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Abstract: In this contribution, the impact dynamic response and failure modes of fibre–metal laminated panels subjected to low velocity impact were investigated and presented. The fibre–metal laminate in this paper comprised of a layer of glass fibre-reinforced plastics sandwiched between two layers of aluminium alloy. Two different types of glass fibre-reinforced plastics were used for the fabrication: unidirectional and woven. A fairly extensive experimental investigation was conducted in conjunction with a detailed finite element analysis. The experiments were conducted using a standard drop-weight test machine and the finite element analysis was carried out using a commercially available finite element software. The results of maximum contact force, contact duration and corresponding failure modes are presented, compared and discussed in this technical paper. [Copyright &y& Elsevier]

Abstract: Detecting impact damage is an important factor in maintaining the structural integrity of aerospace composite structures. Since impacts can cause severe reductions in stiffness and strength of composite structures, there is a need to investigate the material’s stiffness and strength after an impact event. The work discussed in this paper is concerned with one overall goal, which is to determine if an impact has caused damage and then to determine the extent of that damage using only the structural responses that are acquired from low-profile surface-mounted transducers. The work presented in this paper demonstrates that responses recorded from these sensors can provide sufficient information to infer impact damage. Previous work by various authors has established that it is possible to distinguish between damaging and non-damaging impacts if the impact force time-history is available. Therefore, it is the requirement that only response data be used here that distinguishes this work from the previous studies. Following a systematic series of experiments, on the induction of impact damage in Carbon Fibre Reinforced Polymer (CFRP) laminates, a damage model was proposed from this work which can provide preliminary information on the type and extent of damages through observations made from Scanning Electron Microscopy (SEM) and X-ray radiography. The model can be used to provide a general understanding on the prediction of damage and failure progression in CFRP as a function of the number of layers and impact energies. To accommodate the results from SEM and X-ray, discussions on outlier analysis and visualisation, which emphasise the idea of discordancy from the discipline of statistics, are presented. Results from discordancy tests, for both univariate and multivariate data, can be used to clearly separate the data (non-damaging and damaging impacts as well as the separation of the type of failure modes) by a calculated threshold value. Both of these features (univariate and multivariate) can be used as damage indicators and proxies for determining the extent of damage. [Copyright &y& Elsevier]

Abstract: An original, ply-level, computationally efficient, three-dimensional (3D) composite damage model is presented in this paper, which is applicable to predicting the low velocity impact response of unidirectional (UD) PMC laminates. The proposed model is implemented into the Finite Element (FE) code ABAQUS/Explicit for one-integration point solid elements and validated against low velocity impact experimental results. Mesh-regularisation of the impact results is one of the greatest challenges in impact damage modelling of composite targets. Currently published mesh-regularisation strategies can produce mesh-size-independent impact results for a very limited range of mesh-sizes. The originality of the model presented in this paper is based on a novel mesh-regularisation strategy, which is applicable to predicting mesh-size-independent impact results for any target discretisation. Accurate modelling of composites’ impact damage using a relatively coarse mesh, and thus at a much reduced computational cost when compared to existing techniques, can be achieved with the proposed method. [Copyright &y& Elsevier]

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: The subject of this paper is numerical prediction of bird strike induced damage in real aeronautical structures using highly detailed finite element models and modern numerical approaches. Due to the complexity of today’s aeronautical structures, numerical damage prediction methods have to be able to take into account various failure and degradation models of different materials. A continuum damage mechanics approach has been employed to simulate failure initiation and damage evolution in unidirectional composite laminates. Hashin’s failure initiation criteria have been employed in order to be able to distinct between four ply failure modes. The problem of soft body impacts has been tackled by applying the Coupled Eulerian Lagrangian technique, thereby avoiding numerical difficulties associated with extensive mesh distortion. This improvement in impactor deformation modelling resulted in a more realistic behaviour of bird material during impact. Numerical geometrical and material nonlinear transient dynamic analyses have been performed using Abaqus/Explicit. The main focus of the work presented in this paper is the application of the damage prediction procedure in damage assessment of bird impact on a typical large airliner inboard flap structure. Due to the high cost of gas-gun testing of aircraft components, experimental testing on the real flap structure could not have been performed. In order to evaluate the accuracy of the presented method, the bird and composite damage model have been validated against experimental data available in the literature. [Copyright &y& Elsevier]

