JP3025681B1 - Calibration method and potentiometric measuring device in potentiometric measurement - Google Patents

Abstract

An object of the present invention is to improve the accuracy of crack measurement by a potential difference method. SOLUTION: When a constant current is applied to a conductive sample 1 having a thickness T with a crack K, a voltage drop at a crack portion caused by a constant current is represented by E
And Further, a voltage drop at a portion where there is no crack K is defined as Eo. At this time, the potential difference ratio E / E with respect to the change in the crack depth a
The change in o is calibrated by the approximate equation E / Eo = 1 + f (a) / (T
-A). The relationship between the potential difference ratio E / Eo and the crack depth a is calibrated based on this approximate calibration formula.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a calibration method for converting measurement data obtained by a potential difference method into various dimensions, and a potential difference measurement apparatus to which the calibration method is applied.

[0002]

2. Description of the Related Art A measuring method using a potential difference method has been known as a means for inspecting and measuring the presence or absence of a defect such as a crack or a scratch in a conductive material by a nondestructive test.

[0003] This potential difference measurement means, for example, when a crack or the like exists in a conductive material such as a metal, a voltage drop caused by a current flowing across the crack portion when a predetermined voltage is applied to the conductive material. Is a crack measurement made by focusing on the fact that it is related to the crack depth.

For example, as shown in FIG. 4, it is assumed that a rectangular parallelepiped conductive sample 1 has a crack K with a uniform depth.

[0005] A constant current I is applied to the rectangular parallelepiped through the crack K by a constant current power supply, and a voltage drop at a crack portion (a constant section L including the crack K) generated at that time is measured by a voltmeter. Since this voltage drop depends on the size of the crack K, the crack depth a can be converted from the voltage value. This is the basic idea of crack measurement by the potential difference method.

However, in practice, it is difficult to accurately determine the relationship between the voltage drop and the crack depth at that time by simply applying Ohm's law from the dimensions and the resistivity.
This is for the following reason.

That is, when Ohm's law is applied using the electrical resistance value obtained from the thickness of the crack and the cross-sectional area and conductivity of the conducting part, the calculated voltage drop at the crack is extremely small. However, a larger voltage drop actually occurs. This is due to the fact that the crack has virtually no thickness, and the distribution of current across the cross section parallel to the crack near the crack is not uniform, and therefore the conductor cross section near the crack If the electric resistance value is calculated simply assuming that the current density is constant without considering the current distribution, the calculated voltage drop includes the above-described error.

For this reason, usually, a large number of standard samples having different crack depths are measured, and at that time, the potential difference ratio based on the case where there is no crack is calculated from the acquired calibration data (voltage drop at the crack portion). A calibration method has been performed in which a calibration curve representing the relationship between this potential difference ratio and the crack depth is obtained, and the actually measured potential difference ratio of the object to be measured is converted into the crack depth based on this.

For example, when the calibration curve is obtained, the potential difference ratio is E / Eo, and the crack depth is a.
And the coefficients A, B, C, D
Was obtained by the least-squares method.

[Equation 1] E / Eo = Aa3 + Ba2 + Ca + D

When the crack depth a = 0, the potential difference ratio E /
Since Eo = 1 (E = Eo) is clear, the coefficient D is set to 1
A method of obtaining other coefficients A, B, and C has also been performed.

[0011]

However, since the above polynomial 1 is an approximation that disregards the electrical backing, it cannot be used outside the acquisition range of the calibration data. Even within the acquisition range, the calibration curve may be abnormal unless the order of the polynomial 1 is properly selected.

For these reasons, in the approximation using the conventional polynomial, it is necessary to acquire calibration data in advance over a wide range, and the approximation formula used is inappropriate.
High conversion accuracy could not be expected.

The present invention improves the accuracy in converting measurement data obtained by the potential difference method into various dimensions by giving an electrical meaning to the approximation formula of the calibration curve, and performs an arithmetic process at that time. It is intended to simplify and facilitate the calibration operation.

[0014]

According to the first aspect of the present invention, a crack generated when a constant current is applied to a conductive sample (1) having a thickness (T) in which a crack (K) exists. the voltage drop parts and (E), cracking sites voltage drop without (K) and (Eo), crack depth a which does not have a constant term Seki
When the number is f (a), the change in the potential difference ratio (E / Eo) with respect to the change in the crack depth (a) is calculated by a calibration approximation E / Eo =
1 + f (a) / (T−a), and based on this calibration approximation formula, the potential difference ratio (E / Eo) and the crack depth (a)
Is to calibrate the relationship.

Further, according to the present invention, a constant current power supply (2) for supplying a constant current (I) in a direction crossing a crack (K) existing in the conductive sample (1); A voltage measuring device (3) for measuring a voltage drop (E) at a crack portion generated by the current (I) and a voltage drop (Eo) at a portion without a crack (K), and a measurement output ( E, E
An arithmetic processing unit (4) for calculating the crack depth (a) of the sample (1) by the calibration method in the potentiometric method according to claim 1 based on o).

