Nondestructive evaluation of aircraft composites using reflective terahertz time domain spectroscopy (original) (raw)
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Optics Express, 2008
Terahertz time domain spectroscopy (TDS) was assessed as a nondestructive evaluation technique for aircraft composites. Damage to glass fiber was studied including voids, delaminations, mechanical damage, and heat damage. Measurement of the material properties on samples with localized heat damage showed that burning did not change the refractive index or absorption coefficient noticeably; however, material blistering was detected. Voids were located by TDS transmissive imaging using amplitude and phase techniques. The depth of delaminations was measured via the timing of Fabry-Perot reflections after the main pulse. Evidence of bending stress damage and simulated hidden cracks was also detected with terahertz imaging.
Nondestructive evaluation of aircraft composites using terahertz time domain spectroscopy
2008
Terahertz (THz) time domain spectroscopy (TDS) was assessed as a nondestructive evaluation technique for aircraft composites. Material properties of glass fiber composite were measured using both transmission and reflection configuration. The interaction of THz with a glass fiber composite was then analyzed, including the effects of scattering, absorption, and the index of refraction, as well as effective medium approximations. THz TDS, in both transmission and reflection configuration, was used to study composite damage, including voids, delaminations, mechanical damage, and heat damage. Measurement of the material properties on samples with localized heat damage showed that burning did not change the refractive index or absorption coefficient noticeably; however, material blistering was detected. Voids were located by THz TDS transmission and reflection imaging using amplitude and phase techniques. The depth of delaminations was measured via the timing of Fabry-Perot reflections after the main pulse. Evidence of bending stress damage and simulated hidden cracks was also detected with terahertz imaging. v First, I would like to thank my advisor, Lt Col Matt Bohn, for his guidance in this research, offering skillful advice with the laboratory experimental setup and providing expert scientific insight in helping me to resolve research issues. I would also like to thank Dr. Jim Blackshire of AFRL/RXLP for sponsoring this work and for providing us with representative aircraft composite samples which were essential to the research. In addition, I would like to thank Abel Nunez for his help with image processing and to Jeremy Johnson and AFRL/RXLP for the ultrasound and x-ray images. Thanks to Col Brent Richert and my parents for their encouragement to return to school and earn this degree and to my mother for helping to proofread this dissertation. I would like to thank Epiphany Lutheran Church for its spiritual support along the way. I am very grateful for the birth of my two children who have been an inspiration to me and represent promise for the future. Most of all, I would like to thank my wife for her patience and understanding during the difficult days and nights when I was studying, researching, and writing. I also thank her for her strength in enduring an uncomfortable work situation for 4 years.
Aeronautics composite material inspection with a terahertz time-domain spectroscopy system
The usability of pulsed broadband terahertz radiation for the inspection of composite materials from the aeronautics industry is investigated, with the goal of developing a mobile time-domain spectroscopy system that operates in reflection geometry. A wide range of samples based on glass and carbon fiber reinforced plastics with various types of defects is examined using an imaging system; the results are evaluated both in time and frequency domain. The conductivity of carbon fibers prevents penetration of the respective samples but also allows analysis of coatings from the reflected THz pulses. Glass fiber composites are, in principle, transparent for THz radiation, but commonly with significant absorption for wavelengths >1 THz. Depending on depth, matrix material, and size, defects like foreign material inserts, delaminations, or moisture contamination can be visu-alized. If a defect is not too deep in the sample, its location can be correctly identified from the delay between partial reflections at the surface and the defect itself. © 2014 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10 .1117/1.OE.53.3.031208]
Nondestructive monitoring of aircraft composites using terahertz radiation
Saratov Fall Meeting 2014: Optical Technologies in Biophysics and Medicine XVI; Laser Physics and Photonics XVI; and Computational Biophysics, 2015
In this paper we consider using the terahertz (THz) time domain spectroscopy (TDS) for non destructive testing and determining the chemical composition of the vanes and rotor-blade spars. A versatile terahertz spectrometer for reflection and transmission has been used for experiments. We consider the features of measured terahertz signal in temporal and spectral domains during propagation through and reflecting from various defects in investigated objects, such as voids and foliation. We discuss requirements are applicable to the setup and are necessary to produce an image of these defects, such as signal-to-noise ratio and a method for registration THz radiation. Obtained results indicated the prospects of the THz TDS method for the inspection of defects and determination of the particularities of chemical composition of aircraft parts. Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/25/2015 Terms of Use: http://spiedl.org/terms Proc. of SPIE Vol. 9448 94482D-5 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/25/2015 Terms of Use: http://spiedl.org/terms
Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications IX, 2016
A fiber-coupled Terahertz time domain spectroscopy (THz-TDS) system based on photoconductive antennas, pumped by a 100-fs fiber laser, has been used to characterize materials in transmission and reflection mode. THz images are acquired by mounting the samples under investigation on an x-y stage, which is stepped through the beam while the transmitted or reflected THz waveform is captured. The samples include a carbon fiber epoxy composite and a sandwich-structured composite panel with an aramid fiber honeycomb core in between two skin layers of fiberglass reinforced plastic. The former has an artificially induced void, and from a comparison of recorded reflected time-domain signals, with and without the void, a simple model for the structure of the composite is proposed that describes the time-domain signals reasonably well.
