NDT.net • May 2005 • Vol. 10 No.5

A fundamental study of an inspection method for thin-walled structures using lamb waves induced by chirp signals

Yoshihiro Mizutani*, Satoshi Inokawa
Dept. of Mechanical Science and Engineering, Tokyo Institute of Technology/2-12-1-I1-70,
O-okayama, Meguroku, Tokyo, Japan, 152-8552;


*Corresponding Author Contact:
Email: ymizutan@mes.titech.ac.jp, phone 81 3 5734-3195; fax 81 3 5734-2809

© SPIE - The International Society of Optical Engineers.
This paper was originally published in the SPIE Proceedings vol. 5852
The 3rd Int' Conf' on Experimental Mechanics at 29.11.- 1.12.2004 in Singapore
Back To Session: Smart Structures and Non-Destructive Testing


ABSTRACT

In this study, a new NDT-method is proposed for inspecting delaminations in the composite plates (1mm-thickness CFRP layer with 1mm-thickness aluminum liner) Lamb waves (plate waves) generated by chirp signals (250-1000 kHz, 4Vp-p) were used for the inspection. We first studied characteristics of excited signals from the pulser when the chirp signal is used as an input signals. The amplitude of the excited signal from the pulser is changed with frequency due to the resonant characteristics of the transducer, although, wide range of the frequency components can be generated by using chirp signals. Two transducers were mounted on the composite plate, and inspection was conducted by using lamb waves. The amplitude, velocity and scattering of lamb waves on damaged and sound zone were compared. When the transducer is on the delamination area, low frequency components of the lamb wave drastically attenuated and could easily detect the delaminations. While, when the delamination is between two transducers, characteristics of lamb waves are similar to those on the sound area and difficult to separate each other with reasonable accuracy. Further modification of experimental method is needed to accomplish new inspection method.

Keywords: Keywords: Nondestructive testing, Composite plates, Delamination, Lamb wave, Chirp Signal

1. INTRODUCTION

Cryogenic propellant tanks made of carbon fiber reinforced plastics (CFRP) with aluminum liner are supposed to be used for future transportation vehicle for weight reduction. In order to insure soundness of the tanks, it is essential to inspect delaminations between outer-CFRP layer and inner-liner. The traditional pulse-echo ultrasonic inspection method is used at the work site, although, the method takes long time for inspecting whole parts of the tank wall. Therefore, the development of a whole field or a semi-whole field inspection method with a reasonable accuracy is required for shortening inspection time at the work site.

In the several published papers, Lamb waves (plate waves) propagated along CFRP plates were used for shortening inspection times. Tsushima et al. [1] and Toyama et al. [2] inspected delaminations in CFRP plates by using velocity shifting of lamb-waves. Tan et al. [3] used amplitude shifting of lamb-waves for this issue. Mizutani et al. [4] used waveform changing for this issue. Rokhlin [5] showed possibility to use interaction of lamb waves for detecting delaminations. Each of these techniques offers own unique advantages in detecting damages, but these inspection methods are not yet become a common method using at the field. In the previous paper [6], in order to realize more sensitive and clear inspection method, we used burst type signals containing five burst signals of 250 kHz to 650 kHz frequencies at 100 kHz steps for generating lamb waves. Waveforms and FFT of the detected waves were drastically changed by existence of delaminations in the composite plate.

In this study, following the previous paper, we investigated the usefulness of lamb waves generated by chirp signals (signals which frequency continuously changed with time) for the inspection. We first studied characteristics of excited signals from the pulser by putting chirp signals as an input signal. After that, characteristics of lamb waves (amplitude, velocity and scattering) at the damaged zone were compared to those at the sound zone and examined usefulness of lamb waves excited by chirp signals to the inspection.

