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.     

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.     

Flexural properties of sandwich beams consisting of air plasma sprayed alloy 625 and nickel alloy foam

Sandwich structures are considered as viable engineering constructions due to their unique structural, physical, and mechanical properties. An investigation of the mechanical characteristics of sandwich structures suitable for high temperature application is presented. A process has developed to produce high temperature sandwich structures by depositing alloy 625 skins on Ni alloy foam cores using air plasma spraying (APS). The experimental investigation consisted of fabrication of sandwich structures and testing of mechanical performance of sandwich specimens under flexural loading conditions. The responses of the as-fabricated sandwich structure to heat treatment were investigated. The strength of the sandwich structure was significantly increased after heat treatment. The influence of skin thickness on mechanical behavior of sandwich structures was examined by performing four-point bending test on sandwich samples with skin thicknesses of 0.5 and 0.1 mm. The larger the skin thickness, the higher the strength of the sandwich beams. Comparison between the results of four-point bending tests on sandwich structures with different skin thicknesses will help us to understand the effect of skin thickness on the failure mechanism of sandwich structures.

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The effect of temperature on the bending properties and failure mechanism of composite truss core sandwich structures

The effects of temperature on the bending properties and failure mechanism of carbon fiber reinforced polymer composite sandwich structure with pyramidal truss cores were investigated and presented in this paper. The three-point bending tests of composite sandwich structures were performed at seven different temperatures, and the scanning electron microscope was used to examine the fiber-matrix interface properties in order to understand the deformation and failure mechanism. Then the effects of temperature on deformation modes, failure mechanism and bending failure load were studied and analyzed. The results showed that the temperature has visible impact on the deformation modes, failure mechanism, and bending failure load. The bending failure load decreased as temperature increased, which was caused by the degradation of the matrix properties and fiber-matrix interface properties at high temperature. The analytical formulae were also presented to predict the bending stiffness and failure load of composite sandwich structures at different temperatures.     

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。     

阻尼层合板中波传播性能研究

最近许多约束阻尼复合结构的研究中,阻尼层一般都选用粘弹性材料,由于粘弹性材料的刚度一般远小于约束层和基层的刚度,所以大部分研究都在计算和建模时,忽略粘弹材料的弯曲刚度,而不考虑阻尼层的外延和弯曲变形的影响。应用高阶理论研究了敷设具有一定刚度的蜂房型粘弹性材料的约束阻尼层合板结构动力学方程,探讨了复合层合板梁在振动情况时材料的弯曲刚度,剪切刚度对弯曲波在复合梁中传播的能量损耗影响,以及各层厚对层合板梁的阻尼性能的影响。     

蜂窝夹层修理结构的弯曲性能试验分析

采用四点弯加载方式研究分析了含损伤蜂窝夹层修理结构的弯曲性能,该夹层结构由碳纤维增强的聚合物面板和蜂窝芯子组成。进一步分析了挖补斜度、挖补方式、损伤程度、修理设备和修理材料对修理板弯曲性能的影响。研究表明,修理板的破坏模式可分为补片边缘折断、补片中面折断和胶层破坏三种,相同破坏模式修理板的名义弯曲强度相近,其中前两种破坏模式修理板的名义弯曲强度与完好板相近,而第三种破坏模式修理板的名义弯曲强度相对较低。所有修理板的名义弯曲强度恢复率基本处于95%以上,同时修理后抗弯刚度也满足修理准则。     

Micro and macro analysis of sisal fibre composites hollow core sandwich panels

In the view of the growing environmental concerns, hollow cores from recyclable natural fibre composites were manufactured to reduce the undesirable impact on the environment. To evaluate the feasibility of using short sisal fibres as reinforcements in the composites, existing micromechanical models have been used to predict properties starting from the intrinsic properties of its constituents. The stress relaxation behaviour of the composites was examined experimentally by performing tensile stress relaxation tests and to understand the process, it was modelled using variations of Maxwell’s model. A steady-state finite element analysis in the linear range was performed in ANSYS environment to examine flexural properties of the panels, and the shear strength of the hollow cores was experimentally determined by subjecting them to flexural loads in a four-point bending scheme. The micromechanics models indicated that the fibres had failed to provide effective reinforcements with their existing lengths, acting as fillers rather than reinforcements. The stress relaxation models indicated that the formed part needs to be cooled to room temperature within the die under suitable forming loads to avoid local deformations due to warping. The mid-span deflections of the sandwich panels predicted by the FE model agree well with the experimental results, the analysis predicted facing buckling as a mode of failure when wood veneers facings of modulus 4.5 GPa and thickness 1.7 mm were used. The specific shear strengths of the reinforced core are more than twice than those of the unreinforced polypropylene cores, increasing the scope of such panels as structural members in various engineering facets.     

