New technique for evaluating long range internal back stresses

Abstract

A new technique is proposed for determining a measure σ i of the long range internal back stresses in a deformed material. It is based on the inelastic back strain measured as the difference between the maximum observable and the instantaneous back strain accompanying a reduction in stress. Stress reduction tests have been done during steady state creep of Al-11% Zn at 523 K and stress σ to determine the stress dependence of the creep rate at constant structure, the duration of back flow after stress reduction, and σ i. In contrast to the conventional (dip test) technique the new technique yields a constant value of σ i σ ≈ 0.5 for steady state creep. After a large stress reduction σ i falls rapidly below the reduced stress. The stress dependences of the creep rate at constant structure and the duration of back flow can both be described in terms of thermally activated glide with activation areas which depend on σ and are nearly equal to each other at a given σ.