Non-Destructive Testing of Glass-Fibre Reinforced Polymers using Terahertz Spectroscopy
2006
We present THz time-domain spectroscopy measurements on glass-fibre reinforced polymers. The experimental set-up consists of a standard free-space THz spectrometer with photoconductive emitter and detector antennas driven by a femtosecond laser source. Single-point measurements to precisely obtain the THz properties of the sample are performed as well as two-dimensional images by scanning the samples perpendicular to the THz beam. We find that absorption coefficient and refractive index of the fibres significantly differ from that of the polymeric matrix material. Furthermore, the THz waves are strongly attenuated due to scattering off the fibres. The refractive index depends on the fibre concentration and orientation with respect to the linearly polarised THz radiation. With these results we can interpret THz images of a fibre reinforced polymer specimen. The THz images show the textures of the fibre network within the specimens and are compared with x-ray images.
Teraherz frequencies electromagnetic waves - a new tool for investigating composite defects
2011
In this paper the possibility of using a modern NDT technique THz electromagnetic waves for composite materials defects detection and identification was investigated. Two technological defects were investigated voids and internal delaminating. The composite materials were hand lay up made. Several kinds of reinforcing fibers (glass, basalt, jute) and polyester resin were used. Glass micro spheres and mica plates were placed at different depths in order to simulate technological defects. The specimens were tested using terahertz frequencies electromagnetic waves. The goal was to verify if the NDT method is appropriate to identify technological defects, misuse damage as well as determine parameters such as depth or size.
Composites Part B: Engineering, 2015
Glass fiber-reinforced composite laminates in polyetherimide resin have been studied via terahertz imaging and ultrasonic C-scans. The forced delamination is created by inserting Teflon film between various layers inside the samples prior to consolidating the laminates. Using reflective pulsed terahertz imaging, we find high-resolution, low-artifact terahertz C-scan and B-scan images locating and sizing the delamination in three dimensions. Furthermore, terahertz imaging enables us to determine the thicknesses of the delamination and of the layers constituting the laminate. Ultrasonic C-scan images are also successfully obtained; however, in our samples with small thickness-to-wavelength ratio, detailed ultrasonic B-scan images providing quantitative information in depth cannot be obtained by 5 MHz or 10 MHz focused transducers. Comparative analysis between terahertz imaging and ultrasonic C-scans with regard to spatial resolution is carried out demonstrating that terahertz imaging provides higher spatial resolution for imaging, and can be regarded as an alternative or complementary modality to ultrasonic C-scans for this class of glass fiber-reinforced composites.
3D Terahertz Imaging of Hidden Defects in Oxide Fibre Reinforced Ceramic Composites
2012
WHIPOX™ (Wound highly porous oxide composite) is an innovative alloxide fiber-reinforced ceramic matrix composite for high-temperature applications. We are reporting inspection results generated using fully electronic terahertz sources running at 0.1 THz respectively 0.3 THz in a mobile unit. The system operates in reflection as a frequency modulated continuous wave (FMCW) radar. A comparison of the measured results with investigations using established NDT methods like Xray microCT, lock-in thermography and air coupled ultrasound inspection show the great potential of the new method.
Pomiary Automatyka Robotyka
Composites are materials that have replaced traditional construction materials in numerous applications in various fields. Due to the possibility of creating the required material properties, fiber-reinforced composites are most often used. Despite competition from carbon and aramid fibers, the earliest glass fibers produced are used in many applications. One of the areas where glass fiber reinforced composites (GFRP) make a significant contribution to structural applications is aviation. Because both during production and operation, composites are exposed to damage, which often occurs in the internal structure of the composite, works are being carried out to develop the most effective method of non-destructive testing to detect such damage. The article presents a comparison of the results of non-destructive testing of glass fiber-reinforced composite samples. A comparison of the results of the possibility of detecting defects in the form of milled holes of different diameters and d...