Ultrasonic inspection method

Abstract

To provide an ultrasonic inspection method that can reduce the influence of noise without requiring the application of a special device configuration, measurement method, or data processing method, or the use of lower frequency ultrasonic waves.SOLUTION: The ultrasonic inspection method includes a measurement step of irradiating a material to be inspected with an ultrasonic wave and acquiring the change in the intensity of the reflected ultrasonic wave over time as an inspection signal; a gate setting step of setting a gate in a region of the inspection signal where the contribution of reflection or scattering inside the material to be inspected is greater than the contribution of reflection at the interface between the material to be inspected and the external space; a frequency analysis step of frequency-analyzing the inspection signal within the gate to obtain a frequency distribution showing the amplitude for each frequency, and then identifying the peak frequency at which the amplitude is maximized in the frequency distribution; a reference waveform generation step of generating a reference waveform that has a time width shorter than that of the gate and oscillates at the peak frequency; and a convolution step of convolving the inspection signal within the gate with the reference waveform.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an ultrasonic inspection method, and more particularly to an ultrasonic inspection method for performing ultrasonic inspection while reducing the influence of noise. [Background technology]

[0002] Ultrasonic testing is widely used for various inspections, including flaw detection to detect internal defects in materials such as metals. To improve the detectability of ultrasonic testing, it is important to reduce the effects of noise. To reduce noise in ultrasonic testing, improvements and studies have been made on the ultrasonic probes used for testing, such as the applicable frequency, the type and size of the transducer, and the focus configuration. In addition to the commonly used pulse-echo method, new ultrasonic testing methods, such as the phased array method and nonlinear ultrasonic method, have also been proposed.

[0003] Furthermore, signal processing of ultrasound signals obtained during an inspection can reduce the effects of noise and thereby improve detectability. An example of such signal processing is the application of a noise filter. Another type of signal processing method proposed is pulse compression. In the pulse compression method, pulse ultrasound modulated with a chirp wave or the like is used as the incident wave, and the detection wave is demodulated into a pulse wave. The pulse compression method requires special equipment to modulate the incident wave, but a technology that eliminates this modulation is also disclosed, for example, in Patent Document 1. In Patent Document 1, a pulse wave is generated as the incident wave, and waveform data derived from ultrasound reflected or scattered by the object is used. The waveform data is obtained by multiple inspections under the same inspection conditions or by duplication. The modulation process modulates the waveform data using a phase modulation method or a linear frequency modulation method to generate composite waveform data, and the demodulation process demodulates the composite waveform data to generate compressed waveform data. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-85811 Summary of the Invention [Problem to be solved by the invention]

[0005] As described above, various methods have been proposed to reduce the effects of noise and improve detectability in ultrasonic testing. However, many of these methods require specialized probes, flaw detection methods, and data processing methods, making them difficult to generalize. The method of Patent Document 1 also differs from general testing methods in that it requires multiple test results under the same conditions. Furthermore, when flaw detection is performed as an ultrasonic test, noise can arise not only from accidental noise caused by external disturbances, but also from noise caused by minute material structures, such as scattering of ultrasonic waves by grain boundaries in metal crystal structures. Such noise caused by material structures has frequencies close to the reflected and scattered signals from defects to be detected by flaw detection, making it often difficult to distinguish between the two. Lowering the frequency of the ultrasonic waves used can reduce noise caused by material structures, but this makes it difficult to detect minute defects.

