Corrugated all-composite sandwich structures. Part 2: Failure mechanisms and experimental programme

A novel corrugated composite core, referred to as a hierarchical corrugation, has been developed and tested experimentally. Hierarchical corrugations exhibit a range of different failure modes depending on the geometrical properties and the material properties of the structures. In order to understand the different failure modes the analytical strength model, developed in part 1 of this paper, was used to make collapse mechanism maps for the different corrugation configurations. If designed correctly, the hierarchical structures can have more than 7 times higher weight specific strength compared to its monolithic counter part. The difference in strength arises mainly from the increase in buckling resistance of the sandwich core members compared to the monolithic version. The highest difference in strength is seen for core configurations with low overall density. As the density of the core increases, the monolithic core members get stockier and more resistant to buckling and thus the benefits of the hierarchical structure reduces.     

Structural response of all-composite pyramidal truss core sandwich columns in end compression

With the equivalent mechanical properties of composite materials, analytical formulae of critical load for an all-composite sandwich column with pyramidal truss core are derived. Four failure modes are considered: macro Euler buckling, macro shear buckling, face-sheet wrinkling and face-sheet crushing. Failure mechanism maps are constructed with the four competing failure modes, and the relationship between the failure mechanism maps and material mechanical properties is discussed. Selected experiments validate the analytical predictions, and reasonable agreement is obtained. Macro shear buckling is the main failure mode for the sandwich column specimens, which is attributed to the low stiffness of core. The final failure loads is related to the strength of the nodes between face-sheets and truss core, so the node strength is the key of improving the failure load. Given by numerical simulations, the failure loads and failure modes agree well with analytical predictions.     

复合材料点阵夹芯结构平压性能不确定分析与优化

考虑一体化成型工艺制备的复合材料点阵夹芯结构及其不确定性,采用区间向量实现不确定参数定量化,建立复合材料点阵夹芯结构平压性能区间分析模型.考虑结构功能状态判断的模糊性,分别在不考虑设计容差与考虑设计容差情形下,建立了不确定平压载荷作用下含区间参数模糊可靠性分析与优化模型.研究结果表明:材料参数及结构参数不确定性,特别是设计容差对复合材料点阵夹芯结构平压性能影响明显,因此在工程优化中不仅需要充分考虑材料参数与外部载荷等不确定性,而且需要充分重视传统不确定设计方法中未计及的设计容差的影响.本研究实现了理论成果与工程应用的有机结合,为工程领域复合材料点阵夹芯结构平压性能分析与优化提供有效理论方法.     

复合材料点阵夹芯结构平压性能不确定分析与优化

考虑一体化成型工艺制备的复合材料点阵夹芯结构及其不确定性, 采用区间向量实现不确定参数定量化, 建立复合材料点阵夹芯结构平压性能区间分析模型。考虑结构功能状态判断的模糊性, 分别在不考虑设计容差与考虑设计容差情形下, 建立了不确定平压载荷作用下含区间参数模糊可靠性分析与优化模型。研究结果表明: 材料参数及结构参数不确定性, 特别是设计容差对复合材料点阵夹芯结构平压性能影响明显, 因此在工程优化中不仅需要充分考虑材料参数与外部载荷等不确定性, 而且需要充分重视传统不确定设计方法中未计及的设计容差的影响。本研究实现了理论成果与工程应用的有机结合, 为工程领域复合材料点阵夹芯结构平压性能分析与优化提供有效理论方法。     

Homogenization of sandwich structures

A homogenization method is proposed for sandwich structures consisting of two plates interlaced with beams and shells in a periodic, lattice structure. The proposed method is a quasi‐continuum approach where the constitutive response is obtained from the generalized forces of the interlacing elements. Buckling is studied as part of this model. Comparison of the homogenized model with fully discrete models show reasonable to very good agreement. Copyright © 2004 John Wiley & Sons, Ltd.     

