Influence of Terahertz Waves on the Penetration in Thick FRP Composite Materials

Influence of Terahertz Waves on the Penetration in Thick FRP Composite Materials Kwang-Hee Im a, David K. Hsu b, Chien-Ping Chiou b, Daniel J. Barnard b, In-Young Yang c, and Je-Woong Park d a Department of Automotive Eng., Woosuk University, Chonbuk, 565-701, Korea b Center for Nondestructive Evaluation, Iowa State University, Ames, Iowa, 50011,USA c Department of Mechanical Design Engineering, Chosun University, Gwangju 501-759, Korea d Department of Naval Architecture and Ocean Eng., Chosun University, Gwangju 501-759, Korea Abstract. Fiber reinforced plastics (FRP) are increasingly utilized in engineering structures because of their performance and fabrication advantages. With this increased utilization, a technique to gage quality and further characterize the materials would be beneficial. The nondestructive applications for Terahertz (T-ray) methods have also experienced increased utilization for evaluating engineering materials and will be reported on here in applications for the inspection and characterization of FRP materials used in wind energy components. First, refraction and transmission T-ray modes are used to determine the refractive index (n) of a glass fiber reinforced plastic (GFRP) reference sample, and extended for calculating the refractive indices for a sample of GFRP, balsa and epoxy. Additionally, carbon fiber reinforced plastic (CFRP) samples were evaluated with respect to fiber directions versus T-ray electric field polarization direction to evaluate the level of penetration of T-ray energy due to the fiber orientation dependent conductivity of this composite material. Finally, an evaluation of T-ray data was made to evaluate resonance effects, where the resonance frequency was found to agree with that expected from reflections from individual plies in thick GFRP laminates. Keywords: Resonance Frequency, Penetration, Thick FRP Composites, Terahertz Waves PACS: 81.70.-g INTRODUCTION Recent changes of technology and NDE tools in the Terahertz (T-ray) have shown a challenge field on the electromagnetic (EM) spectrum because the terahertz radiation has a shorter wavelength, relatively higher resolution than microwaves, and lower attenuation. The terahertz radiation is of critical importance in the spectroscopy evaluation of airport security screening, medical imaging, polar liquids, industrial systems and composites as well [1]. Also the terahertz time domain spectroscopy (TDz-TDS) is leading noncontact and accurate detection of defects and impact damages in composites [2], in which the TDz-TDS is based on photoconductive switches, which rely on the production of few-cycle terahertz pulses using a femtosecond laser to excite a photoconductive antenna [3]. This can generate sub-picosecond bursts of THz radiation, and subsequently detect them with high signal-to-noise. With the emitted power distributed over several T-rays, they consequently span a very broad bandwidth. A transient change of the emitter occurs in the resistance of a photoconductive switch on a terahertz timescale. An external dc bias can create a current flow that contains components at terahertz frequencies. The current induces a terahertz EM field in the planar and metallic antenna. The terahertz dipole radiation is connected from the antenna into free space as a quasi-collimated beam [4]. Also, another method is optical heterodyne conversion, or photomixing, which can be obtained using two continuous-wave (CW) lasers [5-6]. The mixing of two lasers could produce beating, which can modulate the conductance of a photoconductive switch by the terahertz difference frequency. So, the CW-terahertz (CW-THz) radiation is produced. Here this terahertz T-ray technology constitutes of imaging, real-time acquisition of the T- ray waveforms and advanced signal processing. In some cases, the T-ray images could tell

