Response and Damage Tolerance of Composite Sandwich Structures under Low Velocity Impact

The deformation and failure response of composite sandwich beams and panels under low velocity impact was reviewed and discussed. Sandwich facesheet materials discussed are unidirectional and woven carbon/epoxy, and woven glass/vinylester composite laminates; sandwich core materials investigated include four types of closed cell PVC foams of various densities, and balsa wood. Sandwich beams were tested in an instrumented drop tower system under various energy levels, where load and strain histories and failure modes were recorded for the various types of beams. Peak loads predicted by spring-mass and energy balance models were in satisfactory agreement with experimental measurements. Failure patterns depend strongly on the impact energy levels and core properties. Failure modes observed include core indentation/cracking, facesheet buckling, delamination within the facesheet, and debonding between the facesheet and core. In the case of sandwich panels, it was shown that static and impact loads of the same magnitude produce very similar far-field deformations. The induced damage is localized and is lower for impact loading than for an equivalent static loading. The load history, predicted by a model based on the sinusoidal shape of the impact load pulse, was in agreement with experimental results. A finite element model was implemented to capture the full response of the panel indentation. The investigation of post impact behavior of sandwich structures shows that, although impact damage may not be readily visible, its effects on the residual mechanical properties of the structure can be quite detrimental.     

Three-dimensional Composite Lattice Structures Fabricated by Electrical Discharge Machining

We present a novel method for fabricating carbon fiber composite sandwich panels with lattice core construction by means of electrical discharge machining (EDM). First, flat-top corrugated carbon fiber composite cores were fabricated by a hot press molding method. Then, two composite face sheets were bonded to each corrugated core to create precursor sandwich panels. These panels were transformed into sandwich panels with near-pyramidal truss cores by EDM plunge-cutting the corrugated core between the face sheets with a shaped cuprite electrode. The flat top corrugation permits adhesive to be applied consistently, and the selected dimensions leave a substantial bond area after cutting, resulting in a strong core-to-sheet bond. The crushing behavior of this novel construction was investigated in flatwise compression, and the results were compared to analytical expressions for strength and stiffness.     

铝面板厚度对Nomex蜂窝夹层结构冲击后弯曲性能影响的试验研究

通过低速冲击试验和四点弯曲试验,研究了铝面板厚度对Nomex蜂窝夹层结构抗冲击能力和剩余强度的影响。结果表明:在冲击荷载作用下,面板发生变形的区域大小随面板厚度增加而变大,当面板厚度大于0.5mm时,变形区域直径趋于稳定;无论试件是否受到过冲击,在弯曲载荷作用下,0.2mm厚面板发生芯格内屈曲失稳,而其他厚度面板均发生格间失稳;对无冲击损伤的结构,0.2mm厚面板弯曲强度显著低于其他厚度面板;对含冲击损伤的结构,0.2mm厚面板的剩余强度百分比最高。     

低速冲击后含损复合材料夹层板剩余强度研究进展

综述了受低速冲击后复合材料夹层板的损伤性态研究进展,重点介绍了倍受复合材料工程结构设计师所关注的受损复合材料夹层板的剩余拉/压强度.主要内容为:(1) 复合材料夹层板损伤特征;(2)剩余拉伸强度;(3)剩余压缩强度;(4)相关问题的讨论与研究展望.      

Interaction effects of multiple damage mechanisms in composite sandwich beams subject to time dependent loading

Theoretical models are formulated to explain evolution and interaction of the damage mechanisms for multiple delamination of the face-sheet and core crushing in composite sandwich beams subjected to dynamically applied out-of-plane loading and continuously supported by rigid planes. The models are based on simplified one-dimensional formulations and describe the impacted face of the sandwich as a set of Timoshenko beams joined by cohesive interfaces and resting on a nonlinear Winkler foundation, which approximates the response of the core; the dimensionless formulation highlights the material/structure groups that control the mechanical response. The characteristic features of the problem and transitions in damage progression are explored on varying geometrical parameters and material properties and magnitude and duration of the applied load. For quasi-static loading and low velocity impact, core/face-sheet interactions generate energy barriers to the propagation of delaminations; the efficacy of the barriers in controlling damage in the face-sheets depends on the relative stiffnesses of face-sheet and core and on the foundation yielding strength. For dynamic loading conditions, significant dynamic effects arise in certain regimes and cause substantial changes in behavior: shielding of the crack tip stress fields provided by the foundation is reduced, especially after the load is removed when important delamination openings occur; core plasticity generally opposes this behavior and limits damage in the face-sheet.     

