Fatigue crack closure investigation using Moiré interferometry

An interferometric method was applied to whole-field measurement of in-plane surface displacements adjacent to fatigue cracks in test specimens. The results were analysed to show the development of crack opening profile with increasing load and the interaction of crack closure mechanics with the effective range of stress intensity. A characteristic interference fringe pattern was also identified, associated with localized separation of the crack faces. The main finding of the study was that transition from the fully closed crack state to the point of complete separation seems to occur over a much wider range than noted previously in the literature. Some experimental support was generated for a closure model given by Newman which describes the dependence of the closure point on R ratio.