Experimental and numerical study of interface crack propagation in foam-cored sandwich beams

This article deals with the prediction of debonding between core and face sheet in foam-cored sandwich structures. It describes the development, validation, and application of a FEM-based numerical model for the prediction of the propagation of debond damage. The structural mechanics is considered to be geometrically nonlinear while the local fracture mechanics problem is assumed to be linear. The presented numerical procedure for the local fracture mechanics is a further development of the crack surface displacement method, here denoted as the crack surface displacement extrapolation method. The considered application example is to tear off one of the face laminates from the sandwich. This configuration can be found in many applications but is considered here to be occurring in a ship structure, particularly at the hard spot where the superstructure meets the deck. Face tearing experiments are carried out for structures with three different core densities, material tests are carried out and finally the face tearing tests are simulated with the developed procedure. It is shown that for low core densities, where the crack propagates in the interface immediately below the face sheet, there is fair agreement between experiments and theory. For cores with higher density, the crack tends to propagate in the laminate itself with extensive fiber bridging leading to rather conservative numerical predictions. However, for structural configurations where LEFM can be applied, the presented procedure is sufficiently robust and accurate to be used in a number of important engineering applications, for example risk-based inspection and repair schemes.