Bending fatigue of carbon-fiber-reinforced epoxy composite strands

The bending fatigue behavior of unidirectional, continuous-carbon-fiber/epoxy composite strands has been characterized. The composite strands had a fiber volume fraction of 0.43 and a circular cross-sectional diameter of 0.50 mm. The composite strand was subjected to fully reversed bending fatigue at 3 Hz. A radius of curvature was imposed upon the strand to produce a maximum strain at the outer surface of 2.25%. It was determined that the composite did undergo fatigue, the fatigue damage being manifested as a loss of bending resistance. At high strains, damage occurred through fiber breakage, matrix cracking, and interfacial shear failure. As the number of bending cycles increased, there was crack growth in the matrix along fiber interfaces. At the imposed radius of curvature and up to 106 cycles, the loss in bending moment followed an exponential decay.