As can be seen from the enlarged figure of the end of the main crack, several secondary small cracks are derived in the process of the main crack expanding inward. The generation and expansion of the secondary crack is conducive to the release of the stress of the main crack. The width of the secondary crack is smaller than that of the main crack, and the number of the secondary cracks is larger. With the continuous generation of secondary cracks, the stress is released, and when the residual stress of multiple secondary cracks is less than the tensile strength of the material, they terminate