Abstract: Z-fibre® pinning is a new method of through-thickness reinforcement of laminated composites. This paper presents an experimental test and theoretical analysis on how and why these pins can improve the resistance to impact loading and post-impact compression. Specimens were made of carbon/epoxy T300/914C with nominal thicknesses of 2, 4, and 6mm. For the specimens tested in this study, z-pinning reduced impact damage area by 19–64% depending on the specimen thickness and impact energy. Experimental results obtained also indicate that z-pins can significantly increase the compression-after-impact (CAI) strength by about 45%. In this paper all of these observations are discussed in the context of theoretical and numerical models that have been developed previously to predict the critical impact force and z-pinning performance in terms of the laminate in-plane stiffness and mode I delamination fracture toughness with the z-fibre bridging effect. [Copyright &y& Elsevier]

Abstract: The present paper outlines an unconventional energy based composite damage model capable of modelling woven and unidirectional composite materials. The damage model has been implemented into the LLNL-DYNA3D and LS-DYNA3D finite element codes for shell elements (plane stress), relevant to tensile, compressive and shear damage failure modes. The damage model uses five damage variables assigned to tensile, compressive and shear damage at a laminae level. The evolution of damage in each mode is controlled via a series of damage-strain equations, thus allowing the total energy dissipated for each damage mode to be controlled during a dynamic or impact event. The materials under investigation were a uni-directional (UD) carbon-epoxy and a woven carbon epoxy composite. Recent experimental test results from the CEC HICAS project have also been used to verify the behaviour of the model at elevated strain rates. Experimental data for the 0° and 90° tensile response indicates effectively rate independent, while matrix dominated modes, such as the shear response is highly rate dependent. The 0° and 90° tensile response and the tensile shear response have been modelled at a coupon level, including relevant strain rate effects, with the proposed damage model. Results show very good agreement with the available coupon experimental data. Suggestions are also presented for additional non-standard experimental tests to derive the material model parameters. A follow on paper describes the results of a series of simulations using the proposed material model on a number of experimental plate impact tests performed on the CEC HICAS project. [Copyright &y& Elsevier]

Micro-warpping of sub-scale fibre bundles with thermoplastic fibres has been explored for improved intra-tow hybridisation in comparison to traditional commingling process. This paper investigates matrix toughening of thermoset composites with thermoplastic fibres delivered through textile preforming and hence does not adversely effect resin viscosity during the infusion process. Tow-scale thermoplastic fibres appear to diffuse cracks from inter-laminar to intra-tow due to relatively weak interface, and at the same time protect the reinforcing fibres by acting as elastic foundations during impact loading. In this paper, two hybridisation methods, traditional commingling and novel micro-wrapping, have been compared for assessing the impact damage performance of thermoset composite laminates. It appears that micro-wrapped laminates exhibit better impact and damage tolerance performance than the commingled composite laminates due to the smaller impact induced damage areas and better PP fibre distribution. Improved thermoplastic fibre distribution in micro-wrapped tow architechture has been shown to improve impact damage tolerance through mechanisms of fibre cushioning and intra-tow crack dispersion. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: In this paper, an experimental study is performed to characterise the behaviour of fabric-reinforced composites used in sports products under large-deflection bending in Izod-type impact tests. X-ray micro computed tomography (micro-CT) is used to investigate various damage modes in the impacted CFRP specimens. It revealed that matrix cracking, delaminations, tow debonding, and fibre fracture were the prominent damage modes. Three-dimensional finite-element models are developed to study the onset, progression and interaction of some damage modes such as delamination and fabric fracture observed with micro-CT. A damage modelling technique based on a cohesive-zone method, which is more efficient than continuum damage mechanics approach, is proposed for analysis of interaction of damage modes. The developed numerical models are capable to simulate the damage mechanisms and their interaction observed in the tests. In this study, the pattern of damage formation observed in specimens was front-to-back, unlike bottom-to-top one in drop weight impact tests. The effect of boundary conditions on the dynamic response and damage evolution of composite laminates is also investigated. [Copyright &y& Elsevier]

