Compressive strength of composite laminates following free edge impact

Barely Visible Impact Damage (BVID) can occur when laminated composite material is subject to free edge impact loads in the plane of the laminate and can result in a significant reduction in compressive strength caused by buckle-driven delamination. This paper will report on a semi-analytical fracture mechanics model that predicts the Compression After Impact (CAI) strength of composite laminates subject to in-plane free edge impact. Compression testing has been undertaken on three impacted coupons in order to validate the theoretical results. Analytical results are shown to be on average within 10% of experimental values for the strain at which propagation of damage occurs.     

Damage resistance and damage tolerance of hybrid carbon-glass laminates

The influence of impact energy and stacking sequence on the damage resistance and Compression After Impact (CAI) strength of Carbon and Glass Fibre Reinforced Plastic (CFRP and GFRP respectively) hybrid laminates is investigated. CAI tests demonstrate that, in comparison to fully CFRP laminates, hybrid laminates show increases in structural efficiency of up to 51% for laminates subject to a 12J impact and 41% for those subject to an 18J impact. Laminates displaying the highest stresses at failure are those that exploit stacking sequences and GFRP content to prevent delaminations from forming close to the outer surface of the laminate during impact. This favourable damage morphology inhibits both sublaminate-buckling-driven delamination propagation and anti-symmetric laminate buckling failures.     

低速冲击下复合材料层板压缩许用值

为评估航空结构中常用的T300级和T800级2种碳纤维/环氧树脂复合材料层压板的冲击后压缩许用值,对2种材料体系下具有不同厚度及铺层的层板进行了低速冲击和冲击后压缩试验;讨论了冲击能量、凹坑深度、损伤面积及冲击后剩余压缩强度等之间的关系,以及厚度、铺层、表面防护等因素对其造成的影响;重点关注了2种材料体系下各组层板的目视勉强可见冲击损伤(BVID)形成条件以及含BVID层板的剩余强度.结果表明:厚度及铺层对层板的凹坑深度-冲击能量关系影响较大,而对冲击后压缩强度-凹坑深度及冲击后压缩破坏应变-凹坑深度关系影响较小,且在相同铺层比例下,BVID对应的冲击能量随厚度近似呈线性增长.X850层板的损伤阻抗性能明显优于CCF300/5228层板的,但二者损伤容限性能相当.加铜网、涂漆等表面处理显著提高了层板的损伤阻抗,但对损伤容限性能影响不大;在损伤不超过BVID时,所有CCF300/5228试件的压缩破坏应变均大于4 000 με,而X850材料体系下压缩破坏应变均在3 000 με之上.     

Compressive static strength model for impact damaged laminates

A simple analytical model for the prediction of the compressive strength of composite structures with Barely Visible Impact Damage (BVID) subject to static loading is presented. The model represents the complex damage morphology using circular approximations of the damage area and determines a critical interface for propagation of BVID. Results are compared with experimental values for static strength of a variety of examples reported in the literature. For impacts on the skin under a stiffener the model is accurate to within 5% of the reported experimental result. It is demonstrated how the model can be manipulated for use in laminate optimisation for improved damage tolerance.     

Failure mechanisms on composite specimens subjected to compression after impact

Composite panels are widely used in aeronautic and aerospace structures due to their high strength/weight ratio. The stiffness and the strength in the thickness direction of laminated composite panels is poor since no fibres are present in that direction and out-of-plane impact loading is considered potentially dangerous, mainly because the damage may be left undetected. Impact loading in composite panels leads to damage with matrix cracking, inter-laminar failure and eventually fibre breakage for higher impact energies. Even when no visible impact damage is observed at the surface on the point of impact, matrix cracking and inter-laminar failure can occur, and the carrying load of the composite laminates is considerably reduced. The greatest reduction in loading is observed in compression due to laminae buckling in the delaminated areas. The objective of this study is to determine the mechanisms of the damage growth of impacted composite laminates when subjected to compression after impact loading. For this purpose a series of impact and compression after impact tests were carried out on composite laminates made of carbon fibre reinforced epoxy resin matrix. An instrumented drop-weight-testing machine and modified compression after impact testing equipment were used together with a C-scan ultrasonic device for the damage identification. Four stacking sequences of two different epoxy resins in carbon fibres representative of four different elastic behaviours and with a different number of interfaces were used. Results showed that the delaminated area due to impact loading depends on the number of interfaces between plies. Two buckling failure mechanisms were identified during compression after impact, which are influenced more by the delamination area than by the stacking sequence.     

