Mixed mode fatigue and fracture in planar geometries: Observations on K_(eq) and crack path modelling

摘要

The mixed mode I-II fatigue and fracture is briefly reviewed, addressing experimental and numerical modelling aspects, and focusing on planar specimens. One major challenge concerns the determination of equivalent stress intensity factor (K-eq) in mixed mode situations. Several approaches were compared through the determination of K-eq/K-I over a wide range of values of K-I/K-II or K-II/K-I. Whereas all different approaches converge to the same value as K-I/K-II increases, the same does not happen for large K-II/K-I, where differences between values of K-eq persist. In the regions of 0 K-I/K-II K-II/K-I < 2, no stable trend of results can be defined. Experimental fatigue crack growth results are presented for Al alloy AA6082-T6. Compact tension specimens, modified with holes, and four-point bending specimens under asymmetrical loading promoting mixed mode situations, were subjected to fatigue crack growth tests, where crack path and crack growth rate were measured. The presentation of the fatigue crack growth data was made using a Paris law based upon K-eq. Differences in the Paris law constants were found for the different K-eq criteria. Recent developments in numerical techniques, as the implementation of the extended finite element method (XFEM) in finite element software packages allows to determine accurately crack paths in mixed mode fracture. This article highlights concepts for mixed-mode fatigue and fracture and supporting data, identifying challenges still to be overcome.