POTENTIALS OF TERAHERTZ TECHNOLOGY FOR THE INSPECTION OF PLASTIC PIPES

Transcription

1 POTENTIALS OF TERAHERTZ TECHNOLOGY FOR THE INSPECTION OF PLASTIC PIPES Stefan Kremling SKZ German Plastics Center Würzburg, Germany Thomas Hochrein SKZ German Plastics Center Würzburg, Germany Peter Heidemeyer SKZ German Plastics Center Würzburg, Germany SHORT SUMMARY In plastics industry, the use of terahertz technology opens up opportunities for novel non-destructive and contactless testing applications. For example, the thickness of individual layers of a foamed multilayered plastic pipe can be monitored contactless and continuously during the extrusion process. Furthermore, internal defects and foreign material inclusions can also be detected. Some materials and additives exhibit characteristic fingerprints in the terahertz spectral range allowing identification. KEYWORDS Terahertz, Non-destructive Testing, Inspection ABSTRACT Electromagnetic radiation with terahertz (THz) frequencies exhibit unique properties for the inspection of dielectric materials such as paper, ceramics or plastics. The majority of thermoplastic materials are highly transparent and only slightly absorbing within this spectral range. Thus, THz radiation can pass through and is well suited for nondestructive testing (NDT) applications to detect internal defects as well as for layer thickness determination. Further, the electromagnetic character allows an examination of foamed materials and does not require a coupling medium. THz technology is a novel promising technique which can make a considerably contribution in the field of NDT of plastics. THz radiation can be generated in different ways: Either applying optical technologies utilizing one or more lasers or high frequency electronic microwave systems. THz time domain spectroscopy (TDS) systems emitting short THz pulses in the picosecond (ps) range by exciting a semiconductor switch with ultra-short laser pulses. Time of flight measurements of this short THz pulses allows very precise thickness measurements of single layers down to a few ten microns if there is a noticeable difference in refractive index between adjacent layers. This can be used even for inline monitoring in plastic pipe extrusion for an efficient production process with minimal material consumption. These systems can also enable broadband spectral information about the investigated material allowing the determination of e. g. filler content or the 1 Copyright 2016 by SKZ German Plastics Center (s.kremling@skz.de)

2 mixing ratio of a polymer blend. On the other side the operation principle of most allelectronic THz systems is based on frequency modulated continuous wave (FMCW) radars. Such systems emit chirped electromagnetic radiation in the lower THz range with reduced bandwidth but high output power density. The all-electronic technique allows very fast measurement rates and compact devices. These systems can also be used for inline thickness control of thick pipes, but they are also suited for imaging applications to detect internal defects. NOMENCLATURE THz Terahertz NDT Non-Destructive Testing TDS Time-Domain Spectroscopy FMCW Frequency-Modulated Continuous Wave ps/fs Pico-/Femtosecond PP Polypropylene PE Polyethylene PA Polyamide PMP Polymethylpentene PVC Polyvinylchloride INTRODUCTION Commonly, the THz frequency range is specified between 100 GHz and 10 THz, corresponding to wavelengths of 3 mm to 30 µm, spectrally located between microwaves and infrared radiation (Figure 1). Unfortunately, there was a lack of powerful, feasible and coherent sources and detectors for a long time. But the ongoing technological progress of the past decades enabled appropriate devices [1]. Today, various types of THz systems are established on the market and the technology is currently acquiring more and more industrial applications [2-4]. On the one side, allelectronic THz systems based on microwave technology enter the THz range from low frequencies with high output power. On the other side optical THz systems using lasers in combination with semiconductor antenna modules penetrate from the high frequency range [5]. Biggest advantage of optical systems is the generation of frequencies in a wide range between 30 GHz up to 40 THz [6]. Figure 1: Electromagnetic spectrum between radio frequencies and X-rays 2 Copyright 2016 by SKZ German Plastics Center (s.kremling@skz.de)

