JPH0356848A - Method and device for surface cracking measurement - Google Patents

Abstract

PURPOSE:To measure a surface crack without using any comparison material by detecting a potential difference when a reference current is fed to an object to be measured and a potential difference when a contrast current is fed and calculating the depth of the surface crack from the ratio of the both. CONSTITUTION:A function generation part 1 switches and outputs either of a DC signal and an alternating current of determined frequency to a voltage amplification part 2 according to a signal 10 which is inputted from a control and arithmetic part 4. A current control part 3 outputs a DC or AC constant current 13 according to the signal inputted from the voltage amplification part 2 and the constant current 13 is supplied to the object 22 to be measured. The potential differences Eo and E which are generated with the reference current and contrast current flowing through the object 22 to be measured are outputted to a lock-in amplifier 7. The detection signal 17 of the lock-in amplifier 7 is converted into a digital signal, which is sent to the control and arithmetic part 4; and the control and arithmetic part 4 uses the potential differences Eo and E and calculates the object depth of the crack to output to an output part 5.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method and apparatus for measuring surface crack depth using a potentiometric method, and particularly relates to a method and an apparatus for measuring surface crack depth using a potentiometric method. The present invention relates to a method and device for measuring surface cracks that measure crack depths with high precision without using any surface cracks.

[Conventional technology]

To detect and measure the depth of cracks that occur in metal materials during the use of mechanical products and mechanical structures, there are two methods: direct measurement using a microscope or monitor TV, potentiometric method, ultrasonic method, eddy current method,
There are indirect methods such as the replica method, which are used depending on the material and shape of the target metal and the type of crack. Among them, the potentiometric method is used as a method that can be used in environments such as high temperature, low temperature, high pressure, and corrosive liquid. The potentiometric method is a method of measuring the depth of a crack by using the change in electrical resistance of a metal material caused by a crack. Generally, the depth of a crack is determined from the change in potential difference when a constant current is passed through the object to be measured. The method is roughly divided into the DC potential difference method and the AC potential difference method, depending on the current applied.

The DC potential difference method is a method of determining the crack depth by passing a DC current. The current supply device and potential difference detection device are simple, and measurement is easy. However, since the current flows through the entire cross section of the object to be measured, In order to measure the depth of microcracks in , it is necessary to flow a large current (several amperes) to increase the measurement sensitivity. For this reason, there is a problem in that when a long-time measurement is performed, the object to be measured generates heat and the resistance value changes, causing the potential difference to drift. To avoid this drift in the DC potential difference method, there is an alternating DC potential difference method in which the polarity of the applied current is alternated at regular intervals to shorten the current application time, but there are problems such as noise during alternation.

The AC potential difference method is a method of determining the crack depth by applying an AC current. When an alternating current is passed through a metal material, the current flows through a thin layer on the metal surface. This is called the skin effect, and the relationship between the skin depth δ and the frequency f of the applied alternating current is expressed by the following equation.

δ=(/Laμ.σf)−”−・・・−(
1) Here, μ. is the magnetic permeability in vacuum, μ is the relative magnetic permeability of the conductor, and σ is the electrical conductivity of the conductor. Due to this skin effect, the AC potentiometric method enables highly sensitive crack measurement with a smaller current (less than LA) than the DC potentiometric method.

As mentioned above, the potentiometric method is a method to determine the crack depth by using the change in the electrical resistance of a metal material due to a crack, but the electrical resistance of a metal material is determined not only by the crack but also by the temperature at the time of measurement and changes in the metal structure. , it also changes due to plastic deformation, etc. For this reason,
In order to measure cracks with high precision, it is necessary to remove changes in electrical resistance caused by factors other than cracks, and conventionally, a comparison material with no cracks made of metal with the same structure as the measurement target was measured under the same conditions. Changes in electrical resistance caused by factors other than cracks were eliminated by using the ratio between the measured values and by creating a bridge circuit between the comparison material and the measurement target. However, when measuring cracks that occur in materials that have been used for long periods of time under high temperatures, such as in boilers and turbines, it is difficult to prepare comparative materials such as those mentioned above, and it is difficult to accurately measure crack depth. I couldn't.

