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Laminated composite structures manufactured from pre-impregnated carbon fibre preforms have very high in-plane strength and stiffness. These properties make carbon-fibre composites ideal for withstanding the adverse loading conditions experienced by many aerospace primary load-carrying structures under normal operating conditions. However, such structures are very susceptible to delamination failure in the event of high-velocity impact by a foreign object. This is particularly important for aerospace components where foreign-object damage (FOD) can lead to catastrophic failure of a component due to the low composite material interlaminar fracture toughness. This property depends on the strength of the interfacial layers between plies, which for an unreinforced laminate is governed by the relatively low strength of the highly brittle matrix resin. Lack of fibrous material in the through-thickness direction means this strength is easily overcome by the large interlaminar shear stresses induced by a high-velocity impact event. Z-pin through-thickness reinforcement is an effective solution to this problem by adding fibrous material in the laminate Z-direction to enhance through-thickness impact performance. The ability of Z-pin reinforcement in enhancing the interlaminar fracture toughness of carbon-fibre composites has been well-documented for smaller scales and low strain-rates. However, Z-pins must encounter a delamination crack that is of sufficient scale to invoke large-scale bridging to have maximum efficacy in arresting crack propagation. Existing small-scale test methods do not produce this large-scale bridging effect, and full-scale Z-pinned component testing produces complex failure modes and is often prohibilitively expensive. In this thesis, a new sub-element scale test has been developed which produces large-scale Mode II crack-bridging at high strain-rate in a field of Z-pins. The test was designed using a design-by-analysis approach with the explicit finite element solver LS-DYNA R7.1.3. A cantilevered, tapered plate manufactured from IM7/8552 pre-impregnated carbon-fibre unidirectional material is subjected to impact by a gelatine projectile using a gas-gun. The specimens are produced using a combination of hand layup and autoclave curing with one-sided taper and soft tooling only for simple laminate manufacture. Undesirable additional failure modes such as fibre-failure near the fixture have been completely avoided. Test conditions, configuration and geometry have been well-scaled to produce a large, single delamination near the mid-plane which is driven in both initiation and propagation by structural bending. Experimental data shows that this large delamination crack may be effectively arrested by embedded, normally-aligned carbon-fibre Z-pins via large-scale bridging. Finite element modelling predictions are compared with experimental results and the modelling approach is developed further to establish some predictive ability in terms of predicting Z-pin large-scale bridging behaviour. Novel Z-pin configurations are then assessed using the predictive model.

Developing lighter and more efficient structural members with sufficient seismic resistance will aid in the reduction of casualties and a decrease in structural damage. Application of steel bracing and reinforced concrete (RC) shear walls used for lateral load retaining systems (LLRS) are not always the most efficient and economical solution as they have longer construction times and are heavier compared to steel plate shear walls (SSW). Steel shear walls have shown its superiority over conventional concrete shear walls when implemented in high rise buildings, reducing the self-weight of the structure, as well as having high initial stiffness and energy adsorption. The latest developments in this area have proven that simultaneous application of steel and fibre reinforced polymer (FRP) materials for infill plates results in an increase of energy dissipation and ultimate load capacity. The beneficial influence of those factors for hybrid (FRP & Steel) shear walls will allow for developing of improved characteristics and wider applications in building practice. This research looks at the behaviour of the hybrid steel/FRP and FRP only connections and how variations in certain parameters will affect the load capacity and energy dissipation. This research also looks at the prospect of utilising these FRP materials in SSW infill plates by analysing the ultimate load capacity and energy dissipation of different specimens and evaluating the most efficient system. The FRP materials used are carbon and glass prepreg unidirectional fabric applied as layers on both sides of a thin steel plate. Displacement controlled loading sequence was developed following the ATC-24 loading protocol (Applied Technology Council, 1992). Ultimate load capacity, displacements and strain gauge readings were recorded for all tested specimens. Some of the main findings were that hybrid steel/CFRP infill plates suffered significant delamination, however presented some of the most ductile behaviour amongst the hybrid specimens. It was also concluded that variation in torque influences the mode of failure of SSW connections as well as the energy dissipation and load capacity. Connected experimental research and FEM prove the perspective of further application of indicated innovative variations of SSWs.

This study reports a novel and simple technique for successfully coating multi-walled carbon nanotubes (MWCNTs) on to the surface of carbon fibre (CF) fabric for the production of multi-scale CF-epoxy composites. Initially, epoxy composites with multi-scale reinforcement were produced by resin infusion (RI) using woven CF fabric coated with a dispersion of 1 wt. % MWCNTs in an epoxy binder of low molar mass. The effects of this reinforcement on the CF-epoxy interface with MWCNTs was studied in mode I and mode II interlaminar fracture toughness (ILFT) using double-cantilever beam (DCB) and 4 point end-notch flexure (4ENF) tests, respectively. Relative to an equivalent composite reinforced with non-coated CF reinforcement, the binder/MWCNTs coating increased significantly the ILFT of the CF-epoxy composite; in mode I by 105% and in mode II by 50%. This increase in ILFT was attributed to two main effects: Firstly, the binder alone (without MWCNTs), which has a much lower glass transition temperature (Tg) than that the matrix (45 vs. 140 °C), hindered crack propagation and increased the ILFT of the epoxy matrix by 25% for mode I and 15% for mode II; Secondly, the energy absorbing mechanisms of MWCNTs during fracture particularly pull-out and crack bridging. However the Tg of the matrix epoxy of the multi-scale composites was reduced to 118 °C compared to 140 °C, for the unmodified composite, due to phase mixing with the low Tg binder. For RI processing, the CF volume fraction of the composites prepared using coated CF was ≈50% compare to at ≈55% for the composite with non-coated CF. Curing agents were added to the binder, which not only increased the Tg from ≈50 °C to ≈100 °C, but also increased the Tg of the matrix epoxy of the multi-scale composites to 154 °C. Relative to an equivalent composite reinforced with non-coated CF reinforcement, the curable-binder/MWCNTs coating increased the ILFT of the CF-epoxy composite; in mode I by 120% and in mode II by 90%. A hybrid RI-hot press (HP) process was used to prepare CF-epoxy composites from coated fabrics with CF volume fractions of ≈55%. The damping curves for the HP-composites consisted of a β-peak, due to the formation of a third mixed phase, in addition to a γ-peak (assigned to the Tg of the binder) and an α-peak (assigned to the Tg of matrix epoxy). The β-peak, and the uniformly distributed nodular particles observed on the fracture surface of the matrix, by SEM, for HP-composites, are indicative of the formation of mixed-phase particles due to reaction induced phase separation (RIPS). Relative to an equivalent RI-composite, the curable-binder/MWCNTs treatment increased the ILFT of the CF-epoxy multi-scale composite; in mode I by 134% and in mode II by 15% for HP-composites. Impact test results showed that HP-composites absorbed more energy, due to CF fracture, compared to equivalent RI composites, which showed larger delamination areas after 5 J and 10 J impact. The out-of-plane electrical conductivity and thermal conductivity of the HP-composite with CF coated with curable-binder/MWCNTs was increased by ≈38% and ≈50%, respectively, compared to the composite with non-coated CF, indicating formation of MWCNTs networks in the matrix rich areas of the multi-scale composite.

Engineered timber products are becoming increasingly popular in the construction industry due to their attractive aesthetic and sustainability credentials. Cross-laminated timber (CLT) is one such engineered timber product, formed of multiple layers of timber planks glued together with adjacent layers perpendicular to each other. Unlike traditional building materials such as steel and concrete, the timber structural elements can ignite and burn when exposed to fire, and thus this risk must be explicitly addressed during design. Current design guidance focusses on the structural response of engineered timber, with the flammability risk typically addressed by encapsulation of any structural timber elements with the intention of preventing their involvement in a fire. Exposed structural timber elements may act as an additional fuel load, and this risk must be adequately quantified to satisfy the intent of the building regulations in that the structure does not continue burning. This can be achieved through timber’s natural capacity to auto-extinguish when the external heat source is removed or sufficiently reduced. To address these issues, a fundamental understanding of auto-extinction and the conditions necessary to achieve it in real fire scenarios is needed. Bench-scale flammability studies were undertaken in the Fire Propagation Apparatus to explore the conditions under which auto-extinction will occur. Critical conditions were determined experimentally as a mass loss rate of 3.48 ± 0.31 g/m2s, or an incident heat flux of ~30 kW/m2. Mass loss rate was identified as the better criterion, as critical heat flux was shown by comparison with literature data to be heavily dependent on apparatus. Subsequently, full-scale compartment fire experiments with exposed timber surfaces were performed to determine if auto-extinction could be achieved in real fire scenarios. It was demonstrated that auto-extinction could be achieved in a compartment fire scenario, but only if significant delamination of the engineered timber product could be prevented. A full-scale compartment fire experiment with an exposed back wall and ceiling achieved auto-extinction after around 21 minutes, at which point no significant delamination of the first lamella had been observed. Experiments with an exposed back and side wall, and experiments with an exposed back wall, side wall, and ceiling underwent sustained burning due to repeated delamination, and an increased quantity of exposed timber respectively. Firepoint theory was used to predict the mass loss rate as a function of external heat flux and heat losses, and was successfully applied to the bench-scale experiments. This approach was then extended to the full-scale compartment fire experiment which achieved auto-extinction. A simplified approach based on experimentally obtained internal temperature fields was able to predict auto-extinction if delamination had not occurred – predicting an extinction time of 20-21 minutes. This demonstrates that the critical mass loss rate of 3.48 ± 0.31 g/m2s determined from bench-scale experiments was valid for application to full-scale compartment fire experiments. This was further explored through a series of reduced-scale compartment fire experiments, demonstrating that auto-extinction can only reliably be achieved if burnout of the compartment fuel load is achieved before significant delamination of the outer lamella takes place. The quantification of the auto-extinction phenomena and their applicability to full-scale compartment fires explored herein thus allows greater understanding of the effects of exposed timber surfaces on compartment fire dynamics.

Thin material layers have found various applications with various roles of functions, such as in fibre reinforced laminated composite materials, in integrated electronic circuits, in thermal barrier coating material system, and etc. Interface delamination is a major failure mode due to either residual stress or applied load, or both. Over the past several decades, extensive research works have been done on this subject; however, there are still uncertainties and unsolved problems. This thesis presents the new developed analytical studies on local delamination failure of thin material layers. Firstly, the analytical theories are developed for post-local buckling-driven delamination in bilayer composite beams. The total energy release rate (ERR) is obtained more accurately by including the axial strain energy contribution from the intact part of the beam and by developing a more accurate expression for the post-buckling mode shape. The total ERR is partitioned by using partition theories based on the Euler beam, Timoshenko beam and 2D-elasticity theories. By comparing with independent test results, it has been found that for macroscopic thin material layers the analytical partitions based on the Euler beam theory predicts the propagation behaviour very well and much better than the others. Secondly, a hypothesis is made that delamination can be driven by pockets of energy concentration (PECs) in the form of pockets of tensile stress and shear stress on and around the interface between a microscopic thin film and a thick substrate. Both straight-edged and circular-edged spallation are considered. The three mechanical models are established using mixed-mode partition theories based on classical plate theory, first-order shear-deformable plate theory and full 2D elasticity theory. Experimental results show that all three of the models predict the initiation of unstable growth and the size of spallation very well; however, only the 2D elasticity-based model predicts final kinking off well. Based on PECs theory, the room temperature spallation of α-alumina oxidation film is explained very well. This solved the problem which can not be explained by conventional buckling theory. Finally, the analytical models are also developed to predict the adhesion energy between multilayer graphene membranes and thick substrates. Experimental results show that the model based on 2D elasticity partition theory gives excellent predictions. It has been found that the sliding effect in multilayered graphene membranes leads to a decrease in adhesion toughness measurements when using the circular blister test.

