Multi-scale modeling of time-dependent response of smart sandwich constructions

Polymer and polymer based composite structures exhibit time-dependent response, leading to their being described as viscoelastic bodies. The rate of creep (or stress relaxation) in viscoelastic bodies increases with increasing the temperature of the bodies. In this study, we are interested in analyzing the time-dependent response of smart sandwich composites comprising of glass fiber reinforced polymer (GFRP) skins, polyurethane foam core, and lead zirconate titanate (PZT) wafers embedded in the GFRP skins. The PZT is used to monitor lifetime performance of sandwich composites. A multi-scale model is developed to integrate different constitutive models of the constituents in the sandwich structures. Quasi-static and creep tests are conducted for bulk epoxy, GFRP, polyurethane foam, and sandwich specimens under uniaxial tension and bending. The tests were done at room temperature and at 80 °C. The experimental data are used for material characterization and model verification. The multi-scale model that is developed can be used to understand the effect of different responses of the constituents on the overall time-dependent behavior of sandwich structures and examine the feasibility of using PZT wafers for monitoring lifetime performance of sandwich structures.     

Flexural performance of sandwich panels comprising polyurethane core and GFRP skins and ribs of various configurations

This study explored the feasibility of fabrication and flexural performance of panels composed of low-density polyurethane foam core sandwiched between two GFRP skins. A comprehensive material testing program was first carried out on the constituents. Large scale panels with nominal dimensions of 2500 × 660 × 80 mm were then tested in one-way bending under a simulated uniform load. Various configurations of internal and exterior GFRP ribs connecting the two skins were explored and compared to a panel without ribs. The study showed that, by integrating the ribs, strength and stiffness of the panels increased substantially, by 44–140%, depending on the configuration of the ribs. The maximum gain in strength was equivalent to the effect of doubling the core density in a panel without ribs. Shear deformation of the core contributed over 50% of mid-span deflection in the panel without ribs. By adding ribs, flexure became more dominant and shear deformations of the ribs contributed only 15–20% of the total deflection. Simple analytical expressions have been proposed, and captured these effects reasonably accurately. It was shown that ultimate strengths of the panels were equivalent to those of similar size reinforced concrete panels with moderate to heavy steel reinforcement ratios of 0.6–2.0%, but sandwich panels were 9–14 times lighter in weight.     

Design analysis of three-layered structural composites based on post-consumer recycled plastics and wood residues

Three-layered structural composites were produced from municipal plastic wastes and wood flour residues to investigate the effects of design parameters on their flexural and impact performance. The studied parameters include wood content, thickness of individual composite layers, as well as stacking sequence and configuration (symmetric and asymmetric structures). The results indicate that the core layer has a lower influence on the flexural properties of structural beams in comparison with the skins. But depending on beam configuration (stacking sequence), different flexural characteristics can be obtained using the same composite layers. The classical beam theory was used to predict the flexural modulus with high precision. In addition, performance of the beams under impact tests was shown to be independent from their stacking sequences and layer thicknesses for each configuration.     

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.     

Performance of functionally graded sandwich composite beams under shock wave loading

The dynamic behavior of sandwich composites made of E-Glass Vinyl-Ester (EVE) facesheets and graded Corecell™ A-series foam was studied using a shock tube apparatus. The foam core was monotonically graded based on increasing acoustic wave impedance, with the foam core layer of lowest wave impedance facing the blast. The specimen dimensions were held constant for all core configurations, while the number of core layers varied, resulting in specimens with one layer, two layer, three layer, and four layers of foam core gradation. Prior to shock tube testing, the quasi-static and dynamic constitutive behavior (compressive) of each type of foam was evaluated. During the shock tube testing, high-speed photography coupled with the optical technique of Digital Image Correlation (DIC) was utilized to capture the real-time deformation process as well as mechanisms of failure. Post-mortem analysis was also carried out to evaluate the overall blast performance of these configurations. The results indicated that increasing the number of monotonically graded foam core layers, thus reducing the acoustic wave impedance mismatch between successive layers, helped maintain structural integrity and increased the blast performance of the sandwich composite.     

