Block loading and elemination fatigue studies of notched composite specimens

Notched composite specimens made of HTA7/6376 graphite/epoxy material were tested in fatigue. Constant amplitude fatigue loading was done at R=-1, -5, and -0.2. The results show that the shortest fatigue life was obtained for R=-1 and the longest for R=-0.2. For R=-1 loading the "fatigue threshold" is slightly less than 50% of quasi-static strength. Block loading has been performed by fatigue loading specimens to 50% of their expected fatigue life and then changing load level. When the loading order was high-low the Palmgren-Miner´s rule underestimates the fatigue life whereas for low-high loading the fatigue life is overestimated. Predictions with a delamination growth model are conservative. Elimination studies were performed by adding 100 low amplitude load cycles for each large amplitude load cycles. The low amplitude load cycles had an amplitude of 30% and 50% of the large amplitude load cycle. The results suggest load cycles with 50% amplitude might reduce the fatigue life. The delamination growth model predicts a fatigue life reduction of 20%.