Development of the air-coupled ultrasonic vertical reflection method

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Letitia Cooper
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1 15 th Asia Pacific Conference for Non-Destructive Testing (APCNDT217), Singapore. Development of the air-coupled ultrasonic vertical reflection method M. Endo, M. Ishikawa 1, H. Nishino 1 and S.Sugimoto 2 Graduate School of Advanced Technology and Science, Tokushima University, 2-1 Minamijyousanjima-cho, Tokushima, Japan. c @tokushima-u.ac.jp 1 Graduate School of Advanced Technology and Science, Tokushima University. 2 Japan Aerospace Exploration Agency. Abstract Although air-coupled ultrasonic testing is a convenient non-destructive inspection method because of its non-contact testing application, it is usually performed with a transmission method that uses two ultrasonic probes. In this study, we aim to achieve the air-coupled ultrasonic vertical reflection method. When an ultrasonic wave is incident on a solid from air, most of the wave energy is reflected at the boundary of the solid, and very little energy transmits into the solid because of the large mismatching of acoustic impedances between the solid and air. Thus, there are significantly more waves reflected from the surface of the tested object than are reflected from internal defects, and the surface-reflected wave obscures the defect-reflected wave. To separate and detect the faint defect-reflected wave from the surface-reflected wave, we used ultrasound with linear frequency modulation (chirp signal) and the pulse compression technique. Theoretical and experimental investigations showed that the defectreflected wave could be detected more easily when the bandwidth of the excited chirp signal was larger. Keywords: NDI, CFRPs, Air-coupled ultrasonic, Vertical reflection method 1 Introduction Ultrasonic inspection is the most frequently used non-destructive inspection technique. In conventional ultrasonic testing, ultrasonic probes are in contact with the test objects through couplants such as grease or water, or the test objects are immersed in water during the test. In the immersion method, it is necessary to prepare a water sink, and a drying process after the testing is required. Moreover, it is difficult to test objects where contact with water is prohibitive. On the other hand, air-coupled ultrasonic testing does not require such couplants. In previous papers, air-coupled ultrasonic testing using the transmission method (Fig. 1 (a)) was reported. In the transmission method, two ultrasonic sensors are placed on the test object, one at each side [1-3]. However, under practical inspection conditions, the transmission method cannot always be applied; hence, the vertical reflection method (Fig. 1 (b)) is easier and more desirable. Therefore, in this study, we aim to achieve an air-coupled ultrasonic vertical reflection method. In air-coupled ultrasonic testing, most of the excited wave is reflected at the boundary between the air and objects, and only a faint wave transmits into objects. Thus, detecting signals received after they are propagated in objects is difficult because the signal could easily be buried in a large surface reflection signal [4]. To achieve the vertical reflection method, we examine an application that uses a pulse compression technique combined with broadband chirp signal excitation. [3732] 1

2 Pulse compression technique Because faint signals are detected in air-coupled ultrasonic testing, applying signal processing techniques is essential.

2 15 th Asia Pacific Conference for Non-Destructive Testing (APCNDT217), Singapore. (a) (b) Fig. 1 Configurations of air-coupled ultrasonic testing: (a) transmission method, and (b) vertical reflection method. 2 Pulse compression technique Because faint signals are detected in air-coupled ultrasonic testing, applying signal processing techniques is essential. The pulse compression technique is one of effective signal processing methods for improving the signal to noise (S/N) ratio and time resolution of the detected signals. In the pulse compression technique, the chirp signal is frequently used. The chirp signal is a broadband wave whose frequency is linearly swept, and is expressed as [5, 6] C(t) = H(t) sin (2πF i t + πb T t2 ) t T (1) where t is time, and F i, B, and T are initial frequency, bandwidth, and duration time of the signals, respectively. H(t) is Hanning window, which was used in this study, expressed as H(t) = 1 [1 cos (2πt )]. (2) 2 T The pulse compression technique utilizes correlation processing between the received signal and reference signal. The pulse compression process procedure is schematically demonstrated in Fig. 2. In the process, raw waves detected by ultrasonic probes (Fig. 2(a)) are correlated with reference signals such as the input waveform shown in Fig. 2(b). After processing, the S/N ratio and time resolution are improved (Fig. 2(c)). This technique is effective in detecting very small signals that could not have been detected in raw waveforms. [3732] 2

