Development of New S-N Curves Based on Small Crack Methodology

The propagation behavior of short cracks cannot be studied by linear elastic methods because of large plastic region near the crack tip, as well as a breakdown in correlation of the stress intensity factor with the fatigue crack growth rates. The proposed fatigue design approach incorporates a distance parameter in conjunction with linear elastic fracture mechanics and effectively integrates long and short crack growth test data. This distance parameter is a material constant that allows for the effects of (a) large-scale plasticity, (b) crack closure and (c) fatigue crack threshold. Furthermore, this parameter can be used to successfully predict fatigue crack growth behavior of short cracks. The practical application of this method to study fatigue crack initiation in pressure vessels rests on the concept that initiation occurs only when the material ahead of the crack tip is damaged enough by cyclic straining. The initiation and growth of small cracks have been investigated along with consideration for crack closure.