Dynamic bending behaviour of magnesium alloy rectangular thin-wall beams filled with polyurethane foam

摘要

This study investigates the load-deflection curve characteristics and deformation/fracture modes and energy absorption capacity for polyurethane foam-filled magnesium alloy AZ31B rectangular thin-walled beams under dynamic three-point bending loads, and compares these characteristics with those for mild steel DC04 beams. Different foam-filled AZ31B beams with a variation of foam density (0.05 g/cm3, 0.20 g/cm3 and 0.30 g/cm3) were fabricated through several manufacturing processes: cold bending, tungsten inert gas welding, cathodic dip painting and polyurethane foam injection. It was found that 0.20 g/cm3 and 0.30 g/cm3 foams stabilised the cross sections of the thin-walled AZ31B beams and no inward folds occurred during the bending process, which resulted in significantly higher load carrying capacity than the empty beam. A nonlinear non-monotonic relationship between the specific energy absorption and the foam density was found for the foam-filled AZ31B beams. The AZ31B beam filled with 0.20 g/cm3 foam reached the highest specific energy absorption; moreover, it absorbed nearly 33% more energy and reached nearly 2.9 times higher specific energy absorption than the foam-filled DC04 beam filled with the same foam, although the former one was nearly 54% lighter. This outperformance is associated to the high work hardening rate of AZ31B in compression, where more material is involved in plastic deformation. However, the foam-filled AZ31B beams tend to fracture at the compression and tension walls, because the foam exhibits brittle fracture behaviour in tension and AZ31B exhibits low ductility in compression and plane-strain conditions, which limits their energy absorption at a larger deflection.