Flexural behaviour of glue-laminated fibre composite sandwich beams

This study involved experimental investigation onto the flexural behaviour of glue-laminated fibre composite sandwich beams with a view of using this material for structural beams. Composite sandwich beams with 1, 2, 3, and 4 composite sandwich panels glued together were subjected to 4-point static bending test in the flatwise and edgewise positions to evaluate their stiffness and strength properties. The results showed that the composite sandwich beams in the edgewise position failed with 25% higher bending strength but have 7% lower bending stiffness than beams in the flatwise position. The results however indicated that the bending stiffness of flatwise specimens converges to that of the edgewise specimens with increasing laminations. More importantly, the specimens in the edgewise position failed with greater ductility due to progressive failure of the fibre composite skins while the specimens in the flatwise position failed in a brittle manner due to debonding between the skin and core. Wrapping the glue-laminated sandwich beams with one layer of tri-axial glass fibres did not prove to be effective. Overall, it has been demonstrated that the glue-laminated sandwich beams exhibited better performance than the individual composite sandwich beams.     

Behaviour of glued fibre composite sandwich structure in flexure: Experiment and Fibre Model Analysis

The behaviour of glued composite sandwich beams in flexure was investigated with a view of using this material for structural and civil engineering applications. The building block of this glue-laminated beam is a new generation composite sandwich structure made up of glass fibre reinforced polymer skins and a high strength phenolic core material. A simplified Fibre Model Analysis (FMA) usually used to analyse a concrete beam section is adopted to theoretically describe the flexural behaviour of the innovative sandwich beam structure. The analysis included the flexural behaviour of the glued sandwich beams in the flatwise and the edgewise positions. The FMA accounted for the non-linear behaviour of the phenolic core in compression, the cracking of the core in tension and the linear elastic behaviour of the fibre composite skin. The results of the FMA showed a good agreement with the experimental data showing the efficiency and practical applications of the simplified FMA in analysing and designing sandwich structures with high strength core material.     

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。      

Geometry effect on the behaviour of single and glue-laminated glass fibre reinforced polymer composite sandwich beams loaded in four-point bending

The research investigated the behaviour of single and glue laminated glass fibre reinforced polymer (GFRP) composite sandwich beams considering different spans and beam cross sections. The composite sandwich beams with different thicknesses (1, 2, 3, 4, and 5 sandwich layers) have been tested in four-point static flexural test with different shear span to depth ratio (a/d). The a/d ratios showed a direct effect on the flexural and shear behaviour. The capacity of the beam decreased with increasing a/d. Various failure modes were observed including core crushing, core shear, and top skin compression failure. The failure mode map developed based on the experimental finding and analytical prediction indicated that the failure mode is affected by the a/d with the number of glue laminated panels.     

Compression properties of fire-damaged polymer sandwich composites

The effect of fire-induced damage on the edgewise compression properties of polymer sandwich composites is investigated. Fire tests were performed using a cone calorimeter on sandwich composites with high or low flammability. The highly flammable composite had a poly(vinyl chloride) foam core, while the flame resistant composite had a phenolic foam core. The residual edgewise compression properties of the burnt composites were determined after fire testing at room temperature. The compression stiffness and strength of the two sandwich composites decreased rapidly with increasing heat flux and heating time of the fire due to thermal decomposition of the face skin and foam core. A large reduction to the edgewise compression properties of the phenolic-based sandwich composite occurred despite having good flame resistance, and the reasons for this are described. Preliminary analytical models are presented for estimating the edgewise compression failure load of fire-damaged sandwich composites that fail by core shear or buckling.     

Effect of Bonder at Skin/Core Interface on the Mechanical Performances of Sandwich Structures Used in Marine Industry

The present work is aimed to extend the knowledge of mechanical properties of sandwich structures used for marine applications focusing on the possibility to increase the performances of such structures by adding a bonder at the skin/core interface. Therefore, three sandwich structures that are utilised in different structural components of a yacht were realised by manual lay-up. The mechanical characterisation was performed by flatwise compressive, edgewise compressive and three point flexural tests. The tests execution has allowed both to determine the mechanical performances and to understand the fracture mechanisms that take place when the bonder is added in the stacking sequence of the samples.     