Any composite material is made up from two or more materials and therefore contains interfaces, which usually represent planes of weakness. Interfacial fractures are effectively constrained to propagate along these interfaces as mixed-mode fractures with all three opening, shearing and tearing actions (i.e. mode I, mode II and mode III), instead of kinking to maintain pure-mode-I conditions at the advancing crack front, as would typically happen in an isotropic material. This is significant because mixed-mode fracture toughness is load-dependent and not a purely intrinsic material property (although clearly the pure mode fracture toughnesses are indeed intrinsic material properties that can be determined experimentally). Therefore, in order to know the fracture toughness under general loading conditions, it is necessary to know both the interface failure criterion (that describes the fracture toughness as a function of the mode mixity), and the mode mixity of the crack under the specified loading conditions. This is a complex problem that has occupied researchers in the fracture mechanics community for decades. Consequently, the literature contains a large number of different mixed-mode partition theories. This work appears to show that, of all the partition theories assessed, Wang and Harvey s (2012a) Euler beam partition theory is able to most accurately predict the fracture toughness of a mixed-mode delamination in a fibre-reinforced polymer composite laminate. This statement is based on the outcomes of three separate studies: The first study uses data reported in the literature from a thorough programme of mixed-mode fracture testing of unidirectional and multi-directional laminates. The Euler beam partition theory is able to accurately predict the fracture toughness in all cases. Furthermore, the Euler beam partition theory, which is completely analytical, closely agrees over a large domain with Davidson et al. s (2000) independently-derived non-singular field partition theory, which was derived with the aid of experimental test results. In general, the singular-field approach based on 2D elasticity and the finite element method give poor predictions. In the second study, an original programme of mixed-mode fracture testing is carried out, which incorporates several novel aspects including new test apparatus and a methodology for testing with a wide range of applied pure bending moments. Eighty five fracture tests are performed on unidirectional glass/epoxy laminates to determine the initiation and propagation fracture toughnesses. Although the second study was inconclusive with respect to the correctness of any particular partition theory, the development of the test apparatus and test methodology are considered to be major contributions that will be useful for both design engineers and academic researchers, not only working with fibre-reinforced polymer composite laminates, but also working with other composite materials containing interfacial cracks. The third study uses digital image correlation to investigate the near-crack tip strain fields of mixed-mode delaminations to try to discover the underlying mechanics that govern the selection of a mixed-mode partition theory. The new testing apparatus is used again, and another novel testing methodology is developed. The work appears to confirm (with some caveats) that two sets of pure modes exist, that is, two pure mode I modes, and two pure mode II modes, with their numerical values roughly corresponding to those from Wang and Harvey s (2012a) Euler beam partition theory. It should be noted that, as far as the author s knowledge is concerned, Euler beam partition theory is the only one in the literature to predict the existence of two sets of pure modes. Although this work set out to conclusively determine which mixed-mode partition theory is able to most accurately predict the fracture toughness of a mixed-mode delamination in a fibre-reinforced polymer composite laminate, and also, to discover why, the outcomes cannot truly be called conclusions . Rather, they only offer strong support for Wang and Harvey s (2012a) Euler beam partition theory for predicting the fracture toughness fibre-reinforced polymer composite laminates against delamination. Despite this, the work makes major contributions that will be useful for both design engineers and academic researchers in the field, as described in the above.

Delamination and cracks within the multilayer structure are typical failure modes observed in microelectronic and micro electro mechanical system (MEMS) devices and packages. As destructive detection methods consume large numbers of devices during reliability tests, non-destructive techniques (NDT) are critical for measuring the size and position of internal defects throughout such tests. There are several established NDT methods; however, some of them have significant disadvantages for detecting defects within multilayer structures such as those found in MEMS devices. This thesis presents research into the application of transient infrared thermography as a non-destructive method for detecting and measuring internal defects, such as delamination and cracks, in the multilayer structure of MEMS devices. This technique works through the use of an infrared imaging system to map the changing temperature distribution over the surface of a target object following a sudden change in the boundary conditions, such as the application of a heat source to an external surface. It has previously been utilised in various applications, such as damage assessment in aerospace composites and verification of printed circuit board solder joint manufacture, but little research of its applicability to MEMS structures has previously been reported. In this work, the thermal behaviour of a multilayer structure containing defects was first numerically analysed. A multilayer structure was then successfully modelled using COMSOL finite element analysis (FEA) software with pulse heating on the bottom surface and observing the resulting time varying temperature distribution on the top. The optimum detecting conditions such as the pulse heating energy, pulse duration and heating method were determined and applied in the simulation. The influences of thermal properties of materials, physical dimensions of film, substrate and defect and other factors that will influence the surface temperature gradients were analytically evaluated. Furthermore, a functional relationship between the defect size and the resulting surface temperature was obtained to improve the accuracy of estimating the physical dimensions and location of the internal defect in detection. Corresponding experiments on specimens containing artificially created defects in macro-scale revealed the ability of the thermographic method to detect the internal defect. The precision of the established model was confirmed by contrasting the experimental results and numerical simulations.

Abstract: Over the last two decades, the realm of nanoscience has experienced exceptional expansion, positioning nanotechnology at the forefront of advancements across various sectors such as computing, sensors, biomedicine, and a plethora of additional applications. Within this landscape, the identification and incorporation of graphene into polymer nanocomposites mark a pivotal advancement in nanoscience. Graphene, characterized by its single layer of carbon atoms arranged in a hexagonal lattice, is renowned for its array of extraordinary attributes. Research into functionally graded graphene-reinforced composite (FG-GRC) laminated plates has revealed significant limitations, especially in understanding how interlaminar defects and delamination affect their thermal postbuckling performance and overall instability responses. While the addition of graphene is proven to increase the stiffness and strength of polymeric composite laminates, delamination remains a major obstacle, jeopardizing these improvements. The first two chapters of this thesis thoroughly explores the challenges of graphene distribution within the laminates, highlighting those inconsistencies in the structural integrity and mechanical performance of materials caused by delamination areas and can lead to unexpectable buckling mode shapes under different scenarios. In the first chapter, this study conducts a thorough assessment of how diverse distributions of graphene reinforcement can counteract the negative effects of delamination, employing a refined semi-analytical strategy that leverages the third-order shear deformation theory (TSDT) and the Rayleigh-Ritz approximation method. This approach enables to investigate the complex delamination cases, moving from basic shape assumptions in the past to circular and elliptical shapes of delamination. The study provides insights into local and global buckling in composite delaminated plates, focusing on how graphene distribution patterns, delamination configurations, and external conditions impact thermal instability responses. Chapter two delves into how graphene distribution affects the energy release rate (ERR) around delamination edges in graphene-reinforced laminates. It examines multiple delamination scenarios, which partition the laminate into segments with varied graphene reinforcement patterns. This analysis is key to understanding how these patterns influence the laminate's vulnerability to delamination by studying the ERR across different regions. Moreover, this chapter seeks to understand the effect of both symmetrical and asymmetrical graphene distribution patterns, in conjunction with specific delamination configurations, on the fundamental frequencies of FG-GRC plates in both pre- and post-buckled thermal states, highlighting the complex interplay between material distribution, structural imperfections, and their combined effects on the performance of the laminated plate. Recent advancements in manufacturing technologies have transformed the fabrication of composite laminates. This innovation offers enhanced design flexibility, customization options, and improved production efficiency. The integration of advanced manufacturing with composite materials has opened up new possibilities for constructing complex and functional engineering structures with unique material characteristics, such as thin-walled composite laminated struts. Nonetheless, it is crucial to consider local buckling as a pivotal design factor for these types of structures, regardless of their cross-sectional shape. The final two chapters of this thesis introduce a novel study on the application of graphene sheet reinforcements within composite laminated channel section structures, one of the most practical types of the thin-walled struts. This investigation aims to determine how the integration of graphene, in a variety of symmetric and asymmetric patterns, can enhance the structural performance of channel section struts when subjected to compressive mechanical and thermal loading, while eliminating the need for additional intermediate stiffeners. The method used to study the instability and thermally induced vibration of FG-GRC channel section struts employs the von Karman geometrical nonlinearity and relies on the layerwise, third-order shear deformation theory (LW-TSDT). For the purpose of confirming the precision of the outcomes derived from the LW-TSDT and assessing its computational efficiency, a three-dimensional (3D) finite element model is constructed for comparison, utilizing ABAQUS software. This thesis delivers pivotal numerical insights for solid mechanic designers, highlighting the critical need to weave these findings into their design simulations for augmented performance and enhanced reliability. Specifically, it was found that the application of the FG-X graphene distribution pattern in channel section struts increases their critical buckling resistance by 30%, whereas the FG-O pattern leads to a reduction of about 26% when compared to a uniform graphene dispersion. These results highlight the critical impact of graphene distribution on the mechanical integrity and buckling resilience of composite materials, stressing the value of optimizing material distribution.

Sandwich panels using fibre-reinforced composite skins and low-density cores are being increasingly used in the aerospace industry due to their superior specific strength and stiffness, and increased design flexibility over traditional metallic and composite structures. However, it is well-known that sandwich panels are highly vulnerable to the effects of impact damage, with even low-energy impacts potentially causing very severe reductions in the in-plane compressive strength of these structures. The objective of this project was to produce a faithful and reliable numerical model for the simulation of the compression-after-impact strength of composite sandwich panels. An in-depth literature review revealed that delamination within the skins of a sandwich panel is a damage mechanism that has gone almost entirely neglected in previous efforts at modelling this problem, despite the proven significance of this mechanism in the failure of impact damaged sandwich panels in compression. Consequently, the use of the cohesive zone model for delamination initiation and propagation is the key unique feature of this model, with Hashin s criteria being used for intra-laminar damage formation, and a simple plasticity response capturing core crushing. An experimental study is performed to produce a thorough dataset for model validation, featuring differing levels of damage induced via quasi-static indentation, and novel asymmetric panels with skins of unequal thickness (the thinner skin being on the unimpacted side). The experimental study revealed that the use of a thinner distal (undamaged) skin could improve the strength of mildly damaged sandwich panels over undamaged sandwich panels using the same asymmetric configuration. It is believed that this effect is due to the movement of the neutral plane of the sandwich panel caused by the reduction in the stability of the damaged skin through stiffness reduction and geometric imperfections. This removes the eccentricity of the compressive loading that exists in the undamaged asymmetric panels, which has mismatched axial stiffness between the indented skin and the thinner distal skin, and thus a noticeably lower ultimate strength than the undamaged symmetric panels. The sandwich model is developed using pre-existing experimental and material data, and trialled for a variety of different skin lay-ups, core thicknesses and indenter sizes. The numerical model generally agreed well with the ultimate stress found in the experiments for these different configurations, but is quite poor at estimating the magnitude of the damage induced by the indentation. When used to model the experimental study, the model gave generally good, conservative estimates for the residual compressive strength of both the symmetric and asymmetric panels. The tendency of the asymmetric panels to become stronger with mild damage was not captured by the model per se, with the numerical results instead showing an insensitivity to damage in the asymmetric panels, which was not shared by the symmetric panels. However, the numerical model did exhibit erroneous strain-stress responses for both panel configurations, particularly for the undamaged and mildly damaged cases. Investigations revealed that this erroneous behaviour was caused by inconsistency in the material data, which had been collected partially via experimentation and partly from literature sources. Overall, the model developed here represents a promising advancement over previous efforts, but further development is required to provide accurate damage states.

Modelling damage in composite materials has played an important role in designing composite structures. Although numerical models for the progressive damage in laminated composites (e.g. transverse cracking, delamination and fibre breakage) have been developed in the literature, there is still a need for further improvement. This thesis aimed at developing damage models suitable for predicting intra-laminar and inter-laminar damage behaviour in fibre-reinforced composite materials. Several approaches such as fracture mechanics and continuum damage mechanics have been adopted for constructing the damage model. Meso-macro-mechanics analysis was performed to gain an insight into the entire damage process up to the final failure of the composite laminate under various conditions. Cohesive elements were placed in the finite element model to simulate the initiation and propagation of matrix crack and delamination in cross-ply laminates. This helped to understand the direct interactions between damage modes, i.e. whether one damage mode would initiate the other damage mode. The formation of a single matrix crack and its propagation across the layer thickness was also revealed. A new cohesive zone/interface element model was developed to consider the effect of through-thickness compressive stress on mode II fracture resistance by introducing friction into the constitutive law of the conventional cohesive zone model. Application of the model to practical problem in composite laminates shows that this model can simulate delamination failure more accurately than the cohesive element in ABAQUS.Damage models based on continuum damage mechanics were proposed for predicting intra-laminar damage and interlaminar damage. Five intra-laminar failure modes, fibre tension, fibre compression, matrix tension, matrix compression and shear failure, were modelled. Damage initiation was predicted based on stress/strain failure criteria and damage evolution law was based on fracture energy dissipation. The nonlinear shear behaviour of the material was considered as well. These models have been implemented into ABAQUS via a user-defined material subroutine and validated against experimental/numerical results available in the literature. The issue related to numerical implementation, e.g. convergence in the softening regime, was also addressed. Numerical simulation of the indentation test on filament-wound pipe was finally conducted and damages generated in the pipe were predicted using the above developed damage models. The predictions show an excellent agreement with experimental observations including load/indentation responses and multiple delaminations shape and size. Attempt was made to detect damage-induced leakage path in the pipe after indentation.