Abstract: This paper presents a partial volume correction technique that applies a measurement weighting based on grey scale intensity values, allowing crack opening displacements (CODs) to be better estimated in micro-focus computed tomography (μCT) scans. These were tested on 3D data obtained from two separate μCT scanners on particle toughened and non-particle toughened carbon fibre material subjected to low velocity impact. Direct comparisons of COD estimations were made with higher resolution measurements obtained using synchrotron radiation computed tomography (SRCT) scans taken at the European Synchrotron Radiation Facility (ESRF). In this study, partial volume correction is reported to improve the accuracy of these measurements to within 20% of SRCT measurements, whereas measurements based on counting interconnected voxels representing a detectable crack are reported to consistently overestimate crack openings by up to 500%. Scatter in estimations was dependent on material type, noise, and artefacts associated with μCT volumes. [Copyright &y& Elsevier]

Abstract: Fibre reinforced composites have excellent specific properties and are widely sought after by engineers seeking to reduce mass. However, end of life disposal is a significant problem and so research into more sustainable natural fibre composites is extremely topical. This paper examines the applicability of natural fibre composites for high performance structural applications. Woven flax and regenerated cellulose (Cordenka) textiles were pre-impregnated with commercially available epoxy resins and consolidated into test laminates in an autoclave to determine their static (compressive, tensile, flexural) and dynamic (energy absorption) properties. The range of compressive strengths was 77.5–299.6MPa. Tensile strengths ranged from 63 to 92.6MPa and interlaminar shear strength (ILSS) from 10.7 to 23.3MPa. Specific energy absorption (SEA) varied between 21.2–34.2kJ/kg. Biotex flax combined with MTM49 resin matched the SEA of T300 carbon fibre using the same resin system and layup. This work has demonstrated that natural fibre composites have significant scope for use in structural applications but additional work is required on fibre to matrix bonding in order to maximise their properties whilst remaining an environmentally credible option. [Copyright &y& Elsevier]

Abstract: This paper describes the results of a combined experimental/numerical study to investigate the perforation resistance of sandwich structures. The impact response of plain foam samples and their associated sandwich panels was characterised by determining the energy required to perforate the panels. The dynamic response of the panels was predicted using the finite element analysis package ABAQUS/Explicit. The experimental arrangement, as well as the FE model were also used to investigate, for the first time, the effect of oblique loading on sandwich structures and also to study the impact response of sandwich panels on an aqueous support. Testing has shown that the perforation resistance of the plain foams and their sandwich panels is strongly dependent on the properties of the foam core. For example, increasing the density of the crosslinked PVC foams from 60 to 200kg/m3 yielded an eight fold increase in the perforation resistance of the sandwich panels. At intermediate and higher densities, the crosslinked PVC foams and their associated sandwich structures offered a superior perforation resistance to their linear PVC counterparts. The FE analysis reasonably predicted the impact load–displacement responses and the perforation energies of both the plain foams and the sandwich panels. Finally, it has been shown that sandwich panels impacted in an aqueous environment offer a lower perforation resistance than those tested in air. [Copyright &y& Elsevier]

