Insight into the shear behaviour of composite sandwich panels with foam core

While sandwich construction offers well-known advantages for high stiffness with light weight, the problem of designing the sandwich structure to withstand shear loading remains an important problem. This problem is more difficult with lower stiffness foam cores under high shear loading because the core is typically the weakest component of the structure and is the first one to fail in shear under the assuming of perfect contact between the skin and the foam core. In the present study, the shear response of the composite sandwich panels with Polyvinylchloride (PVC) foam core was investigated. The PVC H100 foam core is sandwiched between Glass Fiber Reinforced Polymer (GFRP) skins using epoxy resin to build a high performance sandwich panel to be investigated. Experiments have been carried out to characterise the mechanical response of the constituent materials under tension, compression and shear loading. Static shear tests for the sandwich panel reveal that the main failure mode is the delamination between the skin and the core rather than shearing the core itself due to the considerable value of the shear strength of the PVC foam. The Finite Element Analysis (FEA) of the sandwich structure shows that shear response and failure mode can be predicted, but that accurate predictions require a consideration of the non-linear response of the foam core. The results have a direct application in predicting the ability of the sandwich structure to withstand the shear loading.     

The influence of core properties on the perforation resistance of sandwich structures – An experimental study

The aim of this study is to investigate the perforation resistance of a range of foam-based sandwich structures. Nine foams, based on a crosslinked PVC, a linear PVC and PET, have been combined with thin glass fibre reinforced plastic skins to produce a range of lightweight sandwich structures, Initially, the mechanical properties of the different foams are characterised. Here, a new test geometry is used to evaluate the toughness characteristics and strengths of the foams under shear loading, a condition similar to that encountered during the impact perforation event.The influence of the plastic collapse stress of the foam in determining the failure thresholds of the front and rear composite skins is established. Here, an existing model has been used to successfully predict failure of the top surface composite skin in the sandwich structures. In addition, the force associated with perforating the lightweight core has been shown to be strongly dependent on the shear strength of the polymer foam. Finally, the perforation resistance of the sandwich structure has been shown to be closely linked to the Mode II work of fracture of the foam material. Here a unique relationship has been established between these two parameters, with all of the experimental points lying on one curve.     

Design of sandwich structures for concentrated loading

While sandwich construction offers well-known advantages for high stiffness with light weight, the problem of designing the sandwich structure to withstand localized loading, such as from accidental impact, remains an important problem. This problem is more difficult with lower stiffness cores, such as expanded foam. In the present study, experiments have been carried out on foam core sandwich beams with carbon/epoxy faces, under conditions of concentrated loading. The variables considered were the density of the foam and the relative thickness of the core. The common failure modes of sandwich structures were observed, including core failure in compression and shear, delamination, and fiber failure in the faces. These failure modes were systematically related to the test variables by means of a detailed stress analysis of the specimen, and a consideration of the failure properties of the constituent materials. The loading is characterized by localized high stress and strain concentrations that are not predicted in first-order shear deformation sandwich beam theory. The three-dimensional elasticity solution of Pagano was used to obtain the stress distributions. The strength prediction requires a detailed consideration of the localized nature of the loading, including the effects of strain gradients in the faces. The results show that failure modes and load levels can be predicted for sandwich structures under concentrated loading, but that accurate predictions require a consideration of the details of the concentrated loading. The results have a direct application in predicting the ability of sandwich structures to withstand localized loading such as from accidental impact.     

Compression, flexure and shear properties of a sandwich composite containing defects

The mechanical properties of a sandwich composite containing interfacial cracks or impact damage are compared when loaded in edgewise compression, flexure or shear. The composite is made from glass fibre reinforced polymer (GFRP) laminate skins over a core of foamed poly vinyl chloride (PVC), and this sandwich material is used in some naval minehunting ships. The properties are reduced with increasing interfacial crack or impact damage length, but only when the defects cause a change in the failure mode, which is dependent on the load state. The principal failure modes under the different load states are compared. The properties are also dependent on the severity of impact damage, with low energy damage to the skin having a smaller effect on stiffness and strength than high energy impacts which damage both the skin and foam core. The implications of these findings on the structural integrity of a minehunting ship made from GFRP/PVC foam sandwich composite is discussed.     

