JP5112261B2 - Phased array probe and method for determining its specifications - Google Patents

Description

  The present invention relates to a phased array probe in which a large number of minute piezoelectric elements are arranged and a defect of an object to be inspected is detected by ultrasonic waves transmitted and received by the piezoelectric element. The present invention relates to a method for determining the specifications of a phased array probe used for detecting a defect of an object to be inspected.

As described above, the phased array probe detects a defect in the inspection object by arranging a large number of minute piezoelectric elements, performing transmission / reception with the piezoelectric elements, and obtaining an ultrasonic tomographic image. The principle will be described with reference to FIG.
In FIG. 9, a phased array probe 100 has a large number of piezoelectric elements 102 arranged with a minute interval, and generates ultrasonic waves from each piezoelectric element 102. Ultrasonic X originating from the piezoelectric element 102 proceeds in the direction of arrow a to form a combined wavefront X _O. Then, the reflected wave reflected and returned from the defective portion is received and analyzed to obtain a tomographic image, and the defective portion is detected.

As shown in FIG. 9B, by providing a difference in the excitation time of each piezoelectric element 102, the traveling direction of the ultrasonic wave can be changed by an angle θ (flaw detection refraction angle). For example, the detection capability of SCC can be improved by making the incident direction a of the ultrasonic wave perpendicular to the direction of stress corrosion cracking (SCC) generated inside the metal member.
Further, by setting the excitation time of each of the piezoelectric elements 102 as shown in FIG. 9 (c), it is possible to set the focusing position b of the combined wavefront X _O appropriately. By matching the focus position b to the defect portion, the intensity of the ultrasonic wave irradiated to the defect portion can be increased, and the defect detection ability can be improved.

  FIG. 10 schematically shows a state where the SCC 108 is generated in the blade implantation groove 106 into which the blade root 104 is inserted in the steam turbine. When the ultrasonic probe 100 is applied to the outer peripheral surface of the blade implantation groove 106 to detect the SCC 108, the base c1 of the SCC 108 reflects the ultrasonic wave well, so that measurement is possible. However, since the SCC 108 is complicatedly cracked and the directions thereof are various, the measurement is not easy.

In particular, since the depth position h and direction of the distal end portion c2 of SCC108 is unclear, even if an attempt to focus the combined wavefront X _O in tip c2, it is difficult to apply accurately.
Conventionally, when the object to be inspected is thick, from experience, the sound pressure is increased or the frequency is decreased, or the probe size is increased. Conversely, when the object to be inspected is thin, The detection accuracy was improved by decreasing the pressure or increasing the frequency.

However, this cannot be expected to greatly improve the detection accuracy, and is not a fundamental solution. Even if the S / N ratio (ratio of signal to noise) is increased, it is not easy to distinguish the difference between noise and reflected echo.
The specifications of the phased array probe have also been determined empirically from the material of the object to be inspected, the flaw detection depth range (beam path length), the degree of the target defect, and the required detection accuracy.

Japanese Patent Laid-Open No. 2007-151561 discloses an ultrasonic probe in which a phased array probe is inserted into a body cavity of a living body, ultrasonic waves are transmitted and received from the body cavity, and an ultrasonic tomographic image is obtained. Is disclosed.
In Patent Document 1, the relative permittivity of the piezoelectric element 102 is set to 2500 or more, and the ratio w / t of the lateral width w to the thickness t of the piezoelectric element 102 is set to 0.6 or less as shown in FIG. The distance between the piezoelectric elements is less than or equal to the wavelength of the ultrasonic wave, thereby proposing an ultrasonic probe with high electromechanical conversion rate, reduced process difficulty, and improved reliability. .

JP 2007-151561 A

In Patent Document 1, since the dielectric constant is different for each organ or fat layer of a living body, the flaw detection accuracy is stabilized by specifying the dielectric constant of the piezoelectric element within a certain range. However, in the case of an object to be inspected made of a single material, such as a steam turbine blade root, the field of application is different from that of Patent Document 1, and is not effective.
Moreover, the ultrasonic probe disclosed in Patent Document 1 improves the detection accuracy for the case where the crack direction is linear and complicated, and the depth position of the defect is unknown, such as stress corrosion cracking. It is not possible.