Bending Response of Foldcore Composite Sandwich Beams

In order to solve bending behavior difference of corrugated structure in L andWorientation, bending response for composite sandwich beams with foldcores of three different wall thicknesses were experimentally and numerically investigated. Effect of the cell walls thickness on the strength and failure behavior of the composite sandwich beams with L and W orientations was also examined. The deformation mode was obtained by the numerical method; a constitutive law of laminated material has been incorporated into a finite element (FE) analysis program. Numerical calculations give accurate prediction to the bending response of foldcore composite sandwich beams comparing with experiments. Structural flexural stiffness, strength and failure mechanism at a given topological geometry depended on the nature of core itself: the bending stiffness and strength of the sandwich beam increased with the core wall thickness (relative density). Also, bending isotropy was shown in this study for foldcore composite sandwich beams with selected core geometry.     

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.     

缝合参数对泡沫夹层结构复合材料力学性能的影响

采用真空辅助树脂注射(VARI)成型工艺制备不同缝合方式和缝合密度的缝合泡沫夹层复合材料, 研究缝合参数对平面拉伸、三点弯曲、芯子剪切以及滚筒剥离性能的影响。结果表明: 缝合使泡沫夹层复合材料的平面拉伸强度和芯子剪切强度明显降低, 可以改善弯曲性能并大幅提高滚筒剥离性能, 改进锁式缝合方式优于临缝式缝合方式; 适当地增加缝合行距对力学性能有一定的积极作用, 但不利于滚筒剥离性能的提高; 与未缝合泡沫夹层复合材料相比, 当缝合密度为30 mm×10 mm时, 改进锁式缝合泡沫夹层复合材料的平拉强度和芯子剪切强度分别降低了14.75%和24.79%, 弯曲强度和平均剥离强度分别提高了7.96%和80.78%。     

Ferrocement encased lightweight aerated concrete: A novel approach to produce sandwich composite

This paper presents the experimental study to investigate the applicability of a novel technique to produce lightweight sandwich composite elements. Sandwich composite is fabricated by encasing lightweight aerated concrete as core with high performance ferrocement box as skin layer. The performance of the sandwich elements is investigated in terms of ultimate compressive strength, flexural strength, water absorption, overall unit weight and the failure mode. The results are compared with control specimens made solely of the aerated concrete. Results showed the remarkable enhancement in the compressive strength and flexural strength while the water absorption is reduced to fractions as compared to that of the control specimens. Overall unit weight of the sandwich composite elements falls in the range of the lightweight structural elements. The failure mode of the sandwich elements reveals the ductile and composite behavior thus transforming a pure brittle material (aerated concrete) into ductile composite material because of the ferrocement encasement.     

The flexural behaviour of aramid fibre hybrid composite materials

In this paper the elastic-plastic model of flexural behaviour of beams is applied to hybrid composites containing aramid fibres. In the hybrids, as in the parent aramid-fibre-reinforced composite, the neutral axis is shifted toward the tensile face. The shift depends on the quantity and placement of the aramid fibre. The analysis and experimental work reported here relate to two fundamental sandwich hybrids, one with aramid fibres in the skins and carbon or glass fibres in the core, and the other with aramid fibre in the core and carbon or glass fibres in the skins. The flexural behaviour of the hybrids is discussed in terms of the effect of the placement of the aramid layer and of the relative thickness of the skin on the ultimate stresses, the elastic-plastic behaviour and the mode of failure.     

FEM analysis of dynamic flexural behaviour of composite sandwich beams with foam core

The dynamic flexural behaviour of sandwich beams, with composite face-sheets and a foam core, was analysed by developing a 3D finite-element model. To model the core behaviour, a crushable foam model was used. The Hou criteria were used to predict the failure of the face-sheets. Dynamic bending tests were performed to validate the numerical model. The comparison between numerical and experimental results in terms of contact-force histories, peak-force values, absorbed energy, and maximum displacement of both face-sheets was satisfactory. It was revealed that the collapse of the foam core under the impact region favoured the failure of the upper face-sheet.     

温度对缝合气凝胶夹芯复合材料面内压缩性能的影响

分别开展缝合气凝胶夹芯复合材料在不同温度下的面内压缩试验,研究材料在室温、300℃、600℃和800℃下的面内压缩力学性能,并采用微焦点工业CT扫描的方法对试样内部结构进行分析,结合有限元分析方法,探究其结构破坏机制。结果表明:在面内压缩载荷作用下,材料存在极限载荷,面板的局部屈曲、芯层的剪切破坏以及缝线柱的断裂是材料破坏的主要方式。随着温度的升高,材料的面内压缩模量和极限载荷也逐渐升高,面板破坏处的断口逐渐呈现出类似脆性的断裂。300℃、600℃和800℃下材料的面内压缩模量分别为室温的1.05倍、1.57倍和1.65倍;极限载荷分别为室温的1.14倍、1.46倍和1.67倍。室温下有限元分析结果和试验结果的对比,验证了缝合气凝胶夹芯复合材料面内压缩破坏模式的合理性。     

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.     