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。      

Flexural behaviour of structural fibre composite sandwich beams in flatwise and edgewise positions

The flexural behaviour of a new generation composite sandwich beams made up of glass fibre-reinforced polymer skins and modified phenolic core material was investigated. The composite sandwich beams were subjected to 4-point static bending test to determine their strength and failure mechanisms in the flatwise and the edgewise positions. The results of the experimental investigation showed that the composite sandwich beams tested in the edgewise position failed at a higher load with less deflection compared to specimens tested in the flatwise position. Under flexural loading, the composite sandwich beams in the edgewise position failed due to progressive failure of the skin while failure in the flatwise position is in a brittle manner due to either shear failure of the core or compressive failure of the skin followed by debonding between the skin and the core. The results of the analytical predictions and numerical simulations are in good agreement with the experimental results.     

Size effect on strength of laminate-foam sandwich plates: Finite element analysis with interface fracture

Recent three-point bend tests of size effect on the strength of geometrically scaled sandwich beams of three types – with no notches, and with notches at the upper or lower skin–foam interface, which were previously evaluated using simplified sandwich beam theory and equivalent linear elastic fracture mechanics, are now reanalyzed more accurately by finite elements. Zero-thickness interface elements with a softening cohesive law are used to model fractures at the skin–foam interface, in the fiber composite skins, and in the foam. The fracture energy and fracture process zone length of a shear crack in foam near the interface are deduced by fitting an analytical expression for size effect to the test data. Numerical simulations reveal that small-size specimens with notches just under the top skin develop plastic zones in the foam core near the edges of the loading platen, and that small-size specimens with notches just above the bottom skin develop distributed quasibrittle fracture in the foam core under tension. Both phenomena, though, are found to reduce the maximum load by less than 6%. Further it is shown that, in notch-less beams, the interface shear fracture is coupled with compression crushing of the fiber–polymer composite skin. For small specimens this mechanism is important because, when it is blocked in simulations, the maximum load increases. The size effect law for notch-less beams is calibrated such that beams of all sizes fail solely by interface shear fracture.     

FLEXURAL FATIGUE CHARACTERISTICS OF FRP SANDWICH BEAMS

The flexural fatigue characteristics of FRP sandwich beams are investigated. The skins of the beams are made from hybrid glass-aramid fibres set in epoxy resin and the core material is made from linear PVC foam. The beams under investigation have practical applications in marine operations.
The applied load is uniformly distributed throughout the length of the beam which is simply supported.
From both static and fatigue tests, it is revealed that the failure occurred in the core due to excessive shear, in turn, resulting in large deflections. An empirical expression is derived to postulate a failure criterion.     

Effects of the core density on the quasi-static flexural and ballistic performance of fibre-composite skin/foam-core sandwich structures

Polymeric foams are extensively used as the core materials in sandwich structures and the core material is typically bonded between relatively thin fibre-composite skins. Such sandwich structures are widely used in the aerospace, marine and wind-energy industries. In the present work, various sandwich structures have been manufactured using glass-fibre-reinforced polymer (GFRP) skins with three layers of poly(vinyl chloride) foam to form the core, with the densities of the foam layers ranging from 60 to 100 kg/m³. This study has investigated the effects on the quasi-static flexural and high-velocity impact properties of the sandwich structures of: (a) the density of the polymeric-foam core used and (b) grading the density of the foam core through its thickness. The digital image correlation technique has been employed to quantitatively measure the values of the deformation, strain and onset of damage. Under quasi-static three-point and four-point bend flexural loading, the use of a low-density layer in a graded-density configuration reduced the likelihood of failure of the sandwich structure by a sudden force drop, when compared with the core configuration using a uniform (i.e. homogenous) density layer. The high-velocity impact tests were performed on the sandwich structures using a gas-gun facility with a compliant, high-density polyethylene projectile. From these impact experiments, the graded-density foam core with the relatively low-density layer located immediately behind the front (i.e. impacted) GFRP skin was found to absorb more impact energy and possess an increased penetration resistance than a homogeneous core structure.     