  In view of the problems of the prior art, the present invention realizes a phased array probe capable of improving the detection accuracy of a defect having a complicated shape and having an unknown depth position, such as stress corrosion cracking. The purpose is to do.

In order to achieve the above object, the method for determining the specifications of the phased array probe of the present invention includes:
In a method for determining the specifications of a phased array probe that is used to detect a defect of an object to be inspected that is made of a single material and has an unknown depth position, in which a large number of piezoelectric elements are arranged to form a flaw detection part.
The evaluation factor is the ultrasonic focusing diameter representing the ultrasonic focusing degree, and the control factor is the total length and width of the probe, the number of piezoelectric elements, and the curvature of the surface of the flaw detection portion that contacts the object to be inspected. The error factors are the flaw detection depth position and the flaw detection refraction angle.
By designing an experiment, setting a plurality of design values with different values of the error factor and the control factor within the range of the expected fluctuation range of the error factor, calculating the convergence diameter calculated from the design value, Using the calculation result, a design value having a small sensitivity obtained from the average of the focusing diameter and a large SN ratio representing the variation degree of the focusing diameter is estimated, and the value of the control factor is determined based on the confirmation calculated design value. Is to be determined.

In the method of the present invention, the above four factors are selected as control factors by the experimental design method, and are obtained from the average of the calculated focal diameters of ultrasonic waves within the fluctuation range of the flaw detection depth position and the flaw detection refraction angle which are error factors. A design value with a small sensitivity and a large S / N ratio representing the variation degree of the focusing diameter is selected, and the value of the control factor is determined based on the design value.
Thus, with respect to variations in the testing depth position and inspection refractive angle, it can be focused degree of the ultrasound focusing size is small combined wavefront X _O of efficiently obtaining a design value of a good probe, the detection accuracy Therefore, a design that is robust against fluctuations in the flaw detection depth position and the flaw detection refraction angle becomes possible.

  “Experimental design” is a method originally developed by Mr. RAFisher in the United Kingdom for rationalizing farm experiments. In the world of technical research, Mr. Genichi Taguchi uses “orthogonal tables” The introduction of the signal-to-noise ratio and examples of application to various experiments have been widely introduced, so it has become popular, and is also known overseas as the “TAGUCHI METHOD” (Taguchi method).

  The Taguchi method is characterized by an experiment using an “orthogonal table” and an evaluation method based on variation (error) called an “SN ratio”. Here, the "orthogonal table" is a table that represents a combination of parameters to be tested, and the tester only needs to select the parameters to be tested and select the "orthogonal table" prepared in advance according to the parameters. The number of experiments and test conditions are determined, and the experiment can be carried out accordingly.

In addition, by applying a statistical analysis process called “variance analysis” to the data obtained as a result, the magnitude of the effect of each parameter can be evaluated as a numerical value. Furthermore, this method can also obtain an estimated value of a characteristic value to be optimized.
In the method of the present invention, the four control factors are selected as parameters, and the Taguchi method can be used to select a control factor value that is robust to variations in flaw detection depth and flaw detection refraction angle, which are error factors. It is a thing.

In the method of the present invention, the design value that minimizes the difference (variation width) of the formula (μ ± σ) composed of the average value μ of the focusing diameter calculated from the set design value and the standard deviation σ of the focusing diameter And the value of the control factor may be determined based on the selected design value.
As a result, it is possible to determine a specification with a high detection accuracy and robustness with respect to the flaw detection depth position and the flaw detection refraction angle, based on the difference of the formula (μ ± σ).

  Therefore, even if a defect to be inspected has a complicated form and a stress corrosion crack whose flaw detection depth is unknown, it can be detected with good detection accuracy.

  Moreover, the specifications of the phased array probe of the present invention are determined by the method of the present invention. Therefore, the detection accuracy can be maintained even for complicated defects such as stress corrosion cracks in which the direction of the crack is not determined, and the detection accuracy does not decrease even if the flaw detection depth or flaw detection refraction angle varies.