3 THz radiation is extremely versatile and can be used in many different fields. Conceivable applications are in the security sector with personal scanners and drug or explosive identification [7], in the medical sector with cancer or caries detection [8] or in data transmission with high rate wireless communication [9], to name just a few. Another promising field is material characterization and non-destructive testing. Most dielectric materials and therefore many types of plastics are transparent and can be well penetrated by THz waves [3]. Mainly plastics with non-polar bindings such as PE or PP are characterized by low absorption losses and thus good transparency [10-13]. In contrast, PA absorbs THz radiation much more strongly with increasing frequency which can be obstructive for the investigation of thicker samples [14]. As a consequence of the high transmissivity of most polymers, THz is a powerful tool for the NDT of plastic parts [15, 16]. Unfortunately, conductive materials like metals or carbon fibers cannot be penetrated and reflects all electromagnetic radiation. Water molecules have a strong absorbing effect on THz waves due to their dipole structure with rotational vibrational transitions in the THz range [17]. Especially at frequencies above one THz, the absorption losses due to humidity in air occurs more apparent. As a consequence, measurements in the laboratory are typically performed under dry air or nitrogen atmosphere. THz technology has some decisive advantages compared to different other NDT methods. In contrast to X-rays there is no ionizing effect on organic matter due to the low photon energy of only several mev implying that no special safety precautions must be considered when working with THz systems. Compared to ultrasonic testing, THz radiation exhibits electromagnetic nature and can be well transmitted through air without the need of a special medium. Furthermore, also air-filled samples like plastic foams or corrugated pipes can be investigated without appreciable damping losses. EXPERIMENTAL In general, two different operation principles for the generation and detection of THz radiation have been established: Either using high frequency electronic techniques entering from low frequencies into the THz range or using optical technologies entering from the opposite side. Both types have specific advantages and disadvantages as explained below. Optical systems are usually distinguished between time domain and frequency domain operation. Here we only focus on time domain systems (TDS). THz-TDS systems use a pulsed fs laser to stimulate photoconductive switches for the generation and detection of broadband THz pulses. The photoconductive antennas are often based on low-temperature grown InGaAs/InAlAs semiconductor structures equipped with a fiber connection for easy handling. In modern systems, all laser beam paths are completely encapsulated in polarization-maintaining optical fibers. In the system used here, a mechanical delay unit is used for an optical sampling of the THz waveform, which has a free laser beam path inside but is encapsulated itself. The mechanical delay line samples the THz pulse in a time range up to 285 ps with a rate of 2 Hz. Today, novel techniques (ECOPS, ASOPS, OSCAT) have developed allowing even a sampling without external moving parts [18]. The fiber coupling allows a high flexibility in the THz beam path. In particular, the antenna module along the optical axis can be rotated very 3 Copyright 2016 by SKZ German Plastics Center (s.kremling@skz.de)

4 easily, which is useful for polarization dependent measurements. Figure 2 shows a sketch of a THz-TDS system in reflection arrangement. Figure 2: Measurement setup of the THz-TDS system in reflection arrangement Almost all THz-TDS systems only have a single pixel transmitter and receiver module. Performing spatially-resolved measurements, a scan unit either for moving the transmitter/receiver or the sample is needed. In order to get a perpendicular incident of the radiation on the sample surface for measurements in 0 reflection arrangement, a 3.5 mm thick silicon wafer was used as THz beam splitter. The emitted THz beam is collimated by a plano-convex lens made out of PMP and focused by a second lens on the sample. The beam width in the focal plane depends on the frequency and measures 0.9 mm between 300 to 400 GHz. The reflected radiation is collimated by a lens, directed via the beam splitter and focused on the detector antenna by another lens. To avoid absorption losses due to humidity, the whole test stand is enclosed and flushed with dry air (dew point < -50 C). All-electronic THz-systems, on the other side, are based on high frequency microwave techniques. The system used here is a FMCW radar system consisting of split block components, as depicted in Figure 3. Figure 3: Schematic of a monostatic w-band FMCW THz system 4 Copyright 2016 by SKZ German Plastics Center (s.kremling@skz.de)

