Extensive programmes of fatigue crack growth tests on many metals in various environments have been conducted to obtain the relationship of da/dN and ΔKI by many organizations over several decades, where da/dN is the fatigue crack growth rate and ΔKI is the stress intensity factor range. The fatigue crack growth threshold ΔKth is an important characteristic of crack growth assessment for the integrity of structural components. If a flaw is detected in a structural component during in-service inspection and its stress intensity factor range is less than the ΔKth, the detected flaw is deemed not to grow, and repair or replacement is not required. Fatigue crack growth thresholds ΔKth are important and valuable issues for integrity assessment of the components.
However, it is not easy to obtain the data of ΔKth experimentally, because of fatigue crack growth rate being significantly small and it takes a long time to determine ΔKth. Almost all materials used in these tests were ferritic steels. The fatigue crack growth thresholds ΔKth for ferritic steels were established as rules and they were provided by many fitness-for-service (FFS) codes. However, threshold values for aluminium alloys and austenitic stainless steels are not well provided in many fitness-for-service (FFS) codes, although extensive amount of fatigue crack growth tests data has been published and these metals are commonly used for structural components.
This presentation focuses on fatigue crack growth threshold values ΔKth for aluminium alloys, ferritc steels and austenitic stainless steels provided by FFS codes. The comparison of the current codes of ΔKth is introduced using the ASME (American Society of Mechanical Engineers) Code Sections VIII and XI, the API (American Petroleum Institute) 579/ASME FFS, the BS 7910, the A.M.E. Codes, etc.
Amongst these FFS codes, it is found that the thresholds Kth are defined differently. It is suggested that predicted fatigue lives of the cracked components will be expected different depending on which Kth is used, although the original crack size and loading conditions are the same. The component containing a crack might be imposed un-necessary repair/replacement, or might cause fatigue failure.
Finally, the threshold ΔKth for austenitic stainless steel provided by the ASME Code Section XI is discussed for codification. Code Case N-809 of the ASME Code Section XI provides fatigue crack growth threshold ΔKth for in PWR (Pressurized Water Reactor) environment. However, Appendix C of the ASME Code Section XI defines fatigue crack growth rate da/dN - ΔKI in air environment at room and high temperatures without the threshold ΔKth. The threshold ΔKth for austenitic stainless steels in air environment is useful for fatigue assessment for not only nuclear power but also other industry components. Based on the test data, literature survey and FFS codes, the fatigue crack growth thresholds ΔKth for austenitic stainless steels in air environment at room and high temperatures has been proposed recently in terms of a simple equation for the ASME Code Section XI.
In the next few years, materials science will undergo major changes as simulation and experimental tools will become more and more integrated with data schemes known from the big data community. In the future we will see the introduction of digital representations of materials - or digital twins - which will follow the materials through all relevant materials processing steps. It will be possible to use these representations for materials design, production control as well as lifetime predictions. In this talk we will review driving factors and the implications on research and teaching.