Fatigue crack tolerance design for stainless steel by crack growth analysis

Fatigue damage of stainless steel was assessed by crack growth analysis. In order to characterize small crack initiation and growth, fatigue cracking behavior was observed by periodical replica investigations during a strain-controlled fatigue test in air at room temperature. It was shown that fatigue cracks with depths of several tens of micrometers were initiated in the early stage of the fatigue test and fatigue life could be estimated as a sum of the cycles for crack initiation of 100 μm in depth and those for growth to 3 mm. Use of the equivalent stress intensity factor allowed prediction of the crack growth for a given strain range. The crack growth analysis was also made to estimate the fatigue life prescribed by the design fatigue curve. Then, the growth prediction was made for thermal transient conditions to take advantage of the stress gradient in the depth direction and the relationship between the cumulative usage factor (CUF) and crack size was shown. It was concluded that fatigue damage accumulated in a stainless steel component can be estimated from an identified crack size, or the maximum CUF can be determined even if no crack was detected by the inspection.