Study on the Collapse of Pin-Reinforced Foam Sandwich Panel Cores

New fabrication technologies now allow for hybrid sandwich structures, known as X-core, to be manufactured. The X-core panels consist of a pin reinforced polymer foam core with carbon fiber face sheets. Carbon fiber or metallic (Titanium/Steel) pins are inserted into the foam core in the out-of-plane direction and extend from face sheet to face sheet. The through thickness three-point simply supported bending behavior of these panels is used to evaluate the collapse characteristics of the panels. Explicit experimental observations are used to calibrate analytical energy balance models describing the panel collapse as a function of geometry and properties. The mechanical response of X-core sandwich panels is compared to current sandwich materials for material selection.     

碳纤维复合材料金字塔点阵结构制备工艺及力学性能研究

本文针对碳纤维复合材料点阵结构,从结构设计、制备工艺、平压性能、剪切性能等方面对其进行试验表征及理论模型研究.设计四种成型碳纤维复合材料金字塔点阵结构的思想,并采用一种新的制备工艺即预浸料二次成型工艺制备试样,试验结果表明,该工艺能最大程度发挥纤维增强潜力.通过实验揭示在平压载荷下杆件屈曲、杆件断裂、杆件分层脱胶失效机理,在剪切载荷下杆件屈曲、杆件分层、杆件脱胶失效机理,基于结构力学基础原理,建立相应理论模型,经过修正之后的理论模型均能较好预报典型载荷下力学性能.本文研究发现碳纤维复合材料金字塔点阵结构具有密度低、比强度大、比刚度高等优点,且芯子中具有大量空间,可以制备轻质多功能结构.     

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.     

双层夹芯复合材料结构横向冲击响应实验

采用玻璃纤维增强环氧树脂复合材料层合板作为内、外面板,以PVC泡沫作为芯材,构造了双层夹芯复合材料结构。采用落锤冲击实验,得到了冲击过程的撞击力历史;研究了在不同的冲击能量下,双层夹芯结构的冲击响应及内面板位置对双层夹芯结构冲击响应的影响。实验结果表明,内面板的引入及内面板的位置显著影响双层夹芯结构的撞击力历史,根据该撞击力历史可以优化设计出抗冲击性能优异的新型双层夹芯复合材料结构。     

Theoretical prediction on corrugated sandwich panels under bending loads

In this paper, an aluminum corrugated sandwich panel with triangular core under bending loads was investigated. Firstly, the equivalent material parameters of the triangular corrugated core layer, which could be considered as an orthotropic panel, were obtained by using Castigliano’s theorem and equivalent homogeneous model. Secondly, contributions of the corrugated core layer and two face panels were both considered to compute the equivalent material parameters of the whole structure through the classical lamination theory, and these equivalent material parameters were compared with finite element analysis solutions. Then, based on the Mindlin orthotropic plate theory, this study obtain the closed-form solutions of the displacement for a corrugated sandwich panel under bending loads in specified boundary conditions, and parameters study and comparison by the finite element method were executed simultaneously.     

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.     

Structural modeling of sandwich structures with lightweight cellular cores

An effective single layered finite element (FE) computational model is proposed to predict the structural behavior of lightweight sandwich panels having two dimensional (2D) prismatic or three dimensional (3D) truss cores. Three different types of cellular core topology are considered: pyramidal truss core (3D), Kagome truss core (3D) and corrugated core (2D), representing three kinds of material anisotropy: orthotropic, monoclinic and general anisotropic. A homogenization technique is developed to obtain the homogenized macroscopic stiffness properties of the cellular core. In comparison with the results obtained by using detailed FE model, the single layered computational model can give acceptable predictions for both the static and dynamic behaviors of orthotropic truss core sandwich panels. However, for non-orthotropic 3D truss cores, the predictions are not so well. For both static and dynamic behaviors of a 2D corrugated core sandwich panel, the predictions derived by the single layered computational model is generally acceptable when the size of the unit cell varies within a certain range, with the predictions for moderately strong or strong corrugated cores more accurate than those for weak cores. The project supported by the National Basic Research Program of China (2006CB601202), the National Natural Science Foundation of China (10328203, 10572111, 10572119, 10632060), the National 111 Project of China (B06024), the Program for New Century Excellent Talents in University (NCET-04-0958), the Open Foundation of State Key Laboratory of Structural Analysis of Industrial Equipment, and the Doctorate Foundation of Northwestern Polytechnical University.     

