Terahertz (THz) applications have emerged as one of the most new powerful nondestructive evaluation (NDE) techniques. A new T-ray time-domain spectroscopy system was utilized for detecting and evaluating orientation influence in carbon fiber-reinforced plastics (CFRPs) composite laminates. Investigation of terahertz time-domain spectroscopy (THz-TDS) was made, and reflection and transmission configurations were studied as a nondestructive evaluation technique. Here, the CFRP composites derived their excellent mechanical strength, stiffness, and electrical conductivity from carbon fibers. Especially, the electrical conductivity of the CFRP composites depends on the direction of unidirectional fibers since carbon fibers are electrically conducting while the epoxy matrix is not. In order to solve various material properties, the index of refraction (
Increasingly, the terahertz radiation has been recognized for their importance in technological applications. Recently, T-ray (terahertz ray) advances of technology and instrumentation has provided a probing field on the electromagnetic spectrum because the terahertz radiation has a shorter wavelength and relatively higher resolution than microwaves. 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 [
The THz TDS setup is shown in Figure
THz TDS system for imaging and material parameter measurement.
In the past years, T-ray technology has received more interest and attention because of its unique properties as a nondestructive evaluation (NDE) tool. The THz frequency region represents an important intersection between spatial resolution and penetration depth. The terahertz 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 techniques for generating, manipulating, and detecting terahertz pulses have been immerged. By obtaining images with THz-TDS, in which the entire terahertz waveform is measured at each pixel of the image, the data acquisition rate could be increased. The TDS system has a frequency range of 50 GHz–4 THz and a fast delay line up to 300 ps. 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, 0.8 mm and 2.5 mm. The TDS system can be configured for through-transmission or reflection (small angle pitch-catch) measurements. The frequency range of the CW system is 50 GHz–1.5 THz, with the best resolution being 100 MHz. The focal lengths of the CW system are also 50 mm and 150 mm. Both the TDS and the CW systems are fully fiber optics connected. Figure
A schematic diagram of a THz-TDS spectrometer.
This method was to determine the index of refraction used to calculate the optical path length difference between the front and back reflections in the time domain. A diagram showing the geometry of the two THz signals is shown in Figure
Diagram showing the geometry of the surface reflection mode.
Time of flight could be solved without sample as follows:
Penetration time (
In through-transmission mode, the index of refraction (
Diagram showing the geometry of the through-transmission mode.
A method to determine the absorption coefficient (
Diagram showing the geometry of the through-transmission mode with two samples in (a) thinner and (b) thicker samples.
In preparation for measuring material parameters, we first measured a THz pulse reflected from 24-ply woven glass epoxy GFRP composites. In Figure
Average THz refractive indices of the material studied.
Materials | Refractive index ( |
Refractive index ( |
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Through-transmission mode | Reflection mode | ||
PMMA |
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Fused quartz |
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GFRP | — |
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THz TDS pulses reflected from the front and back surfaces.
Terahertz waves can readily penetrate nonconducting materials and can therefore be applied to the nondestructive testing (NDT) of glass, quartz, or Kevlar fiber-reinforced polymer matrix composites. Solid laminates and sandwich structures with honeycomb or foam core can all be examined with terahertz waves. In this study, terahertz radiation was explored for the NDT of a variety of non-conducting composite materials and structures. Some preliminary results are summarized later.
The time-domain waveforms of terahertz pulses in the TDS mode bear strong resemblance to ultrasonic signals. Wave propagation concepts such as time of flight (TOF), transmission and reflection coefficients, refraction, and diffraction are common to both waves. However, there are also fundamental differences when materials are probed with terahertz radiation, an electromagnetic wave, and with ultrasound, a mechanical wave. First and foremost, terahertz waves, unlike ultrasound, do not require a material medium to support it and can therefore readily go through vacuum or air. In the ultrasonic inspection of flaws in a solid, the probing field suffers from the “shadow effect,” where a smaller flaw behind a larger flaw could not be detected. In contrast, there is no such limitation with terahertz waves. To demonstrate this effect, two semicircular saw slots were cut parallel to the surfaces into the side of a 28.7 mm thick woven glass composite laminate to serve as simulated delaminations, as shown in Figure
Terahertz TDS through-transmission scan image of double saw slots in a glass composite solid laminate. The results demonstrated that terahertz waves do not suffer from “shadow effect.”
