Abstract
The effect of loading history on the electric resistance in CFRP unidirectional composites was experimentally investigated. First, monotonic tensile tests were conducted for on-axis and off-axis unidirectional specimens to measure the change in the electric resistance together with strain up to failure. The initial gage factor was theoretically derived from piezoresistivity as a function of the off-axis angle and was compared with the experimental results. Next, loading/unloading tensile tests as well as fatigue tests at low load levels were performed to measure the change in the electric resistance due to loading history. It was found that the electric resistance-strain curves for monotonic tensile loading exhibit nonlinear behavior after the linear region, and that the gage factor becomes negative at high strains for some off-axis directions. The initial gage factor becomes the maximum at the angle of 12 degrees according to the present model, and the predicted gage factors are in reasonably good agreement with the experimental ones. The permanent change in the electric resistance, which is not always ascribed to damage, was observed in all the directions after the loading/unloading as well as the fatigue loading.
