner. To facilitate robotic preforming, an end-effector and mould have been designed and developed in this research. The tow placement program was written in the CoDeSys software which is then uploaded into the motion controller to perform specific motions. The cross-ply laminates have been manufactured by the proposed robotic system using split 6k (produced by the tow splitting process) and original 12k carbon tows. Mechanical test of both composites (12k and split 6k) are presented. A tufting process has been developed and conducted by the robotic system in order to manufacture 3D preforms. The tufted composite was compared with 3D woven and stitched 2D broadcloth in terms of the tensile and interlaminar shear strength properties. X-ray tomography has been conducted to investigate preform geometrical variations of manufactured composites. In addition, preforming cost models have been developed for robotic fibre placement and 3D weaving.

This thesis investigates the mechanical properties of the elastic proteins isolated by cyanogen bromide digestion from lamprey cartilages and compares them with the mammalian protein, elastin. Thermomechanical testing and measurements of the effects of hydrophobic solvents on mechanics are used to determine the energetic and entropic contributions to the mechanical properties and the role of solvent interactions. Raman microspectrometry is shown to be a valuable tool in determining the secondary structure of the proteins, their interactions with water and molecular-level effects of mechanical strain. The supramolecular structure of the proteins matrices are investigated using nonlinear microscopy and X-ray diffraction. The mechanical properties of fibrous elastin agreed with those previously reported with elastic moduli in the region of 0.2-0.4 MPa. Elastic moduli decrease by approximately 25% with increased temperature, which was accompanied by a small decrease in hysteresis loss. In agreement with earlier findings, an entropic mechanism of elasticity became dominant only at high temperatures with a major contribution from interactions with solvent water. The lamprey proteins can be divided into two broad groups, the 'soft' branchial and pericardial cartilages resembling elastin, with linear stress-strain behaviour over a range of strains, elastic moduli in the range 0.13 MPa to 0.35 MPa, breaking strains of up to 50% and low hysteresis. Annular and piston proteins showed a very different response having much higher elastic moduli (0.27 MPa to 0.75 MPa), higher breaking strains and large hysteresis. Similarities between elastin and the lamprey matrix proteins extended to their thermomechanical behaviour with a decrease in elastic moduli and a drive towards entropic elasticity at high temperatures, although the annulus and piston were less thermally stable. Raman spectroscopy was able to detect differences between the various proteins and between elastin fibres and fragmentation products. Although no vibrational modes associated with cross-linking of the fibres could be identified, the secondary structure of dehydrated fibrous elastin was significantly different from \alpha -elastin. The former differed from previous experimental measurements, but was close to the theoretical predictions with 36% \beta -structures, 46% unordered and 18% \alpha -helix. \alpha -Elastin contained 29% \beta -structures, 53% unordered and 18% \alpha -helix. Strains of up to 60% in ligament fibre bundles resulted in no significant shifts in peak positions or in secondary structure. Polarization measurements revealed that the peptide bonds and several of the bulky side-chains re-orientated closer to the fibre axis with strain. Heating nuchal elastin fibres to 60^{\circ} C to increase the energetic component of the elasticity was associated with a 30% increase in the proportion of \beta -structures in the amide I band, a 50% increase in the amide III band, and a 50% reduction in the signal from bound water. The Raman spectra of the lamprey matrix proteins are similar both to each other and when compared to fibrous elastin. Only small differences could be detected in side-chain modes consistent with reported biochemical differences. Decomposition of the amide I band indicated that the secondary structures were also very similar to that of elastin, with a preponderance of unordered structures which probably confer the high degree of conformational flexibility necessary for entropy elasticity. Piston and annular proteins, like elastin, showed a strong interaction with water, suggesting a greater role of hydrophobic interactions in their mechanics compared to the branchial and pericardial proteins. Elastin is well known to exhibit autofluorescence. However, only the branchial protein has been reported to autofluoresce. This study shows that all four lamprey matrix proteins investigated exhibit strong autofluorescence which was subsequently exploited to image these tissues using multiphoton microscopy. Microscopic investigations revealed that the architecture of lamprey proteins differ from that of elastin. Nuchal elastin forms bundles of fibres running predominantly parallel to the direction of applied force. The arrangement in lamprey cartilage is very different forming honeycomb structures, which in the case of annular and piston cartilages, is surrounded by a dense sheath of matrix material. Dye injections revealed that the branchial and pericardial form open systems whereas in piston and annular cartilages a closed system exists. These variations in architecture are reflected in their different mechanical properties and in vivo functions.

Plastic packaging waste currently forms a significant part of municipal solid waste and as such is causing increasing environmental concerns. Such packaging is largely non-biodegradable and is particularly difficult to recycle or to reuse due largely to its complex compositions. Apart from limited recycling of some easily identifiable packaging wastes that can be separated economically, such as bottles, most packaging waste ends up in landfill sites. In recent years, in an attempt to address this problem in plastic packaging, the development of packaging materials from renewable plant resources has received increasing attention and a wide range of bioplastic materials based on starch are now available. Environmentally these bioplastic materials also reduce reliance on oil resources and have the advantage that they are biodegradable and can be composted upon disposal to reduce the environmental impact. Many food packaging containers are produced by thermoforming processes in which thin sheets are inflated under pressure into moulds to produce the required thin -wall structures. Hitherto these thin sheets have almost exclusively been made of oilbased polymers and it is for these that computational models of thermoforming processes have been developed. Recently, in the context of bioplastics, commercial thermoplastic starch sheet materials have been developed. The behaviour of such materials is influenced both by temperature and, because of the inherent hydrophilic characteristics of the materials, by moisture content. Both of these aspects affect the behaviour of bioplastic sheets during the thermoforming process. This thesis describes experimental work and work on the computational modelling of thermoforming processes for thermoplastic starch sheets using a commercially available material. The experimental work has been carried in order to characterise the deformation behaviour of the material with regard to different temperature, moisture contents and strain rates. Thermoforming of the material was performed and samples produced were used for comparison and verification of the computational modelling of the thermoforming process. In the first attempt to model the thermoforming process, a hyperelastic constitutive equation was established to approximate the material behaviour taking account of the combined effects of temperature and moisture content and a simple ii membrane model with constrained deformation was used to model an axisymmetric case of thermoforming. Simulations with this model showed that moisture content mostly affects the pressure required to push the sheet into the mould while moisture variation during thermoforming has little effect on the final thickness distribution of the product. Considerable discrepancies were found in the thickness distribution between the predictions from the model and the experimental measurements. Further attempts were made to take account of the elasto-plastic behaviour of the material and a more complex three-dimensional FE model was developed using ANSYS/LS-DYNA. Based on the findings in the simpler modelling work, no attempt was made to incorporate the moisture content effect on material behaviour but the material parameters for the elasto-plastic constitutive equation were obtained from high speed tensile tests so that moisture variation during thermoforming could be minimised and neglected. The predictions from this model have led to significant improvements in prediction of the thickness distribution which has become much closer to the experimental measurements in comparison with the hyperelastic model. This work provides some important insights into thermoforming of thermoplastic starch materials: a) Deformation behaviour of such materials depends strongly on the moisture content and the temperature, both of which affect behaviour during thermoforming processes, including the preheating stage; b) moisture variation during the thermoforming process has a significant effect on the pressure required for the deformation. This also leads to variation of moisture content distribution in the final product, which in turn affects the material properties such as ductility or impact strength at different positions in the thermoformed structure; c) thermoforming of thermoplastic starch materials can be simulated more accurately by an elasto-plastic model and the LS-DYNA algorithm in comparison with a hyperelastic membrane model. This work has provided useful information on thermoforming of thermoplastic starch materials with particular reference to the design of thermoforming tools and to the careful control of processing conditions including preheating. It has also laid a solid foundation for future work on how the moisture variation impacts on the formation of defects such as incomplete forming due to material hardening and fracture due to loss of ductility.