轻质金属点阵夹层板热屈曲临界温度分析

本文针对均匀温度场下四边简支和四边固支金属点阵夹层板的临界热屈曲温度进行了求解和参数影响分析。将点阵夹芯等效为均匀连续体,并且将夹层板的剪切刚度近似为点阵夹芯的抗剪切刚度,忽略夹芯的抗弯刚度且认为夹层板主要由面板来提供抗弯刚度。对于无法获得解析解的四边固支条件,通过对未知变量进行双傅里叶展开的方法求解了Ressiner夹层板模型的临界屈曲温度,理论分析结果与有限元计算结果吻合良好。进一步分析了不同边界条件、点阵胞元构型、点阵材料相对密度、面板厚度等对临界屈曲温度的影响规律。     

Failure behavior of composite sandwich structures under local loading

Usually when analyzing the mechanical response of foam-cored fiber-reinforced composite sandwich structures to localized static loading, the face sheets are treated as a linear-elastic material and no damage initiation and growth is considered. However, practice shows that at higher indentation magnitudes damage develops in the face sheet in the area of contact with the indentor, which could lead to local failure of the face laminate due to the loss of bending stiffness and strength. Therefore, the main objective of the present study is to develop a damage model for predicting the local failure in the composite face sheet and its influence on the load–displacement behavior of sandwich structures under local loading. For this purpose, the Hoffman failure criterion is incorporated into a finite element modeling procedure using the ABAQUS program system. Results deducted from the modeling procedure are compared with experimental data obtained in the case of static indentation tests performed on sandwich beam specimens using steel cylindrical indentors. It is shown that taking into account the damage in the face sheet leads to a substantial improvement in the performance of the model when simulating the mechanical behavior of the sandwich structures at higher indentation values.     

基于红外热成像的编织复合材料低速冲击和冲击后压缩试验研究

针对二维三轴编织复合材料（two-dimensional triaxially braided composite, 2DTBC）在低速冲击和冲击后压缩（compression after impact, CAI）载荷下的损伤失效机理,开展了2DTBC试样的不同能量低速冲击试验以及相应的CAI试验,并采用红外热像仪监测在低速冲击和CAI试验过程中的温升现象。通过C扫描表征了不同能量低速冲击后试样的分层损伤情况,讨论了试样背面温度场分布特性及其随冲击能量的演化规律;对比分析了2DTBC冲击后剩余压缩强度与冲击能量的对应关系,基于数字图像相关（digital image correlation, DIC）技术监测了CAI试验中的全局应变场,结合热成像、变形场和光学图像数据,阐明了不同能量冲击后2DTBC的压缩失效特性,讨论了基于红外热成像技术表征编织复合材料损伤失效行为的有效性。试验结果显示:编织复合材料低速冲击和CAI试验中的温度场分布图与编织几何构型有明显关联度;低速冲击试验的温升幅值随冲击能量的增加而快速上升,CAI试验的温升现象随着冲击能量的增加而减弱;分层面积随冲击能量的增大而增大,冲击后剩余压缩强度随冲击能量的增大而降低。研究结果表明:红外热成像技术能够很好地捕捉试样破坏瞬间释放断裂能所产生的温升现象,温度场图像相较于全局应变场能更好地捕捉破坏的起始位置和失效特征。     

冰雹撞击下泡沫铝夹芯板的动态响应

在传统单层泡沫夹芯结构的上、下面板之间插入中面板,通过移动中面板的位置,获得了外形尺寸相同、质量相等的5种构型夹芯结构,其上层芯材与芯材总厚度比分别为0:30、10:30、15:30、20:30和30:30。在量纲分析的基础上,应用非线性动力有限元程序LS-DYNA对5种构型夹芯结构进行了冰雹撞击数值分析,研究了中面板位置对夹芯板的能量吸收、能量耗散和动态响应的影响。结果表明:中面板的存在对下层芯材能形成有效的保护;随着中面板位置由上向下移动,夹芯板的抗撞击性能呈现由大到小再增大的态势。数值计算结果对抗冰雹撞击夹芯结构的优化设计具有一定的参考价值。     

