An Observation of Brittle Crack Propagation in Coarse Grained 3% Silicon Steel

Brittle fracture in carbon steel affects serious impact for the safety of fracture of in steel. Especially, the arresting technology of crack propagation is the last stand for the structure. It is so important issue that the conditions which can be stopped reliably crack propagation should be clarified thoroughly. On the background of such social importance, lots of experimental and theoretical researches have been conducted from both mechanical and microstructural viewpoints.Though it has been reported that the limit speed of brittle crack propagation is the Rayleigh wave speed and the speed is about 2,900 m/s in steels, the real speed of brittle crack propagation in steels is about 1,000 m/s and lower. The reason for that is considered braking effects with crack propagation, for example unevenness in the facet, tear ridge, microcrack, twin deformation and side ligament, namely the elements that dominate the arresting toughness.In this study, the 3% silicon steel where microstructure is ferrite single phase and its grain size is 4-5mm of very large is used. It is assumed that the brittle crack propagation speed in single crystal grain is very fast because there is no need to consider tear ridge between crystal grains. In this experiment, it is succeeded in shooting of brittle fracture of steel by high speed camera. As a result of that, it was revealed that the brittle crack propagation rate of 3% silicon steel is much lower than ordinary carbon steel. Also, twin deformation and unevenness in the facet are observed by scanning electron microscope (SEM).