Dynamic fracture toughness of magnesium alloy under impact loading conditions

Abstract

In the present study, three-point bend impact experiments on AZ61 magnesium alloy were conducted using an instrumented impact testing machine by Charpy V notch in accordance to ASTM E24.03.03. This test method is to determine dynamic fracture toughness at sharp crack for five different thickness of 2, 4, 6, 8 and 10 mm. A sharp fatigue pre-crack was initiated and propagated to half of specimen width at a constant crack propagation rate of about 1 x 10-8 m/cycle before the specimen was loaded by the impact force until the maximum force is reached and then rapid fracture occurred. The Charpy V notch test was conducted at an impact velocity of 3.85 m/s. The dynamics fracture toughness at sharp crack was determined from the force-displacement history of the load point obtained from measurements of input and reflected strain profiles on the incident specimen. The dynamic fracture toughness Kd values obtained for different thicknesses showed that Kd value decreased with increasing specimen thickness. Scanning electron microscopy was used to elucidate the micro and macro failure mechanism operation during the dynamic fracture event. In particular two micro-mechanisms of failure were of primary interest: (a) fatigue and rapid fracture surface and (b) the development of shear lips at the surface of the specimen follow by fracture.