X状Z向碳pin增强泡沫夹层结构剪切刚度预报

综合考虑了面板对横向增强Z-pin的不同约束情况,结合空间网架结构和等效夹杂方法,提出了X状Z-pins增强泡沫夹层结构剪切刚度的预报模型,经实际计算后与已有的实验值和有限元模拟值比较,证明该方法具有足够的工程精度.计算表明,X状Z-pins能大幅度地提高泡沫夹层结构的剪切刚度,并具有良好的可设计性,可以通过改变Z-pin材料,Z-pin的体积分数、角度、直径等参数改变其力学性能.其中,Z-pin体积分数越大、拉伸模量越高、直径越大,Z-pins增强泡沫夹层结构的剪切刚度越高.     

Investigation of the Sandwich Plate Twist Test

Analysis and test results for the compliance of the sandwich plate twist test are presented. The analysis utilizes classical laminated plate theory (CLPT) and finite element analysis (FEA). It is shown that CLPT greatly underestimates the plate compliance, except when very stiff cores and compliant face sheets are used, as a result of transverse core shear deformation, not accounted for in this theory. Parametric studies are conducted using FEA to examine the influence of transverse shear moduli of the core and specimen dimensions on the plate compliance. The influences of indentation at load introduction and support locations, and overhang (oversized panel) are also examined. A test fixture is designed where two diagonally opposite corners of the panel are loaded, while the other two corners are supported to provide twisting deformation of the panel. Tests were conducted on square sandwich panels consisting of aluminum face sheets over various PVC foam cores. CLPT was found to greatly underestimate the experimental plate compliance. Finite element predictions of the plate compliance were in much closer agreement with the experimental data.     

Assessment of Non-adiabatic Behaviour in Thermoelastic Stress Analysis of Composite Sandwich Panels

Thermoelastic stress analysis (TSA) is used to derive the surface stresses in large sandwich structure panels with honeycomb core and carbon fibre face sheets. The sandwich panels are representative of those used for secondary aircraft structure. The panels were subjected to a pressure load, similar to that experienced in-service, using a custom designed test rig. To achieve the necessary adiabatic conditions for TSA, cyclic loading is regarded as an essential feature. As the panels were full-scale, the maximum loading frequency that could be imparted to the panels by the rig was 1 Hz, which is below the usual range recommended to achieve adiabatic behaviour. To assess the effectiveness of TSA at low frequencies two approaches to calibration are investigated and compared with the stress distribution obtained from independently validated FE models. The thermoelastic response was calibrated into stress data using thermoelastic constants derived experimentally from tensile strips of the sandwich panel face sheet material. It is shown that by using thermoelastic constants obtained from the tensile strips manufactured with the same lay-up as the sandwich panel face sheets, and at the same cyclic load frequency used in the full-scale tests, quantitative stress metrics can be derived from the TSA data. More significantly, a deeper insight into the importance of the thermal characteristics in TSA of laminated materials is provided. It is demonstrated that, for the material used in this work, it is possible to use the global material behaviour to obtain quantitative results when adiabatic conditions do not prevail.     

冲击波与破片对波纹杂交夹层板的联合毁伤数值研究

通过有限元软件LS-DYNA模拟了波纹杂交夹层板在冲击波与破片联合作用下的响应过程,研究了炸药当量、载荷类型和填充方式对波纹杂交夹层板变形与失效模式的影响,并与实体板、间隔板和波纹夹层板的抗联合毁伤性能进行了对比,讨论了波纹杂交夹层板的能量吸收特性。数值计算结果表明:与冲击波单独作用相比,破片群单独作用和冲击波与破片联合作用对结构造成的毁伤更为严重;当药量较小时,波纹夹层板和波纹杂交夹层板的抗联合毁伤性能优于实体板与间隔板,波纹杂交夹层板的抗联合毁伤性能从全填充、迎爆面填充到背爆面填充逐渐降低;当药量较大时,所有结构均产生破口失效;在能量耗散方面,冲击波单独作用时以波纹芯层吸能为主,破片群单独作用和冲击波与破片联合作用时以上面板吸能为主。     