Abstract: This paper compares the mechanical properties of virgin and recycled woven carbon fibre prepreg and goes on to assess the potential for recycled carbon fibre reinforced plastic (rCFRP) to be used in high performance energy absorption structures. Three sets of material were examined: fresh containing virgin fibres and resin, aged which was an out of life but otherwise identical roll and recycled which contained recycled fibre and new resin. The compressive strength and modulus of rCFRP were approximately 94% of the values for fresh material. This correlated directly with the results from impact testing where rCFRP conical impact structures were found to have a specific energy absorption of 32.7kJ/kg versus 34.8kJ/kg for fresh material. The tensile and flexural strength of rCFRP were 65% of the value for fresh material. Tensile and flexural moduli of rCFRP were within 90% of fresh material and ILSS of rCFRP was 75% that of fresh. Overall rCFRP has been proved to remain a highly satisfactory engineering material. This is a significant finding as it proves that carbon fibre can be recycled and reused in high performance applications. This has significant implications for use of carbon fibre in the automotive industry where End of Life Vehicle (ELV) legislation requires 85% of materials to be recyclable. This work opens the door to significantly expanded use of carbon fibre in the automotive industry. Of equal significance is the finding that the mechanical properties of fresh and aged carbon fibre were all within ±6% of one another. This highlights an opportunity to understand resin system life in more detail in order to prevent the creation of composite waste at source. [Copyright &y& Elsevier]

Abstract: In this paper, the damage failure and behaviour of stitched composites under compression after impact (CAI) loading are experimentally investigated. This study focuses on the effect of stitch density and stitch thread thickness on the CAI strength and response of laminated composites reinforced by through-thickness stitching. Experimental findings show that stitched composites have higher CAI failure load and displacement, which corresponds to higher energy absorption during CAI damage, mainly attributed to greater energy consumption by stitch fibre rupture. The coupling relationships between CAI strength, impact energy, stitch density and stitch thread thickness are also revealed. It is understood that the effectiveness of stitching has high dependency on the applied impact energy. At low impact energy range, CAI strength is found to be solely dependent on stitch density, showing no influence of stitch thread thickness. It is however observed that stitch fibre bridging is rendered ineffective in moderately stitched laminates during compressive failure, as local buckling occurs between stitch threads, resulting in unstitched and moderately stitched laminates have similar CAI strength. The CAI strength of densely stitched laminates is much higher due to effective stitch fibre bridging and numerous stitch thread breakages. At high impact energy level, CAI strength is discovered to be intimately related to both stitch density and stitch thread thickness. Since CAI failure initiates from impact-induced delamination area, stitch fibre bridging is considerable for all specimens due to the relatively large delamination area present. Stitch threads effectively bridge the delaminated area, inhibit local buckling and suppress delamination propagation, thus leading to increased CAI strength for laminates stitched with higher stitch density and larger stitch thread thickness. Fracture mechanisms and crack bridging phenomenon, elucidated by X-ray radiography are also presented and discussed. This study reveals novel understanding on the effectiveness of stitch parameters for improving impact tolerance of stitched composites. [Copyright &y& Elsevier]

Abstract: This paper explores the efficacy of polyethylene-co-ethacrylic acid (EMAA), as a thermally activated thermoplastic healing agent embedded within a carbon fibre reinforced epoxy composite. EMAA fibres have been shown to effectively restore mode I properties in a fibre reinforced composite after thermal activation yet other forms of the healing agent or modes of deformation have so far not been studied at all. This work, uses EMAA in the form of a non-woven mesh, rather than a woven fabric to study the healing mechanism and effectiveness of property restoration for mode I (crack opening) and mode II (shear) failure as well as for high speed impact. Property restoration after mode I damage was found to be over 200% and increased with increasing EMAA concentration. For mode II shear failure, the property restoration was reduced to a little over 100% regardless of EMAA concentration. Mode II analysis also showed that the modulus could be restored to about 80% of its original value when modified with EMAA. Repeated impacting using a falling weight test produced no property restoration after healing, yet the modified laminates appeared protected from further damage compared with an unmodified laminate. This was attributed to the formation of a ductile thermoplastic layer mitigating further damage. Scanning electron microscopy revealed that regardless of the extent of healing, the form of the healing agent or the mode of damage, the unique pressure delivery mechanism previously identified, was always observed to occur. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental study of low energy impacts on composite plates covered with a protective layer. In service, composite materials are subjected to low energy impacts. Such impacts can generate damage in the material that results in significant reduction in material strength. In order to reduce the damage severity, one solution is to add a mechanical protection on composite structures. The protection layer is made up of a low density energy absorbent material (hollow spheres) of a certain thickness and a thin layer of composite laminate (Kevlar). Energy absorption ability of these protective layers can be deduced from the load/displacement impact curves. First, two configurations of protection are tested on an aluminium plate in order to identify their performance against impact, then the same are tested on composite plates. Test results from force–displacement curves and C-scan control are compared and discussed and finally a comparison of impact on composite plates with and without protection is made for different configurations. [Copyright &y& Elsevier]

