Terahertz Ray Nondestructive Characterizations in FRP Composites

Je-Woong Park1,a, Kwang-Hee Im2,b*, David K. Hsu3,b , Chien-Ping Chiou 3,b,

and Daniel J. Barnard3,b

1 Dept. of Naval Architecture and Ocean Eng., Chosun University 375 Seosuk-dong, Dong-gu, Kwangju 501-759, Korea 2Department of Automotive Eng., Woosuk University

490, Hujung-ri, Samrae-up, Wanju-kun, Chonbuk, 565-701, Korea 3Center for Nondestructive Evaluation, Iowa State University,IA 50011, USA

ajwpark@chosun.ac.kr , and bkhim@woosuk.ac.kr (corresponding author)

Keywords: Terahertz Ray, Material Characterization, Reflection Mode

Abstract. Recently, terahertz ray imaging has emerged as one of the most promising new powerful nondestructive evaluation (NDE) techniques, and new application systems are under processing development for the area applications.

In this study, a new time-domain spectroscopy system was utilized for detecting and evaluating layup effect and flaw in FRP composite laminates. Extensive experimental measurements in reflection mode were made to map out the T-ray images. Especially in this characterization procedure, we estimated the electromagnetic properties such as the refractive index. Estimates of properties are in good agreement with known data. Furthermore layup effect and flaw of FRP composite laminates were observed in reflection mode and limitations will be discussed in the T-ray processing. Introduction Recent advances of technology and instrumentation in terahertz radiation have provided a probing field on the electromagnetic 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 (THz-TDS) is leading noncontact accurate detection of flaws and impact damages in composites, in which the THz-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. 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 terahertz, they consequently span a very broad bandwidth. T-ray images can show chemical compositions of the object. These features of T-ray imaging have generated interest in commercial applications in diverse areas as moisture analysis, quality control of plastic parts and packaging inspection (monitoring) [2-3].

Especially, in this paper, we will report a few of the most interesting applications of THz ‘T-ray’ radiation for the nondestructive evaluation of composite materials and structures. These methods include both a reflection and through-transmission mode, where the THz pulses was set up

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for scanning image reconstruction. A new measurement technique of refractive index will be discussed, which can be used to resolve on reflection and through-transmission modes. The composite materials and structures investigated include both non-conducting polymeric composites reinforced by glass, Kevlar, and quartz fibers and the conducting carbon fiber composites. The structures used in the experiments included both solid laminates and honeycomb or foam core sandwiches. The defects and anomalies investigated by terahertz radiation were foreign material inclusions, simulated disbond and delamination, mechanical impact damage, fiber orientation and heat damage. The effectiveness and limitations of terahertz radiation for the NDE of composites are discussed.

Fig.1. Schematic of the experimental setup used for terahertz transmission imaging.

Terahertz radiation 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. Fig.1 is a schematic diagram of a THz-TDS spectrometer, used for T-ray transmission imaging [3]. The TDS techniques for generating, manipulating, and detecting terahertz pulses have been immerged. Refractive Index

(a) Through-transmission mode (b) Reflection mode

Fig.2 Measurement of refractive index Refractive index could be considered one of properties in order to estimate materials as shown in Fig.2; so we will suggest a procedure to estimate the electromagnetic properties such as the

840 Multi-Functional Materials and Structures III

refractive index. Refractive index(n) for the through-transmission mode and reflection mode can be solved as Eqs.(1) and (2) respectively.

dtv

n air∆+=∴ 1 (1)

0sin 1224 =−− pAAnn θ (2)

where d is the sample thickness, Vair is a light speed in air and Vs is a light speed in sample, ∆t (T)

is the difference time between with sample and without sample and 22

22

4dVT

A air= .

Imaging of Multiple Delaminations There are fundamental differences when materials are probed with terahertz radiation, an electromagnetic wave, and with ultrasound, a mechanical wave. To demonstrate this effect, two semi-circular saw slots were cut parallel to the surfaces into the side of a 12.5mm thick glass composite laminate to serve as simulated delaminations, as shown in Fig. 3. The smaller slot (101.6mm diameter) was located below the larger slot (127mm diameter). The image of the transmitted terahertz pulse amplitude, also shown in Fig. 3, clearly revealed both saw slots. The right half of the sample contained a single saw slot and some resin-starved region nearby, which appeared in the image above it. The sample was also scanned from the top surface with reflection mode TDS terahertz pulse, and the double saw slots were again imaged.

Fig. 3 Terahertz TDS through-transmission Fig. 4 Embedded defects in Kevlar/Nomex scan image honeycomb sandwich.

Imaging of Embedded Flaws in Kevlar/#omex Sandwich Both the reflection mode (small angle pitch-catch) and the transmission mode TDS terahertz scans were found effective in mapping out defects embedded between the Kevlar composite facesheet and the Nomex honeycomb core of a sandwich panel. In addition to using the peak amplitude of the terahertz pulse reflected from or transmitted through the panel, images can also be generated using the frequency domain FFT signal. Fig. 4 shows C-scan, B-scan images and the FFT of the reflected signal from the top facesheet of the Kevlar/Nomex panel, together with the image based on the peak amplitude of the FFT signal in the frequency window of 0.2 to 0.3 THz. Conducting Effect to Carbon Composites Terahertz waves can penetrate dielectric materials quite easily but not electrically conducting materials. Based on the electrical conductivity in carbon composites, we tried to understand the

Advanced Materials Research Vols. 123-125 841

detectability of shallow embedded flaws in a carbon composite laminate. Brass, 0.025mm thick and measuring 12.5mm and 6.25mm squares, were embedded in a quasi-isotropic carbon composite laminate at depths of 1 ply, 2 plies, 3 plies, and 4 plies, respectively. The fiber orientations in the first four plies of the laminate are respectively -45o, 0o, 45o, and 90o. A series of scan images were obtained with the composite sample rotated every 22.5 degrees with respect to the electric field. Due to space limitation, four of the scans are shown in Fig. 5.

Fig. 5 TDS reflection mode terahertz scan images of embedded flaws and configuration in a quasi-

isotropic carbon composite laminate Conclusions The results were obtained in an exploration of terahertz wave applications as follows. Terahertz time-domain transmission spectroscopy was used to obtain the refractive indices of PMMA, fused quartz and GFRP. THz TDS could be demonstrated for the application to measure one of material properties. Also, for the NDT of composites, the conductivity of carbon fibers will substantially limit the utility of terahertz waves in CFRP. In non-conducting composites of glass, quartz, and Kevlar fibers, terahertz can complement ultrasonic NDT. References [1]. B. B. Hu and M. C. Nuss,: Opt. Lett.(1995), Vol. 20, 17, pp.16-20 [2]. R. Huber, A. Brodschelm, F. Tauser, and A.Leitenstorfer,: Appl. Phys. Lett.(2000), Vol.76,pp.

3191-3198 [3]. I. S. Gregory, C. Baker, W. R. Tribe, I. V. Bradley, M. J. Evans, E. H. Linfield,: IEEE Journal

Of Quantum Electronics, Vol. 41(5), pp.717- 728

842 Multi-Functional Materials and Structures III

Multi-Functional Materials and Structures III 10.4028/www.scientific.net/AMR.123-125 Terahertz Ray Nondestructive Characterizations in FRP Composites 10.4028/www.scientific.net/AMR.123-125.839

DOI References

[1] . B. B. Hu and M. C. Nuss,: Opt. Lett.(1995), Vol. 20, 17, pp.16-20

doi:10.1364/OL.20.001716