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 shearto- 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.