3 15 th Asia Pacific Conference for Non-Destructive Testing (APCNDT217), Singapore. (b) Amplitude (V) Amplitude (V) (a) Correlation Processing Amplitude (V) (c) Fig. 2 Pulse compression technique procedure: (a) detected wave including a faint signal after a large signal, (b) reference wave, and (c) output wave after applying pulse compression. 3 Estimated chirp signal conditions required for the vertical reflection method To achieve the vertical reflection method, it is necessary to separate the signal received after it is transmitted into the objects (S T) from the signal reflected from the surface of the objects (S S). Figure 3 shows the relationship between pulse width after applying the pulse compression technique (W cp) and F i, under a condition where B = 1 khz and T = 2 μs, obtained from analytical autocorrelation calculations. In the same figure, the relationship between the W cp and B calculated under a condition where F i = 1 khz and T = 2 μs is also presented. These results show that, though the W cp is not affected by the F i, W cp decreases with increasing B. Note that smaller W cp leads to higher time resolution of the signal after applying pulse compression and should be more effective in separating the S T from the S S. Initial frequency F i (khz) Pulse width W cp (µs) Dependency on F i Dependency on B 6 8 Bandwidth B (khz) 1 Fig. 3 Calculate half-value width after pulse compression as a function of initial frequency F i and bandwidth B of the excited chirp. [3732] 3

4 15 th Asia Pacific Conference for Non-Destructive Testing (APCNDT217), Singapore. Figure 4 shows analytically calculated signals after applying pulse compression with and without S T (F i = 1 khz, B = 1 khz, and T = 2 μs). In the calculation with S T, the reflected signal after transmitting to the object is assumed to be detected 5 μs after receiving the S S (i.e., time delay to the S S is 5 s). In Fig. 4, a small signal, S T, is obtained at around 25 μs. Thus, Fig. 4 suggests that the S T could be detected in this chirp condition. Figure 5 shows the relationship between the B required to separate the S T from the S S and the time delay between the S T and S S (based on the results in Fig. 3, only B was varied as the parameter which affects W cp). It is found that larger B is required to detect S T with a smaller time delay. 14 Relative Amplitude (-) S S S S + S T Bandwidth B (khz) Delay time (µs) Fig. 4 Internal reflection signal is detected 5 µs after the surface reflection signal. Fig. 5 Bandwidth B at which the internal reflection signal can be detected. 4 Experiment Detection of the vertically reflected signal received after transmitting into the test object was examined through experiments, and the analytical studies discussed in the previous section were verified. As a first step to achieve the air-coupled vertical reflection method, in this study we attempted to detect the reflected signals received after propagating in water. The transmission attenuation at the boundary of the water is relatively small because the acoustic impedance mismatch between water and air is not very large; moreover, the propagation attenuation in water is small. Thus, detecting the reflected signal after its propagation in water should be relatively easy. 4.1 One probe configuration Experimental setup Schematic illustrations of the experimental setup are shown in Fig. 6. Chirp signals were excited by a function generator (AFG312, Tektronix, Inc.) and transmitted from an air-coupled ultrasonic sensor (.4k14 2N-TX, Japan Probe Co., Ltd.) through a power amplifier (BA4825, NF Corporation). The [3732] 4

15 th Asia Pacific Conference for Non-Destructive Testing (APCNDT217), Singapore. F i and T of the excited chirp signal were 3 khz and 1 μs, respectively, and B was varied between 1 khz and 2 khz.

In the water, an aluminum plate was placed as a reflector for the incident wave, and the propagation distance in the water (X) was varied by adjusting the water depth of the plate with a lab jack.

5 15 th Asia Pacific Conference for Non-Destructive Testing (APCNDT217), Singapore. F i and T of the excited chirp signal were 3 khz and 1 μs, respectively, and B was varied between 1 khz and 2 khz. Part of the excited wave was transmitted into the water and the rest was reflected at the water surface. In the water, an aluminum plate was placed as a reflector for the incident wave, and the propagation distance in the water (X) was varied by adjusting the water depth of the plate with a lab jack. The reflected signals both from the water surface (S S) and the reflector (S T) were received by the same sensor, and displayed on a digital oscilloscope (TDS534B, Tektronix, Inc.) through a preamplifer (PR-6A, Japan Probe Co., Ltd.) and a programmable filter (3628, NF Corporation). The experiments were carried out by varying the X from 5 mm to 25 mm. The received signals were processed using the pulse compression technique to detect the S T. In the pulse compression process in these experiments, the S S without S T, which was obtained from a preliminary experiment without the aluminum reflector, was used for the reference signal. Fig. 6 Schematic of the experiment using one probe Experimental results Figure 7 shows the received waveform obtained under the condition where X = 15 mm before and after applying the pulse compression technique. In Fig. 7(a), although S S could be observed, S T could not be observed. On the other hand, the S T and its multiple reflected signals could be clearly observed in the results after pulse compression is applied (Fig. 7(b)). Figure 8 shows the experimental results after pulse compression is applied for X = 5 2 mm. It is observed in Fig. 8 that the first reflected signal approached the S S as X was decreasing, and that it could not be detected when X = 11 mm. This means that in this chirp condition (B = 1 khz), the S T could be detected when the time delay between the S S and S T was smaller than 15 μs (calculated assuming the wave velocity in the water was 148 m/s). In Fig. 9, the experimentally obtained minimum time delay required to separate the S T from the S S was compared with the analytical results (Fig. 5). These results show that the experimental results agree well with the analytical results. [3732] 5

