The influence of specimen thickness on the midplane plastic zone using synchrotron X-ray diffraction data

Abstract

Analysing and measuring the plastic zone surrounding a fatigue crack tip is essential for a comprehensive understanding of fatigue and fracture behaviour. In this work, the strain field near the crack tip was mapped for two samples having different thicknesses, namely 3.3 mm and 12 mm, using in-situ synchrotron diffraction during a fatigue test. Full diffraction pattern refinement enabled strain maps with a resolution of up to 80 µm. Assuming a two dimensional state of stress or strain, depending on the sample thickness and due to the limitations of the experiment, the von Mises equivalent stresses and strains in the region ahead and surrounding the crack tip were mapped at each stage. The equivalent strain energy density rule is used, taking into account cyclic material parameters, to understand the elasto-plastic fields around the crack-tip. The plastic zones were then characterised and compared to simple plane stress and strain analytical models of the plastic zones. The plastic zones closely align with the analytical models in size, though shape variations exist. The sample thickness was found to have a strong influence on the final shape and size of the PZ.