Carbon nanotube epoxy resin composite coupons were subjected to either cyclic tensile loading or crack-like damage while their electrical resistivity was monitored. The main objective of the study was to utilize piezoresistivity of the composite as means of strain and/or damage detection. Volume resistivity was obtained using Kelvin in-line four-probe resistance measurement technique. The technique minimizes contact resistance between electrodes and the composite, which could be orders of magnitude larger than the material resistance being measured. In the cyclic tensile loading study an MTS machine was employed to load the sample to 15 MPa tensile stress and then cycle it between 5 to 15 MPa with a period of 100 seconds. In damage detection part of the study, a CNC mill was used to insert two different types of damages to the sample. The first was a cut along the 25-mm width of the sample, with depth variation of 0.25 to 2.0 mm. The second damage was through thickness cuts that started from one side of the sample and its length changed from 1 to 12 mm. The results of cyclic load tests showed a strong correlation between applied stress and the resistivity changes. The obtained data indicate that resistivity changes may be used as a possible replacement for strain gauges. The damage detection results provide the capability of determining location and quantification of significant damages. Application of these techniques can be useful tools for real time structural health monitoring of composites.

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