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This paper develops a finite element (FE) model of the single fibre fragmentation test designed for direct comparison with experimental results on an E-glass/epoxy system by McCarthy et al. (2015). Interface behaviour is modelled via a cohesive surface, and stochastic Weibull fibre strengths (determined by independent experiments) assigned at random to the elements along the fibre. Predictions from the model agree with experiment for a range of outputs: The evolution of the number of fibre breaks with strain is similar and breaks occur at random locations as required. The model also captures a transition to a Uniform (rather than Weibull) statistical distribution of break locations at later stages of the test consistent with recent experiments. The evolution of the cumulative distribution of fragment lengths is also similar to that of the experiment. In addition, fibre axial stress and interfacial shear stress distributions conform with experimental observation. Correct model predictions of break locations confirm the approach taken on assigning stochastic (Weibull) strengths along the fibre. The effectiveness of the FE model in capturing a number of key aspects of the fragmentation phenomenon suggest its usefulness as a tool in analysing and interpreting fibre fragmentation tests, including back-calculation of interfacial shear strength.

This study proposed a novel approach based on the 3D discrete element method (DEM) to simulate the progressive delamination in unidirectional carbon fibre reinforced polymer (CFRP) composite laminates. A hexagonal packing strategy was used for modelling 0◦ representative plies, the interface between different plies was modelled with one bond and seven bonds following the conservation of energy principle and a power law. The number of representative layers and the stiffness of bonds within these layers were calibrated with a comparison of results obtained from finite element method and theoretical analysis.DEM simulations of delamination with both interface models were conducted on unidirectional composites for double cantilever beam (DCB), end-loaded split (ELS) and fixed-ratio mixed-mode (FRMM) tests. It was found that the seven-bond interface model has a better agreement with experimental data in all three tests than the one-bond interface model by adopting the proposed seven-bond arrangement in terms of the progressive delamination process. The main advantages of the present interface model are its simplicity, robustnessand computational efficiency when elastic bonds are used in the DEM models.

This article describes the use of a novel lactone-layered double hydroxide polymer network (PN), derived from a poly(lactide-co-caprolactone) copolymer, as a controlled ion-release agent for artificial bone tissue regeneration. The osteogenic cell culture Saos-2 is used as a test culture to investigate the PN's performance as an extracellular ion-release agent. The compelling performance of this PN is demonstrated in both growth and osteogenic media compared with a control of cells grown on tissue culture plastic (TCP) without PN. Firstly, the PNs released concentration of magnesium ions over time ranging from 10 to 60 mM after 24 hr, depending on the PN sample. After incubation of Saos-2 with the PN, while no difference was seen in cell number, there was significant upregulation of bone-related gene expression at 14 days-~5fold increase in Bone Morphogenetic Protein 2, ~3fold increase in osteopontin and ~2fold increase in collagen Type I. In addition, normalized alkaline phosphatase activity was seen to significantly increase by ~2fold with PN presence. A ~4fold increase in collagen Type I protein expression (via Gomori Trichrome Stain) was observed with PN presence. In addition, a ~4fold increase in phosphate deposits (as seen with Von Kossa staining analysis) was seen with PN presence. It is found that this novel PN material has a significant potential for bone tissue regeneration.

In this study, two types of single polymer films have been inserted in a composite laminate to examine their toughening effects on mechanical properties. The first is a thermoplastic polyurethane (PU) film, and the second is an adhesive epoxy film featuring a polyester net. The laminates were manufactured either using a co-curing (CC) process or a secondary bonding (SB) process used for the epoxy film. Mode I and mode II interlaminar fracture toughness were measured for laminates manufactured by both processes and compared with the corresponding reference laminate toughness. A significant increase in both mode I and mode II toughness resulted when introducing a single PU film, approximately 290% and 50%, respectively. Similarly, the epoxy film improved the interlaminar fracture properties; the CC process produced an increase of 175% for mode II toughness, while the SB adhesive film showed an increase of 75% for mode II toughness.

