JP4997636B2 - Non-destructive diagnostic method for structures - Google Patents

Description

  The present invention relates to a non-destructive diagnosis method for a structure, in which a defect of the structure is diagnosed nondestructively using a guide wave ultrasonic wave having velocity dispersibility, a so-called guide wave flaw detection method.

  The guide wave flaw detection method is a method in which a guide wave ultrasonic wave is propagated to a structure to be flaw detected, and the defect is detected based on an echo signal reflected by a defect present in the propagation path.

  The guide wave ultrasonic wave essentially has a property called velocity dispersibility in which the propagation velocity differs depending on the frequency. This velocity dispersibility is a property peculiar to a guide wave ultrasonic wave that is not found in a general bulk ultrasonic wave propagating in water or in a large metal block. When guided wave ultrasonic waves are excited in the target structure, the guided wave ultrasonic waves exhibit velocity dispersion with different propagation speeds depending on the frequency, and the low frequency component and high frequency component included in the excitation signal Due to the difference in propagation speed, a phenomenon that gradually separates with propagation occurs. For example, for a guided wave ultrasonic wave that exhibits velocity dispersion such that the propagation speed increases as the frequency increases, when an impulse-like broadband signal is excited, the high-frequency component of the excitation signal propagates faster and the lower frequency component slower Propagation phenomenon occurs. As a result, as the propagation distance becomes longer, the waveform of the guide wave ultrasonic wave gradually collapses from the pulse shape and becomes longer.

  FIGS. 12 to 14 show a case where a hamming burst signal of two waves that is relatively close to an impulse shape is used as an excitation time wave, and an L (0, 1) mode guide wave ultrasonic wave is excited in a steel bar having a diameter of 32 mm. The reception time wave is shown. 12-14, it can be seen that the longer the propagation distance, the more the guided wave ultrasonic wave observed at a later time, the more the waveform is collapsed.

  Waveform collapse due to the velocity dispersion of the guide wave ultrasonic wave as described above is a serious problem in nondestructive diagnosis because it leads to a significant decrease in spatial resolution in defect detection. For example, in the conventional fault diagnosis of thin steel plates using guided wave ultrasonic waves, by restricting the frequency band narrowly using a tone burst signal of 10 to several tens of waves at the time of excitation, significant waveform collapse due to propagation is prevented. I have avoided the influence. However, since the tone burst signal with a narrow frequency band is originally a signal that is long in time, there is a face that sacrifices the spatial resolution. In this case, a diagnosis of about several tens of millimeters is made at the end of the plate. Problems such as the generation of impossible areas have occurred.

  On the other hand, in all ultrasonic techniques not limited to guide wave ultrasonic waves, various efforts have been made to improve the SN ratio of received signals so as to improve diagnostic accuracy. In particular, in diagnosis using guided wave ultrasound, diagnosis is often performed in the vicinity of various high-power devices in the factory and in the vicinity of noise sources such as pumps and conveyors in operation, and improvement of the S / N ratio is required. It has been. For automating diagnosis, the use of non-contact ultrasonic techniques such as electromagnetic ultrasonic waves, air-propagating ultrasonic waves, and laser ultrasonic waves facilitates the diagnosis. The reality is that the sensitivity is low and the S / N ratio is insufficient, so that it has not been put into practical use.

  On the other hand, there is a pulse compression technique which is one of signal processing techniques for improving the SN ratio (for example, see Non-Patent Document 1). This technology has been applied to radar and ultrasonic flaw detection methods for a long time (see, for example, Patent Documents 1 and 2), and pulse compression is also applied to non-velocity dispersive guide wave ultrasonic waves. There is an application example (see Patent Document 3). Pulse compression technology is a pulse-like time resolution that excites a long-time broadband signal instead of a burst wave or pulse wave during transmission, and performs cross-correlation operations on the received waveform using an appropriate reference signal. A good pulse compression signal is obtained. At this time, since the noise component having no correlation with the reference signal is canceled by the cross-correlation calculation, the pulse compression technique has an excellent noise removal effect. The derivation of the dispersion curve of the guide wave ultrasonic wave is also known as a semi-analytic finite element method (Non-Patent Document 2).

