Fiber breakage in polymer-matrix composite during static and fatigue loading, observed by electrical resistance measurem
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Fiber breakage in polymer-matrix composite during static and fatigue loading, observed by electrical resistance measurement Xiaojun Wang and D.D.L. Chung Composite Materials Research Laboratory, State University of New York at Buffalo, Buffalo, NY 14260-4400 (Received 7 June 1999; accepted 25 August 1999)
By measuring the electrical resistance of a continuous unidirectional carbon fiber epoxy-matrix composite along the fiber direction during loading in this direction, fiber breakage was progressively monitored in real time. Fiber breakage occurred in spurts involving 1000 or more fibers. It started at about half of the failure strain during static tensile loading and at about half of the fatigue life during tension–tension fatigue testing. Immediately before static failure, at least 35% of the fibers were broken. Immediately before fatigue failure, at least 18% of the fibers were broken. The fiber breakage was accompanied by decrease in modulus.
I. INTRODUCTION
Continuous fiber polymer-matrix composites are used in aerospace, automobile, marine, machinery and construction industries, due to their low density, high strength, and high modulus. These composites degrade during loading due to fiber breakage, delamination, and other mechanisms that occur prior to failure. Because the degradation can render the composites unreliable or unsuitable for structural use, it is important to characterize and understand the degradation as it evolves during loading. For this purpose, real-time nondestructive monitoring of the degradation is needed. Techniques for this monitoring include acoustic emission detection,1– 4 eddy current testing, and the use of optical fiber sensors.5 In the case of the fibers being electrically conducting and the matrix being insulating, electrical resistance measurement in the fiber direction of the composite provides a way to monitor damage in the form of fiber breakage, because fiber breakage causes the resistance of the composite in the fiber direction to increase.6–10 By using this technique, we have monitored a continuous carbon-fiber polymer-matrix composite, during static and fatigue loading until failure, thereby obtaining information on the evolution of fiber breakage from its onset to composite failure. Prior work did not follow the entire fatigue process at a fixed stress amplitude and did not interpret the resistance changes to obtain the fraction of fibers broken.6–10 Prior work6–10 used the two-probe method for electrical resistance measurement, whereas this work used the four-probe method. The two-probe method suffers from the fact that the measured resistance includes 4224
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J. Mater. Res., Vol. 14, No. 11, Nov 1999 Downloaded: 18 Mar 2015
the resistance of the electrical contacts, whereas the fourprobe method excludes the contact resistance from the measured resistance. Nevertheless, the results of this work are consistent with those of prior work. II. EXPERIMENTAL METHODS
Composite samples were constructe
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