Residual stress in carbon fiber embedded in epoxy, studied by simultaneous measurement of applied stress and electrical resistance

Due to the thermal contraction of the epoxy embedding a carbon fiber after curing, the fiber exhibited a compressive residual stress which caused the electrical resistivity of the fiber to increase by 10%. Subsequent application of tension to the exposed ends of the embedded fiber reduced the residual stress, thereby causing the resistivity to decrease reversibly back to its value prior to embedding. The tensile stress (1320 MPa) required for this to happen was the residual stress in the fiber direction. Excessive tensile stress caused the resistivity to increase, due to fiber damage.