Radar handbook, Skornnik, McGraw-Hill, 1970 (Radar handbook, Skolniket., McGraw-Hill Inc., 1970) Takahiro Hayashi and Joseph L. Rose, Guided wave simulation and visualization by a Semi-Analytical Finite Element Method, Materials Evaluation Vol.61, No.1 (2003) pp.75-79 Japanese Patent No. 3022108 Japanese Patent No. 3036387 JP 2007-121092 A

  However, the conventional pulse compression technology is based on the premise that the sound speed is constant regardless of the frequency, and cannot be applied as it is when it has velocity dispersion like a general guided wave ultrasonic wave. It is.

  That is, in order to detect a structure having a good S / N ratio using a guide wave ultrasonic wave having velocity dispersion, considering the application of a pulse compression technique, a broadband excitation time wave is used for excitation of the guide wave ultrasonic wave. It is a premise to use.

  However, as described in the background art, in a guided wave ultrasonic wave having velocity dispersion, significant waveform collapse occurs due to propagation. When the conventional pulse compression processing is performed, there is certainly a noise removal effect, but it is not possible to remove the influence of significant waveform collapse due to velocity dispersion.

  FIG. 3 shows an example of a group velocity dispersion curve of L (0, 1) mode propagating in a steel bar having a diameter of 32 mm as an example of guide wave ultrasonic waves having velocity dispersion. FIGS. 12 to 14 show reception time waves when a relatively wide-band 2-cycle Hamming burst signal is used as the excitation time wave for the above-described modes. 15 to 17 show examples of time wave pulse compression waveforms processed by a conventional pulse compression technique using a wide band chirp signal as an excitation time wave for the same guided wave ultrasonic mode.

  12 to 14 shows that the reception time wave at the time of humming burst signal excitation has a remarkable waveform collapse due to velocity dispersion, and the resolution is lowered. On the other hand, in FIGS. 15 to 17, the noise level is reduced and the SN ratio is improved by the application of the conventional pulse compression technique, but the significant waveform collapse due to the speed dispersion still remains. From this, it can be seen that it is inappropriate to apply the conventional pulse compression technique as it is to the guided wave ultrasonic wave having velocity dispersion.

  The present invention has been made to solve the above problems, and provides a non-destructive diagnosis method for a structure having a high S / N ratio and excellent spatial resolution.

A non-destructive diagnostic method for a structure according to the present invention is a non-destructive diagnostic method for a structure that uses a guided wave ultrasonic wave having velocity dispersibility to diagnose a defect of a structure non-destructively.
An excitation step of exciting the guided wave ultrasonic wave in the structure by an excitation time wave having a broadband waveform;
A receiving step of receiving the guided wave ultrasonic wave propagating through the structure;
Based on the reception time wave obtained by the reception step, the excitation time wave used in the excitation step, and the velocity dispersion information on the guide wave ultrasonic wave, the spatial distribution of the defects in the structure, A spatial wave pulse compression calculation step for calculating a spatial wave pulse compression waveform corresponding to a convolution waveform with an autocorrelation function of the excitation spatial wave, which is a spatial distribution of the excitation time wave;
A display step of displaying the spatial wave pulse compression waveform on the screen ,
The spatial wave pulse compression calculating step obtains a time Fourier transform signal of the received time wave by time Fourier transforming the received time wave, and a time Fourier transform signal of the excitation time wave by time Fourier transforming the excitation time wave. Seeking
A spatial Fourier based on the received time wave from the time Fourier transform signal of the received time wave by performing variable conversion between angular frequency and wave number using the velocity dispersion information on the guide wave ultrasonic wave. Obtaining a transformed signal, obtaining a spatial Fourier transform signal based on the excitation time wave from the time Fourier transform signal of the excitation time wave;
Next, the spatial wave pulse compression is performed by performing a spatial inverse Fourier transform on the product of the complex conjugate signal of the spatial Fourier transform signal based on the excitation time wave and the spatial Fourier transform signal based on the reception time wave. It characterized that you calculate the waveform.

  In the non-destructive diagnosis method of the present invention, a broadband signal is excited using a guide wave ultrasonic wave having velocity dispersion whose propagation velocity changes depending on the frequency, and signal processing based on the excitation time wave and velocity dispersion information is performed later. To do at high speed. For this reason, according to this nondestructive diagnostic method or nondestructive diagnostic apparatus, it is possible to obtain a pulse compression waveform having a high SN ratio and excellent spatial resolution. That is, this nondestructive diagnostic method is a pulse compression test method adapted to a velocity-dispersive guide wave, and this nondestructive diagnostic device is a device that employs this pulse compression test method.