This study focuses on the effect of tufting on the mechanical and electrical properties of carbon composites using a variety of tuft materials, such as aramid, steel and copper. Several configurations were investigated based on a case study, involving the reinforcement of stiffener-to-skin interfaces of a tail cone. The effect of tuft and base composite material, tufting depth and inclination on the delamination resistance in mode I was evaluated, the associated bridging laws were determined and the failure mechanisms were identified. A simplified superposition model of the delamination response of tufted composites was developed. The electrical performance of tufted composites was determined in simulated lightning strike tests and set against the through-the-thickness electrical conductivity of the materials. The results of mechanical testing showed that the delamination performance depends strongly on the material response of the tufts, with both the bridging behaviour and final toughness levels influenced directly by the strength, ductility and ultimate strain of the tufts. Interactions between the tufts and the surrounding composite, such as interfacial shear and bridging induced by tuft pull-out, play a significant role in the overall behaviour generating a deviation from a simple superposition of the base material and tuft response. The balance between interfacial shear and tuft elongation results in a decreasing trend of delamination toughness with increasing tufting depth for low ductility materials, whilst the trend is reversed for the high ductility copper tufts. This balance is also affected by the properties of the base material, with tougher matrices leading to dominance of shear effects and a weaker enhancement introduced by tufting. Inclination of tufts leads to an increase in crack energy release rate due to the activation of a ploughing mechanism. Metallic and carbon tufts have a positive effect on lightning strike response, with copper tufting offering strike protection at an improved level compared to standard copper mesh solutions.

For CMCs with an electrically conductive matrix, direct current potential drop techniques have the potential to detect composite state such as conductive constituent content (e.g., Si in melt infiltrated composites) or local defects such as delamination or porosity. Melt-infiltrated SiC based composites are an ideal candidate material for such to verify this since the Si content of the matrix is the primary current carrier in the system. In our study, we aimed to evaluate the effectiveness of Electrical Re-sistance (ER) as a NDE method for different 2D woven SiC-based Melt-infiltrated composites, each exhibiting varying degrees and types of processing defects. We con-ducted four types of ER measurements: a. Bulk Resistivity b. Through-thickness c. Axial d. Surface, along the length of the dogbone specimens in the gauge section and on a large plate. Microstructural analysis was performed to correlate observations with microstructure. The bulk resistivity of the specimens in our study exhibited a linear correlation with the infiltrated Si content of the matrix even with different percentage and type of porosities present, allowing us to comment on Si-content of the speci-mens. The Through-thickness set-up incorporates current leads to supply current in a through-thickness manner and determine the nature of current spreading (voltage drop) some distance   away from the current source. The absolute values of the measured through thickness potential represent Si-content, but it is unresponsive for processing defects. The axial set-up is more conventional and can generate local axial current flow. In some cases, such flow of current is affected by and able to locate the local and distinct type porosi-ty. Resistance was sensitive for regions of poor Si-infiltration i.e., “dry-slurry” type defects as well as for isolated larger rounded porosity. It was very sensitive to local surface porosity but not as sensitive for cases where porosity was homogenously pre-sent throughout the specimen. The one-sided axial measurements suit optimally for the cases when only one side of the manufactured specimen or fabricated component is easy to get to or the porosity present is one-sided. Local axial voltages indicate bet-ter ER sensitivity for distinct defects when they are present between I/V leads as op-posed to V/V because of relatively higher change in local current density. The FEA simulations with 2D isotropic planar model successfully validates this finding. Both thickness and axial techniques were effective at locating the pre-existing delamina-tion. The developed ER device enabled on-board measurements and successfully cal-culated bulk resistivities from local current flow for different-sized specimens of lam-inate composites. The device was further employed for non-destructive evaluation of laminate, panels and cut specimens. The results confirm the potential of ER as a valu-able tool for assessing the properties and integrity of Si/SiC matrix CMCs and should be applicable to CMCs where the matrix is more conductive.

As the construction industry increasingly prioritises eco-friendly and sustainable practices, there is a growing demand for engineered wood products (EWP). The Queensland Government Department of Agriculture and Fisheries (QDAF) is working with key industry partners towards the commercialisation of spotted gum (SPG) glulam, a very dense hardwood species in Queensland with high mechanical properties. However, SPG glulam cannot be manufactured confidently since it typically fails the bond integrity test outlined in Australia and New Zealand Standard AS/NZS 1328.1 (1998) for external products. The test, referred to as "delamination test", consists of a vacuum impregnation wetting and a kiln drying process, aiming to assess the glulam's adaptability to environmental change. Due to the SPG species presenting poor adhesive penetration, high level of extractive, low wettability and minimal permeability, SPG boards are difficult to glue (poor adhesion). To promote the development of SPG glulam and help it be confidentially commercialised for external applications, Leggate et al. (2022b) made their best effort to physically improve the SPG gluability. However, and despite significant findings in improving adhesion, the delamination issue still exists. Therefore, the research focuses on a mechanical approach to reduce the stress in the samples during the delamination test to prevent cracks to propagate through the gluelines. This study aims to develop a numerical model that can reproduce the delamination test and analyse the delamination process. The delamination model couples four sub-models (1) a heat and mass transfer model, (2) a moisture-induced stress model and (3) two failure mechanical models. The model required SPG physical parameters, such as porosity, fibre saturation point and diffusion coefficient for the heat and mass transfer equations, were extracted from literature. Similarly, some SPG mechanical properties, such as elastic modulus, shear modulus and strength, are also available from previous research. However, the SPG fracture properties for crack initiation and propagation have not been previously measured, and their correlation with moisture content remains unknown. Consequently, experiments were conducted to investigate the fracture properties of both SPG boards and SPG glulam under various failure modes and moisture content conditions. From these fracture tests, it is found that the SPG and its glulam fracture properties are not sensitive to moisture content. [...]

Carbon fibre-reinforced plastics (CFRP) are widely used in aerospace, automobile and other structural applications due to their superior mechanical and physical properties. CFRP outperform conventional metals in high strength-to-weight ratio. Usually, CFRP parts are manufactured near to net-shape;however,machining is unavoidable when it comes to assembly. Drilling the holes are essential to facilitate riveting and bolting of the components. However, conventional drilling (CD) induces different types of damages such as cracking, fibre pull-out, sprintling and delamination due to the abrasive nature, inhomogeneity and anisotropy of CFRP. A novel technique, ultrasonically-assisted drilling (UAD) is hybrid machining technique in which highfrequency (typically above 20 kHz) vibration are superimposed on a standard twist drill bit in axial direction using ultrasonic transducer. UAD has shown several advantages such as thrust force reduction, improving surface quality and lower bur-formation in drilling of conventional metals. UAD has also effectively been used for drilling brittle materials.

Carbon fabric-reinforced polymer (CFRP) composites are widely used in aerospace, automotive and construction structures thanks to their high specific strength and stiffness. They can also be used in various products in sports industry. Such products can be exposed to different in-service conditions such as large bending deformations caused by quasi-static and dynamic loading. Composite materials subjected to such bending loads can demonstrate various damage modes - matrix cracking, delamination and, ultimately, fabric fracture. Damage evolution in composites affects both their in-service properties and performance that can deteriorate with time. Such damage modes need adequate means of analysis and investigation, the major approaches being experimental characterisation and numerical simulations. This work deals with a deformation behaviour and damage in carbon fabric-reinforced polymer (CFRP) laminates caused by quasi-static and dynamic bending. Experimental tests are carried out first to characterise the behaviour of a CFRP material under tension, in-plane shear and large-deflection bending in quasi-static conditions. The dynamic behaviour of these materials under large-deflection bending is characterised by Izod-type impact tests employing a pendulum-type impactor. A series of impact tests is performed on the material at various impact energy levels up to its fracture, to obtain a transient response of the woven CFRP laminate. Microstructural examination of damage is carried out by optical microscopy and X-ray micro computed tomography (Micro-CT). The damage analysis revealed that through thickness matrix cracking, inter-ply delaminations, intra-ply delamination such as tow debonding, and fabric fracture was the prominent damage modes. These mechanical tests and microstructural studies are accompanied by advanced numerical models developed in a commercial code Abaqus. Among those models are (i) 2D FE models to simulate experimentally observed inter-ply delamination, intra-ply fabric fracture and their subsequent interaction under quasi-static bending conditions and (ii) 3D FE models based on multi-body dynamics used to analyse interacting damage mechanisms in CFRP under large-deflection dynamic bending conditions. In these models, multiple layers of bilinear cohesive-zone elements are placed at the damage locations identified in the Micro CT study. Initiation and progression of inter-laminar delamination and intra-laminar ply fracture are studied by employing cohesive elements. Stress-based criteria are used for damage initiation while fracture-mechanics techniques are employed to capture its progression in composite laminates. The developed numerical models are capable to simulate the studied damage mechanisms as well as their subsequent interaction observed in the tests and microstructural damage analysis. In this study, a novel damage modelling technique based on the cohesive-zone method is proposed for analysis of interaction of various damage modes, which is more efficient than the continuum damage mechanics approach for coupling between failure modes. It was observed that the damage formation in the specimens was from the front to the back at the impact location in the large-deflection impact tests, unlike the back-to-front one in drop-weight tests. The obtained results of simulations showed a good agreement with experimental data, thus demonstrating that the proposed methodology can be used for simulations of discrete damage mechanisms and their interaction during the ultimate fracture of composites in bending. The main outcome of this thesis is a comprehensive experimental and numerical analysis of the deformation and fracture behaviours of CFRP composites under large-deflection bending caused by quasi-static and dynamic loadings. Recommendations on further research developments are also suggested.

