On the Growth of Small Fatigue Cracks in Two Squeeze-Cast Aluminum Alloys

It is now recognized that the fatigue lifetime of many structural components is determined by the rate of growth of small fatigue cracks. However it is not a straightforward matter to analyze the growth of these small cracks through the use of a traditional linear-elastic fracture mechanics approach for the following reasons: (1) on going from the micro-crack growth range to the macro-crack growth range, fatigue crack growth behavior undergoes a transition from stress control to stress intensity factor control; (2) because of the large plastic zone size to crack length ratio in the small crack growth regime, crack growth is elastic-plastic in nature rather than linear-elastic, and (3) the crack closure level increases from zero up to the level associated with a macroscopic fatigue crack as the fatigue crack increases in length in the small crack regime. The present paper presents an analysis of short crack growth data in two squeeze-cast aluminum alloys, and also discusses the Nisitani-Goto elastic-plastic relationship for fatigue crack growth.