On the role of shear strength in sandwich sheet forming

This paper deals with the design of thin, all-metal sandwich sheets for forming applications. Draw bending experiments have been performed on thin prototype sandwich materials with metallic cores of 10% relative density. The experimental results reveal core shear failure as the dominant failure mechanism. Detailed finite element analysis has been carried out to gain further insight in the mechanics of bending and unbending of sandwich sheets. Based on theoretical analysis, design maps have been constructed describing the required core shear strength as a function of the face sheet properties as well as the sandwich core thickness and the draw radius. Furthermore, the relationship between shear strength and relative density has been determined for perforated sandwich core materials. The main result of this study is that the shear strength of formable sandwich sheets should be at least one order of magnitude higher than that of most commercial cellular solids. Here, perforated sandwich cores of relative densities above 25% are suggested to prevent sandwich sheets from core shear failure in forming operations.