Ultra-light asymmetric photovoltaic sandwich structures

This work evaluated the possibility of using silicon solar cells as load-carrying elements in composite sandwich structures. Such an ultra-light multifunctional structure is a new concept enabling weight, and thus energy, to be saved in high-tech applications such as solar cars, solar planes or satellites. Composite sandwich structures with a weight of ～800 g/m² were developed, based on one 140 μm thick skin made of 0/90° carbon fiber-reinforced plastic (CFRP), one skin made of 130 μm thick mono-crystalline silicon solar cells, thin stress transfer ribbons between the cells, and a 29 kg/m³ honeycomb core. Particular attention was paid to investigating the strength of the solar cells under bending and tensile loads, and studying the influence of sandwich processing on their failure statistics. Two prototype multi-cell modules were produced to validate the concept. The asymmetric sandwich structure showed balanced mechanical strength; i.e. the solar cells, reinforcing ribbons, and 0/90° CFRP skin were each of comparable strength, thus confirming the potential of this concept for producing stiff and ultra-lightweight solar panels.     

A.c. characteristics of photosensitive CdSe film sandwich structures

The dielectric properties of photosensitive CdSe sandwich structures were investigated. The devices were prepared by thermal evaporation and condensation of CdSe films in a quasi-isolated volume in vacuum onto transparent conducting SnO₂:Sb films which serve as the base electrodes.The modulus of the impedance and the difference in phase between the current and the voltage were measured over the frequency range 200 Hz to 200 kHz in the dark or under an illumination E of 270 lx and with a steady bias voltage of 0–20 V. We used experimental conditions under which the harmonic frequencies were of small amplitude.A discussion is given on the basis of plots of the complex permittivity ε¹, the admittance G and the impedence Z¹. These plots show serious departures from the ideal semicircular form. The accepted interpretation is founded on the concept of a distribution of relaxation times.     

Moisture diffusion in composite sandwich structures

The mechanical properties of polymer core materials in sandwich structures are often degraded by moisture that is absorbed during storage. To date, there is no reliable model to predict the amount of moisture that is present in these sandwich core materials. A multi-layer diffusion model applicable to these sandwich structures is described in this report. Inputs to this model are: (1) diffusivities of core and face sheet materials as functions of temperature, (2) moisture saturation data as a function of relative humidity, and (3) sandwich structure exposure history. The output is a prediction of the amounts of moisture in the core material and face sheets as a function of time.

In order to validate this model, moisture diffusion experiments were performed on a sandwich material consisting of graphite–epoxy face sheets and a core of Rohacell^® polymethacrylimide 200WF foam. Samples of this material were dried, and then hydrated at either 32 °C or 65 °C at either 83% or 100% relative humidity. The face sheets were separated from the core and each component was weighed, dried, and weighed again in order to determine the moisture distribution in the sandwich structure. The results were then compared with the model predictions.     

Fatigue of sandwich structures with inserts

The main purpose of the study was to evaluate fatigue in sandwich structures with inserts. Cross-linked PVC foam with closed cells as the core material was used in this investigation because this foam type has gained widespread use for maritime applications. Fatigue tests for a four-point bending test on a sandwich-beam containing an insert were investigated using numerical fatigue calculations in addition to the finite element method. A small crack initiated in the numerical model was propagated by reducing the stiffness of the finite elements at the crack tip. The J-integral was used when estimating the energy release rate. The stiffness of the insert material was used as a design parameter. It was seen from the investigation that the stiffness of the insert material had almost no influence on the fatigue crack propagation.     

Behaviour of sandwich panels subjected to intense air blast – Part 1: Experiments

Tests that investigate the inelastic response of blast-loaded sandwich structures, comprising mild steel plates and aluminium alloy honeycomb cores, are reported. The “uniform” loading was generated by detonating a disc of explosive and directing the blast through a tube towards the target. Localised blast loading was generated by detonating discs of explosive in very close proximity to the test structure. The sandwich panels responded in a more efficient manner to the uniformly distributed loading, and hence the majority of the paper is concentrated on uniform loading response. The honeycomb sandwich results are compared to test results on structures with air as the core. The failure modes and interaction between the components are discussed. Three phases of interaction are identified for each sandwich structure, based upon deformation, contact, crushing and tearing responses of the sandwich components. The compromise between load transfer through the core and improved energy absorption is discussed.     

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.     

New peel stopper concept for sandwich structures

The paper addresses the damage tolerance of sandwich structures, where the prevention and limitation of delamination failure are highly important design issues. Due to the layered composition of sandwich structures, face–core interface delamination is a commonly observed failure mode, often referred to as peeling failure. Peeling between the sandwich face sheets and the core material drastically diminishes the structural integrity of the structure. This paper presents a new peel stopper concept for sandwich structures. Its purpose is to effectively stop the development of debonding/delamination by rerouting the delamination, and to confine it to a predefined zone in the sandwich structure. The suggested design was experimentally tested for different material compositions of sandwich beams subjected to three-point bending loading. For all the tested sandwich configurations the suggested peel stopper was able to stop face–core delamination and to limit the delamination damage to restricted zones.     

