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Damage Precursor Investigation of Fiber Reinforced Composite Materials Under Dynamic Cyclic Loads
Abstract
Glass-epoxy composite structures are frequently used in many aerospace applications, such as landing gear struts, satellite frames, aircraft nose cones, and interior fuselage stiffeners. In glass fiber/epoxy composite systems, damage precursors such as micro-cracking have significant implications in impacting the performance and life span of the structure. As a consequence, it is necessary to gain a better understanding of the response under fatigue loading. The focus of this study is to identify the stress levels responsible for crazing, micro-crack formation and coalescence into macrocrack in S2 Glass Cycom381 Epoxy matrices (8 plies woven). Fatigue tests are performed at 1, 100, 1,000, 10,000, 100,000, and 1,000,000 cycles under tensile loading conditions (R=0.1). Strategies for monitoring the damage prior to, during, and after cyclic loading conditions include ultrasound and acoustic emission (AE) non-destructive evaluation techniques. AE is used during the fatigue loading tests to identify and localize real-time the presence of rupture, delamination, and crack formations. The high frequency ultrasound immersion technique is conducted for high fidelity interlaminar evaluation prior to fatigue testing. After each test, an examination of the fracture surface is conducted by several image characterization techniques including optical microscopy and Scanning Electron Microscopy (SEM). By this systematic evaluation, the phenomena of crazing, the propagation of micro-cracks, delaminations and other factors that lead to catastrophic failure are investigated. At the end of this study, an empirical framework is developed that maps the relationship between the sizing and location of the damage pre-cursors and the corresponding matrix/fiber degradation mechanisms.