Defining Yield Stress and Failure Stress (Strength)

Abstract

Definitions of yield stress and failure stress (strength) have always been subject to great variations in understanding and implementation. There is no agreement on the proper definitions of these properties that are needed for the use of failure criteria. There are only individual preferences that usually are not even stated when reporting data-derived properties. Rational definitions are derived for both the yield stress and the failure stress. The yield stress definition is that of the stress at which the second derivative of the stress versus strain attains a maximum. Unfortunately this definition is difficult to implement from typical testing data, so an approximation is derived in an “offset” form. The failure-stress definition is specified in terms of some energy characteristics that are easily found from the stress–strain curves. Both definitions for yield stress and failure stress can be used with the failure criteria derived from relevant failure theories.