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.     

Stability of the face layer of sandwich beams with sub-interface damage in the foam core

This paper addresses the effect of local indentation/impact damage on the bearing capacity of foam core sandwich beams subjected to edgewise compression. The considered damage is in a form of through-width zone of crushed core accompanied by a residual dent in the face sheet. It is shown that such damage causes a significant reduction of compressive strength and stiffness of sandwich beams. Analytical solutions estimating the Euler’s local buckling load are obtained for two typical modes of damage. These solutions are validated through experimental investigation of three sandwich configurations. The results of the analytical analysis are in agreement with the experimental data.     

不同铺层方式下连续玻璃纤维/聚丙烯复合材料波纹夹芯板的力学性能

制备了多种铺层方式的连续玻璃纤维/聚丙烯（GF/PP）复合材料波纹夹芯板,并研究了GF/PP复合材料波纹夹芯板的平压性能和弯曲性能。结果显示:面板相同时,增加芯板厚度可大大增加夹芯板整体的平压性能;芯板相同时,面板的铺层方式对夹芯板的平压性能有一定影响,且面板含有0°和90°铺层的波纹夹芯板具有最高的平压模量,为59.55 MPa,而单纯增加面板厚度对提升波纹夹芯板的平压性能影响不大;面板铺层方式对弯曲性能具有较大影响,面板为0°铺层的波纹夹芯板具有最高的横向弯曲模量,为783.66 MPa,面板为90°铺层的波纹夹芯板具有最高的纵向弯曲模量,为732.09 MPa;面板为单向铺层（0°或90°铺层）时,会使夹芯板另一方向（纵向或横向）的弯曲性能形成短板。      

Finite element simulation and experimental study on mechanical behavior of 3D woven glass fiber composite sandwich panels

The results of finite element simulation followed by an experimental study are presented in order to investigate the mechanical behavior of three-dimensional woven glass-fiber sandwich composites using FE method. Experimental load–displacement curves were obtained for flatwise compressive, edgewise compressive, shear, three-point bending and four-point bending loads on the specimens with three different core thicknesses in two principal directions of the sandwich panels, called warp and weft. A 3D finite element model is employed consisting of glass fabric and surrounding epoxy resin matrix in order to predict the mechanical behavior of such complex structures. Comparison between the finite element predictions and experimental data showed good agreement which implies that the FE simulation can be used instead of time-consuming experimental procedures to study the effect of different parameters on mechanical properties of the 3D woven sandwich composites.     

Pin-Contact Behaviour of Composite Sandwich Structures Under Compressive Bearing Load

The aim this research is to investigate the pin-sandwich contact behaviour of some sandwich composites structures when submitted to compressive bearing loads. A preliminary set of flatwise and edgewise compressive tests and three point flexural tests were performed to get information on the mechanical behaviour of the sandwich structures under different load conditions. The influence of two different manufacturing procedures on the bearing strength is evidenced. The experiments show that the bearing loads increase with the pin diameter, while the bearing stress depends in a different way of the pin diameter for the two kind of procedures employed. In addition a simplified numerical model is proposed to evaluate the stress/strain distribution in the sandwich structure under compressive bearing load, by employing a commercial code. The comparison of numerical results with experiments shows the accuracy of the model.     

Bolted joints of pultruded sandwich composite laminates

This paper describes the tensile behavior of bolted joints of pultruded sandwich composite laminates. The pultruded sandwich laminates have a skin-core-skin structure. Joint strength of longitudinal specimens was independent of specimen width (w), whereas it increased with w in the case of transverse specimens. The joining efficiency of pultruded sandwich laminates was greater in the longitudinal direction than in the transverse direction. The core layer of longitudinal specimens failed by a combination of bearing and shear-out modes, independent of w. The failure mode of skin layers changed from net-tension to bearing mode with increasing w. In transverse specimens, the failure mode of core and skin layers changed from net-tension to bearing with increasing w. Finite element numerical analysis was carried out to predict the failure mode and joint strength. The numerical results were in good correlation with the experimental results.