Cam-type Femoro-Acetabular Impingement (FAI) is a common cause of hip osteoarthritis (OA). In this condition a bony abnormality at the head-neck junction of the femoral head, called the “cam”, abuts against the acetabulum causing labral damage and articular cartilage delamination, which in turn may lead to progressive degeneration and OA. The understanding of the damage mechanism is currently at a conceptual level. The aim of the thesis is to develop a more detailed understanding of the underlying mechanism so as to improve methods of detection and treatment of cam-type FAI and thus to help prevent hip OA. A geometric-kinematic model combining hip joint motion and hip joint geometry was cre- ated to determine what motions, activities or cam shapes give rise to cam-type impingement, which was quantified by the proximity of the acetabular and femoral bony surfaces. Five normal subjects and five symptomatic cam-type FAI patients were modelled. The FAI patients experienced early impingement during the impingement test but did not have impingement during common functional activities. The early impingement was possibly due to the larger coverage and protrusion of their cams and the smaller overall proximity in their hip joints. A 2D finite element (FE) model was created to simulate cam-type FAI. As idealised 2D rectangular and circular geometries did not reproduce the damage seen clinically, subject- specific geometry, loads, and motions were introduced. Under some circumstances, as the cam entered the hip joint, large shear strains developed near the cartilage-bone interface of the acetabulum which would result in cartilage delamination. In vitro experiments were undertaken to validate the FE model and verify the damage mech- anism by which cam-type FAI leads to cartilage delamination. Porcine cartilage-bone samples were loaded under conditions similar to those generated by a cam (shear and compression). A validation FE model was created that used the same material and contact representations and analysis framework as the impingement FE model but mimicked the experimental setup. The cartilage shear strains assessed with a video-based method were similar to predicted FE results. In vitro damage experiments demonstrated that delamination can be caused by repetitive shear and compressive loading that lead to large shear strains near the cartilage-bone interface. The impingement FE model was used to further explore the effect of cam anatomy. In hips with low clearance, cams with large protrusions (75% hip joint clearance) would not enter into the hip joint, but caused high shear strains in the labrum, which would result in labral tears. A narrower cam caused damage to the labral tip, whereas a wider cam caused damage to the labral-bone junction. In contrast, cams with small protrusion (25% hip joint clearance) were able to enter the joint and caused damage at the articular cartilage-bone interface, which would result in cartilage delamination. The wider the cam, the further into the hip joint the damage was initiated. The FE model was used to explore the effect of different labral anatomy and of reshaping surgery. A labrum connected to the articular cartilage resulted in shear strains of up to five times greater in the articular cartilage and labrum compared to an unconnected labrum and was more likely to cause articular cartilage delamination. For a cam that damages the articular cartilage, surgical removal of the cam reduced shear strains. For a cam that abuts the labrum, surgical removal of the cam eliminated labral abutment and increased the range of motion of the hip, but resulted in greater shear strains in the articular cartilage. It is not known whether these shear strains are normal or could possibly be damaging. Also, reshaping the head to be spherical resulted in slightly reduced shear strains in the articular cartilage compared to the current surgical practice of cutting deeper into the femoral head when removing the cam. This study has, for the first time, using a validated FE model demonstrated the mechanism by which a cam can cause articular cartilage delamination and labral tearing. Further analysis using the geometric and FE model should help identify cam deformities that would be likely to cause OA and the best way to treat them surgically so as to prevent OA.

Woven glass composites have been used for many years in commercial applications due to their light weight, competitive price and good engineering properties. Absorption of energy by laminated composite material results in damage in various forms, the most common of which is delamination. Inter-laminar fracture causes the layers of composite to separate, resulting in a reduction in stiffness and strength of the composite structure, matrix cracking and in some cases fibre breakage takes place. The aim of this project was to improve the inter-laminar bond strength between woven glass fabric and resin. Air jet texturing was selected to provide a small amount of bulk to the glass yarn. The purpose was to provide more surface contact between the fibres and resin and also to increase the adhesion between the neighbouring layers. These were expected to enhance the resistance to delamination in the woven glass composites.Glass yarns were textured by a Stähle air jet texturing machine. Core-and-effect yarn was produced instead of a simple air textured yarn. Hand loom and vacuum bagging techniques were used for making the fabric and composite panels from both textured and non-textured yarns. Density and fibre volume content were established for physical characterisation. Breaking strength (tenacity) of the yarns and tensile, flexure, inter-laminar shear strength (ILSS) and fracture toughness (mode 1) properties of the composites were determined. Projection microscopy and SEM imaging techniques were used to assess the fractured surfaces of the composite specimens. The yarn tenacity and the tensile properties of the composites were significantly reduced after the texturing process, whereas flexure properties were unchanged. However, significant improvement was observed in the ILSS and fracture toughness of the composites after the texturing process. It was also observed that the composites made from the fabrics with textured yarns in only the weft direction are the most advantageous as they maintained the tensile and flexure properties but have significantly higher inter-laminar shear strength.

This thesis focuses on the incorporation of carbon nanoparticles within continuous fibre reinforcements by liquid composite moulding processes, in order to provide enhanced electrical and delamination properties to the multiscale composites. The mechanisms controlling the flow and filtration of these nanoparticles during liquid composite moulding are studied, in order to develop a predictive 1-D model which allows design of the processing of these composite materials. Five different carbon nanoparticles at 0.25 wt% loading, three unmodified and one surface modified carbon nanotube systems and one carbon nanofibre system, were utilised to modify a commercial two-component epoxy resin utilised to impregnate carbon and glass reinforcements at high fibre volume fraction by resin transfer moulding. The dispersion of the nanofillers in the prepolymer was carried out by ultrasonication, high shear mixing or triple roll milling or a combination of the three. Electrical conductivity measurements of the carbon nanoparticle liquid suspensions during dispersion, alongside optical microscopy imaging and rheological analysis of these allowed the selection of the concentration of nanofiller and the appropriate dispersion technique for each nanoparticle system. The resin transfer moulding process required adaptation to incorporate the dispersion and modify degassing steps, especially when utilising unmodified carbon nanoparticles suspensions, due to their higher viscosity and tendency to be filtered. Nanoparticle filtration was identified by electrical conductivity measurements and microscopy of specimens cut at increasing distances from the inlet. Cake filtration was observed for some of the unmodified systems, whereas deep bed filtration occurred for the surface modified CNT material. Property graded composites were obtained due to filtration, where the average electrical conductivity of the carbon and glass composites produced increased by a factor of two or one order of magnitude respectively. The effect of filler on the delamination properties of the carbon fibre composites was tested under mode I. The results do not show a statistically significant improvement of delamination resistance with the presence of nanoparticles, although localised toughening mechanisms such as nanoparticle pull-out and crack bridging as well as inelastic deformation have been observed on fracture surfaces. Particle filtration and gradients in concentration resulted in non-linear flow behaviour. An 1-D analytical and a finite difference model, based on Darcy’s law accompanied by particle mass conservation and filtration kinetics were developed to describe the flow and filtration of carbon nanoparticle filled thermosets. The numerical model describes the non-linear problem by incorporating material property update laws, i.e. permeability, porosity and viscosity variations on concentration of retained and suspended particles with location and time. The finite difference model is consistent and converges to the analytical solution. The range of applicability of the analytical model is limited to lower filtration coefficients and shorter filling lengths, providing an approximate solution for through thickness infusion; whereas the numerical model presents a solution outside this range, i.e. in-plane filling processes. These models allow process design, with specified carbon nanoparticle concentration distributions achieved via modifying the nanofiller loading at the inlet as a function of time.

Laminated carbon fibre-reinforced polymer (CFRP) composites are already well established in structural applications where high specific strength and stiffness are required. Damage in these laminates is usually localised and may involve numerous mechanisms, such as matrix cracking, laminate delamination, fibre debonding or fibre breakage. Microstructures in CFRPs are non-uniform and irregular, resulting in an element of randomness in the localised damage. This may in turn affect the global properties and failure parameters of components made of CFRPs. This raises the question of whether the inherent stochasticity of localised damage is of significance for application of such materials. This PhD project is aimed at developing numerical models to analyze the effect of material randomness on delamination damage in CFRP materials by the implementation of the cohesive-zone model (CZM) within the framework of the finite-element (FE) method. Both the unidirectional and cross-ply laminates subjected to quasi-static loading conditions were studied. The initiation and propagation in delamination of unidirectional CFRP laminates were analyzed. The CZM was used to simulate the progress of that failure mechanism in a pre-cracked double-cantilever beam (DCB) specimen loaded under mode-I employing initially, a two-dimensional FE model. Model validation was then carried out comparing the numerical results with experimental data. The inherent microstructural stochasticity of CFRP laminates was accounted for in the simulations, and various statistical realizations for a half-scatter of 50% of fracture energy were performed, based on the approximation of that parameter with the Weibull s two-parameter probability density function. More detailed analyses were undertaken employing three-dimensional DCB models, and a number of statistical realizations based on variation of fracture energy were presented. In contrast to the results of two-dimensional analyses, simulations with 3D models demonstrated a lower load-bearing capacity for most of the random models as compared to the deterministic model with uniform material properties. The damaged area and the crack lengths in laminates were analyzed, and the results showed higher values of those parameters for random realizations compared to the uniform case for the same levels of applied displacement. The effect of material randomness on delamination in CFRP cross-ply laminates was also investigated. Initially, two-dimensional finite-element analyses were carried out to study the effect of microstructural randomness in a cross-ply laminate under bending with the direct introduction of matrix cracks with varying spacings and delamination zones. A considerable variation in the stiffness for cases with different crack spacings suggested that the assumption of averaged distributions of defects can lead to unreliable predictions of structural response. Three-dimensional uniform, deterministic cross-ply laminate models subjected to a tensile load were analyzed to study the delamination initiation and propagation from the tips of a pre-existing matrix crack. The material s stochasticity was then introduced, and a number of random statistical realizations were analyzed. It was observed that by neglecting the inherent material randomness of CFRP laminates, the initiation conditions for delamination as well as the character of its propagation cannot be properly detected and studied. For instance, the delamination crack length value for all the simulated random statistical realizations predicted its higher magnitudes compared to the uniform (deterministic) case for the same value of applied strain. Furthermore, the location of delamination initiation was shown to be different for different random statistical realizations. Another aspect, emphasizing the importance of microstructural randomness, was the scatter in the magnitudes of global strain at the instance of initiation and subsequent propagation of delamination. In summary, the material randomness in CFRPs can induce randomness in localised damage and it can affect the global properties of laminates and critical failure parameters. These effects can be investigated computationally through the use of stochastic cohesive-zone elements.

The proposed research is essentially concerned on numerical simulation of materials and structures commonly used in the aerospace industry. The work is primarily focused on the study of the fracture mechanics with emphasis to composite materials, which are widely employed in the aerospace and automotive industry. Since human lives are involved, it is highly important to know how such structures react in case of failure and, possibly, how to prevent them with an adequate design. It has become of primary importance to simulate the material response in composite, especially considering that even a crack, which could be invisible from the outside, can propagate throughout the structure with small external loads and lead to unrecoverable fracture of the structure. In addition, structures made in composite often present a complex behaviour, due to their unconventional elastic properties. A numerical simulation is then a starting point of an innovative and safe design. Conventional techniques (nite elements for example) are not su-cient or simply not ecient in providing a satisfactory description of these phenomena. In fact, being based on the continuum assumption, mesh-based techniques suer of a native incapacity of simulating discontinuities. Novel numerical methods, known as Meshless Methods or Meshfree Methods (MM) and, in a wider perspective, Partition of Unity Methods (PUM), promise to overcome all the disadvantages of the traditional finite element techniques. The absence of a mesh makes MM very attractive for those problems involving large deformations, moving boundaries and crack propagation. However, MM still have signicant limitations that prevent their acceptance among researchers and engineers. Because of the infancy of these methods, more efforts should be made in order to improve their performances, with particular attention to the computational time. In summary, the proposed research will look at the attractive possibilities offered by these methods for the study of failure in composite materials and the subsequent propagation of cracks.

This thesis investigates the effects of damage characteristics on residual compressive strength (RCS) of 4-mm thick preconditioned carbon/epoxy quasi-isotropic panels through the study of their compressive behaviour. Results of 2-mm thick preconditioned panels mostly from a previous study are also analysed. The preconditions of varying sizes include impact damage, quasi-static damage, single and multiple artificial delaminations of circular and elliptical shapes embedded at different through-the-thickness (TTT) locations, hemispherical-shaped domes of different curvature and depth and open holes. The mechanisms of impact damage and the characteristics of energy absorption were dependent on panel thickness and incident kinetic energy (IKE). A damage threshold for compressive strength (CS) reduction was found at 455-mm2 and 1257 mm2 for 2- and 4-mm thick panels, respectively. Panels affected by the presence of internal delaminations followed a sequence of prebuckling, local and global buckling (mode I) and postbuckling (mode II) in both the longitudinal and transverse directions. Their compressive failure was related to mode I to II transition. Possibility of delamination propagation was examined using response characteristics on the basis of the sequences. Evidence of delamination propagation was found only in panels with large damages and was not sensitive to RCS. For low and intermediate IKEs the effect of impact damage could be simulated with a single delamination (2-mm thick panels) and 3 delaminations of medium size (4-mm thick panels). For high IKEs, the additional effect of local curvature change was significant. The combined effect of delamination number, size and curvature change determines the RCSs. It was demonstrated that the present method of embedding artificial delaminations proves to be very useful for studying RCS of impact-damaged panels via the establishment of response characteristics and their links to the effects of the preconditions on them. This thesis also presents two analytical models, one for deflection of transversely loaded panels and the other one for the prediction of compressive strength retention factor (CSRF) based on the correlation between the ratio of maximum transverse force to initial threshold force and the CSRF, observed experimentally in thick panels.

