１．Introduction

Application of carbon fiber-reinforced plastic (CFRP) is anticipated in various industries such as aerospace, automobiles, vessels, and wind power generation. As for carbon fiber-reinforced thermoplastic resin (CFRTP), for which mass production is anticipated, there are challenges such as uneven fiber orientations, formation of voids, and uneven density during press molding. However, in the past, fiber orientation assessment involved strength tests, X-ray, and CT observations requiring samples to be cut out, with long times for measurements; i.e., there was no simple and practical method. We developed a revolutionary method in which a point on CFRP is periodically spot-heated, with heat diffusion process in the two-dimensional planar direction and the thickness direction then being measured using thermography. Then, phase delays in temperature response and temperature attenuation are analyzed. Based on their dependence on distance, fiber orientation distribution in the in-plane direction is assessed, and based on the heating frequency dependence, density distribution of fiber is noninvasively and nondestructively analyzed in several minutes. This method was then commercialized.

２．Technical contents

The present measurement method uses the principle of 3D thermal diffusivity in both the in-plane and thickness directions, based on the periodic heating method established by Nagoya University and Bethel. One point on the target part is periodically heated, and thermal diffusivity is obtained from time delay in thermal response from the heating point and distance dependence of attenuation in temperature amplitude. By applying this analysis to 360 degrees in the in-plane direction, and fitting with a fiber orientation density function or an elliptic function, the in-plane fiber orientation direction and its dispersion can be identified. In addition, by measuring phase delay and amplitude attenuation while changing the heating frequency, thermal diffusivity in the thickness direction can be obtained, along with density distribution information for fibers. With the present device, we spot-heated one side of a part with a laser and measured temperature propagation information to the back side using lock-in thermography. This allowed simultaneous and rapid measurement of thermal diffusivity in the in-plane and thickness direction, and in turn, fiber orientation analysis at almost the same time, providing fiber orientation information in a noninvasive and nondestructive manner, within several minutes. Figures 1 and 2 show the configuration of the device for the fiber orientation assessment system, and the appearance of the product.

Fig. 1 configuration of the device for the fiber orientation assessment system

Fig. 2 appearance of TEFOD

CFRTP (Figure 3(a)) is irradiated with a laser and periodically heated to obtain the phase delay distribution shown in Figure 3(b), which in turn provides heat diffusivity distribution (Figure 3(c)). By normalizing the present result and performing elliptic approximation, the slope and aspect ratio of an ellipse are obtained, based on the fiber orientation angle and strength determined as shown in Figure 3(d).
For CFRTP samples with intentionally varied fiber content (Figure 4(a)), the relationship between fiber orientation, heat diffusivity and tensile strength, and the relationship between heat diffusivity and fiber content, were examined. Fiber orientation is horizontal (Figure 4(b)). There are correlations between fiber content and tensile modulus, and between fiber content and heat diffusivity (Figure 4(c, d)).

　
(a) CFRTP 　　　　　　　　　　　　(b) Phase-delay distribution

　
(c) Thermal diffusivity and fitting result 　　(d) Fiber orientation evaluation results

Fig. 3 Fiber orientation evaluation for CFRTP

　
(a) CFRTP dumbbell with different VF　 　　　　　　(b) Fiber orientation evaluation results

(c) Relation between thermal diffusivity and Tensile modulus (d) Relation between VF and thermal diffusivity

Fig. 4 Relation between thermal diffusivity, VF and Tensile modulus

３．Summary

In contrast to conventional fiber orientation assessment that only has the options of X-ray and CT analyses, a new rapid method that nondestructively and noninvasively assesses fiber orientation has been developed, allowing for in-line assessment of a composite material production line and nondestructive tests. In addition, this method is garnering attention from companies, universities, and research organizations for research and development of materials, as a revolutionary evaluation method that simultaneously measures thermophysical properties, orientation distribution, and fiber density distribution. We hope that this technology will dramatically accelerate development of new materials, and mass production of composite materials that are the foundation of future industry.

Hosei Nagano*1
Kimihito Hatori*2
Tesuya Otsuki*2
Takaki Awano*2

*1 Member, Nagoya University（〒464-8603 Furo-cho, Chikusa-ku, Nagoya）
*2Bethel Hudson Laboratory（〒300-0037 4-3-18, Tsuchiura brick Bld. 1F, Sakura-machi, Tsuchiura-shi,）