[0006] The problem to be solved by the present invention is to provide an ultrasonic inspection method that can reduce the influence of noise without requiring the application of special device configurations, measurement methods, or data processing methods, or the use of lower frequency ultrasonic waves. [Means for solving the problem]

[0007] In order to solve the above problems, the ultrasonic inspection method according to the present invention has the following configuration. [1] The ultrasonic inspection method of the present invention includes a measurement step of irradiating an ultrasonic wave onto a material to be inspected and acquiring the change in the intensity of the reflected ultrasonic wave over time as an inspection signal; a gate setting step of setting a gate in a region of the inspection signal where the contribution of reflection or scattering within the material to be inspected is greater than the contribution of reflection at the interface between the material to be inspected and the external space; a frequency analysis step of frequency-analyzing the inspection signal within the gate to obtain a frequency distribution showing the amplitude for each frequency, and then identifying the peak frequency at which the amplitude is maximized in the frequency distribution; a reference waveform generation step of generating a reference waveform having a time width shorter than that of the gate and oscillating at the peak frequency; and a convolution step of convolving the inspection signal within the gate with the reference waveform.

[0008] [2] In the aspect of [1] above, in the reference waveform generating step, the reference waveform may be generated as a burst sine wave oscillating at the peak frequency.

[0009] [3] In the aspect of [2] above, in the reference waveform generation step, the phase of the sine wave oscillating at the peak frequency may be shifted by π / 2, and the waveform extracted for a half-integer period may be multiplied by a Gaussian function to obtain the reference waveform.

[0010] [4] In any one of the above aspects [1] to [3], the measurement process is carried out as a flaw detection process, and if there is an area in the waveform obtained in the convolution process where a larger change in intensity occurs than in the surrounding areas, the change in intensity can be considered to be caused by an internal defect in the test material. [Effects of the Invention]

[0011] In the ultrasonic inspection method according to the present invention, which has the configuration described above in [1], frequency analysis is performed on the inspection signal within the gate, which includes contributions from reflections or scattering within the material under test. A reference waveform oscillating at the peak frequency in the resulting frequency distribution is generated, and the inspection signal is then convolved with the reference waveform. The waveform obtained by this convolution emphasizes components oscillating at the same frequency as the reference waveform, i.e., components corresponding to the peak frequency, compared to the original inspection signal. This relatively reduces the influence of noise, including components with frequencies other than the peak frequency and randomly occurring components. By using the convolved waveform instead of the inspection signal for analysis, the detectability of ultrasonic inspections such as flaw detection can be improved. This ultrasonic inspection method can be implemented by performing frequency analysis, generating a reference waveform, and convolution on inspection signals obtained by conventional pulse-echo techniques, etc., and does not require any special equipment configuration, measurement method, or data processing method. The analysis results are less susceptible to arbitrariness by the analyst. Furthermore, since it can be applied regardless of the frequency of the ultrasound used for measurement, it is less susceptible to problems that occur in specific frequency ranges, such as the difficulty of detecting minute defects due to lower inspection frequencies.

[0012] In the above embodiment [2], the reference waveform is generated as a burst sine wave oscillating at the peak frequency of the frequency distribution. In this case, the convolution process using the reference waveform can effectively emphasize the contribution of the component oscillating at the peak frequency.

[0013] In the above-mentioned aspect [3], the reference waveform is generated by adding a phase shift to a sine wave oscillating at the peak frequency, extracting the resulting waveform, and multiplying it by a Gaussian function. This allows for the easy generation of a burst sine wave-like reference waveform. It also makes it easier to eliminate arbitrariness in the generation of the reference waveform and to automate it.

[0014] In the above embodiment [4], the measurement process is performed as a flaw detection process, and if the waveform obtained in the convolution process shows a change in intensity that is greater than that of the surrounding area, it is considered to be caused by a defect inside the material being tested. In other words, information regarding the presence or absence of a defect and its depth location can be obtained based on the waveform that has undergone the convolution process. In flaw detection testing, signals caused by material structure, such as scattering by crystal grain boundaries in the metal structure, act as noise. However, by applying the method of the present invention, the influence of noise caused by material structure can be reduced, thereby improving the detectability of flaw detection testing. [Brief explanation of the drawings]

