INFLUENCE OF STRESS RATIO ON FATIGUE BEHAVIOR IN THE TRANSVERSE DIRECTION OF UNIDIRECTIONAL CFRPS

The dependence of flexural fatigue behavior in the transverse direction of two unidirectional carbon fiber reinforced plastics\(CFRP) upon stress ratio (sigma(min)/sigma(max)) has been studied at several constant temperatures. The two CFRP contain the same type of carbon fiber but different matrix resins, one a general type and one a high glass-transition temperature type matrix resin. Three-point bending fatigue tests were performed at various constant temperatures under various stress ratios. As the stress ratio decreases from one to zero, the flexural fatigue strength near room temperature (well below the glass-transition temperature) decreases while the strength at temperatures close to the glass-transition temperature increases. These observations indicate that the flexural fatigue strength at room temperature depends on stress amplitude and thus is controlled mainly by fatigue damage, while the strength near the glass-transition temperature depends on mean stress and, therefore, is controlled by creep damage.