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The crucial obstacle to design an electrically conducting device is the poor and heterogeneous dispersion of the pristine CNTs in polymer matrices. The best method to overcome these problems is to create a CNT core-polymer shell structure, allowing a dense close-packing of the CNT core-polymer shell units, expecting in the formation of completely homogeneous CNT-polymer films and a big improvement in their physicochemical properties including the electrical conductivity. Poly[3-(5-carboxylpentyl)thiophene-2,5-diyl] wrapped multi-walled carbon nanotubes (P3PT-COOH/CNTs) in core-shell structure were successfully prepared as a bucky paper by simple pH-control of the aqueous solution containing different weight percentages of the both components (P3PT-COOH/CNTs = 50/50, 60/40, 70/30). The core-shell structure of P3PT-COOH/CNT-50 nanohybrid films showed a significantly high electrical conductivity increase of up to 17 fold compared with those of non-core-shell structure prepared by a simple mixing (e.g. from 1.33 to 1.81 S/cm up to 22 S/cm). The result could be considered to be a consequence of a homogeneous P3PT–COOH–wrapping on the surface of CNTs, the regular shell thickness of individual CNT-core/P3PT–COOH–shell nanohybrid fibers, and their three-dimensional close packing in a film. [ABSTRACT FROM AUTHOR]

Numerical models of unidirectional panels of ultra-high-molecular weight polyethylene (UHMWPE), like Dyneema® HB80 or Spectra®, have been a difficult challenge. The problem arises from the intimate structure of the material. It is a huge collection of fibers loosely held together by a matrix. For example, for HB80 the fibers are very small in diameter (∼17 μm) but extremely strong (∼3.5 GPa). The amount of matrix is small (∼20% in volume) and very weak. The “natural” scale to use in the numerical model for this material is the fiber scale (∼17 μm). This scale would provide the right sound speed, transverse wave speed, failure strain, strength of the laminate, deflection, slippage between the fibers/layers, etc. which are all essential details if we want to have predictive capability. But even with the very powerful computers available nowadays we are still very far from being able to simulate a real-size laminated target (for example 30 cm wide and 1 or 2 cm thick) at the fiber scale. This paper proposes an “intermediate” scale that bundles or consolidates many fibers together so that the numerical problem is solvable in a few hours (or maximum one or two days for thick targets) of computation in a single board (24 processors). The scale is chosen precisely so that the essential physics of the ballistics problem is kept and the material properties used are the ones measured in the lab for the fiber and matrix. A similar approach was presented in a previous paper for fabrics (Kevlar, Dyneema, and PBO) where a yarn-level model was able to reproduce both the wave propagation patterns and the ballistic limits. This paper first describes new material test results (shear and compression at low and high confinement pressures). These were essential in properly capturing the delamination of the material. Then, for completeness, transverse wave propagation and ballistics results in Dyneema published elsewhere are briefly presented. Finally the numerical model in LS-DYNA is developed and comparison with deflection history and ballistic limits is conferred. [ABSTRACT FROM AUTHOR]

Abstract In current composite design, stacking sequence symmetry around the laminate mid-plane is an unarguable constraint to avoid warpage during manufacturing. However, several load cases induce unevenly distributed stresses through the laminate thickness, such that symmetric laminates may not be the optimal solution. In this paper, we explore the damage resistance to out-of-plane low velocity impact loading of an unsymmetrical laminate with zero extension-bending coupling matrix ([B]), thereby assuring no undesired coupling deformations during mechanical or thermal loads. Using impact and quasi-static indentation tests, C-scan inspection and numerical modelling, we compare the damage pattern between an unsymmetrical laminate with ply clustering at the impacted face and a laminate with ply clustering at the non-impacted face (produced by flipping the former laminate upside down). The laminate with clusters at the impacted side exhibits better damage resistance for lower impact energies. More importantly, the location of the damage events obeys the predictions assumed when the laminate was designed, demonstrating the room for improvement by tailoring unsymmetrical laminates to particular load cases. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes fractographic observations from the detailed examination of delamination fracture surfaces and offers an interpretation of the key growth mechanisms. Firstly, the relationship between toughness, delamination failure criteria and fracture morphology is presented and the influence of cusp formation and deformation on toughness is discussed. Observations regarding delaminations migrating through the lamina at multidirectional ply interfaces are then discussed. It is demonstrated how this migration process can be avoided in fracture toughness coupons and consequently the toughness of multidirectional ply interfaces can be characterised. The influence of migration on delamination growth from embedded defects in laminates under compression is presented, and these results are extended to demonstrate how migration influences damage growth in structures. The paper concludes by making recommendations for realistic modelling of migration, and suggests how it can be exploited in damage tolerant structural design. [Copyright &y& Elsevier]

Abstract: This paper presents an approach to predict the delamination of braided composite T-piece specimen using cohesive models. As part of an investigation on simulation of delamination in T-piece specimens, cohesive elements from ABAQUS were employed in forming a cohesive model to study the progressive delamination. Predictions given by the model of single delamination together with experimental results are presented. These results suggest that prediction of progressive delamination using cohesive models is feasible. Finally this paper proposes future work for precise prediction of delamination of braided composite T-piece specimens. [Copyright &y& Elsevier]

