Finite element modelling of impact on preloaded composite panels

Composite aircraft structures are susceptible to impact damage during manufacture, maintenance and in-flight. Low energy impact damage is often internal and invisible, but can significantly reduce the stiffness and strength or cause catastrophic failure when the structure is under load during the impact event. This paper describes the development and application of an explicit finite element (FE) model, incorporating a bi-phase material degradation model, to predict the behaviour of loaded carbon/epoxy panels when impacted over a range of low energy levels. Overall, the trends predicted in the FE simulations were consistent with experimental data, although quantitatively the FE results were generally conservative. However, the model greatly underestimated the catastrophic failure boundary. The model was used to investigate the effect of various parameters including magnitude of preload, impact velocity and specimen geometry on the amount of damage and the residual strength of carbon/epoxy panels.     

The low velocity impact response of foam-based sandwich panels

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.     

复合材料层合板低速冲击损伤的有限元模拟

建立了用于预测复合材料层合板在低速冲击作用下损伤的3D有限元模型。采用应变描述的失效判据来判断铺层层内的各类损伤, 如纤维断裂、 纤维挤压、 基体开裂、 基体挤裂, 并结合相应的刚度折减方案对失效单元进行刚度折减。使用界面元模拟层间区域, 结合传统的应力失效判据和断裂力学中的能量释放率准则来定义分层损伤的起始和演化规律, 提出了一种界面元损伤起始强度沿厚度方向的分布函数。通过对数值仿真结果和实验结果的比较, 验证了模型的合理性和准确性。      

Finite element modelling of fibre reinforced polymer sandwich panels exposed to heat

A finite element model that predicts temperature distribution in a composite panel exposed to a heat source, such as fire, is described. The panel is assumed to be composed of skins consisting of polymer matrix reinforced with fibres and a lightweight core (the paper concentrates on the crucial aspect of the problem, i.e. the behaviour of the ‘hot’ skin of the panel. The core is assumed not to decompose, and the ‘cold’ skin is treated exactly as the ‘hot’ skin.) It is assumed that the polymer matrix undergoes chemical decomposition. Such a model results in a set of coupled non‐linear transient partial differential equations. A Galerkin finite element framework is formulated to yield a fully implicit time stepping scheme. The crucial input parameters for the model are carefully identified for subsequent experimental determination. Copyright © 2004 John Wiley & Sons, Ltd.     

Finite element simulation of low velocity impact on shape memory alloy composite plates

Delamination of composite materials due to low velocity impacts is one of the major failure types of aerospace composite structures. The low velocity impact may not immediately induce any visible damage on the surface of structures whilst the stiffness and compressive strength of the structures can decrease dramatically.

Shape memory alloy (SMA) materials possess unique mechanical and thermal properties compared with conventional materials. Many studies have shown that shape memory alloy wires can absorb a lot of the energy during the impact due to their superelastic and hysteretic behaviour. The superelastic effect is due to reversible stress induced transformation from austenite to martensite. If a stress is applied to the alloy in the austenitic state, large deformation strains can be obtained and stress induced martensite is formed. Upon removal of the stress, the martensite reverts to its austenitic parent phase and the SMA undergoes a large hysteresis loop and a large recoverable strain is obtained. This large strain energy absorption capability can be used to improve the impact tolerance of composites. By embedding superelastic shape memory alloys into a composite structure, impact damage can be reduced quite significantly.

This article investigates the impact damage behaviour of carbon fiber/epoxy composite plates embedded with superelastic shape memory alloys wires. The results show that for low velocity impact, embedding SMA wires into composites increase the damage resistance of the composites when compared to conventional composites structures.     

