| An investigation has been conducted into the effect of fatigue loading on the stiffness and strength degradation of composite laminates. A set of fatigue tests was conducted on two orientations of graphite/epoxy laminates: {lcub}0, 90, (+OR-) 45{rcub}(,s) and {lcub}(+OR-) 35{rcub}(,2s). Both static and dynamic stiffness were continually monitored during the fatigue tests, for a wide range of stress levels, and the results presented for each stress level tested. Residual strength tests were also performed on some laminates during this test program.; A model has been proposed which related the residual stiffness degradation to the fractional life expended at a given maximum stress level. This model is restricted to specimens subjected to constant amplitude fatigue loading. This model is based on the assumption that the residual stiffness decreases monotonically as the number of load cycles is increased. It involves three parameters which are determined from base-line data using the statistical method of steepest descent.; The stiffness degradation has also been related to the residual strength based on a strength degradation model proposed by Yang and Jones. Based on this relationship, the statistical distribution of the residual strength can be evaluated. |
