Simple mechanical models of unidirectional metal matrix composites are used to analyze elastoplastic deformation and shakedown in the matrix under cyclic composite loads. Both axial and off-axis loadings of a lamina are considered. It is shown that the fatigue limits of as-fabricated boron-aluminum, beryllium-aluminum, tungsten-copper, and other composites generally coincide with the composite shakedown limits because the matrix yield stresses and fatigue limits are equal.
In heat-treated composites, the matrix yield stress is usually much higher than the fatigue limit, and matrix fatigue failure can take place in the shakedown state. The residual microstresses caused by heat treatment are estimated, and their influence on fatigue is discussed. A method for improvement of the fatigue resistance of heat-treated composites is discussed.
The theoretical predictions of fatigue limits are verified by an extensive comparison with available experiments, and a very good agreement is obtained. It is concluded that, in principle, the composite fatigue failure can be avoided if each of the constituents is stressed within its particular fatigue limits during a cyclic loading program of the composite.