DEVELOPMENT OF ENHANCED GUIDED WAVE SCREENING USING BROADBAND MAGNETOSTRICTIVE TRANSDUCER AND NON-LINEAR SIGNAL PROCESSING

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1 The Open Access NDT Database DEVELOPMENT OF ENHANCED GUIDED WAVE SCREENING USING BROADBAND MAGNETOSTRICTIVE TRANSDUCER AND NON-LINEAR SIGNAL PROCESSING Sergey Vinogradov IHI Southwest Technologies, Inc Culebra Rd, San Antonio, Texas, Tel: (210) , Guided wave screening is quite a new approach and has been rapidly spreading in the industry due to a high effectiveness in finding hidden metal loss. However, a great number of industrial applications of guided waves still require using advanced procedures due to the need for better detection and characterization of anomalies. The applications are mostly associated with situations when the indications from critical defects are buried in the background noise of different origin. The latest efforts were dedicated to the development of techniques for screening components based on utilization of guided waves resonance together with nonlinear signal processing of acquired signals. A recently developed broadband magnetostrictive transducer was used to transmit and receive torsional mode of guided waves. A number of applications of the technology will be presented, including screening of service water piping, screening of dissimilar metal welds for cracking, and screening of tubing incorporated to the complex supporting grids structure. INTRODUCTION Two types of the data presentation linear and non-linear are quite common in medical ultrasound, advanced conventional NDE and can also be utilized when using guided waves (1,2,3). The linear presentation focuses on the energy scatter of guided waves and can provide a rather concise data interpretation methodology. At the same time, a non-linear presentation requires more sophisticated system tuning and interpretation, but can utilize non-linear responses produced by anomalies. There are a number of applications when this particular capability is an indispensable prerequisite of successful NDE. This continues to justify additional research and development in the direction of non-linear guided waves. Scattering of guided waves (GW) on defects is rather complicated phenomena. The rate of scattering is pretty much dependent on the type of the anomaly, operating frequency and the type of guided waves. It has been generally accepted that using torsional modes (T 0,1) of guided waves can provide the maximum benefits due to the low dispersion rate. Manipulation with the frequency regimes is stipulated by essentially more complex logistics depending on the condition of the component and the goal of screening. The most common approach used for guided wave screening includes data acquisition utilizing multiple frequencies with one pre-selected center frequency at a time. Commercially available GW pulse generators can support rather wide frequency ranges (1 250 khz or khz as an example). However, the actual frequency range of a GW signal can be significantly altered by the transfer function of the GW transducer. Obtaining distinctive non-linear responses requires using broadband guided wave transducers capable of generating/receiving the vibrations in possibly a wider frequency range. Development of such transducers, as well as the development of the non-linear approach, proved to be an engineering challenge and will be summarized below.

2 NON-LINEAR EFFECTS ON GUIDED WAVES GW propagation is mostly constituted by non-linear processes of a different nature. As an example, the guided wave energy distribution through the thickness of the component is a non-linear function and can also vary with the frequency of the GW. Another example is the dispersion rate of GWs that can vary from a low to high depending from the frequency-thickness ratio or the mode of GW. Analysis of non-linear responses is mostly associated with the scattering of GWs on defects. All three dimensions of a reflector the length, the depth, and the width contribute to the amplitude and to the shape of the response at different rates. Another strong contributor to the non-linear scattering is the boundary conditions that may result in the effect of trapped acoustical resonance. One of the most pronounced non-linear scattering effects is associated with both the axial extent and the depth of the flaw. Unlike bulk ultrasonic wave, the amplitude of the GW response is determined by two signals produced by the front and the rear side of the discontinuity. As long as the axial extent and the depth of the flaw allow these two signals to interfere, a clear destructive or constructive resonance in the response can be observed. The effect is strongly dependent on the ratio the wavelength over the flaw length and was accurately reproduced on piping and tubing mockups in the past (4,5,6). Figure 1 shows the simulation of GW signal amplitude scattered on a defect with the length measured in the fractions of the wavelength. Figure 1. Non-linear scattering of GW: (Left) - simulation of guided wave signal amplitude scattered on the defect with the length measured in the units of the wavelength; (Right) - simulation of guided wave scattering on small notches. The normal amplitude of the reflection from the front side of the discontinuity is assumed to be 1 volt. The curve deviates up and down from the 1volt level and has the lowest value when the rear side response saturates the front side response as a result of destructive resonance (points A and D). The curve reaches the highest value (point C) when the rear side response forms a constructive resonance with the front side response. When the axial extent of the anomaly is significantly smaller compared to the wavelength (the case of notches and circumferential cracks), the saturation rate can be very high, and reach db (point A). On the other hand, significant gain in the amplitude can be expected when a near through-wall opening turns into the through wall opening. This effect occurs due to the interruption in the propagation of the reflection from the rear side of the opening. The amplitude recovery, in this case, should be equivalent to the voltage drop between points A and B. When the axial extent of the anomaly is larger than the width of the pulse, the response from the front edge is not saturated by the rare edge signal (case E). An example of utilizing a non-linear approach in the application for detection of small ( mm axial extent) notches/cracks is shown on Figure 1 (right). The behavior of responses was simulated assuming the use of 0.06, 0.25 and 1 MHz frequencies of torsional mode GW. For the given axial extent of the anomaly, using essentially higher frequencies could diminish the effect of saturation and even turn it into the constructive