梯度铝蜂窝夹芯板的力学行为

梯度分层铝合金蜂窝板是一种有效的吸能结构,本工作在梯度铝蜂窝结构的基础上根据梯度率的概念,通过改变蜂窝芯层的胞壁长度,设计了4种质量相同、梯度率不同的铝蜂窝夹芯结构。通过准静态压缩实验,并结合非线性有限元模拟准静态及冲击态下梯度铝蜂窝夹芯结构的变形情况及其力学性能,分析对比了相同质量下梯度铝蜂窝夹芯结构在准静态下的变形模式以及冲击载荷下分层均质蜂窝结构和不同梯度率的分层梯度蜂窝结构的动态响应和能量吸收特性。结果表明:在准静态压缩过程中,铝蜂窝梯度夹芯板的变形具有明显的局部化特征,蜂窝芯的变形为低密度优先变形直至密实,层级之间的密实化应变差随芯层密度的增大而逐渐减小;在高速冲击下,梯度蜂窝板并非严格按照准静态过程中逐级变形直至密实,而是在锤头冲击惯性及芯层密度的相互作用下整体发生的线弹性变形、弹性屈曲、塑性坍塌及密实化;另外,在本工作所设计的梯度率中,当梯度率为γ₁=0.0276时,梯度蜂窝夹芯板的吸能性达到最好,相较于同等质量下的均质蜂窝夹芯板,能量吸收提高了10.63%。     

Evaluation of sandwich panels with various polyurethane foam-cores and ribs

The objective of this study was to evaluate three potential core alternatives for glass fiber reinforced polymer (GFRP) foam-core sandwich panels. The proposed system could reduce the initial production costs and the manufacturing difficulties while improving the system performance. Three different polyurethane foam configurations were considered for the inner core, and the most suitable system was recommended for further prototyping. These configurations consisted of high-density polyurethane foam (Type 1), a bidirectional gridwork of thin, interconnecting, GFRP webs that is in-filled with low-density polyurethane foam (Type 2), and trapezoidal-shaped, low-density polyurethane foam utilizing GFRP web layers (Type 3). The facings of the three cores consisted of three plies of bidirectional E-glass woven fabric within a compatible polyurethane resin. Several types of small-scale experimental investigations were conducted. The results from this study indicated that the Types 1 and 2 cores were very weak and flexible making their implementation in bridge deck panels less practical. The Type 3 core possessed a higher strength and stiffness than the other two types. Therefore, this type is recommended for the proposed sandwich system to serve as a candidate for further development. Additionally, a finite element model (FEM) was developed using software package ABAQUS for the Type 3 system to further investigate its structural behavior. This model was successfully compared to experimental data indicating its suitability for parametric analysis of panels and their design.     

Modelling of aluminium foam core sandwich panels under impact perforation

Aluminium foam core sandwich panels are good energy absorbers for impact protection applications, such as light-weight structural panels, packing materials and energy absorbing devices. In this study, the high-velocity impact perforation of aluminium foam core sandwich structures was analysed. Sandwich panels with 1100 aluminium face-sheets and closed-cell A356 aluminium alloy foam core were modelled by three-dimensional finite element models. The models were validated with experimental tests by comparing numerical and experimental damage modes, output velocity, ballistic limit and absorbed energy. By this model the influence of foam core and face-sheet thicknesses on the behaviour of the sandwich panel under impact perforation was evaluated.     

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。      

Ballistic impact experiments of metallic sandwich panels with aluminium foam core

Metallic sandwich structures with aluminium foam core are good energy absorbers for impact protection. To study their ballistic performance, quasi-static and impact perforation tests were carried out and the results are reported and analysed in this paper. In the experiments, effects of several key parameters, i.e. impact velocity, skin thickness, thickness and density of foam core and projectile shapes, on the ballistic limit and energy absorption of the panels during perforation are identified and discussed in detail.     

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.     

Structure and mechanics of the iris leaf

The structure of the iris leaf resembles that of a sandwich beam with fibre composite faces separated by a low-density foam core. Such structures have a high specific stiffness because the separation of the faces by the lightweight core increases the moment of inertia of the section with little increase in weight. In this paper we examine the structure of the leaf of the bearded iris and show that its flexural stiffness can be explained in terms of the mechanics of sandwich beams.     

加筋增强泡沫夹芯结构面内压缩与界面断裂性能试验

为解决复合材料泡沫夹芯结构面板局部屈曲与面芯脱粘的突出问题,提出了一种由筋条增强的玻璃纤维增强树脂基复合材料（GFRP）面板与泡沫芯层组合而成的新型夹芯结构。采用真空辅助树脂导入技术制备试验件,通过面内压缩与双悬臂梁试验,对比分析了加筋增强夹芯板与未加筋夹芯板的受力特性、失效模式和面芯粘结性能。面内压缩试验显示,与未加筋夹芯板相比,加筋增强夹芯板的失效模式由面板局部屈曲转化为面板压缩剪切破坏或整体屈曲,在GFRP材料使用量相同的情况下,试件长度为130 mm的加筋增强夹芯板平均失效荷载提高了40.87%,长度为190 mm试件提高了35.63%。双悬臂梁试验显示,加筋增强夹芯板的裂缝在发展过程中受到筋条与面板之间纤维丝搭接约束,改善了界面粘结性能,与未加筋夹芯板相比,其平均能量释放率提高了57.35%。     

