Effect of braid angle on fatigue performance of biaxial braided composites

Biaxial braided fabric is gaining popularity in primary structural application in small business jets because of its natural ability to conform to complex shapes. This research addresses the effect of braid angle on in-plane mechanical properties and fatigue performance. The carbon/epoxy braided composites were fabricated using low cost vacuum assisted resin transfer molding (VARTM) with different braid angles (25°, 30° and 45°). The static tests were performed to evaluate tensile strength, modulus, and Poisson's ratio. It is observed that as braid angle increases the tensile strength, modulus, and Poisson's ratio decreases significantly. The load controlled tension-tension fatigue tests (R = 0.1) were conducted at 10 Hz frequency with constant amplitude. The endurance limit was defined as the fatigue load that results in a fatigue life of one million cycles. The endurance limit for 25° and 30° braided composites was 40% of UTS whereas for 45° braided composites it was 50% of UTS. However, braid angle did not significantly affect the failure mechanism under fatigue loading. It was very crucial to control the braid angle within a test specimen, as tensile strength is significantly affected by braid angle variation. The special form of biaxial braided fabric termed slit sleeves assures the constant braid angle while handling and processing. It was observed that, a Sigmoidal function could be used effectively to represent the fatigue life behavior. Braided composites exhibited substantially different fatigue failure behavior as compared to conventional angle-ply laminated composites. The major difference being that the failure is sudden. There were hardly any noticeable matrix cracks or delaminations in the first 90% of the fatigue life at all fatigue load levels. There is rapid damage accumulation in the last 10% of the fatigue life.