3 resonance. As an example, the 0.4 mm openings should change the amplitude from 0.1 to 0.4 volts when switching the frequency from 0.06 MHz to 0.25 MHz with further amplitude increase to 1.5 volts at 1 MHz (the area of partial constructive resonance). The amplitude evolution is shown with points A,B and C. According to the simulation, cracks with the axial extent less than 0.01 mm can be better detected when they become a through-wall flaw. The amplitude increment should be close to 12 db and is shown between points D and E. The described above simulation was based on the modeling of interference of two responses and assumed the magnitude of GW vibrations at different frequencies to be equal. In practical NDE applications, utilization of the approach would require the GW responses at different frequencies to be analytically normalized. The described effect of non-linear resonance on notches can be diminished if the GW can reach the rear edge of the anomaly from the sides. This might be the case if the circumferential extent of the anomaly is a small fraction of the wavelength. BROADBAND MAGNETOSTRICTIVE ACTUATORS As it was shown above, utilizing non-linear approach requires using wider frequency ranges of GWs. Using a single narrow frequency range at a time will introduce a significant time delay in the data acquisition. This is the reason why using a broadband actuators has been considered the most practical option. The ideal broadband actuator should be capable of producing all frequencies with equal magnitude of harmonics. It is well known that the pulse having the configuration of the Delta function might be the best choice to introduce a broadband pulse. A single cycle square pulse is the closest approximation to the Delta function and is currently an available option on the current versions of MsS function generators. However, before the GW signal gets recorded, it takes some distortion determined by the transfer function of both the transmitter and the receiver, as shown on Figure 2. Figure 2. Configuration of the guided wave transduction system. The transfer function of the GW transducer is mostly dependent on the transducer design. Two predominant types of magnetostrictive transducers have been used to generate torsional guided waves. In 2003, Southwest Research Institute patented a guided wave actuator (MsS) for inspection of pipes and plates utilizing a ferromagnetic strip and the AC winding covering the strip through the long dimension (7,8). The approach was further developed by IHI Southwest Technologies Inc and was based on the AC winding wrapped through the short dimension of the strip (9,10). Figure 3 illustrates the major difference in the conceptual design of the MsS sensor (left) and the MsT transducer (right). Figure 3. Conceptual designs of the MsS sensor (left) and the MsT transducer (right).