多孔金属夹层板在冲击载荷作用下的动态响应

借助两种有限元软件ABAQUS和LS_DYNA, 模拟和分析了两种厚度不同的泡沫铝合金夹层板(三明治板)、方孔蜂窝形夹层板和波纹形夹层板在冲击载荷下的动态响应. 4种夹层板的单位面积密度相同,冲击载荷分别用泡沫铝子弹与不锈钢子弹模拟. 讨论了泡沫金属夹层板和格构式夹层板在不同冲击载荷作用下的变形机制,重点在于对夹层板的吸能特性及板内各部分吸能变化规律的探讨.研究结果表明: 在泡沫子弹冲击下,夹层板主要是通过自身变形来消耗子弹动能,并转化为自身内能. 厚度为22\,mm的泡沫金属夹层板吸收能量最多,底面变形最小,是结构性能最优的夹层板;在刚性子弹高速冲击穿透过程中,格构式夹层板的吸能性能比单位面积密度相同的泡沫金属夹层板的吸能性能更好. 波纹形夹层板的能量吸收能力在4种板中最高.      

Plasticity of formable all-metal sandwich sheets: Virtual experiments and constitutive modeling

The mechanical behavior of a metallic sandwich sheet material composed of two flat face sheets and two bi-directionally corrugated core layers is analyzed in detail. The manufacturing of the sandwich material is simulated to obtain a detailed unit cell model which accounts for the non-uniform thickness distribution and residual stresses associated with the stamping of the core layers. Virtual experiments are performed by subjecting the unit cell model to various combinations of bi-axial in-plane loading including the special cases of uniaxial tension, uniaxial compression, equi-biaxial tension and shear. The results demonstrate that the core structure’s contribution to the in-plane load carrying capacity of the sandwich sheet material is similar to that of the face sheets. The numerical results are also used to identify the effective yield surface and hardening response of both the core layer and the face sheets. An anisotropic yield function with linear pressure dependency is proposed to approximate the equal-plastic work surfaces for the core structure and face sheets. Furthermore, a new two-surface model with non-linear interpolation based on plastic work density is presented to describe the observed combined isotropic-distortional hardening of the core structure.     

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.     

Three-dimensional thermoelastic analysis of finite length laminated cylindrical panels with functionally graded layers

Based on the 3D thermoelasticity theory, the thermoelastic analysis of laminated cylindrical panels with finite length and functionally graded (FG) layers subjected to three-dimensional (3D) thermal loading are presented. The material properties are assumed to be temperature-dependent and graded in the thickness direction. The variations of the field variables across the panel thickness are accurately modeled by using a layerwise differential quadrature (DQ) approach. After validating the approach, as an important application, two common types of FG sandwich cylindrical panels, namely, the sandwich panels with FG face sheets and homogeneous core and the sandwich panels with homogeneous face sheets and FG core are analyzed. The effect of micromechanical modeling of the material properties on the thermoelastic behavior of the panels is studied by comparing the results obtained using the rule of mixture and Mori–Tanaka scheme. The comparison studies reveal that the difference between the results of the two micromechanical models is very small and can be neglected. Then, the effects of different geometrical parameters, material graded index and also the temperature dependence of the material properties on the thermoelastic behavior of the FG sandwich cylindrical panels are carried out.     

Influence of imperfections on the performance of metal foam core sandwich panels

Sandwich panels and beams are used in bending and compression dominated components. The retention of their load capacity in the presence of imperfections is a central consideration. To address this issue, sandwich beams with metallic foam cores have been tested in four-point bending following the introduction of imperfections, created by impressing the face sheets. Limit load expressions for face yielding, core shear, and indentation failure have been developed and used to construct failure mechanism maps. From these maps, specimen designs were determined. Imperfections were introduced by indenting to varying penetrations. The indents were located on both the compressive and tensile side of bending configurations. Experimental measurements of the load/deflection response are obtained and compared with finite element results.     