2. SPECIMEN AND EXPERIMENTAL SETUP

Two types of composite plates (820mmLX300mmWX2mmT) comprising CFRP layer (1mm thickness) and aluminum layer (1mm thickness) were prepared for this study. The stacking sequence of CFRP layer is [90° [7]]. These composite plates simulate sidewall of composite propellant tank for future space transportation system except fiber direction. Artificial delamination of 30 and 10 mm length were introduced between CFRP layer and aluminum layer at the center of each plate (named type-I specimen and type-II specimen respectively). Figure 1 shows schematic illustration of experimental setup and specimen with coordinate system. Inspection zones and propagation direction of lamb waves are indicated by arrows at over the x-axis. The origin of x-axis is set at the center of plates. Fiber direction of the top layer (CFRP layer) is perpendicular to the x-axis. Two 500 kHz-resonant type transducers (Physical Acoustic Corp., type: pico, 5.0 mm-diameter) were put on the front surface at 100 mm distance. One of them was used for a pulser and the other was used for a receiver. Chirp (sweep) signals were used as input signals to the pulser. Elastic waves generated from the pulser propagated as lamb wave (plate wave) because the thickness of composite plates is much smaller than wave length. Output signals from the receiver were amplified 46 dB by preamplifier (NF corp., Low Noise Preamplifier, Type: 9193) and digitized by A/D converter (Keyence corp., Type: NR-350) at an interval of 40 nsec with 8192 points. Digitized data was stored to a personal computer for analyzing. A set of transducers on the surface were moved along x-axis at 50 mm step. The distance between two transducers was kept constant during the experiment. The inspection was started from x=-150 mm as the pulser position and x=-50 mm as the receiver position. Inspection zone for this situation is defined as zone "A" (see Fig. 1). Zones A and E were categorized as sound zones, while, B, C and D were categorized as damaged zones.

Fig. 1 Schematic illustration of experimental setup and specimen with inspection zones.

3. CHARACTERISTICS OF INPUT SIGNALS AND EXCITED SIGNALS

The displacement of the pulser excited by a chirp signal was investigated by facing pulser and receiver as shown in Fig. 2. Figure 3 shows waveforms of chirp signal (the upper of Fig. 3) put into the pulser and output signal (the bottom) from the receiver. The results of FFT and continuous wavelet transform of each signal are shown in Fig. 4 and 5 respectively. We used the free software [7] for the wavelet transform and the software used gabor function as a mother wavelet. The amplitude of input signal is constant and the frequency is linearly changed from lower frequency to higher frequency (see the upper of Fig.3 and Fig. 5).

Fig. 3 Input signal (the upper) and excited signal (the bottom).

Fig. 4 FFT of the input signal (the upper) and the excited signal (the bottom).


Fig. 2 Experimental setup for investigating characteristics of excited signals.
Fig. 5 Wavelet contour map for the input signal (the upper) and the excited signal (the bottom).

On the other hand, the amplitude of the excited signal is changed with frequency due to the resonant characteristics of the sensor. The result of FFT and wavelet transform shows that the dominant component of excited signal is 250-600 kHz. It is noted that even in a small amplitude, although, signals of 600 - 1000 kHz are also excited. Therefore, wide range of the frequency components of 250-1000 kHz can be used for the inspection.

4. DAMAAGE INSPECTION USING LAMB WAVES EXCITED BY CHIRP SIGNALS

4.1 The inspection focused on amplitude and waveform changing
Figure 6 represents lamb waves detected at zone A to E shown in Fig. 1 on the type-I specimen (composite plates with 30 mm-length delamination) and the type-II specimen (10 mm-length delamination). The FFT of detected lamb waves are shown in Fig. 7. As shown in Fig. 6 and 7, waveforms, amplitude and Fourier spectrums are changed by inspection zone even in the sound area (zone A and E). When the sensor is on the delamination zone (zone B and D), low frequency components (initial part of the waves, indicated by dotted rectangles in Fig. 6) is drastically attenuated and easy to separate from other waves. Though, when the delamination is between two transducers (zone C), low frequency component is not attenuated as zone B and D, but middle frequency component (at around 100µsec, indicated by dotted circles in Fig. 6) is attenuated. This result indicates that the inspection method which uses amplitude changing of constant frequency lamb waves (involve only a single frequency component) is not adequate for inspecting delaminations in thin plates. Waveforms and FFT of detected lamb waves at zone C are similar to those at the sound area (zone A and E) and hard to separate each other. We then conclude that, in this stage, it is difficult to conduct the inspection with reasonable accuracy only by using amplitude information.

Fig. 6 Lamb waves on the Type-I specimen (the left) and the Type-II specimen (the right). The Type-I specimen involves 30 mm-length artificial delamination while the Type-II involves 10mm-length delamination. The inspection zones are indicated in Fig. 1.

Fig. 7 FFT of detected lamb waves (Fig. 6). The left: lamb waves on the Type-I specimen. The right: lamb waves on the Type-II specimen.

4.2 The inspection focused on arrival time changing
Lamb waves have two propagation modes of symmetric (S-mode) and anti-symmetric mode (A-mode), and the velocities of both modes are changed with frequencies. The velocity of lamb waves is also changed by thickness of plates. Figure 8 shows example of dispersion curves calculated for the lamb waves propagated along 2 mm-thickness and 1 mm-thickness aluminum plates. When the plate thickness becomes thinner (from 2mm to 1mm), the velocity of both propagation mode is changed. As it can be expected from this result, when the delamination exists in the composite plates as shown in Fig. 1, the velocity is changed at the damaged zone.

