A GENERAL METHOD TO INTRODUCE PRE-CRACK IN BULK METALLIC GLASSES FOR PLANE STRAIN FRACTURE TOUGHNESS TEST

We report a simple, low-cost but robust method to introduce pre-crack in bulk metallic glasses (BMGS) for plane strain fracture toughness test. In recent years, bulk metallic glasses have shown potential and promising engineering application due to their excellent properties such as high elasticity, high strength, wear resistance and soft magnetism. As an important material parameter for engineering application, fracture toughness has also attracted great attention in the BMG community. However, there is still challenge on the fracture toughness test of BMGs because of the metastable nature and limited maximum castable size of BMGs. On the one hand, the casting induced thermal history difference and the defects such as internal micropores and impurity inclusions in BMGs, and the way of crack prefabrication will significantly affect the reliability of the fracture toughness test. On the other hand, limited by the sample size, most of the measured fracture toughness of BMGs are not the plane strain fracture toughness, resulting in the reported values showing large deviation, even for the same amorphous alloy. In this work, pre-notched BMG samples were thermoplastically compressed at the notched region to form ideal crack by creep flow. As an example, the fracture toughness of Zr-based amorphous alloy is tested by this method. The experimental results show that as the sample thickness increasing, the measured toughness decreases quickly and tends to a constant value. It should be pointed out that in the experiments, the local thermoplastic compression is designed to obtain a neck shaped pre-crack, which makes the minimum thickness of the specimen being far less than the required thickness by the plane strain fracture toughness testing standard.