Damage Precursor Investigation of Fiber-Reinforced Composite Materials Under Fatigue Loads.

摘要

Glass-epoxy composite structures (common aerospace applications) exhibit damage precursors such as micro-cracks, which significantly impact structural performance and life span. This study focused on identifying the stress levels responsible for crazing, micro-crack formation, and coalescence into macro-cracks in S2 glass and Cycom-381 epoxy matrices (8 plies woven). Fatigue tests were performed at 1, 100, 1,000, 10,000, 100,000, and 1,000,000 cycles under tensile loading conditions (R=0.1). Damage monitoring strategies prior to, during, and after cyclic loading conditions include ultrasonic nondestructive evaluation (NDE), acoustic emission (AE), and scanning electron microscopy (SEM). Macro-scale inhomogeneities in the as-fabricated composite structures, such as porosity, debonding, macro-cracking, and ply distortions, were evaluated using an ultrasonic NDE technique. AE was used during the fatigue loading tests to identify and localize the presence of rupture, delaminations, and micro-crack formations in real time. Post-damage microscale examination of the fracture surfaces was conducted using SEM. By this systematic evaluation, the phenomena of crazing, the propagation of micro- cracks, delaminations, and the factors that lead to catastrophic failure were investigated. Based on this study, an empirical framework was developed to map the relationship between the sizing and location of damage precursors and the corresponding matrix/fiber degradation mechanisms.