Abstract

The effect of loading frequency and loading level on the fatigue behavior of ($\rm{\pm}45\rbrack\sb{4s}$ carbon/PEEK composite was investigated. Tention-tention load controlled fatigue tests were performed on straight sided samples using a 100-KN MTS machine. Three loading frequencies (1Hz, 5Hz, and 10Hz) and three load levels (60%$\rm\sigma\sb{ult.}$, 70%$\rm\sigma\sb{ult.}$, and 80%$\rm\sigma\sb{ult.}$) were used, with a load ratio of R = 0.13. During each fatigue test, the cyclic load, axial strain, and the sample's surface temperature were recorded through a data acquisition system. It was found that the fatigue life of this material decreases as loading frequency increases. At the same frequency, the fatigue life decreases as the loading level increases, with more decline at higher frequencies. High levels of temperature rise were obtained during fatigue tests at high frequencies, which increase as loading level increases. Consequently, it can be said that the major effect of loading frequency on the fatigue life is the temperature rise and the consequent thermal degradation. High levels of creep deformation were observed throughout the tests. At high frequencies, the high levels of temperature rise accelerate the creep deformation rates. A model for the effect of loading frequency on the fatigue life was suggested, based on the general degradation of the material's strength under fatigue loading. Also, the variation in viscoelastic properties during fatigue were investigated, and the thermal behavior was analyzed