chemical compositions from the objects. These features of the T-ray imaging have generated interest in commercial applications in diverse areas as moisture analysis, quality control of plastic parts and packaging inspection (monitoring) [7-10]. Many T-ray NDE reports, however, have rarely made for materials evaluation and structural testing regardless of recent advances of technology and instrumentation in terahertz radiation. An investigation of terahertz radiation was made for the NDE of composite materials and structures. The T-ray can readily penetrate some thickness of dielectric materials; so that non-conducting polymer composites reinforced with glass, quartz, or Kevlar fibers are well suited for the T-ray inspection. Structures such as aircraft radomes, designed for passing radar signals, are good cases for the T-ray inspection. Also, wind energy turbine blades, being constructed of glass fiber composites, balsa wood, and adhesive, can also be penetrated with terahertz radiation. The T-ray was investigated as an NDE tool for detecting and characterizing flaws and damage in nonconducting composites. Carbon fiber reinforced polymer composites, on the other hand, are generally considered as some conducting material due to carbon fibers. However resin is considered as nonconducting material; so the T-ray goes to penetrate the resin. The degree of penetration in carbon composites by the T-ray, especially as a function of fiber orientation, is handled quantitatively in this study. The degree of penetration was defined based on the angle of function between the carbon fiber direction and the E-field direction of T-ray. Also, the surface fiber angle plays an important role to penetrate the carbon composites. Additionally in a piece of GFRP composites for a use of wind energy, T-ray C-scan images were made and analyzed for detecting the defects in GFRP composites; also, T-ray A-scan data were measured; a regular difference of time (Δt) was shown, which was related to the thickness of each ply in the GFR P composites. Also, a relation between T-ray TOF and FFT data were analyzed in order to measure the thickness of each ply in the GFRP composites. EXPERIMENT The terahertz instrumentation systems used in this research were provided by TeraView Limited. The instrumentation includes a time domain spectroscopy (TDS) pulsed system and a frequency domain continuous wave (CW) system. The TDS system has a frequency range of 50GHz 4 THz and a fast delay line up to 300ps. The beam is focused to focal lengths of 50 mm and 150 mm and the full width at half maximum (FWHM) beam widths are respectively 0.8mm and 2.5mm. The TDS system can be configured for through-transmission or reflection (small angle pitch-catch) measurements. The frequency range of the CW system is 50GHz 1.5THz, with the best resolution being 100MHz. The focal lengths of the CW system are also 50mm and 150mm. Both the TDS and the CW systems are fully fiber optics connected [1, 11, 12]. Reflection Mode This method was to determine the index of refraction used to calculate the optical path length different between the front and back reflections in the time domain. A diagram showing the geometry of the two THz signals is shown as in Fig. 1. First of all, recall from a normal incident echo, a regular difference of time, Δt will be below [11];

t = 2d v (1) Where d is the sample thickness and v is the sample velocity. Consider both the geometry time delay, δ as shown in Fig.1 and oblique T-ray trace length, l in the reflection mode above a regular difference of time, Δt between front echo and back surface echo will be obtained below [11]; t = 2l v δ C a Where l = d, δ = 2lsin 2 θ cosθ a = 2 sin 2 θ r cosθ a r d Where Ca is the velocity in air, d is the sample thickness, v is the sample velocity, θa is the angle of reflection mode, θr is the refractive angle in sample. Resonance frequency, Δf is obtained based on considering geometry time delay and oblique T-ray trace length below; f = 1 = 1 2d = ( v cosθr δ Ca ) 2d ( v cosθr 2d sin2 θa cosθr Ca ) 1 2d cosθr (1 v sin2 θa Ca ) (3) (2) FIGURE 1. Diagram showing the geometry of the reflection mode. Where the transmission time of the sample, d is the sample thickness, V is the speed in sample, Ca is the light speed in air, θr is the incident angle of the sample and θa is the incident angle in air ; so we will suggest a procedure to estimate the electromagnetic properties such as the refractive index. Refractive index (n) for the reflection mode can be solved as Eq.(4) [11-12]. 4 2 2 n An Asin p1 0 (4) where d is the sample thickness, Vair is a light speed in air and Vs is a light speed in sample, t (T) 2 2 T Vair is the difference time between with sample and without sample and A. 2 4d 2

Through-Transmission Mode In through-transmission mode, the index of refraction (n) could be calculated using the following equation [11]. n = 1 + t v air (5) d Where Δt is the difference time between with sample and without sample, d is the sample thickness, Vair is the light speed in air. RESULTS AND DISCUSSION Measurement of Refractive Index In order to measure parameters of T-ray that shows the material's physical property, THz pulse was obtained from GFRP in both reflection mode and through-transmission mode. Table 1 shows refractive indices of GFRP composites, Balsa, PMMA, Fused quartz and Epoxy samples measured in the both modes. Standard deviation of data did not spread away from 1 to 2 %. When measuring the refractive index, the through-transmission mode is being used a lot because terahertz measuring technique in through-transmission mode is relatively easy in experiment compared to other methods even though there are many parameters to consider in actual measuring. Materials TABLE 1. Average THz refractive indices of the material studied. Through- Reflection transmission mode mode PMMA * 1.60 ± 0.05 1.61 ± 0.01 Fused quartz * 1.92 ± 0.04 1.93 ± 0.03 GFRP 2.18 ± 0.06 2.19 ± 0.06 Epoxy 1.77 ± 0.02 1.76 ± 0.04 Balsa 1.19 ± 0.09 1.19 ± 0.06 *PMMA and Fused quartz were known data by References 8-9 In case of test specimen of PMMA and fused quartz materials; however, there was difficulty in comparing the result to the existing data because the method of fabrication and properties were different among them [8-9]. E-Field Characterization in Carbon Fiber T-ray waves can penetrate dielectric materials quite easily but not electrically conducting materials. The application of terahertz waves to the inspection of carbon composites is mentioned in the literature [8-10] but there has not been in depth studies. Carbon fiber reinforced polymer composites (CFRP) are poor conductors for electricity and the conductivity is anisotropic, so it is worthwhile to quantify the penetration of terahertz waves in carbon composites. The carbon fibers used in the manufacturing of CFRP are highly anisotropic microscopically; the electrical conductivity along the fiber axis is about three orders of magnitude greater than that in the radial

