Cutout reinforcements for shear loaded laminate and sandwich composite panels

This paper presents the numerical and experimental studies of shear loaded laminated and sandwich carbon/epoxy composite panels with cutouts and reinforcements aiming at reducing the cutout stress concentration and increasing the buckling stability of the panels. The effect of different cutout sizes and the design and materials of cutout reinforcements on the stress and buckling behaviour of the panels are evaluated. For the sandwich panels with a range of cutout size and a constant weight, an optimal ratio of the core to the face thickness has been studied for the maximum buckling stability. The finite element method and an analytical method are employed to perform parametric studies. In both constant stress and constant displacement shear loading conditions, the results are in very good agreement with those obtained from experiment for selected cutout reinforcement cases. Conclusions are drawn on the cutout reinforcement design and improvement of stress concentration and buckling behaviour of shear loaded laminated and sandwich composite panels with cutouts.     

等腰梯形蜂窝芯玻璃钢夹芯板的面内压缩性能

为研究等腰梯形蜂窝芯玻璃钢夹芯板面内压缩破坏机制, 利用材料试验机对夹芯板面内压缩性能进行了试验测试, 并开展了模拟研究。结果表明: 夹芯板的面内压缩破坏方式主要有面板折断、夹芯板屈曲失稳和夹芯板中面板与蜂窝芯脱粘3种类型。面板为夹芯板面内压缩的主要承载构件, 蜂窝芯对面板起到固支作用。面板结构参数与材料参数为影响夹芯板面内压缩抗压强度与抗压刚度主要因素, 多数蜂窝芯的结构参数与材料参数对夹芯板面内压缩抗压强度的影响微弱, 而个别蜂窝芯的结构参数对夹芯板面内压缩抗压刚度的影响比较显著。夹芯板体积一定时, 随着蜂窝芯胞体单元数量的增加, 夹芯板面内压缩的抗压强度与抗压刚度逐渐增大。      

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.     

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。      

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。     

Buckling,collapse and failure analysis of FRP sandwich panels

Rectangular orthotropic fiber-reinforced plastic (FRP) sandwich panels were tested for buckling in uni-axial compression. The panels, with 0.32 cm (0.125 in.) face sheets and a 1.27 cm (0.5 in.) core of either balsa or linear poly(vinyl chloride) (PVC) foam, were tested in two sizes: 154×77 cm² (72×36 in.²) and 102×77 cm² (48×36 in.²). The sandwich panels were fabricated using the vacuum-assisted resin transfer molding process. The two short edges of the sandwich panels were clamped, while the two long edges were simply supported for testing. The clamped panel ends were potted into a steel frame. The experimental elastic buckling loads were then measured using strain gauges fixed to both sides of the panels. A total of 12 panels were tested under uni-axial compression. Bifurcation in the load versus engineering strain curve was noted in all cases. For all six sandwich panels tested using balsa core, the type of failure was easily identified as face sheet delamination followed by core shear failure. For all six PVC foam core sandwich panels tested, the type of failure consisted of core shear failure with little or no face sheet delamination. In the failed balsa core panels there was little or no evidence of balsa remaining on the FRP face sheet, however, in the PVC foam core panels there were ample amounts of foam left on the FRP face sheet. It was concluded that although the buckling loads for the foam core panels were not as high as those for the balsa core panels, PVC foam core bonding to the FRP face sheets was superior to balsa core bonding.     

Parametric study of the post-buckling strength of structural core sandwich panels

Post-buckling strength of simply supported orthotropic corrugated board panels subjected to edge compressive loading has been investigated using geometrically non-linear finite element analysis (FEA). Adjustments of the transverse shear stiffnesses in the FEA were necessary and performed by comparing the critical buckling load calculated by FEA with a closed form solution. The collapse load of the sandwich plate was calculated based on material failure of the facings predicted from Tsai-Wu failure theory. Parametric studies were performed to investigate the sensitivity of the collapse load to changes in the transverse shear stiffnesses of the core, initial out-of-plane imperfections, asymmetry in board construction, slenderness ratio and eccentric loading of the plate. It was found that a reduction of the transverse shear stiffnesses of the core below a certain limit produces a significant reduction in the collapse load. Panels are said to be insensitive to imperfections and this holds true when the imperfections are the same as or lesser than the thickness of the panel, but a 40% reduction of the collapse load is observed for imperfections that are ten times the panel thickness. From a design point of view it is shown that a symmetrical board is preferred because an asymmetric board as well as eccentric loading of the panel significantly reduce the collapse load. It is also shown that the critical buckling load is directly related to the slenderness ratio of the panel whereas the collapse load is not.     

