Low-velocity Impact Damage Analysis of Composite Laminates Using Self-adapting Delamination Element Method

On the basis ofa 2D 4-node Mindlin shell element method, a novel self-adapting delamination finite element method is presented, which is developed to model the delamination damage of composite laminates. In the method, the sublaminate elements are generated automatically when the delamination damage occurs or extends. Thus, the complex process and state of delamination damage can be simulated practically with high efficiency for both analysis and modeling. Based on the self-adapting delamination method, linear dynamic finite element damage analysis is performed to simulate the low-velocity impact damage process of three types of mixed woven composite laminates. Taking the frictional force among sublaminations during delaminating and the transverse normal stress into account, the analytical results are consistent with those of the experimental data.

Nonlinear progressive damage model for composite laminates used for low-velocity impact

In order to effectively describe the progressively intralaminar and interlam- inar damage for composite laminates, a three dimensional progressive damage model for composite laminates to be used for low-velocity impact is presented. Being applied to three-dimensional （3D） solid elements and cohesive elements, the nonlinear damage model can be used to analyze the dynamic performance of composite structure and its failure be- havior. For the intralaminar damage, as a function of the energy release rate, the damage model in an exponential function can describe progressive development of the damage. For the interlaminar damage, the damage evolution is described by the framework of the continuum mechanics through cohesive elements. Coding the user subroutine VUMAT of the finite element software ABAQUS/Explicit, the model is applied to an example, i.e., carbon fiber reinforced epoxy composite laminates under low-velocity impact. It is shown that the prediction of damage and deformation agrees well with the experimental results.

Influence of Low-velocity Impact on Damage Behavior of Carbon Fiber-reinforced Composites

A combination of experimental measurements and numerical analysis was utilized to study the low-velocity impact damage of domestic carbon fiber-reinforced composites(CFRCs).The results indicated that the low-velocity impact damage induced pits and longitudinal cracks on the front side,oblique cracks and delaminationin on the back side.The pit depth increased with the increasing impact energy.It was demonstrated that the numerical analysis strain history curve was similar to the experimentally measured strain history curve,which verified the accuracy of numerical analysis in which the Hashin failure criterion was used.The work provides basic data and theoretical basis for the promotion and application of the domestic carbon fiber,and demonstrates the feasibility of replacing imported carbon fibers with domestic carbon fibers.

STUDY ON THE RESPONSE TO LOW-VELOCITY IMPACT OF A COMPOSITE PLATE IMPROVED BY SHAPE MEMORY ALLOY

Improvement from the pseudo-elastic effect of shape memory alloy （SMA） on the low-velocity impact （LVI） resistance of a composite plate is investigated by the finite element method （FEM）. The stiffness matrix of the dynamic finite element equation is established step by step and the martensite fraction is obtained at each time step. The direct Newmark integration method is employed in solving the dynamic finite element equation, while the impact contact force is determined using the modified Hertz＇s law. It is found that SMA can effectively improve the performance of a composite structure subjected to low-velocity impact. Numerical results show that the deflection of a SMA-hybrid composite plate has been reduced approximately by thirty percent when the volume fraction of the embedded SMA reaches 0.3.

Low-Velocity Impact and Compression after Impact(CAI)Behaviors of Carbon-Aramid/Epoxy Hybrid Braided Composite Laminates

Low-velocity impact and in-plane axial compression after impact(CAI)behaviors of carbon-aramid/epoxy hybrid braided composite laminates were investigated experimentally.The following three different types of carbon-aramid/epoxy hybrid braided composite laminates were produced and tested:(a)inter-hybrid laminates,(b)sandwich-like inter-hybrid laminates,and(c)unsymmetric-hybrid laminates.At the same time,carbon/epoxy braided composite laminates were used for comparisons.Impact properties and impact resistance were studied.Internal damages and damage mechanisms of laminates were detected by ultrasonic C-scan and B-scan methods.The results show that the ductility index(DI)values of three kinds of hybrid laminates aforementioned are 37%,4%and 120%higher than those of carbon/epoxy laminates,respectively.The peak load of inter-hybrid laminates is higher than that of sandwich-like inter-hybrid laminates and unsymmetric-hybrid laminates.The average damage area and dent depths of inter-hybrid laminates are 64%and 69%smaller than those of carbon/epoxy laminates.Those results show that carbon-aramid/epoxy hybrid braided composite laminates could significantly improve the impact properties and toughness of non-hybrid braided composite laminates.