  According to the method of the present invention, a specification of a phased array probe used for detecting a defect of an object to be inspected, which is formed of a single material and has an unknown depth position, in which a large number of piezoelectric elements are arranged to form a flaw detection portion. In the determination method, the evaluation factor is an ultrasonic focusing diameter representing the ultrasonic focusing degree, and the control factor is the total length and width of the flaw detection unit, the number of piezoelectric elements, and the surface of the flaw detection unit that contacts the object to be inspected The error factor is the flaw detection depth position and the flaw detection refraction angle, and a plurality of error factors and control factors having different values within the range of the expected fluctuation range of the error factor according to the experimental design method. A design value is set, a design value having a low sensitivity obtained from the average of the focusing diameters calculated from the design value and a large SN ratio representing the variation degree of the focusing diameter is selected, and based on the selected design value To determine the value of the control factor And, even in the defect inspection depth with an unknown complex form as stress corrosion cracking, regardless the variations in flaw depth, can maintain a high detection accuracy.

  In addition, the phased array probe of the present invention whose specifications have been determined by the method of the present invention can be used to detect fluctuating flaw depth even if the flaw detection depth is unknown and the defect has a complicated shape. Regardless of, high detection accuracy is possible.

  Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the component parts described in this embodiment are not intended to limit the present invention to that only, unless otherwise specified.

  An embodiment in which the present invention is applied to detection of defects such as stress corrosion cracks occurring in a blade implantation groove of a steam turbine will be described with reference to FIGS. FIG. 1 shows the procedure of this embodiment. First, in step 1, information on the specification of the object to be inspected such as material, flaw detection depth range (beam path length) and target defect, and necessary detection accuracy are set.

Next, a control factor (design parameter) of the probe 10 of the phased array probe is selected. As shown in FIG. 2, four factors are selected as control factors: the total probe length L of the flaw detection portion, the probe width l of the flaw detection portion, the number of piezoelectric elements 12, and the curvature r of the contact surface of the piezoelectric element 12 with the object to be inspected.
Next, the basic design value of the phased array probe is determined (step 2).

As a basic design value determination method, for example, the probe (flaw detection part) total length L shown in FIG. 11 is obtained from the following equation (1).
L = d · D _max / e · f (1)
Here, d is the speed of sound in the ultrasonic mode (longitudinal wave or transverse wave), D _max is the maximum focusing distance (maximum path length), e is the beam size (beam spot), and f is the frequency.

ここで、Ｄ_minは必要最小フォーカシング距離（最小路程）、Ｄ_maxは必要最大フォーカシング距離（最大路程）である。 Further, the element length w of the piezoelectric element is obtained from the following equation (2).

Here, D _min is a necessary minimum focusing distance (minimum path), and D _max is a necessary maximum focusing distance (maximum path).

After determining the basic specifications of the flaw detection unit 10 in this way, next, an orthogonal table based on the selected four control factors is created (step 3). In FIG. 3, three levels 1 to 3 are set as the values of the four control factors.
In FIG. 4, the flaw detection refraction angle θ and the flaw detection depth position h are set as error factors, and three levels 1 to 3 are set as these values. Note that the flaw detection refraction angle θ is adjusted to match the SCC direction in a direction perpendicular to the SCC direction, and the flaw detection depth position h is adjusted to match the SCC depth.

FIG. 5 is an orthogonal table showing the calculation results of the ultrasonic beam diameter when the values of levels 1 to 3 of the four control factors and levels 1 to 3 of the two error factors are assigned in 18 ways. In the orthogonal table, the sensitivity S is the test number. The average value of the respective beam diameters 1 to 18 is obtained from the following equation (3), and the SN ratio η is obtained from the following equation (4).
Sensitivity S = 10 logm ² (dB) (3)
SN ratio η = 10 log (m / σ) ² (db) (4)
Here, m is an average of the beam diameters, and σ is a standard deviation of the beam diameters.