High-Entropy Alloys have been a highly researched area of metals ever since their introduction in 2004 by Brian Cantor and Jien-Weh Yeh. In the continued research of High-Entropy Alloys (HEAs), a specific area concerning Refractory High-Entropy Alloys (RHEAs) has emerged for their high-temperature applications. Although RHEAs have maintained high strength and toughness at high temperatures, their low ductility still needs to be addressed. A dataset was created to find correlations between various characteristics of RHEAs and their composition. A set of seven compositions were selected and fabricated. Mechanical tests were run on the seven compositions, and a proposal was written for neutron diffraction tests at Oak Ridge National Lab. In-situ neutron diffraction was performed during mechanical testing on the seven compositions. X-ray diffraction was performed on as-cast samples and post-mortem samples of each composition. The microstructural analysis led to one out of the seven RHEAs being confirmed to contain Transformation Induced Plasticity (TRIP). The six remaining RHEAs were confirmed as a single-phase BCC. Nanoparticles were fabricated from three of the confirmed single-phase compositions and characterized using X-ray diffraction and Scanning Electron Microscope (SEM) Imaging. All three were found to be a single-phase FCC nanoparticle.

The plantar plate is a fibrocartilaginous tissue beneath the metatarsophalangeal joint (MTPJ). Plantar plate function centers around maintaining the static stability of the MTPJ, and its integrity facilitates the dynamic stabilizing functions of surrounding soft tissue structures. Injury to the plantar plate can cause significant forefoot discomfort focused around the MTPJ, swelling, and altered forefoot biomechanics from toe instability. Significant injury like either partial or complete tear of the plantar plate commonly requires surgical intervention to repair the tissue. As fibrocartilage, the plantar plate lacks an intrinsic capacity for robust healing, thus requiring a surgical repair aiming to restore proper function. Existing plantar plate repair techniques afford different perspectives for restoring plantar plate biomechanical function, though room for improvement exists for an enhanced repair. Our senior design team developed a novel approach for plantar plate repair using a two-piece snap fitting permanent implant. This novel technique was reduced to practice and required further experimental analysis of its functional capacity to inform future development. Two methodologies were used to evaluate the novel implant device designed for plantar plate repair. An implant isolated mechanical testing protocol was developed to evaluate the implant and suture construct of the repair in anatomically relevant orientations. A human cadaver tissue model protocol was employed to evaluate the integrity of the native plantar plate tissue, a simulated conventional repair, and our novel implant fixation repair. These methodologies used uniaxial tensile testing with custom test configurations to evaluate the structural integrity and properties of the implant-suture construct and simulated tissue only or tissue-repair constructs, respectively. Our results provided encouraging support for the use of mechanical testing and the continued development of this novel implant device for plantar plate repair. Additionally, qualitative outcomes from this testing revealed additional avenues to improve the novel implant device in support of further advancing the product for future use in the field of podiatric medicine.

This thesis presents a comparative analysis of the dissimilar laser welding of Ti6Al4V and Ni-Ti with two different interlayers, namely vanadium and niobium, using a continuous fiber laser welding machine. This study attempts to solve the problem associated with dissimilar welding of the Ni-Ti and Ti6Al4V with the use of interlayers specimen. The objective of this study is to improve the welding strength between Ni-Ti and Ti6Al4V in comparison to previous research and to investigate the effect of interlayer composition on the quality of the weld joint. The welding process was performed using identical laser power, welding speed, and focal position, and the quality of the weld joint was evaluated through scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) analysis, hardness testing, and tensile testing.The welding was successfully performed using both interlayers. The tensile strength of the welded samples with niobium interlayer was found to be 100 MPa greater than that of the samples with vanadium interlayer. Scanning electron microscopy images showed that the fracture occurred at the welding region interface between the Ni-Ti-interlayer in both cases due to the dendritic structure, which caused the region to be more brittle. Furthermore, the hardness of the Ni-Ti-interlayer interface was higher, resulting in brittle fracture at the same interface in both cases during tensile testing.The findings suggest that the use of niobium interlayer produces a higher quality weld joint with improved mechanical properties under the same laser welding parameters compared to the vanadium interlayer. These results are significant for designing the laser welding process and selecting the appropriate interlayer for specific applications. Further research can be conducted to optimize the laser welding parameters and explore the impact of different interlayer thicknesses on the welding behavior.

Concrete is the most widely used building material in the world and has many different applications and uses, all requiring specific properties and mix designs. While concrete is an incredibly valuable and versatile material for construction, it has a major drawback in the form of low tensile strength compared to its compressive strength. One solution for this is Fiber-Reinforced Concrete (FRC), where short fibers are randomly dispersed into the concrete during mixing to carry the tensile loads in the concrete. Many fibers have been used, with steel being one of the most common; however, carbon fiber shows particular promise due to its superior mechanical properties. Recycled carbon fiber is of particular interest due to its comparable mechanical properties to virgin carbon fiber but at a potentially much lower cost and additional sustainability benefits, potentially making this a novel form of reinforcement for FRC. In this study, ultra-high-performance concrete reinforced with recycled carbon fiber was evaluated for its mechanical performance and compared with two commercially available carbon fiber types and steel fibers using the same concrete mix. Additionally, high-resolution micro X-ray computed tomography was performed on the concrete samples to investigate microstructure features of interest spatially including defects such as pore size distribution which can compromise the structural integrity of the concrete. Scanning electron microscopy were performed to investigate failure mechanism of the tensile concrete samples. Important properties including thermal stability of the carbon fibers and fiber density are reported. The study further investigated the mechanical performance and effectiveness of recycled carbon fiber when mixed into high strength, structural lightweight and ultra lightweight concrete. The mechanical performance of the recycled carbon fiber reinforce concrete was compared to steel fibers. This study showed that recycled carbon fiber can significantly improve the tensile and flexural strength of neat concrete serving as a potential replacement for steel fiber reinforced concrete in certain applications. This study will help provide critical information for the development of process-structure relationships for recycled carbon fiber reinforced concrete for structural applications.