This study investigates the effects of Z-pinning on the delamination performance in opening and shear loading modes in woven fabric reinforced / epoxy composite materials, as well as the effects of friction between specimen crack faces and the Z-pin failure mechanisms involved in mode II delamination. Mode I and mode II delamination tests are carried out on Z-pinned unidirectional (UD) and woven laminates. Both UD and woven laminates exhibit enhanced delamination resistance and crack propagation stability through Z-pinning. The effects of various structural and Z-pin parameters on the mode I and mode II delamination behaviour are separately assessed. The 4ENF testing configuration is deemed as the appropriate mode II configuration for the testing of Z-pinned laminates. A new basic friction rig is used to measure the friction coefficient between crack faces in woven laminates. An additional friction effect attributed to fibre architecture is identified. A specially designed delamination specimen is used to overcome the difficulty of accurately measuring crack propagation in Z-pinned woven fabric materials and aid data reduction using the available analytical methods. The failure mechanisms involved in the mode II delamination of Z-pinned laminates have been investigated with the implementation of a new test. Z-pins fail under shear loading through a combination of resin crushing, laminate fibre breakage, pin shear, pin bending and pin pullout. The balance of the failure mechanisms is shown to be a function of the crack opening constraint, material type, stacking sequence, Z-pin angle and insertion depth to Z-pin diameter ratio. Z-pin and material parameters influencing Z-pinning quality are identified, categorised and quantified. The importance of controlling Z-pin insertion depth is underlined and updated manufacturing procedures are proposed. Partial pinning appears as an advantageous alternative. A reduction in in-plane stiffness and in-plane strength in UD and woven fabric composites is measured, whilst no significant change of in-plane shear stiffness of UD materials is observed. A reduction in the fibre volume fraction is the single most important parameter affecting the in-plane stiffness. The performance of a Z-pinned sub-structural component is investigated. Enhanced loading carrying capacity and damage tolerance is achieved through Z-pinning.

In this thesis layerwise B-spline finite strip method approaches have been developed for free vibration and geometrically nonlinear static analyses of laminated composite plates. In order to account for delamination kinematics, the layerwise computational model has been extended with step function supplements to allow the possible displacement jumps across delamination interfaces. On the basis of such a model, layerwise Bkk and Bkk-i spline finite strip methods have been developed for buckling and postbuckling analysis of delaminated composite laminates. In conjunction with the contact laws the computational model has been used in the development of both Bkk and Bkk-i spline finite strip method capabilities for geometrically nonlinear transient analysis of composite laminates subjected to low-velocity impact. The geometrical non-linearity has been taken into consideration by use of von Karman's non-linear strain-displacement relationship and a total Lagrangian formulation. The failure analysis of composite laminates has been incorporated into the layerwise Bkk and Bkk-i spline finite strip method capabilities for static, postbuckling and impact analyses. In order to account for the nature of the progressive failure, stress-based failure criteria and material degradation rules have been accommodated into each loading/time increment according to failure mechanisms. The presence of delamination in the laminates will cause stress singularity at the delamination tip and therefore make a stress-based failure criterion inappropriate for predicting delamination propagation. The fracture mechanics-based criterion by means of energy release rate has been employed for the prediction of delamination propagation. For the purpose of efficiently and effectively modelling an arbitrary delamination, point and distributed interface spring models have been developed by the introduction of inplane and out-of-the-plane imaginary springs. A wide range of numerical applications has been conducted and comparison has been made with available analytical and experimental results. The accuracy, efficiency, convergence and versatility of the layerwise Bkk and Bkk-i spline finite strip methods have been demonstrated.

This project was sponsored by two companies interested in promoting the use of pultruded glass fibre/polyester composites in the construction of freight containers. Thus, the research was to understand and quantify the damage mechanisms caused by low velocity impact on the composite system and to produce a finite element impact model to further the understanding of these events.

This thesis investigates the smart damage sensing capabilities of the electrical resistivity measurement technique in carbon fibre reinforced composite materials. The static and fatigue damage mechanisms of unidirectional and cross-ply laminates are reviewed. The electrical resistivity measurement investigations of carbon fibre, epoxy resin and carbon fibre reinforced composites are described. Theoretical models related to damage and electrical conduction modelling are reviewed. The material properties, laminate fabrication, specimen design, tab design and electrode design details are presented. The experimental test set-up and test programme details are presented. The details of the formulation and algorithms of a proposed new random resistor network model and the three-dimensional unit cell assemblage model are described. The results related to the applicability and the validation of these models are presented. The influence of electrode effects on the electrical resistance are investigated. The relevant results are presented. The electrical properties of carbon fibre reinforced composites independent of specimen geometry and electrode configuration are established. The dependence of electrical resistance on the specimen geometry was investigated and the relevant results are presented. The smart damage sensing capabilities are demonstrated using computational simulation studies using the proposed new three-dimensional model. The strain sensing and damage sensing capabilities of electrical resistivity measurement technique under static and fatigue loading conditions are presented. The dependence of electrical resistance to the applied stress and fatigue life of the samples subjected to fatigue loading is presented. The smart sensing capabilities are highlighted based on the experimental and theoretical results. The static and fatigue behaviour of epoxy 914 and epoxy 920 based CFRP laminates is investigated. The failure mechanisms are established using the electrical resistance and other non-destructive test methods.

Thin layered systems can be considered as a solution to the repair and protection of concrete structures. This subject was studied in the current investigation. Some common uses of these systems include protection, upgrading and rehabilitation of the floor slabs,restoration of appearance of the structures, impermeability, skid resistance, wear resistance, and protection of the reinforcing steel of concrete structures against atmospheric or chemical attack. However it can be said that protection, upgrading and rehabilitation of floor slabs are the main uses for the design of modem thin layered systems. For example a thin layer of a polymer concrete with a thickness of less than 3 nun can resist a very highly concentrated load resulting from a steel wheel rolling load of 5000 N without any sign of defect. Thin layer systems therefore include traditional screeds, externally bonded steel plates, plasters and coatings as well as the more recent hi - tech. systems. Like any other structure, a thin layered system may break down as the result of many causes. Among other types of failures, delamination defect is the most common mode of failure and particularly relevant to a thin layered system. This phenomenon which mostly occur between the upper layer directly subjectedto the load and the subsequent layer, is due to debonding or slippage at the interface of the two layers. A delamination may occur at the interface of a thin layered system even without any sign of failure in other parts of the structure. Steel wheeled trolleys and fork - lift trucks are among the most anticipated types of loading and causes of failures in the thin layered systems. Different combinations of thin layered systems were prepared using some special flooring materials, both in small and large scales. Despite the lack of any standard test, the action of a steel wheel rolling load on the ready made and purpose made specimens of thin layered systems was well simulated using the Steel Wheel Rolling Load Rig. The NUROLF, Newcastle University Rolling Load Facility, was also used for simulating the action of a tyred vehicle wheel rolling load on the thin layered systems of large scale. Some simple ways for detecting any possible delamination at the interface of the thin layered systems were examined. In addition to the available material characteristics tests, a relatively new simple shear box test was proposed for defming the relationship between normal stress and the correspondings hear strength for each combination of the materials at each age of the test. The results were then used as the basis for the subsequent structural analysis. The structural analysis of the systems was carried out for both of the experiments using the finite element method and the interface technique. In spite of the simplifications made in the solution, the analytical results were consistent with the experimental results to a considerable extent. Based on the results of this investigation, a relatively constitutive procedure was concluded for predicting the behaviour of a thin layered system under the action of a wheel rolling load with regard to the delamination defect.

Fundamental understanding of contact interactions between two surfaces is of paramount importance as surface-surface contact phenomena can be found in a wide range of applications, such as microelectromechanical systems (MEMS), wire bonding in electronic packaging, total joint replacements (TJR), oscillating-slide actuators, bolted and riveted joints, shroud and snubber in turbine blades, and components operating in a microgravity environment. As result of contact interactions, material loss occurs and can lead to undesirable outcomes. Therefore, the primary objective of this dissertation was to develop a finite element method (FEM) based framework to investigate the effects of cyclic normal and shear (friction) traction, coefficient of friction, and surface topography on material damage, removal rate, and failure mechanisms.First, the problem of a rigid flat and a patterned surface pressed against an elastic-plastic half-space exhibiting isotropic strain hardening was analyzed using the FEM to elucidate the development of plasticity. Simulation results in dimensionless form were obtained and discussed to illuminate the effects of geometry, imprint depth, and coefficient of friction on the evolution of plasticity. The deformation due to the impression of the patterned surface was largely affected by the interaction of the stress and strain fields produced by neighboring protrusions, resulting in a three-stage normal force response. Examination of plastic flow of the half-space material into the pattern cavities revealed that cavity filing became prominent with increasing protrusion distance of the patterned surface and decreasing coefficient of friction. This study introduced a computational methodology for fine-tuning key design and process parameters aimed to enhance the efficiency of metal imprinting. Next, a plane-strain FEM model of a rigid cylinder in reciprocating sliding contact with an elastic-plastic half-space exhibiting isotropic strain hardening was introduced to investigate plasticity-induced damage leading to material loss in oscillatory sliding contact. By incorporating a quasi-static, isothermal damage model based on a ductile material failure criterion into the developed FEM model, plasticity-induced cumulative damage was tracked in terms of a dimensionless damage parameter. Numerical results yielded insight into the effects of normal load and coefficient of friction on material loss due to the accumulation of plasticity with oscillation cycles. Specifically, plastic deformation and wear increased with the number of cycles and coefficient of friction due to the intensification of plastic shearing. A non-monotonic increase of wear with normal load was observed, which was explained by the distribution of plastic shear strain produced under high- and low-load oscillatory sliding conditions and the decrease of the fraction of contact area where slip occurred with the increase of the normal load. The developed computational methodology for exploring the evolution of plasticity, damage, and material loss in reciprocating sliding contacts is an effective tool for assessing the effects of load, friction, and material behavior on the mechanical performance of mechanical systems with components experiencing oscillatory contact. Although most engineering surfaces are nominally smooth, they demonstrate random roughness over a wide range of nano/micro-scales. Henceforth, it is imperative to develop numerical models of the material removal rate for engineering interfaces undergoing reciprocating sliding that take into account the effect of the interface topography. To this end, an elastic-plastic contact mechanics analysis of an isotropic strain hardening half-space in oscillatory sliding contact with a rigid surface exhibiting multi-scale roughness characterized by fractal geometry was performed with the FEM. Cumulative damage was tracked by a dimensionless damage parameter and material stiffness degradation was modeled by a degradation parameter depending on fracture energy. Aside from the subsurface stress and plastic strain fields, the effects of fractal parameters (roughness) on the material removal rate were investigated. Contrary to the classical wear law, which predicts a liner dependence of wear rate on normal load, the material removal rate was found to exhibit a nonlinear dependence on normal load due to the occurrence of material interlocking introduced by the increase of surface conformity and the evolution of material loss. The developed model can be used to perform parametric studies of the material loss in mechanical devices operating in reciprocating sliding contact mode. Delamination is a common failure process in layered materials. Thus, to provide insight into this fundamental problem, a contact mechanics analysis of interfacial delamination in elastic and elastic-plastic homogeneous and layered half-spaces in sliding contact was performed. A surface-based cohesive zone model was implemented in the FEM analysis to model surface separation at the interface. Complete delamination was determined by the critical separation distance of interfacial node pairs in mixed-mode loading based on a damage initiation criterion, exemplified by a quadratic relation of the interfacial normal and shear tractions. Linear stiffness degradation was accounted for by a scalar degradation parameter based on the effective separation distances corresponding to the critical effective cohesive strength and the fully degraded stiffness, defined by a mixed mode loading critical fracture energy criterion. Numerical results of the delamination profiles, stress fields, and plastic strain in both the surface layer and underlying half-space illuminated the effects of indentation depth and sliding distance on interfacial delamination for different combinations of elastic-plastic properties, cohesive strength, and layer thickness. The introduced model provides a capability for analyzing plasticity-induced cumulative damage in multilayered structures. The investigations comprising this dissertation provide fundamental understanding of the evolution of stresses, plasticity, material loss, and delamination in contact surfaces subjected to normal loading, sliding, and small-amplitude oscillatory contact. The computational models of this work can be extended to study material loss (wear) in various applications involving contact interfaces, such as MEMS, TJR, and high-efficiency gas turbines.