  The spatial wave pulse compression calculation step is configured based on the following concept.

  FIG. 1 is a time-space diagram simulating the state of propagation of guided wave ultrasonic waves excited at time t = 0 and position x = 0. Guide wave ultrasonic waves that emit t = 0 and x = 0 propagate while spatially spreading due to the difference in propagation speed due to velocity dispersion. After that, the guide wave ultrasound reflected by the reflection source distribution z (x) and reaching the position x = 0 again is equal to the guide wave ultrasound excited from the virtual source distribution y (x) at time t = 0. Become. Here, the real reflection source distribution z (x) and the virtual transmission source distribution y (x) satisfy the relationship of the following Expression 1.

  By the way, if the velocity dispersion information of the guide wave ultrasonic wave is known, based on the time wave of the guide wave ultrasonic wave observed at a certain place or the spatial wave of the guide wave ultrasonic wave at a certain time, A spatial waveform and a temporal waveform of the guide wave ultrasonic wave at the position are obtained by calculation.

  Consider a case where a broadband excitation time wave s (t) is excited all at once from time t = 0 from the virtual source distribution y (x). The reception time wave r (t) at the position x = 0 is expressed as the following Expression 2.

  Here, S (ω) is the Fourier transform of s (t). K (ω) is dispersion information of wave number k obtained from velocity dispersion information of the excited guide wave ultrasonic wave. Here, by comparing the Fourier transform R (ω) of r (t) with Equation 2, the relationship of Equation 3 below is obtained.

  On the other hand, if the dispersion information k (ω) of the wave number k is known, S (ω) and R (ω) that have been functions of ω can be replaced with functions of the wave number k. In actual signal processing, both S (ω) and R (ω) are discrete data for equidistant ω, and this is replaced with discrete data S (k) and R (k) for equidistant k. become.

  Each replacement is performed by interpolation. This replacement can be performed only in a region where k and ω are monotonously increasing. Deviating from this range is dω / dk ≦ 0, that is, a special region having a group velocity of 0 or less, but in the practical nondestructive diagnosis range, in most cases, it is a positive group velocity. , Don't worry too much. Therefore, from Equation 3, the following Equation 4 holds for equally spaced k.

  Here, the following equation 5 is defined as the spatial wave pulse compression waveform c (x). Here, corr. Indicates a correlation operation, and conv. Indicates a convolution operation.

  In Equation 5, s (x) corr. The part of s (x) is an autocorrelation function for the excitation spatial wave, and corresponds to the signal processing part of pulse compression. In addition, the entire expression 5 is a convolution of the above-described autocorrelation function and the virtual source distribution, and the spatial wave is increased as the excitation spatial wave is sufficiently wide with respect to the wave number k and the autocorrelation function becomes more pulsed. The pulse compression waveform c (x) is closer to the virtual source distribution. Further, when Formula 5 is expanded and Formula 4 is substituted, Formula 6 below is obtained. Here, the symbol * indicates a complex conjugate.

  Finally, in order to obtain z (x) corresponding to the actual defect distribution from the spatial wave pulse compression waveform c (x) corresponding to the virtual source distribution, the following Expression 7 is calculated with reference to Expression 1.

  In order to finally obtain z (x) from the reception time wave r (t) and the excitation time wave s (t), it is only necessary to perform two FFT operations and one interpolation operation. The time is short.

  For this reason, the spatial resolution in the defect detection is improved by removing the influence of the waveform collapse due to the velocity dispersion according to the present invention. Thereby, the recognition of the defect signal becomes clear and the diagnostic accuracy is improved. In addition, it is easy to separate large reflection signals and defect signals that are caused by the shape of the structure, such as reflection from the end face of the structure and reflection from the weld. The dead zone near the end face is reduced, and the effective diagnosis area is expanded.

  Further, the effect of improving the SN ratio according to the present invention makes it possible to clearly capture a minute defect echo signal that has been buried in noise and could not be detected, thereby improving the flaw detection accuracy.