Continuously expanding urban environments compete with and negatively impact natural ecosystems. Building processes consume numerous resources including materials which need to be extracted, transported, shaped, and disposed of. To reduce this negative impact and instead potentially positively affect natural ecosystems, the built environment needs to perform ecosystem services originally in place and sustainable building practices should be employed for their entire life cycle. The principle of biological growth has architectural potential including adaptation, resilience, dynamics, differentiation, and continuous functionality. Biotechnology and biomimicry serve as integrated bioscience methods to transfer biology into architecture by integrating living organisms into the manufacturing process and applying abstracted biological principles into technological systems. This PhD explores the solidification of aggregate materials through both methods to show their potential and limitations: growth of fungal mycelium on agricultural byproducts to produce building materials and design of foundation systems inspired by tree roots. Mycelium-based materials are of increasing interest for their potential in generating biodegradable building materials with tunable properties from waste products through clean manufacturing processes. Before their implementation into permanent buildings, numerous aspects have yet to be researched. This PhD primarily focuses on: the optimization of large-scale manufacturing processes, effects of the manufacturing process and material compositions on mechanical properties and outdoor durability, and utilization of local species.Root-inspired foundation systems can enhance traditional foundations by reducing resources utilized and integrating new functions to serve the built environment and natural ecosystems. Conceptual designs present how root strategies are abstracted and transferred towards the future of building foundations. Biological research on tree roots is performed to gather emerging knowledge about morphological adaptation across species and environments. An innovative process is employed to generate 3D models of tree roots in the field and develop an algorithm which extracts morphological traits. Civil engineering tests are performed to evaluate the effect of various root traits on foundation.Overall, this research serves the fields of integrated bioscience by providing new methods and insights of biological knowledge while abstracting and applying it towards the future of architecture.

Heart failure (HF) currently affects over 6 million Americans, 50% of whom die within 5 years of their initial diagnosis. A major contributor to the onset of HF is cardiac fibrosis in the myocardium, which arises when fibroblasts (FBs) are activated in response to heightened mechanical stress from overload conditions like hypertension. Activated FBs remodel the extracellular matrix (ECM) and secrete ECM proteins including collagen. FB remodeling has been studied in the past by applying forces and/or deformations to three-dimensional, cell-seeded gels and tissue constructs in vitro. Unfortunately, previous stretching platforms have traditionally not enabled mechanical property assessment to be performed with an efficient throughput, thereby limiting the full potential of in vitro mechanobiology studies. We developed a novel in vitro platform to study cell-populated tissue constructs under dynamic mechanical stimulation while also performing repeatable, non-destructive stress-strain tests in living constructs. The multi-well platform is 3D printed out of a Dental LT resin with each well housing a magnetic piston grip. Cell populated tissue constructs were grown within the wells to attach to magnetic grips opposite from stationary grips. Uniaxial mechanical tension was applied to the tissues by a computer-controlled electromagnet placed beside the well plate, which enabled both dynamic stimulation during long-term culture as well as repeatable, non-destructive stress-strain tests for simultaneously comparing tissue stiffnesses across all wells. We have successfully utilized our platform to stretch cell-populated tissue constructs composed of both primary murine and primary human cardiac fibroblasts within 3D fibrin gels. Cyclically stretching cardiac fibroblasts within a 3D fibrin matrix led to collagen accumulation and increased tissue stiffness. Results from these studies validated our novel platform’s ability to enable dynamic mechanical characterization of cell-mediated tissue remodeling in vitro - a capability not possible in standard 3D tissue culture. This multiwell platform offers potential as a high-throughput screening tool for assessing the effect of different treatments on clinically relevant metrics of tissue stiffness using automated measurements.

Nuclear thermal propulsion (NTP) utilizes nuclear fission to double the efficiency ofin-space propulsion systems compared with traditional combustion rocket systems.NTP systems are limited primarily by the fuel material choice, due to the extremeconditions they will need to endure, including temperatures up to 3000 K, multiplethermal cycles with rapid heating and cooling, exposure to hot flowing hydrogen,large thermal gradients, and high neutron flux. Particle based fuels, namely ceramic-metallic (cermet) and ceramic-ceramic (cercer) composites are both promising fuelelement material candidates for NTP. Given the high temperature nature, thesematerials are difficult to fabricate and very little thermophysical and mechanicalproperty data exists. The fabrication of a cermet with a molybdenum-tungsten matrixand a cercer with a zirconium carbide matrix using surrogate fuel particles have beenoptimized utilizing spark plasma sintering to produce materials > 97% theoreticaldensity. Thermophysical properties including the coefficient of thermal expansion,thermal conductivity, and heat capacity were determined up to 1000 °C. Compressionand notched bend bar tests up to 800 °C also provided information on the effect ofthe particles to the strength of the composites. From this data, predictive modelswere created to approximate the thermophysical and mechanical properties of fueledmaterials for use in NTP design.