Abstract: In the given research paper, the effects of reinforcing polylactid (PLA) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) biopolymers on the mechanical performance were studied. Both PLA and PHBV were compounded with man-made cellulose, jute and abaca fibres. The test bar specimens were processed via injection moulding. Various testing methods, including tensile and impact tests, were used to investigate the composites’ mechanical performance. Scanning electron microscopy was carried out to study the fibre–matrix interphasial adhesion. To determine the fibre-size distribution, optical microscopy was used. Finally, the obtained results were compared to composites on PP basis with the same reinforcing fibres. The reinforcing with fibres increased the tensile stiffness and strength significantly; however, depending on the fibre type, different improvements of the mechanical parameters were achieved. The main enhancement was realised in impact and tensile strength by reinforcing biopolymers with man-made cellulose fibres. SEM photographs show a largely differing fibre/matrix bonding for PLA and PHBV compared to PP composites. The broadest fibre-size distribution can be found in abaca composites. [ABSTRACT FROM AUTHOR]

Abstract: In the paper, data of repeated impact tests performed on seven laminates of different lay-up and thickness are used to illustrate how the damage index (DI), a damage variable recently introduced by the authors to monitor the range of the penetration process in thick laminates, can be applied in case of repeated impact tests to get information on the rate of initial steady damage accumulation as well as on the onset of severe damage modes. Curves of the rate of growth of the DI in the steady phase (ΔDI/ΔN) vs. the normalised impact energy (ratio of the impact energy E i and the laminate penetration threshold P n) show no significant damage accumulation besides initial specimen indentation for impact energies below 0.2P n. For intermediate levels of impact energy, repeated impact tests are characterised by an initial region of steady damage accumulation followed by an abrupt change in the rate of damage growth a few impacts before laminate perforation. For higher impact energies (E i/P n >0.4–0.45), no phase of steady damage accumulation is present, suggesting that severe damage mechanisms take place from the very first impacts. Values of the DI at the end of the steady phase (DIunsteady) are shown to be rather peculiar to each laminate regardless of the impact energy used in the tests and therefore may be used to get a first indication of the laminate performance to repeated impacts. The extent of the steady phase may also be used to compute the total energy absorbed by the laminate in the steady damage accumulation phase (E aTOT_steady). [Copyright &y& Elsevier]

Abstract: The paper investigates the capability of a finite element model based on cohesive interface elements to simulate complex three-dimensional damage patterns in composite laminates subjected to low-velocity impact. The impact response and the damage process of cross-ply laminated plates with grouped ([03/903]s and [903/03]s) and interspersed ([0/90]3s) ply stacking was simulated using a FE model developed by the authors in a previous study and the numerical results were compared to experimental observations. The model provided a correct simulation of size, shape and location of the principal fracture modes occurring in impacted [03/903]s and [903/03]s laminates. In [0/90]3s laminates, characterized by a complex spatial damage distribution, the model was able to predict the approximately circular shape of the overall projected damage area and to capture the typical shape features of individual delaminations; significant discrepancies between experiments and predictions were however observed in terms of delamination sizes at single interfaces. Further investigations are needed to clarify the main reasons of these discrepancies. [Copyright &y& Elsevier]