Support structure fatigue is a key component in determining the structural lifetime of an offshore wind turbine (OWT). As the currently installed assets age, turbine operators are exploring options for lifetime extension to potentially increase the long-term financial return. Strain monitoring at critical points on a turbine is commonly performed to improve understanding of structural integrity and ultimately reassess its remaining useful life. Reliable application of the findings of a time-limited structural monitoring programme in predicting structural response over a turbine’s lifetime requires a good understanding of the representativeness of the dataset. Uncertainties arise in fatigue damage estimations due to the stochastic nature of the environmental loading. Statistical treatment of the environmental loads and the corresponding structural response is made by defining measured load cases (MLCs), within which the turbine operational state and associated range of environmental parameters are specified. For OWTs, this leads to a multi-dimensional problem, as both wind and wave parameters need to be accounted for. The complexity of the analysis is thus increased, requiring identification of the critical external and operational parameters that influence overall fatigue. The associated statistical uncertainty can then be estimated by considering repeated measurements throughout the monitoring period. The work presented in this paper investigates the application of statistical resampling techniques in evaluating the uncertainty in total measured fatigue damage experienced by an offshore wind turbine. Direct fatigue computation over the given measured dataset has been contrasted with statistical approaches applying probability distributions of MLCs to give indication of the influence of the key environmental parameters. Strain monitoring data from a 2.3MW OWT was utilised in conjunction with the corresponding operational and environmental measurements. The methods and outcomes of this study can be used to improve the remaining fatigue life prediction of installed turbine foundations, by assessing the representativeness of strain measurements. As structural design uses industry defined safety margins, comparison of design predictions against operational measurement data will allow verifying that these safety margins are not exceeded, within the bounds of the given uncertainties. Finally, an understanding of data uncertainties will allow estimates to be made regarding the reliability of the consequent fatigue lifetime reassessment or of the numerical model validation procedures. Such information is useful to wind turbine operators as it provides the first step towards data-driven lifetime extension and informs on measurement campaign utilisation.

A novel material and process was developed using fibre-reinforced powder-epoxy to produce unidirectional towpreg with a pilot-scale towpregging line, for cost-effective production of large composite structures for the renewable energy market, specifically for wind and tidal turbine blades. Electrostatic attraction was used to coat fibre tows with powder epoxy and either joule or radiant heating employed to heat and melt the polymer, followed by consolidation between rollers. Unidirectional carbon-fibre and basalt-fibre reinforced polymer laminates (UD-CFRP and UD-BFRP, respectively) were manufactured from the towpreg. Tensile test results showed that the towpregging process could be employed to achieve high performance UD-CFRP with 0° tensile properties that are similar or better than commercially-available UD-CFRP systems. The competitive advantages of the powder-epoxy towpreg system include lower cost, better overall manufacturing control for vacuum-bag-only manufacturing and the ability to co-cure parts together at a later stage. Mechanical test results showed some variation between two types of UD-BFRP, but the results compared well with published data on UD-BFRP and equivalent glass-fibre reinforced polymer (GFRP) systems. Finally, the influence of hygrothermal ageing due to water immersion on the tensile properties of the materials was investigated, with tests revealing that the water ageing effect was more severe in the case of UD-BFRP than for UD-CFRP.

The fibre-matrix interfacial properties of glass-fibre/polyamide-6 (GF/PA-6) composites produced by thermoplastic resin transfer moulding (TP-RTM) were investigated. Two different fibre sizings were compared, a specially-developed novel reactive fibre sizing and a standard silane glass fibre sizing. Scanning electron microscopy, atomic force microscopy and a number of mass-loss techniques were employed to study the form, distribution, quantity and degradation temperature of the fibre sizings. Activated PA-6 monomer precursor materials with viscosities of ∼10 mPa s were injected into the glass-fibre fabrics, contained between heated press platens, and polymerisation occurred in-situ within several minutes. Glass-fabric laminates with fibre volume fractions of ∼53% and low void content were produced at a pressure of ∼4 bar, with the low viscosity of the monomer negating the need for expensive high-pressure injection. Similar quality between the laminates was demonstrated by measuring density, thickness, fibre volume fraction, void content and fibre bundle distribution. Transverse mechanical properties of the composites reinforced with the novel reactive sizing were 20–28% higher than those with the standard fibre sizings, demonstrating improved fibre-matrix interfacial properties. Average mode I fracture toughness was also measured to be 10–30% higher than with the standard fibre sizing. Scanning electron microscopy and 3D depth composition were used to investigate fracture surfaces and determine the surface roughness. The novel reactive fibre sizing resulted in improved fibre-matrix adhesion and improved fracture toughness.