We report the synthesis and characterization of PEEK-MAX (Ti3SiC2, Ti3AlC2,and Cr2AlC), and PEEK-MoAlB, (DDGS500 – pyrolyzed Distiller’s dried grains with soluble (DDGS)), metal (Mo) and PEEK-ceramic (Xonotlite) based PEEK composites, by hot-pressing. A systematic study was done to understand the effect of these additives on functional and mechanical properties. Initially, these composites were synthesized by hot pressing. Detailed microstructure analysis by scanning electron microscopy showed that at higher concentration (≥10 vol%), the Ti3SiC2, Ti3AlC2 and PEEK-MoAlB particles segregated at the phase boundaries and formed interpenetrating micro-networks. PEEK-Cr2AlC were well dispersed in the PEEK matrix. Microstructure study of the composites showed that DDGS500 powder particulates were well dispersed within the PEEK matrix after the addition of 5 vol%. However, at higher concentration of 10 and 20 vol%, the particles segregated at the phase boundaries. Mo and Xonotlite particles were segregated at the phase boundaries even at 5 vol% concentration. The addition of MAX phases or MoAlB reduced the crystallization temperature and melting point of these composites. PEEK reinforced with 10 vol% Ti3SiC2, Ti3AlC2 and MoAlB showed plastic failure, and higher strength than PEEK. Comparatively, PEEK reinforced with 10 vol% Cr2AlC did not show any enhancement. All the PEEK-MAX and PEEK-MoAlB composites showed triboactive behavior and enhanced wear resistance. The addition of DDGS500, PEEK-Mo and PEEK-Xonotlite particulates lowered the wear rate until 10 vol% by several orders of magnitude and stabilized the friction coefficient (μ) versus distance profile. PEEK-20 vol% Xonotlite which showed hydrophilic behavior with a decrease in contact angle as a function of time which shows that Xonotlite can be a promising additive for increasing hydrophilicity of PEEK based composites for biomedical applications. PEEK composites fluid lubrication tribology behavior was also studied in the presence of distilled deionized water and simulated body fluid (SBF). PEEK composites showed improvement in the tribological behavior. In addition, PEEK composites immersed in SBF, also shown formation of bone growth.

The lead-cooled fast reactor (LFR) is a fourth-generation reactor design characterized by high temperatures, neutron dosage, and highly corrosive and erosive environment due to high flow velocities. Therefore, materials testing must be performed to find structural and system components that withstand this harsh environment. These candidate alloys must have high-temperature creep strength, resistance to damage from fast-spectrum neutrons, and must grow protective oxide layers. FeCrAl alloys have been identified as possessing all these qualities, however, there are limitations due to erosion and the joining of these materials. In this work, mechanical and microstructural properties of base and conventionally joined FeCrAl-based ferritic alloys as well as additively manufactured alternatives before and after molten lead exposure at 500 C will be presented. Analysis of dissimilar joints of candidate alloys was also performed.

The effects of changes in process parameters, heat treatment, build orientation, and resulting defects on fatigue behavior of selective laser melting-processed AlSi10Mg have been determined. Samples were prepared under distinct P-V parameters and heat treatments for the following classifications: A- nominal with SR+HIP+T6, B- large defects with SR+HIP+T6, G- small to medium defects with SR+HIP+T6, D- many defects with SR+HIP+T6, E- large defects with SR+T6, and F- small to medium defects with SR+T6. Fatigue crack growth (FCG) testing of bend bars and high cycle fatigue (HCF) fatigue testing of cylindrical samples occurred at a stress ratio R = 0.1 and 20 Hz according to appropriate ASTM standards. This is reviewed along with ASTM-standardized tension testing of cylindrical samples. Increased defects typically reduced UTS, ductility, and fracture toughness particularly in the Z orientation. The S-N performances of X/Y orientation were improved or similar to the Z orientation in HCF as a result of having smaller or similar fatigue-initiating defect sizes, that were quantified for all failed S-N samples. HIP-processing generally reduced fatigue-initiating defects sizes and improved most S-N performances. Compared to SLM AlSi10Mg from literature, the nominal (A) build had an increased HCF performance. HCF sample life was estimated by using fracture surface defect measurements and FCG data obtained for those process conditions.

While wood construction may be as ancient as humanity itself, wood remains one of the most utilized structural materials because of it’s availability, low cost, durability, surprisingly high specific mechanical properties, and renewability. Recent research into wood-based materials has focused on strategies for enhancing the strength and stiffness of as-grown materials. We fabricated densified wood by thermochemically treating raw white oak wood in Na2SO3 and NaOH solutions, followed by compressing the treated wood at 5MPa at high temperature. Treating the raw wood in NaOH and Na2SO3 cleaves the alpha-O-4 ether bond and beta-O-4 ether bonds, thus dissolving most of the lignin and hemicellulose, which allows the wood cell walls, vessels, and pits to collapse when compressed. This allows the crystalline cellulose microfibril to align themselves adjacent to the neighboring crystalline cellulose, which increases the strength, crystallinity percentage, and crystallite size. The resulting enhanced wood microstructures were characterized using optical and scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and mechanical testing. Under optimal processing conditions, the thermochemically densified wood exhibited a maximum strength of 350 MPa – 370 MPa, which is 5-fold higher than the raw wood. This process results in a wood material that is comparatively as strong as some structural aluminum alloys and has potential as a future material for high-performance engineering applications.

Additive manufacturing (AM) empowers the creation of novel open-celled architectures that can achieve dramatic performance gains through topology. Lattice structures with millimeter-scale features, specifically, yield the most notable performance gains in applications that require high specific strength and stiffness, enhanced energy absorption ability, and improved heat transfer. For metal AM features at this length scale, process-structure-property relationships become intrinsically linked to geometry because the widths of the struts in the lattices consist of only several melt pools. This process-properties coupling greatly complicates the design and development cycle. One strategy to speed up the design process is to characterize lattice sub-components, or “primitives”, and use their effective properties in process-informed models of larger scale structures. To explore the feasibility of this approach, extensive characterization of the effective properties of both lattice primitives and multi-celled lattices is needed.In this work, experimental and computational techniques are used to advance the understanding of the mechanical response of metal AM lattices. The effective properties needed to predict the response of lattice primitives made of AM Ti-6Al-4V were determined using CT measurements and validated against experimental and FEA. The effective properties are shown to be highly dependent on printed geometry. The methods used to define effective properties are then applied to AM SS316L to quantify the effect of processing conditions upon effective properties of struts and nodes. Samples built with processing conditions that resulted in a higher energy density were found to have increased distributed porosity but not necessarily decreased strength. In addition, thin strut-based samples were found to display increased hardening relative to bulk properties. After understanding how to determine effective properties in struts and nodes, AM Ti-6Al-4V lattices are examined with CT and tested using in situ DIC and evaluated to determine if lattice primitives behave as expected in larger scale lattice structures. The results of these tests demonstrate some unexpected localizations in the lattices, warranting further exploration of local material properties.This work makes several critical advances towards improving the design of printed lattice structures. Its foremost contribution is a method of determining the effective geometry of printed strut-based structures based on intuitive, direct measurements of the structure from CT. By using this method, the effective mechanical properties can be inferred, and advances in the understanding of the influence of print orientation, processing condition, and sample size upon the mechanical properties of lattice primitives are made. This work also provides insight into the behavior of strut intersections (nodes) in isolation and within lattice structures. Nodal behavior within lattice structures was found to heavily depend upon degree of constraint, orientation within the lattice, and nodal geometry.

