Analysis and Compression Testing of Laminates Optimised for Damage Tolerance

Barely Visible Impact Damage (BVID) can occur when laminated composite material is subject to out-of-plane impact loads and can result in a significant reduction in compressive strength. This paper reports on three compression tests of laminates optimised to maximise damage tolerance. Results from these tests were analysed using a semi-analytical, fracture mechanics based method that predicts the strain below which laminated coupons containing BVID subject to axial compression will not fail. A further experiment was conducted on an artificially delaminated coupon in order to validate the modelling methodology. Results from one of the two optimised stacking sequences considered show an increase of over 40% in Compression After Impact (CAI) strength compared with a baseline configuration. Analysis of results has indicated that CAI strength is dependent to a great extent on damage morphology and stability of damage growth, both of which are functions of laminate stacking sequence.     

复合材料加筋板低速冲击损伤及剩余压缩强度试验研究

采用落锤法对复合材料加筋板进行了低速冲击损伤（LVI）试验,根据复合材料加筋板构型,设计了冲击支持支架,研究了支持支架的间距对冲击结果的影响;用相同的冲击能量对复合材料加筋板结构中3处典型位置进行冲击,得到不同位置的损伤形貌;分别对完好件和损伤试验件进行压缩试验,将试验结果进行对比,分析不同位置的冲击损伤对结构压缩性能的影响。试验结果表明：在相同的冲击能量下,支持支架间距越小,所造成的冲击损伤越严重;在50 J冲击能量下,筋条区蒙皮处的冲击所造成的损伤不易观察,筋条间蒙皮处的冲击所造成的损伤最为明显,而筋条边缘蒙皮处的冲击可以导致筋条边缘的脱粘;冲击损伤会使加筋板屈曲载荷轻微下降,筋条间蒙皮和筋条区蒙皮冲击损伤对压缩结果影响相对较小,筋条边缘处的冲击会引起损伤处蒙皮的子层屈曲,并影响结构破坏形式,使结构压缩承载能力有较为明显的下降。     

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.     

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.     

Impact Damage to Composite Laminates: Effect of Impact Location

Accidental impact loading of Composite laminates during manufacture and in-service can occur in different locations including near the edge or on the edge of a composite structure. This paper describes investigation of the effect of impact to composite laminates and compares the damage arising from central, near edge and on edge impact events. The damage tolerance of impact damaged laminates using both compression and tension tests has been measured. These results reveal the different damage mechanisms arising from different locations of impact. These different damage mechanisms have been investigated using X-Ray computed tomography. Impact on the edge of composite laminates is found to lead to smaller damage area, but more fibre failure; the severity of this damage is not revealed in standard compression after impact tests.     

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.     

Impactor diameter effect on low velocity impact response of woven glass epoxy composite plates

In this study, the effect of impactor diameter on the impact response of woven glass–epoxy laminates has been investigated. Impact tests were performed by using Fractovis Plus test machine with four different impactor nose diameters as 12.7, 20.0, 25.4 and 31.8 mm. Specimens were impacted at various impact energies ranging from 5 J to perforation thresholds of the composite at room temperature. Variation of the impact characteristics such as the maximum contact load, maximum deflection, maximum contact time and absorbed energy versus impact energy are investigated. Results indicated that the projectile diameter highly affects the impact and Compression After Impact (CAI) response of composite materials.     

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.     

Optimizing the effect of reinforcement,particle size and aging on impact strength for Al 6061-red mud composite using Taguchi technique

Aluminium 6061-red mud composite has been successfully casted by using stir casting. Fairly uniform distribution of red mud reinforcement has been observed by SEM micrographs. The presence of red mud particles inside the matrix has been confirmed by energy dispersive spectroscopy. Impact testing has been done on impact testing machine with a range of 1–300 J (Charpy), 1–175 (Izod) and least count of 0.5 J. Impact strength shows a decreasing trend with increase in percentage reinforcement, aging time and is increased with increase in particle size. ANOVA analysis found that the effect of particle size and aging time are significant but the effect of percentage reinforcement has not been found significant. From the analysis, it can be observed that the particle size influences the impact strength most followed by aging time and percentage reinforcement. Optimum value of impact strength has been predicted by using Taguchi technique and confirmed experimentally by confirmation experiment results.     