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.     

Failure behaviour of honeycomb sandwich corner joints and inserts

In nearly all sandwich constructions certain types of joints have to be used for assembly, but little is known about their failure behaviour. This paper deals with the investigation of the mechanical behaviour of three different corner joints as a right-angled connection of two sandwich panels and of two different potted inserts as a localised load introduction in Nomex^® honeycomb sandwich structures with glass fibre-reinforced composite skins. For this purpose, experimental test series were conducted including shear tests and bending tests of the corner joints and pull-out as well as shear-out tests of the threaded inserts. The failure mechanisms and sequences are described for each load case and the influence of the different designs and of the loading rate is discussed. Based on these characteristics, finite element simulation models were developed in LS-DYNA, which are able to represent the respective failure behaviours.     

Bending response of sandwich panels with discontinuous wire-woven metal cores

The effects of a gap between discontinuous WBK (Wire-woven Bulk Kagome) cores on the bending properties of mild steel sandwich panels were elaborated. Analytic solutions were derived, and the experimental and numerical results of the bending response of sandwich panels with continuous and discontinuous WBK cores were presented. The analytic solutions of sandwich panels with continuous or discontinuous WBK cores under bending load provided good estimations of the failure mode, peak load, and bending stiffness in comparison with the experimental results. The strength and stiffness of sandwich panels with discontinuous WBK cores under bending load often substantially deteriorated depending on the gap width between the cores and on the detailed geometry near the gap. The analytic solutions successfully explained how the deterioration of the bending strength or stiffness could be minimized, when two separate sandwich panels or cores are to be joined.     

Effect of a Thin Soft Core on the Impact Behaviour of CFRP Laminates

Low-velocity impact tests were carried out on sandwich plates having CFRP facings and thin rubbery core. Two types of cores, differing in the material nature and thickness, were used. For comparison, similar tests were performed on the monolithic laminate. Various impact parameters, among which indentation, first failure energy, perforation energy, absorbed energy and maximum contact force, were analyzed, to highlight the effect of the core on the material response. The influence of the core on the macroscopic behaviour of the panels was quite limited, except in the elastic phase, where the lower stiffness of the sandwich configurations resulted in a higher energy at first failure. More relevant differences were found from the study of failure modes, carried out combining ultrasonic C-scan and a limited number of microscopic observations. In particular, in correspondence of the energy for barely visible impact damage, besides considerable facing-core debonding, both the facings of the sandwich structures exhibited fibre breakage at their back side.     

复合材料夹层结构平接胶结节点抗弯性能试验研究

试验研究钢型材插入式泡桐木夹层结构平接胶结节点的抗弯性能,包括荷载-变形特性、应变发展规律和破坏模式。平接胶结节点采用工字钢型材,型材翼缘取3种不同内斜角,考察内斜角对平接胶结节点强度的影响规律。试验研究表明:平接胶结节点能够满足建筑结构变形限值所对应的承载力要求,该平接胶结节点形式可以应用于泡桐木芯材夹层结构;连接工字钢型材的翼缘内斜角对连接强度影响不明显。分析了胶结节点的变形特性、破坏机理和强度影响因素。研究成果可供复合材料夹层板结构的钢型材胶结节点设计应用参考。     

Energy Absorption Characteristics of Web-Core Sandwich Composite Panels Subjected to Drop-Weight Impact

The objective of the study was to characterise the energy absorption of composite panels with tied cores, subjected to a drop weight impact test. Numerical simulations based on explicit finite element analysis have successfully modelled low velocity impact tests carried out on sandwich panels with web-core structure and plastic foam. The numerical model has been validated in terms of the failure behaviour of the panel and the variation of the contact force after the initial peak load corresponding to flexural failure. The numerical model is used for a better interpretation of the test results and of the failure mechanisms within the structure. The contribution to the overall energy absorption of the different parts composing the panels has been studied, with the aim of evaluating the feasibility of using low density foam in combination with web-core reinforcement in structural applications.     