Imperfection sensitivity of pyramidal core sandwich structures

Lightweight metallic truss structures are currently being investigated for use within sandwich panel construction. These new material systems have demonstrated superior mechanical performance and are able to perform additional functions, such as thermal management and energy amelioration. The subject of this paper is an examination of the mechanical response of these structures. In particular, the retention of their stiffness and load capacity in the presence of imperfections is a central consideration, especially if they are to be used for a wide range of structural applications. To address this issue, sandwich panels with pyramidal truss cores have been tested in compression and shear, following the introduction of imperfections. These imperfections take the form of unbound nodes between the core and face sheets—a potential flaw that can occur during the fabrication process of these sandwich panels. Initial testing of small scale samples in compression provided insight into the influence of the number of unbound nodes but more importantly highlighted the impact of the spatial configuration of these imperfect nodes. Large scale samples, where bulk properties are observed and edge effects minimized, have been tested. The stiffness response has been compared with finite element simulations for a variety of unbound node configurations. Results for fully bound cores have also been compared to existing analytical predictions. Experimentally determined collapse strengths are also reported. Due to the influence of the spatial configuration of unbound nodes, upper and lower limits on stiffness and strength have been determined for compression and shear. Results show that pyramidal core sandwich structures are robust under compressive loading. However, the introduction of these imperfections causes rapid degradation of core shear properties.     

泡沫金属固支夹芯圆板的大挠度变形分析

论文分析了大挠度情况下固支夹芯圆板在准静态中心荷载作用下的承载能力.提出了考虑芯层强度影响的夹芯结构屈服条件,应用该条件,并基于边界条件和平板的初始变形机制假设夹芯板的速度场,推导了夹芯圆板考虑弯曲和膜力联合作用的大挠度响应,并应用数值算例验证了分析模型的合理性,在此基础上,分析和讨论了面板厚度、芯层厚度和芯层强度对夹芯圆板承载能力的影响.     

Higher-order impact modeling of sandwich structures with flexible core

In this study, a higher-order impact model is presented to simulate the response of a soft-core sandwich beam subjected to a foreign object impact. A free vibration problem of sandwich beams is first solved, and the results are validated by comparing with numerical finite element modeling results of ABAQUS and the solution by Frostig and Baruch [Frostig, Y., Baruch, M., 1994. Free vibration of sandwich beams with a transversely flexible core: a high order approach. Journal of Sound and Vibration 176(2), 195–208]. Then a foreign object impact process is incorporated in the higher-order model, and the contact force and deflection history as well as the propagation of transverse normal, shear, and axial stresses during the impact are analyzed and discussed. The validity of the model in the impact response predictions is demonstrated by comparing with finite element solutions of LS-DYNA. The calculated stresses caused by a foreign object impact are then used to assess failure locations, failure time, and failure modes in sandwich beams, which are shown to compare well with the available experimental results. The effects of impact mass, initial velocity, core stiffness, and core height on the impact stresses generated in the beams are discussed. The influences of impact mass and initial velocity on the contact force history are close to those by the linearized impact solution, but the proposed higher-order impact model captures the non-linear impact process and different generated stresses. Compared to the fully backed sandwich case, the core height shows a great influence over the impact process of a simply supported sandwich system, in which the global behavior of the sandwich is dominant; while the core stiffness shows minor effect over the impact process. The higher-order impact model of sandwich beams developed in the study provides accurate predictions of the generated stresses and impact process and can be used effectively in design analysis of anti-impact structures made of sandwich materials.     

爆炸载荷下双向梯度仿生夹芯圆板的力学行为

基于王莲仿生面内梯度芯层,通过引入面外梯度,设计了一种双向梯度仿生夹芯圆板。在此基础上,运用ABAQUS有限元软件,对不同排列方式的双向梯度夹芯圆板在不同爆炸载荷作用下的响应进行了数值仿真,着重分析了不同仿生夹芯圆板的前后面板挠度、芯层压缩量、变形模式和能量吸收等特性,得到了一种抗爆性能较好的芯层排列方式。结果表明:相较于单一的面外梯度夹芯圆板,合理设计的双向梯度仿生夹芯圆板可以有效降低后面板挠度,并提高芯层的能量吸收。     

Special behavior of unidirectional sandwich panels with transversely flexible core under statical loading