Abstract: In the current paper, a series of high velocity impact tests using ϕ50 and ϕ25mm ice spheres and 0.32g granite stones on non-crimp fabric (NCF) composite plates are reported. The impact tests were performed using an air gun and velocities between 100m/s and 199m/s. The impact events were monitored using a high-speed camera, with a 20 million frames per second capacity, as well as by a displacement transducer for out-of-plane displacement measurements of the impacted plates. NCF composite plates of two different thicknesses were impacted. The composites were manufactured from carbon fibre and epoxy resin by vacuum infusion. Engineering type models were employed to predict impact response and impact damage formation. Comparison between predicted and resulting damage for the impact test validates the application of a semi-empirical model for predicting impact velocity thresholds for damage formation. Analytical models relying on the assumption of solid impact bodies cannot be employed for these types of impact. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental investigation concerning low-velocity impact and quasi-static indentation tests on highly oriented laminates used in aeronautical and aerospace applications. The damage observed in such laminates is very particular. Post mortem analysis were carried out which helped to define an impact damage scenario. Microscopic observations led to explain the mechanism of permanent indentation formation which is a fundamental point of damage tolerance justification. Equivalence between static and dynamic is also discussed. [Copyright &y& Elsevier]

Abstract: This paper reports on experimental and numerical investigations into the response of flexible sandwich-type panels when subjected to blast loading. The response of sandwich-type panels with steel plates and polystyrene cores are compared to panels with steel face plates and aluminium honeycomb cores. Panels are loaded by detonating plastic explosive discs in close proximity to the front face of the panel. The numerical model is used to explain the stress attenuation and enhancement of the panels with different cores when subjected to blast induced dynamic loading. The permanent deflection of the back plate is determined by the velocity attenuation properties (and hence the transmitted stress pulse) of the core. Core efficiency in terms of energy absorption is an important factor for thicker cores. For panels of comparable mass, those with aluminium honeycomb cores perform “better” than those with polystyrene cores. [Copyright &y& Elsevier]

Abstract: In this paper, the correlation between interlaminar fracture toughness and impact energy absorption for the fracture of epoxy–carbon laminates was studied. Carbon fibres–epoxy cross-ply prepreg layers were interleaved with thin (26μm) poly(ethylene-terephthalate) (PET) films. Before the composite preparation, circular holes 1mm in diameter were drilled in the PET films at several densities (from 0 up to 44holes/cm2) in order to selectively increase the interlaminar contact area between the epoxy–carbon laminae. In this way, the interlaminar contact area was gradually varied from 0%, corresponding to the case in which non-perforated PET films were used, up to 100% in the case of non-interleaved laminates. The Mode I interlaminar fracture toughness of the resulting laminates was determined according to the ASTM D-5528-01 standard test method on double cantilever beam (DCB) specimens. The critical values of the strain energy release rate determined at the point at which the load versus opening displacement curve becomes non-linear (G IC,NL) resulted to vary from 40 up to 260J/m2, depending on the interlaminar contact area. All the laminates were then characterized by three point bending tests performed both under quasi-static (5mm/min) and impact (2m/s) loading conditions. The elastic modulus of the laminates resulted to be practically independent of the level of interlaminar adhesion, while the bending strength decreased as the interlaminar fracture toughness decreased. The total energy to fracture evaluated under impact conditions showed a non-monotonic correlation with the interlaminar fracture toughness, reaching a maximum level in correspondence of a G IC,NL value of about 60J/m2. At the same time, the ductility index, i.e. the ratio between the propagation and the initiation energies, evaluated by instrumented Charpy impact tests, markedly increased as the interlaminar fracture toughness decreased. [Copyright &y& Elsevier]

