Ultrasonic flaw detection method

Abstract

To provide an ultrasonic flaw detection method that has improved flaw detection accuracy by eliminating the influence of scattering noise due to crystal grains.SOLUTION: An ultrasonic flaw detection method includes irradiating an object under test 2 with a flaw detection ultrasonic wave Ps by a vibrator 1 and receiving a reflected wave with the vibrator 1, separating from each other a positive-side waveform and a negative-side waveform of a reception signal Sr obtained by the vibrator 1, inverting the negative-side waveform Srn and shifting the negative-side waveform Srn by half the wavelength of the flaw detection ultrasonic wave Ps on a time axis, superimposing it on the positive-side waveform Srp, averaging a resultant waveform, and obtaining a flaw signal portion Sd.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an ultrasonic flaw detection method, and in particular to an ultrasonic flaw detection method that aims to improve flaw detection accuracy by eliminating the influence of scattering noise caused by crystal grains. [Background technology]

[0002] In ultrasonic testing, ultrasonic waves are emitted from the transducer of a flaw detection probe toward the test object, and the presence of a defect is detected when the ultrasonic waves are reflected by a defect in the test object and received back by the transducer, and a defect signal above a predetermined level appears in the received signal.

[0003] However, when the test object is a polycrystalline body such as steel, the ultrasonic wave for flaw detection is scattered by the crystal grains, resulting in scattering noise in the received signal, which reduces the S / N ratio of defect detection. Therefore, the ultrasonic wave for flaw detection is made to have a long wavelength and a low frequency to suppress scattering by the crystal grains, but this makes it difficult to detect small defects.

[0004] Therefore, a phased array probe equipped with multiple transducers is used, and the effects of scattering noise are reduced by phasing and combining the received signals received by each transducer after the ultrasonic waves incident from the multiple transducers are reflected within the test object.

[0005] Patent Document 1 proposes a method for ultrasonic flaw detection using a phased array probe in which the area where a defect exists is identified by the first inspection, and then only the identified area is re-inspected the second time, thereby enabling highly accurate defect detection in a short period of time. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2011-2360 Summary of the Invention [Problem to be solved by the invention]

[0007] However, even in flaw detection methods using phased array probes, each of the multiple flaw detection ultrasonic waves is affected by crystal grains, causing a slight change in sound speed, making it difficult to achieve sufficient phase matching, and a decrease in the S / N ratio due to scattering noise may be unavoidable.

[0008] Therefore, the present invention is devised to solve such problems, and has an object to provide an ultrasonic flaw detection method that eliminates the influence of scattering noise caused by crystal grains and improves flaw detection accuracy. [Means for solving the problem]

[0009] In order to achieve the above object, the first invention is characterized in that a flaw detection ultrasonic wave (Ps) is transmitted from a transducer (1) to a test object (2) and the reflected wave is received by the transducer (1), the positive and negative waveforms of the received signal (Sr) obtained by the transducer (1) are separated, the positive and negative of one of the separated waveforms (Srn) are inverted, and the inverted one of the waveforms (Srn) is sequentially shifted in one direction, either the positive or negative direction, on the time axis at least once by half the wavelength of the flaw detection ultrasonic wave (Ps), and each time it is superimposed on the other separated waveform (Srp), and these are averaged to obtain a flaw signal portion (Sd).

[0010] According to the first invention, the flaw signal part of the received signal has the same frequency as the frequency of the flaw detection ultrasonic wave, so when one inverted waveform is shifted by half a wavelength and superimposed on the other waveform, the phases of the two waveforms match in the flaw signal part, so they reinforce each other. On the other hand, the frequency of the scattered noise part is random, so the phases of the one inverted waveform and the other waveform rarely match and reinforce each other. This greatly improves the S / N ratio of the flaw signal part, eliminating the effect of scattered noise due to crystal grains and improving flaw detection accuracy.

