Relaxation Behaviors of Adhesive Bonded Carbon Fiber Reinforced Composites Using Dielectric Relaxation Spectroscopy and Thermally Stimulated Depolarization Current

Abstract

Improving the reliability and durability of adhesive bonds is one of the biggest challenges in critical composite structures in industries such as Aviation, Automotive, Marine, and so on. Characterizing the interfacial region of the adhesive and composite adherends can yield vital information in this regard. Dielectric Relaxation Spectroscopy (DRS) is a technique used to investigate relaxation behaviors in terms of dielectric constant and loss tangent as a function of frequency. The material's response to polarization mechanisms at different frequencies can be captured by conducting Broadband Dielectric Spectroscopy (BbDS). Among these polarization mechanisms, interfacial polarization is most relevant to adhesively bonded joints. Thermally Stimulated Depolarization Current (TSDC) is also an effective tool that provides information about interfacial polarization and polarization of polar groups in epoxy-based polymers. The relaxation behaviors of the specimen are described by the depolarization current that is emitted as the polarized dipoles are relaxed in relation to temperature. Adhesively bonded carbon fiber composite adherends with different surface properties were manufactured and studied using TSDC and DRS techniques. Different polarization current spectra were observed with bonded specimens of varying surface characteristics. The morphological changes made to the adhesive-adherend interfaces are found to be reflected in the relaxation times and corresponding frequencies at different temperatures. These two complementary techniques portray conclusive relaxation mechanisms triggered by the external electrical fields.