On perfectly plastic flow in porous material

Abstract

Experimental observations suggest that for perfectly-plastic materials containing pores, the (small) strain at which significant macroscopic yielding occurs is relatively insensitive to porosity, for volume fractions below approximately 15–20% (although the yield stress drops significantly with increasing porosity). Another observation is that, at these porosity levels, the stress–strain curve remains approximately linear almost up to the yield point. Based on these observations, Sevostianov and Kachanov constructed yield surfaces that explicitly reflect the shapes of the pores and their orientation. The underlying microscale mechanism is that local plastic “pockets” near pores blunt the stress concentrations; as a result, they remain limited in size and well contained in the elastic field until they connect and almost the entire matrix plasticizes within a narrow interval of stresses that can be idealized as the yield point. The present paper provides direct insight into the micromechanics of poroplasticity through direct microscale numerical simulation. Besides confirming the basic microscale mechanism, these simulations reveal that the reduction of the macroscopic poroplastic yield stress is approximated quite closely by 1−v2 times the dense nonporous yield stress, where v2 is the volume fraction of the pores.