[0015] [Figure 1] 1A and 1B are diagrams illustrating an ultrasonic inspection method according to one embodiment of the present invention. (a) shows the state in which a gate setting step is performed on an inspection signal obtained in a measurement step. (b) shows the frequency distribution obtained in a frequency analysis step. (c) shows the reference waveform generated in a reference waveform generation step. (d) shows the processed waveform obtained in a convolution integration step. [Figure 2] An example of applying the ultrasonic inspection method described above is shown below. The processed waveforms obtained from the inspection signals in (a) and (c) are shown in (b) and (d), respectively. DETAILED DESCRIPTION OF THE INVENTION

[0016] An ultrasonic inspection method according to an embodiment of the present invention will be described below. The ultrasonic inspection method according to this embodiment includes a measurement step and subsequent waveform processing. The waveform processing involves performing a gate setting step, a frequency analysis step, a reference waveform generation step, and a convolution step in this order to reduce the influence of noise in the inspection signal obtained in the measurement step. The specific type of inspection performed in the ultrasonic inspection method according to this embodiment is not particularly limited, but the following description will be given taking a flaw detection inspection as an example. Each step of the ultrasonic inspection method according to this embodiment will be described in order below.

[0017] (1) Measurement process In the measurement process, ultrasonic waves are applied to the material to be inspected, and the time change in the intensity of the reflected ultrasonic waves is acquired as an inspection signal. Figure 1(a) shows an example of an inspection signal acquired in the measurement process.

[0018] The ultrasonic wave incidence and detection in the measurement process can be performed using the measurement devices and methods used in conventional ultrasonic testing; the details of the measurement devices and methods are not particularly limited. Here, we assume that ultrasonic flaw detection testing is performed using the pulse-echo method. That is, a probe that transmits and receives pulsed ultrasonic waves is used to irradiate pulsed ultrasonic waves into the material under test, and the ultrasonic waves reflected by the material under test are acquired as an inspection signal. The pulsed ultrasonic waves contain frequency components across a certain frequency range. While the frequency range is not particularly limited, for example, frequencies within the range of 0.5 to 75 MHz can be suitably applied. The inspection signal shown in Figure 1(a) is also obtained using the pulse-echo method. When performing flaw detection, the measurement process can be performed at multiple positions on the surface of the material under test as appropriate. In this case, the inspection signals are stored in association with each position and are each subjected to subsequent waveform processing independently.

[0019] (2) Gate setting process In the gate setting process, a gate to be subjected to waveform processing is set for the inspection signal obtained in the measurement process. Specifically, the gate is set in a region of the inspection signal where the contribution of reflection or scattering within the test material is greater than the contribution of reflection at the interface between the test material and the external space (e.g., atmosphere). Preferably, the gate is set in a region where the contribution of reflection at the interface between the test material and the external space can be ignored. Figure 1(a) shows an example of a gate to be set, indicated by a double arrow. In this example, the region of ultrasonic intensity with large amplitudes that exists before the set gate corresponds to the reflection of the incident ultrasonic waves from the surface of the test material. Also, although not shown in the figure, there is a region with similarly large amplitudes after the set gate, and this region corresponds to the reflection of ultrasonic waves from the surface of the test material opposite the incident side, i.e., the bottom surface. By setting a gate that excludes the region corresponding to the reflection at the interface and subjecting it to subsequent processing, the behavior of relatively small amplitude ultrasonic waves that exist within the gate region can be analyzed.

[0020] (3) Frequency analysis process In the frequency analysis process, frequency analysis is performed on the inspection signal within the gate set above to obtain a frequency distribution. The frequency distribution indicates the amplitude of ultrasonic waves for each frequency and can be obtained using known frequency analysis methods such as Fourier transform methods such as fast Fourier transform (FFT) and short-time Fourier transform (STFT), or wavelet transform methods. There are no particular restrictions on the frequency analysis method used, but when acquiring multiple inspection signals, such as in flaw detection testing, it is preferable to apply STFT or wavelet transform methods. As an example, Figure 1(b) shows the frequency distribution obtained by extracting the gate region of the inspection signal in Figure 1(a) and performing FFT.