Strength of a Composite Sheet Material in Impact Tension

A procedure and results of studies on the strength of a 2-mm PA6 shock-resistant composite sheet in impact tension are briefly outlined. Experimental investigations include static and impact tensile tests of specimens with a short test portion and sharp symmetrical edge notches. Acoustic emission methods used in the tests of specimens in static tension revealed acoustic signals that point to the development of damages in the material under loads much lower than the ultimate one. Test procedures are similar to those used earlier for testing sheet metals in tension. The wave process in the specimens upon impact tension was analyzed using the viscoelastic model. The influence of viscosity on the width of the wave front, determining stress growth rates upon its propagation, was established. According to the test results, the change from quasistatic to impact tension slightly changes maximum stress levels. Impact tension of notched specimens in the first wave of loading lower than the ultimate one produces a decrease in stresses with time that can be determined by viscosity and damage effects near sharp notches.     

Impact and post impact behavior of composite sandwich panels

Assessing the residual mechanical properties of a sandwich structure is an important part of any impact study and determines how the structure can withstand post impact loading. The damage tolerance of a composite sandwich structure composed of woven carbon/epoxy facesheets and a PVC foam core was investigated. Sandwich panels were impacted with a falling mass from increasing heights until damage was induced. Impact damage consisted of delamination and permanent indentation in the impacted facesheets. The Compression After Impact (CAI) strength of sandwich columns sectioned from these panels was then compared with the strength of an undamaged column. Although not visually apparent, the facesheet delamination damage was found to be quite detrimental to the load bearing capacity of the sandwich panel, underscoring the need for reliable damage detection techniques for composite sandwich structures.     

复合材料层板冲击后压缩强度经验预测公式

复合材料被广泛应用于飞机结构中,其损伤容限性能是飞行器设计的重要指标。提出了一种基于冲击能量预测复合材料层板受冲击后剩余压缩强度的经验预测公式,并通过试验方法分析了剩余强度的影响因素,确定了经验公式形式与参数,对经验公式预测结果进行了验证。经过一定量的试验验证,本文所提出的经验预测方法的预测结果与试验结果吻合良好,研究结果表明,该方法是一种简便迅速的剩余强度工程预测方法。试验结果表明：剩余强度与冲击能量的关系适合采用分段幂函数进行拟合;冲击能量、层板厚度、铺层比例和韧性对复合材料层板冲击后压缩强度有较明显的影响,而冲头直径对剩余强度的影响不显著。     

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.     

An engineering approach for predicting residual strength of carbon/epoxy laminates after impact and hygrothermal cycling

A semi-empirical analysis on residual compressive strength (RCS) of carbon/epoxy woven composite laminate was developed which included the damage effects caused by impact and hygrothermal cycling. Impact damage is modelled as a soft inclusion with an exponentially reduced stiffness and the stiffness is further reduced due to hygrothermal cycling. A complex variable method was used to determine the in-plane stress distribution near the impact-induced damage and point stress failure criterion is then used to predict the failure load. Based on the semi-empirical model, the RCS can be related to damage width, damage intensity, undamaged strength and a degradation factor due to hygrothermal cycling. The results from the analysis coincide reasonably well with the experimental data for the plain-woven fabric laminates.     

碳纤维复合材料假脚工艺参数对其冲击后疲劳性能的影响

基于三维逐渐损伤理论和有限元法,对碳纤维复合材料假脚的冲击及冲击后疲劳破坏过程进行分析,研究了不同的复合材料体系、几何尺寸、纤维铺设方式等工艺参数对碳纤维假脚的冲击损伤及疲劳性能的影响规律。结果表明,在冲击载荷作用下,碳纤维复合材料假脚的损伤模式主要为基体开裂、纤维压缩和分层;复合材料体系的横向和法向拉伸强度以及剪切强度等参数越小,假脚的冲击损伤面积越大,所能承受的疲劳循环次数越低;随着后龙骨厚度的增加,基体开裂损伤面积越来越大,分层损伤面积略有减小,而纤维压缩损伤几乎没有变化。尽管随着后龙骨厚度的增加,假脚的疲劳循环次数逐渐增大,但是相对于厚度的增加量,疲劳循环次数的增加量相对较小;不同铺层参数对碳纤维复合材料假脚的冲击损伤模式几乎没有影响。适度增加0°铺层的含量,可有效提高碳纤维复合材料假脚的疲劳性能。     