Thousands of kilometers of possible subduction sites have been identified on Venus near certain quasi-circular surface features called corona and near branches of rift zone trenches called chasmata. Subduction on Earth is driven by negative plate buoyancy with respect to the underlying mantle, however lithosphere on Venus may be significantly more buoyant than on Earth. On Earth, only 6-7 km of positively buoyant crust compete with the negative thermal buoyancy of the lithospheric mantle, but on Venus the enhanced positive buoyancy from 30 km of globally-averaged crust would significantly inhibit subduction initiation. Plume-lithosphere interactions have been proposed as a mechanism for subduction initiation, but the relatively high bending moments and elastic thicknesses near subduction sites indicate the lithosphere is thick and may not be easily penetrated by thermal upwellings. The following chapters present a series of 2D and 3D numerical models using the code, StagYY, to investigate the dynamics of regional-scale lithospheric recycling initiated at chasmata trenches on Venus with realistic crustal densities. Rather than subduction, a tectonic regime called ``peel-back delamination" (PBD) was observed in which the lithospheric mantle decouples and peels away from the overlying positively buoyant crust. Though net-negative lithospheric buoyancy is not a requirement for delamination, PBD is driven by the negative thermal buoyancy of the lithospheric mantle and resisted by the positive compositional buoyancy of the crust and the strength of the lithosphere. Aided by the pre-existing lithospheric weakness at chasmata, plume-rift interactions may accelerate timescales of delamination initiation. A thermal upwelling is shown to destabilize even thinner, more positively buoyant rift-adjacent lithosphere than in models without plume-rift interactions. Plume-induced PBD may require a minimum lithospheric thickness of approximately 150-200 km, and therefore may be most applicable to the inferred thick lithosphere near the Dali-Diana chasmata system. 3D delamination models show a radial retrograde migration of a trench and flexural bulge, compatible with the style of surface deformation observed at Artemis Corona in the Dali-Diana chasmata region of Venus.

Despite a high strength to weight ratio, composites are susceptible to impact damage which can be barely visible while still adversely affecting their strength, therefore detecting and characterizing damage is important. Non-destructive evaluation, specifically single-sided pulse-echo ultrasonic C-scans, can be used to detect damage. The goal is to develop a code that aids the quantitative assessment of damage within laminates by segmenting C-scans of damaged regions into 3D reconstructions. The code is open source to create a free alternative to commercial ultrasonic C-scan software and allow users to adapt the code for their own applications. The code processes C-scans and calculates time-of-flight (TOF), then uses unique characteristics in C-scans to create a map of boundaries where the damage changes from one layer to another. Using this map, the code processes the TOF to create a 3D reconstruction of damaged layers. When both front and back side C-scans of a sample are available, the code combines them to produce a hybrid C-scan that merges information from both. The code is developed using a dataset of 25-ply composite panels with varying layups and levels of impact damage. A 24-ply composite sample impacted at 48 Joules was also processed and compared with an X-ray CT dataset. The code's 3D reconstruction output is useful for an enhanced understanding of the impact damage state (e.g., area of damage at specific depth), provides the capability to extract quantitative metrics of the damage, and can be used to create actual-geometry structural analysis model representations of the damage state.

\textit{What is the fate of the lower crust during continental collision?} Chapter 2 incorporates geologic constraints of lower crustal dynamics. Specifically, crustal xenoliths from the Pamir are used to constrain the geodynamic style of the loss of a lithospheric root. Delamination—the peeling back of a dense layer—is favored based on xenolith pressure-temperature-time paths. Moreover, delamination is caused by feedbacks between the lateral escape of the lower crust and cooling at the margins of the orogen. This contrasts with the assumption that delamination and channel flow are decoupled mechanisms. Instead, laterally flowing buoyant lower crust can founder after cooling. \textit{How do sediment diapirs differentiate?} Recent models of sediment diapirs in subduction zones assume diapirs are perfectly round with no internal transport of solid-state material. Chapter 3 tests these assumptions by applying porous and solid-state flow to diapiric differentiation. Not only can diapirs split into a buoyant portion and a dense residue but mushy material flows upward along the diapir margins, driving convection \textit{within} the diapir. Such churning could promote reactions between different lithologies in diapirs, meaning that hybridized material need not only be created in a subduction channel but inside the diapir itself. More broadly, however, geodynamic models of diapirism, delamination, and tectonic processes in general often require many more input parameters than can be tested individually, limiting the robustness of results. \textit{How then can the sensitivity of many input parameters in geodynamic models be tested?} This work streamlines the calculation of linearized sensitivities. Specifically, the technique of \textit{reverse automatic differentiation} is applied to models with many (154) inputs to obtain sensitivities of \textit{all} inputs for a reasonable computational cost ($

Fibre reinforced laminated composites offer many benefits over conventional materials and hence are now used in most engineering sectors. However, their intrinsic in-homogeneity and anisotropic properties make modelling damage initiation and propagation a challenging task. This thesis explores a relatively new semi-analytical approach, Scaled Boundary Finite Element Method (SBFEM), for accurate and efficient modelling of inter-ply damage in thick laminated composites, which is sometimes referred to as interfacial imperfection, debonding or delamination. SBFEM is a tool which combines the benefit of finite element method (FEM) and boundary element method (BEM). In SBFEM, boundaries are solved numerically, whereas an analytical solution is obtained inside the domain. This considerably reduces the computational effort required to solve the problem. In its framework, a 2D plane strain approach is first considered to model the laminated composite plates undergoing cylindrical bending. The work is then extended for 3D modelling to model the bi-axial bending of plates. Unlike 2D plate models, these approaches do not make any assumption on the displacement field and hence, provides superior results. Modelling has been performed in perfectly bonded conditions as well as with interfacial imperfections. Often thick laminates make up the load bearing components of engineering structures; however, they are challenging to model accurately, particularly in cases where out-of-plane loads are significant. Thus, the applicability of the presented modelling technique using SBFEM is assessed over traditional numerical approaches for modelling thick laminated composites. The model is then expanded to study progressive delamination growth using cohesive zone modelling, in pure as well as mixed mode fracture conditions. Finally, for experimental validation of the SBFEM predictions and to justify the application of theoretical approach in practical scenarios, in-house experimentations performed for the fracture studies are modelled. The SBFEM approach for analysis of inter-ply damage in laminated composites is found to be in good agreement with traditional methods while achieving significant reduction in the computational cost. Precise behaviour of laminates can be captured without the necessity of multiple sub-divisions in the throughthickness direction of plies. Moreover, the requirement of having small interface elements can be fulfilled without refining the mesh in the adjoining regions. In this way, SBFEM reduces the computational cost of the model many folds without any compromise in the accuracy. The thesis provides a basis for future research on the application of SBFEM to model inter and intra-ply damage in complex laminated composite structures.

Following a comprehensive literature review on the progress of abrasive waterjet (AWJ) machining, an experimental study of the AWJ machining of carbon fibre-reinforced polymers (CFRPs) of various thicknesses was conducted, showing that clean cuts can be achieved with good processing rates. The effect of process parameters on the machined kerf and hole characteristics is amply discussed in the thesis. It was demonstrated that AWJ machining is a good process for thick CFRPs that other processes may be unable to cut. However, material delamination in the form of edge pop-up in the jet entry and push-out at the jet exit caused by the initial pure waterjet impact of an AWJ piercing operation was observed. It was experimentally shown that using a steel mask on top of the workpiece can eliminate pop-up delamination, while push-out delamination at the jet exit can be reduced or eliminated by proper process parameters. However, the mechanisms involved require further investigation. Mathematical models for predicting the major machining performance indicators were developed using dimensional and regression analysis. Experimental verification confirms that the predictive models are reasonable and reliable for assisting in the planning of AWJ machining processes. A computational model is developed and verified experimentally to study the interaction between a pure waterjet and CFRPs. The behaviour of the waterjet is modelled using the smoothed particle hydrodynamics method while the CFRP is modelled by finite element using a continuum damage material model and cohesive zone method. A computational study using the developed model reveals that the material pop-up delamination is initiated due to the material’s elastic response to a rapid release of shock pressure to stagnation pressure and the traverse shear stresses induced by the downward bending of the laminated layers. The pure waterjet impact causes flow divergence and a hydro wedging effect between the material plies, which propagates the delamination. The delamination magnitude is found to increase initially with waterjet pressure up to a threshold after which a change in pressure does not affect the pop-up delamination significantly. The smallest pop-up delamination area occurs on the [0]12 laminate, followed by the [0/45/90/-45/0/45]s and [0/90]3s laminate. It is also found that the push-out loading towards the jet exit and the hydro wedging effect act jointly to result in push-out delamination.

America has a rapid growth in the development of renewable power generation in the twenty-first century. Hydrogen energy especially is the major topic in development of renewable power generation because it has the ability to provide for energy in transportation, combined heat and power generation and energy storage systems with little or no impact on the global ecosystem. Electrolysis and fuel cell technology for hydrogen energy storage has brought interest to the researchers worldwide. Solid oxide electrolysis cells (SOECs) and Solid oxide fuel cells (SOFCs) are promising hydrogen energy technologies that can utilize chemical energy to produce and store electricity. SOEC can produce hydrogen at a higher chemical reaction rate with a lower electrical energy requirement. SOFCs offer significant advantages in the variety of fuels, quiet operation, low or zero emission and high efficiency. The performance of SOECs and SOFCs is the key for commercializing SOECs and SOFCs. This work presents the methods and theory of improving the performance of SOECs and SOFCs. A thermodynamic and electrochemical equilibrium model is established for SOECs by considering a cathode/electrolyte interfacial porous layer. By using numerical experiments, the one dimensional thermodynamic and electrochemical equilibrium model with porous layer provides a guideline for designing and optimizing SOECs. The mechanisms of SOFCs degradation are reported and the ideal maximum oxygen partial pressure is calculated to avoid SOFCs degradation.

Fiber reinforced polymer matrix composites are a class of structural materials that have gained high desirability in a wide range of applications over the past few decades. Due to their high specific strength and toughness, low density, and design flexibility, fiber reinforced composites have been preferred over traditional homogenous materials, such as ceramics and metals, in structures and components within the military, aerospace, automotive and marine industries. The two constituents of these composite materials are typically the rigid fibers and compliant polymer resin, acting as reinforcement and matrix phases, respectively. Yet unlike biological multicomponent materials, such as bones, teeth, and bamboo, composites are heterogeneous materials suffering from failure-prone, discontinuous, and discrete fiber-matrix interfaces that limit them from achieving their ideal theoretical mechanical properties. Therefore, improving interfacial adhesion and the load transfer mechanism between the fiber and the matrix, while simultaneously maintaining the structural integrity and light weight of composite structures, is of great importance for the fabrication of high performance composites and has been a long-lasting challenge in the field of composite materials. This dissertation is an effort to experimentally investigate hierarchical and multifunctional fiber reinforced polymer matrix composites with improved interfacial and interlaminar adhesion through the integration of nanoscale interphases and interlayers. As nanotechnology regularly introduces new functional building blocks, many promising and lightweight nano-reinforcement approaches are continuously emerging and integrated into composite materials. Here, the potential and role of chemical interactions between nanomaterials (aramid nanofibers (ANFs) and nanofibrils, zinc oxide nanowires (ZnO NWs), laser induced graphene (LIG)) and fiber surfaces (aramid, carbon, glass), along with their impact on the morphology and adhesion quality of the resulting interphases and interlayers are initially investigated. As a result, it is demonstrated that well-adhered nanostructured interphases and interlayers can be achieved in various fiber reinforced composites through a number of chemical processes, which include fibrilization, physical and electrostatic adsorption, surface functionalization, hydrothermal growth, and laser-induced graphitization, as well as other mechanical approaches, such as transfer printing and spray-coating. Further research is then performed to thoroughly investigate and optimize the effect of the introduced nanostructured interphases and interlayers on the interfacial and interlaminar properties of both fabrics and composites under quasi-static and dynamic loading conditions, all while maintaining their structural integrity, light weight, and flexibility. The obtained results conclusively indicate that aramid nanostructured interphases are capable of enhancing the interfacial shear strength (IFSS) and interlaminar properties of quasi-statically loaded aramid and glass fiber reinforced composites, while also improving the impact response and stab resistance of ballistic protection aramid fabrics. Moreover, ceramic zinc oxide interphases are studied using a novel experimental setup and are shown to allow for the tailoring of composite interfacial properties as a function of the applied strain rate. Finally, ANF and LIG nanostructured interlayers are demonstrated to suppress delamination and improve interlaminar fracture toughness in both aramid and carbon fiber reinforced polymer matrix composites. The nanomaterial reinforced interlaminar regions exhibit improved toughening mechanisms that increase energy absorption, and thus delay catastrophic failure due to delamination in composite structures. The research presented in this dissertation provides a multitude of scalable and efficient approaches for the grafting of nanostructured interphases and interlayers capable of yielding hierarchical and multifunctional fiber reinforced polymer matrix composites with improved mechanical performance and maintained light weight and flexibility.

