Fabrication and electromagnetic characteristics of electromagnetic wave absorbing sandwich structures

The radar absorbing structures (RAS) having sandwich structures in the X-band (8.2–12.4 GHz) frequencies were designed and fabricated. We added conductive fillers such as carbon black and multi-walled carbon nanotube (MWNT) to composite prepregs and polyurethane foams so as to efficiently increase the absorbing capacity of RAS. In order to improve the mechanical stiffness of RAS, we adopted the sandwich structures made of composite face sheets and foam cores. Glass fabric/epoxy composites containing conductive carbon black and carbon fabric/epoxy composites were used for the face sheets. Polyurethane foams containing MWNT were used as the core material. Their permittivity in the X-band was measured using the transmission line technique. The reflection loss characteristics for multi-layered sandwich structures were calculated using the theory of transmission and reflection in a multi-layered medium. Three kinds of specimens were fabricated and their reflection losses in the X-band were measured using the free space technique. Experimental results were in good agreement with simulated ones in 10-dB absorbing bandwidth.     

Localized indentation of sandwich panels with metallic foam core: Analytical models for two types of indenters

Sandwich structures with metallic foam core are sensitive to local indentation because of the low strength of the core and low bending stiffness of the thin face sheets. In this paper, local indentation response of sandwich panels with metallic foam core under a flat/spherical indenter was analyzed. The composite sandwich is modeled as an infinite, isotropic, plastic membrane on a rigid-plastic foundation. For simplicity, a quadratic polynomial displacement field was employed to describe the deformation of the upper face sheet. By using the principle of minimum work, explicit solutions for the indentation force and the sizes of the deformation regions were derived. The analytical results were verified by those from simulation by using the ABAQUS code, and they are in close agreement. Distribution of radial tensile strain of the upper face sheet and the ratio of energy dissipation of foam core to that of the upper face sheet were analyzed.     

Moisture diffusion in composite sandwich structures

The mechanical properties of polymer core materials in sandwich structures are often degraded by moisture that is absorbed during storage. To date, there is no reliable model to predict the amount of moisture that is present in these sandwich core materials. A multi-layer diffusion model applicable to these sandwich structures is described in this report. Inputs to this model are: (1) diffusivities of core and face sheet materials as functions of temperature, (2) moisture saturation data as a function of relative humidity, and (3) sandwich structure exposure history. The output is a prediction of the amounts of moisture in the core material and face sheets as a function of time.

In order to validate this model, moisture diffusion experiments were performed on a sandwich material consisting of graphite–epoxy face sheets and a core of Rohacell^® polymethacrylimide 200WF foam. Samples of this material were dried, and then hydrated at either 32 °C or 65 °C at either 83% or 100% relative humidity. The face sheets were separated from the core and each component was weighed, dried, and weighed again in order to determine the moisture distribution in the sandwich structure. The results were then compared with the model predictions.     

钢板夹泡沫铝组合板抗爆性能研究

鉴于泡沫铝材料优异的吸能特性和夹层结构在强度、刚度上的优势,提出了分层结构为钢板-泡沫铝芯层-钢板的抗爆组合板。对厚度为10 cm、7 cm和5 cm的组合板进行了5组不同装药量的爆炸试验,考察了各板在不同装药量爆炸条件下的变形及破坏情况,并对变形破坏过程进行了理论分析。研究表明:组合板承受爆炸冲击荷载时,通过局部压缩变形和整体弯曲变形吸收能量。钢板相同时,适当增大泡沫铝芯层厚度,增强面板与芯层间连接,可提高该组合板的抗爆性能,防止组合板发生剥离,减小其承受爆炸冲击荷载时产生的变形。     

Experimental investigation of compression failure of sandwich specimens with face/core debond

An experimental study of the in-plane compressive failure mechanism of foam cored sandwich specimens with an implanted through-width face/core debond is presented. Tests were conducted on sandwich specimens with glass/vinylester and carbon/epoxy face sheets over various PVC foam cores. Observation of the response of the specimens during testing showed that failure occurred by buckling of the debonded face sheet, followed by rapid debond growth towards the ends of the specimen. The compression strength of the sandwich specimens containing a debond decreased quite substantially with increasing debond size. A high-density core resulted in less strength decrease at any given debond size. Examination of the failure surfaces after separation of the face sheet and core revealed traces of core material deposited on the face sheet evidencing cohesive core failure. The amount of core material adhered to the face sheet decreased with increasing foam density indicating increasing tendency for core/resin interfacial failure.     

