Improvement of damage evolution equation and method for fatigue life prediction of axial-symmetrical specimens

Based on irreversible thermodynamics, damage evolution equation of fractional form is constructed by introducing damage driving force. The effective strain and effective stress are introduced under the condition of axial symmetry. By the theory of conservative integral, the strain with damage is expressed by the strain without damage and the damage extent for the case of small-scale damage. The integral of the damage evolution equation is obtained via variable separation, series of power function is derived to express the relation between the stress and the mean values of fatigue life under the repeated loading with constant amplitude. The material parameters in the damage evolution equation can be obtained by the mean values of experimental fatigue lives of axially symmetric specimens with K_T = 1 and K_T = 3. The correctness of the damage evolution equation and the method of fatigue life prediction can be checked by the mean values of experimental fatigue lives of axially symmetric specimens with K_T = 5. Fatigue life prediction of the flared type pipeline connection components is conducted.