Third‐Power Stress Dependence in Creep of Polycrystalline Nonmetals

Published results on the high‐temperature creep of polycrystalline nonmetals indicate that under high stresses many of them obey the equation εασ n , where ε is the steady‐state creep rate, σ the applied stress, and n a constant which is usually either 3 or 5. A fifth‐power stress dependence is observed in most pure metals; the creep rate is thought to be controlled by a slip creep process involving dislocation climb. Third‐power dependence is not merely a combination of fifth‐power‐dependent slip creep and first‐power‐dependent Nabarro‐Herring creep but appears to be controlled by a separate mechanism. This mechanism may be associated with Nabarro's dislocation climb model or with Weertman's microcreep model involving charged dislocations. It appears that the ratio of the anion radius to the cation radius is usually >2 in nonmetals for which n =3 and >2 in those for which n =5.