Mechanical properties and design of sandwich materials

Sandwich materials consisting of a low density core with stiff skins offer considerable potential for weight saving in panel applications, where the main loads are flexural. Sandwich materials of interest for car and van body panels, seat shells, etc, include steel/plastic laminates, integral skinned plastic foams and glass fibre-reinforced polyester skins with foamed plastic cores. In this paper, basic design formulae for the flexural stiffness and strength of such sandwich materials are reviewed and a method for designing optimum sandwich structures for least weight or cost is given. Mechanical property data are presented on a range of sandwich materials of potential interest for vehicle panel applications. It is then shown how use of the least-weight design method enables core and skin thicknesses to be determined and gives a means of improving the flexural properties of existing sandwich constructions.     

Numerical modeling of impact on wood-based sandwich structures

Abstract

The impact behavior of innovative wood based sandwich structures with plywood core and skins made either of aluminum or of fiber reinforced polymer (carbon, glass, or flax composite skins) was investigated numerically. The wood based sandwich structures were subjected to low-velocity/low-energy impacts. An explicit nonlinear numerical model based on volume elements with a cohesive layer was developed. A plastic wood law already implemented in LS-DYNA was used in association with composite type damage criteria. Comparisons with experiments in terms of layer deformations and overall contact laws during impact showed satisfactory results.     

On the impact response of sandwich composites with cores of balsa wood and PVC foam

This paper presents an experimental investigation on impact response of sandwich composite panels with PVC foam core and balsa wood core. A number of tests were performed under various impact energies. Damage process of the sandwich composites is analyzed from cross-examining load–deflection curves, energy profile diagrams and the damaged specimens. The primary damage modes observed are; fiber fractures at upper and lower skins, delaminations between adjacent glass–epoxy layers, core shear fractures, and face/core debonding. After visual inspection of the top and bottom face-sheets, initial examination, damage mechanisms at the interior layers and cores were ascertained through destructive analysis, i.e. sectioning by an abrasive water-jet machine, of samples. In addition to the single impacts, repeated impact response of the samples is also investigated.     

Experimental and numerical evaluation of the mechanical behaviour of GFRP sandwich panels

This work deals with the analysis of the mechanical behaviour of a class of sandwich structures widely employed in marine constructions, constituted by fiber-glass laminate skins over PVC foam or polyester mat cores. In detail, a systematic experimental study and numerical simulations have shown that the theoretical prediction of the strength and the actual failure mechanism of these sandwich structures can be affected by significant errors, specially in the presence of prevalent shear loading. Moreover, because of the low shear stiffness and the elastic constants mismatch of the skins and core material, failure modes and strength are strongly influenced by eventual stresses orthogonal to the middle plane of the sandwich. In particular, for the sandwich structures with a PVC foam core, such a stress interaction leads to early skin–core delamination failure, whereas for those with a polyester core it leads to core shear-cohesive failure. By means of accurate non-linear simulations, accurate failure criteria, that can be used at the design stage in the presence of complex loading, have also been developed.     

Modelling of composite sandwich structures with honeycomb core subjected to high-velocity impact

In this study the perforation of composite sandwich structures subjected to high-velocity impact was analysed. Sandwich panels with carbon/epoxy skins and an aluminium honeycomb core were modelled by a three-dimensional finite element model implemented in ABAQUS/Explicit. The model was validated with experimental tests by comparing numerical and experimental residual velocity, ballistic limit, and contact time. By this model the influence of the components on the behaviour of the sandwich panel under impact load was evaluated; also, the contribution of the failure mechanisms to the energy-absorption of the projectile kinetic energy was determined.     

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.     

GFRP约束微珠泡沫柱准静态受压性能

本研究对2根微珠泡沫柱及5根玻璃纤维复合材料(GFRP)约束微珠泡沫组合柱开展准静态轴压试验,探讨了GFRP层数、横向纤维与纵向纤维比例、泡沫密度等参数对组合柱极限承载力和吸能效应的影响,并与静态试验结果进行对比,研究不同加载速率对构件受压性能的影响规律.结果表明:准静态压缩作用下GFRP层数和泡沫密度的增加均提高了构...