Abstract: A novel procedure for the identification of delaminations in composite laminated panels from surface displacement measurements is presented in this paper. The damage identification procedure is formulated as an inverse problem where experimental data from digital speckle pattern interferometry (DSPI) out-of-plane displacement measurements, are used to quantify the planar location, size, orientation and depth of the flaw. The inverse problem is then reduced to an optimization problem where the optimum crack parameters are found by minimizing the objective function through the use of a real-coded low-population genetic algorithm. DSPI measurements of ten delaminated T700/LTM-45EL carbon/epoxy laminate panels are used to validate the methodology presented in this paper. [Copyright &y& Elsevier]

A variational approach is used to model the behaviour of composite cross-ply laminates damaged by transverse, longitudinal cracking and delamination. An energetic criterion is proposed. It is based on the strain energy release rate associated with each of the three damage modes. The first part of this paper is concerned with the modelling of the transverse and longitudinal cracking. In the second part, a model for studying delamination damage is presented. The numerical results show that these models provide a consistent level of accuracy for a variety of thin laminate material systems and configurations, with various combinations of delaminations and matrix cracks. In this paper several numerical simulations meant to describe initiation for each damage mode are proposed. The estimation of damage modes contributions is achieved for two thin laminates in order to predict the evolution of damage mode transition. [Copyright &y& Elsevier]

Delamination is a significant defect of laminated composites. The present study employs an electric potential change method in an attempt to identify internal delaminations experimentally. The method adopts reinforcing graphite fibres as sensors. In our previous paper, a two-prove method was adopted for the electric-resistance change measurements because of the simplicity. The two prove method is not appropriate for a precise measurement, and it tends to include a large experimental error in the electric resistance change owing to the electric resistance change at the electrodes. Instead, the present paper employs a multiple-prove method for the measurements. In the present study, high precise measurement system of electric voltage change is developed, and the electric voltage measurement method is adopted for identification of embedded delamination location and size. Measured electric potential data are normalized for creations of response surface to identify the delamination. As a result, the new multi-prove electric potential method is shown to be effective for the identifications of embedded delamination cracks of graphite/epoxy composite laminates. [Copyright &y& Elsevier]

Abstract Self-sensing capability of carbon nanotube buckypapers (BPs), played as in situ sensor on delamination damage in multi-directional laminates, was investigated in this paper. BPs were interleaved into these interfaces of the laminates, where delamination tends to happen firstly under tensile loading. The inserted BPs are porous and conductive, which can benefit infiltration of epoxy resin and give contribution to damage monitoring. Systematic analysis of resistance variation (Δ R/R 0 %) against strain has demonstrated that BPs are sensitive to the initiation and growth of delamination. When delamination initiates, a sudden rise of the Δ R / R 0 % or the slope of the Δ R / R 0 %-strain curves is observed and corresponding delamination initiation stresses are simultaneously obtained. Moreover, effect of thicker laminates on the sensitivity is also examined and the self-sensing ability of BPs has been further proved. Finally, tensile properties of the BPs interleaved laminates slightly change compared with these of base laminates. [ABSTRACT FROM AUTHOR]

The damage evolution study on 3D woven carbon-fiber reinforced composites has always been a challenge to the research community. In this paper, a new in situ experimental method is proposed to realize the non-destructive observation of the failure process of 3D woven carbon-fiber reinforced composites. Synchrotron radiation computer tomography (SRCT), an in situ load frame and new-style specimens are integrated. With the method, the 3D crack initiation and propagation can be continuously recorded by the SRCT scanner. Through analyzing the experimental results, the out-plane failure mechanism under tension and shear loads is found. Particularly, it is shown that the interface property is the leading factor affecting out-plane strengths. Moreover, the effect of braiding angle on the out-plane strengths in different directions is discussed. According to the failure modes, it can be concluded that with the increasing of the braiding angle, the out-plane tension strength will increase but the out-plane shear strength will decrease. The proposed in situ experimental method and the above findings are significant to calibration of failure prediction model and the novel design of woven composites. [ABSTRACT FROM AUTHOR]

Multi-Directional hybrid composites can suffer from free-edge delamination, a damage mode that doesn't exist in Uni-Directional hybrid composites and can hinder the pseudo-ductility that can be achieved with thin-ply hybrids. This paper presents a new lay-up concept called ‘ orientation-dispersed ’ laminates to avoid this mode of damage in quasi-isotropic hybrids. It is shown that the energy release rates at the free-edges of orientation-dispersed layups are significantly lower than those in ‘ orientation-blocked ’ laminates. Finally, the experimental results from two quasi-isotropic layups with π/3 and π/4 intervals are presented showing a good pseudo-ductility with no free-edge delamination. [ABSTRACT FROM AUTHOR]

Proper modeling of the interplay between transverse cracks and delamination is crucial in accurately simulating degradation in laminated composites. However, up to now, there is no computationally friendly model to account for this intra/inter laminar coupling. In this paper, a new hybrid cohesive law has been proposed in which the damage could be activated by both out-of-plane separation and in-plane strains of the interlaminar interface. By doing so, the in-plane strain can be used to estimate the damage state of the adjacent plies to the interface, so the response of the cohesive element is modified accordingly. This provides an efficient framework to account for the effect of intralaminar transverse cracks onto the delamination resistance. Through the authors' earlier work, the influence of intralaminar damage on interlaminar properties is twofold: harmful (through transverse crack-induced delamination) or beneficial (transverse crack-induced bridging). Both coupling effects are embedded in the hybrid cohesive elements. The modeling parameters are calibrated using rigorous experimental data. The simulation results demonstrate the potential of the pragmatic cohesive element proposed in this paper. Image 1 [ABSTRACT FROM AUTHOR]