全厚度缝合复合材料泡沫芯夹层结构力学性能研究与损伤容限评定

探索了全厚度缝合复合材料闭孔泡沫芯夹层结构低成本制造的工艺可行性及其潜在的结构效益。选用3 种夹层结构形式, 即相同材料和工艺制造的未缝合泡沫芯夹层和缝合泡沫芯夹层结构及密度相近的Nomex 蜂窝夹层结构, 完成了密度测定、三点弯曲、平面拉伸和压缩、夹层剪切、结构侧压和损伤阻抗/ 损伤容限等7 项实验研究。结果表明, 泡沫芯夹层结构缝合后, 显著提高了弯曲强度/ 质量比、弯曲刚度/ 质量比、面外拉伸和压缩强度、剪切强度和模量、侧压强度和模量、冲击后压缩(CAI) 强度和破坏应变。这种新型结构形式承载能力强、结构效率高、制造维护成本低, 可以在飞机轻质机体结构设计中采用。      

Dynamic response of anisotropic sandwich flat panels to explosive pressure pulses

This paper addresses the problem of the dynamic response in bending of flat sandwich panels exposed to time-dependent external blast pulses. The study is carried out in the context of an advanced model of sandwich structures that is characterized by anisotropic laminated face sheets and an orthotropic core layer, and a closed form solution of the dynamic response to a variety of blast pulses is provided. A detailed analysis of the influence of a large number of parameters associated with the particular type of pressure pulses, panel geometry, fiber orientation in the face sheets and, presence of tensile uni/biaxial edge loads on dynamic response is carried out, and pertinent conclusions are outlined.     

Composite wrist blocks for double arm type robots for handling large LCD glass panels

Recently, robot structures handling liquid crystal display (LCD) glass panels are increased in size as the size of LCD is increased. In order to handle large LCD panels, the robot structures should have high stiffness to reduce the deflection of robot end effector under the weights of LCD. The LCD manufacturing industries have a trend to employ double arm type robots rather than single arm type robots to increase productivity. Currently, two aluminum wrist blocks that have different configurations not to interfere with each other are mounted on the robot arms. The aluminum wrist block becomes one of the largest deflection sources as the size of the robot structures increases. Since the size of the wrist block can not be increased indefinitely to increase the stiffness due to the limitation of driving motor power, the best way to increase the stiffness of the wrist block is to employ carbon fiber epoxy composite material for structural material of the wrist block because the carbon fiber epoxy composite material has much higher specific stiffness and damping than aluminum. In this work, the two wrist blocks for the double arm type robot for handling large LCD glass panels were designed and manufactured using foam core sandwich structure. Finite element analysis was used along with an optimization routine to design the composite wrist blocks. Box type sandwich structures were employed to reduce shear effect arising from the low modulus of polyurethane foam core. The weight reduction of the composite wrist blocks was more than 50% compared to those of comparable aluminum wrist blocks and found that the composite wrist blocks had much improved performances compared to those of the aluminum wrist blocks from the static and dynamic tests.     

Non-linear indentation behavior of foam core sandwich composite materials—A 2D approach

Light weight high performance sandwich composite materials have been used more and more frequently in various load bearing applications in recent decades. However, sandwich materials with thin composite face sheets and a low density foam core are notoriously sensitive to failure by localized external loads. These loads induce significant local deflections of the loaded face sheet into the core of the sandwich composite material, thus causing high stress concentrations. As a result, a complex multiaxial stressed and strained state can be obtained in the area of localized load application. Another important consequence of the highly localized external loads is the formation of a residual dent in the face sheet (a geometrical imperfection) that can reduce significantly the post-indentation load bearing capacity of the sandwich structure.This paper addresses the elastic–plastic response of sandwich composite beams with a foam core to local static loading. The study deals with a 2D configuration, where a sandwich beam is indented by a steel cylinder across the whole width of the specimen. The ABAQUS finite element package is used to model the indentation response of the beams. Both physical and geometrical non-linearities are taken into account. The plastic response of the foam core is modeled by the ¹CRUSHABLE FOAM and the ¹CRUSHABLE FOAM HARDENING option of the ABAQUS code. The purpose of the numerical modeling is to develop correct 2D simulations of the non-linear response in order to further understand the failure modes caused by static indentation. In order to verify the finite element model, indentation tests are performed on sandwich composite beams using a cylindrical indentor. The numerical results show good agreement with experimental test data.     

