Alternating ductile and brittle cracking mode in medium manganese steel sheets

Medium manganese steel sheets exhibit an unusual alternating failure mode transition involving an arrowhead fracture pattern under various loading conditions. Tests were performed using an Arcan setup with different shear-to-tension ratios and specimen orientations to explore the failure mechanisms and unravel the root causes of the transition. The fracture surfaces, characterized by optical and electron microscopy, show a periodic switchover from ductile damage to quasi-cleavage, organized into repeating arrowhead zones pointing towards the crack propagation direction. The step-by-step crack propagation leaves a signature on the load-displacement curve matching the discontinuous cracking events found on the fracture surfaces. A reduction of the stress-intensity factor due to sudden crack advance and associated load drop causes the brittle crack to arrest. Periodic porosity clusters under the fracture surface indicate re-blunting after each cycle of stable/unstable fracture.