Thin ply laminates are a new class of composite materials with great potential for application in the design of thinner and highly optimized components, resulting in potential weight savings and improved mechanical performance. These new composites can stir the development of lighter structures, overcoming current design limitations as well as notably reducing the onset and development of matrix cracking and delamination events. This paper presents the application of two recent modeling methods for the failure analysis and strength prediction of open-hole thin ply laminates under tensile loading, which exhibit a brittle response upon failure: (i) the analytical coupled energy-stress Finite Fracture Mechanics (FFMs) technique, and (ii) the FE-based Phase Field (PF) approach for fracture that is incorporated into an enhanced assumed solid shell element. The predictions obtained using both strategies are compared with experimental data. These correlations exhibit a very satisfactory level of agreement, proving the robustness and reliability of both methods under consideration. [ABSTRACT FROM AUTHOR]

In cross-ply laminates, the shape of delamination areas, which form due to low velocity impact, have two subtle features, which have been observed consistently in numerous experiments. Those are the pointed delamination tips and the intact zone between the lobes of delamination. However, there have not been any account available in the literature how they can be consistently captured through numerical modelling, and hence these features in published modelling results were often absent. It is the objective of this paper to identify the underlying modelling considerations so that these features can be captured with confidence. A key and unique reason has been identified in each case. Namely, inclusion of intra-laminar damage allows to reproduce the pointed delamination tips, while the gap between the lobes of delamination can be captured by models with sufficiently refined mesh, where friction between the laminas is taken into account. The capability of capturing these subtle features helps to raise the level of fidelity on the simulation of delamination due to impact. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of the delamination migration in angle-ply laminates observed in experiments reported in the literature, where the delamination originally propagates along the lower, 0 ∘ / 60 ∘ interface and later migrates onto the upper, 60 ∘ / 0 ∘ interface. The recently-developed Floating Node Method (FNM) is used for modelling this problem. The initiation and propagation of both delamination and matrix cracks are modelled within the FNM elements. Experimentally-observed phenomena such as the numerous kinking attempts and the multiple onset locations of migration are successfully predicted. The effect of load offset on the locations of migration is captured. In addition, this work tries to shed light on the proper use of standard cohesive elements in cases where delamination migration is expected. [ABSTRACT FROM AUTHOR]

This paper focuses on the effect of interfacial fibre orientation and interleaved veil on the delamination migration of carbon fibre reinforced polymer laminates under Mode I loading. Double cantilever beam specimens with midplane interfacial fibre orientations of 0/0, 90/90, 0/90, 0/45 and 90/45 were tested under two conditions: one with interleaved thermoplastic polyphenylene sulfide veil at the midplane and one without. Results show that, except for the 0/0 configuration, all other orientations exhibit varying levels of migration associated with the interfacial fibre orientation and veil interleaving. The apparent fracture toughness determined with the modified compliance calibration method is closely related to the delamination migration and hence a structural energy dissipation measure dependent on interfacial fibre orientation and the interleaved veil. Distributions of the fibre and matrix materials around the delamination front are found to be closely related to the delamination migration behaviour along its path. The experimental observation and rationalisation presented in this paper provide further knowledge regarding delamination migration and its correlation to the apparent fracture toughness, which is of direct relevance to the damage tolerance design of laminated composite components. [ABSTRACT FROM AUTHOR]

Thermoplastic composites are likely to emerge as the preferred solution for meeting the high-volume production demands of passenger road vehicles. Substantial effort is currently being directed towards the development of new modelling techniques to reduce the extent of costly and time consuming physical testing. Developing a high-fidelity numerical model to predict the crush behaviour of composite laminates is dependent on the accurate measurement of material properties as well as a thorough understanding of damage mechanisms associated with crush events. This paper details the manufacture, testing and modelling of self-supporting corrugated-shaped thermoplastic composite specimens for crashworthiness assessment. These specimens demonstrated a 57.3% higher specific energy absorption compared to identical specimen made from thermoset composites. The corresponding damage mechanisms were investigated in-situ using digital microscopy and post analysed using Scanning Electron Microscopy (SEM). Splaying and fragmentation modes were the primary failure modes involving fibre breakage, matrix cracking and delamination. A mesoscale composite damage model, with new non-linear shear constitutive laws, which combines a range of novel techniques to accurately capture the material response under crushing, is presented. The force-displacement curves, damage parameter maps and dissipated energy, obtained from the numerical analysis, are shown to be in a good qualitative and quantitative agreement with experimental results. The proposed approach could significantly reduce the extent of physical testing required in the development of crashworthy structures. [ABSTRACT FROM AUTHOR]

This paper presents detailed experimental information on mode-II delamination development in fibre/epoxy composite materials and provides observations about the process zone in the vicinity of the crack tip. It is shown that the energy dissipated in delamination propagation is spent on two ways (i) creating new fracture surfaces (delamination) and (ii) nonlinear shear deformation in the composite plies adjacent to the delaminating interface. Therefore, the nonlinear process zone is not restricted to the resin-rich interface between the layers, but also extends into the fibre/epoxy composite layers. This is different from the conventional assumption in modelling delamination using cohesive elements where the fibre/epoxy layers are fully linear-elastic and the process zone is lumped at the plane of fracture. Based on the accurately measured displacement field around the crack tip, the experimental traction-separation relation at the interface is found to be trapezoidal which is again different from the conventional bilinear cohesive law. [ABSTRACT FROM AUTHOR]

