说明：  一受均匀拉力的板，中心带贯穿裂纹，裂纹长2a，使用4节点单元计算裂尖应力分布和应力强度因子
(A uniform tensile plate with a through crack in the center and a crack length of 2a is used to calculate the stress distribution and stress intensity factor at the crack tip using a four node element)