Experimental Investigation on Low-Velocity Impact Response and Residual Compressive Bearing Capacity of Composite Stringers

Three types of composite stringers were impacted from two different directions.Relationships between impact energy and visible defect length were found.The critical impact energy corresponding to barely visible impact damage(BVID)of each stringer was determined.Specimens with BVID were then compressed to obtain the residual strength.Experimental results showed that for all types of stringers,the critical energy of in-plane impact is always much lower than out-plane ones.In-plane impact causes much more decrement of stringers'bearing capacity than outplane impact.For both impact directions,I-stringers own the highest defect detectability,T-stringers come second.Meanwhile,I-stringers own the better residual strength ratio than I-stringers and I-stringers.Synthetic considering impact defect detectability and residual bearing capacity after impact,T-stringers own the best compression-afterimpact(CAI)behaviors.

Experiments and simulation of low-velocity impact characteristics on carbon fiber composite lattice core sandwich structures

Since composite sandwich structures are susceptible to low-velocity impact damage,a thorough characterization of the loading and damage process during impact is important.In the present paper,the low-velocity impact response of carbon fiber composites lattice structures is investigated by experimental and numerical methods.Impact tests on composite plates are performed using an instrumented drop-weight machine(Instron 9250HV)and a new damage mode is observed.A three-dimensional finite element model is built by ABAQUS/Explicit and user subroutine(VUMAT)to predict the peak loading and simulate the complicated damage problem.The numerical predictions are in good agreement with the experimental results.

Low-Velocity Impact Response of Stitched Multi-layer Foam Sandwich Composites

Low-velocity impact damage known as“imperceptible”damage usually destroys the structural integrity of the material and seriously affects the service life of the materials.To improve the low-velocity impact resistance of foam sandwich composites,an innovative concept of a stitched multi-layer sandwich structure by organically combining the discrete splitting of foam layer with full thickness stitching was proposed,and its low-velocity impact resistance obtained through drop-hammer impact tests was explored.The results showed that the multi-layer foam sandwich structure acted as a stress disperser and reduced the irreversible impact damage.The depth and area of low-velocity impact damage of multi-layer foam sandwich composites gradually decreased with increasing the number of the layers.The stitched structure would improve the integrity of the foam sandwich composites and inhibit the propagation of cracks.The maximum impact load of the stitched foam sandwich composite increased by approximately 5% compared with that of the non-stitched material.In addition,the low-velocity impact damage depth,damage area and absorbed energy of the stitched three-layer foam sandwich composite were reduced by 37.7%,34.6% and 20.7%,respectively,compared with those of the non-stitched single-layer sandwich material.

Design Optimization of CFRP Stacking Sequence Using a Multi-Island Genetic Algorithms Under Low-velocity Impact Loads

A method to improve the low-velocity impact performance of composite laminate is proposed, and a multi-island genetic algorithm is used for the optimization of composite laminate stacking sequence under low-velocity impact loads based on a 2D dynamic impact finite element analysis. Low-velocity impact tests and compression-after impact（CAI） tests have been conducted to verify the effectiveness of optimization method. Experimental results show that the impact damage areas of the optimized laminate have been reduced by 42.1% compared to the baseline specimen, and the residual compression strength has been increased by 10.79%, from baseline specimen 156.97 MPa to optimized 173.91 MPa. The tests result shows that optimization method can effectively enhance the impact performances of the laminate.

Low-Velocity Impact and Compression-After-Impact Behaviour of Flax Fibre-Reinforced Composites

This study details the low-velocity impact and compression-after-impact(CAI)behaviour of flax fibre-reinforced polymer(FFRP)composites.The impact resistance,energy absorption efficiency and residual compressive strength as well as the damage pattern of the FFRP composites are compared with the corresponding features of glass fibre-reinforced polymer(GFRP)composites,and the effect of the stacking sequence of FFRP composites is also investigated.The results show that the cross-ply FFRP composites have the highest impact resistance,whereas the multi-directional ply composites have the lowest impact resistance but the highest energy absorption efficiency.The energy absorption efficiency of the FFRP composites is greater than that of the GFRP composites,but the penetration resistance and residual compressive strength of the FFRP composites are lower than those of the GFRP composites with the same stacking sequence,mainly due to the lower tensile strength and elongation at fracture of the FFRP composites.It is also reported that the damage pattern of the FFRP composites is localised cracking and delamination,unlike the overall delamination failure exhibited in the GFRP composites after CAI testing.Finally,the failure mechanisms of the FFRP and GFRP composites are detailed.