An analytical and experimental comparison of orthotropic sandwich panels using the Hydromat Test System

A new two-dimensional test system, called the Hydromat Test System, simulates the hydrostatic and hydrodynamic loading conditions which are often present in actual sandwich structures, such as marine hulls. The test fixture uses a square 24 inch×24 inch panel sample which is simply supported all around and has a distributed load provided by a water-filled bladder.

In this study, the Hydromat Test System has been used to obtain data on sandwich panels with orthotropic face sheets and isotropic cores. This data has been compared to analytical expressions for the deflection and the in-plane strains based on small deflection sandwich panel theory. The engineering constants needed for the analytical solution were obtained from characterization tests of the face sheet materials. Core shear properties were obtained experimentally using two different ASTM standards. Four panels, with two different core materials and two different face sheets, were tested. Face sheet properties varied from slightly orthotropic (plane weave) to highly orthotropic (unidirectional), with an axial to transverse tensile moduli ratio of 1.2 and 3.9, respectively. The cores were closed cell foams with both a low and a high shear stiffness.

The analytically obtained center panel deflection varied from 1 to 10% of that obtained by experiment. Most of the analytical tensile strains were less than 10% different from the measured ones. Both experimental deflection and strain data are in excellent agreement with the small deflection theory. It was concluded that the Hydromat Test System provides predictable and repeatable boundary conditions and loading mechanism and is a suitable method for testing soft cored, highly orthotropic sandwich panels.     

The effect of pin reinforcement upon the through-thickness compressive strength of foam-cored sandwich panels

Titanium and carbon fibre pins have been inserted into the polymethacrylimide foam core of a sandwich panel (with carbon fibre face sheets) in order to increase the through-thickness strength. The elevation in compressive strength has been measured both quasi-statically and dynamically using a direct Kolsky bar, and the sensitivity of strength to the relative density and thickness of foam have been determined. An X-ray CAT scan machine was used to examine the deformed shape of the pins during interrupted compression testing of the sandwich specimens. It was found that the foam core stabilises the pins against elastic buckling, and the pin-reinforced core has a strength and energy absorption capacity in excess of the individual contributions from the foam and unsupported pins. It is shown that the compressive strength is governed by elastic buckling of the pins, with the foam core behaving as an elastic Winkler foundation in supporting the pins. The peak strength of the pin-reinforced core is increased by a factor of about four when the speed of loading is increased from the quasi-static rate of about 10⁻⁶ ms⁻¹ to the dynamic value of 10 ms⁻¹; it is concluded that the micro-inertia of the pins stabilises them against elastic buckling and leads to the observed elevation in strength.     

Response of metallic pyramidal lattice core sandwich panels to high intensity impulsive loading in air

Small scale explosive loading of sandwich panels with low relative density pyramidal lattice cores has been used to study the large scale bending and fracture response of a model sandwich panel system in which the core has little stretch resistance. The panels were made from a ductile stainless steel and the practical consequence of reducing the sandwich panel face sheet thickness to induce a recently predicted beneficial fluid-structure interaction (FSI) effect was investigated. The panel responses are compared to those of monolithic solid plates of equivalent areal density. The impulse imparted to the panels was varied from 1.5 to 7.6 kPa s by changing the standoff distance between the center of a spherical explosive charge and the front face of the panels. A decoupled finite element model has been used to computationally investigate the dynamic response of the panels. It predicts panel deformations well and is used to identify the deformation time sequence and the face sheet and core failure mechanisms. The study shows that efforts to use thin face sheets to exploit FSI benefits are constrained by dynamic fracture of the front face and that this failure mode is in part a consequence of the high strength of the inertially stabilized trusses. Even though the pyramidal lattice core offers little in-plane stretch resistance, and the FSI effect is negligible during loading by air, the sandwich panels are found to suffer slightly smaller back face deflections and transmit smaller vertical component forces to the supports compared to equivalent monolithic plates.     

Compression-after-Impact Strength of Sandwich Panels with Core Crushing Damage

Compression-after-impact (CAI) strength of foam-cored sandwich panels with composite face sheets is investigated experimentally. The low-velocity impact by a semi-spherical (blunt) projectile is considered, producing a damage mainly in a form of core crushing accompanied by a permanent indentation (residual dent) in the face sheet. Instrumentation of the panels by strain gauges and digital speckle photography analysis are used to study the effect of damage on failure mechanisms in the panel. Residual dent growth inwards toward the mid-plane of a sandwich panel followed by a complete separation of the face sheet is identified as the failure mode. CAI strength of sandwich panels is shown to decrease with increasing impact damage size. Destructive sectioning of sandwich panels is used to characterise damage parameters and morphology for implementation in a finite element model. The finite element model that accounts for relevant details of impact damage morphology is developed and proposed for failure analysis and CAI strength predictions of damaged panels demonstrating a good correlation with experimental results.     

