Through-thickness stress sensing of a carbon fiber polymer--matrix composite by electrical resistance measurement

Through-thickness stress self-sensing in a quasi-isotropic carbon fiber epoxy–matrix composite by in-plane electrical resistance measurement is effective. The resistance decreases reversibly upon through-thickness compression conducted up to 67 MPa, due to an increase in the proximity between adjacent laminae. The sensing can be attained by measuring the surface resistance in the direction of the surface fibers or by measuring the volume resistance in essentially any in-plane direction. The sensing is ineffective if the transverse surface resistance is the quantity measured, due to the dominance of the surface fibers in governing the surface resistance. In the case of the longitudinal surface resistance, the decrease in resistance upon compression has a slight irreversible component, due to an irreversible increase in the proximity between adjacent laminae and the consequent increase in the degree of current penetration. This effect is smaller for the longitudinal or transverse volume resistance. The variability of the resistance from area to area in the same laminate is larger for the surface resistance than the volume resistance, due to its higher sensitivity to current spreading. The sensitivity of stress sensing, as described by the fractional change in resistance per unit through-thickness compressive stress, is −10−5 MPa−1. The magnitude of the effectiveness is lower for the resistance away from the stressed region than that at the stressed region.