The characterization of the damage state of a system provides insight into its performance and safety during operation. In composite materials, specifically, fiber-reinforced polymers, delaminations form from the evolution of cracks in a matrix that leads to adhesion failure between adjacent laminae. Nondestructive evaluation (NDE) seeks to characterize the state of a material or system during non-operational times. A previously proposed NDE method employs embedded magnetostrictive particles between laminae of carbon fiber reinforced polymer (CFRP) for damage sensing. The phenomenon of magnetostriction couples the mechanical state of a material with its magnetic state so that a change in the local stress field alters its magnetic susceptibility. The change in magnetic susceptibility is measured using an induced sensing voltage. This work aims to provide a preliminary exploration of machine learning to predict the presence of a delamination using the embedded magnetostrictive particle NDE method with CFRP laminates. Machine learning algorithms' ability to decipher and develop relationships among input features attracted its use since the visual examination of the experimental induced sensing voltage plots yielded inconsistent delamination predictions. This work investigated the feasibility of an analytical model based on the Euler-Bernoulli beam theory to generate data. This model utilized functional relationships to characterize the nonlinear behavior of the magnetostrictive material Terfenol-D. Fourier series relationships reduced the error in the characterization of the properties over the previously proposed functions. The previously proposed derived model failed to converge for the calculation of stress within the magnetostrictive material. Eight machine learning algorithms were employed using a Python script to classify the presence of a delamination within a unidirectional HexPly AS4/3501-6 CFRP embedded with Terfenol-D particles. The maximum accuracy achieved was approximately 80, whereas the average accuracy for all the models was just below 71%. The multi-layer perceptron (MLP) models, a neural network algorithm, produced the highest prediction accuracy for this two-class classification problem because of their ability to account for nonlinear relationships. A parametric study involving the architecture and activation function was necessary for the MLP models because of the range of obtained prediction accuracies. A direct relationship was observed between the number of hidden layers and the accuracy outliers. Despite the accuracy of the examined machine learning models' being less than that of other NDE applications, this preliminary investigation demonstrated that machine learning could be paired with previously indiscernible experimental data to detect delaminations.

The concrete delamination induced by prestressing forces is a potential problem of post-tensioned concrete containments with curved geometry. To investigate the concrete delamination mechanism, the researchers of the University of Texas at Austin completed an experimental research project in 2017. In this research project, the academic finite element method (FEM) program VecTor4, which has powerful shell-type elements for cracked reinforced concrete shells, was modified and proved to be an appropriate tool to predict the level of applied prestressing load at the delamination failure. VecTor4 has a well-defined reinforced concrete material model and capability to analyze the structures on an overall system level with low computational resources. However, its user interface has limitations, and the details of the local behavior of structures may not be captured due to its simplicity. On the other hand, commercial FEM programs generally provide a convenient graphical-based user interface and various analysis features. Still, typical commercial FEM programs are not designed to analyze the concrete structures. Some commercial FEM programs provide concrete material models; however, they tend to require significant calibration analyses to capture the behavior of reinforced concrete structures. Specifically, to capture the delamination of concrete walls with those programs, solid elements should be selected, which should increase the computational costs. However, commercial FEM programs can potentially provide the details of the local behavior of structures. Recently, an experimental test investigating the concrete delamination phenomenon was conducted. In this recent test, numerical analyses with the commercial FEM program Abaqus ware conducted as well as the analysis with VecTor4 to explore other options in commercial FEM programs. This paper will show the analysis results of both Abaqus and VecTor4 for the recent test. Additionally, the discussion on the relative merits and limitations of these programs obtained from the experience of the analyses for the recent test is presented.

Monolayer molybdenum disulfide (MoS2) is a three-atom-thick direct band gap semiconductor, which has received considerable attention for use as a channel material in atomically thin transistors, photodetectors, excitonic LED’s, and many other potential applications. It is also a mechanically exceptional material with a large stiffness and flexibility, and can withstand very large strains (11%) before rupture. In this dissertation we investigated the mechanics of the stiffness and adhesion forces in atomically thin MoS2 membranes, and how biaxial strains can be used to induce large modulations in the band structure of the material. First, we used chemical vapor deposition (CVD) to grow MoS2 crystals that are highly impermeable to gas, and used a pressure difference across suspended membranes to induce large biaxial strains. We demonstrated the continuous and reversible tuning of the optical band gap of suspended monolayer membranes by as much as 500 meV, and induced strains of as much as 5.6% before rupture. We observed the effect of strain on the energy and intensity of the peaks in the photoluminescence (PL) and Raman spectra and found their linear strain tuning rates, then report evidence for the strain tuning of higher level optical transitions. Second, we determined the Young’s modulus and works of separation and adhesion of MoS2 membranes, and found that adhesion hysteresis is an important effect in determining the behavior of our systems. Finally, we investigated the use of atomically thin materials as nanofiltration membranes, by perforating the material with nanopores which selectively permit the transport of smaller molecules while rejecting larger ones. We studied ion transport through nanopores in graphene membranes and demonstrate that in-situ atomic force microscope measurements in liquid are a powerful way to reveal occlusions and contaminants around the pores - work which will aid future researchers in further unveiling the properties of these fascinating systems.

Experimental study accomplished the characterization of fatigue crack growth rates and mechanisms in both hand and die forged Aluminum 7085-T7452. Testing was conducted at various positive and negative loading ratios, primarily focused on L-S and T-S orientations to discover a correlation between crack tip branching or turning mechanisms and stress intensity. Interior delaminations were found to originate in the interior of the specimen and propagate outward to the surface and manifested as splitting cracks parallel to the loading direction. Stress intensity ranges have been correlated with the onset of crack deviation from the primary crack plane, as well as, the transition to branching dominated fatigue crack growth.

Many residential buildings in the United States of America are light wood structures which get damaged considerably due to high-speed winds, hurricanes, and tornadoes all year-round. Specifically, the roof-to-wall connections are very vulnerable when subjected to these extreme loadings. The objective of this study is to evaluate the application of FRPs as a roof-to-wall connection using finite element analysis (FEA) ANSYS Mechanical APDL software program. The FEA models of the wood roof-to-wall GFRP connection were validated with an experimental study in the literature. The experiment involved component level shear and uplift tests under static loading representing the effect of high-speed wind on the connection. The FEA models and experimental results were in good agreement which were further validated through Monte Carlo Simulation. The validated models were then employed to perform comprehensive parametric study. To mitigate the contact failure, anchorages were added to GFRP ties and their performances were compared with the FEA validated models of shear and uplift tests which had no anchorage and was under static load. Besides the validated model of the GFRP connection, uplift model was examined under monotonic cyclic loads with various intensities to better simulate the effect of wind load. Furthermore, carbon and basalt FRP sheets were introduced as wood roof-to-wall connections and their performances were examined under cyclic loads. To validate the efficiency of GFRP ties with and without anchorages, the shear and uplift design loads specified in ASCE 7-16 were calculated for comparison purposes. Similarly, the shear and uplift capacities of typical hurricane clips evaluated in the same experimental study used for validated FEA model were also compared with the GFRP ties with and without anchorages. Furthermore, a formula was proposed to calculate the shear strength of the GFRP connection in comparison with double shear bolted metal plate connections using National Design Specifications for wood construction. The finite element results revealed that anchorage increased the uplift load capacity by 15% but the increase in shear capacity was insignificant. Comparing glass, carbon and basalt FRP ties, BFRP was superior in deformation capability and CFRP provided more stiffness on uplift test simulation. GFRP ties were found to be approximately nine times stronger in shear and two times stronger in uplift than hurricane clips and superseded the ASCE design load requirements by a large margin. Finally, the proposed formula would assist in calculation of shear strength of the proposed GFRP tie for future use or research.

Structural response of composite structures subjected to different impact loadings is studied. The low-velocity and high-velocity ballistic impacts, and residual structural response are analysed. First, low-velocity impact response of the composite laminates and composite sandwich panels are investigated. The composite honeycomb sandwich panels are composed of facesheets made of carbon fibre reinforced facesheets and the Nomex honeycomb core. Composite laminates and sandwich panels are subjected to six different impact tests performed with impact energy levels ranging from 17.13 to 154.18 J. Two types of loading conditions have been implemented: (a) impacts by the impactors having the same mass and variable impact velocities resulting in different impact energy levels; (b) impacts with the same level of energy (equivalent energy impacts) and different impactor masses with corresponding (different) velocities. The experiments probed the effects of impactor mass, velocity, and different impact energy levels by analysing the patterns of induced damage, energy absorption, penetration resistance and force-displacement response. Microscopic non-intrusive damage assessment procedures have been employed to investigate various damage modes induced by the drop weight impact depending on the impact of velocities and impactor masses. Also, ballistic damage effects on the structural performance of composite honeycomb sandwich panels are investigated. Ballistic tests have been conducted in a shooting range using live projectiles of three different calibres. The sample panels are subjected to normal, oblique, and multiple oblique impacts to simulate the damage to composite sandwich structures that can be inflicted by the use of small weapons or by fragments of anti-aircraft ammunition. The modes and extent of the damage induced have been analysed using optical microscopy, X-ray inspections, and computer tomography methods. Structural performance of the intact and ballistically damaged panels has been investigated experimentally: using in-plane compression tests, and numerically, based on the finite-element modelling and analysis. The reduction in the load-carrying ability caused by the ballistic damage has been assessed for the panels under consideration and compared with that predicted by numerical simulations.

Primary aerospace composite structures are commonly assembled with bolted joints due to their ability to transfer high loads and ease of assembly. However, when bolted joints are used beyond their originally intended design life, joint strength can be significantly reduced due to the accumulation of internal damage, necessitating frequent inspections. Furthermore, internal damage in composites (delamination, matrix cracks) can continue to propagate without visual indications, thus nondestructive testing is required. As a result, maintenance can become very costly, particularly for aircraft that are in-service beyond their designed life expectancy. By establishing a comprehensive understanding of damage propagation behavior, engineers can determine which damage modes to inspect for and reduce inspection frequency. This research aims to support and improve maintenance operations, fleet management, and aircraft design practice by investigating delamination initiations and propagations in the vicinity of fastener holes within fiber-reinforced composite materials. Static and fatigue bearing were performed using novel test methods developed as part of this research for countersunk fastener joints: the modified countersunk double lap shear (DLS), single lap shear (SLS), and semi-circular notch (SCN) test configurations. DLS and SLS static and fatigue experimental test results were compared to study joint configuration, laminate stacking sequence, and loading condition effect on bearing damage initiation and growth under both static and fatigue loading. From static and fatigue tests, it was observed that major bearing damage accumulates in the straight shank region of the countersunk hole indicating most of the bearing load is carried by the straight shank region. Fatigue bearing test data showed that when the bolted hole elongates, stiffness decreases and internal delamination damage area growth becomes detectable through C-scan. Stated in reverse, if no measurable hole elongation is found, significant delamination is not expected. Complex damage morphology forms in this region, emanating from the loaded bearing face, and creating large wedge-shaped regions that drive delamination propagation with additional loading cycles. Additionally, optical microscopy observations indicated that pin bending might have affected bearing damage growth. In order to understand the effect of pin bending, a custom designed semi-circular notched experiment was performed on the countersunk hole geometry and compared to the DLS static experiments. Results indicated that the pin bending had no strong effect on the bearing failure morphology for the selected diameter to thickness ratio. Finite element analysis using Virtual Crack Closure Technique (VCCT) and Hashin failure criteria in Abaqus was used to further understand the internal stress state of the specimen configurations and to investigate the rate of delamination growth and arrest in the SCN and DLS configurations. Results from FEA were used to more comprehensively understand the observations from static and fatigue experiments and to verify hypotheses formulated to explain these observations.

