Mixed-mode (I and II) fracture behavior of a basal-textured magnesium alloy

Abstract

Mixed-mode (I/II) fracture experiments on rolled AZ31 Mg alloy are conducted using notched four point bend specimens along with in-situ imaging. Digital Image Correlation (DIC) technique is used to analyze the images and map out the deformation and strain fields. With increase in mode II component of loading, a monotonic reduction in the notched fracture toughness (Jc) is observed. Fractographs reveal that the fracture mechanism transitions first from (ductile) micro-void growth and coalescence to twin induced quasi-brittle failure and then to shear cracking as the loading changes from mode I to II. Detailed optical micrographs and EBSD maps show profuse tensile twinning in the ligament, especially near the far-edge of the specimen leading to pronounced texture changes. However, the micro void size near the notch tip as well as the twin density in the ligament reduce with higher mode II component. The pronounced drop in fracture toughness and the changes in the associated mechanism with increase in mode-mixity are rationalized from the combined effects of hydrostatic stress, plastic strain localization and tensile twinning.