Design fatigue curves based on small crack growth and crack closure

The propagation behavior of short cracks cannot be studied by linear elastic methods because of the large plastic region near the crack tip, as well as a break down in correlation of the stress intensity factor with the fatigue crack growth rates. The proposedfatigue design approach incorporates a distanceparameter in conjunction with linear elastic fracture mechanics and effectively integrates long and short crack growth test data. This distance parameter is a material constant, which allows for the effects of (a) large-scale plasticity, (b) crack closure,and (c) fatigue crack threshold. Furthermore, this parameter successfully predicts fatigue crack growth behavior of short cracks. The practical applicationof this method is for studying fatigue crack initiation in pressure vessels and is based on the concept that initiation occurs only when the material ahead of the crack tip is damaged enough by cyclic straining. In this paper, the initiation and growth of small cracks have been investigated along with consideration for crack closure. These results provide the design fatigue curves for some typical structural materials and lead to realistic estimates of fatigue lives for materials used in pressure vessels.The techniques outlined in this paper are equally applicable to materials used in aerospace and automotive industries.