Effect of notch acuity on the fracture behavior of AZ31 Mg alloy

In this work, the effect of notch acuity on fracture behavior of rolled AZ31 Mg alloy is investigated by conducting mode-I fracture experiments using four point bend specimens with different notch root radii. Digital Image Correlation (DIC) technique is employed to determine the angle of rotation of the specimen and the fracture toughness is obtained from moment versus rotation curves. It is found that the fracture toughness Jc increases linearly with notch root radius beyond a threshold value and is almost constant below this limit. This trend is rationalized from fractographic observations near the crack initiation region which show predominantly quasi-brittle features for fatigue pre-cracked specimen and dimples of increasing size in notched specimens with enhancement in notch root radius. Significant tensile twinning is noticed in the ligament for all specimens, especially near their far edge. This is found to impart substantial toughening even in the fatigue pre-cracked specimen which fails by a locally quasi-brittle mechanism. Finally, the strong increase in Jc with notch root radius beyond a threshold is interpreted using two ductile fracture initiation models.