Prediction of the Thermal Conductivity of the Constituents of Fiber-Reinforced Composite Laminates: Voids Effect

Abstract

There are inherent difficulties in the direct measurement of the thermal conductivity of fibers. The need to determine the thermal conductivity of fibers for design purposes has been the motivation of the present work. Four empirical formulas are developed to predict the thermal conductivities of fiberreinforced composite laminates (FRCLs) and their constituents. Two of these models utilize the parallel and series thermal models of composite walls in predicting the thermal conductivity of the fibers. The models are tested at different fiber-to-resin volume ratios (30/70, 45/55, 50/50, 60/40, and 75/25) and various fiber-to-resin thermal conductivity ratios (0.5, 1, 2, 3, 4, and 5). These ranges indicate the physically possible fiber to resin volume thermal conductivity ratios including the extreme possible cases. The effect of the air void volume ratio on the thermal conductivity of the composite laminates is investigated. The range of the investigated void fractions also represent the practical ranges. The predicted thermal conductivity of the fiber can be accurately predicted throughout the spectrum via three models. The first model is a first-order formula (R 2 = 0.9148) based on the parallel series structure of the constituents within the composite laminate. The second model is a second-order formula (R 2 = 0.9308) which is also based on the parallel series structure of the constituents. A third model is developed to predict the fiber thermal conductivity as a direct function of the composite thermal conductivity and other composite constituents and volume ratios. This correlation has a coefficient of determination (R 2 = 0.9632). A fourth model is developed to predict the effective thermal conductivity of the laminate. The effective thermal conductivity of the FRCL is predicted with very high accuracy (R 2 = 0.9948). Another use of these models is to determine the fiber to resin volume ratio or air void volume fraction (if all thermal conductivities of fiber, resin, and laminate are known).