Self-Monitoring of Strain and Damage by Carbon Fiber Polymer-Matrix Composite
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ABSTRACT Self-monitoring of damage and dynamic strain in a continuous crossply (00/90') carbon fiber polymer-matrix composite by electrical resistance (R) measurement was achieved. With a static/cyclic tensile stress along the 00 direction, R in this direction and R perpendicular to the fiber layers were measured. Upon tension to failure, R in the 0' direction first decreased (due to increase of degree of 00 fiber alignment) and then increased (due to 00 fiber breakage), while R perpendicular to the fiber layers increased monotonically (due to increase of degree of 00 fiber alignment and delamination). Upon cyclic tension, R (00) 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. For a 90' unidirectional composite, R (00) increased reversibly upon tension and decreased reversibly upon compression in the 00 direction, due to piezoresistivity. INTRODUCTION Although the placement of strain/damage sensors is common in smart structures, it suffers from poor durability, limited sensing volume, degradation of mechanical properties, and high cost. Self-monitoring refers to the ability of the structural material to monitor itself. By using a self-monitoring material, the disadvantages mentioned above are removed. A common method of fatigue monitoring is acoustic emission [1,21, which suffers from its inability to monitor dynamic strain. Much less common is the method involving the measurement of the electrical resistance, which increases due to damage and provides a mechanism for self-monitoring. Previous work using electrical resistance to monitor fatigue was carried out on a CaF 2-matrix SiCwhisker composite [3], but dynamic strain monitoring was not performed, probably because this composite's electrical resistivity did not change reversibly with reversible strain. In general, dynamic strain monitoring requires a measurand which changes in value reversibly during reversible straining. In addition, in order for both dynamic strain and damage to be simultaneously monitored with a single method, that method must involve a measurand which changes in value reversibly during reversible straining and changes irreversibly during damage. In this work, we have achieved this by using the electrical resistance as the measurand and continuous carbon fiber polymer-matrix composite as the material. Continuous carbon fiber polymer-matrix composites are advanced composites which are attractive in that they combine high strength, high modulus and low density. Previous work on a polymer-matrix composite containing a combination of continuous glass and carbon fibers showed that the electrical resistance of this composite increases irreversibly upon damage (due to the fracture of the carbon fibers) [4], but fatigue monitoring and reversible resistivity changes (dynamic strain 171 Mat. Res. Soc. Symp. Proc. Vol. 459 ©1997 Materials Research Society
monitoring) were not explored. Our previous work on continuous unidire
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