JPS5850457A - Detection of crack - Google Patents

Abstract

PURPOSE:To detect the shape and dimensions of a crack on the surface at a high accuacy by determining the surface length of the crack from the range of 2sigma and the depth thereof from the area ratio thereof and the ratio of potential difference through approximation of the distribution of the potential difference ratio to the normal distribution. CONSTITUTION:Direct current runs between A-A' from a D.C. power source (c) to measure a potential difference at upper and lower positions sandwitching a crack (a) between terminals 1-1', 2-2', 3-3'... and the results are inputted into a crack shape analyzer (f) through a micro voltometer (e) to compute the shape of the crack. In the analysis of the crack shape, the ratio V/V0 is obtained between a potential difference V0 without crack and a potential difference V after a crack develops to prepare a distribution of the potential difference ratio from which the shape and dimensions of the crack are determined. In approximation of the distribution of the potential difference ratio to the normal distribution, the surface length of the crack corresponds to the position of 2sigma (sigma: standard deviation). To determine the depth of the cracking, an equation of the cracking area S is established from a relationship of the ratio betwen the area S of the cracking and the total sectional area S0 thereof with the ratio of potential difference and calculated by substituting the surface length thereof previously obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for detecting cracks occurring in equipment or piping.

Conventionally, there has been no example of crack detection using the electric potential method in which only one-dimensional dimension (length or depth) of a crack was measured. Detection techniques used so far include AE and ultrasound methods. However, the number of cracks that occur in the actual machine is 2
This is a crack that has a dimensional spread, and currently there is no measurement data analysis method that can accurately detect the nine dimensions of such a crack.

The purpose of the present invention is to detect surface cracks occurring in equipment and piping in a non-destructive manner using the electric potential method.
The purpose is to determine the dimensions.

The present invention deals with statistical processing of measurement data, specifically, approximating the potential difference distribution to a normal distribution, and calculating from the range of 2σ (σ: standard deviation) one of the two degrees of freedom of a two-dimensional crack (surface crack). The surface length C is determined, and the crack depth a of the remaining one degree of freedom is determined from the crack area ratio and the potential difference ratio.

As an example of the present invention, consider a flat plate having surface cracks as shown in FIG. The figure also shows a diagram of the apparatus of the present invention. A DC current is applied from the DC power supply to A in the figure between '' and the crack is sandwiched between them.
, ■-■', ■-■'... The crack shape is measured via a microvoltmeter and then entered into a fabrication shape analysis device (microcomputer) to calculate the crack shape. The calculation results can be displayed on a CRT screen or other display methods, and crack growth can be monitored online.

The method of analyzing the woodworking shape from the measured potential difference ratio distribution is performed as follows.

Figure 2 shows the potential difference V when there is no crack. A distribution diagram of the ratio V/Vo to the potential difference V after the crack grows is shown from the crack core, and the crack shape and size are determined from this potential difference ratio distribution.

The crack shape is generally determined by the crack surface length C and the crack depth a. Therefore, first determine the surface length C, and then determine the depth a. C is the second
As shown in the lower half of the figure, it can be clearly seen that the potential difference ratio at the crack edge on the surface is V/Vo=1.
It is not o. In other words, a=Q, Q r in the figure
The position where ran (corresponding to the position of the surface length C) intersects with the broken line corresponds to the surface length C of the crack. FIG. 3 schematically shows this relationship. The distance at the position of a-0,0 in the figure is equal to the surface length C of the crack.

The position of Ll in L, = is the position of 2σ when the potential difference ratio distribution is approximated to a normal distribution, and the potential difference ratio distribution is v/vo-1
, o, and the distance (Ll+L2) at position t is divided 2:1.

Once the crack surface length C is determined by this method, the rest is to find the crack depth a.

Now, the area occupied by the crack S and the total cross-sectional area S. FIG. 4 shows the relationship tlj between the ratio of tlj and the potential difference ratio. From this relationship, the crack area S is determined from the measured potential difference ratio.

Assuming that the crack shape is an ellipse, its area is S−πaC Therefore, the crack depth a is a=S/πC Calculated by

The shape and dimensions of surface cracks occurring in equipment or piping can be detected with high accuracy.

It can also be used for online lifespan diagnosis of equipment, and the health of the equipment can be constantly monitored.

Furthermore, the electric potential method used here has a more stable output than other non-destructive testing techniques, can be measured with a simple device, and is highly economical.

According to the present invention, the size and shape of a crack can be detected with high accuracy.

[Brief explanation of the drawing]

Figure 1 is a system diagram of measurement using DC potential.
The figure shows the potential difference ratio distribution, Figure 3 shows the position of the crack surface length C in the potential difference ratio distribution, and Figure 4 shows the relationship between the covered area ratio and the potential difference ratio. . A, A'...Current input potential, ■, ■'...■'...
・Potential detection terminal. Agent Patent Attorney Akio Takahashi Me 1 Figure 3 Figure Saarmor f (Ka5ku 1) Utami Hato Uma - Distance L No.
da figure

Claims (1)

[Claims] 1. In a method of detecting a crack based on a potential difference between measuring terminals arranged at arbitrary intervals by passing a direct current through a metal structure as an object to be measured, the potential difference at the start of measurement is used as a reference potential difference. Then, the potential difference ratio is obtained by dividing the potential difference continuously measured at a predetermined time interval by the reference potential difference, and the area between the measurement terminals where the value is greater than 1 is determined to be a crack occurrence or crack propagation location. Then, the surface length of the crack is determined by statistical processing of the potential difference ratio, and the crack depth is determined from the relationship between the crack area ratio and the potential difference ratio, which is determined by approximating the crack to an ellipse or semi-ellipse. A crack detection method that detects the forged shape by determining. 2. The crack detection method according to claim 1, characterized in that a position twice the standard deviation σ of the potential difference ratio distribution is set as the surface length.