Interactive buckling in sandwich structures

Abstract

A progressive destabilization in compressed elastic structures of sandwich construction is identified, whereby a triggering bifurcation into an overall mode of buckling is rapidly followed by secondary bifurcation into an unstable combination of at least two local modes. A six degree-of-freedom analysis for a sandwich strut is provided, which tracks equilibrium paths for a perfect system, specifically pinpointing states of secondary bifurcation. The analytical model allows for the independent variation of bending and shear in the section, components that are found in significantly different combinations in the overall and local forms of buckling that make up the interaction. An optimum wavelength for local buckling is identified, which maximizes the interactive effect by bringing the primary and secondary bifurcations together. Examples of sandwich sections drawn from the literature are tested. Although the critical loads of two of the contributing modes are often considerably higher than the third (typically by multiples of two or more), the preference for the broken symmetry of the interactive form is such that secondary bifurcation into the combination of modes is always to be expected.