In situ surface displacement analysis on sandwich and multilayer beams composed of aluminum foam core and metallic face sheets under bending loading

Deformation and damage mechanisms of sandwich and multilayer beams composed of aluminum foam core and metallic face sheets under four-point bending condition have been investigated in situ by surface displacement analysis software (SDA). Plastic collapse of the sandwich beams within the foam core is by two basic modes, indentation (ID) and core shear (CS), while failure of the multilayer beams depends on if their face sheets show ID or not. If a sandwich fails by ID, the multilayer beams with similar face sheets show ID mode in the first core and CS mode in the other cores of the beam. However, multilayer beams with face sheets that only display CS mode in sandwiches may fail in pure CS in every core. SDA results illustrated that indentation is locally compressive deformation on the beam beneath the loading rollers, where displacement and compressive strain are the maximum. In the case of CS that happens in the core between inner and outer rollers that corresponds to the maximum shear strain, discontinuously horizontal and vertical displacement is the mechanics factor for shear crack initiation, growth and broadening, and beam collapse. A simple criterion is given as F ID > F CS CS mode taking place, and F ID < F CS ID mode taking place, for prediction of the failure mode, where F ID and F CS are the loading limits of the indentation and core shear, which can be expressed as 4 t( σ cy × σ fy) 1/2 and 2( c + d) τ cy, respectively ( σ cy: yield strength of foam, σ fy: yield strength of face sheet, τ cy: shear yield strength of foam, c: thickness of core, d: thickness of beam and t: thickness of face sheet). Beams tend to fail in ID mode when the thickness and strength of face sheet decreases. The CS mode may dominate failure when the thickness of core increases.