[0021] The frequency analysis step further detects a peak frequency in the obtained frequency distribution. The peak frequency is the frequency at which the amplitude is maximum in the frequency distribution. In the frequency distribution of Figure 1(b), as indicated by the bar at the top, the peak frequency exists at approximately 20 MHz. The peak frequency may be identified after applying processing such as smoothing to the frequency distribution as appropriate.

[0022] (4) Reference waveform generation process Once the peak frequency has been identified, a reference waveform generation process is then performed based on the peak frequency information to generate a reference waveform to be used in the subsequent convolution integration process. Specifically, a waveform having a time width shorter than the gate set in the inspection signal and oscillating at the peak frequency is generated as the reference waveform. Figure 1(c) shows an example of a reference waveform. In this example, the reference waveform has a time width of 0.125 μs and oscillates at a frequency of 20 MHz, which is the peak frequency identified in Figure 1(b).

[0023] While the specific shape of the reference waveform is not particularly limited, a preferred form is a burst sine wave-like waveform oscillating at a predetermined peak frequency, i.e., a waveform oscillating sinusoidally at the peak frequency for only a short period of time, as shown in Figure 1(c). This allows for effective noise reduction in the subsequent convolution process. A burst sine wave can be generated based on a burst wave. One specific method for generating a burst sine wave-like reference waveform is to shift the phase of a sine wave oscillating at the peak frequency by π / 2, extract a waveform equivalent to half an integer period (n+1 / 2 periods, where n is an integer greater than or equal to 0), and multiply the resulting waveform by a Gaussian function. This allows for the easy generation of a burst sine wave-like reference waveform that decays smoothly and symmetrically on both sides along the time axis. The number of periods (n+1 / 2) of the sine wave is not particularly limited, and a preferred example is approximately 1.5 to 5.5. The waveform shown in Figure 1(c) is 2.5 periods. If the frequency distribution has a plurality of peak frequencies, the reference waveform may be generated by combining waveforms that oscillate at the plurality of peak frequencies.

[0024] (5) Folding process In the convolution process, the inspection signal (f(t)) in the area within the gate set in the gate setting process is convolved with the reference waveform (g(t)) generated above. In other words, the processed waveform h(t) is obtained by calculating h(t) = f(t) * g(t). As an example, Figure 1(d) shows the processed waveform obtained by convolving the inspection signal in Figure 1(a) with the reference waveform in Figure 1(c).

[0025] By performing this convolution, the intensity of components in the test signal that have the same periodicity as the reference waveform, i.e., components oscillating at the peak frequency of the frequency distribution, is relatively increased and assumes a large value. This is because such components have a high correlation with the reference waveform. On the other hand, components oscillating at other frequencies or components that do not have periodicity assume only small intensities. As a result, in the resulting processed waveform, the components oscillating at the peak frequency are emphasized more than in the original test signal. In the example shown in Figure 1, the oscillation component indicated by the symbol S is emphasized in the processed waveform (d) compared to the surrounding noise in the test signal (a). In other words, the S / N ratio is improved. The period of this emphasized oscillation is approximately 0.05 μs, which corresponds to the peak frequency of 20 MHz.

[0026] By performing the convolution process using the reference waveform in this way, a processed waveform can be obtained that reduces the influence of noise in the inspection signal. Therefore, the obtained processed waveform can be used instead of the inspection signal to subsequently perform the analysis process desired in the ultrasonic inspection. For example, if the measurement process is performed as a flaw detection process, the obtained processed waveform can be used as the target for flaw detection analysis. Specifically, if there is a region in the processed waveform where a larger intensity change occurs than in the surrounding region (time region on the waveform), the intensity change can be considered to be caused by a defect inside the inspected material. In other words, a flaw detection inspection can be performed on the surface of the inspected material, and the presence of a defect can be identified within the position where the inspection signal was acquired. Furthermore, by converting the time during which the intensity change occurs in the processed waveform into distance based on the ultrasonic velocity, the depth location of the defect in the inspected material can be identified. By performing each process from the gate setting process to the convolution process on inspection signals obtained at multiple positions on the surface of the inspected material to obtain a processed waveform and then analyzing the processed waveform, flaw detection inspection can be performed over a wide area of ​​the inspected material with high detectability.