The definition of failure is fundamental to the characterisation of fatigue strength of components and structures and is often expressed as a percentage of stiffness degradation. This article proposes a new method to capture damage initiation and structural degradation during a fatigue test, by exploiting resonance vibrations. The method entails monitoring dynamic parameters, including the phase angle between excitation and response, which suddenly changes as soon as the overall stiffness changes as a consequence of damage. In this paper, a theoretical approach based on equations of motion is presented in order to describe the possible scenarios in which a component undergoes structural degradation. In the proposed approach, an abrupt change in the time history of the stiffness of the component is assumed and analysed in both the quasi-static case at low excitation frequencies and the dynamic case, when the excitation frequency is close to the resonance frequency. The resulting analytical solution shows that any small stiffness variation is amplified by the dynamic response of the component as a large phase change, even when it is too small to be captured by the quasi-static response. The idea is successfully applied to capture the initiation of delamination in CFRP laminates, and the relative S-N curve to initiation is drawn. The experimental evidence of a critical event in the fatigue life of a component is found both in the dynamic phase response and in the base acceleration. The critical event is captured from a sudden change in the dynamic parameters of the structure as soon as a delamination is initiated. Thermography, C-scan and micrograph confirmed that the onset of delamination occurs at the moment of the critical event, after an initial stage of microcracking. [ABSTRACT FROM AUTHOR]

This paper describes the results related to the crack growth rate measurement on double cantilever beam (DCB) specimens made of a carbon-fibre fabric-reinforced multifunctional epoxy composite. Two plates were evaluated where the resin was enhanced using a combination of 0.5% carbon nanotubes (CNTs) and Glycidyl POSS (GPOSS) flame retardant for one of the plates. Loading in mode I was displacement-controlled with R = 0.1, f = 4 Hz and a constant maximum strain energy release rate ( G I ) max during the cycle. The value of ( G I ) max was defined to be from 20% up to 60% of the mode I interlaminar fracture toughness. The specimens enhanced by CNTs and GPOSS highlighted a significant decrease in the fatigue crack growth rate of approximately 80%. The crack growth rate was also observed to be significantly related to the interface of the weft and warp tows of the plain weave. [ABSTRACT FROM AUTHOR]

This paper investigates for the first time the usage of through-thickness reinforcement for delamination detection in self-sensing composite laminates. Electrically conductive T300/BMI Z-pins are considered in this study. The through-thickness electrical resistance is measured as the delamination self-sensing variable, both for conductive and non-conductive laminates. The Z-pin ends are connected to a resistance measurement circuit via electrodes arranged on the surface of the laminate. The delamination self-sensing function enabled by conductive Z-pins is characterised for Mode I/II delamination bridging, using single Z-pin coupons. Experiment results show that, if the through-thickness reinforced laminate is electrically conductive, the whole Z-pin pull-out process associated with delamination bridging can be monitored. However, for a non-conductive laminate, delamination bridging may not be sensed after the Z-pin is pulled out from one of the surface electrodes. Regardless of the electrical properties of the reinforced laminate, the through-thickness electrical resistance is capable of detecting Mode II bridging, albeit there exists an initial “blind spot” at relatively small lateral deformation. However, the Z-pin rupture can be clearly detected as an abrupt resistance increase. This study paves the way for exploring multi-functional applications of through-thickness reinforcement. [ABSTRACT FROM AUTHOR]

This paper presents a FEM model for the simulation of delamination in composite materials under fatigue loads using extended cohesive interface elements. A virtual fatigue damage variable is proposed to continually locate the crack tip elements with local element information only. The proposed method allows for a fully automatic identification of crack tip elements without any global information. Once crack tip elements are identified, the fatigue damage accumulation is determined by a modified Paris-law with a strain energy release rate correction method to reduce mesh sensitivity. The predicted delamination growth rates of different mesh sizes were compared with experimental results from open literature using a mode-I Double Cantilever Beam specimen and a mode-II 4-point End Notch Flexure specimen respectively. In all cases good agreement was obtained with weak mesh dependency. [ABSTRACT FROM AUTHOR]

In this paper a model for predicting fatigue delamination growth in laminated composites under high cycle fatigue is proposed. The model uses the cohesive zone approach and a two-scale continuum damage mechanics model. The behavior of the interface material is considered quasi-brittle at the macro scale while plastic deformations are allowed at the scale of micro-defects. The validity of the proposed model is investigated through several standard tests using experimental data from literature. Good agreement between the numerical and experimental results is observed. The model is also capable of simulating fatigue under variable amplitude loading. This feature of model is shown through several sample simulations. [ABSTRACT FROM AUTHOR]

Fatigue delamination growth in composite laminates is strongly influenced by mean loads or load ratios. Description of this behaviour currently relies on empirical curve fitting, which renders it difficult to predict fatigue lives of composite structures subjected to variable amplitude fatigue loading. This paper presents a new scaling parameter that is consistent with the similitude concept and incorporates the crack-tip shielding effects of fibre bridging under fatigue loading. Static and fatigue experiments were carried out on IM7/977-3 composite laminates under mode I and mode II. Large-scale fibre bridging was observed as a major toughening mechanism under both static and fatigue loading. To correctly account for the effect of fibre bridging, an inverse method was developed to determine the traction stresses acting in the crack wake. The new scaling parameter, accounting for the effect of bridging by cross-over fibres, is shown to unify the fatigue growth rates under different load ratios obtained in this study. [ABSTRACT FROM AUTHOR]

