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Fatigue has often been described as the most common cause of failure in engineering structures, and designers of pressure vessels and piping have incorporated fatigue considerations into design requirements since the first edition of Section III in 1963. The development of this technology and its application in Section III are discussed in Chapter 39 of the third edition of this publication. Its application in Section XI is discussed in Section 32.3 of this chapter.
With the advancement of the state of the art, the ability to recognize how the growth and size of a crack can lead to failure has been enhanced. This technology has been a key aspect of the Section XI flaw evaluation procedures since the 1974 edition was published and will be discussed thoroughly herein.
Further advancements in the state-of-the-art in fracture evaluation resulted in the introduction of models for stress corrosion cracking into Section XI in 2001, and since that time additional models have been added and more are planned. This addition gives Section XI flaw evaluations a complete treatment of all the potential modes of cracking that can occur in components, and details will be provided in Section 32.2.
Since additional crack growth data, for both fatigue and stress corrosion cracking, is periodically made available on a regular basis from additional research testing, the models contained in Section XI are regularly reviewed and updated to ensure their applicability, accuracy, and reflection of the latest known material behaviors. This improved knowledge may lead to more conservative models for crack growth over time. To accommodate such situations, Section XI has adopted an approach to conservatively account for potential increases in crack growth predictions with augmented examinations. The rules of IWB-3600 require that augmented (additional) examinations must be performed at an accelerated pace for situations where a flaw indication was accepted by analytical evaluation. This rule also protects against NDE uncertainties at the location of interest.
The nomenclature used here will be identical to that used in Section XI; any nomenclature that is not present in Section XI will be explained as it appears.
This chapter has been updated using the 2019 Code edition.
Warren H. Bamford is the original author of this chapter and has since revised it for all of the subsequent editions including the current online edition.