Evaluation of Fiber Orientation in Carbon Fiber-Reinforced Polymer Composites Using a Simple, Low-Cost Infrared Measurement System: Application to Unidirectional Carbon Fiber Composites ^†

Abstract

Carbon fiber-reinforced polymer (CFRP) composites exhibit higher strength in the longitudinal direction than in the transverse direction of the fibers, making fiber orientation evaluation crucial. In this study, a method for evaluating fiber orientation using halogen spot-periodic heating and a non-lock-in-type infrared camera is employed and applied to a unidirectional carbon fiber composite—a CFRP composite with unidirectional continuous fibers. Consequently, the in-plane thermal diffusivity in the fiber direction is significantly higher than that in other directions. Therefore, the employed method successfully evaluates fiber orientation in unidirectional carbon fiber composites.

1. Introduction

Recently, carbon fiber-reinforced polymer (CFRP) composites have attracted interest in the automotive industry [1]. These composites exhibit higher strength in the longitudinal direction than in the transverse direction of the fibers [2], making fiber orientation evaluation crucial. Carbon fibers have higher thermal diffusivity than resin [3]. Consequently, the thermal diffusivity along the carbon fibers in CFRP composites is high. As a pioneering study on fiber orientation evaluation in CFRP composites, Fujita and Nagano [3] proposed a method based on the in-plane thermal diffusivity distribution obtained using laser spot-periodic heating and a lock-in-type infrared camera. However, this method requires an expensive infrared measurement system to synchronously obtain the laser heating signal and the temperature response. Therefore, the development of a simple, low-cost method for evaluating fiber orientation is beneficial.

In this study, a method for evaluating fiber orientation using halogen spot-periodic heating and a non-lock-in-type infrared camera was employed and applied to a unidirectional carbon fiber composite—a CFRP composite with unidirectional continuous fibers.

2. Evaluation of Fiber Orientation

The employed method requires no synchronous measurement of the heating signal and the temperature response. Figure 1 depicts the procedure for measuring the in-plane thermal diffusivity to evaluate fiber orientation. As depicted in Figure 1a, a plate is subjected to spot-periodic heating using a lamp controlled by a function generator, and its periodic temperature response on the rear surface is measured using an infrared camera. As depicted in Figure 1b, the slope $\mathrm{d}\varphi /\mathrm{d}r$ for a certain in-plane direction is calculated. In Figure 1b, the center corresponds to a point directly behind the heating spot. Furthermore, the thermal diffusivity for a certain in-plane direction is calculated using the following equation [3]:

$${\left.{\alpha }_{\mathrm{i}\mathrm{n}}\right|}_{\omega }={\displaystyle \frac{\pi f}{{\left(\mathrm{d}\varphi /\mathrm{d}r\right)}^2}}$$

(1)

where ${\left.{\alpha }_{\mathrm{i}\mathrm{n}}\right|}_{\omega }$ is the in-plane thermal diffusivity for the direction at an angle $\omega$ from the vertical axis and $f$ is the heating frequency. Herein, the phase lag ($\varphi$) between the periodic heating signal and the temperature response is calculated using the following equation:

$$\varphi =\theta -{\theta }_0$$

(2)

where $\theta$ and ${\theta }_0$ are the initial phases of the fundamental harmonic in the periodic temperature response at any point and the center, respectively.

In this study, fiber orientation in CFRP composites is evaluated based on their in-plane thermal diffusivity distribution.

3. Materials and Methods

A square plate of a unidirectional carbon fiber composite, fabricated by laminating 10 plies of prepreg sheets (Toray, Tokyo, Japan, T700SC/#2300) via autoclave molding, was utilized. This square plate had a side length of 150 mm and a thickness of 2 mm. The mass content of polyacrylonitrile-based continuous carbon fibers in the epoxy resin of the square plate was 67%, and the carbon fibers were aligned unidirectionally. Herein, this square plate is referred to as a UD specimen. Additionally, a blackbody paint was sprayed on the UD specimen’s surface.

A halogen lamp (Fintech Tokyo, Tokyo, Japan, HSH12) capable of heating a spot on the UD specimen’s surface with a diameter of 1.5 mm was utilized. The halogen lamp, controlled by a function generator, provided sinusoidal spot-periodic heating to the central portion of the UD specimen at a frequency of 0.01 Hz. On the rear side, temperature-distribution images on the UD specimen’s surface, subjected to the sinusoidal spot-periodic heating, were captured using a non-lock-in-type infrared camera (NEC Avio Infrared Technologies, Tokyo, Japan, InfReC R300SR) with a spatial resolution of 0.2 mm/pixel. The time-series temperature-distribution images, consisting of 100 frames recorded at a frame rate of 0.2 Hz, were then analyzed in the frequency domain. Consequently, the amplitude and initial phase of the fundamental harmonic in the periodic temperature response were obtained pixel by pixel.

4. Results and Discussion

In the experiments to evaluate fiber orientation in CFRP composites, the UD specimen was placed with its carbon fibers oriented horizontally. Figure 2 shows the distributions of the amplitude and initial phase of the fundamental harmonic in the periodic temperature response of the UD specimen. As shown in Figure 2, both the amplitude and initial phase exhibit anisotropic distributions.

The in-plane thermal diffusivity distribution was obtained. First, a point at which the maximum amplitude of the fundamental harmonic in the periodic temperature response was observed in Figure 2a was regarded as the center. Subsequently, the phase lag ($\varphi$) between the periodic heating signal and the temperature response was calculated pixel by pixel using the initial phases shown in Figure 2b and Equation (2). Finally, the thermal diffusivities for specific in-plane directions were calculated using Equation (1),and the results are shown in Figure 3. The x-axis values in the figure represent the angles of in-plane directions measured clockwise from the vertical axis. The y-axis values in the figure represent the in-plane thermal diffusivities normalized to the maximum thermal diffusivity among all in-plane directions. The slope $\mathrm{d}\varphi /\mathrm{d}r$ was calculated using data in the straight section ($2 \mathrm{m}\mathrm{m}<r<5 \mathrm{m}\mathrm{m}$) for each in-plane direction. As shown in Figure 3, the in-plane thermal diffusivity in the fiber direction is significantly higher than that in other directions. This anisotropy is attributed to the higher thermal diffusivity of carbon fibers compared with that of resin. Consequently, the employed method successfully evaluated fiber orientation in unidirectional carbon fiber composites.

5. Conclusions

In this study, a method for evaluating fiber orientation using halogen spot-periodic heating and a non-lock-in-type infrared camera was employed and applied to a unidirectional carbon fiber composite. The employed method required no synchronous measurement of the heating signal and the temperature response. Experimental results showed that the in-plane thermal diffusivity in the fiber direction was significantly higher than that in other directions. This anisotropy is attributed to the higher thermal diffusivity of carbon fibers compared with that of resin. Therefore, the employed method successfully evaluated fiber orientation in unidirectional carbon fiber composites.