Transient heat flow in unidirectional fiber–polymer composites during laser flash analysis: Experimental measurements and finite element modeling

Laser flash analysis (LFA), an unsteady-state technique originally developed for measuring the thermal diffusivity of homogenous materials, was used to estimate the thermal conductivity of carbon fibers consolidated in an epoxy matrix to form axially aligned unidirectional composites. Experimental studies were conducted for P-25 and K-1100 mesophase pitch-based carbon fibers whose conductivity values bracket almost two orders of magnitude (∼10 and 1000 W/m·K). Experimentally determined fiber thermal conductivity values were generally consistent with those cited in the literature after appropriate corrections were applied to account for the extremely high conductivity (low thermal resistance) of highly graphitic fibers, relative to the graphite coating. Finite element analysis was used to simulate heat flow patterns that may occur in a uniaxial fiber–polymer composite due to the large differences in the thermal conductivities of carbon fibers and polymer matrices. Simulations reveal that fiber thermal conductivity is accurately determined from composite response for high volume fraction of fibers (≥0.6) regardless of fiber conductivity, or for lower conductivity fibers (10–100 W/m·K) regardless of volume fractions. However, for composites containing high thermal conductivity fibers (100–1000 W/m·K) at low volume fractions (≤0.2), fiber thermal conductivity may not be accurately determined due to transverse heat flow within the graphite layers that channel heat through the highly conductive fiber. Thus, under certain conditions, heat flow paths deviate from the one-dimensional heat flow assumption inherent to laser flash analysis and rule-of-mixtures.