Carbon fibres with three different sizing agents were used to manufacture unidirectional composites based on a powder epoxy resin. Powder epoxy processing was investigated as a route for fast, cost-effective manufacturing of out-of-autoclave composites compared to more time-consuming vacuum infusion technologies. In this work, a heat-activated epoxy powder was used as a resin system in low-cost vacuum-bag-only prepregs for thick composite parts that are required in the renewable energy industry (e.g. wind turbine blade roots). The importance of interfacial bonding between fibres and the matrix is shown and the impact on the ultimate mechanical performance of the manufactured composites demonstrated. The surface characteristics of the sizing on the carbon fibres were investigated using atomic force microscopy (AFM) and Raman spectroscopy. Results showed that the amount of sizing on the fibres' surfaces was inextricably linked with surface roughness and coverage. This in turn influenced the mechanical and chemical interlocking phenomena occurring at the fibre/matrix interface. The composites’ mechanical performance was evaluated using tensile, flexural and interlaminar fracture toughness tests. Fractographic analysis using optical and scanning electron microscopy (SEM) was likewise employed to analyse the fracture surfaces of the tested/failed composites. Interlaminar fracture toughness testing (DCB Mode-I) revealed that the interfacial adhesion differences could alter the fracture resistance of the composites, hence emphasizing the importance of the interfacial bonding strength between the polymer matrix and the carbon fibres.

In a fibre-reinforced polymer matrix composite (PMC), the function of the fibre is to bear the applied load that is transferred via shear stresses through the fibre-matrix interface from the polymer matrix. The fibre absorbs stress by progressively fragmenting along its axis until a critical fibre fragment length is realized. After this no further fragmentation is possible, the fibre is said to be “saturated” in stress, and it provides no strengthening upon further deformation. A critical and intrinsic material property of the composite that determines the failure of the fibre is the interfacial shear strength (IFSSh) between matrix and fibre. This review presents the multi-fibre fragmentation technique (MFFT) and Laser Raman Spectroscopy (LRS) which are used for fibre-matrix interface testing. The limitations of MFFT are summarised and ideas for improvement are proposed. The key findings of this review are: (1) MFFT equipment and protocols vary considerably and require standardisation. (2) Existing models for stress transfer between fibres rely on geometrical models that do not capture the material properties or the constitutive models of the transmitting matrix. Comprehensive constitutive matrix stress transfer models are needed. Current MFFT models do not incorporate terms for matrix vibration as a function of fibre fracture shock. It is clear that more work can also be done to characterize microcomposite systems in compression, at various angles to the fibre axis, and under various combinations of cyclic loading, but arguably such work should be pursued after the uniaxial tensile fibre fragmentation problem has been better understood.

This paper for the first time presents experimental results for fibre flow and void formation during 3D printing of short fibre reinforced thermoplastic composites by fused filament fabrication (FFF). Short carbon fibre T300 reinforced nylon-6 composite is selected as the printing material. X-ray micro-tomography (µCT) scans are performed on the raw filament, in-nozzle melted filament, extruded printing bead and on-bed printing bead to trace the through-process evolution of fibres and voids for the specific nozzle used therein. Qualitative visualisation of voids fraction and fibre orientation, length and fraction, as well as quantitative analysis are carried out using image processing techniques. The results show that the orientation and volume fraction of fibres vary with different internal geometry of the nozzle and fibre misalignment occurs in the on-bed printing bead because the relative motion between the nozzle and the print bed disturbs the flow field. Also the fibre length decreases slightly during the printing process due to the collision between fibre and nozzle wall when the melted materials pass the nozzle. For the void volume fraction, most voids are generated when the melted filament is extruded from the nozzle, and porosity decreases in the on-bed printing bead. The reported experimental data of through-process evolution of fibre flow and void formation can also be used for benchmarking and/or validating computational models for 3D printing of short fibre reinforced polymer composites.

The mechanical integrity of a structural composite is strongly affected by the strength and toughness of the fiber–matrix interface/interphase (Norwood, 1994), with interfacial shear strength (IFSS) being generally accepted as the best quantifying metric. The value of the IFSS is not directly measurable, but it can be approximated by several micromechanics based test methods with the value obtained being dependent on the choice of the model. The most popular of these test methods is the embedded single fiber fragmentation test (SFFT) which provides the experimental data needed to estimate the IFSS: (a) mean fragment length at saturation and (b) fiber strength at the critical fragment length. Because the IFSS is used in unidirectional composite models to predict strength and failure behavior, where the interaction between fibers can be important, the validity of extrapolating from test results based upon the repeated failure of a single isolated fiber has often been questioned. In this paper, the spatial distribution of fiber breaks in a 2-D array of glass fibers is compared with break locations observed from SFFT specimens. In both cases, the break locations in each fiber were found to evolve to a uniform distribution, thereby confirming that the ordered fragment lengths from the repeated fracture process conforms for both SFFT and multi-fiber fragmentation test (MFFT) specimens to a cumulative distribution function (CDF) derived by Whitworth (1887) and cited by others (Read, 1988; Pyke, 1988; Holst, 1980). The array break density was also observed to be less than the break density in isolated fibers, and break locations across array fibers were observed to be highly coordinated and mostly aligned.