  For automating product diagnosis and diagnosis of high-temperature structures, it is practical to use non-contact diagnostic methods such as electromagnetic ultrasonic waves (EMAT) and air-propagating ultrasonic waves (ACT) for transmitting and receiving ultrasonic waves. Although it is necessary, these non-contact diagnostic methods are uniformly low in sensitivity, and automatic diagnosis or diagnosis of a high-temperature body is not easily realized in many cases. According to the present invention, since the signal-to-noise ratio is improved by signal processing, the above-described non-contact diagnosis method can be adopted, and thus it is possible to automate diagnosis and diagnosis of a high-temperature body that have been difficult to realize so far.

  Here, as the ultrasonic wave incident means used in the excitation process, a signal generator and a known ultrasonic sensor capable of generating ultrasonic waves can be used. Usually, a signal amplifying device is used to amplify the voltage level required for the ultrasonic sensor. Further, it is desirable that the signal generator has a function of generating an arbitrary waveform so that an arbitrary excitation time wave can be generated. As a specific ultrasonic sensor, a general-purpose piezoelectric ceramic probe, an electromagnetic ultrasonic probe (EMAT), a piezoelectric resin film (PVDF), or the like may be employed.

  Moreover, as an ultrasonic wave reception means used at a receiving process, the well-known ultrasonic sensor which can receive an ultrasonic wave can be used. Usually, a signal amplifier is used to amplify to an observable voltage level, or a frequency filter device is used to remove noise outside the frequency band of the excitation time wave. As a specific ultrasonic sensor, a general-purpose piezoelectric ceramic probe, an electromagnetic ultrasonic probe (EMAT), a piezoelectric resin film (PVDF), a laser vibrometer, or the like can be employed.

  In the excitation process, it is preferable to use a chirp time wave as the excitation time wave. This is because the spatial resolution of the finally obtained spatial wave pulse compression waveform is determined by the frequency bandwidth of the excitation time wave and the propagation speed of the guide wave ultrasonic wave, but the chirp time wave can be obtained by simply changing the frequency bandwidth. This is because the spatial resolution of the spatial wave pulse compression waveform can be controlled.

  In the excitation process, it is preferable to use a pre-defined excitation spatial wave having a broadband property with respect to the wave number converted by velocity dispersion information about the guide wave ultrasonic wave as the excitation time wave. The conversion from the predefined excitation space wave to the excitation time wave can be performed as follows.

That is, the excitation time wave s (x) and its Fourier transform S (ω) and the excitation space wave s (x) and its Fourier transform S (k) have the following relationship.

  From the predefined excitation space wave s (x), S (k) is obtained using Equation 8. Thereafter, using the dispersion information k (ω) of the wave number k, replacement of the angular frequency ω with the function S (ω) of the angular frequency ω with respect to S (k) as a function of the wave number k is performed by an interpolation method. Thereafter, an excitation time wave s (t) is obtained by Equation 9.

  The spatial resolution of the finally obtained spatial wave pulse compression waveform is determined by the wave number bandwidth of the excitation spatial wave. Since the wave number bandwidth of the excitation spatial wave depends on both the frequency bandwidth of the excitation time wave and the propagation speed of the guide wave ultrasonic wave, the velocity dispersion of the guide wave ultrasonic wave (variation of the propagation speed with respect to the frequency) is particularly important. In a large region, it becomes difficult to roughly control the spatial resolution of the spatial wave pulse compression waveform from the frequency bandwidth of the excitation time wave. If the pre-defined excitation spatial wave converted from the velocity dispersion information about the guide wave ultrasonic wave is used as the excitation time wave, the spatial wave pulse compression waveform regardless of the velocity dispersion of the guide wave ultrasonic wave It becomes possible to control the spatial resolution.

  In the excitation process, a chirped spatial wave is used as the defined excitation spatial wave when a predefined excitation spatial wave that has been defined in advance as the excitation time wave is converted using velocity dispersion information about the guide wave ultrasound. preferable. If a chirped spatial wave is used as a predefined excitation spatial wave, the wavenumber bandwidth of the chirped spatial wave can be easily controlled by simple parameter changes, thereby enabling the spatial resolution of the spatial wave pulse compression waveform to be controlled. .

  Embodiments embodying the present invention will be described below.

  In the first embodiment, an experiment is performed by using a guide wave ultrasonic wave propagating in the longitudinal direction in a steel bar as an example, and a pulse wave compression technique of a guide wave ultrasonic wave having a velocity dispersion, a conventional pulse compression technique, and a conventional burst excitation method A comparison was made.