[0016]

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The calibration based on the approximation polynomial 1 described above disregards the electric support and is likely to cause an error when obtaining the calibration approximation from the calibration data. In the embodiment, a new calibration approximation formula different from the conventional one is set. Therefore, first, the electrical confirmation of this approximate expression will be performed.

The details will be described below. In the conductive sample 1 shown in FIG. 4, the potential difference ratio E / Eo based on the voltage drop when no crack is present becomes 1 when the crack depth a = 0. When the crack depth a reaches the plate thickness T, it becomes infinite.

Therefore, in the present invention, in addition to the condition that the crack depth a = 0 and the potential difference ratio E / Eo = 1, the crack depth a = the plate thickness T
In consideration of the condition of the potential difference ratio E / Eo = ∞, an approximate expression shown in the following expression 2 was set. In Equation 2, E is a voltage drop at a crack, Eo is a voltage drop at a portion where no crack exists, f
(A) is a function of a having no constant term, T is the thickness of the sample,
a is the crack depth.

[Equation 2] E / Eo = 1 + f (a) / (Ta)

Incidentally, the electric resistance Ro of a rectangular parallelepiped sample having a thickness T, a width W and a length L can be expressed by the following formula 3 where the electric resistivity of the sample is ρ.

[Formula 3] Ro = ρL / (TW)

Next, the electric resistance R in the case where there is a slit K having a depth a in the thickness direction at the center in the longitudinal direction of the rectangular parallelepiped sample is determined. In addition, the fact that the current distribution in the conductor cross section is not uniform near the slit K is taken into account here.

Since the slit K does not conduct current, FIG.
As shown in (a), it can be assumed that there are portions where the current does not easily flow as shown by hatching on both sides of the slit K. The portion where the current hardly flows is equivalent to the rectangular parallelepiped sample being cut into a triangular cross section as shown in FIG. 3 (b). Therefore, as shown in FIG. The electric resistance R to be obtained can be expressed by the following equation (4).

Equation 4 R = ρ (L−b) / (TW) + ρb / (W
(Ta))

When the current is constant, the voltage drop is proportional to the electric resistance, and the potential difference ratio E / Eo can be expressed by the following equation (5).

Equation 5 E / Eo = R / Ro = 1 + ab / (L (T
-A))

Here, assuming that the slit width b is a missing portion of the sample corresponding to a portion where the current is interrupted by the slit K, b is a function f dependent only on the depth a of the slit K.
(A).

Therefore, Equation 2 and Equation 4 are equivalent, and the following Equation 6 holds.

[Equation 6] E / Eo = 1 + ab / (L (T−a)) =
1 + f (a) / (Ta)

From this, it can be said that the above-mentioned calibration approximation formula 2 is an approximation formula that is electrically meaningful.

Next, a description will be given of the potential difference method crack measurement according to the present invention.

FIG. 1 is a schematic block diagram showing an embodiment of a crack measuring device to which a calibration method in the potential difference method according to the present invention is applied.

In the drawing, reference numeral 1 denotes a conductive rectangular parallelepiped sample 1 having a thickness T in which a crack K exists. This crack measuring device uses this sample 1
A constant current power supply 2 for supplying a constant current in a direction crossing the crack K, a high-precision voltmeter 3 as a voltage measuring device for measuring a voltage drop of a crack portion generated at this time, and the high-precision voltmeter 3 And an arithmetic processing unit 4 for calculating the crack depth from the measured values of the above. In addition, one of a standard sample for deriving a calibration approximation formula to be described later and a sample to be actually measured for crack measurement is appropriately connected to the crack measuring device as the sample 1.

The constant current power supply 2 may be a DC power supply or an AC power supply. The current may be made to flow uniformly over the entire sample 1 or a local current may be applied only to the portion to be measured. You may make it flow. However, in any case, the measurement of the standard sample and the measurement of the measurement target sample must be performed under the same conditions.

Next, the processing operation of crack measurement in the present crack measuring device will be described based on the data processing flowchart of FIG.

This measurement processing is performed by the arithmetic processing unit 4, and is roughly divided into a calibration section and a measurement section in terms of function.

The calibrating unit executes a process for creating a calibration approximation formula represented by the above formula 2 based on calibration data obtained by measuring a large number of standard samples 1 having known dimensions such as cracks.

The details will be described below. First, the potential difference between the standard samples 1 prepared in a large number in step 11, that is, the potential difference Es of the crack portion of each of the standard samples 1 having cracks K and the standard difference without cracks K The potential difference Eso of the sample 1 is measured. In step 12, the crack depth as (the slit depth in the case of an artificial slit) of each of the standard samples 1 is measured.

Next, in step 13, the potential difference Eso when the standard sample 1 having no crack K (or slit) was measured.
Is calculated, the ratio of the potential difference Es when the standard sample 1 having the crack K (or slit) is measured, that is, the potential difference ratio Es / Eso is calculated. A correspondence table of the actually measured values “as” of the crack depths of the respective standard samples 1 obtained in the above is prepared.

In step 15, the crack depth a in equation (2)
As a function f (a), an appropriate mathematical expression having no constant term is substituted (in the present embodiment, a quadratic expression is used). Potential difference ratio Es / Eso and crack depth a prepared in 14
With reference to the correspondence table of s, the undetermined coefficients α and β of Equation 7 are calculated by the least squares method.