Advanced TTT composite materials for aeronautical purposes: Compression after impact (CAI) behaviour

The purpose of this paper is to examine the behaviour of the dry stitched composites when they are subjected to low velocity impact loads. Such composites have been laminated in combination with the Resin Film Infusion (RFI) technique.The experimental results exposed in this document are obtained from the Compression before Impact (CBI) and Compression after Impact (CAI) test efforts.The CAI behavioural determination of such composites is very important in terms of residual strength status after impact because in this case their use in primary aeronautical structures has also to satisfy the stringent Certification Requirements for Airworthiness.Traditional stitching methodologies, using Kevlar 29 threads as a 3-directional reinforcement, are compared to other advanced techniques such as the “tufting” and z-pins insertion.Following the optimization of the needle pass and the pitching line, normalized compression tests have been performed on a number of specimens which have been obtained through the utilization of different fabrication techniques in both before and after impact conditions. This has been done so as to evaluate the strength decay due to the impact damage. The subjec tests¹ were performed at the Alenia Aeronautica laboratories and in accordance with the AECMA ST Std prEN 6038 standards. Finally a review and a discussion of the experimental results conclude the paper.     

Impact damage with compressive preload and post-impact compression of carbon composite plates

This paper examines the effect on laminated composites of in-plane compression followed by impact damage, and the coupling between the two, on compression-after-impact (CAI) performance. It is found that preload can actually raise the CAI strength if the load approaches the initial buckling value, since the plate loses stiffness and the impact-induced force is reduced and so is the consequent damage. However, as the preload approaches the CAI strength the induced delamination can propagate catastrophically during the impact, but at a preload value below the CAI strength. The coupling between impact and preload is simulated using the equations of motion, and the same dynamic solver is then used to capture the snap-through when a plate is loaded beyond its initial buckling load. A special ‘dynamic relaxation’ routine is shown to be robust and reliable when handling these ‘snap-through’ problems which can be formidable challenges to conventional static incremental loading.     

The damage tolerance of GRP laminates under biaxial prestress

The damage tolerance of E-Glass reinforced/polyester laminated plates subjected to a low velocity impact while under an in-plane prestress is investigated. Prior to test, the plates are subjected to either uniaxial or biaxial loading by means of a specially designed test rig which enables independent tension/tension, tension/compression or compression/ compression testing in all stress/strain quadrants. Impact tests are carried out for a single strike energy for a comprehensive range of pre-strains. Absorbed energy, damage area, peak impact loads and the maximum permanent indentation depth are assessed in characterising the resulting damage.

It is shown that the shape, orientation and size of the damage zone is strongly influenced by the nature and magnitude of the pre-strain. Impact specimens subject to shear loading give rise to the largest increase in damage area when compared to unstressed plates.     

Enhancement of mechanical performance of epoxy/carbon fiber laminate composites using single-walled carbon nanotubes

Carbon nanotubes (CNT) in their various forms have great potential for use in the development of multifunctional multiscale laminated composites due to their unique geometry and properties. Recent advancements in the development of CNT hierarchical composites have mostly focused on multi-walled carbon nanotubes (MWCNT). In this work, single-walled carbon nanotubes (SWCNT) were used to develop nano-modified carbon fiber/epoxy laminates. A functionalization technique based on reduced SWCNT was employed to improve dispersion and epoxy resin-nanotube interaction. A commercial prepregging unit was then used to impregnate unidirectional carbon fiber tape with a modified epoxy system containing 0.1 wt% functionalized SWCNT. Impact and compression-after-impact (CAI) tests, Mode I interlaminar fracture toughness and Mode II interlaminar fracture toughness tests were performed on laminates with and without SWCNT. It was found that incorporation of 0.1 wt% of SWCNT resulted in a 5% reduction of the area of impact damage, a 3.5% increase in CAI strength, a 13% increase in Mode I fracture toughness, and 28% increase in Mode II interlaminar fracture toughness. A comparison between the results of this work and literature results on MWCNT-modified laminated composites suggests that SWCNT, at similar loadings, are more effective in enhancing the mechanical performance of traditional laminated composites.     

