Dynamic Fracture Toughness of Magnesium Alloy under Impact Loading Conditions

摘要

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 K_d values obtained for different thicknesses showed that K_d 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.