Interlaminar shear property and high-velocity impact resistance of CFRP laminates after cyclic hygrothermal aging

T700/TDE-85 composites were aged in cyclic hygrothermal tests in accordance with the service environment of military aircraft, and the variations in interlaminar shear strength (ILSS) and ballistic impact resistance during the aging cycles were determined. It was found that cycling hygrothermal aging induced the damage of the resin surface, long cracking along the fibre/matrix interface, and interlayer interface due to the combined action of swelling stress and thermal stress. ILSS first increased by 12.4% after 14 cycles due to the plasticising effect and then reduced due to the damage caused to the fibre/matrix interface. The critical energy absorption after 280 cycles was 15.6% higher than that of the unaged composite, suggesting the improvement effect of cyclic hygrothermal aging on the impact performance from the aspects of energy absorption and ballistic limit. The maximum energy absorption was achieved by the specimen aged for 70 cycles. Cyclic hygrothermal aging altered the damage morphology and failure modes dramatically. The unaged composite target had the lowest delamination area among all impact velocities, which explains the improvement in impact resistance and energy absorption capability after cyclic hygrothermal aging.