Bending and vibration of multilayer sandwich beams and plates

A finite element displacement analysis of multilayer sandwich beams and plates, each with n stiff layers and n?1 weak cores, is presented. Each layer of the sandwich structure may have individual orthotropic properties of its own and the bending rigidities of the stiff layers are taken into account while direct stresses in cores are neglected in the analysis. The condition of common shear angle for all cores, which has been used by several authors is not implied in the formulation. Several examples on bending problems have been solved using lower-order elements and the accuracy of the results has been shown to be excellent. Two higher-order elements have also been developed but have not been found to yield much better results. The free vibration problems of multilayer sandwich structures have also been solved, and good accuracy is demonstrated.     

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.     

Compression and bending performances of carbon fiber reinforced lattice-core sandwich composites

To restrict debonding, carbon fiber reinforced lattice-core sandwich composites with compliant skins were designed and manufactured. Compression behaviors of the lattice composites and sandwich columns with different skin thicknesses were tested. Bending performances of the sandwich panels were explored by three-point bending experiments. Two typical failure mechanisms of the lattice-core sandwich structures, delaminating and local buckling were revealed by the experiments. Failure criteria were suggested and gave consistent analytical predictions. For panels with stiff skins, delamination is the dominant failure style. Cell dimensions, fracture toughness of the adhesives and the strength of the sandwich skin decide the critical load capacity of the lattice-core sandwich structure. The mono-cell buckling and the succeeding local buckling are dominant for the sandwich structures with more compliant skin sheets. Debonding is restricted within one cell in bending and two cells in compression for lattice-core sandwich panels with compliant face sheets and softer lattice cores.     

Failure In metal honeycombs manufactured by selective laser melting of 304 L stainless steel under compression

ABSTRACT

Cellular structures, specifically honeycombs, are commonly used as core materials in sandwich structures. This is especially true in aerospace applications where high bending and out-of-plane compressive stiffness coupled with low component weight is required. Additive manufacturing techniques are well suited for the manufacture of such cellular structures in a cost-effective manner. The current work focuses on honeycombs using selective laser melting of 304?L stainless steel. The mechanical behaviour of honeycombs was evaluated using out-of-plane compression tests. A numerical model was built to describe failure of the additively manufactured honeycombs. Compression tests were performed, on cylindrical samples to build the nonlinear material model. The material behaviour was found to be dependent on the build direction. Results of experiments and simulation show that failure occurs through a plastic buckling mechanism.     

Examination of the failure of sandwich beams with core junctions subjected to in-plane tensile loading

The paper concerns local effects occurring in the vicinity of junctions between different cores in sandwich beams subjected to tensile in-plane loading. It is known from analytical and numerical modelling that these effects display themselves by an increase of the bending stresses in the faces as well as the core shear and transverse normal stresses at the junction. The local effects have been studied experimentally to assess the influence on the failure behaviour both under quasi-static and fatigue loading conditions. Typical sandwich beam configurations with aluminium and glass-fibre reinforced plastic (GFRP) face sheets and core junctions between polymer foams of different densities and rigid plywood or aluminium were investigated. Depending on the material configuration of the sandwich beam, premature failure accumulating at the core junction was observed for quasi-static and/or fatigue loading conditions. Using Aluminium face sheets, quasi-static loading caused failure at the core junction, whereas no significance of the junction was observed for fatigue loading. Using GFRP faces, a shift of the failure mode from premature core failure in quasi-static tests to face failure at the core junction in fatigue tests was observed. In addition to the failure tests, the sandwich configurations have been analysed using finite element modelling (FEM) to elaborate on the experimental results with respect to failure prediction. Both linear modelling and nonlinear modelling including nonlinear material behaviour (plasticity) was used. Comparing the results from finite element modelling with the failure behaviour observed in the quasi-static tests, it was found that a combination of linear finite element modelling and a point stress criterion to evaluate the stresses at the core junction can be used for brittle core material constituents. However, this is generally not sufficient to predict the failure modes and failure loads properly. Using nonlinear material properties in the modelling and a point strain criterion improves the failure prediction especially for ductile materials, but this has to be examined further along with other failure criteria.     

Experimental analysis of the flexural properties of sandwich panels with cellular core materials

This paper addresses the flexural properties of sandwich structures with cellular core materials. Experimental three point bending tests are conducted in order to determine the flexural stiffness and the load‐carrying capacity of these advanced composites. In addition, the significant failure modes after exceeding the load‐carrying capacity are identified. The results of these analyses are compared for sandwich structures containing various core materials. These core materials comprise two aluminium foams, namely M‐Pore® and Alporas®, honeycomb structures and novel metallic hollow sphere structures (MHSS).