Three-dimensional morphology of fracture surfaces generated by modes II and III fatigue loading in ferrite and austenite

This work is focused on an experimental study of shear-mode crack propagation in the ARMCO iron and the X5CrNi18–10 austenitic steel near the crack-growth threshold. Three experimental setups are used for a generation of cracks in simple-shear, compact tension-shear and torsion specimens. The crack path and the surface topography are studied by means of 3-dimensional reconstruction of fracture surfaces using stereophotogrammetry in a scanning electron microscope. Measurements by means of the profile analysis are used to determine local deflection and twisting angles of the crack with respect to the remote shear direction. The tendency to mode I crack branching is found to be much higher in austenite than in ferrite where both modes II and III cracks propagate close to the plane of maximal shear stress.