Fatigue and fracture

Composite materials are inhomogeneous and anisotropic, and exhibit several modes of damage including delamination, matrix crazing, fiber failure, void growth, matrix cracking, and composite cracking. A particular structure may exhibit any or all of these damage modes and it is difficult to predict, a priori, which mode will dominate and cause failure. The fatigue phenomenon is therefore phenomenological/empirical with microcracks and damage in the material volume not considered in the relevant equations. The fatigue properties should therefore be analyzed statistically due to the large variations exhibited during testing.
Nevertheless, in other cases where the fatigue/failure is dominated by simple or multiple cracks, such as in adhesively bonded composite joints, the fatigue fracture phenomenon is approached by fracture mechanics methods that are based on the actual measurement of the damage and the establishment of empirical models thereof, e.g, the fatigue crack growth curves, for the analysis of the fatigue life.