The beam diameter can be obtained by sound field simulation based on the design value of each design parameter (step 4). FIG. 6 shows a screen display when the beam diameter is obtained by sound field simulation, and the intensity of the ultrasonic wave is shown by shading.
The intensity (dB) of the ultrasonic wave is obtained by the following formula (5).
dB = 20 log ₁₀ A / A ₀ (5)
Here, A indicates the echo height, and A ₀ indicates the reference echo height.

The beam diameter shown in FIG. 5 is a beam range in which the beam intensity is halved with respect to the peak position where the beam intensity is the highest as shown in Expression (6) in the simulation obtained in FIG. That is, in the sound field simulation, the beam range corresponding to the effective beam range is obtained with the beam range between [peak value to −6 dB] until the intensity of the ultrasonic wave is halved from the peak value. To do.
_{20log 10 A / A O = 20log} 10 1/2 = -6dB (6)

Phased array probes have the effect of focusing beams using multiple piezoelectric elements, so the beam diameter at the focusing position varies depending on the probe dimensions and the time difference applied to each piezoelectric element. It is difficult to obtain the beam diameter by a simple theoretical formula.
In FIG. 6, (a) shows the beam diameter g ₁ when the ultrasonic wave is focused at a shallow position of the test subject portion, (b), when the ultrasonic wave is focused at a deep position of the test subject portion It shows the beam diameter _{g 2} of. In the drawing, the numerical value on the right side indicates the ultrasonic intensity (dB).

  Next, a factor effect diagram regarding the SN ratio η and sensitivity S is created from the orthogonal table shown in FIG. FIG. 7 shows the factor effect diagram. From the factor effect diagram of FIG. 7, it can be seen that the curvature r of the contact surface has a great influence on the output fluctuations of the SN ratio η and sensitivity S.

  Next, the values of the four design parameters are determined by confirmation calculation (step 6). FIG. 8 shows estimated calculation values and confirmation calculation values using the Taguchi method for the beam diameter. In the figure, the numbers entered in the upper factor rows indicate the numbers of the control factors of levels 1 to 3 in FIG. For example, in the current condition column, the numerical values of level 2 are used for all four control factors. In the column of improvement 1, the numerical value of level 3 is used for the probe total length L and the numerical value of level 1 is used for the probe width l. The number n of piezoelectric elements and the curvature r of the contact surface indicate that the numerical values of level 2 were used.

  The estimated calculation values in the specification determination table of FIG. 8 are obtained by using the S / N ratio η and the average value of the sensitivity S when the control factors are at the same level. On the other hand, for the confirmation calculation value, the sound field simulation of the probe is performed according to the set level of each control factor (implemented for the error factor case in FIG. 5). Based on these results (such as the factor effect diagram of FIG. 7) and the maximization / minimization conditions of FIG. 8, each control factor is set in the items of improvements 1 to 5, and the target SN ratio η and sensitivity S An estimation calculation is performed and a confirmation calculation is performed.

This result is a confirmation calculation value, and considering the conditions to be improved while observing these results, the specifications of the probe are brought closer to the optimum design with improvements 1 to 5.
In FIG. 8, the sensitivity S having the smallest sensitivity is the improvement 4, but since the improvement 4 has a small SN ratio η, the improvement 5 having the second smallest sensitivity S and the largest SN ratio η is selected.

  Then, with the control factor of improvement 5 as a reference, by adjusting these values as appropriate, the robustness that is not affected by fluctuations in the flaw detection refraction angle θ and the flaw detection depth h, which are error factors, The specifications that can maintain good detection accuracy can be determined.

That is, improvement 5 which is a design value brought close to the optimum design by the Taguchi method is obtained, and (7) gain (S / N ratio) is improved to 1.81 for the gain shown in FIG. ) Gain (sensitivity) decreases to -1.83.
Thus, robustness has been improved by finding a better condition than the 18 cases of FIG. 5, that is, a condition in which the beam diameter is small and there is little variation with respect to the flaw detection depth h and the flaw detection refraction angle θ. be able to. Furthermore, for improvement 5, the level can be set with design parameters, and the Taguchi method can be repeated to make it closer to the optimum design.

