The effect of shear on the local indentation and failure of sandwich beams with polymeric foam core loaded in flexure

Three-point and four-point bending tests were carried out on sandwich beams consisting of glass-fibre-reinforced plastic facings and a Divinycell H60 foam core. During the tests, final failure was invariably precipitated in the upper facing at the points of load introduction, whereas no other failure modes typically exhibited by a sandwich structure were observed. From the experimental results, the critical load causing final failure was found to be independent of the overall bending moment M and local force Pl alone, in contrast with the hypotheses of the theoretical models presently available in the literature. On the contrary, for a given M/Pl ratio adopted in the tests, the critical conditions were achieved more easily in the case of four-point than under three-point bending. A possible explanation of the observed phenomenon was given by calling into question the influence of the shear force, that may contribute to the sandwich local collapse. An empirical failure criterion, taking into account the bending moment, the local force and the shear force, was found to reasonably match the experimental results, although the physical meaning of one of the parameters appearing in the model formulation is matter of further research.