Fatigue crack initiation can be regarded as a generalized phase transformation from the view of breaking energy barrier. A new phase transformation model is proposed to predict fatigue crack initiation based on the second law of thermodynamics. Evolution of dynamic energy during crack initiation and the damage under fatigue loading is considered combining with the entropy theory. The system becomes unstable when the Helmholtz free energy reaches the extremum value, which is adopted to represent fatigue crack initiation. The effects of grain size on the crack initiation and the asymptote of S-shaped S-N curve are discussed. The theoretical predictions are compared with the experimental data and show reasonable accuracy for different metallic materials.

从打破能垒的角度来看，疲劳裂纹萌生可以看作是广义的相变。根据热力学第二定律，提出了一种新的相变模型来预测疲劳裂纹萌生。结合熵理论，考虑了裂纹萌生过程中动能的演化以及疲劳载荷下的损伤。当亥姆霍兹自由能达到极值时，系统变得不稳定，该极值用于表示疲劳裂纹的萌生。讨论了晶粒尺寸对S形SN曲线裂纹萌生和渐近线的影响。理论预测值与实验数据进行了比较，并显示了不同金属材料的合理精度。