The double cantilever beam (DCB) tests are widely used to assess the interfacial delamination properties of laminated composites. For quasi-static loads, the DCB tests are standardized based on the beam mechanics; for dynamic loads, however, such as high-loading-rate impact and cyclic loads, there is no established analytical theory. This presents a significant obstacle preventing the research community from assessing the delamination behavior of composites or adhesives for their application under complex in-service loads. In this paper, the theory of evaluating dynamic mode-I delamination driving force for DCBs under general displacement loads is developed for the first time, accounting for structural vibration effects. The developed theory is demonstrated by two examples: high-loading-rate split Hopkinson bar impact and cyclic fatigue loads. The analytical solutions are validated by published experiment results and in-house tests. This work provides a fundamental analytical tool to study and assess the fracture behavior of fiber reinforced-polymer composite and adhesive materials under various loading conditions. [Display omitted] • Analytical theory developed for mode-I dynamic energy release rate. • Quasi-static, dynamic, and general cyclic loads solved analytically. • Structural vibration included to account for dynamic effect. • Dynamic effect studied and quantified with induced displacement. • Finite-element-method simulation and in-house fatigue test verify the analytical theory. [ABSTRACT FROM AUTHOR]

This paper seeks to further investigate a recently proposed novel empirical-based Delamination Reduction Trend (DRT) for stitched composites. The DRT is capable of predicting the effective reduction in impact-induced delamination area due to the influence of stitching. DRT simply relates two parameters: normalised delamination area and stitch fibre volume fraction, to characterize the effectiveness of stitching in impact damage suppression. In this work, quasi-static indentation (QSI) test is performed on stitched composites with the aim to offer new observations, provide validations and understand limitations in DRT. Test data coupled with experimentally observed damage mechanisms reveal limitations of DRT, particularly across two regions: first, when damage is initiated and delamination growth has not been developed; second, when delamination propagation is influenced by boundary conditions and saturation of delamination damage has occurred. Validation of DRT is demonstrated in domain outside of these regions. The investigation is further analysed by statistical approach using a three-factorial design of experiments (DoE). The main influence and coupling interaction of stitch density, stitch thread thickness and indentation displacement on indent-induced delamination area are established and discussed. Graphical and statistical results provide further insights and validation to DRT. The novelty of this work lies in showing validation and understanding limitation of DRT for its effective use and accurate prediction of delamination damage in stitched composites. [ABSTRACT FROM AUTHOR]

This paper investigates kinematic coupling approaches for FE simulation of the mechanical behavior of complex composite structures. Two coupling methods belonging to the family of kinematic coupling approaches, namely the point-wise kinematic coupling and the weighted residual kinematic coupling are analyzed and assessed with respect to their application to non-linear structural FE analyses. The investigated kinematical coupling approaches have been implemented in the in B2000++ FEM code developed by SMR® and used to couple global–local domains characterized by different mesh refinements. A stiffened composite panel with an artificial bay delamination under compressive load has been adopted as numerical benchmark. Results in terms of stress distributions at domains interfaces, obtained with several FE models characterized by different coupling approaches and different mesh refinements, have been compared with the results obtained adopting a reference test-case model with mesh continuity and merged modes. Furthermore, the influence of the coupling approach on the delamination growth simulation has been assessed. Finally considerations about the computational effectiveness of the coupling approaches with respect to the reference test-case are introduced. [ABSTRACT FROM AUTHOR]

Predictive models have struggled to accurately simulate progressive delamination growth in composite structures, often due to the challenges associated with modelling delamination migration phenomenon. This paper presents a methodology with which to model such migration. Firstly, the interlaminar shear at a delamination front were partitioned into axial and transverse modes, the mode-mixity of which was controlled by the mismatch between this front and the ply directions. An element formulation was presented which utilised this mismatch. Consequently, delamination migration was shown to have ensued when the transverse mode exceeded a critical mode-mixity G iii /G T = 0.22. This approach was verified against experimental studies on width tapered end loaded split coupons by correlating the fracture morphology against mode-mixity. In particular, the orientation of the cusps on the delamination surface were shown to be controlled by the relative dominance of the axial and transverse modes. This methodology provides a means to accurately and robustly model progressive delamination growth processes in composite structures. [ABSTRACT FROM AUTHOR]

For carbon fibre reinforced composites, two different delamination processes are identified; one at the crack-tip with a sub millimetre sized process zone and one with a large-scale process zone associated with fibre bridging. In the present paper we focus on the crack-tip process that is important when studying initiation of cracks and when crack growth has to be avoided. Methods to measure the cohesive laws associated with the fracture processes are developed for loading in mode I and II using the DCB- and ENF-specimens. An evaluation method involving finite element simulations and an optimization procedure is proposed and verified. Experiments performed on a CFRP are evaluated using the proposed method. For loading in mode I, the fracture energy and cohesive strength are in the order of 300J/m2 and 75MPa, respectively. The corresponding values for mode II loading are 1200J/m2 and 80MPa. [ABSTRACT FROM AUTHOR]

