A comparative investigation of electrical resistance and acoustic emission during cyclic loading of CFRP laminates

This paper reports on simultaneous monitoring of electrical resistance and acoustic emission (AE) during cyclic tensile loading of cross-ply carbon-fibre-reinforced plastics (CFRP). The detection of the AE allows further insight into the damage mechanisms causing the electrical response. During loading and unloading the samples the electrical measurements show a hysteresis in the corresponding resistance vs strain plot. When the previous load maximum is exceeded in the subsequent load cycle a characteristic increase in the measured slope of the resistance vs strain curve appears to combined with a sudden rise in the AE (Kaiser effect). After unloading the resistance relaxes to a new equilibrium value. This behaviour is consistent with the formation, opening and closing of cracks in the CFRP samples. The observed time dependence in the resistance hysteresis and relaxation can be attributed to the influence of the stressed polymeric matrix. Thus, the reported in situ observation of the electrical resistance allows virgin and damaged CFRP samples to be identified and the previous load maximum to be determined by the characteristic change in the resistance-strain slope. (C) 2001 Elsevier Science Ltd. All rights reserved.