Fatigue strength and damage progression in a circular-hole-notched GRP composite under combined tension/torsion loading

Fatigue strength and failure processes in a hole-notched glass-fiber-reinforced plastic (GRP) material under combined tension/torsion cyclic loading have been investigated. Thin-walled tubular specimens were used in the experiment. The reinforcing fibers (plain-weave glass cloth) were aligned parallel to the longitudinal and circumferential axes of the specimen. The damage state at each biaxiality stress ratio (: the ratio of normal stress to shear stress) was observed macroscopically and microscopically.

The fatigue damage progression is dependent on the combined stress ratio. In the case of = 1/0, damage spreads radically from the hole, but covers an area of only π/4 on either side of the circumferential axis. Damaged areas are symmetrical and opposite each other. When = 0/1, damage progresses radially along both longitudinal and circumferential axes. These areas of damage progression are called ‘fatigue bands’. Macro- and microscopical observations reveal that the damage state under combined loading has mixed features from the damage states at = 1/0 and = 0/1.