Special features inherent in the response of ordinary (fully bonded) and delaminated sandwich panels with a transversely flexible (“soft”) core subjected to external in-plane and vertical statical loads are analyzed. The analytical formulation is based on a higher-order theory for sandwich panels with non-rigid bond layers between the face sheets and the core. The central finite difference scheme is used for discretizing the continuous formulation. The deflated iterative Arnoldi scheme for solution of a large-scale generalized eigenvalue problem is employed, as well as the quasi-Newton global framework for the natural parameter and the arc-length continuation procedures. The numerical higher-order analysis reveals that the ordinary sandwich panel behaves as a compound structure in which the local/localized, overall or interactive forms of the response can take place depending on the geometry, mechanical properties, and boundary conditions of the structure. The non-sinusoidal modes confined to the support zones of the panel may occur at critical loads much lower than those predicted on the basis of presumed sinusoidal modes. Soft-core sandwich panels possess a complex branching behavior with limit points and secondary bifurcations. The thin-film-delamination approach used in the field of the composite plates is unsuitable for the analysis of delaminated sandwich panels and consideration of the interaction between the face sheets and the core is required. The complex response of the soft-core sandwich panels can be predicted only with the aid of the enhanced higher-order theory.     

Element-failure concepts for dynamic fracture and delamination in low-velocity impact of composites

An element-failure algorithm is proposed and incorporated into a finite element code for simulating dynamic crack propagation and impact damage in laminated composite materials. In this algorithm, when a crack is propagating within a finite element, the element is deemed to have partially failed, but not removed from the computations. Consequently, only a fraction of the stresses that were computed before the crack tip entered the element contribute to the nodal forces of the element. When the crack has propagated through the element, the element is completely failed and therefore can only resist volumetric compression. This treatment of crack propagation in isotropic solids allows fracture paths within individual elements and is able to accommodate crack growth in any arbitrary direction without the need for remeshing. However, this concept is especially powerful when extended to the modeling of damage and delamination in fibre-reinforced composite laminates. This is because the nature of damage in composite laminates is generally diffused, characterized by multiple matrix cracks, fibre pullout, fibre breakage and delaminations. It is usually not possible to define or identify crack tips in the tradition of fracture mechanics. Since parts of a damaged composite structure are often able to partially transmit load despite the presence of some damage, it is advantageous to model the damaged portions with partially failed elements. The damage may be efficiently modeled and tracked using element-failure concepts, with the application of appropriate failure criteria and damage evolution laws. The idea is to embody the effects of damage into the effective nodal forces of the finite element. In this paper, we report the novel use of element-failure concepts in the analysis of low-velocity impact damage of composite laminates. The initiation and propagation of delaminations arising from the impact are predicted and the results show qualitative agreement with experimental observation of the formation of multiple delaminations in impact-damaged specimens. While such delaminations do not permit transmission of tensile stress waves across the cracked surfaces, transmission of compressive stress waves are allowed in the simulation. It is further shown that, when elements are allowed to fail, the dynamic stress wave distributions are altered significantly. In the element-failure algorithm, the issue of interpenetration of delamination surfaces in the model does not arise. This is a significant advantage over the conventional method of explicitly modeling the delamination surfaces and crack front, where generally, much computational time must be spent in employing contact algorithms to ensure physically admissible solutions. Finally, we also demonstrate the simulation of crack propagation of pre-notched specimens of an isotropic material under initial conditions of mode II loading using the element-failure algorithm. The numerical results showed that the cracks propagated at an angle of about 70° with respect to the notches, in agreement with the experimental results of Kalthoff.     

泡沫铝夹层结构抗冲击性能的近场动力学模拟分析

针对某光学舱所采用的泡沫铝夹层防护结构在破片冲击下的抗冲击性能问题，采用Monte-Carlo方法创建了泡沫铝结构的二维细观模型，在常规态型近场动力学理论中引入了Mises屈服准则和线性各向同性强化模型，建立了近场动力学塑性本构的数值计算框架。基于近场动力学计算程序模拟了低速冲击作用下泡沫铝夹层结构的塑性变形以及有机玻璃背板的裂纹扩展形态，分析了泡沫铝芯材孔隙率对该夹层结构抗冲击性能和损伤模式的影响规律。结果表明：泡沫铝夹层结构良好的塑性变形能力是其发挥缓冲与防护作用的主要因素，并且在一定范围内，泡沫铝芯材孔隙率越高，则夹层结构具有更好的抗冲击性能；当泡沫铝孔隙率从0.4提升到0.7时，泡沫铝对冲击物的动能吸收率从90%提高到99%；模拟结果与实验结果具有较好的一致性，验证了模拟结果的准确性和分析结论的有效性。通过数值模拟，预测了有机玻璃背板的裂纹扩展形态，发现提高泡沫铝的孔隙率能获得更好的防护效果。     

