Strain hardening reduces energy absorption efficiency of austenitic stainless steel foams while porosity does not

Abstract

Strain hardening significantly affects the mechanical function of steel foams. We compare hardening and failure of two commercially available austenitic stainless steel foams (316L and 310) spanning strut porosities of 9.4 to 14.4%. Damage is correlated with strut microstructure and geometry, combining in-situ quasi-static compression testing in the SEM, 3D-evaluation by synchrotron μCT and bending simulations. We provide an analytical model for the experimentally observed strain hardening. Upon compression, 316L steel foams exhibit a plateau regime of continuously increasing stress due to the hardening effect, whereas 310 steel foams show almost constant plateau stress. This is explained by the much less ductile behaviour of the 310 steel foam struts as compared to 316L steel foam struts. Finite element modelling suggests that significant stress concentrations develop around microporosities in the 310 struts. Due to its finer and less porous microstructure, the 316L foam exhibits a larger energy absorption capacity than the 310 foams. This results in distinctly different efficiency-strain curves. However, up to about 25% strain, the efficiency values are surprisingly similar. Thus, modification of microstructure and/or pore micro-geometry can be used to optimise the stress-strain response to achieve the desired energy absorption property of steel foams.