From this point of view, we investigated velocities of lamb waves at damaged and sound zones. In this study, lamb waves were generated by chirp signals with long duration (more than 200 µsec) and then special technique is needed for determining velocities. In other words, excitation timing of lamb waves of each frequency components should be accounted. In order to determine excitation timing of each frequency components of lamb waves, wavelet transform of the input signal (chirp signal) is conducted. Wavelet coefficients of the input signal are shown in the top-left of Fig. 9. Peak arrival times of each frequency components (shown by solid line in Fig. 9) were supposed to be excitation timings. Wavelet coefficients of lamb waves at zone-A to -E on Type-I specimen are shown in Fig. 9. Excitation timing estimated from top-left of Fig. 9 is overlapped in the figures. As you can see from the figures, time resolution of each frequency component is bad. Another problem is that some frequency components excited at not predicted timing and arrived before the estimated excitation timing. We need some modification of experimental procedure for measuring velocities of each frequency components of lamb waves excited by long duration chirp signals.

Fig. 8 Dispersion curves of the velocity calculated for the 1mm and 2 mm-thickness aluminum plates.

4.3 The inspection focused on scattering of lamb waves
When the delamination exists in the propagation pass, lamb waves may scatter at the delamination points. We then investigated scattering of lamb waves. Figure 10 shows wavelet contour map of zone A to E of Type-I and II specimen. When the delamination is exist between two sensors, delayed components are observed (indicated by dotted circle in Fig. 10) followed by main components. It is also obtained that linear relationship between frequency and arrival time is affected when the damage is existed (circled by solid line in Fig. 10). It should be useful to use this scattering information for the inspection, although, further experiments by changing width and depth of the delamination is needed before utilizing.

Fig. 9 Wavelet coefficients of chirp signal and lamb waves at zone A to E on Type-I specimen.Fig. 10 Wavelet contour maps of lamb waves at Type-A to E for Type-I (the left) and Type-II (the right) specimens.

5. CONCLUSION

In this study, a new NDT-method using lamb waves excited by chirp signal was proposed for the inspection of composite thin plates. Results are summarized below;
  1. Characteristics of excited signals from the pulser by putting chirp signal as an input signal were investigated. The amplitude of the excited signal from the pulser was changed with frequency due to the resonant characteristics of the transducer, although, wide range of the frequency components could be generated by using chirp signals.
  2. Lamb waves were generated on the composite plates with artificial delamination. When the transducer was on the delamination area, low frequency components of the lamb wave drastically attenuated and could easily separate from those at sound area.
  3. When the delamination was between two transducers, characteristics of lamb waves were similar to those on the sound area and difficult to separate these waves with reasonable accuracy.
  4. Velocity measurement of lamb waves excited by chirp signal is difficult due to the lack of time resolution. Some modifications for the experimental procedure is needed for this issue.
  5. Scattering of lamb waves at the delamination may be useful for the damage inspection. Further experiment is needed before utilizing.

REFERENCES

  1. S. Tsushima et. al., Nondestructive Testing of FRP Plates by Elastic Waves, Symposium on Nondestructive Evaluation for New Materials, 1990, pp. 105-110 (written in Japanese).
  2. N. Toyama and J. Takatsubo, Lamb Wave Method for Quantitative Inspection of Impact-Induced Delamination in CFRP Plates, J. of JSNDI, 52-11, 2003, pp. 633-638 (written in Japanese).
  3. K. S. Tan et. al., Experimental Evaluation of Delaminations in Composite Plates by the Use of Lamb Waves, Composites Science and Technology, 53-1, 1995, pp. 77-84
  4. Y. Mizutani et. al., Non-contact Detection of Delamination in Impacted Cross-ply CFRP Using Laser Generated Lamb Waves, 10th-International Conference on Fracture, Hawaii, 2001
  5. S. Rokhlin, Interaction of Lamb Waves with Elongated Delaminations in Thin Sheets, International Advances in Nondestructive Testing, 6, 1979, pp. 263-285
  6. Y. Mizutani et. al., A Fundamental Study of Detecting Delaminations in Composite Tanks by Using Lamb Waves, Key Engineering Materials, 270-273, 2004, pp. 1898-1904
  7. http://www.vallen.de/wavelet/index.html
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