Theoretical study of failure in composite pressure vessels subjected to low-velocity impact and internal pressure

A theoretical solution is aimed to be developed in this research for predicting the failure in internally pressurized composite pressure vessels exposed to low-velocity impact.Both in-plane and out-of-plane failure modes are taken into account simultaneously and thus all components of the stress and strain fields are derived.For this purpose,layer-wise theory is employed in a composite cylinder under internal pressure and low-velocity impact.Obtained stress/strain components are fed into appropriate failure criteria for investigating the occurrence of failure.In case of experiencing any in-plane failure mode,the evolution of damage is modeled using progressive damage modeling in the context of continuum damage mechanics.Namely,mechanical properties of failed ply are degraded and stress analysis is performed on the updated status of the model.In the event of delamination occurrence,the solution is terminated.The obtained results are validated with available experimental observations in open literature.It is observed that the sequence of in-plane failure and delamination varies by increasing the impact energy.

Low-velocity impact response and infrared radiation characteristics of thermoplastic/thermoset composites

The low-velocity impact response and infrared radiation characteristics of composites have rarely been focused on simultaneously.This study aims to investigate the low-velocity impact response and infrared radiation characteristics of the glass fiber reinforced thermoplastic polypropylene and carbon fiber reinforced thermosetting epoxy resin laminates wildly used in the aircraft industry.The impact tests were conducted at five energy levels.Characterization parameters such as impact load,displacement,and absorbed energy were measured.The damage evolution and damage modes of the laminates were analyzed through active and passive thermography,ultrasonic C-scan,and optical microscope.The results indicate that Thermosets(TS)laminates exhibit better impact resistance,while Thermoplastics(TP)laminates show higher delamination ductility,and the maximum contact force of TP laminates is much smaller than that of the TS laminates under lowvelocity impacts,but the low bending stiffness and low ductility of the TP matrix cause the difference in energy absorption level between the two not significant.The temperature characteristic changes of passive infrared thermography heat maps could characterize the damage mode of the laminates.The correlation between the heat maps and the impact characteristic curves is explained;the fluctuation of the impact characteristic curves is directly related to the hot spot characteristics changes of the heat maps.More frequent curve fluctuations correspond to a larger and brighter hot spot on the heat map,which peaks at the maximum impact load after the impact force versus time curve fluctuation cutoff point,the maximum center displacement of the impact force versus displacement curve,and the maximum absorbed energy of the absorbed energy versus time curve.

T800复合材料层压板损伤引入技术研究

本文针对国产T800级碳纤维增强复合材料开展多次低速冲击损伤与准静态压痕损伤及最终剩余强度对比研究,以提出一种高效的复合材料损伤引入方式。首先开展多次冲击及静压痕损伤引入试验,在达到目标凹坑深度时,通过目视观测及超声C扫分析不同损伤引入方式产生损伤的机理及损伤模式;其次,对达到目标凹坑深度的试验件进行压缩剩余强度测试,分析不同损伤引入方式、不同凹坑深度、不同损伤面积与剩余强度之间的关系;通过DIC(Digital Image Correlation)、声发射系统监测对剩余强度试验过程进行监测,确定不同损伤引入方式的试验件在压缩中的损伤演化过程及机理。通过以上研究,最终确定一种高效的损伤引入方式。

