A simple fatigue failure criterion for unidirectionally fiber reinforced laminae under oscillatory states of combined plane stress has been es tablished. The criterion is expressed in terms of three S-N curves which are easily obtained from fatigue testing of off-axis unidirectional specimens under uniaxial oscillatory load. An extensive series of tests have demonstrated good agreement of the failure criterion with experimental data.

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An experimental technique was developed to determine the condi tions for the onset of reinforcement unbonding in composite materials. The technique is based upon the continuous measurement of the dynamic moduli and damping of tensile specimens. The samples are subjected to low amplitude longitudinal and torsional oscillations superimposed on the tensile loading. Experiments were performed at room temperature on quartz particle filled resin specimens. The results for specimens with epoxy binder indicated that unbonding did not occur prior to overall failure. Those for the polyester specimens, however, showed that early particle separation occurred under certain conditions.

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