齿板-玻璃纤维混合夹层结构弯曲性能试验

提出了一种齿板-玻璃纤维混合面板和泡沫芯材组成的新型混合夹层结构,齿板通过齿钉与泡沫芯材相连。该结构采用真空导入成型工艺制备,通过三点弯曲试验研究该结构在不同跨度以及不同芯材密度情况下的破坏模式和弯曲性能,并与普通泡沫夹层结构进行对比分析,同时探究了齿板对该结构界面性能的影响。结果表明:在泡沫芯材密度为35kg/m~3、80kg/m~3和150kg/m~3情况下,齿板-玻璃纤维混合泡沫夹层梁弯曲承载能力与普通泡沫夹层梁相比分别提高了168%、211%和258%,其界面剪切强度依次为0.09 MPa、0.21 MPa和0.45 MPa;随着芯材密度和跨度的变化,该结构主要产生芯材剪切和芯材凹陷两种破坏形态,齿板的嵌入有效抑制界面的剪切失效。另外,利用理论公式估算了试件受弯极限承载能力,理论值与实测值吻合较好。     

Approximate elastic-plastic analysis of the static and impact behaviour of polymer composite sandwich beams

An elastic-plastic beam bending model has been developed to simulate the post-upper skin failure energy absorption behaviour of polymer composite sandwich beams under three-point bending. The beam skins consist of woven and chopped strand glass, while the core is a resin impregnated non-woven polyester material known as Coremat. A polyester resin was used for the construction. The theoretical model consists of a central hinge dominated by a crushing core and tensile elastic strains in the lower skin. Experimental measurements of the non-linear force-deflection characteristics for the beam are compared to the theoretical predictions from the model, and it is shown that the shear crushing of the core has an important effect on the behaviour of the beam. The model shows that the most important material properties are the lower skin tensile failure strain and the core crushing strength. Dynamic effects are included in the model in the form of a strain rate dependence of the core crushing stress and the strain rate dependence of the failure strain in the lower skin. The increase in material strength with strain rate gives rise to an improved energy absorption capacity for the beam under impact loading.     

Face/core interface fracture characterization of mixed mode bending sandwich specimens

Debonding of the core from the face sheets is a critical failure mode in sandwich structures. This paper presents an experimental study on face/core debond fracture of foam core sandwich specimens under a wide range of mixed mode loading conditions. Sandwich beams with E‐glass fibre face sheets and PVC H45, H100 and H250 foam core materials were evaluated. A methodology to perform precracking on fracture specimens in order to achieve a sharp and representative crack front is outlined. The mixed mode loading was controlled in the mixed mode bending (MMB) test rig by changing the loading application point (lever arm distance). Finite element analysis was performed to determine the mode‐mixity at the crack tip. The results showed that the face/core interface fracture toughness increased with increased mode II loading. Post failure analysis of the fractured specimens revealed that the crack path depends on the mode‐mixity at the crack tip, face sheet properties and core density.     

The structural response of clamped sandwich beams subjected to impact loading

The structural response of dynamically loaded monolithic and sandwich beams made of aluminum skins with different cores is determined by loading the end-clamped beams at mid-span with metal foam projectiles. The sandwich beams comprise aluminum honeycomb cores and closed-cell aluminum foam cores. Laser displacement transducer was used to measure the permanent transverse deflection of the back face mid-point of the beams. The resistance to shock loading is evaluated by the permanent deflection at the mid-span of the beams for a fixed magnitude of applied impulse and mass of beam. It is found that sandwich beams with two kind cores under impact loading can fail in different modes. Experimental results show the sandwich beams with aluminum honeycomb cores present mainly large global deformation, while the foam core sandwich beams tend to local deformation and failure, but all the sandwich beams had a higher shock resistance, then the monolithic beam. For each type of beams, the dependence of transverse deflection upon the magnitude of the applied impulse is measured. Moreover, the effects of face thickness and core thickness on the failure and deformation modes were discussed. Results indicated that the structural response of sandwich beams is sensitive to applied impulse and structural configuration. The experimental results are of worth to optimum design of cellular metallic sandwich structures.     