复合材料层压板低速冲击和准静态压痕损伤等效性的研究

通过对低速冲击试验和准静态压痕试验进行对比,获得了冲击能量(准静态压痕力)与层压板损伤面积、损伤宽度和凹坑深度的三组对应关系。分析表明,损伤面积、损伤宽度和凹坑深度均可作为损伤参数来建立低速冲击和准静态压痕损伤的等效性。当冲击能量或准静态压痕力达到一定值后,三组对应关系曲线上出现拐点,两类试验的拐点相差很小,且两类试验的变化趋势相同,初步说明了用准静态压痕试验替代低速冲击试验是可行的,同时在较低冲击能量(拐点值之前)下准静态压痕力近似等于相对应的冲击能量下冲击过程的最大接触力。对两类试验过程进行分析,准静态压痕试验的初始分层载荷较冲击试验稍低,但两类试验过程中载荷的变化趋势相同,进一步说明了低速冲击试验和准静态压痕试验损伤的等效性。

厚度对混合机织复合材料低速冲击和准静态横向压缩性能的影响

将二维机织碳布和单向碳布预浸料按照一定比例铺设成的混合机织复合材料兼具比刚度、比强度高、抗低速冲击损伤以及工艺性好等优点。试验研究了3个铺层结构相似,但总厚度不同的混合机织复合材料的低速冲击性能,采用超声波C扫描法记录了各层合板损伤面积,测量了剩余压缩强度,并进行了准静态横向压缩试验。实验结果表明,厚度增加会使层合板内部的超声波C扫描损伤面积增加,但并不会明显影响其剩余压缩强度。

复合材料低速冲击永久凹坑深度预测方法

基于赫兹接触定律,建立了复合材料层板低速冲击模型,在此基础上提出了复合材料层板冲击后永久凹坑的理论计算方法。通过对T700/5428和T300/5428两种材料层板低速冲击永久凹坑计算结果与试验结果的对比,证实了这种方法在较小冲击损伤情况下的可行性与有效性。通过这种方法,可以为复合材料冲击性能的预测提供一种参考。

低速冲击下复合材料加筋板的损伤阻抗性能

为确定冲击能量、几何尺寸对低速冲击下复合材料加筋板损伤阻抗性能的影响,对3组工型加筋板进行了试验和数值模拟研究。通过落锤式低速冲击试验,得到了试验件的接触历程、凹坑深度和分层面积等损伤特征。基于引入纤维断裂损伤的各向异性弹塑性理论建立了有限元(FE)模型,对试验件凹坑深度进行了模拟预测,模拟结果与试验结果吻合较好。研究表明,复合材料加筋板凹坑深度随冲击能量的变化曲线存在拐点,拐点后表面冲击部位出现纤维断裂。随着冲击能量的增大,试验件的最大接触力不断增大,而分层起始载荷及分层面积则变化不大。含1.5 mm深凹坑试验件对应的冲击能量和最大接触力随筋条或蒙皮厚度的增大而不断增大,而分层起始载荷仅随蒙皮厚度的增大而增大。

层合板静压痕及压缩强度试验与其数据统计分析

复合材料层合板在低速能量冲击后的压缩强度变化是航空器结构设计应用的重要依据。本研究运用标准静压痕及冲击后压缩强度试验方法,对工程中两类典型的碳纤维及玻璃纤维树脂基复合材料层合板进行了静压痕及压缩强度试验研究,并采用数理统计方法,对试验数据进行了处理,给出了冲击后有效弹性模量及压缩极限强度的分散性表征及其拟合分布,计算了A、B基准值。研究结果表明:冲击后两类复合材料层合板的有效模量与强度特性仍遵从正态分布的概率推断;但较玻璃纤维增强的树脂基复合材料层合板,碳纤维增强复合材料呈现更明显的脆性。

复合材料薄板在低速冲击下的破坏分析:试验和数值方法

在冲击荷载作用下,复合层压板破坏过程中有很多并发现象,因此,复合层压板的动力性能十分复杂。当采用复合材料结构时,对于接触和较大的位移导致的纤维破坏、分层、模具破裂、塑性变形应给予重视。对碳纤维环氧树脂加固的层压复合薄板进行低速冲击下的研究。采用荷载时间序列、位移时间序列、能量时间序列以及来自NDE的图像,分析叠加系列和能量响应的影响。当层压复合薄板受到低速外来目标的冲击时,由压痕试验结果与动力学的结果比较,验证了迟滞效应。采用希尔模型和ABAQUS软件中由UMAT补充的材料模型,通过有限元分析以模拟压痕试验的破坏机制。