Self-monitoring of fatigue damage and dynamic strain in carbon fiber polymer-matrix composite

Self-monitoring of static/fatigue damage and dynamic strain in a continuous crossply [0/90] carbon fiber polymer-matrix composite by electrical resistance (R) measurement was achieved. With a static/cyclic tensile stress along the 0 degrees direction, R in this direction and R perpendicular to the fiber layers were measured. Upon static tension to failure, R in the 0 degrees direction first decreased (due to increase of degree of 0 degrees fiber alignment and fiber residual compressive stress reduction) and then increased (due to 0 degrees fiber breakage), while R perpendicular to the fiber layers increased monotonically (due to increase of degree of 0 degrees fiber alignment and delamination). Upon cyclic tension, R (0 degrees) 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% of the fracture stress, R (0 degrees) irreversibly increased both in spurts and continuously, due to 0 degrees fiber breakage, which started at 15% of the fatigue life, while R (perpendicular to the fiber layers) irreversibly increased both in spurts and continuously, due to delamination, which started at 33% of the fatigue life. The peak R (0 degrees) in a cycle irreversibly decreased, while the minimum R (perpendicular to the fiber layers) at the end of a cycle irreversibly increased during the first 0.1% of the fatigue life, due to irreversible increase in the degree of 0 degrees fiber alignment. R (0 degrees) became noisy starting at 87% of the fatigue life, whereas R (perpendicular to the fiber layers) became noisy starting at 50% of the fatigue life. For a [90] unidirectional composite, R (0 degrees) increased reversibly upon tension and decreased reversibly upon compression in the 0 degrees direction, due to piezoresistivity. (C) 1997 Elsevier Science Ltd. All rights reserved.