Modeling and characterization of electrical resistivity of carbon composite laminates

Abstract

In the past few decades, composite materials especially carbon fiber reinforced polymers (CFRP) have been widely used as structural materials for its high strength to weight ratio, tailorable properties, and excellent corrosion properties. Applications that require better understanding of the electrical properties of CFRP laminates include carbon fiber assisted heating during composites manufacturing, self-sensing of damage of composite structures, integrated electromagnetic shielding, and lightning strike protection. Accurate predictive model describing the electrical conduction behavior of CFRP laminates is the key for them to be used for such applications. ☐ Different approaches have been explored to model the electrical conduction of CFRP under various current conditions. A comprehensive literature review revealed that most methods used to model electrical conduction of CFRP fail to capture the impact of micro-structure of CFRP, especially the fiber-fiber contact, and resin-rich layer between plies, which can drastically change the conduction pattern. ☐ The aim of this dissertation work is to develop a model that capture key electrical conduction mechanisms of CFRP, which address the impact of the micro-structure and geometrical parameters. The model is constructed in a modular fashion by validating the model with experimental validation after the addition of each key mechanism module. First, the model constructs a resistor network framework for describing electrical conduction behavior of UD laminas and fiber tows subjected to low DC currents. The model is validated with reported experimental results, and by characterization of resistivity of dry carbon fiber tows. ☐ The next module investigates the specific features of a multi-ply laminate such as: varying ply orientation, existence of resin-rich layer, and dependence on geometric parameters that influence the local resistivity. A meso-scale fiber bundle model is proposed to strike a balance between the level of details modeled and the computational cost. Influence of the resin-rich layer is described with an inter-ply connectivity term. Expressions for estimating contact resistance from multiple sources including direct fiber-fiber contact and tunneling resistance across thin resin layer are introduced. The refined model is compared against experimental results and finite element model. A parametric study is conducted to investigate the impact of geometrical parameters. ☐ Finally, the dissertation work investigates the impact of high current density both numerically and experimentally. Simplified analytical model examining the impact of localized Joule heating revealed that current concentrations due to microstructure constraints can introduce excessive Joule heating at contact spots. Thus, it is vital not to under-estimate the temperature rise at contact points, even at seemingly small overall applied currents. Based on these analysis, the model is further refined with the implementation of the module that introduces Joule heating. Both reversible change in resistivity such as temperature dependent resistivity and irreversible change such as thermal and electric degradation of resin matrix is considered. ☐ Electrical characterization under high current density is carried out for dry fiber tows and cured composites experimentally. The contributions of reversible and irreversible resistivity change are identified with carefully designed repetitive current tests. It is found that for dry fiber tows with sizing and for cured composites, thermal breakdown of the thin resin/sizing layer contributes significantly to the nonlinear conduction behavior under high current density. The developed model captures important characteristics of the electrical conduction behavior when compared with experimental results. Possible explanations are offered for cases and regions where the model shows discrepancies with experimental results. This model should prove useful to address and design and fabricate composite components in which electric and thermal conductivity play a key role in defining their functional properties.