Residual Strength in Fatigue Based on the Strength-Life Equal Rank Assumption

Equations for the distribution of the residual strength after fatigue are derived. They are compared with experimental data for a few graphite/ epoxy composites. The theory is based on an assumption first introduced by Hahn and Kim, which states that for a given specimen its rank in static strength is equal to its rank in fatigue life, or the strength-life equal rank assumption. The equations contain a free parameter which is more versatile in matching experimental data. It is found that two processes are present during fatigue, one degradates individual specimens and tends to lower the mean strength, the other is the weeding out of weak specimens by fatigue failure which tends to increase the residual mean strength. The change of residual strength can be of weak degradation, strong degradation, or in crease in strength. Among the test data studied, two have weak degrada tion, two have strong degradation and two have increase in strength.