direction. In a unidirectional laminate of carbon fiber composite, the transverse electrical conductivity is further impeded by the lack of continuity. The conduction mechanism in the transverse direction (perpendicular to the fiber axis) is a percolation process that relies on the random contact between adjacent fibers. In the literature, the electrical conductivity data for carbon composites are somewhat sparse [9]. Experimentally, we have measured the angular dependence of the power transmission through a 1-ply unidirectional carbon composite laminate using the CW terahertz system. Near the low end of the frequency spectrum (f ~ 0.1 THz), the transmitted power is more than 30 db above the noise floor. The angular dependence of the transmitted power at 0.1 THz is shown in Fig. 2. Figure 2(a) shows asset up for T-ray testing based on E-field direction; so the transmission powers were measured at every 15 degree angles. Figure 2(b) shows the highest transmission power amplitude at around 90 and 270 deg. and does lowest amplitude at around 0 and 180 deg. And the measured power amplitudes were plotted as a function of angles as shown in Fig. 2(b). FIGURE 2. Angular dependence of transmitted power of THz terahertz waves through a 1-ply unidirectional CFRP laminate. When compared to the theory prediction [10] based on the angular dependent conductivity, the measured power transmission at angles away from 90 degree much higher the predicted. The value would have the unidirectional carbon composites behaving like a polarizer with a sharp cut-off under the assumptions that the incident terahertz ray is linearly polarized and that the fiber axes in the laminate are all parallel. It seems that the discrepancy in some angles contributes to the above involved things. However, it is found that the transmission of terahertz power depend on the fiber direction of conducting CFRP composite laminates. So, it is assumed that the E-field direction is normal to the carbon fiber direction as shown in Fig.2 (b). However, there is no difference in case of GFRP composites as shown in Fig. 2(b) due to non-conducting materials.

THZ Images in Thick GFRP Composites Figure 3 shows two photos (a) and (b) of wind turbine blade and Terahertz system. Experimentation of a through-transmission mode was made as shown in Fig. 3 (a). Figure 4 exhibits a C-scan image and B-scan image in the blade. A red area could be observed with the relatively high amplitude, which area can be expected to some defects. FIGURE 3. Wind turbine blade and setup for scanning the blade. FIGURE 4. T-ray scan images in the wind turbine blade (a) C-scan in through-transmission mode (b) B-scan image at A-A line (c) B-scan image at B-B line.

FIGURE 5. A-scan and FFT image in the thick GFRP plate. So a shorter TOF (time-of-flight) was measured. Figure 4(b) displays a B-scan image from Fig. 4(a). See a point A; there is a line around the time of 240ps. We expected that that area could be very uniform at the A-A section. However, take a look of Fig. 4(c); there is some lines on a point A at the right side, which means the uniform region. At a point B, there is a declined line at the right side. This means that the blade are consisted of GFRP, balsa and epoxy with shapes of roundness and curves; so it seemed that the shapes were corresponding with that of the blades. Also, see a point C; a yellow line with high amplitude was observed. This line was corresponding with that in Fig. 4(a). Thus, it was thought that the yellow line could be related with the defects in the wind turbine blade. THz Signal Based on Resonance Frequency A GFRP plate for a use of wind turbine blade was scanned using T-ray system. Also, Fig. 5 shows a A-scan and FFT images respectively. Notice that a T-ray time domain data in Fig. 5(a) seemed to show peaks with a regular spacing (see a Δt). We estimated in the lab and found that the value of delta-t (Δt= 12ps) seemed to be consistent with a ply thickness as measured in the lab. One thing was done for a FFT on the data and resonance frequency (Δf=0.08THz) was obtained as shown in Fig. 5(b), which was obvious with the close relation between TOF and FFT. This does prove that there are reflections or echoes at the ply interfaces in the GFRP plate due to the regularly spaced peaks. CONCLUSION Recently, T-ray waves can show unique characteristics for nondestructive evaluation on the conducting and non-conducting composites. It was found that the index of refraction of samples could be easily measured using both a reflection mode and a transmission configuration as a study for application of T-ray.

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