DAMAGE MECHANICS MODEL FOR OFF-AXIS FATIGUE BEHAVIOR OF UNIDIRECTIONAL CARBON FIBER-REINFORCED COMPOSITES AT ROOM AND HIGH TEMPERATURES

SUMMARY: Tension-tension fatigue tests were performed for unidirectional T800H/Epoxy at room temperature (RT) and 100°C, respectively, using seven kinds of plain coupon specimens with different fiber orientation angles. The characteristic nature of the fiber-matrix interface controlled off-axis fatigue failure was similar at RT and 100°C. Irrespective of off-axis angle, almost linear S-N relationships were obtained in the whole range of fatigue testing up to 106 cycles. Normalizing the maximum fatigue stress with respect to the static tensile strength at test temperature, the off-axis fatigue data eventually fell on a single S-N relationship. A non-dimensional effective stress defined on the basis of the classical static failure theory succeeded in coping with the directional nature of the off-axis fatigue behavior in a unified manner. By using the non-dimensional effective stress in the continuum damage mechanics approach, a general failure model was developed to describe the observed off-axis S-N relationships