Cyclic and static fatigue crack growth thresholds

The mathematical foundation for the new double threshold concept is investigated for the analysis of fatigue crack growth. The model consists of two major elements; an intrinsic crack growth threshold which corresponds the material resistance to the fatigue crack growth due to the reverse yielding at the crack tip, and a maximum stress intensity factor threshold which contributes to the possible change of crack growth mode and the crack closure mechanism when the tensile plastic deformation ahead of crack tip is very small. These two thresholds are proposed as material parameters to determine fatigue threshold condition. The mathematical model is developed based on the double threshold concept so that only three parameters are required to determine the threshold for various different materials. The model is successfully used to characterise the crack growth threshold for varieties of materials with significantly different features. The model is also randomised to account for the scatter in fatigue crack growth thresholds. The statistical model has successfully accounted for and explained the widely observed phenomenon that the scatter in the experimentally measured threshold may increase considerably for low stress ratios.