[0011] In the second invention, a flaw-detection ultrasonic wave is transmitted from a transducer to a test object and the reflected wave is received by the transducer. The received signal obtained by the transducer is separated into a positive waveform and a negative waveform. The positive and negative signs of one of the separated waveforms are inverted, and the inverted one of the waveforms is sequentially shifted in two directions, the positive and negative directions, on the time axis by at least one half-wavelength of the ultrasonic wave. Each of the waveforms shifted in the two directions is superimposed on the other waveform and these are averaged to obtain a flaw signal portion.

[0012] According to the second invention, one waveform is shifted in two directions, the positive direction and the negative direction, on the time axis, and each of the waveforms shifted in the two directions is superimposed on the other waveform and averaged to obtain a flaw signal portion. This further improves the S / N ratio of the flaw signal portion and further improves the flaw detection accuracy by eliminating the effects of scattering noise due to crystal grains.

[0013] In the third invention, a plurality of the transducers (61) are provided, and a composite received signal (CSr) is obtained from the received signal (Sr) obtained by each transducer (61). The composite received signal (CSr) is then subjected to the following processes to obtain a composite flaw signal (CSd): separation of the waveform of the positive portion and the waveform of the negative portion, inversion of the positive and negative of one of the separated waveforms, shifting the inverted waveform on the time axis, superimposition on the other waveform, and averaging.

[0014] According to the third aspect of the present invention, the present invention can be applied to flaw detection using a phased array probe equipped with a plurality of transducers.

[0015] The reference characters in parentheses indicate, for reference, the correspondence with specific means described in the embodiments described later. Effect of the Invention

[0016] As described above, according to the ultrasonic flaw detection method of the present invention, it is possible to eliminate the influence of scattering noise due to crystal grains and to significantly improve the flaw detection accuracy. [Brief description of the drawings]

[0017] [Figure 1]1 is a cross-sectional view illustrating an ultrasonic flaw detection method according to a first embodiment of the present invention. [Diagram 2] FIG. 4 is a waveform diagram of a received signal. [Diagram 3] FIG. 4 is a partially enlarged waveform diagram of a received signal. [Figure 4] 4 is a conceptual waveform diagram illustrating signal processing within the flaw detection device. FIG. [Diagram 5] 4 is a conceptual waveform diagram illustrating signal processing within the flaw detection device. FIG. [Figure 6] 4 is a conceptual waveform diagram of a flaw detection signal obtained as a result of signal processing. FIG. [Figure 7] 4 is a waveform diagram of a flaw detection signal obtained by signal processing of a received signal. FIG. [Figure 8] 1 is a cross-sectional view illustrating an ultrasonic flaw detection method according to a first embodiment of the present invention. [Figure 9] FIG. 4 is a waveform diagram of a composite received signal. [Figure 10] 4 is a conceptual waveform diagram illustrating signal processing within the flaw detection device. FIG. [Figure 11] FIG. 4 is a waveform diagram of a flaw detection signal. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] It should be noted that the embodiment described below is merely an example, and various design improvements made by those skilled in the art without departing from the gist of the present invention are also included in the scope of the present invention.

[0019] (First embodiment) An example of a flaw detection method using an ultrasonic probe consisting of a single transducer is shown in Figure 1. In Figure 1, a thick plate-like specimen 2 placed in water has an internal defect 3, and the defect 3 is detected by an ultrasonic probe 1 placed above the surface of the specimen 2 at a specified water distance.

[0020] That is, the transducer of the ultrasonic probe 1 is excited by an excitation signal output from the flaw detector 4, and a pulsed flaw detection ultrasonic wave Ps is emitted toward the flaw 3. The flaw detection ultrasonic wave Ps is reflected by the flaw 3 and returns to the ultrasonic probe 1, where it is received. An example of the received signal is shown in Figure 2. In this case, the transducer size of the ultrasonic probe 1 is 6 mm, the ultrasonic frequency is 10 MHz, and the water distance is approximately 10 mm.

