As can be seen from the enlarged view of the end of the main crack, some secondary cracks are generated during the process of the main crack expanding inward. The generation and propagation of the secondary crack are beneficial 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 secondary cracks is larger. As the secondary cracks are generated, the stress is released, and when the residual stress of multiple secondary cracks is less than the tensile strength of the material, they terminate