GFRP sample
C-scan image
B scan at B–B′
The sample was also scanned from the top surface with reflection mode TDS terahertz pulse, and the double saw slots were imaged.
Based on the capability described easlier, the multiple delaminations in solid composite laminates were imaged with reflection mode TDS terahertz waves. The specimen was a 24-ply woven glass epoxy solid laminate, and the impact was made with a 50.8 mm diameter tup at an impact energy of 16 Joules. The reflection mode TDS scan was made on the back side of the sample so that the smaller delaminations were shadowed by the larger and shallower delaminations. Figure
Time-domain TDS waveform of glass composite laminate (a) and impact delamination images using different time gates (from left: (b) 43–69 ps, (c) 47–69 ps, and (d) 56–69 ps).
T-ray A-scan
43–69 ps
47–69 ps
56–69 ps
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 (skin: 0.38 mm Kevlar, 12.7 mm Nomex honeycomb, and cell size: 5.08 mm). 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. Figure
Embedded defects in Kevlar/Nomex honeycomb sandwich. (a) C-scan image based on amplitude and (b) B-scan image in the horizontal middle location of image (a).
C-scan image
B-scan image
Terahertz waves can penetrate dielectric materials quite easily but not electrically conducting materials. Also, the application of electrical resistance technique could be found for impact damage detection of multilayer woven carbon composite [
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 [
The value of transverse conductivity in a unidirectional laminate is highly dependent on the manufacturing process and the quality of the composite. In a unidirectional carbon composite, the in-plane conductivity with the electrical current flowing at an angle
Because of the highly anisotropic electrical conductivity (
Angular dependence of transmitted power of THz terahertz waves through a 22-ply unidirectional carbon composite laminate.
When compared to the theory prediction based on the angular-dependent conductivity, the measured power transmission at angles away from 90 degrees much higher this 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 contributes to the previous involved things.
A reflective TDS system was utilized in order to evaluate the effect of
Sample’s layup is [45/0/-45/90
Modeled conductivity of the first ply.
Scan 0° | Scan 22.5° | Scan 45.0° | Scan 67.5° | Scan 90° | |
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0 | 22.5 | 45 | 67.5 | 90 |
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1.0 | 0.85 | 0.5 | 0.15 | 0 |
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1 | 0.73 | 0.25 | 0.022 | 0 |
Diagram showing a sample set up system.
Diagram showing a scan configuration of TDS for scanning and direction of
As described easlier, being based on the electrical conductivity in carbon composites, we tried to understand the penetration of fiber orientation in a carbon composite laminate. As shown in Figure
We then correlated the S/N of the flaw images to the conductivity of the first two plies. Since
Diagram showing TDS scan images at the angles (
Photo of sample surface and microscopic photo of in-plane fiber waviness in CFRP composites.
Photo on the surface of the sample
Macroscopic photo on waviness
Initial results were obtained in an exploration of terahertz wave applications for the NDT of composites. A composite with various thicknesses was examined using a THz TDS system. Index of refraction in the terahertz frequency range for composite was measured using a reflection and transmission configuration. THz TDS reflection imaging on the composite with flaw was performed.
First of all, it was also easier to measure the refractive index of the composite using a transmission configuration. A reflection mode is certainly a more likely case for a maintenance technique. And, in non-conducting composites of glass, Kevlar/Nomex sandwich panels, Fused quartz, and PMMA, terahertz can complement ultrasonic NDT, especially with its penetration ability. The conductivity of carbon fibers will substantially limit the utility of terahertz waves even in one- or two-ply PPS CFRP and CFRP laminates; therefore, it is found that the penetration of the fiber depends on the relation between the
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (no. 2011-0008391).