Mechanical behavior of sandwich panels with hollow Al–Si tubes core construction

A new type of lightweight sandwich panels consisting of vertically aligned hollow Al–Si alloy tubes as core construction and carbon fiber composite face sheets was designed. The hollow Al–Si alloy tubes were fabricated using precision casting and were bonded to the face sheets using an epoxy adhesive. The out-of-plane compression (i.e. core crushing), in-plane compression, and three-point bending response of the panels were tested until failure. The hollow Ai–Si alloy tubes core configuration show superior specific strength under crushing compared to common metallic and stochastic foam cores. Under in-plane compression and three-point bending, the buckling of face sheets and debonding of hollow cores from the face sheets were observed. Simple analytical relationships based on the concepts of mechanics of materials were provided for the compression tests, which estimate the sandwich panels’ strength with high fidelity. For three-point bending, detailed finite element analysis was used to model the response and initial failure of the sandwich panels.     

Mechanical degradation of foam-cored sandwich materials exposed to high moisture

Sandwich specimens composed of E-glass/polyester face sheets bonded to a PVC foam core were exposed to high moisture (95% RH) and immersed in sea-water for extended periods of time. Degradation of mechanical properties of the face sheets, foam core and face/core interface were progressively evaluated using flexural testing of the laminates, through-thickness tension of the foam core and interfacial sandwich DCB fracture testing. Testing reveals substantial flexural stiffness and strength reductions for the laminated composites, and only minor reduction in the tensile stiffness and strength of the foam. Degradation of the interfacial face/core fracture toughness is weak for specimens subjected to elevated moisture and more pronounced for sandwich specimens immersed in sea-water. After 30 days of exposure to high moisture, foam damage is visible in the form of cracks and pits on the cell walls. Optical examinations of expansional strains show that moisture absorbed by the foam penetrates only about to 2–3 mm from the core free surface for the 95% RH condition, while penetrates deeply for the immersed condition.     

Experimental determination of torsion and shear properties of sandwich panels and laminated composites by the plate twist test

A recently developed sandwich plate twist test is employed here for determination of the transverse shear modulus of the core and twist stiffness (D₆₆) of a sandwich panel consisting of a soft (H45 PVC foam) core and glass/vinylester face sheets. The shear modulus of the H45 PVC foam core extracted from the twist test was in good agreement with shear modulus obtained from ASTM plate shear testing of the foam core. D₆₆ values obtained from the sandwich twist test were in good agreement with predictions from classical laminated plate theory. In addition, the twist test was used to determine the in-plane shear modulus of glass/vinylester laminates isolated and as face sheets in sandwich panels with a stiff (plywood) core. The in-plane shear modulus of the face sheets, isolated and as part of a sandwich panel, was in good agreement with shear modulus determined using the Iosipescu shear test. The results point to the potential of the twist test to determine both in-plane and out-of-plane shear moduli of the constituents of a sandwich structure, as well as D₆₆.     

泡沫铝衰减冲击波峰值压力的理论及数值分析

为比较系统地了解表面粘贴泡沫铝及其夹芯层对结构上作用冲击波峰值压力的衰减性能与影响因素,运用理论及数值模拟方法分析了泡沫铝及其夹芯层衰减冲击波峰值压力的性能。并讨论了影响泡沫铝及其夹芯层衰减冲击波峰值压力的几个主要因素。研究结果显示,在达到压实应变之前,表面粘贴泡沫铝及其夹芯层能有效地衰减冲击波的峰值压力。达到压实应变后,泡沫铝及其夹芯层对冲击波峰值压力的衰减性能下降。孔洞形式、相对密度对泡沫铝衰减冲击波峰值压力具有明显地影响,面板材料对泡沫铝夹芯层衰减冲击波峰值压力的性能也有一定的影响。要取得较好地衰减冲击波峰值压力的性能需综合考虑以上因素进行优化设计,否则可能出现粘贴的泡沫铝或其夹芯层达不到衰减结构上冲击波峰值压力的目的。     