6 15 th Asia Pacific Conference for Non-Destructive Testing (APCNDT217), Singapore. (a) Amplitude (-) (b) Amplitude (-) SS ST Propagation time (ms) Propagation time (ms) Fig. 7 An experimentally observed signal (a) before applying pulse compression, and (b) after applying pulse compression (X = 15 mm). Relative Amplitude (-) Propagation time (ms) 2 mm 15 mm 1 mm 5 mm 2. Bandwidth B (khz) Theoretical caluculation Experimental result Bandwidth B (khz) 2 Fig. 8 Experimentally obtained waves after applying pulse compression for X = 5 2 mm. Fig. 9 Comparison between the experimentally obtained minimum time delay and analytical results. 4.2 Bandwidth expansion of the excited chirp signal (using multiple probes) The analytical result (Fig. 5) suggests that using chirp signals with higher B enables the detection of reflected signals with a smaller time delay. In other words, the vertical reflection method using the broadband chirp signals could be applied to objects with higher wave velocity or to thinner objects. However, it is difficult to achieve the broadband condition using only one air-coupled ultrasonic probe. One promising method to overcome the problem is to use multiple probes to excite the chirp signals with different frequency ranges and merge the received waves. Figure. 1 demonstrates the concept of the suggested method and Fig. 11 shows the example of the merged wave (calculated by merging the chirp signals with a frequency range of 1 3 khz and 2 4 khz). Applying the pulse compression technique to the merged wave could be similar to the experiment using a broadband chirp signal. [3732] 6

15 th Asia Pacific Conference for Non-Destructive Testing (APCNDT217), Singapore. Amplitude (-).8.4. -.4 -.8 1 2 Fig.

Conclusions In this report, we examined the feasibility of the air-coupled ultrasonic vertical reflection method by combining excitation of a broadband chirp signal and the pulse compression

7 15 th Asia Pacific Conference for Non-Destructive Testing (APCNDT217), Singapore. Amplitude (-) Fig. 1 Concept of the experimental setup for vertical reflection method using multiple probes. Fig. 11 Example of broadband chirp signal obtained by merging two chirp signals with different bandwidths. 5. Conclusions In this report, we examined the feasibility of the air-coupled ultrasonic vertical reflection method by combining excitation of a broadband chirp signal and the pulse compression technique. Through simple calculations, it was found that a chirp excitation signal with a larger bandwidth was required to detect the internal reflection signal received with smaller delay time. This tendency was also observed in the experimental results. This means that using a broadband chirp signal is necessary for inspecting thinner objects or objects with higher wave velocity. However, exciting a broadband wave using only one aircoupled ultrasonic probe is difficult. One solution for the problem is to use multiple probes and excite and merge the various chirp signals with different bandwidths. Acknowledgements This work was supported by a research grant from The Mazda Foundation. References [1] R. Stoessel, et al., Air-coupled ultrasound inspection of various materials, Ultrasonics, vol. 4, no. 1, pp [2] M.Takahashi, M. Noji, and K. Kiryu, Development of Non-Contact Air Coupled Ultrasonic Testing and its Application, Journal of the Japanese Society for Non-Destructive Inspection, vol. 6, no. 9, pp , 211. [3] K. Kawashima, Nondestructive Material Evaluation and Testing of Structures with Air-Coupled Transducers, Journal of the Japanese Society for Non-Destructive Inspection, vol. 58, no. 7, pp , 29. [3732] 7

8 15 th Asia Pacific Conference for Non-Destructive Testing (APCNDT217), Singapore. [4] H. Nishino, M. Takahashi and Y. Ogura, Characteristics of Ultrasonic Transmission Coefficients of Air/Solid Interface for Non-Contact Air-Coupled Ultrasonic Testing, Journal of the Japanese Society for Non-Destructive Inspection, vol. 58, no. 7, pp , 29. [5] E. Blomme, D. Bulcaen, and F. Declercq, Air-coupled ultrasonic NDE: experiments in the frequency range 75 khz-2 MHz, NDT&E International, vol. 35, pp , 22. [6] T.H. Gan et al., The use of Broadband acoustic transducers and pulse-compression techniques for air-coupled ultrasonic imaging, Ultrasonics, vol. 39, pp , 21. [3732] 8

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