Abstract: A damage variable (damage degree DD) representing the ratio between the absorbed energy and the impact energy was introduced in 1998 by Belingardi and Vadori to assess damage accumulation caused by low velocity impacts. More recently, repeated impact tests carried out on thick laminates pointed out the significance and extent of the penetration process in thick laminates, which the DD is unable to monitor as, by definition, it reaches the value of one at penetration to remain unchanged over the entire penetration process. In the present paper, a new damage variable (damage index DI) is proposed to overcome shortcomings of the DD with respect to thick laminates. By introducing the displacement of the impactor into the definition of the DI, the depth of the penetration process is taken into account. Normalization by the displacement of the quasi-static perforation test allows for a non-dimensional damage variable which is shown to reach the value of one at perforation. Validity of the approach is proven against impact data obtained for different fibre-matrix architectures and laminate thickness. Results show that the DI can effectively differentiate between penetration and perforation thresholds, increasing monotonically within the range of the penetration process. In particular, the DI is shown to increase linearly with the impact energy up to penetration and to rise quite abruptly within the range of the penetration process. Results for repeated impact tests prove that at first the DI increases quite linearly impact after impact, owing to a steady accumulation of damage. A few impacts before penetration a sudden rise in the DI value points out a change in the rate of damage accumulation. [Copyright &y& Elsevier]

Abstract: The paper investigates the potential of cohesive interface elements for damage prediction in laminates subjected to low-velocity impact. FE models with interface elements adopting a bilinear cohesive law were first calibrated and validated by simulation of standard fracture toughness tests and then employed to model the impact response of cross-ply graphite/epoxy laminated plates. The developed model provided a correct simulation of the impact response of laminates in a wide range of energy values and successfully predicted size, shape and location of main damage mechanisms. The results of the analyses also pointed out the importance of employing a damage criterion capable of accounting for the constraining effect of out-of-plane compression on the initiation of the decohesion phase. [Copyright &y& Elsevier]

Abstract: After careful tailoring, composite structures can provide a reasonable well response to impact loads with the additional advantages of weight savings and structural stiffness. In order to efficiently design composite structures for impact loads, reliable numerical models are required and sufficiently accurate material codes are necessary. This paper deals with the experimental investigation of carbon reinforced composites under low-velocity impact and its numerical modelling with an orthotropic continuous damage-based material approach available in LSDYNA-3D. Experimental investigations in a Charpy test rig were conducted in order to identify key parameters influencing the impact damage resistance of composite structures. Using the experimental results, a numerical study of the impacted specimens was performed in LSDYNA-3D. Solid elements in conjunction with a damage-based composite material model were used to perform the calculations. A good correlation between experimentally and numerically obtained forces and failure modes has been achieved. Furthermore, the practical numerical modelling of composite materials under low-velocity impact, together with recommendations and achievements towards the effort to model their complex behaviour under high dynamic impact is discussed. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to present an experimental study of impact and compression after impact (CAI) tests performed on composite laminate covered with a cork thermal shield (TS) intended for launchers fairing. Drop weight impact tests have been performed on composite laminate sheets with and without TS in order to study its effect on the impact damage. The results show the TS is a good mechanical protection towards impact as well as a good impact revealing material. Nevertheless, totally different damage morphology is obtained during the impact test with or without TS, and in particular at high impact energy, the delaminated area is larger with TS. Afterwards, CAI tests have been performed in order to evaluate the TS effect on the residual strength. The TS appears to increase the residual strength for a same impact energy, but at the same time, it presents a decrease in residual strength before observing delamination. In fact, during the impact tests with TS, invisible fibres’ breakages appear before delamination damage contrary to the impacts on the unshielded sheets. [Copyright &y& Elsevier]

Abstract: The influence of the loading rate on the material behaviour of glass fibre reinforced phenolic composites and phenolic resin-impregnated aramid paper (Nomex®) honeycomb structures was investigated experimentally. The composite specimens were made of woven fabric plies and loaded in tension and shear. Two types of Nomex® honeycomb specimens (hexagonal and over-expanded) were loaded in uniaxial compression in all three material directions. Quasi-static test results were compared to dynamic test data obtained on a drop tower, where different strain rates from 10s−1 to 300s−1 were tested. The glass/phenolic composite material showed a remarkable strain rate effect at higher loading rates with over 80% increase in tensile strength. Also for the Nomex® honeycomb an increase of the stress level of up to 30% was observed. These material characteristics should be taken into account in case of dynamic analysis (e.g. crash, impact) of honeycomb sandwich panels for public transport applications, which are usually made from these phenolic materials. [Copyright &y& Elsevier]

