Abstract: The inconsistency of shear stress positive directions in planar and spatial stress states in current Material Mechanics is indicated．The shear stress positive directions of planar stress state in current Material Mechanics is suggested to be reversed． Based on the new direction，the formulas of stresses on an incfine and the corresponding Mohr circle are presented． To introduce stress matrix to calculate its main stresses．main directionS and the stresses in any coordinates are advised．The teaching practice proves its feasibility．

Key words: MateriM Mechanics, shear stress, planar stress state, stress matrix

摘要： 指出了现有《材料力学》平面应力状态分析中剪应力正向与空间单元体的正向不一致问题，提出了调整剪应力 正向的建议，介绍了调整剪应力正向后平面应力的计算公式和应力圆的相应画法，提出了在调整剪应力正向后在应 力分析教学中引入矩阵计算方法的建议．教学实践证明了改进方法的可行性．

关键词: 材料力学, 剪应力, 平面应力状态, 应力矩阵