[Problem to be solved by the invention]

As mentioned above, the electrical resistance of a metal material changes depending on the presence or absence of cracks, structural changes, and plastic deformation. Therefore, to measure the depth of a crack in a metal material that has undergone fatigue, creep, or age damage using the conventional potentiometric method, it is necessary to Comparative materials were required, which were extremely difficult to obtain.

An object of the present invention is to measure the depth of a crack generated on a metal surface with high precision without using a comparison material.

[Means to solve the problem]

The above problem arises when a surface crack measurement method involves applying current to a metal material to be measured and detecting the potential difference in the energized area of the metal material to measure the crack depth on the surface of the metal material. Detect the potential difference when current is applied and the potential difference when a contrasting current is applied, and detect the ratio of the two, the surface crack depth measured in advance on the test piece, the frequency of the current to be applied, and the time when the current is applied. The function with the electronic difference that was
This is achieved by calculating the surface crack depth of the measured object from

In the above method, even if the reference current is DC].
O O K H Alternating current with a frequency exceeding Z or 1
It may be an alternating current with a frequency less than 0OHZ.

The above object is also achieved by the method for measuring surface cracks according to claim 1, 2 or 3, wherein the contrast current is alternating current.

In the above method, the frequency of the contrast current is 10 to 90K.
It may be HZ.

The above problem further requires a current supply means for supplying a constant current, a power supply terminal for supplying the supplied current to a metal material to be measured, a detection terminal for detecting a potential difference in an energized area of the metal material, and a detection In the surface crack measuring device, the current supply means is connected to a reference current and a comparison current. shall be supplied alternately, and the control/calculation means is controlled to control the voltage difference detected when the reference current is energized, the potential difference detected when the comparison current is energized, and the crack depth, frequency of the energizing current, and potential difference measured in advance. This can also be achieved by calculating and outputting the crack depth from the function.

In the above device, even if the reference current is DC and the contrast current is AC, the reference current is AC with a frequency of 100KHZ or more or 10OHZ or less, and the comparison current is 10KHZ to 90K.
It may also be HZ AC.

[Effect]

Metal material +: *When a current is passed, the way the current flows differs depending on the type of current (direct current, alternating current) as shown in FIGS. 3 to 5. FIG. 3 shows the flow when a direct current is applied, and the current flows uniformly over the entire cross section of the object to be measured. Fig. 4 shows the flow when an alternating current of 10 KHZ and Fig. 5 shows the flow when an alternating current of 10 OK HZ is applied, respectively, and the current flows in a thin layer on the metal surface due to the skin effect. The depth of this thin layer on the metal surface (skin depth) depends on the frequency of the current flowing as shown in equation (1) above, and the higher the frequency, the shallower the skin depth, and the potential difference due to cracks. The rate of change in will increase.

However, if the frequency of the applied current is extremely high, the skin depth becomes extremely shallow and the crack surface acts like a capacitor, making crack measurement impossible, as shown in Figure 5. Become. Figure 6 shows the potential difference ratios when direct current, alternating current with a frequency of 10 KHz, and alternating current with a frequency of 100 KHz are applied.
It shows the relationship between crack depth and shows that the crack depth measurement sensitivity differs depending on the type of current.

The present invention takes advantage of the fact that the potential difference change due to a crack differs depending on the type of current. Potential difference E detected when a constant current (current with a constant value) is passed through the measurement target
The relationship shown in FIG. 7 exists between the frequency f of the constant current and the frequency f of the constant current. Even if the electrical resistance of the metal to be measured itself changes due to structural changes or plastic deformation, the electrical difference changes only on average, and the frequency dependence does not change. In addition, the potential difference E0 detected when a DC current is passed as a reference current is a reference potential difference that reflects the metal structure, plastic deformation, etc. of the object to be measured. The detected potential difference E is a potential difference that largely reflects the effects of cracks as well as metal structure and plastic deformation. Therefore, by taking the ratio of the two (potential difference ratio) E/E, only the potential difference change due to the influence of the crack is extracted, excluding the influence of the temperature and metallographic structure at the time of measurement. is measured.

〔Example〕

FIG. 1 shows the overall structure of a surface crack measuring device that is an embodiment of the present invention. This device includes a current supply and voltage difference detection device 20 and a terminal section 21.