In the past few decades, Laminated composites have become the most favored material system for a wide range of industrial applications. Factors such as the potential for structural tailoring, superior strength, and stiffness for a given weight along with higher toughness and mechanical damping etc., have justified their increased use. Due to this high usage, the accurate behavior of laminated composites under various loading conditions remains a concern and hence this dissertation focuses on building simple and accurate uni-directional micro-mechanical material models.Firstly, an elastic micromechanical model based on the physically viable sub-cell boundary conditions is developed and implemented for use with uni-directional composite laminates in the explicit finite element software LS-DYNA®. Stress-strain relations have been presented in a three-dimensional context and hence can be used with solid elements. The effect of these boundary conditions in accurately estimating the elastic properties has been studied. Transverse and shear modulus have been analyzed in detail alongside popular analytical methods and verified against available experimental results for various volume fractions. Good agreements have been observed for the presented model in comparison with the experimental results.Secondly, a non-linear micro-mechanical composite material model is developed to simulate the behavior of uni-directional composites under impact loading conditions. The strain-rate and pressure dependency in the composite material model is accounted by the resin, which uses previously developed state-variable viscoplastic equations. These equations were modified to account for the significant contributions of hydrostatic stresses typically observed in polymers. The material model also uses a continuum damage mechanics (CDM) based failure model to incorporate the progressive post-failure behavior. A set of Weibull distribution functions are used to quantify this behavior and an iterative methodology of characterizing the softening parameters used in these functions is presented. The impact response of composite laminate plates has been simulated and compared to the experiments. It has been observed that the predicted results compare favorably to the experiments. In addition, the effectiveness of studying inter-laminar delamination in laminated composites with the help of thickness-stretch shell elements which utilize a 3-D material model sub-routine as compared to the traditional plane-stress shell elements has also been investigated. Ply-level progressive failure criteria have been used to simulate the initiation and propagation of delamination. The numerical delamination growth has been qualitatively analyzed against the experimental C-scan images for multiple impact events on different composite plates.Lastly, since it is well known that strain-softening behavior in composite material models leads to a pathological mesh sensitivity in finite element simulations. Non-local continuum theory has been used and three non-local damage models with isotropic weight function have been implemented to work in conjunction with the non-linear composite micro-mechanical material to address this problem. The effect of weighing function on each of these formulations has also been studied in detail. All three non-local formulations have been observed to produce a nice smeared effect of damage, unlike the local damage models.

Laminar, fiber reinforced polymer matrix composites have been widely and increasingly used in industries where the strength-to-weight ratio (or specific strength) is important. These types of composites exhibit impressive specific mechanical properties due to the high tensile strength of the reinforcing fibers, combined with the low overall density. While the in-plane properties for laminar composites are comparable to those found in other engineering materials such as metals, the out-of-plane properties do not receive reinforcement from the fibers and therefore tend to be significantly lower. As a result, the weak failure mode in such composites is commonly in the form of delamination, in which the composite laminae separate in the weaker, matrix-dominated interlaminar phase.There is also an increasing demand in multifunctionality in engineering materials – that is, the ability for materials to serve more than one purpose simultaneously. As the demands for weight reduction in structures increase (particularly in the aerospace industry), one promising frontier to enable such reductions to use materials that behave as both a structural component (i.e. have sufficient stiffness, strength, etc.) and as an electrical component. These two types of components have classically existed as separate entities, and therefore if they can be efficiently combined into one material system, there can be significant improvements in areas of weight, cost, simplicity and reliability. For example, in aircrafts, the structural components of the fuselage and the wiring for onboard electronics exist separately – if they could be integrated into one structural conductor that sacrifices neither on conductivity nor on mechanical properties, there is a potential to significantly reduce the weight of the aircraft and thereby increase the fuel efficiency.The approach of this work is to improve both the interlaminar mechanical properties and the electrical conductivity of a laminar, fiber reinforced composite using Dyneema thread stitching and carbon nanotube interleaves. Sheets of carbon nanotubes were laid between layers of the composite, and the layers were stitched together in the through-thickness direction to increase the resistance to delamination. In order to validate this as a viable technique to make delamination resistant, multifunctional composite materials, the mechanical properties were tested to ensure there were no significant reductions in tensile or interlaminar shear strength. The improvement to delamination resistance was confirmed with mode I fracture testing, and the electrical conductivity was tested to demonstrate that the carbon nanotube sheets between each layer impart a significant increase in the in-plane conductivity. The combined effects of the presented method of interleaving carbon nanotube sheets between laminae and stitching through the thickness results in a delamination resistant, highly conductive laminated polymer composite that can be readily manufactured at the scale required for large components such as structural paneling in aerospace applications.

A comprehensive literature review on the development of the abrasive waterjet (AWJ) machining technology and the knowledge relating to the AWJ machining processes has been conducted. It is revealed that the AWJ machining has been proven to be an effective tool for processing composite materials due to its distinct advantages. However, the understanding of the AWJ machining performance on polymer matrix composites (PMCs), particularly the woven carbonfibre reinforced polymers (CFRPs) that are gaining increasing industrial applications, remains unclear, while the knowledge of delamination induced by the machining process is even in more dearth. Furthermore, there is little reported study on the jet impact process and phenomenon under the ultrahigh velocity conditions relevant to AWJ machining. AWJ cutting and drilling (or piercing) processes for through holes on woven CFRPs are experimentally investigated to understand the characteristics of machined holes and the influencing parameters. It is shown that small holes whose diameter is slightly greater than the nozzle diameter can be effectively generated by AWJ drilling, while relatively large holes of a desired diameter can be produced by AWJ cutting. The holes machined by both processes exhibit similar geometrical features, where the hole top diameter is greater than the bottom, so that a hole wall inclination angle is formed. Plausible trends of the hole characteristics with respect to the process parameters are discussed. In the cutting process, a higher water pressure or a slower nozzle traverse speed causes a larger hole diameter and a smaller hole wall inclination angle. In the drilling process, the hole diameter increases with an increase in standoff distance or water pressure, while the hole wall inclination angle increases with an increase in standoff distance or a decrease in water pressure. The shock wave impact of the waterjet at the initial jet-workpiece interaction is found to be the major cause for the formation of hole defects, such as delamination and chipping. The drilled holes have more severe defects than the cut holes because the defects caused by the initial jet impact stay with the drilled holes, while the hole cutting process can start at somewhere on the workpiece that eventually becomes a scrap. Nevertheless, the defects can be alleviated or eliminated by selecting the recommended appropriate process parameters. Mathematical models for predicting the relevant machined hole characteristics, namely hole diameter and hole wall inclination angle, have been developed. It has been shown that the predicted results from the models agree well with the experimental data under the corresponding conditions. In order to understand the jet flow and impact characteristics under the ultrahigh velocity conditions relevant to AWJ machining, a computational fluid dynamics (CFD) model has been developed. A CFD model for the AWJ flow characteristics is first developed, experimentally verified, and used to simulate and study the jet and particle flow characteristics typified by velocities. This simulation study shows that the waterjet and particle velocity profiles across the jet evolve from the initial one-seventh power law distribution to a flatter cap distribution with an increase in downstream distance from the nozzle exit. The change rate of the water velocity is higher than that of the particles due to the less density and momentum of the water. However, a small difference between the waterjet and particle velocities is found even at the 70 mm downstream location. The AWJ flow model is then extended to allow the study of the AWJ impact characteristics on a solid surface. It is revealed that stagnation is developed at the jet impact site, which has a significant effect on the energy of the waterjet and particles that can be transferred to the target surface. The average particle velocity across the jet is reduced by approximately 33% by the damping effect of the stagnation for the conditions considered in this study. The stagnation also induces the particles to deviate from their moving trajectory, so that the particle impact angle is changed and the jet impact area is enlarged. The formation of stagnation is found to be influenced by the jetting and impact parameters. An increase in standoff distance or nozzle diameter results in an increase in the magnitude of the stagnation zone. An oblique jet impact yields an asymmetric stagnation zone, where the stagnation centre is away from the jet axis. Mathematical models for the relevant particle impact characteristic quantities, namely the particle impact velocity, particle translation in the jet radial direction and particle impact angle, are finally developed based on the understanding gained from the CFD simulation studies. The models are verified by comparing the model predictions with the corresponding numerical data. It is shown that the developed particle impact models can provide adequate predictions for use in future studies of the particle impact micro-mechanics and impact erosion.

Abstract: The central Andes (18° - 26°S) is an active Cordilleran orogen, formed through the subduction of the Nazca oceanic plate beneath the South American continent. Despite significant tectonic crustal shortening and thickening (60 -70 km), deep seismic imaging indicates a thin to absent mantle lithosphere beneath the central Andean plateau (Altiplano-Puna plateau). In order to explain this as well as regional geological observations (e.g. widespread mafic volcanism, and along-strike timing variations in uplift across the plateau), lithosphere removal has been proposed to have occurred within the central Andean system within the last 20 Ma. Conditions within Cordilleran systems, such as the central Andes, can produce density inversions (i.e. roots) within the plateau mantle lithosphere, including: changing pressure-temperature conditions induced by subduction, crustal tectonic shortening, and cyclical magmatic processes. The presence of gravitationally unstable mantle lithosphere drives its removal. Motivated by these observations, the present study uses thermal-mechanically coupled geodynamic models in order to study gravitational removal of single and multiple dense roots within an active subduction zone setting, representative of the central Andes. Both lithosphere removal via large-scale coherent delamination and localized Rayleigh-Taylor (RT)-type instabilities are investigated. Emphasis is placed on i) the dynamics and timing of removal events, ii) the topographic response to lithosphere removal, and iii) the predicted seismic velocity structures generated as lithosphere removal proceeds. In general, subduction-induced flow aids the removal process, perturbing the lithosphere and entraining the foundering material to remove it from the system. For the modelled lithospheric structure, delamination causes mantle lithosphere to coherently peel away from the overlying crust towards the subducting oceanic slab. The numerical modelling experiments show this delamination occurs within 5 – 12 m.y., resulting in 0.5 – 0.7 km isostatic uplift, with removal proceeding faster for decreased root density and increased mantle lithosphere strength (i.e. dehydrated conditions). In comparison, RT-type instabilities of greater density and decreased strength are more readily destabilized. Mantle lithosphere is locally removed within 2 – 12 m.y., and the root’s down-going trajectory is largely dependent on its relative position to the trench and the rate at which is descends. Note that lithosphere removal of a single RT instability results in local uplift of 0.5 km, whereas destabilization of multiple roots positioned across the extent of the plateau causes regional uplift of 1 – 1.2 km. This study shows that present-day lithosphere structure across the Altiplano-Puna plateau can be reproduced through episodes of gravitationally driven lithosphere removal. Overall, for both removal mechanisms (i.e. delamination and Rayleigh-Taylor-type instabilities), predicted seismic velocity structures are able to reconcile previously identified upper mantle velocity anomalies to a certain degree, supporting the idea that low velocity zones beneath the central Andean plateau represent high temperature areas of thinned mantle lithosphere. However, regardless of the manner of removal, the resulting surface uplift is limited to ~1 km, suggesting the modern Altiplano-Puna plateau topography is likely not solely linked to removal-induced isostatic and dynamic elevation changes.