Composite members in which the outer layer is a relatively thin metallic skin, when subjected to compressive and/or flexural loads are likely to exhibit skin buckling in which sections of the skin become separated (delaminated) from the core material and buckle away from it. This type of problem may be modelled as plates resting on tensionless rigid foundations. This paper examines the elasto-plastic post-buckling behaviour and the ultimate strength of such unilaterally constrained skin sheets under compressive loads. Plates with different dimensions are analysed using FE package ABAQUS. The results show that unilaterally constrained plates exhibit three distinct behaviour regimes: initial elastic buckling, elasto-plastic post-buckling and secondary buckling. The secondary buckling phenomenon occurs in either the elastic stage or the plastic stage depending on the thickness-width ratio and the yield stress of the plate. Based on a comparative study of the behaviour of constrained and unconstrained plates, it will be shown that essential aspects of the elasto-plastic post-buckling response of a clamped plate resting on a tensionless rigid foundation can be reproduced by the simpler case of a clamped plate without foundation restraint. As a consequence, it becomes possible to use simple effective width-based formulas from AS/NZS 4600:1996 (for unconstrained plates) to deduce ultimate strengths for the more complex restrained case.
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