Functionally graded porous structures (FGPSs) are gaining interest in the biomedical sector, specifically for orthopaedic implants. In this study, the compressive behaviour of seven different FGPSs comprised of Face Centred Cubic (FCC) and the Octet truss unit cells (OCT) were analysed. The porosity of the structures were graded in different directions (radially, longitudinally, laterally and longitudinally & radially) by varying the strut diameters or by combining the two types of unit cells. The structures were manufactured by laser power bed fusion and compression tests were performed. Radially and laterally porous graded structures were found to outperform uniform porous structures with an increase in stiffness of 13.7% and 21.1% respectively. The experimental and finite element analysis (FEA) results were in good agreement with differences in elastic modulus of 9.4% and yield strength of 15.8%. A new FEA beam model is proposed in this study to analyse this type of structures with accurate results and the consequent reduction of computational time. The accuracy of the Kelvin-Voight model and the rule of mixtures for predicting the mechanical behaviour of different FGPSs was also investigated. The results demonstrate the adequacy of the analytical models specifically for hybrid structures and for structures with smooth diameter transitions.

There is an ongoing need for the design and development of metal alloys with improved properties for extreme environment applications. High entropy alloys (HEAs) are a group of metal alloys that in contrary to conventional metal alloys can have multiple principal elements in high concentrations. HEAs show promising properties better than or comparable to conventional metal alloys for a range of temperature down to cryogenic temperature. HEAs are good candidates to be used as structural materials for extreme environments applications such as in aerospace, automotive, transportation, and energy industries, among others. Mechanical behavior and the underlying plastic deformation mechanisms and the factors affecting HEAs need to be fully understood to be able to use these alloys for the mentioned applications and to design and develop further improved metal alloys.Low stacking fault energy face centered cubic (fcc) HEAs show simultaneous high strength and ductility and specially by the decrease in temperature down to cryogenic temperatures, whereas there is usually a tradeoff between strength and ductility in conventional metal alloys. Plastic deformation in low stacking fault energy fcc HEAs starts with dislocation slip and with the increase in stress, deformation twins nucleate and grow as an additional mode of deformation. There have been studies that experimentally and computationally looked at slip and deformation twins and the effect of different parameters on their nucleation and growth in HEAs. However, the critical resolved shear stress for slip which indicates the beginning of the plastic deformation region in some of these HEAs has not been found. Also, different factors in deformation twin nucleation and growth have been studied but the effect of grain boundary (GB) types and elemental segregation at GBs have not been fully investigated. In this research experimental and computational approaches are used to further identify the underlying plastic deformation mechanisms in HEAs giving rise to their improved properties. High resolution digital image correlation and electron backscatter diffraction have been used to find the dislocation slip critical resolved shear stress (CRSS) in Al0.3CoCrFeNi polycrystalline under tension. Molecular dynamics (MD) simulations and Monte Carlo molecular dynamics (MCMD) simulations have been used to identify the effect of different symmetric twist GB types and elemental segregation on deformation twins in CoCrFeNi bicrystals at three different temperatures 77 K, 100 K, and 300 K. Experimentally Al0.3CoCrFeNi polycrystalline was tested under tension at room temperature slip CRSS was found to be 63±2 MPa based on the activated slip system of (-1 1 1)[-1 -1 0] which also had the highest Schmid factor of 0.42. The MD simulations and the MCMD simulations studies on the CoCrFeNi HEA bicrystals confirmed GBs as deformation twin nucleation sites. The mechanical properties and deformation twin nucleation changed with different symmetric twist GBs having different sigma values and misorientation angles. MCMD simulations revealed GBs becoming Cr-rich and Ni-deficient which matches the results from experimental observations and MCMD simulations of HEAs of similar compositions. Temperature also was shown to influence the material properties in this alloy. With the decrease in temperature from 600 K, to 300 K, to 77 K, the yield strength and stress, and the overall plastic flow stress increased, and the modulus of elasticity decreased. The mentioned scientific contributions guide HEA design and development with improved properties through GB engineering by populating the polycrystals with symmetric twist grain boundaries of high angle misorientation angles and segregation engineering and designing chromium-rich GBs. As a next step to this research, experimentally, tensile tests at cryogenic temperatures with further post-mortem microscopy can be performed to find the CRSS at cryogenic temperatures and characterize the slip and deformation twins. Computationally, MCMD chemical equilibrium can be continued and reinforcement learning algorithms can be implemented to optimize the process. Furthermore, other types of GBs can be considered and the effect of GB geometry on the elemental segregation itself can be another route branching from this research.

There is an increasing interest in biodegradable metal implants made from magnesium (Mg), iron (Fe), zinc (Zn) and their alloys because they are well tolerated in vivo and have mechanical properties that approach those of non-degradable metals. In particular, Mg, Zn, and their alloys show the potential to be the next generation of biodegradable materials for medical implants. We have employed photo-chemical etching for fabricating different designs of AZ31 magnesium (Mg), Zn (99.95% pure) stents. The stents were fabricated with different strut patterns, made by photo-chemical etching, and mechanically tested to evaluate radial forces. AZ31 Mg Stents with rhombus design patterns showed a promising Mg- 0.20 N/mm and for Zn 0.167 N/mm radial force. Thus, fabricated stents and ribbon of AZ31 and Zn were tested in vivo, in vitro and histology. In this study, we present three stages of creating and validating a stent prototype, finally, in vivo testing in peripheral arteries of domestic pigs. Six stents were implanted in two domestic farm pigs weighing 25-28 kg and were allowed to survive for 28 days, with an interim follow-up on day 14. The left and right superficial femoral, the left iliac, and the right renal artery were selected for this study. The diameters of the stented segments were evaluated at the time of implantation, on day 14 and then, finally, on day 28, by quantitative vessel analysis (QVA) using fluoroscopic imaging. Optical Coherence Tomography (OCT) confirmed some malposition, breaks, stacking, and protrusion into the lumen at the proximal, distal, and mid-sections of the stented arteries. After 28 days, the animals were euthanized, and explanted vessels were fixed for micro-CT imaging and histology studies. Micro-CT imaging revealed stent morphological and volumetric changes due to the in-body degradation. An in the vivo corrosion rate of 0.75 mm/year was obtained by the CT evaluation. The histology suggested no-life threatening effects, although moderate injury, inflammation, and endothelialization scores were observed. The extracted remains of the stents were found to be covered with Mg corrosion by-products typical of the physiological environment. Photo-chemically etched zinc rhombus design stents and Zn ribbons were utilized to asses the corrosion behavior via in vitro and cytocompatibility of Zn metal by subcutaneously inserting them in mice. The Parylene C coating reduced the corrosion rate by 50% compared to uncoated stents. In vivo studies were carried out by implanting photo-chemically etched, uncoated Zn stent segments subcutaneously in a C57BL/6 mice model. Histological analyses provided favorable data about the surrounding tissue status, as well as nerve and blood vessel responses near the implant.After performing in vivo experiments to evaluate the performance of the photo-chemically etched AZ31 rhombus design stents, it was found that AZ31 had a higher corrosion rate than the Zn and degrade fast. In this study, we have designed and fabricated biodegradable helical stents made of AZ31 magnesium alloy, and have explored theirs in vitro corrosion behavior in DMEM. Materials coated with Parylene C coating revealed a maximum corrosion rate reduction of 70% to 85% in DMEM solution.