5 The baseband signal (Ku-band between 12,5 GHz and 18,3 GHz) is generated in a voltage controlled oscillator (VCO) were a time-varying voltage sweep is converted into a time-varying frequency sweep, also called chirp. After amplifying and filtering, the chirp is frequency converted into the w-band between 75 GHz and 110 GHz using different multiplier steps. Realizing a monostatic system, the HF signal is transferred in a 10 db broadband coupler. Using a horn antenna allows an efficient and directed out coupling of the radiation into free space. After hitting a sample a part of the radiation is transmitted and a part is reflected back. The reflected part is mixed with an internal reference signal resulting in specific beat frequencies for each distance between transceiver and sample interfaces. Today, ultra-wideband on chip FMCW systems based on high frequency SiGe technology are available promising lower system costs and high packing density up to several 100 GHz [19]. RESULTS THz technology is extremely versatile and can be used for different measurement task for the inspection of plastic parts. First application presented here are thickness measurements, for example, on plastic pipes. This measurement task can be done either using THz-TDS-Systems or FMCW-Systems. In principle, a time (or frequency) difference between reflections from layer interfaces is measured which can be used to determine the thickness between these interfaces if knowing the dielectric material properties. Figure 4 a) shows a sketch for illustration and the measured waveform for TDS (b) and FMCW (c) systems, respectively. Each technology has specific advantages and disadvantages doing this task. Figure 4: a) measurement principle for thickness determination using THz technology. B) TDS waveform signal; c) FMCW waveform signal In THz-TDS systems, a short ps-thz pulse is emitted and focused on the sample surface. Considering Fresnel equations, a part of the incident radiation is transmitted (T) into the sample and a part is reflected (R) back, depending on the refractive index n 1 of the material. The Fresnel equations for a perpendicular incident are given by: ; 1; The transmitted part propagates through the sample and on the next interface, again, a part is transmitted and another part is reflected back. This repeats on all interfaces along the propagation direction. Note, depending on the dielectric material 5 Copyright 2016 by SKZ German Plastics Center (s.kremling@skz.de)

6 properties there can be absorption losses during material propagation which are often frequency dependent and increasing with frequency. Due to the short pulse duration of only a few ps and an optical sampling in fs range, THz-TDS technology allows clear pulse separation in this time scale and thus very precise measurements. The thickness can be determined by the time difference t between two reflections in consideration of the refractive index n: Δ 0 2 c 0 is the speed of light in vacuum. This allows measurement down to 100 µm for a single plastic layer without special evaluation algorithm. Using advanced data analyzation techniques allows the determination down to a few µm [20]. Figure 5 shows the results from TDS measurements on a single layer PE pipe (a) and a three layer PVC pipe with a foamed core (b). Figure 5: Thickness measurement of a single layer PE pipe (a) and a three layer PVC pipe with foamed core (b) The measured time differences between the reflection pulses allow the determination of the wall thickness if the refractive index is known. The results from THz measurements and reference measurements are listed in Table 1. Table 1: Calculated thickness of the measurements shown in Figure 5 and reference measurements. refractive index determined thickness with THz-TDS system reference measurement PE Pipe (a) mm 4.0 mm PVC Pipe (b) 1.7 (outer layer) 1.3 (core) 1.7 (inner layer) 0.62 mm 2.35 mm 0.69 mm 0.6 mm 2.4 mm 0.7 mm On the other side, FMCW radar systems using a frequency chirp to measure distances. Again, each interface with a difference in refractive index leads to a reflected and transmitted part of the incident radiation regarding Fresnel equations. The reflected part is frequency shifted depending on the distance of the reflection and mixed with a 6 Copyright 2016 by SKZ German Plastics Center (s.kremling@skz.de)

