Sensing damage in carbon fiber polymer-matrix composites during fatigue by electrical resistance measurement

Self-monitoring of static/fatigue damage and dynamic strain in a continuous crossply carbon fiber polymer-matrix composite by electrical resistance (R) measurement was achieved. With a static/cyclic tensile stress along the 0 degree direction, R in this direction and R perpendicular to the fiber layers were measured.Upon static tension to failure, R in the 0 direction first decreased and then increased, while R perpendicular to the fiber layers increased monotonically. Upon cyclic tension, R decreased reversibly, while R perpendicular to the fiber layers increased reversibly, though R in both directions changed irreversibly by a small amount after the first cycle. Upon fatigue testing at a maximum stress of 57 percent of the fracture stress, R irreversibly increased both in spurts and continuously, due to 0 degree fiber breakage, which started at 15 percent of the fatigue life, while R irreversibly increased both in spurts and continuously, due to delamination, which started at 33 percent of the fatigue life. The peak R in a cycle irreversibly decreased, while the minimum R at the end of a cycle irreversibly increased during the first 0.1 percent of the fatigue life, due to irreversible increases in the degree of 0 fiber alignment. R became noisy starting at 87 percent of the fatigue life, whereas R became noisy starting at 50 percent of the fatigue life. For a unidirectional composite, R increased reversibly upon tension and decreased reversibly upon compression in the 0 direction, due to piezoresistivity.