Abstract
The fatigue failure of materials, which usually occurs on the loading of cyclic stress, almost starts on the initiation and growth of small fatigue crack. This study investigates the growth of fatigue cracks, and analyzes the phenomena of crack growth in different materials, different fiber orientations, different initial crack lengths, and different loadings. Owing to anisotropy of materials, the tensile strengths of different orientations are not equivalent, and this greatly affects the growth of fatigue crack. So the Normal Stress Criterion is assumed in this study. The probabilities of crack growth in different directions are assumed to be proportional to the ratio of the normal tensile stress and tensile strength in the prospect direction. In this study, the thickness of each ply is supposed to be the same and cohere to a composite Laminate symmetrically, and can be taken as an anisotropic plate. Using the formulae of composite material mechanics to calculate the material constants necessary for computer simulations. In order to simplify the problem, it is assumed the cracks may extend in one of several possible directions, and let the random numbers generated by random number producer to select the direction of crack growth. The experimental data of the material T300/5208 has been used to calculate the material constants that are needed for computer simulation. In the computer simulation, we can get the da/dN vs. ΔK curve and predict the fatigue life conveniently. The results indicate that fatigue cracks still extend toward the orientation of main crack. In the initial stage of crack growth, the fatigue crack grows very slowly and steady. As the crack length increasing, the growth of the fatigue crack becomes quickly and unsteady. Finally, the results of computer simulation are summarized through Least-Square Method to find the regression formulae of the life of fatigue cracks.