[0027] By using a processed waveform in which the influence of noise is reduced compared to the inspection signal as the object of analysis, it is possible to improve the detectability of ultrasonic testing, such as the accuracy and sensitivity of flaw detection testing. Furthermore, the ultrasonic testing method according to this embodiment performs waveform processing on inspection signals acquired by any method, such as the conventional pulse-echo method, through each step from the gate setting step to the convolution step. This method can reduce the influence of noise without requiring a special device configuration, measurement method, or data processing method, and is therefore highly versatile. Furthermore, since the noise reduction process does not require a new measurement step and processing is performed using a reference waveform generated based on the frequency distribution of the inspection signal itself to be analyzed, it can be applied not only to inspection signals acquired by performing a new measurement step, but also to inspection signals that have already been acquired and stored.

[0028] In the ultrasonic inspection method according to this embodiment, as long as a peak frequency can be identified from the frequency distribution, a reference waveform can be generated using that peak frequency and then convolution integrated, thereby reducing the influence of noise. Therefore, this method can be applied regardless of the frequency of the ultrasonic waves used in the measurement process, the frequency corresponding to the phenomenon to be detected (e.g., the frequency of a signal originating from a defect in a flaw detection test), or the frequency of noise (e.g., the frequency corresponding to scattering by a grain boundary in a flaw detection test). In conventional flaw detection tests, the frequency of the ultrasonic waves used for measurement is sometimes lowered to reduce the influence of noise. However, when using the inspection method according to this embodiment, such frequency lowering is not necessary. By performing flaw detection tests without excessively lowering the frequency of the ultrasonic waves, even small defects can be easily detected. For example, the method according to this embodiment is particularly suitable for flaw detection in a frequency range higher than that required for flaw detection tests, such as 20 MHz or higher.

[0029] In the ultrasonic inspection method according to this embodiment, if the step of forming a reference waveform based on the peak frequency in the reference waveform generation step can be automated, all waveform processing can be performed based on automatic calculations. Therefore, even when a large number of inspection signals are obtained consecutively, such as when an inspection signal is obtained at each inspection position in a flaw detection inspection, automatic processing can quickly obtain processed waveforms. The generation of the reference waveform can also be easily automated using the above-described method of generating a burst sine wave-like waveform by shifting and extracting the phase of a sine wave and multiplying it by a Gaussian function. Automating the generation of the reference waveform in this way also eliminates arbitrariness in data processing.

[0030] In the above, an example has been described in which the measurement process is performed as a flaw detection process using a simple pulse-echo method. However, other flaw detection techniques may be combined with the measurement process. Furthermore, ultrasonic testing may involve testing other than flaw detection. For example, testing of the state of the crystalline structure, such as grain size, may be performed. In this case, the state of the crystalline structure is analyzed by analyzing components of the inspection signal that are caused by the structure of the crystalline structure, such as scattering from the grain boundaries of the material being tested. In this case, the noise components whose influence should be reduced through the processes from the gate setting process to the convolution process are noise components other than those caused by the structure of the crystalline structure, such as noise due to disturbances from the device. [Example]

[0031] Examples of the present invention are shown below. However, the present invention is not limited to these examples. Here, it was actually verified whether the influence of noise can be reduced by using the ultrasonic inspection method according to the embodiment of the present invention.