4 Changes in the design of magnetostrictive actuator allowed targeting two major goals - increasing the power output of the transducer and also optimizing the transfer function of the transducer. The transfer function was altered on both the transmitter and the receiving side. Figure 4 (left) shows the results of impedance measurements in the AC circuits of MsS and MsT transducers. Both transducers were designed to fit a 12 (300 mm) OD pipe. Measurements were made in the frequency range of khz. The diagram represents the values of the phase angle ϕ mostly related to the reactance of the circuits. A clear resonance peak at 28 khz can be noticed on MsT transducer and do not exist on the MsS curve. The peak exists due to the fact that the AC winding of MsT transducer together with some capacitors in the circuit forms a typical series RLC resonator with the power load. The resonance occurs at a frequency ω0 = 1/ LC and establishes a band-pass filter for the resonant frequency. The presence of the resonance effect in the transfer function of the transmitter can be utilized for obtaining a short duration powerful pulse needed for the generation of broadband ultrasonic vibrations. The transfer function of the receiver determines the results of GW convolution on the receiver. The qualitative simulation of the convolution was done taking into account a known shape of the test pulse and the configuration of the receiving coil. The MsS receiver design typically incorporates two ribbon cable wraps with turns of wire in each wrap. The width of each ribbon is typically selected equal to the quarter of GW wavelength, and each sensor setup is configured to be a band-pass filter for pre-selected center frequency. Reading higher frequency vibrations with the MsS receiver would be a challenge until the receiver geometry is reconfigured to the designated frequency range. Figure 4 (right) shows the results of the simulation of the 1 cycle square pulse convolution on the MsS receiver having 6 turns of wires in the ribbon with the total width of the ribbon in the order of the wavelength. Figure 4. Performance of magnetostrictive transducers: (Left) - transfer function effects on MsS and MsT transmitters; (Right) a simulation of a test pulse convolution on the receivers of MsS and MsT receivers. It can be noticed that the MsS ribbon acts primarily as a low-pass filter and turned the square shaped pulse into a triangle pulse. This kind of evolution can also significantly undermine the magnitude An of the higher order 1 1 harmonics given as A n = A 1 for the square pulse and calculated as A n = 2 A 1 for a triangle pulse. n n Unlike from the MsS receiver, the MsT circuit is formed from multiple individual single wire receivers connected in the sequence. Since the thickness of each individual wire is very small ( mm), it can accurately sample signals with frequencies up to 5 mhz with minimal distortion. The length of the receiver is determined by the width of the ferromagnetic strip and will elongate the test pulse as shown on the same figure. The gap between the single wires can also be adjusted to support or reject certain frequencies in the lower frequency range ( khz) An actual transfer function of the MsT transducer was assessed on the mockup. The testing arrangement included the MsS 2020 electronic equipment, MsT transducer dry coupled to the mockup tube and the acquisition software with the standard DSP options. Figure 5 (right) illustrates a response obtained from axial notch machined in 3/8 tubing after applying a 64 khz single cycle square pulse to the MsT transmitter. The same transmitter was used as a receiver. The short time Fourier transform performed over the response illustrated that the response is a synthetic product of a superposition of multiple waveforms with discrete frequencies covering the range khz. According to classic theory, the Fourier transformation of such a

5 pulse should produce the first harmonic at central frequency (64 khz) together with multiple odd harmonics (the theoretical pattern is shown on the left side of Figure 5). The actual frequency of harmonics were found to be 64, 192, 320, 448, 576, 704, 832, 960, 1088 and 1216 khz. This frequencies distribution very accurately correlates with theoretical data. The magnitude of harmonics did not follow the theoretical pattern due to the use of a band-pass filter centered at 700 khz. The higher frequency harmonics (above 1216 khz) might be presented in the mockup tube, but were not sampled correctly due to the limitations associated with the Nyquist sampling criterion. Figure 5. The structure of a broadband guided wave signal: (Left) theoretical pattern of frequency content of the response produced by the square pulse; (Right) actual frequency spectrum of the response obtained from axial notch machined in 3/8 tubing. The overall testing results of the MsT transducer revealed its ability to generate/read the torsional GW vibrations in the frequency range khz during single data acquisition session. The synthetic nature (composed from multiple discrete frequencies) of GW signal opens a rather wide window for the utilization of non-linear effects associated with guided waves. APPLICATIONS There is a number of applications when using linear guided wave either do not provide the reliable detection of anomalies or introduce significant time delays in the acquisition due to the need to use multiple frequency settings. Figure 6 shows the predominant applications such as screening of tubing incorporated to the complex supporting grids structure (A,B), screening of service water piping (C), screening of stainless steel and centrifugally cast stainless steel piping for cracking (D).