The current supply and potential difference detection device 20 includes a function generation section 1, a voltage amplification section 2 connected to the output side of the function generation section 1, and a current control section 3 connected to the output side of the voltage amplification section 2. , a lock-in amplifier 7 connected to the function generating section 1 and into which the detected potential difference is input; an A/D converter 6 connected to the output side of the lock-in amplifier 7;
/Control/calculation unit 4 connected to the output side of the D converter 6
and an output section 5 connected to the output side of the control calculation section 4. The function generating section 1, the voltage amplifying section 2, and the current controlling section 3 constitute current supply means. The control/calculation section 4, which serves as control/calculation means, is also connected to the function generation section 1 and the current control section 3. The output section 5 is equipped with a display tube that numerically displays the crack depth. The terminal section 21 includes a power supply terminal 9 connected to the output side of the current control section 3 and a detection terminal 8 connected to the input side of the lock-in amplifier 7.

In the surface crack measuring device having the above configuration, the function generating unit 1 generates either a DC signal or an AC signal at a predetermined frequency f (about several tens of KHz) based on the signal 10 input from the control/calculation unit 4. The voltage amplifying unit 2 amplifies the input DC signal or AC signal and outputs the amplified DC signal or AC signal to the current control unit 3. The current control unit 3 outputs a DC or AC constant current 13 controlled within a predetermined error range based on the signal input from the voltage amplification unit 2, and the constant current 13 is measured via the power supply terminal 9. Swept away by target 22.

Potential differences E0 and E generated in the measurement object due to the constant DC current that is the reference current and the constant AC current that is the contrast current flowing through the measurement object 22 are detected by the detection terminal 8 and output to the lock-in amplifier 7. As a detection signal 17 with the same frequency as the constant current 13, A/
It is output to the D converter 6. The A/D converter 6 is
The detection signal generator 7, which is an analog signal, is converted into a digital signal 18 and sent to the control/calculation section 4, which converts the potential difference E, which is the input digital signal 18, into a digital signal 18.

and E, the crack depth is calculated and output to the output section 5. The output unit 5 numerically displays the input crack depth on a display tube.

Once the measurement target is selected, test pieces with artificial cracks of various depths are prepared using the same metal material as the measurement target, and the test pieces are subjected to direct current and various frequencies. A constant current is applied and the potential difference is measured. Figure 7 was created based on the measurement results, and the frequency of the contrast current and Figure 7 were also created.
The chart shown in FIG. 8 is created based on the diagram. The functions shown in FIG. 8 are stored in advance in the control/calculation unit 4 as calculation data. When the digital signal 18 representing the potential difference EQ, E is inputted from the A/D converter 6, the control/calculation unit 4 calculates the electrical potential difference ratio E/Ea from this,
Next, the crack depth C is calculated from the calculated potential difference ratio E/E and the above function. The calculated crack depth is output as a numerical signal to the output section 5 and displayed numerically on the display tube.

In the above embodiment, the base $ current is a direct current,
I measured the potential difference by flowing an alternating current as a contrast current, but
As shown in Figure 7, the potential difference measured when a constant current is applied is 10 OHZ or less,
At OKHZ or higher, it is similar to when the flowing current is direct current, and the potential difference detected when an alternating current of 10 OHZ or less or an alternating current of 100 KHz or more is passed as the reference current instead of direct current is used as the reference (E0). However, crack measurement is possible in the same manner as in the above embodiment. However, in this case, when creating the function shown in Figure 8 based on Figure 7 in advance, instead of the potential difference obtained by flowing a direct current, the voltage obtained by flowing an alternating current at the reference frequency is used. It is necessary to use the A6 difference as E0.

Moreover, instead of creating the function shown in FIG. 8 in advance, the data of frequency, voltage, η difference, and crack depth shown in FIG. When measuring cracks and cracks,
Reference potential difference E. The frequency of the current to be passed for measurement is specified, and the control/calculation unit 4 calculates the function shown in FIG. 8 from the frequency, potential difference, and crack depth data each time, and the obtained function The crack depth may be calculated from the potential difference ratio E/EO.

