Mixed Mode Fatigue Crack Growth Related to Widespread Fatigue Damage
The problems associated with fatigue were brought to the forefront of research by the structural failure of a transport category aircraft in 1988. This failure was attributed to the occurrence of multiple cracking which has since been termed “Widespread Fatigue Damage” (WFD). Figure 1 is a photograph of the multiple cracking that occurred in the upper rivet row of a lap splice joint located in the vicinity of the fuselage crown. Apparently, the fatigue cracks propagate at an inclined angle rather than purely horizontal. The inclined angle of the cracks implies that the fuselage lap joint is subjected to mixed mode loading. The source of the mixed mode loading in the fuselage is assumed to be biaxial stress due to cabin pressurization combined with transverse shear due to body bending. In this paper, a methodology to perform mixed mode fatigue crack growth calculations is proposed to predict the growth of inclined cracks emanating from an open hole in an infinite medium subjected to remote stress. This methodology is based on an engineering approximation of a curved crack modelled as a straight crack of equal stress intensity. Two different criteria are considered to determine the crack trajectory or angle of crack propagation. Also, different crack growth rate models are assumed for each criterion. In particular, the maximum principal stress criterion  is used in conjunction with an effective Paris-Walker type equation, and the strain energy density criterion  is used with a corresponding crack growth model.
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