On the Relation Between Engineering Load-Displacement Curves and True Stress Strain Behavior in Tests of Flat Tensile Specimens

Abstract

The effects of irradiation on true stress-strain constitutive laws associated with large increments of hardening, apparent yield drops, a significant loss of uniform strain and a propensity to microscopic flow localization as derived from the tensile test are evaluated. Analytical and finite element simulations of engineering stress strain curves for flat tensile specimens were performed with systematic variations in the true stressstrain laws. Coupled with large yield stress increases, relatively modest reductions in strain hardening can result in significant reductions in uniform strain. True strain softening eliminates uniform strains, produces apparent yield drops and greatly reduces the post-necking strains. Thus, constitutive laws with varying combinations of yield stress increases and reductions in strain hardening, or modest softening over a limited strain range, rationalize all the major observed engineering stress-strain trends following irradiation. Notably, the true flow stress is persistently increased following irradiation, even at high true strain. Continuum constitutive laws and J2 incremental flow plasticity theory appear to be adequate for modeling the deformation of irradiated materials. While source hardening or micro-localization mechanisms probably play some underlying role, they are neither a necessary nor sufficient condition for producing the behavior observed in tensile tests.