6 Figure 6. Applications of non-linear guided waves: A - screening of heat exchanger tubing, B - screening of fuel rods, C screening of service water piping; D - screening of stainless steel and centrifugally cast stainless steel piping for cracking. The challenges associated with each application are listed in the Table 1. Table 1 Application Challenges Screening of heat exchanger tubing Screening of fuel rods Screening of service water piping Screening of centrifugally cast stainless steel piping The areas under tubes support plates might be saturated with the high amplitude stress-related responses. Supporting grids produce significant reverberation noise disguising potential damage indications. Through-wall defects need to be sorted out from near through-wall ones. Intensive scale and deposits produce high reverberation noise disguising potential damage indications. Intensive back-scattering noise produced by uneven surfaces together with large grains structure saturate the potential cracking indications. As it can be noticed, all the listed applications are associated with the situations when indications from anomalies are buried in the background noise of different origin. Another important application of non-linear guided waves might be associated with any kind of GW-related R&D work. As it was shown above, the MsT transducers are capable of introducing a pronouncedly broadband GW pulse in the component. This can help to quickly identify the shortest path to the solution. Detection of de-laminations in multi-layered composite or metal structures, detection of stress-corrosion cracking are just a few applications that require extensive R&D efforts. Enhanced screening of heat exchanger tubing Guided waves screening of heat exchanger tubing has been rapidly developed last decade and exhibited robust potential to become a stand-alone technique in the near future (11-19). The predominant damage mechanism in the area under tube support plates is wear and tear. This wear typically cause wall thinning having rather pronounced edges. The known axial extent of the wall thinning (the thickness of TSP) can be utilized for nonlinear analysis using the range of frequencies with the wavelength close to the axial extent of the anomaly. Figure 7 show the responses obtained form a 7 m long, ¾ thick CS tube with four circumferential grooves all simulating 20% wall thinning. The grooves have different axial extent. As it can be noticed, up to 18 db changes in the amplitude of the response was accomplished (transition between response A to response D) by adjusting the ratio of the wavelength over the flaw length. In this particular case, the flaw length was a variable parameter. In practical NDE, the process is based on identification of resonant frequency for each particular discontinuity. As an example, the second odd harmonica (90 khz) of 30 khz T-mode excitation should produce a resonance response when passing the wear type of damage under TSP with the axial extent 3/8 (9 mm).

7 Figure 7. The effect of non-linear scattering of GW on circumferential grooves having the same depth, but different axial extent. Screening of fuel rods Screening of fuel rods using guided waves introduced a high complexity due to the presence of multiple grids structure in the fuel bundle. The grids apply some pressure to the tubing causing a dumping effect. They also produce a pronounced reverberation noise due to the GW energy coupling to the walls of the grids. The most challenging task regarding the fuel rod NDE was identification of through-wall discontinuities under the grids. Both a linear and a non-linear GW approaches were applied to the mockup tubing with the number of circumferential EDM notches. The results are presented on Figure 8. Figure 8. A linear (gray color) and non-linear presentations (black color) of GW data obtained from the fuel rod mockup tubing with the number of circumferential EDM notches. The mockup had two grids (marked Grid 1 and Grid 2) placed in the areas free of any defects. When applying a linear presentation (gray color), both grids and a through-wall notch produced rather pronounced reflections, along with an intensive reverberation noise. The amplitude of the reflection from the through-wall notch was about 7 db higher compared to the largest reflection from the grid (grid 2). A response from the 75% deep notch turned out to be buried in the reverberation noise. Applying a non-linear processing allowed improving the ratio through-wall notch over the grid up to 19 db. This kind of difference in the amplitude should bring the reliability of detection of the through-wall notch to a rather high level even if when it occurs in the area under the grid. The amplitude of reverberation was also reduced up to the level when a response from the 75% notch could be seen. It should also be noticed that when applying a non-linear presentation, the response from the end of the tube turned out to be lower in the amplitude compared to the response from the through-wall notch. This effect demonstrates a significant energy drop of selected harmonics of GWs on this particular type of anomaly. Screening of service water piping