Graded conventional-auxetic Kirigami sandwich structures: Flatwise compression and edgewise loading

The work describes the manufacturing and testing of graded conventional/auxetic honeycomb cores. The graded honeycombs are manufactured using Kevlar woven fabric/914 epoxy prepreg using Kirigami techniques, which consist in a combination of Origami and ply-cut processes. The cores are used to manufacture sandwich panels for flatwise compression and edgewise loading. The compressive modulus and compressive strength of stabilized (sandwich) honeycombs are found to be higher than those of bare honeycombs, and with density-averaged properties enhanced compared to other sandwich panels offered in the market place. The modulus and strength of graded sandwich panel under quasi-static edgewise loading vary with different failure mode mechanisms, and offer also improvements towards available panels from open literature. Edgewise impact loading shows a strong directionality of the mechanical response. When the indenter impacts the auxetic portion of the graded core, the strong localization of the damage due to the negative Poisson’s ratio effect contains significantly the maximum dynamic displacement of the sandwich panel.     

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。      

Global buckling of composite structural insulated wall panels

This paper presents a new type of composites structural insulated panels (CSIPs) for structural wall applications. The proposed composite panel is made of low-cost thermoplastic orthotropic glass/poly-propylene (glass–PP) laminate as a facesheet and expanded polystyrene foam (EPS) as a core with very high facesheet/core moduli ratio. The proposed CSIP walls are intended to overcome problems of traditional Structural Insulated Panels (SIPs) such as termite attack, mold buildups and poor penetration resistance against wind borne debris. This paper investigates the behavior of CSIPs under concentric and eccentric loading. CSIPs specimens failed by global buckling mode in which no debonding was observed. The eccentric specimens failed at load 35% lower than that of the concentric ones. Global buckling formulas for concentric and eccentric loading were presented and validated using the experimental results and were in a good agreement. An equivalent stiffness formula was also developed for sandwich wall under in-plane loading considering the shear deformations effect of the core. Design study for CSIP walls is also presented to help in designing this new type of composite panels.     

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.     

Experimental,Theoretical and Numerical Investigation of the Flexural Behaviour of the Composite Sandwich Panels with PVC Foam Core

This study presents the main results of an experimental, theoretical and numerical investigation on the flexural behaviour and failure mode of composite sandwich panels primarily developed for marine applications. The face sheets of the sandwich panels are made up of glass fibre reinforced polymer (GFRP), while polyvinylchloride (PVC) foam was used as core material. Four-point bending test was carried out to investigate the flexural behaviour of the sandwich panel under quasi static load. The finite element (FE) analysis taking into account the cohesive nature of the skin-core interaction as well as the geometry and materials nonlinearity was performed, while a classical beam theory was used to estimate the flexural response. Although the FE results accurately represented the initial and post yield flexural response, the theoretical one restricted to the initial response of the sandwich panel due to the linearity assumptions. Core shear failure associate with skin-core debonding close to the loading points was the dominant failure mode observed experimentally and validated numerically and theoretically.     

含不同形状分层缺陷蜂窝夹层板的压缩性能

对含面板/夹芯界面中央分层缺陷复合材料蜂窝夹层板的压缩性能进行了试验研究和理论分析,考察了一种圆形分层和2种矩形分层缺陷对其压缩强度的影响,并采用子层局部屈曲模型对压缩强度进行了计算。结果表明：无缺陷夹层板表现为总体失稳破坏,而对于含分层缺陷的夹层板,则视分层形状及其大小的不同而表现出不同的破坏机制。对于矩形缺陷的长边与载荷方向垂直的夹层板,一般情况下面板子层局部屈曲对夹层板的最终破坏不起控制作用;对于矩形缺陷的长边与载荷方向平行的夹层板,表现为总体失稳破坏。压缩破坏过程中,面板子层屈曲起控制作用的夹层板,子层局部屈曲模型能够比较精确地预测其压缩强度。     

Buckling analysis of rectangular composite plates with rectangular cutout subjected to linearly varying in-plane loading using fem

A numerical study is carried out using finite element method, to examine the effects of square and rectangular cutout on the buckling behavior of a sixteen ply quasi-isotropic graphite/epoxy symmetrically laminated rectangular composite plate [0°/+45°/-45°/90°]_2s, subjected to various linearly varying in-plane compressive loads. Further, this paper addresses the effects of size of square/rectangular cutout, orientation of square/rectangular cutout, plate aspect ratio(a/b), plate length/thickness ratio(a/t), boundary conditions on the buckling bahaviour of symmetrically laminated rectangular composite plates subjected to various linearly varying in-plane compressive loading. It is observed that the various linearly varying in-plane loads and boundary conditions have a substantial influence on buckling strength of rectangular composite plate with square/rectangular cutout.