7 reference signal resulting in a specific beat frequency. Due to the finite signal length, the resulting pulse width in time domain after Fourier transformation is broadened. The minimal distance between two pulses is then limited by the bandwidth B of the frequency chirp: 2. For example, the minimal distance for a clear pulse separation of a FMCW system working in the W-band (75 GHz 110 GHz, B = 35 GHz) is limited to about 6 mm in plastic with a refractive index of n = 1.5. But these systems often have higher output power then TDS systems, do not need complex fs lasers, provide high measurement rates and are very compact. Figure 6 shows thickness measurement on plastic pipes made out of different materials and with varying wall thickness. In Figure 6 a) shows a PVC pipe with a nominal thickness of 14 mm. The depth scale in the graph is already recalibrated with the refractive index of n = 1.7, so one can directly measure the wall thickness between the maxima of the two reflection pulses which results in a thickness of 14 mm. In Figure 6 b) three PE pipes with different wall thicknesses are depicted. Again, the depth scale is recalibrated by the refractive index n = The measured thicknesses of 6 mm, 23 mm and 49 mm correspond all to the nominal thicknesses measured with a caliper. Figure 6: Thickness measurement of plastic pipes using FMCW systems: a) PVC pipe with 14 mm wall thickness and b) different PE pipes with wall thickness of 6 mm, 23 mm and 49 mm. The transparency pf plastics for THz radiation allow another promising application field, the detection of internal structures and defects. Again, this task can be done either by using TDS or FMCW systems. Similar to the thickness measurement, the test object is irradiated and the reflected signal is monitored. If there are no internal defects or differences, only one pulse from the front and another one from the back surface can be observed. If there is a defect, additional pulses emergence between them; if there are changes in internal structure, the reflected signal waveform changes. For this measurement task an imaging technique is often implemented to visualize the position of the defects or changes of the internal structure in a 2D scan. As mentioned above most systems only have single pixel emitter and receiver. Thus a pixel wise 2D scan has to be implemented. Today there are also THz cameras available but 7 Copyright 2016 by SKZ German Plastics Center (s.kremling@skz.de)

8 the only measure the amplitude of a signal without phase information [21]. This significant limit the application options for such systems. In our TDS system the sample is mounted on two linear stages for movement while the antenna modules are fixed. In Figure 7 a) a photograph (top) and a THz time delay image of a pressed PP plate with locally varying CaCO 3 filler content is presented. The plate has a filler content of 16 wt.-% and the internal letters of 26 wt.-%. The varying filler content locally changes the refractive index which can be detected by phase sensitive measurements. This can be also transferred to other matrix materials as well as other additives. Furthermore, if the correlation between changes in refractive index with varying content is known a quantitative identification is also possible. Another example is shown in Figure 7 b). Two injection molded PA parts filled with short glass fibers are shown as a photograph (top) and THz images (bottom). One of the parts has an additional mold flow obstacle inside (right) and the injection direction was from top to bottom. Here we have measured the orientation of the glass fibers which is encoded in the color as well as vector arrows. Without the obstacle there is nearly homogeneous orientation along the injection direction, only close to the back surface the orientation changes due to damming of the melt front. On the other side, the same effect can be observed in front of the flow obstacle. The measurement of the orientation is based on birefringence due to the elongation of the fibers and done with polarization sensitive detection in multiple directions. Special evaluation algorithms allow the calculation of the fiber direction [22]. Figure 7: a) Photograph (top) and THz-image (bottom) of a pressed PP plate with locally varying filler content of CaCO 3. b) Photographs (top) and THz-images (bottom) of an injection molded PA plate with short glass fibers with (right) and without (left) a mold flow obstacle. Imaging with FMCW-systems is performed by the opposite way where the sample is fixed and the measurement unit moved by a 2D raster scan unit. Here we present two glass fiber reinforced parts with internal defects, shown in Figure 8. 8 Copyright 2016 by SKZ German Plastics Center (s.kremling@skz.de)