This paper describes the conversion of a layered anionic initiator (carbonate-intercalated layered double hydroxide, (LDH-carbonate)) into a self-assembled resin-embedded network during the in-situ polymerization of one or more lactone monomers using the LDH-carbonate as the sole initiator. Uniquely in this paper, no long-chain acid intercalant is present in the LDH-carbonate to act as an additional initiator species, and this is the first known report of a copolymerisation of these lactones using LDH as an initiator. The formation of a network is in marked contrast to the behavior of most in-situ polymerisations using layered species, where the latter retains its layered structure at the molecular level and is either intercalated or exfoliated to form a nanocomposite. The molecular disintegration of the LDH sheets is unusual. Nine new insoluble materials (scaffolds) are isolated from various l , d -lactide & e-caprolactone (LC) and l , d -lactide & δ-valerolactone (LV) copolymer hybrids. The latter hybrids are polymerised using the LDH-carbonate as initiator at 150 °C for 24 h without using conventional metal catalysts. Each insoluble phase is isolated from each primary hybrid product using dichloromethane (DCM) to selectively dissolve the soluble polymer phase. X-ray diffraction (XRD) is used to verify the morphology of the insoluble phases. This demonstrates that the molecular sheets of the LDH-carbonate are fully dismantled during the polymerization. Porous, network morphology is established for some of the insoluble phase structures using scanning electron microscopy (SEM). This indicates potential suitability of these self-assembled insoluble phase materials as bioscaffolds for artificial cell growth. Nuclear magnetic resonance spectrometry (NMR) was used to determine the ratio of ester to acidic carbonyls in the insoluble phase. Energy Dispersive X-ray spectroscopy (EDX) was also used to determine the ratio of magnesium to aluminium in the insoluble phases.

Poly(lactic acid), PLA, was extruded with intumescing flame retardant formulations based on ammonium polyphosphate, APP. Nanofibrillated cellulose fibers (NFC), POSS-modified NFC (PNFC), and pentaerythritol (PER) were used as the additional carbon source. The effects of each additive and their intumescing combinations on polymer degradation, flammability, crystallization, melt rheology, and tensile properties were systematically examined. APP and PER catalyzed the degradation of PLA during extrusion, which increased the crystallinity of PLA, lowered the viscosity of the melt, reduced the moduli, and decreased the tensile strength of the composite. APP had the largest effect on physical properties, acting as a nucleating agent, decreasing tensile strength, and increasing elongation to break. The POSS moieties on PNFC acted as a slight plasticizer in melt rheology studies, but did not affect the glass transition temperature. PNFC formed a cross-linked network with APP when melt-blended with PLA, which reduced polymer degradation, decreased PLA crystallinity, reduced the melt viscosity, and improved composite stiffness relative to the neat extruded PLA. The PLA composites containing APP and PNFC had the best tensile properties of all the intumescing composites studied. The cross-linked network formed between cellulose, POSS, and PLA helps produce composites with superior flame retardant, rheological, and mechanical properties relative to other intumescing formulations.

This paper presents a comprehensive experimental study of impact damagedetection for carbon fibre-reinforced polymer (CFRP) composites using anelectromagnetic (EM) sensor with coupled spiral inductors (CSI). Tworepresentative types of damage are detected and evaluated, i.e., barely visibleimpact damage (BVID) and delamination. A multi-frequency inspection isperformed, where the resultant images indicate the potential of the CSI sensor inthe characterisation of damage extent. The accuracy and efficiency of the CSIsensor are compared with the open-ended waveguide imaging, near-fieldmicrowave microscopy, microwave time-domain reflectometry, thecomplementary split-ring resonator and ultrasonic scanning. Applications andlimitations of these non-destructive testing (NDT) methods for identifying impactdamage are discussed. There is a free edge effect on the electromagnetic signal,which is illustrated for the first time with the proposed EM technique. Detectionof the air gap produced by inserting a thin piece into a machined subsurfacegroove is carried out. It is found that the developed CSI sensor is able toaccurately resolve the location and extent of the air gap. The experimentalresults demonstrate that the sensor could offer an alternative relatively low costmethod that can be fully automated for structural monitoring of aircraft andother composite structures.

