MECHANICAL STRENGTH DEGRADATION OF CARBON FIBER POLYMER MATRIX COMPOSITES EXPOSED TO CONSTANT LOW-DENSITY DIRECT CURRENT

Carbon fiber reinforced composites (CFRP) can experience two dissimilar magnitudes of direct current during a lightning strike on an aircraft, a concentrated catastrophic high current followed by low direct current spread across the surface. Low density direct current can also occur in multifunctional composite structures for resistive heating, energy harvesting and storage. These direct currents lead to material degradation. Since CFRP structures are more susceptible to a lightning strike due to weak electrical and thermal conductivity compared to metallic bodies, considerable amount of research has been done to study the effects of instantaneous high current on mechanical properties. With the ever-growing demand for tailorable multifunctional composites, the effect of low direct current on mechanical properties of CFRP should be investigated. An experiment is designed to study the long-term exposure of low-density electric field effects on CFRP which are often coupled with detrimental thermal effects. In this study, experiments have been performed using an in-house setup to study the electrical effects of low constant direct current (DC) on cross-ply CFRP laminates. A constant current study has been conducted to characterize the voltage across the laminate over a period. The strength of the polymer depends on the integrity and type of bonds, the observed resistance change is a perceptible way of demonstrating the change in mechanical properties. The combined effect of electrical and thermal fields has been studied by mapping the surface temperatures continuously on the entire length of the laminate. Preliminary research showed that the presence of non-conducting epoxy undergoes localized dielectric breakdown near the carbon epoxy interface. In order to quantify the degradation, combined loading compression (CLC) and dynamic mechanical analysis (DMA) tests have been performed for coupon size samples which have been electrically degraded for a definite period. Compression test results are compared with electrical characterization and glass transition temperatures from DMA results