新型复合材料点阵结构的研究进展

复合材料点阵结构是一种具有轻质、高比强、高比刚以及多功能潜力的新型结构材料, 近几年受到国外学者的极大关注, 是新一代结构材料一体化的理想结构材料. 本文概述了点阵复合材料及结构的发展历程, 包括复合材料点阵结构的拓扑构型设计、制备工艺研究、力学性能表征、失效模式分析、预报模型评价等方面的工作, 并给出了复合材料点阵结构的力学性能、失效模式和理论数值模型汇总表以及修正后的材料强度与密度关系图. 同时, 本文对复合材料点阵结构可能应用的领域进行预测, 并对其未来发展进行了展望.      

The blast resistance of sandwich composites with stepwise graded cores

Shock tube experiments were performed to study the dynamic response of sandwich panels with E-Glass Vinyl Ester (EVE) composite face sheets and stepwise graded styrene foam cores. Two types of core configurations, with identical areal density, were subjected to the shock wave loading. The core layers were arranged according to the density of the respective foam; configuration 1 consisted of low/middle/high density foams and configuration 2 consisted of middle/low/high density foams. The method to calculate the incident and reflected energies of the shock wave, as well as the deformation energy of the specimen, were proposed based on the shock wave pressure profiles and the high speed deflection images that were obtained. The experimental results showed that configuration 1 outperformed configuration 2 in regards to their blast resistance. Significant core material compression was observed in configuration 1, while in configuration 2 the core layers disintegrated and the front skin (blast side) fractured into two pieces along the midsection. The estimated energies were then calculated for both configurations. The total energy difference between the incident and reflected energies was almost identical, even though the deformation energy for configuration 2 was larger.     

Analysis and active control of nonlinear vibration of composite lattice sandwich plates

This paper is devoted to investigate the nonlinear vibration characteristics and active control of composite lattice sandwich plates using piezoelectric actuator and sensor. Three types of the sandwich plates with pyramidal, tetrahedral and Kagome cores are considered. In the structural modeling, the von Kármán large deflection theory is applied to establish the strain–displacement relations. The nonlinear equations of motion of the structures are derived by Hamilton’s principle with the assumed mode method. The nonlinear free and forced vibration responses of the lattice sandwich plates are calculated. The velocity feedback control (VFC) and H_∞ control methods are applied to design the controller. The nonlinear vibration responses of the sandwich plates with pyramidal, tetrahedral and Kagome cores are compared. The influences of the ply angle of the laminated face sheets, the thicknesses of the lattice core and face sheets and the excitation amplitude on the nonlinear vibration behaviors of the sandwich plates are investigated. The correctness of the H_∞ control algorithm is verified by comparing with the experiment results reported in the literature. The controlled nonlinear vibration response of the sandwich plate is computed and compared with that of the uncontrolled structural system. Numerical results indicate that the VFC and H_∞ control methods can effectively suppress the large amplitude vibration of the composite lattice sandwich plates.

     

嵌锁式CFRP方形蜂窝夹芯梁低速冲击响应及失效机理

采用嵌锁组装工艺制备了碳纤维/树脂基复合材料方形蜂窝夹芯梁,实验研究了低速冲击载荷下简支和固支夹芯梁的动态响应及失效机理,获得了不同冲击速度下夹芯梁的失效模式,分析了其损伤演化过程和失效机理,探讨了冲击速度、边界条件、面板质量分布以及槽口方向等因素对夹芯梁破坏模式及承载能力的影响。研究结果表明,芯材长肋板槽口方向对夹芯梁的失效模式有较大影响,槽口向上的芯材跨中部分产生了挤压变形,而槽口向下的芯材跨中部分槽口在拉伸作用下出现了沿槽口开裂失效,继而引起面板脱粘和肋板断裂;同等质量下,较厚的上面板设计可以提高夹芯梁的抗冲击能力,冲击速度越大,夹芯梁的峰值载荷和承载能力越高;固支边界使得夹芯梁的后失效行为呈现出明显的强化效应,在夹芯梁跨中部分发生初始失效后出现了后继的固支端芯材和面板断裂失效模式。