Compressive behaviour of thin-skin stiffened composite panels with a stress raiser

The in-plane compressive behaviour of thin-skin stiffened composite panels with a stress concentrator in the form of an open hole or low velocity impact damage is examined analytically. Drop weight impact in laminated polymer composites causes matrix cracking, delaminations and fibre breakage, which together can seriously degrade the laminate compressive strength. Experimental studies, using ultrasonic C-scan images and X-ray shadow radiography, indicated that the overall damage resembles a hole. Under uniaxial compression loading, 0° fibre microbuckling surrounded by delamination grows laterally (like a crack) from the impact site as the applied load is increased. These local buckled regions continued to propagate, first in discrete increments and then rapidly at failure load. The damage pattern is very similar to that observed in laminated plates with open holes loaded in compression. Because of this resemblance, a fracture mechanics model, developed initially to predict notched compressive strength, was applied to estimate the compression-after-impact (CAI) strength of a stiffened panel; in the analysis the impact damage is replaced with an equivalent open hole. Also, the maximum stress failure criterion is employed to estimate the residual compressive strength of the panel. The unnotched compressive strength of the composite laminate required in the analysis is obtained from a three-dimensional stability theory of deformable bodies. The influence of the stiffener on the compressive strength of the thin-skin panel is examined and included in the analysis. A good agreement between experimental measurements and predicted values for the critical failure load is obtained.     

Failure analysis of CFRP laminates subjected to compression after impact: FE simulation using discrete interface elements

This paper presents a model for the numerical simulation of impact damage, permanent indentation and compression after impact (CAI) in CFRP laminates. The same model is used for the formation of damage developing during both low-velocity/low-energy impact tests and CAI tests. The different impact and CAI elementary damage types are taken into account, i.e. matrix cracking, fiber failure and interface delamination. Experimental tests and model results are compared, and this comparison is used to highlight the laminate failure scenario during residual compression tests. Finally, the impact energy effect on the residual strength is evaluated and compared to experimental results.     

三维五向编织复合材料低速冲击及冲击后压缩性能实验研究

通过实验研究三维五向碳纤维/环氧树脂编织复合材料低速冲击及其冲击后压缩(CAI)性能。测试试件虽然有不同的编织角度,但承受相同的冲击能力。采用冲击后压缩测试表征不同编织结构的冲击后剩余力学性能。结果表明:编织角较大的试件由于其更紧密的空间结构,能承受更高的冲击载荷且冲击损伤区域更小。CAI强度和损伤机理主要取决于编织纤维束的轴向支撑。随着编织角的增加,CAI强度降低,材料的破坏模式也由横向断裂转变为剪切破坏。     

Effect of resin and fibre properties on impact and compression after impact performance of CFRP

The effect of resin and fibre properties on composite impact, compression after impact (CAI) and mode II energy release rate (G_IIC) performance has been studied. Impact events were instrumented to record values of P_c, the critical load for initiation of impact damage. Impact response of the laminates was strongly influenced by the fracture toughness of the resin. In contrast, use of high strength and high stiffness fibres did not improve the resistance to impact. The differences in impact and CAI response of the laminates were largely a consequence of the impact damage created at the damage threshold, P_c, rather than of the differences in delamination growth. As a strong correlation was found between G_IIC values measured by delamination tests, and those calculated from measurements of P_c, it is suggested that instrumented impact testing may be a more convenient way of determining G_IIC in CFRP laminates than delamination tests.     

Impact and post impact behavior of layer fabric composites

In this study, the effect of impact and post impact behavior of E-glass/epoxy composite plates having different layer fabrics were investigated by considering energy profile diagram and the related load–deflection curves. Different impact energies (5 J–60 J)were subjected to the plates consisting of eight layers of plain weave (1D), double (2D) and triple (3D) layer fabrics. The impact tests were continued until complete perforation of layer fabrics. The damage modes and damage processes of layer fabrics under varied impact energies were also discussed. At the end of the impact tests, the damaged samples were mounted into a compression apparatus to determine the Compression After Impact (CAI) strength of layer fabric samples. The results of these impact and post impact tests showed that contact force occurring between the impactor and the composite specimen increased and the CAI strength reduced by increasing the impact energy. The objective of this study was to determine the perforation threshold of E-glass/epoxy composite plates having different layer fabrics as plain weave (1D), double (2D), and triple (3D) layer fabrics.     

