JPH01169348A - Cracking detector - Google Patents

Abstract

PURPOSE:To judge the presence of cracking, by measuring potential differences in two directions orthogonal to each other at the same point on the surface of a member to compare both voltage differences. CONSTITUTION:A probe 4 is equipped with two sets of terminals (feed terminals 7 and 7' and detection terminals 8 and 8'), four per set. The terminals 7 and 7' and 8 and 8' are connected to a constant current device 2 and a fine voltmeter 1 respectively through a switching circuit 3. With such an arrangement, first, a potential difference (V) is measured between the terminals 8 and 8 with energization between the terminals 7 and 7. Then, a potential difference (V') is measured between the 8' and 8' with energization between the terminals 7' and 7'. Normally when a fine cracking is generated in a member, current detours around the cracking so that the potential difference increases as compared with a case having no cracking. Thus, when there is no cracking, V V' and when there is any cracking, V>V' or V<V'. This enables detection of the presence of a fine cracking.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a crack detection device, and particularly to a crack detection device suitable for detecting the occurrence of minute fatigue cracks on the surface of a metal member.

[Conventional technology]

Metal components used for long periods in thermal power plants, chemical equipment plants, etc. are often subjected to repeated stress as the plant starts and stops, causing fatigue damage that can cause cracks to develop and develop, leading to serious accidents. Sometimes.

- In the initial stage of fatigue damage before macrocracks with a length of several meters or more occur in the IG, as shown in Figure 8, the member 6
A large number (several 10s to several 100s) of microcracks 9 with a length of several 10 to several 100 μm are generated on the surface of the substrate.

If the occurrence of microcracks can be detected, countermeasures can be taken at a stage where the damage is small, before macrocracks develop.

By the way, even if a general non-destructive testing method is used, such as fluorescent magnetic particle testing which is effective for detecting surface cracks, it is almost difficult to detect cracks smaller than about 0.5 mm in length. Therefore, in order to investigate the presence or absence of the above-mentioned microcracks, the surface of the member was carefully polished to a mirror finish, a replica of the acetyl cellulose film, etc. was taken, and the replica was observed under a microscope at a magnification of about 50 to 200 times.

[Problem that the invention seeks to solve]

In the conventional inspection using replicas, the dimensions of one replica are approximately 1010×10 at most, and a huge number of man-hours are required to investigate a wide range of parts of an actual machine member.

In addition, crack detectors that use electrical resistance or magnetic changes that are generally available on the market target a single macro crack with a length of several meters or more;
It cannot be applied when many microcracks occur.

As mentioned above, in the conventional technology, the length is several tens to several hundred μm.
microcracks could not be easily detected.

An object of the present invention is to provide a crack detection device that can easily detect the presence or absence of such micro fatigue cracks.

[Means for Solving the Problems] The present invention has been made to solve the above problems, and the present invention is made by bringing two power supply terminals into contact with each other at two points on the surface of a member to be inspected for cracks, and Two detection terminals are brought into contact with the member at two points within the line connecting the surface contact points of the two members, a constant current device supplies a predetermined current to the two power supply terminals, and a microvoltmeter is used to detect the voltage between the two detection terminals. In a device that detects cracks on the surface of a member by detecting a potential difference, the potential difference is measured in two orthogonal directions at the same location on the surface of the member, and the presence or absence of a crack is determined by comparing both potential differences. It is characterized by the fact that

[For production]

Micro fatigue cracks usually occur as cracks all in the same direction, so if many cracks occur, the potential difference in the direction perpendicular to the crack surface will be larger than the potential difference in the direction parallel to the crack surface. Therefore, by comparing both values, it is possible to detect whether or not a crack has occurred.

In the case of the present invention, the stress acting on the member may be in any direction since the potential difference in two orthogonal directions is measured and compared.

〔Example〕

Hereinafter, the present invention will be explained in more detail with reference to Examples.

FIG. 1 is a diagram showing a system of a crack detection device that is an embodiment of the present invention. In this device, a constant current device 2 and a microvoltmeter 1 are connected to a probe 4 via a switching circuit 3.

FIG. 2 shows the basic structure of the probe 4, in which the member 6
Shows the state in which it is in contact with the surface. Further, FIG. 3 shows the state of the probe 4 seen from the lower side in FIG. Two sets of four terminals (power supply terminal 7, detection terminal 8) shown in FIG. 2 are attached to the probe 4. These two sets of terminals are
As shown in FIG. 3, they are fixed so that their respective axes are perpendicular to each other.

Further, the power supply terminal 7.7° and the detection terminal 8.8' are connected to the constant current device 2 and the microvoltmeter 1 via the switching circuit 3, respectively.