NONLINEAR BUCKLING BEHAVIOR OF DAMAGED COMPOSITE SANDWICH PLATES CONSIDERING THE EFFECT OF TEMPERATURE-DEPENDENT THERMAL AND MECHANICAL PROPERTIES

On the basis of the first-order shear deformation plate theory and the zig-zag deformation assumption, an incremental finite element formulation for nonlinear buckling analysis of the composite sandwich plate is deduced and the temperature-dependent thermal and mechanical properties of composite is considered. A finite element method for thermal or thermo-mechanical coupling nonlinear buckling analysis of the composite sandwich plate with an interfacial crack damage between face and core is also developed. Numerical results and discussions concerning some typical examples show that the effects of the variation of the thermal and mechanical properties with temperature, external compressive loading, size of the damage zone and ply angle of the faces on the thermal buckling behavior are significant. Project supported by the National Natural Science Foundation of China (No. 59975013).     

Progressive failure analysis of tow-placed, variable-stiffness composite panels

The past developments on tow-placement technology led to the production of machines capable of controlling fibre tows individually and placing them onto the surface of a laminate with curvilinear topology. Due to the variation of properties along their surface, such structures are termed variable-stiffness composite panels.In previous experimental research tow-steered panels have shown increased buckling load capacity as compared with traditional straight-fibre laminates. Also, numerical analyses by the authors showed that first-ply failure occurs at a significant higher load level. The focus of this paper is to extend those analyses into the postbuckling progressive damage behaviour and final structural failure due to accumulation of fibre and matrix damage. A user-developed continuum damage model implemented in the finite element code ABAQUS^® is employed in the simulation of damage initiation and material stiffness degradation.In order to correctly predict the buckling loads of tow-steered panels under compression, it is of crucial importance to take into account the residual thermal stresses resulting from the curing process. Final failure of tow-steered panels in postbuckling is predicted to within 10% difference of the experimental results. Curvilinear-fibre panels have up to 56% higher strength than straight-fibre laminates and damage initiation is also remarkably postponed. Tow-steered designs also show more tolerance to central holes than traditional laminates.     

Metallic sandwich panels subjected to multiple intense shocks

The mechanical response and fracture of metal sandwich panels subjected to multiple impulsive pressure loads (shocks) were investigated for panels with honeycomb and folded plate core constructions. The structural performance of panels with specific core configurations under multiple impulsive pressure loads is quantified by the maximum transverse deflection of the face sheets and the core crushing strain at mid-span of the panels. A limited set of simulations was carried out to find the optimum core density of a square honeycomb core sandwich panels under two shocks. The panels with a relative core density of 4%–5% are shown to have minimum face sheet deflection for the loading conditions considered here. This was consistent with the findings related to the sandwich panel response subjected to a single intense shock. Comparison of these results showed that optimized sandwich panels outperform solid plates under shock loading. An empirical method for prediction of the deflection and fracture of sandwich panels under two consecutive shocks – based on finding an effective peak over-pressure – was provided. Moreover, a limited number of simulations related to response and fracture of sandwich panels under multiple shocks with different material properties were performed to highlight the role of metal strength and ductility. In this set of simulations, square honeycomb sandwich panels made of four steels representing a relatively wide range of strength, strain hardening and ductility values were studied. For panels clamped at their edge, the observed failure mechanisms are core failure, top face failure and tearing at or close to the clamped edge. Failure diagrams for sandwich panels were constructed which reveal the fracture and failure mechanisms under various shock intensities for panels subjected to up to three consecutive shocks. The results complement previous studies on the behavior and fracture of these panels under high intensity dynamic loading and further highlights the potential of these panels for development of threat-resistant structural systems.     

EXPERIMENTAL AND NUMERICAL STUDIES OF THE ANTI-PENETRATION PERFORMANCE OF SANDWICH PANELS WITH ALUMINUM FOAM CORES

Effects of face-sheet thickness and core thickness of sandwich panels, and shape of projectiles on the penetration resistance of sandwich panels were discussed, while typical penetration failure modes were presented. It was shown that the anti-penetration performance of sandwich panels was enhanced with the increase of face-sheet or core thickness; The penetration resistance of sandwich panels was shown to be strongest to blunt-shaped projectile impacts,weaker to hemispherical-nose-shaped projectile impacts, and weakest to conical-shaped projectile impacts. The corresponding numerical simulation was carried out using the finite element code LS-DYNA V970. Numerical results showed that the penetration time decreased with the increase of projectile impact velocity.