Abstract: Thin-ply based laminates are a promising development in composite materials and are expected in the near future to outperform conventional laminates in mechanical performance. A rational design with thin plies requires understanding the effect of ply thickness on each damage mechanism. This paper presents an experimental investigation into damage occurrence in a quasi-isotropic laminate made from thin-ply, bi-axial, Non-Crimp-Fabric (NCF), under different off-axis uniaxial loadings. The NCF layers are positioned through the laminate thickness creating two regions, namely THICK and THIN (with and without ply clustering). Then, the onset and progress of three damage mechanisms (transverse matrix cracking, matrix crack induced delamination and free-edge delamination) for both regions are analyzed by monitoring the specimen’s free-edge. The results show that the critical region where damage occurs is that with ply clustering (THICK), whereas delamination originating from matrix cracks or free edge effects are delayed or even suppressed in the THIN region. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental investigation into the mode I interlaminar fracture toughness and fatigue resistance of carbon fibre–epoxy laminate reinforced in the through-thickness direction with z-pins. The effects of the volume content, diameter and length of z-pins on the interlaminar toughness, fatigue resistance, and crack bridging toughening mechanisms are determined. The delamination crack growth rate under cyclic interlaminar loading slowed rapidly with increasing z-pin content up to a limiting concentration (∼2% by volume), above which no further improvement to the fatigue resistance was achieved due to a transition in the fatigue cracking process. The delamination growth rate also slowed when the z-pin diameter was reduced or the z-pin length was increased. The dependence of the fracture toughness and fatigue resistance on the volume content, diameter and length of the z-pins is related to the crack bridging toughening processes induced by the z-pins. [Copyright &y& Elsevier]

Abstract: This paper presents a finite element model (FEM) to analyse the structural deformation, strength properties and delamination fracture behaviour of composite T-joints reinforced with z-pins. The FE modelling involves multi-level analysis of a pinned joint using unit cell and macro-scale structural models. Unit cell model is used to calculate the crack bridging traction loads generated by the elastic and interfacial friction (pull-out) stresses of a single pin within a representative unit cell of the joint. Macro-scale analysis involves modelling delamination crack growth in the pinned joint using a cohesive zone model, which is based on the traction load analysis of a single pin. The FE model was validated using experimental results for a pinned carbon fibre/epoxy T-joint subjected to tensile (stiffener pull-off) loading. The model accurately calculated the crack initiation load, ultimate load, and fracture mode of the pinned joint. [Copyright &y& Elsevier]

Abstract: This paper addresses the issue of using energy balance methods and crack closure concepts to predict the growth of delaminations associated with ply cracks during the progressive loading of cross-ply laminates subject to a combination of in-plane biaxial stresses and thermal residual stresses. When the effective applied stresses and the temperature are held fixed during delamination growth, and there is negligible interaction of the delamination tips with the ply cracks, very simple analytical formulae for the energy release rate can be derived for unconstrained and generalised plane strain conditions, which are exact when the ply crack separation tends to infinity. In some practical applications, such as for wide plates, delamination growth is constrained transversely by surrounding undamaged laminate and this has an effect on energy release rates. An analytical method is described that determines the energy release rate for delamination growth under such constrained conditions. This result is also exact in the limit of ply crack separations tending to infinity. The boundary element method (BEM) has been used to investigate energy release rates for constrained conditions for a range of ply crack densities, both in the presence and absence of thermal residual stresses, for a special case where the delamination tips are mid-way between neighbouring ply cracks. BEM results show a slight dependence of the energy release rate on ply crack separation, and predictions for the maximum ply crack separation considered are within 1.6% of the exact limiting analytical predictions valid for infinite separation. This agreement indicates the validity of both the analytical methods used, and the numerical analysis based on the BEM. [Copyright &y& Elsevier]

Abstract: During the experimental characterization of the mode I interlaminar fracture toughness of multidirectional composite laminates, the crack tends to migrate from the propagation plane (crack jumping) invalidating the tests. In an earlier numerical study , we reported that this problem could be eliminated by choosing the appropriate bending stiffness of the beam arms. In the current paper, six stacking sequences were defined numerically and checked experimentally to validate the methodology presented in the numerical study. The results showed that crack jumping could be avoided by increasing the stiffness of the crack arms. Micrographies of the tested specimens showed that the delamination was not a perfect interlaminar fracture, as is usually considered. Instead, the propagation of delamination occurred with fiber tearing. The toughness values showed a dependency upon both the amount of fiber bridging and the interface angles. [Copyright &y& Elsevier]

Abstract: In this paper, an application of the Extended Finite Element Method (XFEM) for simulation of delamination in fibre metal laminates is presented. The study consider a double cantilever beam made of fibre metal laminate in which crack opening in mode I and crack propagation were studied. Comparison with the solution by standard Finite Element Method (FEM) as well as with experimental tests is provided. To the authors’ knowledge, this is the first time that XFEM is used in the fracture analysis of fibre metal laminates such as GLARE. The results indicated that XFEM could be a promising technique for the failure analysis of composite structures. [Copyright &y& Elsevier]

Abstract: The paper presents a shell element based unit cell approach for numerical homogenization of fiber reinforced textile composites. The modeling strategy is set up within the framework of the Finite Element Method with special emphasis on numerical efficiency, which becomes particularly important in the perspective of non-linear implicit simulations. A single layer of fabric is considered and the tows as well as the unreinforced matrix pockets are discretized by shell elements which are coupled appropriately allowing for delamination between the constituents. The approach is discussed, and the linear as well as the geometrical non-linear mechanical response of an example fabric is compared to a continuum based discretization of the same fabric. Additionally, simulations including delaminations are conducted. The substantially improved numerical efficiency of the shell based approach is shown. [Copyright &y& Elsevier]