A heat transfer model based on the well-known Henderson equation has been modified to allow for self-sustained ignition and the flaming combustion phenomena of E-glass fibre-reinforced epoxy composites to be predicted from first principles using known thermal-physical and thermodynamic data for their constituents. The modifications consider: (1) the assignment of thermodynamic conditions (e.g. ignition temperature and mass flux of volatiles) necessary and sufficient to trigger self-sustained ignition, and (2) the inclusion of an integrated loop allowing the heat energy generated from the flaming combustion process to be fed back into the burning laminate. The model compares moderately well with experimental results obtained from cone calorimetric measurements. The additional modelling capabilities considered in this study provide the basis for an analytical model that can more accurately predict the thermal response and flaming combustion of glass fibre-reinforced polymer composites exposed to a one-sided radiant heating environment in the presence of an ignition source.

This article is part of a series on the thermo-mechanical responses of fiber-reinforced composites at elevated temperatures and it follows the first part containing experimental results. A flame-retardant system consisting of a cellulosic charring agent and an interactive intumescent additive (melamine phosphate) has been used in order to improve the post-fire mechanical performance of glass fiber-reinforced epoxy composites. The effect of one-sided radiant heat on the residual flexural stiffness of laminate coupons exposed to incident heat fluxes of 25 and 35 kW m-2 was investigated. The flame-retarded coupons retained a higher percentage of their original room temperature flexural modulus after heat exposure while the control specimens showed inferior material property retention over the same exposure periods. A heat transfer (thermal) model based on Henderson’s equation is used to predict the through thickness temperature profiles and subsequently the mass loss due to the resin matrix decomposition at elevated temperatures. The theoretical results from the heat transfer model are validated against experimentally obtained data and then coupled with a mechanics model that describes material property-temperature dependence in order to predict the residual flexural stiffness, after heat damage. The accuracy of the thermo-mechanical model was validated against the experimental data and a good agreement was observed.

The high thermal expansion coefficient of perfluoroelastomer materials can cause problems with the design of adequate seal grooves to cover both high and low temperature operation. Variations in filler shape and proportions have been shown to have a significant effect on thermal expansion over part of the temperature range. This feature describes an investigation of relevant literature that has been compared with experimental verification.

We report the formation of an ionomeric network in a poly( l , d -lactide) hybrid nanocomposite, (PLDLA-HYB) during in-situ melt polymerization of l , d -lactide in the presence of magnesium/aluminum layered-double-hydroxide (LDH) without added catalyst. The effect of LDH mass loading and reaction time on molecular mass and yield of soluble poly( l , d -lactide) (PLDLA-SOL) present in the hybrid was investigated for a better understanding of the conflicting roles of LDH in polymerization and degradation of PLDLA-SOL. High molecular mass PLDLA-SOL is obtained through initiation of polymerization by LDH. However an additional insoluble organic–inorganic fraction, INSOL, is also observed within the product when PLDLA-SOL is extracted using methylene chloride as solvent. It is proposed that INSOL is an ionomeric network comprising hydrogen-bonded, or otherwise co-ordinated, lactic acid monomer salts of magnesium, together with PLDLA in a 24%–76% mass ratio.

A hardware-in-the-loop simulation has been developed to eval uate the performance of active missiles from prelaunch to inter cept. The facility is used for system integration, design verifica tion, preflight and postflight analysis, and demonstrations of systems performance. A CDC Cyber 175 general-purpose computer interfaced with an Applied Dynamics International AD-10 multiprocessor solves the six-degree-of freedom simulation in real time. The simulation includes the target model, the missile model, and relative motion. The Cyber 175 also drives the model of the RF environment through a Hewlett Packard 1000 computer. The model of the RF environment provides the necessary phase and amplitude control to reproduce a 10-point glint model. The facility also can receive an active transmission and can supply four independent returns separated in range, angle, and velocity. The simulation has not yet undergone a formal validation program, but the facility results have been compared to an all- mathematical simulation of an active missile. Initial comparisons with flight results support the credibility of the facility.