Impact damage to thick carbon fibre reinforced plastic composite laminates

Recently, very thick section laminates, up to 20 mm in thickness, have been proposed for the wing skins of large aircraft. Composite components in all aircraft have concerns relating to the presence of accidental damage, but there has been little work to investigate the mechanisms and effects of damage in such thick sections. In this work, carbon fibre composite laminates of up to 12 mm thickness have been subjected to dropped-weight impacts of at most 375 J. Two types of impacts were considered. The first is a central impact where the laminate is completely supported and the second a near edge impact where the laminate is partially supported so that one of its edges is free. The geometry of the damage has been studied using C-scan and deply techniques. The residual strengths of the impact-damaged laminates have been measured in tension and compression. The geometry of damage and level of strength reduction is different for central and edge impacts. Generally, an edge impact causes a greater reduction in compressive strength while a central impact causes more tensile strength reduction.     

Effect of autofrettage on durability of CFRP composite cylinders subjected to out-of-plane loading

The objective of this study is to evaluate effects of autofrettage on durability of composite cylinders subjected to impact loading. Impact tests and damage observation were performed on the composite cylinders with and without autofrettage. Internal pressure tests were also conducted on the cylinders after impact tests. Simulated cycle tests were conducted on the rectangular plate specimens simulated as composite cylinders after impact tests. For the composite cylinder with autofrettage, absorbed energy became smaller, which was due to suppression of the damages, such as delamination. As a result, residual strength became larger and fatigue life became longer. It was clarified that CFRP composite cylinders with proper autofrettage were superior in the impact resistance.     

超支化聚酰胺酯/环氧树脂力学性能的研究

采用超支化聚酰胺酯作为增韧剂,制备环氧树脂复合材料.测试其力学性能、热学性能,并用扫描电镜对冲击断面进行了观察比较.结果表明,超支化聚酰胺酯的加入使环氧树脂的冲击强度从12.27kJ/m2提高到29.78kJ/m2,并且其拉伸和弯曲强度都没有下降.弯曲测试和冲击断面都表现出明显的应力发白现象,进一步验证了超支化聚合物的空穴化理论在提高高聚物韧性中的作用.同时,扫描电镜图片显示复合材料呈现蜂窝形态,有大量撕裂物,应力条纹趋于分散.复合材料的耐热性能变化不大.     

A Discrete Model for Simulation of Composites Plate Impact Including Coupled Intra- and Inter-ply Failure

Laminated composites can undergo complex damage mechanisms when subjected to transverse impact. For unidirectional laminates it is well recognized that delamination failure usually initiates via intra-ply shear cracks that run parallel to the fibres. These cracks extend to the interface of adjacent orthogonal plies, where they are either stopped, or propagate further as inter-ply delamination cracks. These mechanisms largely determine impact energy absorption and post-delamination bending stiffness of the laminate. Important load transfer mechanisms will occur that may lead to fibre failure and ultimate rupture of the laminate. In recent years most Finite Element (FE) models to predict delamination usually stack layers of ply elements with interface elements to represent inter-ply stiffness and treat possible delamination. The approach is computationally efficient and does give some estimate of delamination zones and damaged laminate bending stiffness. However, these models do not properly account for coupled intra-ply shear failure and delamination crack growth, and therefore cannot provide accurate results on crack initiation and propagation. An alternative discrete meso-scale FE model is presented that accounts for this coupling, which is validated against common delamination tests and impact delamination from the Compression After Impact (CAI) test. Ongoing research is using damage prediction from the CAI simulation as a basis for residual strength analysis, which will be the published in future work.     

复合材料层板低速冲击后疲劳性能实验研究

通过对T300/5405复合材料层板进行低速冲击后的压-压疲劳实验,研究含不同冲击损伤层板的压缩性能与其在多级应力水平下的疲劳寿命与损伤扩展,并讨论冲击能量、应力水平、损伤扩展对层板疲劳寿命的影响。结果表明:冲击损伤明显降低层板的剩余强度;在低应水平下,冲击能量越大,含冲击损伤层板的疲劳寿命越小;疲劳实验中损伤经历平稳扩展和快速扩展两个阶段,其中平稳扩展阶段约占总体寿命的80%,快速扩展阶段约占总体寿命的20%,损伤扩展速率随着应力水平降低而减小。