Testing of composite materials using advanced NDT methods

COMPEL: The International Journal for Computation and Mathematics in Electrical and Electronic Engineering

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ISSN: 0332-1649

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Subject Area: Electrical & Electronic Engineering
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Testing of composite materials using advanced NDT methods

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DOI (Permanent URL): 10.1108/03321641111133172

Article citation:
Przemyslaw Lopato, Tomasz Chady, Ryszard Sikora, (2011) "Testing of composite materials using advanced NDT methods", COMPEL: The International Journal for Computation and Mathematics in Electrical and Electronic Engineering, Vol. 30 Iss: 4, pp.1260 - 1270

The Authors

Przemysław Łopato, Department of Electrical and Computer Engineering, West Pomeranian University of Technology, Szczecin, Poland

Tomasz Chady, Department of Electrical and Computer Engineering, West Pomeranian University of Technology, Szczecin, Poland

Ryszard Sikora, Department of Electrical and Computer Engineering, West Pomeranian University of Technology, Szczecin, Poland

Acknowledgements

The authors would like to express sincere thanks to Professor Joao M.A. Rebello (Metallurgy and Materials Department, Federal University of Rio de Janeiro, Brasil) for providing the composite protective coating specimens.

Abstract

Purpose – The purpose of this paper is to present capabilities of terahertz imaging technology in case of various composite materials and to propose a new defect detection algorithm.

Design/methodology/approach – This paper first discusses an applicability of the terahertz technique in composite materials inspection. It then describes source of terahertz radiation (photoconductive antenna) and general structure of terahertz time domain imaging system. Next the terahertz imaging results of composite anticorrosion coating, glass- and carbon-fiber-reinforced laminates are presented. Then the signal processing and identification scheme based on time domain A-scan signal equalization and C-scan thresholding is presented. Data processed in this way are parameterized and defect identification database is prepared. The proposed procedure is verified using the exemplary inspection results of glass-fiber laminate delamination. Finally, some comparison of terahertz time domain inspection with low energy digital radiography is presented.

Findings – This paper shows terahertz imaging as a well-suited technique for composite structures inspection. The terahertz imaging results of composite anticorrosion coating, glass- and carbon-fiber-reinforced laminates are presented. An application of proposed signal processing algorithm enables accurate defects detection and effective data collection for identification database purpose.

Originality/value – The paper provides an insight into the potential of terahertz imaging of various composite structures. Proposed signal processing and defects detection scheme is applicable to wide range of composite structures.

Article Type:

Research paper

Keyword(s):

Non-destructive testing; Composite materials; Image processing; Signal processing.

Journal:

COMPEL: The International Journal for Computation and Mathematics in Electrical and Electronic Engineering

Volume:

30

Number:

4

Year:

2011

pp:

1260-1270

Copyright ©

Emerald Group Publishing Limited

ISSN:

0332-1649

1 Introduction

Because of high corrosion resistance, sufficient stiffness and high strength to weight ratio today modern industry make very extensive use of various composite materials. Composites occur in wind turbines, tanks, automotive, maritime and aeronautical constructions. Ultrasonics, eddy current method, microwave technique, optical methods and thermography are commonly used techniques for non-destructive testing (NDT) of composite materials (Summerscales, 1990). Owing to heterogeneous structure of composites (multi-layer nature and fiber waviness), the defects differs from those typically found in metals. Detection and identification of discontinuities is more complicated task in this case. Advanced NDT techniques such as a terahertz spectroscopy or a low energy digital radiography (DR) make possible very precise characterization of defect location because of high spatial resolution. Especially, the terahertz technique is able to evaluate interior of layered composite laminates. In this paper, T-Ray (terahertz radiation) method is described and compared with low energy DR for the purposes of various composite materials evaluation.

2 Terahertz technique in composites inspection

Terahertz electromagnetic radiation enables non-invasive, non-ionizing and non-contact examination of dielectric materials such as: plastics, dry wood, explosives ceramics, foams and composites – especially non-conducting fiber reinforced. The T-Rays are sensitive for refractive index. Any defect, which disturbs refractive index, e.g.:

• void;
• delamination;
• inclusion;
• material inhomogeneities (fiber/matrix distribution);
• surface roughness;
• fiber waviness; and
• internal interfaces between layers (in layered structures),

can be observed. In most cases, defects are detected by reflection and transmission imaging based on pulsed terahertz time domain spectroscopy (TDS) (Mittelman et al., 1999). The method is well suited for evaluation of layered materials. Each interface between separate layers causes reflection of an incident terahertz pulse and attenuation of transmitted one. Differences in delays of the propagated pulses and their echo (delayed layer reflections) enable characterization of the thicknesses and state of inner structure. Very short pulses (order of picoseconds) contain wide frequency bandwidth (0.05-3 THz), and therefore, it is possible to carry one single point broadband measurements.
The main components of a terahertz TDS system are:

• pair of transducers (transmitter and receiver);
• ultra fast laser; and
• optical delay line.