Cellular materials are gaining interest thanks to developments in additive manufacturing. Whilst Finite Element Analysis (FEA) is commonly used to obtain the mechanical behaviour of these structures, different modelling and simulation methodologies are followed in literature. Consequently, there is not a clear procedure to accurately evaluate the mechanical properties of porous structures. This study presents a method to perform FEA of lattice structures with accurate results. All inputs required to simulate compression testing of lattices in FEA were investigated, these included the modelling type, element size, number of unit cells required, boundary conditions and the material model. The effect of these variables on the modulus and yield strength of a lattice structure was studied. Lattices with two unit cell structures, varying unit cell sizes and relative densities were additively manufactured in stainless steel, compression tested and compared to FE simulations. The material model for the FE simulations was obtained from tensile testing individual micro-struts of varying diameters. FE simulation results were in good agreement with the experimental results with an average error for the modulus and yield strength of ~10% and 17% respectively. The methodology presented should provide a foundation to accelerate development and adoption of these structures.

Structural alloys find use in a wide variety of areas; depending on the area of application these alloys can be subjected to considerably harsh environments. In this study the mechanical behavior of structural alloys of interest and the effect of fire damage on material properties was investigated. Face centered cubic aluminum alloys AA6061 and AA5083, and hexagonal closed packed titanium alloy Ti-6Al-4V find applications in marine environments and in the aerospace industry. These alloys perform in circumstance where fire outbreaks commonly occur. Understanding of residual mechanical properties of structural materials and characterization of changes induced by fire damage are needed to accurately estimate life of structural components. Mechanical properties of undamaged materials were characterized using non-invasive diffraction tools (neutrons and x-rays); additional techniques such as digital imaging correlation and microscopy were employed to complement diffraction data. Samples were subjected to fire damage and material strength reduction was quantified. Mechanical testing involved tensile, torsional and combined tension/torsion loading measurements. Due to the source aluminum material, samples had a rectangular cross section; this introduced non-uniform stress/strain distributions during torsion and combined loading tests. Results show significant strength reduction in aluminum alloys due to recrystallization, grain re-orientation and loss of strengthening mechanisms. Neutron diffraction was found to be a useful tool in understanding deformation and yielding mechanisms. For structures in the field such as naval structures, a non-reactor based neutron diffractometer for stress measurements could be critical to rapidly assess state of damaged components. One of the main steps towards achieving a compact, portable diffraction instrument for bulk stress measurements is the development and integration of an appropriate detector. The detector of choice was an ANGER logic, scintillator-based detector capable of pulse shape discrimination to reduce effects of high background environments. Development is similar to what is required for a conventional neutron diffraction instrument. Components such as shielding, and slits have to be designed and detector operation has to be calibrated to accurately quantify acquired data. Instrument design and operation was successful. Data analysis showed high two-dimensional resolution; future work will aim at recording and quantifying strain induced diffraction peak shifts.

Methods for tracheal repair and regeneration are necessary due to the limitations of tracheal resection and reconstruction for certain disorders such as tracheal stenosis, tracheomalacia, and tracheal tumors. Additionally, pulmonary injuries such as airway burns do not have effective treatment options aside from supportive care. The feasibility of a cell spraying device is investigated here as a system for applying human bronchial epithelial cells (HBECs) to decellularized porcine trachea matrices for creation of engineered grafts or as a minimally invasive method for delivering cells for wound healing. HBECs show viability greater than 90% after spraying onto cell culture media or tissue culture plastic. Similarly, one day after spraying onto decellularized trachea, viabilities are seen to be around 90%. Around day three, viabilities were slightly decreased to around 80%. After culturing for over one week, HBECs sprayed onto decellularized trachea displayed a basal cell marker (cytokeratin-5, CK5) and a club cell marker (uteroglobin). Markers for ciliated cells and goblet cells that are crucial for tracheal epithelium could not be found, but this needs to be investigated further. To validate the mechanical performance of the decellularized trachea, compressive resistance testing was performed before and after decellularization of tracheal rings. Results were generally inconclusive with high degrees of variability. A paired sample test conducted with 4 tracheas provided the most interesting results and showed that the decellularization process produced a significantly different compressive resistance compared to the native samples. In practice though this did not seem to be noticeable as the variability found within tracheal samples masked the difference. This would suggest that the decellularization process is not detrimental to the compressive resistance of trachea rings. Based on the results reported here, using a cell spraying device for engineering tracheal grafts and airway epithelial repair seems achievable.

The use of electrospun polystyrene (PS) fibers to create continuous long range ordered multi-scale porous structures in titanium diboride (TiB2) is investigated in this work. The introduction of electrospun PS fibers as a sacrificial filler into a colloidal suspension of TiB2 allows for easy control over the pore size, porosity, and long range ordering of the porous structures of the sintered ceramic. Green bodies were formed by vacuum infiltrating an electrospun-fiber-filled mold with the colloidal TiB2 suspension. The size, volume, distribution, and dispersion of the pores were optimized by carefully selecting the sacrificial polymer, the fiber diameter, the solvent, and the solid content of TiB2. The green bodies were partially sintered at 2000 C in argon to form a multiscale porous structure via the removal of the PS fibers. Aligned continuous cylindrical pores were derived from the PS fibers in a range of ~5 - 20 μm and random porosity was revealed between the ceramic particles with the size of ~0.3 - 1 μm. TiB2 near-net-shaped parts with the multi-scale porosities (~50 to 70%) were successfully cast and sintered. The multi-scale porous structure produced from electrospun fibers was characterized both thermally and mechanically, at room temperature. The conductivity ranged from 12-31 W m^(-1) K^(-1) at room temperature and the compressive strength ranged from 2-30 MPa at room temperature. Analytical thermal and mechanical models were employed to understand and verify he processing-structure-properties relationship. Finally, a method was devised for estimating the effective thermal conductivity of candidate materials for UHTC applications at relevant temperatures using a finite difference model and a controlled sample environment. This low-cost processing technique facilitates the production of thermally and mechanically anisotropic structures into near-net shape parts, for extreme environment applications, such as ultra high temperature insulation and active cooling components.