Abstract: Highly oriented polypropylene (PP) tapes, with high tensile strength and stiffness achieved by molecular orientation during solid state drawing are consolidated to create high performance recyclable “all-polypropylene” (all-PP) composites. These composites possess a large temperature processing window (>30°C) and a high volume fraction of highly oriented PP (>90%). This large processing window is achieved by using co-extruded, highly drawn PP tapes. This paper investigates the impact resistance of these all-PP composites, and the relationship between penetrative and non-penetrative impact behaviour, and composite consolidation conditions. The response of all-PP composites to falling weight impact is reported together with a comparison to conventional commercial glass reinforced polypropylene composites. A model for energy absorption is proposed by comparison with previous studies based on interfacial and tensile failure of tapes and composites. [Copyright &y& Elsevier]

Abstract: In this paper, the feasibility of using optical methods to detect impact damage in thin laminates is investigated and discussed. Specimens impacted with different energy levels were analysed by a holographic procedure and by an electronic speckle pattern interferometry technique; the results obtained with the two techniques were compared with those acquired by a pulse-echo full-volume ultrasonic technique. The investigation showed that both optical methods were able to identify the presence of impact damage, with an efficiency dependent on the through-thickness location of the delaminations produced by impact. The adoption of the ESPI technique allowed significant reductions in inspection times, but quantitative estimates of impact damage were drastically impaired by the high level of speckle noise typical of the technique. [Copyright &y& Elsevier]

The nature of the failure of composite materials subjected to impact loading makes it a very difficult process to model. For example, in the central impact loading of a composite laminated multi-layer panel, a marked difference is evident between the results obtained from experiments and from a FE explicit code simulation. The observed difference is probably related to matrix fracture, creating numerous irregularities in the fragmented zone which is held together by reinforcing fibres. The compacted shattered zone, as well as separating from each other, also penetrates themselves with likely consequences of strengthening the panel. The apparent anomaly of the failed zone being associated with stiffness enhancement is addressed in this paper. The problem seems to arise from the fact that failure criteria incorporated in some FE codes employ a failure indicator whose value is not restricted to disengage the failed zone from the model. A modelling procedure is developed which incorporates a combination of elasto-plastic sink, non-linear spring and dashpot elements to reduce the difference between the results obtained from experimental and numerical methods. The load-displacement and the load-velocity characteristics of the spring and dashpot elements need to be obtained and this is achieved by a simple graphical procedure, which is discussed in detail. The process is a rapid convergence iterative procedure, and in most cases only needs to be repeated twice. [Copyright &y& Elsevier]

In the quest to improve the compression after impact (CAI) strength of thin laminates, ply-hybrid laminates (where plies of different thicknesses are mixed) have been used in a previous study to mitigate the fibre failure and, consequently, improve the CAI strength. In the same study, hybrid laminates were proposed following qualitative design rules. In this paper, we systematically look for hybrid stacking sequences with improved damage tolerance by virtually testing all the laminates in a defined design space. While the laminates in the design space are made of intermediate and thick ply grades, the baseline laminate has only intermediate grade plies. Using an in-house numerical model, we virtually tested, (impact and CAI at two impact energies), all the candidate stacking sequences. The best hybrid laminates considerably improved the CAI strength over the baseline (31% and 40% improvement for the symmetric and unsymmetrical hybrid laminates, respectively). One of the best hybrid laminates was then manufactured and tested experimentally to validate the approach. Through virtual testing, this study demonstrates the benefits of using ply thickness hybrid laminates and the feasibility of optimizing the stacking sequence for impact damage tolerance. [ABSTRACT FROM AUTHOR]