Influence of cyclical hygrothermal aging on mechanical response and structural durability of composite bolted interference-fit joints

Abstract

The paper deals with the long-term performance and durability of composite bolted interference-fit joints subjected to harsh environmental loads in flight-cycles. An accelerated cyclical hygrothermal aging spectrum with repeated temperature and moisture variation is specially designed to simulate real service condition. The single-lap composite joints with various stacking sequences, interference-fit sizes and tightening torques are aged in designed hygrothermal environments with different time duration, in which the material deterioration, stress relaxation and intrinsic frequency of joints are monitored to characterize structural evolution. Tensile tests are conducted to aged composite joints to obtain joint stiffness, bearing strength and failure modes to quantitatively evaluate their residual mechanical properties and structural degradation regulation. The results show that the moisture absorption rate presents a sharp rise and slow decent trend with a peek value of 0.213%. The aging process causes weathering phenomenon and reduces the structural benefits induced by such assembly technologies as interference fitting and bolt preloading. The reduction of joint stiffness and bearing strength exhibits a negative exponential decline trend with the growth of aging time. The matrix plasticization and interlaminar debonding are main hygrothermal damages. The safety factor of 0.7 to 0.8 is suggested on bearing strength value to reach hygrothermal aging bearing strength allowable in joint application.