9 Figure 8: Imaging of two glass fiber reinforced plastic parts with internal defects and different reinforcements with different orientations. Two main insights can be observed from the FMCW-Images: First, the position of the defects can be clearly determined. Second, the orientation of the fiber reinforcement can also be identified. Here the defects were metallic parts and therefore exhibit a strong reflectivity (red color). But also other material inclusions can be found if there is a noticeable difference in refractive index. The FMCW images here are a projection of the maximum reflectivity along the full depth profile in each point. It is also possible to pick out and visualize single depth layers. DISCUSSION THz-TDS systems as well as THz-FMCW systems feature high potentials for the contactless and non-destructive testing of plastic parts. Both have specific advantages and disadvantages, depending on the measurement task. For example, precise thickness measurements are only possible using TDS technology due to the much higher bandwidth and narrow pulses. On the other side for measuring thick samples, FMCW systems are preferred due to higher output power and a larger measurement range along propagation direction. It is also possible using THz systems for an inline wall thickness monitoring during the extrusion of plastic pipes. For this task, first commercial systems are available on the market [23]. For imaging application more aspects has to be considered for the selection of the most suitable system. The lateral resolution in imaging applications is determined by the wavelength and the numerical aperture of the used optics. FMCW systems usually work in the low frequency range corresponding to long wavelength resulting in a lateral resolution of only a few mm. TDS systems have a broad spectrum with frequencies up to several THz, corresponding to wavelength of only a few µm. Wavelength sensitive analyzation allows to display images were only high frequencies are considered for high resolution. But be careful about absorption losses which usually increase with higher frequencies. All measurements presented here were done in reflection arrangement. In contrast to transmission arrangement, reflection has much more potential in NDT since only a one-sided access to the sample is needed. But on the other side, compared to 9 Copyright 2016 by SKZ German Plastics Center (s.kremling@skz.de)

10 transmission a lower signal to noise ratio (SNR) as well as a strong dependence of the measured signal on the incident angle and surface inhomogeneity occurs. This can be a major challenge for non-planar samples. The systems shown here are all stationary, meaning the test object has to come to the system. Especially FMCW systems have great potential for mobile devices and applications because no complex fs laser systems are needed. First systems are already on the markets which have been used for the inspection of pipe fittings of plastic casing pipes in the field [24]. In the coming years we expect further mobile devices for flexible use out of the labs. CONCLUSIONS THz technology is a fairly new technology with barely ten years of industrial research. Nevertheless, even today there are a lot of potential application fields especially in plastic industry for which this technology is well suited. One of the most promising application is thickness measurements in various industries. The advantages compared to established technologies are obvious, for example foamed or air filled samples can be measured contactless without ionizing effect of the radiation. In this field new commercial available systems for the thickness control in plastic pipe extrusion will be on the markets soon. New ultra-wideband on chip FMCW systems based on high frequency SiGe technology promising low system costs and high packing density. This is crucial for multiple pixel systems which provide the full amplitude and phase information. In the near future first multiple input multiple output (MIMO) systems will be also available on the market for compact and fast imaging systems. ACKNOWLEDGMENTS We gratefully acknowledge the Research Association "Fördergemeinschaft für das Süddeutsche Kunststoff-Zentrum e. V." via the AiF (Industrial Research Alliance) within the framework of the program for promotion of joint industrial research and development (IGF) for supporting the research projects 16546N and 17277N of the German Federal Ministry of Economics and Technology (BMWi) in accordance with a resolution of the German Parliament. REFERENCES [1] K.-E. Peiponen, J. A. Zeitler, M. Kuwata-Gonokami, Springer Series in Optical Science 171, Berlin Heidelberg (2013) [2] J. Hauck, D. Stich, P. Heidemeyer, M. Bastian, T. Hochrein, PPS 29 Proceedings, Nuremberg/ Germany (2013) [3] N. Krumbholz, T. Hochrein, N. Vieweg, T. Hasek, K. Kretschmer, M. Bastian, M. Mikulics, M. Koch, Polymer Testing, 28, 1 (2009) [4] O. Peters, M. Schwerdtfeger, S. Wietzke, S. Sostmann, R. Scheunemann, R. Wilk, R. Holzwarth, M. Koch, B. M. Fischer, Polymer Testing, 32, 5 (2013) [5] C. Fattinger, D. Grischkowsky, Appl. Phys. Lett., 54, 490 (1989) 10 Copyright 2016 by SKZ German Plastics Center (s.kremling@skz.de)

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