Another measure of subcritical crack growth is the exponent n defined in Eq.(12.30). To measure n directly from velocity versus K1 curves is often difficult and time consuming Fortunately, a simpler and faster technique, referred to as a dynamic fatigue test (not to be confused with the normal fatigue test discussed below), is available. In this method, the strain rate dependence of the average strength to failure is measured;i.e.,the samples are loaded at varying rates and their strength at failure is recorded. It can be shown (see app.12A) that the mean failure stress @1 at a constant strain rate &1 is related to the mean failure stress @2 at a different strain rate &2 by