Environmental and safety concerns have necessitated a phase-out of lead-based alloys, which are often used in electronics solder applications. In order to properly assess suitable replacement materials, it is necessary to understand the deformation mechanisms relevant to the application. In the case of electronics solder, creep is an important mechanism that must be considered in the design of reliable devices and systems. In this study, Power-Law and Garofalo constitutive creep models were derived for two medium temperature solder alloys. The first alloy is known by the commercial name Indalloy 236 and is a quaternary alloy of lead, antimony, tin, and silver. The lead-free alternative is a binary alloy of tin and antimony known by the trade name Indalloy 264. Constant strain rate tests were conducted at temperatures from -20 to 175 Celsius using constant strain rate tensile testing in the range of e-5 s-1 to e-1 s-1. Creep constants were defined for use in materials selection and design analysis activities.

In this study, the formation, evolution, and failure of dissimilar metal welds (DMWs) involving Grade 91 steel using nickel based filler metals were evaluated. First, the curiosity of stable d ferrite found in the heat affected zone (HAZ) of Grade 91 DMWs was investigated since this phase was not present in the HAZ of matching filler metal welds. This difference could have signified a change in the thermal histories of the weld, a change in the chemical potential gradients present, or a combination of both. In the first investigation, it was found that the thermo-mechanical properties of the nickel based filler metal contributed to longer dwell times within the temperature range of stable d ferrite within the HAZ as compared to the autogenous and matching filler metal welds. This occurred because solidification temperature range of nickel based filler metals overlaps the stable d ferrite temperature range and because of the lower thermal conductivity of the nickel based filler metal. These factors enabled carbide dissolution and carbon diffusion, if the presence of a chemical potential gradient existed. Since these welds involved steel base metal and nickel based filler metals, the chemical potential gradients were relevant and were also investigated.The effect of the chemical potential gradient across the dissimilar fusion boundary was also investigated. It was found that there was a strong carbon chemical potential gradient between the Grade 91 base material and the nickel based filler metal caused by a difference in carbon concentration and carbide forming elements. A diffusion simulation was used to predict the magnitude of carbon migration during welding, which resulted in a carbon depleted HAZ. Carbon depletion in the HAZ stabilized the d ferrite phase, shown through statistical analysis of the hardness distribution and a strong correlation between the carbon concentration and amount of d ferrite found in the HAZ. A mechanism was proposed for the retention of d ferrite, which included the driving force from the carbon chemical potential gradient and the long dwell times at high temperatures.After that, the solidification mechanism was investigated, since the multiple phase transformations of the base metal on cooling hide the evidence of solidification. However, the phase boundary was identified and the details of the initial fusion boundary were uncovered using a variety of characterization techniques and the reconstruction of prior austenite grain orientations. As a result, two different mechanisms were proposed for DMW solidification, and each of them identified the fusion boundary to be within, or at the edge of, the narrow band of martensite grains found between the band of d ferrite and the fusion boundary. Fresh martensite was also identified within the highly diluted region of the partially mixed zone (PMxZ) after the PWHT.In order to explain the presence of fresh martensite in the PMxZ after the PWHT, phase transformations within the PMxZ were studied. In particular, the composition gradient of the PMxZ was reproduced at intervals, which determined that the A1 and A3 temperatures were locally reduced, resulting the region transforming to and from austenite during the PWHT. Because of this, the martensitic PMxZ would have reduced toughness. Additionally, a narrow region of the PMxZ operated as austenite during service, but martensite at room temperature. This region experienced cycling phase transformations and moving phase boundary that had unknown consequences on mechanical properties. The premature damage region was also found within the PMxZ and was characterized using transition electron microscopy (TEM), energy dispersive spectroscopy (EDS), and transmission kikuchi diffraction (TKD). This region contained coarse nickel-rich carbides, which may have contributed to the failure mechanism. Finally, the high temperature mechanical behavior of cross-weld specimens was investigated. It was found that under high levels of restraint (as in a weld), that the thermal expansion coefficient of the materials was sufficient to cause yielding in a cross weld specimen. This was problematic, since these regions of the power plant cycle on and off, potentially on a daily basis. Additionally, mechanical testing at 650 °C indicated strain concentration at grain boundaries near the fusion boundary. Finally, during the high temperature tensile test, strain localization was found to be higher in the PMxZ than the HAZ.

Lower-limb energy storage and return (ESAR) prostheses are constructed of carbon-fibre composite to approximately replicate the lower-limb of an able-bodied athlete during the stance phase of dynamic activities, such as running and jumping. However, the dynamic properties of ESAR prostheses means they adhere poorly to the underlying rigid body assumptions of conventional link segment models, and the associated impact on calculated mechanical behaviour is not well documented. As such, the purpose of this thesis was to investigate the types and limitations of existing mathematical models of lower-limb prostheses, and then develop alternative models of ESAR prosthesis mechanical behaviour - both independent from and during gait. Firstly, mathematical models of prosthetic feet independent from gait were developed. Finite element (FE) models of four ESAR prostheses were developed from mechanical testing, presenting a new and robust method of comparing the dynamic behaviour of different ESAR prostheses without resorting to destructive testing. To describe prosthesis behaviour with reduced computational cost compared to FE models, three lumped-parameter models of four ESAR prostheses were also developed that enable the simulation of ground reaction force for a given prosthesis translation and rotation. Mathematical models of prosthesis behaviour during gait were then developed. A multibody FE model was proposed, using experimental marker trajectories to prescribe the displacement of the rigid skeletal bodies as well as the proximal and distal ends of the prosthesis. Additionally, the effect of a link segment model’s marker-set and geometry on calculated lower-limb kinematics, kinetics and energetics during amputee sprinting was investigated. Five different link segment models of the Ottobock 1E90 Sprinter were developed and statistically compared. The results indicated that the omission of prosthesis-specific spatial, inertial and elastic properties from full-body models significantly influence the calculated amputee gait characteristics. The newly developed methodologies enable the analysis of ESAR prostheses without using a two-link segment model to describe the mechanical behaviour. This research has the potential to optimise sporting technique as well as to provide more accurate data for health care professionals, improving the athletic performance and well-being of individuals with lower-limb amputation.