[0032] [Test method] The test material was a steel material with artificial defects created in it, and a flaw detection test using the pulse-echo method was performed to obtain test signals. Two types of test signals with different noise intensities were prepared. Figure 2(a) shows test signal 1, and Figure 2(c) shows test signal 2. Test signal 2 contains noise with greater intensity than test signal 1. The noise intensity of these test signals was changed by varying the state of the crystalline structure of the steel material and the ultrasonic frequency. Specifically, test signal 2 was targeted at steel material containing coarse crystalline structures. The ultrasonic frequencies (center frequencies) used were 20 MHz for test signal 1 and 10 MHz for test signal 2.

[0033] As explained above, the gate setting process, frequency analysis process, reference waveform generation process, and convolution process were performed on each of test signal 1 and test signal 2 to obtain a processed waveform. In the gate setting process, a gate was set in the area indicated by the double arrow in Figure 2. In the reference waveform generation process, the phase of a sine wave oscillating at the peak frequency was shifted by π / 2, and 2.5 periods of the waveform were extracted, similar to the waveform shown in Figure 1(c), and multiplied by a Gaussian function to obtain the reference waveform.

[0034] [Test Results] Figure 2 shows the test signals used for processing and the processed waveforms obtained from those test signals. (a) is test signal 1, and (b) is processed waveform 1 obtained from test signal 1. (c) is test signal 2, and (d) is processed waveform 2 obtained from test signal 2.

[0035] Comparing (a) Test Signal 1 with (b) Processed Waveform 1, and (c) Test Signal 2 with (d) Processed Waveform 2, it is clear that in both cases, waveform processing has made the vibration intensity, indicated by "S" in the figure, larger relative to the surrounding noise. The quantitative results of the S / N ratio are shown in the lower right corner of each figure, and these values ​​also confirm that waveform processing improves the S / N ratio. This demonstrates that the effects of noise in test signals can be reduced by performing waveform processing that includes the gate setting process, frequency analysis process, reference waveform generation process, and convolution process.

[0036] Because noise reduction effects were observed for both test signal 1 and test signal 2, which have different levels of noise, it can be said that the above waveform processing is effective regardless of the level of noise. Comparing the noise reduction effects, test signal 1, which had little noise before processing, saw an improvement in the S / N ratio of 37% after processing, while test signal 2, which had a high level of noise before processing, saw an improvement of more than 140% in the S / N ratio. From this, it can be said that applying waveform processing to test signals that are heavily affected by noise can achieve particularly significant noise reduction effects.

[0037] Although the embodiments of the present invention have been described above, the present invention is not limited to these embodiments and various modifications can be made.

Claims

1. a measuring step of irradiating an ultrasonic wave onto a test material and acquiring a time change in the intensity of the reflected ultrasonic wave as an inspection signal; a gate setting step of setting a gate in a region of the inspection signal where the contribution of reflection or scattering within the inspection material is greater than the contribution of reflection at the interface between the inspection material and an external space; a frequency analysis step of performing a frequency analysis on the inspection signal within the gate to obtain a frequency distribution indicating an amplitude for each frequency, and then identifying a peak frequency at which the amplitude is maximum in the frequency distribution; a reference waveform generating step of generating a reference waveform having a time width shorter than that of the gate and oscillating at the peak frequency; and a convolution step of convolving the inspection signal within the gate with the reference waveform.

2. 2. The ultrasonic inspection method according to claim 1, wherein in the reference waveform generating step, the reference waveform is generated as a burst sine wave oscillating at the peak frequency.

3. 3. The ultrasonic inspection method according to claim 2, wherein in the reference waveform generating step, the phase of a sine wave oscillating at the peak frequency is shifted by π / 2, and a waveform extracted for a half integer period is multiplied by a Gaussian function to obtain the reference waveform.

4. The measurement step is carried out as a flaw detection step, 4. The ultrasonic inspection method according to claim 1, wherein, when there is a region in the waveform obtained in the convolution process in which a change in intensity is greater than that in surrounding regions, the change in intensity is deemed to be caused by a defect inside the material being inspected.

Images