Unprecedented sensing of the twist level in fiber tows, as shown for carbon fiber by inductance-based self-sensing, which provides fast, low-cost and large-format sensing

Abstract

Twisting is commonly imposed on fiber tows to improve the processability, as needed for manufacturing composites and other products. Twist level (number of twists per unit length) determination commonly involves visual inspection. This work provides a method of sensing the twist level, as shown for carbon fiber tows up to 4 twists/inch. This method involves measuring the inductance (0.5–2.0 kHz), which increases monotonically with the twist level, due to Faraday's Law. The method is fast, low-cost, high-durability and large-format (scanning not needed). Uncoated and nickel-coated carbon fiber tows (PAN-based, 12K) give similar fractional increases in the inductance (≤46%) due to twisting. Magnetic character (as provided by nickel) is not required. A larger tow (24K) increases the inductance, but reduces the fractional increase due to twisting. Increasing the voltage enhances the inductance, but the fractional increase due to twisting is not affected. An increase in frequency also enhances the inductance, but the fractional change in inductance due to the twisting is diminished. The resistance decreases monotonically with the twist level (≤14% and ≤4% decrease for uncoated and nickel-coated carbon fibers, respectively), due to the increasing number of fiber-fiber contacts, which enable current detour when defects are encountered. Inductance-based sensing is superior in twist-level sensitivity to resistance-based sensing. Both sensitivities decrease monotonically with increasing twist level, with highest sensitivity below 1 twist/inch. The lowest twist level with demonstrated sensing is 0.1 twist/inch. This work also provides the first report of inductance-based structural self-sensing, which differs from the widely reported resistance-based structural self-sensing.