2004 Volume 47 Issue 3 Pages 357-364
Carbon Fiber Reinforced Plastic (CFRP) is composed of electric conductive carbon fibers and electric insulator resin. Self-monitoring system has been reported utilizing electric resistance change of unidirectional CFRP due to fiber breakages and to applied strain. Piezoresistivity is electric resistance change with applied strain. Many researchers have already reported the piezoresistivity of unidirectional CFRP. There is, however, large discrepancy in the measured piezoresistivity even in the fiber direction during tensile loading: both positive piezoresistivity (electric resistance increase) and negative piezoresistivity (electric resistance decrease) are reported during tensile tests. Electric resistance change at electrodes due to poor electric contacts are reported to be a main cause of this large discrepancy. In the present study, therefore, basic properties of piezoresistivity were measured with specimens made from single-ply and multi-ply laminates using a four-prove method. Many cases of electric resistance changes in the fiber direction transverse direction were measured during tensile loading. Effect of shear loading was also investigated using a shear test. To investigate the effect of poor electric contact at the electrodes, electrodes were made without polishing specimen surface and a tensile test was performed with measuring piezoresistivity. After the test, the specimen surface was polished, and a tensile test was performed again using the identical specimen. As a result, positive piezoresistivity was obtained for both single-ply and multi-ply specimens and negative piezoresistivity is confirmed that it was caused by the poor electric contact at electrodes.