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

The dependence of flexural fatigue behavior in the transverse directio n of two unidirectional carbon fiber reinforced plastics\(CFRP) upon s tress ratio (sigma(min)/sigma(max)) has been studied at several consta nt temperatures. The two CFRP contain the same type of carbon fiber bu t different matrix resins, one a general type and one a high glass-tra nsition temperature type matrix resin. Three-point bending fatigue tes ts were performed at various constant temperatures under various stres s ratios. As the stress ratio decreases from one to zero, the flexural fatigue strength near room temperature (well below the glass-transiti on temperature) decreases while the strength at temperatures close to the glass-transition temperature increases. These observations indicat e that the flexural fatigue strength at room temperature depends on st ress amplitude and thus is controlled mainly by fatigue damage, while the strength near the glass-transition temperature depends on mean str ess and, therefore, is controlled by creep damage.