Imperfection-induced Wrinkling Material Failure in Sandwich Panels

In this paper, sandwich wrinkling or local face sheet instability is treated in the context of material failure. Traditionally, test results rarely complied well with the predicted failure load and a knock down factor often had to be used. The reason for this is often referred as the effect of initial geometric imperfections. In this paper, imperfections are included in the natural waveform given by the linear stability analysis, i.e., a short wavelength sinusoidal buckling shape. These initial imperfections lead to increased displacements during loading giving rise to both, an in-plane compressive strain and a varying bending strain. These strains can then be related to material failure criteria, one for the face sheet compressive strain and one for the core normal strain. An analytical model is derived and compared with experimental results and several issues are revealed. The panel strength measured using a realistic initial imperfection amplitude agrees very well with the derived model, giving a prediction somewhat below the values obtained from the traditional approach. This verifies that the actual wrinkling failure is below the theoretical instability load. The model is able to distinguish between different failure modes, face sheet compression failure or core-adhesive joint tensile failure, giving good correlation with the experimental findings. Thus, it appears that using initial imperfections as a basis for wrinkling analysis provides a better foundation for the failure analysis than the ordinary stability analysis, and it also allows to determine which failure mode is predominant. Finally, it is shown that the choice of the core material can be made based on the theory presented to obtain a more efficient sandwich panel.