The terahertz pulses are generated and picked up by photoconductive antennas (PCA) (Gregory et al., 2005). A simplified view of such device is shown in Figure 1. The main part of the terahertz transmitter is a bowtie antenna with photoconductive gap, which is illuminated by femtosecond laser pulses. Such excitation and application of external DC bias causes a pulsed current flow through a metallic part of antenna. The currents induce an electromagnetic wave. The resulting bowtie antenna radiation is collimated by a high resistivity hemispherical silicon lens and then focused on the examined object surface by a high-density polyethylene lens. Simplified scheme of the pulsed terahertz TDS system is shown in Figure 2 and a view of equipment utilized during our measurements is shown in Figure 3. The TRay4000 imaging system of Picometrix offers the frequency resolution of 3 GHz and average power of terahertz beam – less than 500 μW. A lateral resolution of TDS technique is less than 200 μm and depth resolution is less than 50 μm (Zimdars et al., 2005).

3 Results of terahertz TDS measurements and data analysis

The terahertz time domain imaging similarly to the ultrasonic technique delivers a wide variety of data collection types: A-scan, B-scan and C-scan. All of them are gathered in Figure 4. The A-scan is a signal acquired during a single point (x, y) measurement – time dependent waveform S(x = const., y = const., t _D) consisting of reflected or transmitted pulses. The amplitude and phase of the pulses as well as time distance between them carry an information about the examined structure. The B-scan S(x, y = const., t _D) consists of set of the A-scans acquired during single line (1D) scan (Figure 4).
During surface inspection, a 3D signal S(x, y, t _D) is acquired. A slice for given t _D value constitutes the C-scan – S(x, y, t _D=const.). Depth z in the examined specimen corresponds to value of delay time t _D, thus one can say that, the C-scan is a depth-specific slice.
The pulsed terahertz TDS is very well suited for non-destructive inspection of non-conducting composites. During our experiments various materials were examined. The first experiment concerned a composite material sample consisting of a paint layer, polymeric layer, ceramic layer and a steel plate in its interior (Figure 5). Such structures are used in the oil and chemical industry as a protective coating against corrosion in metallic materials. The evaluated test specimen contains artificial flaws in the metal/ceramic layer interface. The defects cause additional reflection pulse in the vicinity of steel plate reflection pulse, thus are easily detectable.
The other type of examined materials were fiber reinforced laminates. One of the most important information about fiber-reinforced composite, which can be gained using the presented technique, is a fiber orientation and fiber/matrix distribution. The exemplary C-scan signals obtained for regular and irregular fiber structures are shown in Figure 6. The B-scan signal obtained by reflection inspection of delaminated glass fiber-reinforced composite using terahertz technique is shown in Figure 7. A mechanical damage (impact) was introduced to the examined specimen (Figure 7) before testing procedure. The surface waviness and delamination between interior layers are easily detectable in resulting signal.
Delamination is one of the most serious problems that can occur in the laminates. It can affect a structural integrity of material by reducing mechanical stiffness and compressive strength. That is why effective flaw detection procedure must be utilized. The block scheme of a proposed flaw detection algorithm is shown in Figure 8. For each position (x, y), the measured signal is aligned in delay time domain in order to simplify further signal processing algorithms and reduce an influence of surface roughness. The detailed description of applied time domain equalization procedure is presented in the previous paper of authors (Lopato et al., 2009). Signals are aligned to position of terahertz pulses' minimum. After that several time delay values (t _D1, t _D2 and t _D3) are chosen and the corresponding C-scan signals are thresholded in order to detect high probability of defect detection areas. Next, thresholded C-scans are parameterized. Selected features like:

• defected area;
• shape coefficients; and
• position – depth in examined specimen which corresponds to value of delay time t _Dn are calculated.

On the basis of above-mentioned information, a database of defects' parameters is created. The decisions about membership to specific class of flaws (e.g. delamination, void, inclusion, etc.) are done using comparison with database records. A quality of classification depends on size of database, thus an extensive atlas of possible defects must be prepared.
In Figure 9, acquired terahertz C-scan signals and results of thresholding operation are presented. One can observe, the delamination profiles differ significantly depending on depth of examination, thus pulsed terahertz TDS method may be a very effective tool in non-destructive evaluation of multi-layered composite materials.

4 Comparison of terahertz and DR techniques

Because of the skin effect, in case of terahertz technique, only a surface inspection of conducting fibers reinforced materials is possible. Therefore, for the carbon-fiber-based composite structures an application of terahertz radiation-based NDT is reduced to a surface roughness evaluation. This limitation can be omitted by application of the low energy radiographic inspection. The results obtained using both techniques (pulsed terahertz TDS and DR) are shown in Figures 10 and 11. Results of carbon-fiber composite evaluation using the low energy DR are shown in Figure 10. A fiber/matrix distribution and porosity are clearly visible. As we predicted, in case of pulsed terahertz TDS, only a fiber waviness is detectable and no information about interior part of the specimen is accessible (Figure 11).
The terahertz TDS imaging requires time consuming scanning procedure, thus the low energy DR is much faster method of examination (expense of inspection safety – ionizing radiation).

