Characterisation of the crack tip plastic zone in fatigue via synchrotron X-ray diffraction

Abstract

This paper describes a new methodology for characterising the plastic zone ahead of a fatigue crack. This methodology is applied to a set of experimental data obtained by synchrotron X-ray diffraction on a bainitic steel compact tension specimen. The methodology is based on generating the equivalent Von Mises strain field from the X-ray experimental elastic strain maps. Based on the material response, a threshold is then applied on the equivalent strain maps to identify the size and shape of the plastic zone. The experimental plastic zone lies between the plane strain and plane stress Westergaard's bounds but closer to the plane strain theoretical prediction confirming that the volume analyzed is predominantly subjected to plane strain conditions. However, the observed plastic zone has a somewhat flatter shape, extending further from the crack plane but less extended in the crack growing direction.