Degradation mechanisms in carbon fiber–epoxy laminates subjected to constant low-density direct current

Abstract

This paper reports on an experimental investigation on the mechanical degradation of carbon fiber reinforced composites (CFRP) subjected to long-term exposure of low-density electric field by passing direct current (DC) through cross-ply laminates. In this study, an in-house fabricated experimental setup is used to study the effects of long-duration electrical conduction through cross-ply CFRP laminates. The experiments are conducted under constant current conditions. The change in laminate resistance is measured by monitoring the voltage across the specimen. Meanwhile, the effects of Joule heating are measured by monitoring the surface temperature across the length of the composite sample. Both voltage and temperature measurements are made continuously through the duration of the experiment. Experiments are conducted for continuous three (3) and twenty-four (24) hour duration and for cyclic three-hour repeated duration. Combined loading-compression (CLC) mechanical testing, ultrasonic pulse-echo test and dynamic mechanical analysis (DMA) is performed after electrical degradation to quantify mechanical and thermo-physical changes in the material. It is found that the electrical degradation leads to a 13%–15% reduction in compressive strength , 6%–7% reduction in Young’s modulus in thickness direction, 3%–4% decrease in glass transition temperature , and 41%–45% increase in tan δ . Post-degradation electrical resistivity change in through-thickness and in-plane directions is also correlated to damage. Comparison of change in resistivities of electrically degraded samples to thermally loaded samples indicates that the primary degradation is in the form of delamination caused by Joule heating and localized dielectric breakdown of epoxy near the carbon fiber–epoxy interface. Additionally, X-ray computed tomography (CT) images confirm that damage accumulated in electrically degraded samples is a combination of thermal damage due to Joule heating and dielectric degradation between plies due to passage of electric current.