Efficient computational modelling of carbon fibre reinforced laminated composite panels subjected to low velocity drop-weight impact

This paper reports on the actual and virtual low velocity impact response of carbon fibre composite laminates. It utilises the contribution of through-thickness stresses, in the prediction of the onset of internal damage created by this type impact scenario.The paper focuses on the damage imparted by the flat nose impactor since this induces a different type of damage and structural response compared to that of the standard test method of using a round nose impactor.Vulnerability of the fibrous composites to vertical drop-weight impact can result in premature failure which is a major concern in their widespread usage. The topic has been of intense research to design more damage tolerant and resistant materials. However, due to materials’ anisotropic and three-dimensional nature and complicated damage mechanisms no standard model could have been achieved. Designers predict consequences of a local impact within the global structural context without full-scale testing.Majority of the existing simulation models neglect through-thickness stresses that are regarded as the major cause of catastrophic failures. Efficient and reliable investigations are required to reduce testing and include through-thickness stresses. Drop-weight impact simulation models were developed herein using ABAQUS™ software. Simulations were carried out to compute in-plane stresses subjected to flat and round nose impacts on laminates of differing thicknesses. These stresses once computed were numerically integrated employing the equilibrium equations to efficiently predict through-thickness stresses. The predicted stresses were then utilised in failure criteria to quantify the coupled and embedded damage. This provides a quick insight into the status and contribution of through-thickness stresses in failure predictions. The computed values were compared to the experimental results and found to be in good agreement.     

钢板夹钢管组合板抗低速冲击性能的优化研究

试验共设计了三块钢板夹钢管组合板,分别为净距90 mm的三钢管组合板(G3)、净距30 mm的四钢管组合板(G4)与净距0 mm的五钢管组合板(G5);研究了组合板在落锤冲击作用下的抗冲击性能及破坏情况。利用有限元软件对试验组合板及不同厚度钢板、钢管的组合板进行模拟,分析钢管(钢板)位置、钢板厚度、钢管壁厚对组合板抗冲击性能的影响。提出单位增加质量贡献比γ来衡量组合板抗冲击性能优化的程度。结果表明:钢管分布连续的组合板抗冲击性能最好;在组合板变形不太大的情况下,钢板在抗冲击过程中起主要吸能作用;只增加中钢管壁厚能更快地提高组合板抗冲击性能,但材料利用率降低;只增加上钢板厚度能提高组合板边钢管抗冲击过程的参与度,增强组合板的整体性,并提高材料利用率。     

复合材料加筋板低速冲击损伤的数值模拟

建立了复合材料加筋板在横向低速冲击载荷作用下的渐进损伤有限元模型。该模型考虑了复合材料加筋板受低速冲击时的纤维断裂、基体开裂及分层脱粘等五种典型的损伤形式, 在层内采用应变描述的失效判据, 结合相应的材料性能退化方案, 通过编写VUMAT用户自定义子程序以实现相应损伤类型的判断和演化。在层间以及筋条与层板间加入界面元, 模拟层间区域的情况, 结合传统的应力失效判据和断裂力学中的能量释放率准则来判断分层损伤的起始和演化规律。通过对数值模拟结果与实验数据的比较, 验证了模型的合理性和有效性。同时探讨了不同位置、不同冲击能量以及含初始损伤(脱粘)等因素对复合材料加筋板低速冲击性能的影响。     

复合材料加筋板低速冲击损伤的数值模拟

建立了复合材料加筋板在横向低速冲击载荷作用下的渐进损伤有限元模型.该模型考虑了复合材料加筋板受低速冲击时的纤维断裂、基体开裂及分层脱粘等五种典型的损伤形式,在层内采用应变描述的失效判据,结合相应的材料性能退化方案,通过编写VUMAT用户自定义子程序以实现相应损伤类型的判断和演化.在层间以及筋条与层板间加入界面元,模拟层间区域的情况,结合传统的应力失效判据和断裂力学中的能量释放率准则来判断分层损伤的起始和演化规律.通过对数值模拟结果与实验数据的比较,验证了模型的合理性和有效性.同时探讨了不同位置、不同冲击能量以及含初始损伤(脱粘)等因素对复合材料加筋板低速冲击性能的影响.     

Prediction of impact damage on sandwich composite panels

Sandwich structures are extensively employed in the aerospace and automobile industries. The understanding of their behaviour under impact conditions is extremely important for the design and manufacturing of these engineering structures since impact problems are directly related to structural integrity and safety requirements. This paper investigates the damage behaviour of composite sandwich panels with aramid paper honeycomb (NOMEX) and polyetherimide (PEI) foam cores under transverse impacts at high velocities. A numerical model was developed using the dynamic explicit finite element (FE) structure analysis program PAM-CRASH. For both sandwich structures numerical analysis reproduces physical behaviour observed experimentally in high velocity impact tests.     

