The main features of physical modeling of the growth of delaminations under cyclic tension of quasi-isotropic composite materials are considered using the model of decreasing the initial stiffness. It is noted that these features are significantly different from the provisions of the classical mechanics of interlayer fracture of composite materials. As a definite development of this direction, based on the use of the stiffness degradation model, a number of new relationships have been proposed, which make it possible to more accurately than the existing models to carry out calculated estimates of the growth rate of delamination. Verification of the proposed relations is carried out on the example of calculated estimates of the growth rate of delaminations in a quasi-isotropic CFRP laminate XAS/914 [45/90/-45/0]s.