It is generally believed that multiple fatigue crack growth prediction is difficult with the use of conventional stress intensity factor (SIF) solution calculations because of issues such as SIF magnification and shielding effects. Therefore, almost all the existing Fitness for Service (FFS) rules such as the ASME Code Section XI and JSME Code adopt the procedure whereby multiple cracks grow independently after applying a certain alignment rule based on the initial crack configuration and are combined immediately into an enveloping crack when the crack tips touch. In some cases, the results of the procedures in the existing FFS rules are less accurate in predictions of the service life of cracked components. Therefore, there is still room for improvement, although the procedures are simple for utilities. This paper describes a new approach to predict fatigue crack growth life of multiple nonaligned cracks by the use of SIF solutions modified by empirical interaction factors. Several examples of two nonaligned cracks illustrate the accuracy and effectiveness of the procedure by comparison with numerical analysis by the body force method for two-dimensional problems and with the experimental results given in the literature for three-dimensional problems.

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