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.     

Compression and impact testing of two-layer composite pyramidal-core sandwich panels

Quasi-static uniform compression tests and low-velocity concentrated impact tests were conducted to reveal the failure mechanisms and energy absorption capacity of two-layer carbon fiber composite sandwich panels with pyramidal truss cores. Three different volume-fraction cores (i.e., with different relative densities) were fabricated: 1.25%, 1.81%, and 2.27%. Two-layer sandwich panels with identical volume-fraction cores (either 1.25% or 2.27%), and also stepwise graded panels consisting of one light and one heavy core, were investigated under uniform quasi-static compression. Under quasi-static compression, load peaks were identified with complete failure of individual truss layers due to strut buckling or strut crushing, and specific energy absorption was estimated for different core configurations. In the impact test, the damage resulting from low-velocity concentrated impact was investigated. Our results show that compared with glass fiber woven textile truss cores, two-layer carbon fiber composite pyramidal truss cores have comparable specific energy absorptions, and thus could be used in the development of novel light-weight multifunctional structures.     

Experimental and modelling study of low velocity impacts on composite sandwich structures for railway applications

This paper aims to evaluate the damage resistance of sandwich composites used in train structures subjected to low velocity impacts. These impacts can be induced by railway ballast projection incidents underneath the floor of the train. A high-speed and low weight test bench to simulate such impacts was developed in our laboratory. After the impact, the damage mechanism evolution as a function of solicitation conditions was inspected. Three-dimensional finite element models were implemented to analyze the impact response. Results obtained from numerical simulations have confirmed experimental observations. Shock wave propagation localized around the impact point confined the damage. Visual inspection identified delamination in composite skin, fiber breakage, and indentation. Skin/core debonding, core crushing and shear failure were analyzed. Damages intensified proportionally with the number of successive projectile launches. Correlations between impact energy and damage mode are discussed.     

Damage prediction in composite sandwich panels subjected to low-velocity impact

The paper illustrates the application of a finite element tool for simulating the structural and damage response of foam-based sandwich composites subjected to low-velocity impact. Onset and growth of typical damage modes occurring in the composite skins, such as fibre fracture, matrix cracking and delaminations, were simulated by the use of three-dimensional damage models (for intralaminar damage) and interfacial cohesive laws (for interlaminar damage). The nonlinear behaviour of the foam core was simulated by a crushable foam plasticity model. The FE results were compared with experimental data acquired by impact testing on sandwich panels consisting of carbon/epoxy facesheets bonded to a PVC foam. Good agreement was obtained between predictions and experiments in terms of force histories, force–displacement curves and dissipated energy. The proposed model was also capable of simulating correctly nature and size of impact damage, and of capturing the key features of individual delaminations at different depth locations.     

Computer modelling of impact on curved fibre composite panels

This paper presents results of a finite element (FE) analysis study into low energy impact on curved composite panels. The aim of this study was to determine the accuracy and efficiency of a non-linear explicit FE code, MSC.Dytran, and compare results to published experimental data. The study also looked at impact response as a function of composite panel curvature, composite mesh density, impactor weight, velocity and size, and various suggestions are made for improving the accuracy and efficiency of FE analysis procedures in composite low energy impact studies. The paper presents 265 explicit computer simulation results, which show that non-linear FE analysis does provide accurate, efficient and conservative solutions provided various guidelines are followed.     

Damage growth analysis of low velocity impacted composite panels

Low velocity impact loading in aircraft composite panels is a matter of concern in modern aircraft and can be caused either by maintenance accidents with tools or by in-flight impacts with debris. The consequences of impact loading in composite panels are matrix cracking, inter laminar failure and, eventually, fiber breakage for higher impact energies. Even when no visible impact damage is observed on the surface at 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 limit loading capacity and the damage growth mechanisms 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 fiber reinforced epoxy resin matrix. Four stacking sequences representative of four different elastic behaviours were used. Results show that the compressive, after impact, failure stress is influenced by the stacking sequence but a relatively independent strain to failure is observed.     