The Effect of Load and Geometry on the Failure Modes of Sandwich Beams

Facing compressive failure, facing wrinkling and core shear failure are the most commonly encountered failure modes in sandwich beams with facings made of composite materials. The occurrence and sequence of these failure modes depends on the geometrical dimensions, the form of loading and type of support of the beam. In this paper the above three failure modes in sandwich beams with facings made of carbon/epoxy composites and cores made of aluminum honeycomb and two types of foam have been investigated. Two types of beams, the simply supported and the cantilever have been considered. Loading included concentrated, uniform and triangular. It was found that in beams with foam core facing wrinkling and core shear failure occur, whereas in beams with honeycomb core facing compressive failure and core shear crimping take place. Results were obtained for the dependence of failure mode on the geometry of the beam and the type of loading. The critical beam spans for failure mode transition from core shear to wrinkling failure were established. It was found that initiation of a particular failure mode depends on the properties of the facing and core materials, the geometrical configuration, the type of support and loading of sandwich beams.     

复合材料双向波纹夹层结构力学性能

为改进传统单向波纹夹层结构横向力学性能较差的缺点,设计了一种新型复合材料双向波纹夹层结构。考虑复合材料双向夹层结构制备困难,研究了整套真空辅助成型工艺(VARI)工艺制备方案,实现双向波纹夹层结构的高效制备,以满足工程应用的需要。对制备出的复合材料双向波纹夹层结构与单向波纹夹层结构分别进行面外压缩、弯曲和剪切实验,分析了双向波纹夹层结构在不同载荷下的破坏模式及其失效机制,计算了该结构在不同荷载条件下的强度和模量,并将其与单向波纹夹层结构进行对比分析。结果表明,在压缩荷载作用下,玻璃纤维/环氧树脂芯子为主要承载部分,结构的失效主要体现在芯子的屈曲、断裂和分层;在弯曲荷载的作用下,由于纤维的抗压强度远小于抗拉强度,所以压头下方的上面板最先达到破坏荷载,结构的弯曲失效形式主要为上面板的断裂和脱粘;结构的剪切失效主要以泡沫与面板的脱粘和压溃为主,芯子和面板未见明显的破坏现象;与单向波纹夹层结构相比,双向波纹夹层结构力学性能显著提升。      

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.     

An analytical solution for the large deflections of a slender sandwich beam with a metallic foam core under transverse loading by a flat punch

The large deflections of slender ultralight sandwich beams with a metallic foam core are studied under transverse loading by a flat punch, in which interaction of bending and stretching induced by large deflections is considered. Firstly, a unified yield criterion for metallic sandwich structures considering the effect of core strength is proposed, which is valid for metallic sandwich cross-sections with various core strengths and geometries. This can reduce to the yield criterion for a solid monolithic cross-section and the classical yield criterion for sandwich cross-sections with a weak core, respectively. Then, analytical solutions for the large deflections of fully clamped and simply supported metallic foam core sandwich beams are derived under transverse loading by a flat punch, respectively. Comparisons of the present solutions with experimental results are presented and good agreements are found. The effects of the core strength, the size of loading punch and the boundary conditions on the structural response of sandwich beams are discussed in detail. It is shown that the axial stretching induced by large deflections has significant effect on the load-carrying and energy absorption capacities of sandwich structures in the post-yield regime, and the load-carrying and plastic energy absorption capacities of metallic foam core sandwich beams may be underestimated as the core strength is neglected in analysis, especially for the sandwich beams with a strong core.     

X状Z-pin增强泡沫夹层结构的剪切性能

通过不同Z-pin角度(15°和25°)和夹芯厚度(8mm和12.7mm)的X状Z-pin增强泡沫夹层材料的剪切性能试验, 与相同材料同尺寸的未增强件进行对比, 考察X状Z-pin对泡沫夹层结构的增强作用。试验结果表明, X状Z-pin增强使材料的剪切强度和刚度都有较大幅度的提高; 同时, Z-pin的加入使该结构具有与传统泡沫夹层材料不同的剪切破坏形式。在此基础上, 结合空间网架结构和等效夹杂方法, 提出了X状Z-pin增强泡沫夹层结构剪切刚度模型, 计算结果与试验值符合良好。结果表明, X状Z-pin增强不仅能大幅度提高泡沫夹层结构的剪切性能, 并具有良好的可设计性, 可以通过改变Z-pin角度和材料等改变其力学性能。