An extension of the validity of a theory proposed at the Crack Propagation Symposium in Cranfield, September 1961, has been performed by introducing a new parameter, the endurance limit n,, i&o the basic formula, which thus takes the form dx/&? = k(o -a,)@.
The relationship between crack length z and number of cycles N has beeu derived for two alternative cases, viz. constant stress cycles (u = constant) and constant load cycles (a(1 --x) = constant). In the first, case the result implies that r be a linear function of N, which has amply been verified by tests under various conditions. In the second case, a quantity y, which is a function of 2 involving ,!l and a, as parameters, and N are linearly related. The successive increase in the stress amplitude produces sudden changes in the mode of failure, which can be observed in the fractured specimens and are indicated by diseontinuit,ies in t,he curves, if plotted on appropriate scales.
The decisive influence of t.he stress level on the relative lengths of t,he different stages of propagation is demonstrated by a diagram. Results from various test series emphasize t,he necessity of splitting up the crack propagation period into three stages. Within each of them y is a linear function of&X, provided proper parameter values are used.