Two types of tension-tension fatigue tests were conducted on quasi-isotropic laminates and fatigue damage was measured in the form of ply cracks. Baseline fatigue tests were run at constant-amplitudes ranging from 20% to 40% of the ultimate tensile strength (UTS) of laminates while specimens with preloads of 50% to 80% UTS were tested in fatigue at the same amplitudes. The ply crack densities determined from edge replications in preloaded specimens were compared to those without preload. The effect of preloads on fatigue damage in composite laminates has been examined in order to better understand the modes of damage development and to assess the effect of proof testing. The effect of a preload on subsequent fatigue damage growth depended on the combination of both the preload level and the subsequent fatigue stress level. Since the tension preload was always higher than the tension-tension fatigue stress level, damage in preloaded specimens was more severe in the low-cycle region than nonpreloaded specimens, However, this preload-induced damage did not grow any more if the fatigue stress level was kept low, Preloads just above levels that cause significant damage (70% UTS and above here) appeared to retard fatigue damage development. At the highest tension-tension fatigue stress level of 40% UTS, the high-cycle damage in the form of matrix cracking rather decreased with increasing preload level, Thus, preloading could be beneficial at this stress level as far as the high-cycle ply crack damage is concerned.

Two types of tension-tension fatigue tests were conducted on quasi-isotropic laminates and fatigue damage was measured in the form of ply cracks. Baseline fatigue tests were run at constant-amplitudes ranging from 20% to 40% of the ultimate tensile strength (UTS) of laminates while specimens with preloads of 50% to 80% UTS were tested in fatigue at the same amplitudes. The ply crack densities determined from edge replications in preloaded specimens were compared to those without preload. The effect of preloads on fatigue damage in composite laminates has been examined in order to better understand the modes of damage development and to assess the effect of proof testing. The effect of a preload on subsequent fatigue damage growth depended on the combination of both the preload level and the subsequent fatigue stress level. Since the tension preload was always higher than the tension-tension fatigue stress level, damage in preloaded specimens was more severe in the low-cycle region than nonpreloaded specimens, However, this preload-induced damage did not grow any more if the fatigue stress level was kept low, Preloads just above levels that cause significant damage (70% UTS and above here) appeared to retard fatigue damage development. At the highest tension-tension fatigue stress level of 40% UTS, the high-cycle damage in the form of matrix cracking rather decreased with increasing preload level, Thus, preloading could be beneficial at this stress level as far as the high-cycle ply crack damage is concerned.

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