  As described above, according to the present embodiment, the Taguchi method is adopted, and the four control factors (the probe total length L, the probe width l, the number of piezoelectric elements n, and the curvature r of the contact surface of the piezoelectric elements with the object to be inspected). Is selected, and the flaw detection refraction angle θ and the flaw detection depth position h are used as error factors, and the control factor value is determined so that the detection accuracy is not affected by fluctuations in these error factors. In addition, it is possible to obtain high detection accuracy even for a defect having a complicated shape whose depth position and crack direction are not determined.

In the above embodiment, the design specification is determined based on the values of the SN ratio η and the sensitivity S. Instead, the one having the smallest difference (variation width) of the output (μ ± σ) is selected. Also good. In FIG. 9, the output (μ ± σ) having the smallest difference is the improvement 5, and the same result as that obtained by selecting from the SN ratio η and the sensitivity S can be obtained.
When using the difference in output (μ ± σ), the control factor is selected using one variable as an index. Therefore, the selection becomes easy and no error occurs.

  According to the present invention, the Taguchi method can be used to efficiently obtain an optimum specification of a phased array probe that is robust and has high detection accuracy.

It is a flowchart which shows the specification determination procedure which concerns on one Embodiment of this invention. It is a perspective view of the flaw detection part which shows the control factor selected in the said embodiment. It is a graph which shows the setting value (level 1-3) of the control factor of the said embodiment. It is a graph which shows the setting value (level 1-3) of the error factor of the said embodiment. It is a graph which shows the orthogonal table | surface of the said embodiment. It is a figure which shows the screen display which performed the sound field simulation in the said embodiment. It is a factor effect figure of SN ratio (eta) and sensitivity S of the said embodiment. It is a chart which determines the specification of a probe by confirmation calculation in the embodiment. It is explanatory drawing which shows the flaw detection principle of a phased array probe. It is explanatory drawing in the case of flaw-detecting the stress corrosion crack which generate | occur | produced in the blade implantation groove part of the steam turbine. It is a perspective view of the piezoelectric element which comprises a phased array probe.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Phased array flaw detection part 12,102 Piezoelectric element 100 Phased array probe 106 Steam turbine blade implantation groove part (inspection object)
108 Stress corrosion cracking L Probe total length X _O Synthetic wavefront a Synthetic wavefront traveling direction b Focusing position of synthetic wavefront h Defect depth l Probe width r Contact surface curvature t Piezoelectric element thickness w Piezoelectric element length θ Deflection angle

Claims (4)

In a method for determining the specifications of a phased array probe that is used to detect a defect of an object to be inspected that is made of a single material and has an unknown depth position, in which a large number of piezoelectric elements are arranged to form a flaw detection part.
The evaluation factor is the ultrasonic focusing diameter representing the ultrasonic focusing degree, and the control factor is the total length and width of the flaw detection part, the number of piezoelectric elements, and the curvature of the surface of the flaw detection part in contact with the object to be inspected. Yes, the error factors are flaw detection depth position and flaw detection refraction angle,
A plurality of design values with different values of the error factor and the control factor are set within the range of the expected fluctuation range of the error factor by the experimental design method, and obtained from the average of the focused diameters calculated from the design value. A design value having a small sensitivity and a large S / N ratio representing the variation degree of the focusing diameter is estimated, and the value of the control factor is determined based on the design value selected by the confirmation calculation. Specification method for phased array probe. Select the design value that minimizes the difference in the formula (μ ± σ) consisting of the average value μ of the focusing diameter calculated from the set design value and the standard deviation σ of the focusing diameter,
The method for determining the specification of a phased array probe according to claim 1, wherein the value of the control factor is determined based on the selected design value.   The method for determining the specifications of a phased array probe according to claim 1 or 2, wherein the defect to be inspected is a stress corrosion crack formed in the object to be inspected. A phased array probe characterized in that the control factor is manufactured by the specification determining method according to claim 1.

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