Measured Through-the-Thickness Thermal Diffusivity of Carbon Fiber Reinforced Composite Materials

The thermal diffusivity of carbon fiber reinforced composite materials is dependent on fiber, matrix, and porosity volume fractions. Since the carbon fiber thermal conductivity is much greater than the epoxy resin matrix thermal conductivity, thermal diffusivity is a good indicator of the fiber volume fraction. In this study, through-the-thickness thermal diffusivity images were obtained nondestructively on carbon fiber reinforced composite plates, and the values varied from 0.0035 to 0.0062 cm2/s. The fiber and porosity volume fractions were measured destructively, and their values varied from 56.0 to 70.5% and from 0.9 to 7.2%, respectively. The destructive test data were used as a standard for comparison of three commonly used transverse equivalent thermal conductivity models: the Stacked Plate, the mixed flow Cylindrical Fiber model, and the Composite Circular Assemblage model. In these models, a porosity correction is also studied. There was very good agreement between the Composite Circular Assemblage and Cylindrical Fiber model when porosity levels were less than 2%. The porosity correction helped to reduce the chi-squared value by 24% in the Cylindrical Filament model. Finally, the potential is discussed to image fiber volume fraction nondestructively using the described thermal measurement system.