In such a configuration, first one set of power supply terminals 7-7
During this time, a constant alternating current is applied from the constant current device 2, and the potential difference (2) between the detection terminals 8-8 is measured.

Next, electricity is applied between the power supply terminals 7° and 7'' of the other set, and the potential difference (1) between the detection terminals 8° and 8° is measured.

The energization conditions at this time are set so that the skin thickness δ determined by the following equation is approximately the same as the depth of the microcrack (approximately several 10 to several 100 μm).

1-′A δ=(−σωμμ.)−−−−−−−−・−・・・−(
1) Here, σ is the electrical conductivity, ω is the frequency, and μ is the magnetic permeability of the member. is the vacuum permeability.

When microcracks occur in normal members, fatigue cracks are
Since the cracks occur in a direction perpendicular to the direction of the tensile stress acting on the member, they appear as a group of cracks 9 oriented in almost the same direction as shown in FIG. When the potential difference is measured on the (b)-(b) line in Fig. 4, the current flows in a direction that crosses the cross section shown in Fig. 5, that is, in a direction that crosses the crack surface at right angles.As mentioned above, Since the skin thickness δ is made small to the same level as the crack depth, the current bypasses the crack, and the potential difference Vl becomes thicker (increases) compared to the case where there is no crack.

In this case, the larger the number of microcracks, the larger Vl becomes.

On the other hand, (B) - (口°) direction is perpendicular to (A) - (A")
When the potential difference is measured on the line, the current flows across the cross section shown in FIG. 6 in a direction parallel to the crack surface, so the potential difference v2 is not affected by the crack.

Therefore, if there is no crack, ■Iζ■, but if there is a crack, V,>V! becomes.

In actual investigation, since the stress direction is often unknown, the probe 4 shown in FIGS. 2 and 3 is brought into contact in any direction to measure the potential difference (2) and vl in the two directions. Next, the probe 4 is rotated 45 degrees and the measurement is performed again. In either measurement, if VIζ■2, it is determined that no crack has occurred, and if v>v' or v<v', it is determined that a micro-crack has occurred, and detailed replica observation is performed.

However, in the case of an actual member, even if microcracks occur, the exact depth thereof is unknown, so the optimal value of the skin thickness δ cannot be determined in advance. Therefore, the frequency of the applied current is changed continuously between about 10 and 1000 μm so that the value of δ can cover the target crack depth.

Make a comparison.

By using this device in this way, the occurrence of microcracks can be easily detected.

FIG. 7 is a diagram showing the configuration of a probe in another embodiment of the present invention. In this embodiment, four sets of four terminals are fixed so that their axes intersect at the center. Although this probe has a slightly more complicated terminal configuration than that shown in Fig. 3, the switching circuit 3 can switch the probe 4 without rotating it 45 degrees, allowing the V and ■
1 can be measured.

〔Effect of the invention〕

According to the present invention, the presence or absence of microcracks can be detected by bringing a probe into contact with the surface of the member without requiring mirror-like finishing after removing oxide scale from the surface of the member, as in replica observation.

Furthermore, since the evaluation is performed by comparing the potential difference in the orthogonal directions at the same location, rather than the absolute value of the potential difference, it is not affected by the material of the member or the surface roughness. Therefore, microcracks can be detected more efficiently than in the past, which has a significant effect on plant safety management.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the system of a crack detection device showing an embodiment of the present invention, Fig. 2 is a diagram showing the configuration of the probe, and Fig. 3 is a diagram showing the probe contact on the surface of the specimen material shown in Fig. 1. FIGS. 4 to 6 are explanatory diagrams showing the occurrence of microcracks on the surface of the test material. FIG. 7 is a diagram showing the configuration of a probe according to another embodiment of the present invention. 8 are explanatory views showing examples of occurrence of microcracks. 1: Microvoltmeter, 2: Constant current device, 3: Switching circuit, 4 nib lobe, 5: Lead wire, 6: Component, 7: Power supply terminal, 8
: Detection terminal. Agent Patent Attorney Takeshi Kawakita 1: Microvoltmeter 2: Constant current device 3: Switching circuit 4 Nib lobe 7: Power supply terminal 8: Detection terminal 6: Parts Figure 4 (A)

Claims (1)

[Claims] Two power supply terminals are brought into contact with the member at two points on the surface of the member to be inspected for the presence or absence of cracks, two detection terminals are brought into contact with the member at two points within a line connecting the two member surface contact points, and a constant current device is installed. In an apparatus for detecting cracks on the surface of a member by supplying a predetermined current to the two power supply terminals and detecting the potential difference between the two detection terminals using a microvoltmeter, two directions perpendicular to each other at the same location on the surface of the member are provided. A crack detection device characterized in that the above-described potential difference is measured and the presence or absence of a crack is determined by comparing both potential differences.