In situ surface displacement analysis of fracture and fatigue behaviour under bending conditions of sandwich beam consisting of aluminium foam core and metallic face sheets

The present paper deals with the study of the mechanical behaviour of sandwich beams under monolithic and cyclic bending. The strains and displacements were monitored by in situ surface displacement analysis (SDA) software. The sandwich beams consisted of an aluminium foam core covered by metallic face sheets. For a certain sandwich geometrical configuration, the operative failure mode is the same under both monolithic and cyclic bending conditions. Indentation (ID) and core shear (CS) are the basic failure modes, which depend entirely on the geometry of the sandwich beam. The SDA results show that ID is localised compression of the beam adjacent to the inner rollers, where the displacement and strain are at a maximum. As for the CS failure mode, failure happens within the core between the inner and outer rollers, where shear crack initiation and growth correspond to the maximum shear strain, accompanied by discontinuous displacements in both vertical and horizontal directions. Owing to constraint of the weak foam core imposed by the strong metallic face sheets in a sandwich beam, the core can bear stresses higher than the yield strength of monolithic foam by a factor of 1.6-3.2 for both CS and ID failure modes. This factor depends mainly on the geometry of the sandwich beam. Similarly, the fatigue limit of the core in the sandwich beam is also higher than the fatigue strength of monolithic foam. The stronger is the constraint, the higher is the fatigue limit of the core. The fatigue limit of a sandwich beam also relates to the failure mode. At fatigue ratio R=0, the fatigue limit of sandwich beams failing by the ID mode is higher than that of beams which fail by the CS mode.