Measurement of thermal diffusivity and evaluation of fiber condition of discontinuous fiber CFRP

Abstract

This paper proposes a new method to evaluate the degree of resin impregnating into carbon fiber in carbon fiber-reinforced plastic (CFRP) before molding based on the measurement of thermal diffusivity distribution in the thickness direction. The sample’s surface is periodically line-heated at an arbitrary frequency and the phase lag distribution is measured by a lock-in thermography. The thermal diffusivity distribution in the thickness direction is obtained from the frequency dependence of the phase lag. The degree of resin impregnating into carbon fiber in CFRP is evaluated by analysing the range of the measured thermal diffusivity distribution. First, a reference material with known thermal diffusivity is used to confirm the validity of the thermal diffusivity measurements. Measurement results are within 10% range relative to the reference value. The proposed method is then applied to the CFRP before molding process. Two types of CFRPs are measured with different degrees of resin impregnating into carbon fiber: impregnated CFRP and non-impregnated CFRP. Results show that the value of the thermal diffusivities for the impregnated CFRP is mainly distributed between those of the resin and the carbon fiber in the radial direction. On the other hand, thermal diffusivity values for the non-impregnated CFRP are lower than those of the resin, suggesting the presence of many voids in the specimen. This is consistent with the material structure of CFRP in each degree of resin impregnating into carbon fiber. Results of this study demonstrate the validity of the proposed method for evaluating the degree of resin impregnating into carbon fiber in CFRP before molding.