Novel micro-scale specimens for mode-dependent fracture testing of brittle materials: A case study on GaAs single crystals

To study microscale fracture behavior under mode II and mode III loading new sample geometries are proposed. Finite element simulations were carried out to determine the mode I, II and III stress intensity factor distributions over the sample width as well as the geometry factors. Notched samples were prepared in GaAs single crystals by focused ion beam milling. Two crack systems were investigated to improve the understanding of the cleavage behavior in this anisotropic material. Critical stress intensity factors were measured for mode I, II and III loading for the first time on the microscale. Fracture toughness under mode I loading was measured using micro-cantilevers and found to be 0.54 ± 0.04 MPaSRm and 0.50 ± 0.05 MPaSRm for the {100}〈110〉 and {110}〈100〉 crack systems, respectively. In-plane shear loading (mode II) led to crack kinking and a fracture toughness of 0.48 ± 0.05 MPaSRm and 0.72 ± 0.01 MPaSRm. Out-of-plane shear loading (mode III) led to multiple kinking processes and the highest fracture toughness of 0.80 ± 0.13 MPaSRm and 0.84 ± 0.07 MPaSRm. This is most likely due to the pronounced fracture anisotropy of GaAs single crystals and the comparably low resolved stresses on the {110} type cleavage planes, which demand higher critical loads for failure.