A Review of Fatigue Crack Growth Thresholds for Metals in Fitness-for-Service Codes: On the Uncertainty of Using the Thresholds at Negative Stress Ratios

Fatigue crack growth thresholds Delta K-th for metals are provided in many fitness-for-service codes. However, fatigue crack growth thresholds at negative stress ratios are not consistently defined. There are two forms of thresholds at negative stress ratios: constant thresholds irrespective of stress ratios, or increasing thresholds with decreasing stress ratios. The definitions of the thresholds at negative stress ratios also take two forms: either Delta K-th = K-max-K-min, or Delta K-th = K-max. ASME Section VIII, Section XI (ferritic steel) and International Institute of Welding (IIW) give constant thresholds expressed by Delta K-th = K-max. American Petroleum Institute (API) 579 and ASME Section XI (stainless steel) give increases in thresholds with decreasing stress ratios and the thresholds are expressed by Delta K-th = K-max-K-min. British Standard (BS) 7910 gives constant thresholds expressed by Delta K-th = K-max-K-min. The fatigue crack growth thresholds differ significantly among different fitness-for-service (FFS) codes. Appropriate thresholds for ferritic steels, stainless steels, and aluminum alloys are demonstrated in the literature survey.