Effect of Manufacturing on the Dielectric Properties of Composite Materials

Abstract

From golf clubs to airplanes, in today’s world there is a great need for lighter, stronger, and more durable high performance composite structures. Partially cured epoxy with fiber reinforcement, commonly referred to as prepreg material, is routinely used in the manufacturing of these composite materials. In general the laminae can be formed from multiple prepreg by the lay-up method either manually or by machine. These stacked materials then go through a highly controlled cure cycle consisting of a combination of pressure and temperature to allow the thermoset epoxy enough time to fully cure. Defects or air voids are inherent in composite structures and depend greatly on the cure cycle used and the manufacturing process. Normally, the defects are distributed and have an important influence on the final material performance. Local defects have different electrical properties, e.g., dielectric constant, than the surrounding fiber reinforcement and matrix materials. In this type of complex, three phase material system, polarization in an electric field is typically observed as an interfacial effect due to the build-up of electric charge on the constituent material interfaces and interfacial polarization induced by distributed internal defects. For decades Broadband Dielectric Spectroscopy, BbDS, has been a well-known and established method of determining polymer matrix cure kinetics. Likewise, BbDS can be used to extract material-level information, including morphological changes caused by microdefect generation and configuration. This paper presents an experimental investigation based on measuring dielectric properties of carbon fiber reinforced composites (CFRP) manufactured with different cure cycles. We will show how the relationship between the mechanical strength of the composite structure is uniquely linked with changes in the bulk dielectric response that reflect the internal defect state of the as-manufactured material.