The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section XI utilizes reference fatigue crack growth rate (FCGR) curves for flaw evaluations. The current ASME reference curve for austenitic stainless steels in air environments is a Paris-Law relation with a single ΔK exponent that covers the entire ΔK range. Since generation of the model that became the ASME reference curve, extensive additional FCGR testing of Type 304, Type 304L, and Type 304/304L dual-certified stainless steel and the corresponding weld metal has been performed in an elevated temperature air environment. This testing revealed fatigue crack growth (FCG) behaviors that were not adequately captured by the ASME reference curve. In particular, the ASME reference curve failed to capture a flattening of the FCGR curve in the intermediate ΔK range before the FCGRs sharply dropped off as the threshold behavior is approached. Additionally, the FCGR data showed a slight frequency-dependence. Based on this new data, a new FCGR model was generated for Type 304 austenitic stainless steels in air environments between 250°C and 338°C. A tri-linear Paris-Law style correlation was chosen for the updated FCGR model to accommodate both the flattening of the FCGR curve at intermediate ΔK levels and the sharp downturn in the near-threshold ΔK regime. Each of the three branches of the FCGR curve exhibit a different R-ratio dependence, with the near-threshold regime being the most sensitive to changes in the R-ratio.

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