  An outline of the experimental apparatus is shown in FIG. As the target structure 1, a round bar having a diameter of 32 mm and a length of 2 m was used. A magnetostrictive L-mode guided wave ultrasonic transmission sensor T and reception sensor R are arranged at a position 500 mm from one end E1 of the structure 1. Further, a slit-like defect 1a is provided at a position 500 mm from the other end E2 of the structure 1. The transmission sensor T and the reception sensor R are sensors of a type that preferentially imparts or captures an axially symmetric longitudinal strain to the structure 1 by the magnetostrictive effect of iron, and substantially receives axially symmetric L-mode guided wave ultrasonic waves. It is characterized by being able to excite or receive in a single way.

  FIG. 3 is a group velocity dispersion curve of an axially symmetric mode guided wave ultrasonic wave propagating through the structure 1. In the region below 100 kHz, there are a non-velocity dispersive T (0,1) mode and a dispersive L (0,1) mode. Here, the dispersive L (0,1) mode Is used. Among these, three center frequencies of A: 36 kHz, B: 50 kHz, and C: 64 kHz having different dispersive strengths (dispersion curve slopes) were selected, and the respective results were compared.

  As the excitation time wave s (t) used in the dispersive guide wave ultrasonic pulse compression technique and the conventional pulse compression technique, a chirp signal represented by Expression 10 was used. Note that there are various expressions representing the chirp signal, and the present invention is not limited to the definition of Expression 10.

  The center frequency fc was set to three types of A: 36 kHz, B: 50 kHz, and C: 64 kHz having different dispersibility strengths. The sweep frequency band Bw and time length Tw of the chirp signal were 2fc and 10 / fc, respectively. A Hamming window was used as the window function W (t). Further, as a comparative burst signal, a hamming burst having 2 cycles at each center frequency A to C was used. Here, the semi-analytical finite element method which is a well-known technique according to Non-Patent Document 2 is used to derive the dispersion curve k (ω) of the guide wave ultrasonic wave.

  4 to 6 show the final output results according to the first embodiment. On the other hand, FIGS. 12 to 14 show the final output results by the conventional burst excitation method. 15 to 17 show the final output results by the conventional pulse compression technique. The symbol attached to each waveform indicates the propagation path of the guide wave ultrasonic wave (for example, the symbol “E1E2” indicates the path of T / R → E1 → E2 → T / R).

  In the conventional burst excitation method and the conventional pulse compression technique, the waveform distortion due to the speed dispersion is a conspicuous result. However, in the pulse compression technique of the guide wave ultrasonic wave having the speed dispersion of the first embodiment, the dispersibility is low. Even in the case of strong C: 64 kHz, it is shown that the influence of dispersibility is eliminated, and the spatial resolution is improved as a clean pulse signal.

  Further, it is also shown that noise is removed and the SN ratio is improved in the pulse compression technique of the guide wave ultrasonic wave having the velocity dispersion of the first embodiment as compared with the conventional burst excitation method.

  Next, an embodiment will be described below in the case of using an excitation time wave converted from a predefined excitation spatial wave defined in advance using dispersion information of guide wave ultrasonic waves. Note that the same environment as that of the first embodiment is used for the experimental apparatus and the inspection object.

  As the defined excitation spatial wave s (x), a chirped spatial wave represented by Expression 11 was used. The center wave number is kc, the wave number bandwidth is Kw, the space width is Lw, and the window function is W (x). Note that there are various expressions representing the chirp signal, and the definition of Expression 11 is not limited.

  Here, the center wavelength λc (= 2π / kc) is 70 mm, the wave number bandwidth Kw of the chirped spatial wave is 2 kc, and the spatial width Lw is 29 λc (about 2 m). A Hanning window was used as the window function W (t).

