On the Growth of Small : Short Fatigue Cracks

Abstract

In this paper a review is made of the stress concentration and the stress intensity factor approaches to dealing with fatigue crack growth. For small cracks, i.e., those which initiate in unnotched specimens, and for short cracks, i.e., those which initiate at notches, the stress intensity factor approach is not useful because the factor goes to zero for such cracks. On the other hand in the stress concentration approach the limiting stress is always the applied stress. A modification to include a material constant in the expression for the stress intensity factor is proposed based upon an analogy with the stress concentration approach. The modified stress intensity factor is used in the analysis of surface crack growth. The growth of small/short cracks is also analyzed with consideration being given to the effects of crack closure, and the observation that at a given ΔK level in the short crack range the shorter the crack the higher the rate of growth is discussed. Comparisons are also made between experimental results for short cracks growing from notches and predictions based upon the proposed method of analysis.