Mechanical performance of through-thickness tufted sandwich structures

This paper presents a study about the influence of through-thickness tufted fibres on compression and bending properties of sandwich structures. The tufting process aims to avoid the delamination between the skin and core in order to improve the performance of sandwich structures, increase the interlaminar strength and damage tolerance of sandwich structures.     

A new hybrid concept for sandwich structures

Sandwich structures are considered as optimal designs for carrying bending loads and can be either metal (aluminium faces and honeycomb or metal foam cores) or polymer structures (composite faces with polymer foam cores). In this paper, a new hybrid sandwich structure has been developed by combining most of the advantages of metallic and polymeric materials while avoiding some of their main disadvantages. For this new concept metal sheets are used at the outer surfaces to maximize rigidity while introducing in between lightweight cores adhesively bonded to keep the whole structure together. Furthermore, composite or wood layers may be used as intermediate layers to improve impact resistance. Potential methods for the manufacturing of this new structure are based on compression under vacuum. The results include the study of several panel configurations theoretically based on Finite element analysis and on the modified simplified equations and experimental results in the most representative cases of the study.     

A study of damage development in a weft knitted fabric reinforced composite. Part 1: Experiments using model sandwich laminates

Coupons consisting of single layers of Milano weft knitted glass fabric reinforced epoxy resin have been tested with the knitted fabric oriented at various angles to the loading direction, alone and sandwiched between outer layers of unidirectional glass/epoxy reinforcement. The sandwich coupons enable the initiation of damage to be observed directly. Tensile tests have shown that the first damage occurs as microdebonding between the loop cross-over points in the knitted fabric structure. Matrix cracking damage develops from these initiation sites and the pattern of cracking is intimately related to the fabric architecture and the fabric orientation with respect to the loading direction. Cyclic tests of sandwich specimens, and a representation of the results in terms of cumulative strain, indicate that some of the acoustic emission activity during loading and unloading of the specimens is likely to be associated with the debonding and pulling-out of knitted fabric tows across the fracture surfaces of the matrix cracks.     

Loss behavior of viscoelastic sandwich structures: A statistical-continuum multi-scale approach

This work presents a multi-scale model of viscoelastic constrained layer damping treatments for vibrating plates/beams. The approach integrates a finite element (FE) model of macro-scale vibrations and a statistical-continuum homogenization model to include effects of micro-scale structure and properties. The statistical-continuum homogenization model makes the micro- to macro-scale transition to approximate the effective behavior of the heterogeneous core by using n-point probability functions. A simple sound transmission model is used to show the effect of material microstructure on the sound transmission loss of the sandwich structure. The damping behavior resulting from the presence of voids and negative stiffness regions in the core material is modeled. This study clearly shows that, it is of high interest to research either material structures or processing techniques which lead to negative stiffness behavior. The results also poignantly show that the proposed multi-scale model yields insight on heterogeneous material behavior leading to increased damping properties and ultimately enhances the ability to design sandwich beam/plates.     

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.     

Actuation and sensing of piezolaminated sandwich type structures

A new piezolaminated sandwich type structure to be eventually used as a smart wall for active control of sound radiated by harmonically excited thin walled structures is proposed. The present study presents the equations of motion of the new adaptive sandwich structures in a sufficiently accurate model in a form readily for solution either in closed-form or by approximate methods. The theoretical natural frequencies are compared with an approximate evaluation and test results yielding a good correlation. It also yields the axial strains and the curvature of the composite beam leading to the calculation of equivalent mechanical loads produced by the piezoceramic actuator for inclusion in a finite element code. The numerical results are compared with experimental ones obtained during a test series on a cantilever sandwich beam equipped with piezoceramic sensors and actuators and constructed according to the new proposed concept. The influence of the input voltage on the performance of the new sandwich structure is investigated. The beam tip deflection induced by the piezoceramic actuators is measured and compared with numerical and finite element predictions to yield a very good match. Both the numerical and the experimental results show the applicability of the new proposed concept.