Delamination growth and thresholds in a carbon/epoxy composite under fatigue loading

This paper presents a study on delamination growth in Mode I, Mode II and m ixed mode under fatigue loading in an HTA/6376C composite. The computed slo pes of the modified Paris plots were high. Therefore, threshold values of t he strain energy release rate for delamination growth were determined. Low fatigue threshold values revealed a significant effect of fatigue loading. The largest effect was found for the ENF test (Mode II) for which the fatig ue threshold value was only 10% of the critical strain energy release rate in static tests. Threshold values for MMB (mixed mode) and DCB (Mode I) tes ts were 15% and 23% of the static values, respectively. Fractographic evaluation revealed identical initial failure mechanisms in f atigue and static loading conditions for the ENF specimen. The ENF specimen failed by formation and coalescence of microcracks. The low fatigue thresh old for the ENF specimen was explained by microscopical observations on the specimen edge. It was also shown that the fracture surfaces generated in s tatic and fatigue DCB and MMB tests were similar.