Abstract: This paper presents a cohesive zone model based finite element analysis of delamination resistance of z-pin reinforced double cantilever beam (DCB). The main difference between this and existing cohesive zone models is that each z-pin bridging force is governed by a traction–separation law derived from a meso-mechanical model of the pin pullout process, which is independent of the fracture toughness of unreinforced laminate. Therefore, two different traction–separation laws are used: one representing the toughness of unreinforced laminate and the other the enhanced delamination toughness owing to the pin bridging action. This approach can account for the large scale bridging effect and avoid using concentrated pin forces, thus removing the mesh dependency and permitting more accurate analysis solution. Computations were performed using a simplified unit strip model. Predicted delamination growth and load vs. displacement relation are in excellent agreement with the prediction by a complete model, and both models are in good agreement with test measured load vs. displacement relation. For a pinned DCB specimen, the unit strip model can reduce the computing time by 85%. [Copyright &y& Elsevier]

Abstract: The machining of carbon fibre reinforced plastics (CFRPs) is often accompanied by delamination of the top layers of the machined edges. Such damage necessitates time-consuming and costly post-machining and in some cases leads to rejection of components. The work described in this paper systematically investigates the occurrence of delamination of the top layers during the machining of CFRP tape, with the focus being on the process of contour milling. The occurrence and propagation of delamination were studied by milling slots in unidirectional CFRP specimens having different fibre orientations and mainly analysing the slot tip. This allowed the key mechanisms to be clarified. The results show that delamination is highly dependent on the fibre orientation and the tool sharpness. The experiments allow derivation of a novel system for describing the occurrence and propagation of delamination during milling. Furthermore, the principles also apply for drilling. The results allow customisation of the machining procedure to reduce and in some cases totally avoid delamination, leading to a significant increase in the quality of components. [Copyright &y& Elsevier]

Abstract: Results from a series of split Hopkinson pressure bar (SHPB) tests on 3D woven tetxile composites (3DWC) are presented. These tests were done to determine the rate dependent compression response of 3DWC. Three different configurations of the 3DWC, corresponding to compression response in the plane of the material and through-the-thickness direction (out-of-plane) were studied. The rate dependent responses were compared against quasi-static test results and it was found that 3DWC showed an increase in strength in all three directions studied, however, accompanied by a transition in the failure mechanism. The in-plane orientations showed the largest increase in (about 100%) strength at the elevated rates of loading. A follow-on paper provides finite element based results that correspond to the experimental results presented here. [Copyright &y& Elsevier]

Abstract: This study explores the feasibility of using a non-contact guided wave imaging system to detect hidden delamination in multi-layer composites. The study is conducted in two phases. In the first phase, Lamb waves are excited by a lead (Pb) Zirconate Titanate transducer (PZT) mounted on the surface of a composite plate, and the out-of-plane velocity field is measured using a one-dimensional (1D) scanning laser Doppler vibrometer (LDV). From the scanned time signals, wavefield images are constructed and processed to study the interaction of Lamb waves with delamination. The paper presents additional signal and image processing techniques used to highlight the defect in the scanned area. The techniques are demonstrated using experimental data collected from a 1.8mm thick multi-layer composite. In the second phase, a completely non-contact system is described to excite and measure guided waves. A modulated continuous wave (CW) laser source in conjunction with a photodiode is used to wirelessly excite an attached PZT and the resulting waves are again sensed using the vibrometer. The non-contact system is used to excite and measure elastic waves in a composite channel test article. The elastic wave propagation image sequences are created from the non-contact excitation system. [Copyright &y& Elsevier]

Abstract: The experimental study of composite delaminations and adhesive joints is usually carried out using beam-type specimens with bonded metallic fixtures to transfer the applied load. However, bonded joints between the composite specimen and the metallic parts might fail, especially under extreme temperature or fatigue test conditions. Moreover, the quality and alignment of the bonding procedure highly depends on the operator skills and is time consuming. In this paper the design of an improved mechanical hinge is introduced. The new hinge eliminates bonded joints by mechanically clamping the sides of the specimen and avoids the misalignment effects between specimen and fixture. The new system has proven to be easy to use, fast and reliable. [Copyright &y& Elsevier]

Abstract: This paper presents a study of the effects of ply clustering on polymer-based laminated composite plates subjected to a drop-weight impact loading. The tools used to define the impact configurations, as well as the experimental results obtained, are described in detail. These tools are simplified analytical models for the description of the impact behavior and of the damage thresholds that result in a significant reduction on the structure stiffness and strength, caused by delamination. The results obtained demonstrate that the analytical tools are useful to define the impact configurations, to obtain a preliminary understanding of the effects of each parameter that can influence the response, and to interpret the experimental results. It is concluded that ply clustering reduces the damage resistance of the structure. However, the damage tolerance assessed by the compression after impact tests is unaffected by ply clustering. [Copyright &y& Elsevier]