  Next, the chirp space wave is converted into an excitation time wave using the dispersion information of the guide wave ultrasonic wave. The predefined chirp space wave at this time is shown in FIG. 7, and the converted excitation time wave is shown in FIG. Further, FIG. 9 shows the wave number spectrum of the amplitude of the chirped spatial wave, and FIG. 10 shows the frequency spectrum of the amplitude of the converted excitation time wave. If there is no velocity dispersion in the guide wave ultrasonic wave, the chirp space wave, the converted excitation time wave, and the amplitude spectrum of each will be similar, but in reality, due to the existence of velocity dispersion, the waveform and amplitude spectrum Are very different. When the excitation time wave shown in FIG. 8 is excited, the wave number spectrum of the amplitude with respect to the spatial wave of the guide wave ultrasonic wave propagating through the inspection object becomes a symmetric shape as shown in FIG. FIG. 11 shows the final output result (spatial wave pulse compression waveform) when the guide wave ultrasonic wave is excited using the converted excitation time wave.

  The structure non-destructive diagnosis method of the present invention can be used for periodic inspection of structures such as pipes, plates, and bars.

It is the time-space diagram which showed the mode of propagation of a guide wave ultrasonic wave. It is a model side view which shows the outline | summary of the experimental apparatus of Example 1,2. In Example 1, 2, it is a group velocity dispersion curve of the guide wave ultrasonic wave which propagates in the iron bar. It is a final output result in the case where the center frequency is 36 kHz according to the first embodiment. It is a final output result in the case where the center frequency is 50 kHz according to the first embodiment. It is a final output result in the case where the center frequency is 64 kHz according to the first embodiment. It is a waveform of a defined chirp space wave according to the second embodiment. 6 is a waveform of a converted excitation time wave according to the second embodiment. It is a wave number spectrum of the amplitude of a defined chirp space wave according to the second embodiment. It is a frequency spectrum of the amplitude of the converted excitation time wave according to the second embodiment. It is a last output result at the time of exciting the converted excitation time wave concerning Example 2. FIG. This is a final output result when the center frequency is 36 kHz according to the conventional burst excitation method. This is the final output result when the center frequency is 50 kHz according to the conventional burst excitation method. This is the final output result when the center frequency is 64 kHz according to the conventional burst excitation method. This is the final output result when the center frequency is 36 kHz according to the conventional pulse compression technique. This is a final output result when the center frequency is 50 kHz according to the conventional pulse compression technique. This is a final output result when the center frequency is 64 kHz according to the conventional pulse compression technique.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Structure 1a ... Defect T ... Transmission sensor R ... Reception sensor

Claims (4)

A non-destructive diagnostic method for a structure that diagnoses a defect of a structure non-destructively using guided wave ultrasonic waves having velocity dispersion,
An excitation step of exciting the guided wave ultrasonic wave in the structure by an excitation time wave having a broadband waveform;
A receiving step of receiving the guided wave ultrasonic wave propagating through the structure;
Based on the reception time wave obtained by the reception step, the excitation time wave used in the excitation step, and the velocity dispersion information on the guide wave ultrasonic wave, the spatial distribution of the defects in the structure, A spatial wave pulse compression calculation step for calculating a spatial wave pulse compression waveform corresponding to a convolution waveform with an autocorrelation function of the excitation spatial wave, which is a spatial distribution of the excitation time wave;
A display step of displaying the spatial wave pulse compression waveform on the screen ,
The spatial wave pulse compression calculating step obtains a time Fourier transform signal of the received time wave by time Fourier transforming the received time wave, and a time Fourier transform signal of the excitation time wave by time Fourier transforming the excitation time wave. Seeking
A spatial Fourier based on the received time wave from the time Fourier transform signal of the received time wave by performing variable conversion between angular frequency and wave number using the velocity dispersion information on the guide wave ultrasonic wave. Obtaining a transformed signal, obtaining a spatial Fourier transform signal based on the excitation time wave from the time Fourier transform signal of the excitation time wave;
Next, the spatial wave pulse compression is performed by performing a spatial inverse Fourier transform on the product of the complex conjugate signal of the spatial Fourier transform signal based on the excitation time wave and the spatial Fourier transform signal based on the reception time wave. nondestructive diagnostic method of the structure which is characterized that you calculate the waveform.   The non-destructive diagnosis method for a structure according to claim 1, wherein a chirp time wave is used as the excitation time wave in the excitation step.   In the excitation step, the excitation time wave is obtained by converting a predefined excitation spatial wave having a broadband property with respect to the wave number according to the velocity dispersion information on the guide wave ultrasonic wave. A nondestructive diagnosis method for a structure according to Item 1. The non-destructive diagnosis method for a structure according to claim 3, wherein a chirped spatial wave is used as the predefined excited spatial wave in the excitation step.

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