Observation of micro-cracks beneath fracture surface during dynamic crack propagation

In this study, micro-cracks generated during dynamic crack propagation were investigated for an ESSO specimen and V-notched Charpy specimen. The ESSO test with temperature gradient and the Charpy impact test were conducted for steel for shipbuilding. The macroscopic roughness of the fracture surface increases with increasing temperature. The section beneath the fracture surface generated during dynamic brittle crack propagation was observed using a scanning electron microscope, after cutting with a focused ion beam or grinding. Many micro-cracks can be found beneath the brittle fracture surface in the V-notched Charpy specimen. In the ESSO specimen, which had a macroscopic flat surface, micro-cracks can be detected beneath the main crack propagated in the temperature range from −140 °C to −100 °C. On the other hand, there were a few micro-cracks beneath the main crack propagated at relatively high temperature, at which the fracture surface was macroscopically rough. The number of micro-cracks in the Charpy specimen is fewer than those in the ESSO specimen at the same temperature. The observations indicated that the correlations of Charpy impact properties with crack arrest performance for the ESSO specimen was inconvincible in terms of the differences in the micro-scale fracture behavior as well as lack of theoretical basis.