Abstract: This paper investigates the fatigue delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates under mixed-mode I/II conditions at cryogenic temperatures. Fatigue delamination tests were performed with the mixed-mode bending (MMB) test apparatus at room temperature, liquid nitrogen temperature (77K) and liquid helium temperature (4K), in order to obtain the delamination growth rate as a function of the range of the energy release rate, and the dependence of the delamination growth behavior on the temperature and the mixed-mode ratio of mode I and mode II was examined. The energy release rate was evaluated using three-dimensional finite element analysis. The fractographic examinations by scanning electron microscopy (SEM) were also carried out to assess the mixed-mode fatigue delamination growth mechanisms in the woven GFRP laminates at cryogenic temperatures. [Copyright &y& Elsevier]

Abstract: Use of the strain energy release rate for characterizing delamination growth in composite and bonded structures is now commonplace. Analogous to the use of the stress intensity factor range for fatigue crack growth in metals, the strain energy release rate range, defined as the arithmetic difference between maximum and minimum values, is often used to characterize fatigue delamination growth behaviour. The basis for similitude for these two parameters, however, is different and can lead to misinterpretations of delamination growth behaviour if this difference is not understood. This paper examines the basis of similitude for the strain energy release rate range and how it can be redefined in order to avoid potential misinterpretations of fatigue delamination growth. [ABSTRACT FROM AUTHOR]

Abstract: Numerical procedures based on the combined use of the Virtual Crack Closure Technique and of a fail release approach have been widely used to simulate delamination growth phenomena of composite material structures. This paper starts explaining why this kind of methodologies might not be robust due to mesh and load step size dependency and introduces a novel approach able to cope with the problems identified. Finally the effectiveness and robustness of the proposed procedure, implemented into a commercial finite element software by means of user subroutines, are assessed by comparing the obtained numerical results for a delamination growth phenomenon against literature experimental data on a stiffened panel with a circular embedded delamination under compressive load. [Copyright &y& Elsevier]

Abstract: This paper considers damage development mechanisms in cross-ply laminates using an accurate numerical method that assumes a Generalized Plane Strain (GPS) state. A 2D Boundary Element Method (BEM) model is generated to investigate the two types of damage progression in a [0/90]S laminate: transverse cracks in the 90° lamina and delamination between both laminae. The model permits the contact between the surfaces of the cracks. The study is carried out in terms of the dependence of the Energy Release Rates (ERR) of the two types of crack on their respective lengths. A special emphasis is put on the mechanisms of the joining of the two aforementioned types of crack, including the study of the distribution of the stresses along the interface between the two plies when the transverse crack is approaching this interface. [Copyright &y& Elsevier]

Abstract: This paper focuses on the latest development of a solid hexahedron element for composite delamination analysis. The 8-node solid is derived from a 20-node hexahedron. It is transformed into two physical independent 4-node shell elements according to the propagation of delamination process within the element. A global–local finite element approach coupled with the traditional VCCT method allows to calculate the energy release rates and to control the propagation of cracking in the element. According to the position of the crack in the element, one parameter defines the degradation of the transverse properties at the Gauss point as well as the transfer of the volume element towards the bi-plate formulation. Classical experimental tests are carried out on composite structures such DCB (mode I), ELS and ENF (mode II), ADCB (combined mode) and modelled using the developed element. The numerical simulation properly correlates with the experimental results. [Copyright &y& Elsevier]

Abstract: This paper presents the experimental and analytical investigation of the effect of variable amplitude load sequences on delamination behavior in Fibre Metal Laminates . Delamination tests were performed and results are compared with linear damage accumulation predictions. Scanning Electronic Microscopy was used to analyse the delaminated surfaces to study the delamination growth rate under VA loading in more detail. The correlation between test results and predictions highlighted the absence of load sequence and interaction effects in delamination growth rate under VA loading. This correlation is supported by the SEM observations. [Copyright &y& Elsevier]

Abstract: In this paper, a modified adaptive cohesive element is presented. The new elements are developed and implemented in LS-DYNA, as a user defined material subroutine (UMAT), to stabilize the finite element simulations of delamination propagation in composite laminates under transverse loads. In this model, a pre-softening zone is proposed ahead of the existing softening zone. In this pre-softening zone, the initial stiffness and the interface strength are gradually decreased. The onset displacement corresponding to the onset damage is not changed in the proposed model. In addition, the critical energy release rate of the materials is kept constant. Moreover, the constitutive equation of the new cohesive model is developed to be dependent on the opening velocity of the displacement jump. The traction based model includes a cohesive zone viscosity parameter (η) to vary the degree of rate dependence and to adjust the maximum traction. The numerical simulation results of DCB in Mode-I is presented to illustrate the validity of the new model. It is shown that the proposed model brings stable simulations, overcoming the numerical instability and can be widely used in quasi-static, dynamic and impact problems. [Copyright &y& Elsevier]

Abstract: Composites produced by resin infusion techniques will inevitably suffer from variation in resin distribution due to imprecise fibre placement and distortion of the preform during mould closure and infusion. This paper describes an investigation into the effect of variations in fibre volume fraction (FVF) on mode I and mode II delamination behaviour for 5 harness satin (5HS) woven carbon–fibre/epoxy resin composites manufactured by resin transfer moulding (RTM). Additionally, the effect of satin face tow orientation on interlaminar toughness was investigated. In mode I, it was found that toughness increased with increasing FVF and that a strong correlation between fracture surface damage and measured interlaminar fracture toughness was observed. In mode II, measured toughness values were higher than expected and tests were repeated using a mixed-mode rig with 5% mode I. It was found that fracture toughness measurements in pure mode II are significantly affected by friction or mechanical interlocking between the delamination surfaces. [Copyright &y& Elsevier]