Characterization of elastic-plastic fracture toughness of AM60 Mg alloy under mixed-mode loading conditions

Abstract

In this paper, an estimation of the elastic-plastic fracture of the AM60 alloy under pure mode-I, mixed-mode, and pure mode-II loading conditions has been investigated. For this purpose, a modified Arcan-type device and a butterfly specimen were used. Fracture tests were conducted according to ASTM standard E1820; however, due to the standard being limited to some specific geometries and the mode-I loading, the relationships and parameters presented in the standard, including the non-dimensional stress intensity factor, the geometric work factor, and other parameters were extracted and analyzed using finite element analysis for the butterfly specimens and under different loading conditions and relationships developed for them. In the following, the fracture tests for the butterfly specimen were carried out under mode-I, 45° mixed-mode, and mode-II loading conditions and the material resistance curves against crack growth (J-R) were extracted. As a result, the material fracture toughness was extracted for each. Also, for the verification of the fixture used in this study, a beam-shaped specimen test was arranged, and its results were compared with the values of mode-I obtained from the modified Arcan fixture. The results indicated that, by changing the loading angle from mode-I to mode-II, the critical values of the fracture took on a decreasing trend, which indicated the greater resistance of the material to crack growth under the conditions of mode-I (tension) compared to mode-II (shear). On the other hand, the difference between the results obtained from the fixture used and the beam-shaped specimen was less than 9%, which indicates the correct performance of the fixture. Finally, in order to get a deeper understanding of the material fracture mechanism under different loading conditions, the fracture surfaces were examined using scanning electron microscopy (SEM).