[0021] 2, a surface reflection part Sur and a bottom reflection part Sbr appear due to the reflected waves from the surface and bottom of the specimen 2, and a flaw signal part Sd appears between them due to the reflected waves from the flaw. And before and after the flaw signal part Sd, there appear scattering noise parts Sn where the flaw detection ultrasonic waves are scattered and reflected by the crystal grains and returned to the ultrasonic probe 1. The S / N ratio of the flaw signal part in this case was 2.54.

[0022] The received signal Sr obtained by the ultrasonic probe 1 is input to the flaw detector 4, which performs signal processing as described below. Here, Fig. 3 shows an enlarged waveform of the received signal Sr near the flaw signal portion Sd. The flaw detector 4 separates the positive part waveform (positive side waveform) Srp and the negative part waveform (negative side waveform) Srn of the received signal Sr (Fig. 4), inverts the separated negative side waveform Srn to the positive side, and shifts the inverted waveform Srn in the positive direction by half the wavelength of the flaw detection ultrasonic Ps on the time axis (Fig. 5), superimposes it on the positive side waveform Srp, and averages (additive averages) to obtain the flaw detection signal Sdd (Fig. 6). In this case, it may be shifted in the negative direction on the time axis.

[0023] In this way, the flaw signal portion Sd of the received signal Sr has a frequency equal to that of the flaw detection ultrasonic wave Ps, so when the inverted negative waveform Srn is shifted by half a wavelength and superimposed on the positive waveform Srp, the phases of the positive waveform Srp and the inverted negative waveform Srn match in the flaw signal portion Sd, so they reinforce each other. On the other hand, the frequency of the scattered noise portion Sn is random, so the phases of the positive waveform Srp and the inverted negative waveform Srn rarely match and reinforce each other. Therefore, by performing the above-mentioned signal processing, the S / N ratio of the flaw signal portion Sd in the flaw detection signal Sdd is greatly improved. The positive waveform Srp may be inverted with respect to the negative waveform Srn.

[0024] Fig. 7 shows the flaw detection signal Sdd after the above-mentioned signal processing is performed on the received signal Sr shown in Fig. 2. The S / N ratio of the flaw signal portion Sd in the signal Sdd is improved from 2.54 before the signal processing to 3.07.

[0025] Second embodiment The flaw detection method of the present invention can also be applied to phased array flaw detection shown in Fig. 8. In Fig. 8, a flat-bottom hole defect 5 is formed on the bottom surface of a thick plate-like test object 2 placed in water, and this defect is detected by a phased array probe 6 provided above the test object 2 at a predetermined water distance. The phased array probe 6 is provided with a plurality of ultrasonic transducers 61 adjacent to and parallel to the surface of the test object 2 at equal intervals. Each transducer 61 is excited with a predetermined delay time by an excitation signal output from the flaw detection device 4, and the composite wavefront of the pulsed flaw detection ultrasonic waves Ps incident from each transducer 61 is focused on the top surface of the defect 5.

[0026] Each flaw detection ultrasonic wave Ps is reflected by the flaw 5 and returns to each transducer 61, where it is received. The received signals obtained by each transducer 61 are appropriately time shifted in the flaw detection device 4 so that the flaw signal portions are synchronized, and are superimposed on each other. The flaw detection device 4 divides the superimposed received signals by the number of signals (arithmetic mean) to obtain a composite received signal CSr (FIG. 9). The flaw signal portion in the composite received signal CSr is then regarded as a composite flaw signal portion CSd, and the presence or absence of a flaw in the specimen 2 is determined based on the magnitude of this signal.

[0027] For example, a phased array probe 6 with 252 channels (252 transducers 11) was installed at a water distance of approximately 50 mm, and the results of inspection for a flat-bottom hole defect 5 with a diameter of 5 mm located at a focus position of 150 mm within the specimen 2 are shown below.

[0028] In the composite reception signal CSr (FIG. 9) obtained by averaging all the reception signals Sr together and dividing them by the number of signals, a composite flaw signal portion CSd appears. Before and after the composite flaw signal portion CSd, there appear scattering noise portions Sn, which are the flaw detection ultrasonic waves Ps scattered and reflected by the crystal grains and returned to each transducer 61. The S / N ratio of the composite flaw signal portion CSd in this case was 3.96.