8 The majority of service water piping produces a high rate of internal reverberation due to the presence of thick deposits. This is the case when introducing more energy into the pipe wall will not help to improve the SNR. The deposits form a second layer on the pipe ID surface, but the layer by itself cannot support GW due to an extremely coarse structure. As a result of this, the energy scatters on the layer returns in the pipe as mode converted vibrations. Since the dispersion rate of mode converted GWs differs from the dispersion rate of T- mode, a non-linear processing should be applicable to the case. Figure 9 (right) shows the typical condition of a 89 mm OD service water pipe after 25 years of operation. Like the majority of service water piping, the above pipe had rather thick deposits covering deep pockets with metal loss. Manual ultrasonic testing performed over a long period of time identified some suspect areas marked with grids and painted circle. The pipe was taken out of service after 25 years of operation as a result of leakage through the pinhole that appeared outside of the suspect area (marked with the small circle). The section of the pipe was cut off and tested in the lab using guided waves at 60 khz. The results of initial testing and the level of internal reverberation caused by deposits are shown on the left side of the picture. It should be noted that the level of the internal reverberation would make it difficult to call any of the known metal loss indications as suspect due to rather low SNR. Improving the SNR in this case was accomplished by applying non-linear signal processing. As a result of the advanced processing, the majority of internal reverberation signals were effectively rejected and all three indications including the indication produced by the pinhole could be detected. Figure 9 - Advanced GW screening of service water piping: (Left) - results of advanced conditioning of guided wave signal; (Right) - typical condition of service water pipe after 25 years of operation Screening of centrifugally cast stainless steel piping Centrifugally cast stainless steel piping (CCSS) is one of the major components of the primary coolant circuit of Nuclear Power Plants. The coarse-grained and anisotropic microstructure of CCSS material makes it difficult to inspect CCSS piping due to severe attenuation, change in velocity, and scattering of ultrasonic energy. At the same time, keeping the track of the integrity of such a component is an outmost importance (20). Figure 10 (left) shows GW data obtained from the mockup representing a heavy wall pipe joint with the dissimilar metal weld. The geometry of mockup is shown on the right side of the figure.

9 Figure 10 - Advanced GW screening of CCSS piping: (Left) - data obtained from the mockup representing a heavy wall pipe joint with the dissimilar metal weld using linear and non-linear presentation; (Right) the mockup and the testing arrangement using segmented MsT. The data were acquired using segmented MsT transducer allowing making a C-scan imaging of the component. The transducer was installed from the OD of the CCSS section of the mockup. Results of two types of presentation - linear and non-linear are both presented in the unwrapped C-scan view. As it can be noticed, two circumferential notches machined in the area of DM weld both produced indications. However, the linear presentation was significantly contaminated with the backscattering noise from the grains and reflections from uneven surfaces. The overall SNR accomplished using a linear presentation was about 12 db on 79% circumferential notch and about 4 db on 20% notch. A non-linear concept in this particular case was based on the analysis of high frequency non-linear responses introduced to a low frequency transmission pulse by notches. The final SNR accomplished using a non-linear GW was 18 db for 79% notch and 10 db for 20% notch. The effect of trapped acoustical resonance was observed on the 79% notch. As a result of this, the response exhibited essentially longer ringing time compared to the response from the 20% notch. CONCLUSIONS Non-linear effects on guided waves were utilized for development of enhanced GW screening procedures. The approach was based on using magnetostrictive transducers with the specific transfer function allowing reading the vibrations in the frequency range khz. The enhanced GW screening procedure was shown to be capable of improving the flaw detection capability in the presence of high reverberation noise and with the minimal impact on the speed of the data acquisition. The further development of this approach should be based on utilizing the broader range of non-linear effects such as a function of GW energy distribution through the thickness of the component, non-linear effects in multi-layered structures and other effects. AKNOWLEGMENTS Authors wish to thank the Electric Power Research Institute for supporting the development of the technology and also for providing centrifugally cast stainless steel piping and fuel rods mockups. REFERENCES 1. Theodore L. Rhyne, Richard Y. Chiao. Nonlinear imaging using orthogonal transmit and receive codes. U.S. Patent , William T. Yost, John H. Cantrell. Nonlinear ultrasonic scanning to detect material defects. U.S. Patent , 1998

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