5 Conclusions

Both described techniques enable accurate evaluation of composite structures. The pulsed terahertz TDS is meant rather for dielectric composites like glass-fiber reinforced ones. This method offers very wide and unique compared to other common methods abilities of inspection: high resolution, no need to use any additional coupling medium, availability of spectroscopic information, estimation of dielectric parameters of examined materials and finally a defect depth information is also provided. The proposed algorithm (Figure 8) is very effective tool for non-destructive evaluation of multilayered composite materials. The low energy DR is more universal (application in case of medium, low and non-conducting materials) and enables fast inspection, but it is less safe and do not provide as much information about internal structure (e.g. depth of defects) as the pulsed terahertz TDS.

Figure 1Terahertz photoconductive transmitter (PCA bowtie antenna)

Figure 2Scheme of pulsed terahertz TDS system

Figure 3View of pulsed terahertz TDS system

Figure 4Data collection types in terahertz time domain method

Figure 5View of composite protective coating and its terahertz inspection results (B-scans) in case of non-defected and defected area

Figure 6Regular versus irregular fiber orientation evaluated using terahertz TDS (material: glass-fiber composite)

Figure 7Results of glass-fiber composite evaluation using terahertz beam (B-scan) and photo of delamination in glass-fiber laminate

Figure 8Block scheme of signal processing and defect detection algorithm

Figure 9Evaluation of laminated glass-fiber composite with delamination type defect. C-scan signal in the vicinity of: (a) front surface (t _D1), (b) middle surface (t _D2), (c) back surface of the plate (t _D3)

Figure 10Results of carbon-fiber composite evaluation using low energy X-ray beam

Figure 11Results of carbon-fiber composite evaluation using pulsed terahertz TDS

References

Gregory, I.S., Baker, C., Tribe, W.R., Bradley, I.V., Evans, M.J., Linfield, E.H., Davies, A.G., Missous, M. (2005), "Optimization of photomixers and antennas for continous-wave terahertz emission", IEEE Journal of Quantum Electronics, Vol. 41 pp.717-28.
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Lopato, P., Chady, T., Goracy, K. (2009), "Image and signal processing algorithms for THz imaging of composite materials", in Thompson, D.O., Chimenti, D.E. (Eds),Review of Quantitative Nondestructive Evaluation, Vol. Vol. 29 pp.766-73.
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Mittelman, D.M., Gupta, M., Neelamani, R., Baraniuk, R.G., Rudd, J.V., Koch, M. (1999), "Recent advances in terahertz imaging", Applied Physics, Lasers and Optics, Vol. B68 pp.1085-94.
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Summerscales, J. (1990), Non-destructive Testing of Fibre-reinforced Plastic Composites, Elsevier Applied Science, New York, NY, .
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About the authors

Przemysław Łopato received his MSc degree in telecommunication from Szczecin University of Technology (Poland) in 2004. He received the PhD degree in Electrical Engineering from the West Pomeranian University of Technology (Poland) in 2008. His research interests involve terahertz tomography, numerical methods for high frequency electromagnetic field simulations and artificial neural networks applications. He works as an Assistant Professor in the Department of Electrical and Computer Engineering in West Pomeranian University of Technology. He is participating in several national and international research projects.
Tomasz Chady is a Professor at West Pomeranian University of Technology (previously Technical University of Szczecin). He received a PhD in Electrical Engineering from Technical University of Szczecin, where he has been working since 1987. From 1997 to 1999, he was employed at Oita University, Japan. In 1999, he joined Oita Industrial Research, Japan under the STA fellowship. At the end of his work in Japan, Professor Tomasz Chady received Best Technical Contribution Award of the Japan Society of Applied Electromagnetics and Mechanics. Since 2001, he has been with the Department of Theoretical Electrotechnics and Computer Engineering. His research focuses on the electromagnetic non-destructive testing, digital signal processing and neural computing. He is coordinating several national and international research projects.
Ryszard Sikora received an MSc from Silesian Technical University of Gliwice (in 1956) and PhD from the Technical University of Gdansk (in 1967). He was an Organizer and long time Chairman of Department of Theoretical Electrotechnics and Computer Engineering in West Pomeranian University of Technology (former Technical University of Szczecin). He is currently a Full Professor of the Electrical Department (WPUT). He is a member of World Federation of Non-destructive Evaluation Center (Iowa State University, USA), He has published over 300 technical journals and conference papers and chaired various international conferences. In 1999, Professor Ryszard Sikora was awarded the Volta Silver Medal (University of Pavia, Italy) and in 2007 JAEM Award (Journal of the Japan Society of Applied Electromagnetics and Mechanics). His current research focuses on the electromagnetic non-destructive testing, DR and intelligent systems. He is coordinating several national and international research projects.