说明：  摩尔应力圆 根据给定的正应力和切应力画出摩尔应力圆 程序在一个数值的计算上可能有BUG 但一般计算时没有问题
(Mohr stress circle to draw molar stress circle program according to a given normal stress and shear stress in the calculation of a value may have BUG, ​ ​ but there is no problem in the general calculation)