Nanoindentation has been used for decades to assess materials on a local scale and to obtain fundamental mechanical property parameters. Nuclear materials research often faces the challenge of testing rather small samples due to the hazardous nature, limited space in reactors, and shallow ion-irradiated zones, fostering the need for small-scale mechanical testing (SSMT). As such, correlating the results from SSMT to bulk properties is particularly of interest. This thesis compares macroscopic tensile (σy, σflow, and σuts) and shear punch (τy and τmax) test results to nanoindentation hardness data (H) obtained on a number of different neutron-irradiated materials in order to understand the relationship and scaling behavior on radiation-damaged samples. Various empirical relationships are employed and compared. In addition, multiple methods for obtaining hardness numbers from the load-displacement (LD) curves generated during instrumented indentation testing (IIT) are analyzed. After investigating many permutations of LD curve analysis methods and empirical relationships, combinations for specific cases are suggested based on expected material behavior. Utilizing these suggestions, efficacious relationships are found between hardness, tensile yield, tensile flow, and shear max.

Hot stamped boron steels, such as Usibor® 1500, have been increasingly used in automotive structural components for light-weighting and impact resistance. Classified as an ultra-high strength steel, these alloys have superior strength with tensile strengths exceeding 1500 MPa. The rapid heating and cooling thermal cycle during resistance spot welding can significantly alter the martensitic base metal microstructure, resulting in formation of coarse-grained and subcritical heat-affected zones (CGHAZ and SCHAZ) with inferior mechanical properties. The martensitic CGHAZ is adjacent to the weld nugget and experiences the most time above the AC3, which allows for austenite grain growth. The SCHAZ is next to the unaffected base metal and does not reach the AC1¬ during welding, thus the base metal microstructure is over-tempered into cementite and ferrite. The present research aims at developing the fundamental knowledge of plastic deformation and fracture behaviors of ultra-high strength steel resistance spot welds. As a resistance spot weld comprises highly inhomogeneous microstructure, the overall research approach is based on studying the local (or microstructure-dependent) mechanical properties for individual regions in the weld as well as their interactions with weld geometry on the deformation behavior. Specifically, optimal welding parameters are determined to produce welds of appropriate nugget diameter for 2T Usibor 1500 with a gauge thickness of 1.5 mm. Micro-hardness mapping and metallographic analysis allow for characterization of the weld metal, CGHAZ, SCHAZ, and base metal of the spot weld. Quasi-static tensile testing with digital image correlation (DIC) is used to determine the local stress-strain behaviors of each region using bulk microstructural samples created in a Gleeble® thermal-mechanical simulator. Conventional and innovative resistance spot weld mechanical testing methods are used to generate more knowledge on the deformation of joints under various loading conditions. Sectioned tensile shear testing and single-sided wedge testing procedures have been established to use 2-D DIC for in situ observations of local deformation on the exposed weld cross-section during testing. A mechanical model, developed using Abaqus finite element analysis (FEA) code by incorporating the local constitutive behaviors of RSW joints, is used to better understand the effect of weld nugget profiles on the stress state present during loading. The FEA model is validated by comparing the simulated strain fields to the experimentally measured strain fields. The knowledge generated in this study can help improve the accuracy of predicting spot weld fracture of ultra-high strength steels in the automotive industry. Particularly, the fine-resolution, coupon-scale model developed in this research will be useful for implementation into coarse-resolution, full-scale models for crash simulation and optimization of vehicle components.

Ultra-High Performance Concrete (UHPC) is an emerging material technology that exhibits a combination of high compressive strength, high tensile strength, high toughness and ductility, and improved durability when compared to normal concrete. These characteristics lead to applications in bridge structures across the various department of transportations in the country. The objective of this research was to characterize proprietary and non-proprietary UHPC mixtures in the laboratory, with the ultimate goal aimed at implementing UHPC in bridge applications in Texas, particularly for field-cast connections and closure pours.

Traumatic nerve injury is generally permanent and debilitating. There is no available therapy primarily owing to the lack of spontaneous axon growth in the adult human central nervous system. In this doctoral work, an interventional technology was investigated to promote and guide axons through nerve gaps to provide nerve repair. Previously, agarose hydrogel microchannel scaffolds linearly guided axons through lesion gaps of spinal cords in rats. However, these scaffolds were non-degradable. In this work, first the efficacy of degradable hydrogels such as alginate, chitosan and poly(ethylene glycol) (PEGDA) as nerve guidance scaffolds was studied. All the hydrogels, however, were concluded unstable in vivo and provided limited axon growth. To fabricate scaffolds effective for nerve repair poly caprolactone (PCL) with slow degradation rate (reported over 8 months) was selected and investigated. In addition, to increase the open volume of scaffolds, a novel scaffold architecture and fabrication process were introduced in which, both the channels open space and the interstitial space between the channels could be utilized for axon growth. A salt-leaching process was developed to optimize PCL properties such as porosity, stiffness and cell adhesion. The scaffold design entailed the fabrication of PCL tubes and their assembly inside a PCL outer tube resulting scaffolds with >60% open volume (a 3-fold improvement to state-of-the-art microchannel scaffolds). When implanted in transected spinal cords in rats, linear axon growth inside and between the channels was observed. The PCL scaffolds, with 3 orders of magnitude higher stiffness than the nerve tissue, provided the highest axon integration and growth in close proximity to the scaffold walls when compared to soft hydrogels. This observation is contradictory to the general belief that an implant with stiffness more closely matching the tissue is more effective. Indeed, this doctoral work is the first study that suggests axon/implant integration is enhanced in vivo when the substrate stiffness is orders of magnitude higher than the host tissue. This technology was translated to poly lactic-co-glycolic acid (PLGA), for a higher degradation rate, and was fabricated to clinically-relevant dimensions. Overall, this dissertation introduces a promising microchannel scaffold for its translation to human nerve repair.

Mechanical tests have been used in industry and academia for the characterization of ceramics, metals, polymers, and biological tissues. While standard testing protocols have been established for manufacturable materials, only adaptations of these procedures exists for biologics. Simulation studies have been conducted to show the performance of these adaptations, focusing on the effect of boundary conditions and the homogeneity in loading distribution. However, these studies have not been used to optimize controllable variables such as the specimen geometry, cutting mechanisms, and gripping methodologies, with the goal of improving experimental outcomes. Moreover, previous studies have not include structural material models that may represent the microstructure of biological tissues more accurately. Using a novel Finite Element Analysis (FEA) platform for biomechanics, our goal is to optimize the geometry of inhomogeneous soft tissue samples, even stress distribution and avoid early material failure at intensities.