[Equation 7] Es / Eso = 1 + (αas + β) as /
(T-as)

On the other hand, the measuring section executes a process for converting the actual crack depth from the potential difference ratio in the sample 1 to be measured.

The details will be described below. First, in step 21, the potential difference of the sample 1 to be measured, that is, the potential difference E of the crack portion and the potential difference Eo of the portion having no crack are measured, and in step 22, the measured value E is measured. Potential difference ratio E / Eo from Eo
Is calculated.

Next, at step 23, the calibration approximate expression 7
Then, the crack depth a is calculated using the same equation (8), and the value of the crack depth of the sample 1 to be measured calculated in step 24 is output. At this time, the numerical values calculated in step 15 are substituted for the undetermined coefficients α and β.

[Equation 8] E / E0 = 1 + (αa + β) a / (T−a)

The above is the calculation processing operation of the present invention. If the regularity between “a” and “b” can be confirmed experimentally or analytically in the above equation 5, the calculation processing approximates the measured value of the standard sample. It is also possible to obtain an approximate calibration equation from measurement data such as the shape and dimensions of the sample to be measured without performing the above.

In the present embodiment, the measurement of the crack depth existing in the conductive material has been described. However, the object to be measured by the potential difference measurement according to the present invention is not limited to this, and the following measurement is performed. It can be applied sufficiently to

A description will be given of some concrete examples. First, one application example is measurement of dimensions of a conductive article.

For example, when measuring the size of a notch (notch) provided in a conductive article, the depth of a hole, and the like by a potential difference method, highly accurate measurement is possible by applying the calibration method of the present invention. It is.

Another application example is measurement of void fraction.

Since the void of the conductive material does not conduct current, it is considered that the void is equivalent to a crack or a notch. For example, use this approximate calibration formula by using the void fraction instead of the crack depth. It is possible to measure the void ratio at the joint surface when a conductive substance such as a metal is joined by the method.

As still another application example, it is a characteristic measurement of a liquid, a solid, or a gas.

For example, when examining the properties of a conductive solid cut by a cut (or crack) or foreign matter, or a conductive liquid or gas (plasma) in such a container, the cut (or crack) or foreign matter , The conductivity of the substance can be determined from the comparison with the potential difference of the standard sample using the approximate equation for calibration. Furthermore, if the association between the conductivity and the property of the substance is clear, the property of the substance can be measured.

[0048]

As described above, according to the first aspect of the present invention, by setting an approximation formula having electrical backing as a calibration approximation formula, the calibration accuracy of the crack measurement by the potential difference method is set. Is improved, and accurate measurement can be performed even outside the acquisition range of the calibration data.

According to the present invention described in claim 2,
Since the calibration approximation formula can be expressed by a relatively simple formula, when calibration is performed using a computer or the like, the processing program can be simplified and the calibration operation is also facilitated.

[0050]

[Brief description of the drawings]

FIG. 1 is a schematic block diagram showing an embodiment of a crack measuring device according to the present invention.

FIG. 2 is a data processing flowchart showing a processing operation of the crack measuring device.

FIG. 3 is an explanatory diagram for calculating an electric resistance value of a slit portion of a conductive sample.

FIG. 4 is a diagram showing the principle of crack measurement by a potential difference method.

[Explanation of symbols]

 Reference Signs List 1 sample 2 constant current power supply 3 voltage measuring device 4 arithmetic processing unit K crack

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Hiroshi Fukasaku 4002 Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki Japan Nuclear Fuel Cycle Development Organization Oarai Engineering Center Inside Toko Machinery Co., Ltd. (JP, A) JP-A-4-186102 (JP, A) JP-A-6-201632 (JP, A) Proceedings of the Japan Society of Mechanical Engineers Tohoku Regional Lectures (1996), p. 28-30, Paper No. 111, "Improvement in Sensitivity of Crack Measurement by DC Potentiometric Method Using Proximity Terminals" (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 27/00-27/24 JICST File (JOIS)

Claims (2)

(57) [Claims] 1. A voltage drop at a crack portion generated when a constant current is applied to a conductive sample (1) having a thickness (T) in which a crack (K) exists, is defined as (E), and there is no crack (K). A function of the crack depth a without the constant term, where the voltage drop at the site is (Eo)
Where f (a) is the change in the potential difference ratio (E / Eo) with respect to the change in the crack depth (a), the calibration approximation E / Eo = 1 + f (a) / (T−
a) a calibration method in potentiometric measurement, wherein the relationship between the potential difference ratio (E / Eo) and the crack depth (a) is calibrated based on the calibration approximation formula. 2. A constant current power supply (2) for supplying a constant current (I) in a direction crossing a crack (K) existing in a conductive sample (1), and a voltage at a crack generated by the current (I). A voltage measuring device (3) for measuring a voltage drop (Eo) at a portion where there is no drop (E) and a crack (K), and a measurement output (E, Eo) of the voltage measuring device (3). And an arithmetic processing unit (4) for calculating the crack depth (a) of the sample (1) by the calibration method in the potential difference measurement described in (1).