应用弹塑性力学的基本原理，推导了由材料单调拉伸应力─应变（σ－ε）曲线求得三向应力状态下应力－应变（σ1－ε1）曲线的表达式。实验结果验证了理论分析结果的正确性，并用σ1－ε1曲线解释了材料在受三向拉应力时机械性能的变化以及带裂纹构件在裂纹扩展时裂纹前缘形状变化的原因。 Based on the basic principle of elasto-plastic mechanics, the expression of stress-strain curve (σ1-ε1) under three-stress condition is deduced from the monotonous tensile stress-strain curve. The experimental results verify the correctness of the theoretical analysis and explain the change of the mechanical properties of the material under three-direction tensile stress and the reason of the change of the crack leading edge shape when the cracked member expands with the σ1-ε1 curve.