Sandwich Contruction and Core Materials Part III. Instability of Sandwich Struts and Beams

Abstract

The purpose of the core in a sandwich structure is to stabilize the skins against local failure and to enable them to work together as a single beam, having a moment of section relatively much greater than that of the two skins separately. To fulfil these functions the core must restrain the skins from relative lateral movement and also limit relative shear movement between them; a core which resisted shear only would permit out-of-phase buckling at a load determined purely by instability of the skins as struts; whereas a core which resisted only lateral separation of the skins would permit in-phase buckling at the same load. In Ref 2 the incidence of these two types of instability was examined in respect of an artificial structure, in which in effect the core had an infinite stiffness longitudinally. This analysis indicated that the in-phase type of instability was of the modified Euler type over a long wavelength, the true Euler load being reduced by shear deformation of the core; and that the out-of-phase mode should occur over a short wavelength and should depend principally on the modulus of the core material in the transverse direction. The formula derived for the critical strain in this out-of-phase mode included also an additive term proportional to the shear stiffness of the core; owing to the inherent assumption that the core material was infinitely stiff longitudinally this effect was known to be overestimated.