The role of microstructural dissimilitude in fatigue and fracture of small cracks

Abstract

The use of the stress intensity factor, K, as the characteristic driving force parameter for crack extension requires that the conditions of small scale yielding and microstructural similitude be met. The specific effect of microstructural similitude, or its lack, on the crack driving force of both small and short, versus large, cracks is examined in this article. The concept of microstructural dissimilitude is introduced, and its relevance to small crack behavior illustrated by considering the dependence of the number of grains interrogated by the crack front on crack size, and the resulting yield strength variation within the crack tip process zone. It is demonstrated that microstructural dissimilitude can lead to characteristic small crack behavior (anomalous rapid growth) by altering (1) the intrinsic fracture toughness and cyclic crack growth resistance of the process zone, and (2) the local crack driving force. The latter effect is examined in detail by using a modified Barenblatt-Dugdale model to deduce the dissimilitude-induced local crack driving force. The proposed model is then applied for predicting the crack growth and fracture behavior of short and small cracks on the basis of large crack data for a number of alloys. It is shown that the approach may be relevant to subcritical crack growth and fracture in brittle materials like ceramics, as well as to fatigue crack growth in metals.