According to the gradient plasticity theory, we introduced two- class strain gradient into yield function, derived from the formula of deformation localization band width by using Drucker - Prager yield criterion and gave the internal length of coal.

• 并通过应变梯度塑性理论，在屈服函数中引入应变梯度二阶项。 采用Drucker－Prager准则解析得出了煤试件的变形局部化带宽度公式并根据试验确定煤的材料内部长度。