FIG. 2 shows the above-mentioned detection procedure. During measurement, the control/calculation section 4's output signal switch ○ is used as shown in Fig. 2! ．． IN
1 0 2 to 107 are repeated at predetermined time intervals,
The crack depth output from the control/calculation unit 4 is continuously displayed on the display tube.

According to the above embodiment, the potential difference includes conditions such as the metallographic structure and fatigue of the metal material to be measured, and the electric potential difference has a small effect of cracks, and the electrical potential difference has a large effect of cracks. Since the difference between the two is measured, it is now possible to measure cracks without using a comparison material. In addition, since no comparison material is used, there is no need to polish the surface of the material to eliminate measurement errors due to differences in surface roughness between the measurement target and the comparison material, and even if the materials to be measured are different, the crack depth and potential difference Since the ratio relationship does not change, measurements are quick and easy.

〔Effect of the invention〕

According to the present invention, the fatigue, creep, aging damage, etc. of the metal material to be measured is reflected, but the electrical profile difference due to the reference current, which is less susceptible to the influence of surface cracks, and the fatigue, creep, or aging damage mentioned above are reflected. etc. are reflected, and the potential difference due to the contrast current, which is greatly affected by surface cracks, is measured.
Since the potential difference as a function of the surface crack is detected from both of the above, the effects of fatigue, creep, or aging damage are eliminated, so it is possible to detect the surface crack of the object to be measured without using a comparison material. , it has become easier to detect surface cracks in the measurement target during service.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the main structure of a surface crack depth measuring device which is an embodiment of the present invention, FIG. 2 is a procedure diagram showing an example of the method of the present invention, and FIGS. 3 and 4. , Figure 5 is a schematic diagram showing how various currents flow when applied to a material with surface cracks, and Figure 6 shows the frequency dependence of the potential difference ratio for surface cracks at various depths. The graph shown in Figure 7 is a conceptual diagram showing the relationship between the frequency of the current applied to surface cracks at various depths and the detected electrical difference, and Figure 8 is a conceptual diagram showing the relationship between the potential difference ratio and the crack depth. It is a conceptual diagram showing a relationship. 1, 2.3... Current supply means, 4... Control/calculation means, 5... Output means, 8...
Detection terminal, 9...Power supply terminal.

Claims (1)

[Claims] 1. In a surface crack measurement method in which a metal material to be measured is energized and a potential difference in a energized area of the metal material is detected to measure the depth of a crack on the surface of the metal material, The potential difference when the reference current is applied and the potential difference when the contrast current is applied are detected, and the ratio of the two, the surface crack depth measured in advance on the test piece, the frequency of the applied current, and the current A surface crack measuring method characterized by calculating the surface crack depth of a measurement target from a function of a potential difference detected during energization. 2. The method for measuring surface cracks according to claim 1, wherein the reference current is a direct current. 3. The method for measuring surface cracks according to claim 1, wherein the reference current is an alternating current with a frequency exceeding 100 KHz or an alternating current with a frequency below 100 Hz. 4. The method for measuring surface cracks according to claim 1, 2 or 3, wherein the contrast current is alternating current. 5. The method for measuring surface cracks according to claim 4, wherein the frequency of the contrast current is 10 to 90 KHz. 6. A current supply means for supplying a constant current, a power supply terminal for passing the supplied current to the metal material to be measured, a detection terminal for detecting the potential difference in the current-carrying area of the metal material, and a detection terminal for detecting the potential difference in the current-carrying area of the metal material. control/calculation means for converting into a depth signal;
In the surface crack measuring device, the current supply means alternately supplies a reference current and a comparison current, and the control/calculation means comprises: The crack depth is calculated and output from the potential difference detected when the reference current is applied, the potential difference detected when the comparison current is applied, the crack depth measured in advance, the frequency of the applied current, and a function of the potential difference. A surface crack measuring device characterized by the following. 7. The surface crack measuring device according to claim 6, wherein the reference current is a direct current and the comparison current is an alternating current. 8. The reference current has a frequency of 100 KHZ or higher or 100
It is an alternating current below HZ, and the contrast current is 10KHZ to 90K.
The surface crack measuring device according to claim 6, characterized in that it uses HZ alternating current.