Strain sensing using carbon fiber
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Strain sensing using carbon fiber Xiaojun Wang, Xuli Fu, and D. D. L. Chung Composite Materials Research Laboratory, State University of New York at Buffalo, Buffalo, New York 14260-4400 (Received 15 September 1997; accepted 17 August 1998)
Carbon fiber provides strain sensing through change in electrical resistance upon strain. Due to piezoresistivity of various origins, a single carbon fiber in epoxy, an epoxy-matrix composite with short carbon fibers (5.5 vol%), a cement-matrix composite with short carbon fibers (0.2–0.5 vol%), and an epoxy-matrix composite with continuous carbon fibers (58 vol%) are strain sensors with fractional change in resistance per unit strain up to 625. A single bare carbon fiber is not piezoresistive, but just resistive.
I. INTRODUCTION
II. A SINGLE BARE CARBON FIBER
An important aspect of a smart structure is structural control, which typically involves the sensing of strain, displacement, or derivative quantities and the use of the signal from the sensor to activate certain actuators, which bring about the desired intelligent response of the structure. Thus, strain sensing is a key function in structural control. Numerous types of strain sensors are available, including optical fibers, piezoelectric sensors, electrostrictive sensors, magnetostrictive sensors, and piezoresistive sensors. The sensors are usually attached to or embedded in the structure. Composite materials involving fiber reinforcements have become common structural materials. Among the various types of fibers, carbon fibers have become quite dominant due to their high strength, high modulus, low density, and temperature resistance. Carbon fibers are used to reinforce polymers, carbon, cement, and metals. If the carbon fibers in the composite provide strain sensing, then the conventional attached or embedded sensors are not necessary. This would mean reduced cost, greater durability, larger sensing volume, and absence of mechanical property degradation (due to embedded sensors). Therefore, this paper addresses strain sensing using carbon fibers. Carbon fibers are electrically conductive. This behavior causes a change in electrical resistance in response to strain, thus enabling strain sensing. The nature of the electromechanical behavior depends on whether the fibers are continuous or discontinuous and depends on the matrix around the fibers. This paper provides a systematic study of the electromechanical behavior by considering (i) a single bare carbon fiber, (ii) a single carbon fiber in a matrix, (iii) a polymer-matrix composite containing randomly oriented short carbon fibers, (iv) a cement-matrix composite containing randomly oriented short carbon fibers, and (v) a polymer-matrix composite containing continuous unidirectional carbon fibers.
Previous electromechanical study of carbon fibers reported that, for low-modulus carbon fibers, the electrical resistance increases reversibly with tensile strain and decreases reversibly with compressive strain, mainly due to di
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