明胶鸟弹撞击复合材料蜂窝夹芯板试验

开展明胶鸟弹撞击复合材料蜂窝夹芯板试验,研究夹芯结构在软体高速冲击下的损伤形式,分析相关因素对结构动态响应结果的影响。通过CT扫描对复合材料蜂窝夹芯板内部进行检测可知,面板出现分层、基体开裂、纤维断裂、凹陷、向胞内屈曲等损伤形式,蜂窝芯出现芯材压溃、与面板脱粘的损伤形式;分析复合材料蜂窝夹芯板后面板的动态变形过程及撞击中心处位移-时间数据可知,复合材料蜂窝夹芯板在撞击过程中出现由全局弯曲变形主导和局部变形主导的两种变形模式;通过对比不同工况下的复合材料蜂窝夹芯板损伤程度可知,复合材料蜂窝夹芯板损伤程度随鸟弹撞击速度的增加而增大;蜂窝芯高度为10 mm的复合材料蜂窝夹芯板较蜂窝芯高度为5 mm的复合材料蜂窝夹芯板的损伤程度大;初始动能较大的球形鸟弹较圆柱形鸟弹对复合材料蜂窝夹芯板造成的冲击损伤程度更大。      

Finite element simulation and experimental study on mechanical behavior of 3D woven glass fiber composite sandwich panels

The results of finite element simulation followed by an experimental study are presented in order to investigate the mechanical behavior of three-dimensional woven glass-fiber sandwich composites using FE method. Experimental load–displacement curves were obtained for flatwise compressive, edgewise compressive, shear, three-point bending and four-point bending loads on the specimens with three different core thicknesses in two principal directions of the sandwich panels, called warp and weft. A 3D finite element model is employed consisting of glass fabric and surrounding epoxy resin matrix in order to predict the mechanical behavior of such complex structures. Comparison between the finite element predictions and experimental data showed good agreement which implies that the FE simulation can be used instead of time-consuming experimental procedures to study the effect of different parameters on mechanical properties of the 3D woven sandwich composites.     

Adaptive multi-scale modeling of high velocity impact on composite panels

A computationally efficient adaptive multi-scale methodology for modeling composites under high rates of loading is proposed. The physically based model relies on micromechanical properties of the constituents only. The adaptive algorithm switches between two different constitutive laws. Initially, the material response is calculated based on effective linear-elastic, orthotropic material properties at the ply scale which are calculated using the rule of mixtures. A modified Hashin–Rotem criterion is then used to identify the switch to a more accurate micromechanical analysis based on the generalized method of cells (GMC). The methodology is verified by simulating tensile tests on laminates with different stacking sequences. Finally the model validated against experimental data for high-velocity impact on quasi-isotropic composite targets taken from the literature in order to illustrate the efficiency and accuracy of the proposed methodology.     

Finite element modelling of the progressive crushing of braided composite tubes under axial impact

Composite tubular structures are of interest as viable energy absorbing components in vehicular front rail structures to improve crashworthiness. Desirable tools in designing such structures are models capable of simulating damage growth in composite materials. Our model (CODAM for COmposite DAMage), which is a continuum damage mechanics based model for composite materials with physically based inputs, has shown promise in predicting damage evolution and failure in composites. In this study, the model is used to simulate the damage propagation, failure morphology and energy absorption in triaxially braided composite tubes under axial compression. The model parameters are based on results from standard and specialized material testing and a crack band scaling law is used to minimize mesh sensitivity (or lack of objectivity) of the numerical results. Axial crushing of two-ply and four-ply square tubes with and without the presence of an external plug initiator are simulated in LS-DYNA. Refinements over previous attempts by the authors include the addition of a pre-defined debris wedge, a distinguishing feature in tubes displaying a splaying mode of failure, and representation of delamination using a tiebreak contact interface that allows energy absorption through the un-tying process. It is shown that the model adequately predicts the failure characteristics and energy absorption of the crushing events. Using numerical simulations, the process of damage progression is investigated in detail and energy absorptions in different damage mechanisms are presented quantitatively.