| Summary: | 博士 === 雲林科技大學 === 工程科技研究所博士班 === 96 === It has been confirmed that polymer matrix composites possess viscoelastic behavior. This means that one could accelerate the fatigue testing by changing the stress amplitude, frequency or temperature. This study is to invest the accelerated fatigue properties, which resulted from the viscoelastic behavior, of carbon/epoxy and glass/epoxy composites and to predict their fatigue life. For this purpose, a series of fatigue tests of unidirectional specimens are conducted at room temperature under different stress ratios and stress frequencies. A group of sigmoid S-N curves which are suitable for the whole fatigue life and the corresponding parameters are developed for different cyclic loading conditions. A transformation method, which can transform a reference S-N curve to the corresponding S-N curve of the assigned fatigue conditions, is established by the parameters. And this S-N curve can be utilized to predict the fatigue life of the composite at the assigned stress ratio or stress frequency. The comparison between the linear and sigmoid S-N curves is also carried out to show the advantages of the latter model in the whole fatigue life. Following, a nonlinear damage model, which was based on a physical phenomenon of damage growth, for the composites that includes the nonlinear effects of stress ratio and loading frequency on the damage processes was developed. A damage index was defined and the corresponding parameters were decided. A series of two-stress level fatigue tests were conducted to confirm the validity of this model. The results reveal that the model of this study is benefit to assess the fatigue damage caused by different stress ratios and frequencies.
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