An analysis of sustained mixed mode fatigue crack growth

This paper presents the results of a continued study to better understand the causes of fatigue crack growth under mixed mode loading [Magill, M. A. and Zwerneman, F. J., Sustained Mode I and Mode II fatigue crack growth in flat plates. Engineering Fracture Mechanics, 1996 (in press)]. The study involved analyzing a specimen capable of maintaining crack growth on a plane oblique to the direction of the applied load. For the ''Mode I/Mode II'' specimens analyzed, the crack was forced to grow in a direction other than perpendicular to the load. The resulting crack front did not remain straight and flat, but stabilized into a curved or warped shape. Based on finite element analyses of these curved specimen cracks, it is concluded that the stress intensity factors (SIF) were predominantly Mode I with the Mode II and III SIFs being negligible. Further study has revealed that the ''mixed mode'' cracks grew perpendicular to the maximum principal stress. A ''Mode I/Mode III'' specimen was also developed and tested. A ''rough'' finite element model of the specimen computed mode III SIFs that are approximately one-half of the mode I values. Copyright (C) 1996 Elsevier Science Ltd