钢板夹泡沫铝组合板抗爆性能数值模拟

鉴于泡沫铝材料良好的吸能特性和三明治型组合构件在强度、刚度上的优势,通过有限元分析软件ANSYS/LS-DYNA对钢板-泡沫铝-钢板三明治型组合板进行了装药量为10.0kgTNT的非接触爆炸数值模拟,考察组合板在爆炸荷载作用下的动力响应。研究表明:钢板夹泡沫铝组合板承受爆炸冲击波荷载时,响应方式主要为组合板整体弯曲变形和泡沫铝芯层局部压缩变形,芯层压缩变形是组合板吸收耗散能量的主要途径;适当地增加泡沫铝芯层厚度和面板厚度能够提高组合板的抗爆性能,同时使组合板充分发挥耗能作用。     

Compressive and shear buckling analysis of metal matrix composite sandwich panels under different thermal environments

Combined inplane compressive and shear buckling analysis was conducted on flat rectangular sandwich panels using the Rayleigh-Ritz minium energy method with a consideration of transverse shear effect of the sandwich core. The sandwich panels were fabricated with titanium honeycomb core and laminated metal matrix composite face sheets. The results show that slightly slender (along the unidirectional compressive loading axis) rectangular sandwich panels have the most desirable stiffness-to-weight ratios for aerospace structural applications; the degradation of buckling strength sandwich panels with rising temperature is faster in shear than in compression; and the fiber orientation of the face sheets for optimum combined-load buckling strength of sandwich panels is a strong function of both loading condition and panel aspect ratio. Under the same specific weight and panel aspect ratio, a sandwich panel with metal matrix composite face sheets has a much higher buckling strength than one having monolithic face sheets.     

Hybrid carbon fiber composite lattice truss structures

Carbon fiber reinforced polymer (CFRP) composite sandwich panels with hybrid foam filled CFRP pyramidal lattice cores have been assembled from linear carbon fiber braids and Divinycell H250 polymer foam trapezoids. These have been stitched to 3D woven carbon fiber face sheets and infused with an epoxy resin using a vacuum assisted resin transfer molding process. Sandwich panels with carbon fiber composite truss volumes of 1.5–17.5% of the core volume have been fabricated, and the through-thickness compressive strength and modulus measured, and compared with micromechanical models that establish the relationships between the mechanical properties of the core, its topology and the mechanical properties of the truss and foam. The through thickness modulus and strength of the hybrid cores is found to increase with increasing truss core volume fraction. However, the lattice strength saturates at high CFRP truss volume fraction as the proportion of the truss material contained in the nodes increases. The use of linear carbon fiber braids is shown to facilitate the simpler fabrication of hybrid CFRP structures compared to previously described approaches. Their specific strength, moduli and energy absorption is found to be comparable to those made by alternative approaches.     

Experiments on curved sandwich panels under blast loading

In this paper curved sandwich panels with two aluminium face sheets and an aluminium foam core under air blast loadings were investigated experimentally. Specimens with two values of radius of curvature and different core/face sheet configurations were tested for three blast intensities. All the four edges of the panels were fully clamped. The experiments were carried out by a four-cable ballistic pendulum with corresponding sensors. Impulse acting on the front face of the assembly, deflection history at the centre of back face sheet, and strain history at some characteristic points on the back face were obtained. Then the deformation/failure modes of specimens were classified and analysed systematically. The experimental data show that the initial curvature of a curved sandwich panel may change the deformation/collapse mode with an extended range for bending dominated deformation, which suggests that the performance of the sandwich shell structures may exceed that of both their equivalent solid counterpart and a flat sandwich plate.     

Fatigue crack growth and life prediction of foam core sandwich composites under flexural loading

Fatigue crack growth of foam core sandwich beams loaded in flexure has been investigated. Sandwich panels were manufactured using an innovative co-injection resin transfer molding process. S2-glass fiber with epoxy resins was used as face sheets over a PVC foam core. Testing was performed in a three-point flexure mode utilizing a newly designed fixture such that the localized indentation damage was minimal. Extensive fatigue data were generated for the S–N diagram and crack growth was monitored to develop a model for life prediction. The first visible sign of damage initiation was a core–skin debond parallel to the beam axis. This debond propagated slowly along the top interface and eventually kinked into the core as shear crack and then grew in an unstable manner resulting in total specimen collapse. A fatigue model based on this crack growth has been developed and validated with experiments.