[0029] As explained in the first embodiment, the positive and negative waveforms of the composite received signal CSr are separated, the separated negative waveform is inverted to the positive side, and the waveform is shifted by half the wavelength of the flaw detection ultrasonic Ps on the time axis, superimposed on the positive waveform, and averaged to obtain the flaw detection signal Sdd. Note that in this embodiment, focusing on the fact that the composite flaw signal portion CSd is symmetrical in the time axis direction, as shown in Fig. 10, the negative waveform inverted to the positive side is shifted by half the wavelength in the positive direction on the time axis to obtain a waveform Srn+, and the waveform Srn- is shifted by half the wavelength in the negative direction on the time axis to obtain a waveform Srn-, which are each superimposed on the original waveform CSd and averaged (additive average) to obtain the flaw detection signal Sdd (Fig. 11).

[0030] This further emphasizes the effect described in the first embodiment that in the flaw signal portion, the positive side waveform and the inverted negative side waveform reinforce each other, while in the scattered noise portion, the phases of the positive side waveform and the inverted negative side waveform hardly reinforce each other. As a result, the S / N ratio of the synthetic flaw signal portion CSd in the flaw detection signal Sdd is greatly improved from 3.96 before signal processing to 5.93.

[0031] (Other embodiments) In the first embodiment, a single transducer ultrasonic probe is used, but a phased array probe having multiple transducers may be used. In the second embodiment, a phased array probe is used, but a single transducer ultrasonic probe may be used.

[0032] When a burst wave is used as the ultrasonic flaw detection wave, the inverted waveform may be shifted by half a wavelength on the time axis not only once as shown in the above embodiments, but also three or more odd numbers of times in sequence to take the average. Of course, in this case, the shift is made in the positive or negative direction on the time axis, or in both directions. Depending on the waveform, the shift may be made an even number of times by half a wavelength. [Explanation of symbols]

[0033] 1...transducer, 2...test object, 6...phased array probe, 61...transducer, CSr...synthetic received signal, CSd...synthetic flaw signal, Ps...flaw detection ultrasound, Sd...flaw signal portion, Sr...received signal, Srn...negative part waveform (one waveform), Srp...positive part waveform (the other waveform).

Claims

1. An ultrasonic flaw detection method comprising the steps of: transmitting flaw detection ultrasonic waves to a test object from a transducer; receiving the reflected waves by the transducer; separating the waveform of a positive portion from that of a negative portion of the received signal obtained by the transducer; inverting the positive and negative signs of one of the separated waveforms; shifting the inverted waveform in one direction, either the positive or negative direction, on the time axis by at least half the wavelength of the flaw detection ultrasonic waves at least once in succession; superimposing the shifted waveform on the other separated waveform each time; and averaging these to obtain a flaw signal portion.

2. An ultrasonic flaw detection method in which a transducer transmits flaw detection ultrasonic waves to a test object and receives the reflected waves by the transducer, separates the waveform of a positive portion from the waveform of a negative portion of the received signal obtained by the transducer, inverts the positive and negative signs of one of the separated waveforms, and sequentially shifts the inverted waveform in two directions, the positive and negative directions, on the time axis by at least one half-wavelength of the flaw detection ultrasonic waves, superimposing each of the waveforms shifted in the two directions on the other waveform and averaging them to obtain a flaw signal portion.

3. 3. The ultrasonic flaw detection method according to claim 1 or 2, wherein a plurality of the transducers are provided, a composite received signal is obtained from the received signals obtained by each transducer, and the composite received signal is processed to separate the waveform of the positive portion and the waveform of the negative portion, invert the positive and negative of one of the separated waveforms, shift the inverted waveform on the time axis, and superimpose and average the inverted waveform on the other waveform to obtain a composite flaw signal portion.

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