Electro-thermal influencing mechanism of carbon fiber/epoxy unidirectional laminated composites based on dielectric properties

Direct currents (DC) ranged from 0.05 A·mm^-2 to 0.2 A·mm^-2 were subjected through carbon fiber/epoxy (CF/EP) unidirectional composites with a 1.5 h duration by the electro-thermal tester,meanwhile the tem-perature fields were measured on the surface of samples under different electro-thermal treatments.Changes in the longitudinal volume resistance and dielectric properties were analyzed respectively after electro-thermal treatments.In order to characterize the flexural property,the three-point flexure test was conducted before and after electro-thermal treatments.Preliminary experimental results show that temperature fields of all samples are rising rapidly with the duration before steady-state equilibrium.Treatments at less than 0.125 A·mm^-2 lead to a uniform temperature field,and make the longitudinal volume resistance decrease and enhance dielectric properties.Treatments at more than 0.125 A·mm^-2 come to a contrary result.The dielectric constant and loss tangent increase with the increment of test frequency,whereas the AC conductivity remains unchanged and then rises.The loss tangent affected mostly by electro-thermal treatments can be used as characteristic parameter of electro-thermal influence.The flexural property test demonstrates that treatments less than 0.125 A·mm^-2 make flexural strength and modulus increase by 11.8% and 7.32% mostly,but they reduce by 8.26% and 6.52% maximally after treating more than 0.125 A·mm^-2.