I~ UTTE RWORTH E I N E M A N N Composites 26 (1995) 803-814 9 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0010-4361/95/$10.00 Approximate elastic-plastic analysis of the static and impact behaviour of polymer composite sandwich beams R. A. W. Mines* and Norman Jones Impact Research Centre, Department of Mechanical Engineering, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK (Received 29 March 1995; revised 16 June 1995) 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 theor- etical 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. (Keywords: elastic-plastic analysis; sandwich beam; impact behaviour) INTRODUCTION Structural members using sandwich construction are designed usually in terms of flexural stiffness and strength. However, some structures must be capable of sustaining large overloads, e.g. foreign object impact 1 or impact with solid objects 2. In these cases, the non-linear (post-failure) behaviour of the structure is important and the parameters of interest are the integrity of the structure and the energy absorbing capabilities of the structure before total collapse. Given the complex- ity of structural response in the general case, it was decided to study a simple structural element (e.g. a beam) in the first instance. Previous experimental work 3 has examined the behav- iour of selected polymer composite sandwich beams under static and dynamic loading in a three-point bend configuration. A number of different failure modes were found which depend on the beam construction and span. In general, up to eight possible modes of failure may occur for sandwich beams 4, including upper skin wrink- ling, upper skin compression failure, lower skin tensile failure and core shear. For thin skinned and large spanned beams, the first three failure modes are the most likely, and, for thick skinned and short spanned beams, * To whom correspondence should be addressed the last failure mode is the most likely. Ref. 3 compared constructions having different polymer composite skins and with two core materials, namely aluminium honey- comb and Coremat. The latter is a proprietary material and is a resin impregnated non-woven polyester with a binder and 50% microspheres. Upper skin compression failure in the vicinity of the central loading point of the beam was the most common failure mode observed in this experimental work and this failure mode gave stable energy absorbing behaviour after initial skin failure. The beam materials and geometries selected for study are typical of transport and lightweight marine applications, viz. thin skins and intermediate values of beam span-to- depth ratio. The elastic analysis of sandwich beam construction is well documented 4-6. However, little post-failure, non-linear, theoretical work for composite beams has been published except for aramid-reinforced laminated beams. Zweben 7, Reedy s and Fischer and Marom 9 used simple beam models in which the compression plasticity of aramid fibre gives rise to an elastic-plastic hinge at the loading point of a three-point bend specimen. In the case of sandwich construction, the non-linear, crushing behaviour of the core contributes to the behav- iour of the elastic-plastic hinge at the loading point. The crushing behaviour of foams has been studied in some detail 1~ and the failure of foams under multi-axial COMPOSITES Volume 26 Number 12 1995 803