Low velocity impact and compression after impact characterization of woven carbon/vinylester at dry and water saturated conditions

The low velocity impact response and compression after impact strength of dry and water saturated plain weave carbon/vinylester composites have been determined. The composites employed T700 carbon fibers and vinylester 510A and 8084 resins. Quasi-static impact tests were conducted on dry C/VE510A and C/VE8084 to estimate the threshold impact force required to initiate damage in the composites. Falling-weight impact tests were conducted on the composites over a range of impact energies from 6.7 to 47 J. Destructive inspection of damaged panels revealed damage in the form of matrix cracks as well as delamination between fiber bundles. The quasi-static estimation of the threshold impact force was in reasonable agreement with that measured in the impact test. To examine structural degradation due to impact loading, impacted panels were tested in compression (CAI). The CAI strength decreased with increasing impact energy. Absorbed moisture caused further reductions of the CAI strength.     

Effect of stitch density and stitch thread thickness on compression after impact strength and response of stitched composites

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.     

Compression fatigue properties of z-pinned quasi-isotropic carbon/epoxy laminate with barely visible impact damage

This paper presents an experimental study into the use of z-pins to improve the compression fatigue properties of quasi-isotropic carbon fibre–epoxy composite containing barely visible impact damage (BVID). The study investigates the effect of increasing volume content of z-pins (up to 4%) on the barely visible impact damage resistance, post-impact compression fatigue properties, and fatigue damage mechanisms of a quasi-isotropic carbon–epoxy material. The study reveals new insights into the impact damage resistance of z-pinned composites. Z-pins induce different responses in the compression fatigue properties of the quasi-isotropic composite following low or high-energy impact loading. Z-pins proved ineffective at increasing the fatigue properties when the quasi-isotropic composite contained low-energy BVID. However, z-pins were effective at improving the fatigue performance of the composite with high-energy BVID, with the post-impact fatigue life and fatigue endurance limit increasing with the pin content. The improvement in fatigue performance is due solely to the increased resistance against high-energy impact damage imposed by the z-pins. It is also found that z-pins do not affect the fatigue mechanism or fatigue damage growth rate of the composite containing BVID.     

Predicting low velocity impact damage and Compression-After-Impact (CAI) behaviour of composite laminates

Low-velocity impact damage can drastically reduce the residual strength of a composite structure even when the damage is barely visible. The ability to computationally predict the extent of damage and compression-after-impact (CAI) strength of a composite structure can potentially lead to the exploration of a larger design space without incurring significant time and cost penalties. A high-fidelity three-dimensional composite damage model, to predict both low-velocity impact damage and CAI strength of composite laminates, has been developed and implemented as a user material subroutine in the commercial finite element package, ABAQUS/Explicit. The intralaminar damage model component accounts for physically-based tensile and compressive failure mechanisms, of the fibres and matrix, when subjected to a three-dimensional stress state. Cohesive behaviour was employed to model the interlaminar failure between plies with a bi-linear traction–separation law for capturing damage onset and subsequent damage evolution. The virtual tests, set up in ABAQUS/Explicit, were executed in three steps, one to capture the impact damage, the second to stabilize the specimen by imposing new boundary conditions required for compression testing, and the third to predict the CAI strength. The observed intralaminar damage features, delamination damage area as well as residual strength are discussed. It is shown that the predicted results for impact damage and CAI strength correlated well with experimental testing without the need of model calibration which is often required with other damage models.     

Impact and compression after impact experimental study of a composite laminate with a cork thermal shield

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.     

Investigation of knitting architecture on the impact behavior of glass/epoxy composites

In this study, the effect of impact and post-impact behavior of E-glass/epoxy composite plates having different knitted fabrics were investigated by considering energy profile diagram and the related load–deflection curves. Different impact energies (5–25 J) were subjected to the plates consisting of eight layers of Plain [P]₈, Milano [M]₈, and Rib [R]₈ knitted fabrics. The impact tests were continued until complete perforation of layer fabrics. The damage modes and damage processes of layer fabrics under varied impact energies were also discussed. At the end of the impact tests, the damaged samples were mounted into a compression apparatus to determine the Compression after Impact (CAI) strength of layer fabric samples. The results of these impact and post-impact tests showed that the maximum contact force was observed in the [R]₈ fabric and the minimum contact force was observed in the [P]₈ fabric and the CAI strength reduced by increasing the impact energy.