JP3427737B2 - Prediction method of fatigue crack growth direction - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention uses a magnetostrictive sensor for measuring a stress state acting on a steel structure or the like,
The present invention relates to a method of predicting the propagation direction of a fatigue crack generated by stress repeatedly applied.

[0002]

2. Description of the Related Art Conventionally, cracks generated on or near the surface of steel structures or mechanical parts have been detected by "visual inspection", "penetration flaw detection test", "magnetic particle flaw detection test", "eddy current flaw detection test". , Etc.

On the other hand, a built-up member in which a web and a flange are assembled by welding, which are used in each member of a truss structure among steel structures, a built-up longe for a ship double bottom, a stiffener with a flange for a ship, a bridge, etc. There is a demand for a method of predicting the direction of fatigue crack propagation that occurs due to repeated loading so that measures can be taken to prevent serious accidents in the assembled state.

[0004]

However, although the above-mentioned conventional crack detection method can detect the crack itself, it is difficult to predict the propagation direction of the crack.

The present invention has been made to solve the above problems, and an object thereof is to provide a method for predicting the propagation direction of a fatigue crack occurring in a member that is repeatedly subjected to an axial load.

[0006]

SUMMARY OF THE INVENTION The above-mentioned problems are solved by the first aspect of the present invention, in which a U-shaped yoke having an exciting coil wound around it and a U-shaped yoke having a detecting coil wound around the center of the yoke saddle portion. Stresses acting on the object to be measured by measuring the electromotive force induced in the detection coil by exciting an object to be measured by passing an alternating current through the excitation coil. When the magnetostrictive sensor for determining the state is rotated on the object to be measured in a non-contact manner and the waveform of the electromotive force induced in the detection coil is represented by the following formula (1), the parameter C
The maximum principal stress direction obtained from the above is measured at a plurality of points of the object to be measured, and the fatigue crack growth direction prediction method connecting the convergence points of the plurality of points in the maximum principal stress direction is solved.

V = A + B · COS [2 · (θ−C)] (1) where V is the rectified value of the AC electromotive force induced in the detection coil, and θ is the detection coil. The angle between the straight line connecting the open ends of the wound U-shaped yoke and the direction of maximum principal stress, CO
S [2 · (θ−C)] is a cosine function, and A, B and C are parameters.

A second aspect of the present invention is a structure in which the structure of the object to be measured has a fillet weld that repeatedly receives an axial force. For example, the first aspect of the present invention is a built-up member in which a web and a flange are assembled by welding. Is a method of predicting the direction of fatigue crack growth.

First, the principle of the magnetostrictive sensor used in the method of the present invention will be described. As a method for measuring the stress acting on a ferromagnet such as a steel material, there is a stress measuring method using a magnetostriction effect, that is, a phenomenon in which magnetic properties change due to the stress. Among them, the stress measuring method utilizing the magnetic anisotropy caused by the magnetostrictive effect is a method for non-destructively measuring the stress acting on a steel structure or a mechanical part, and is relatively simple. No. 121325, Japanese Utility Model Publication No. 1-135338, and Japanese Patent Laid-Open No. 7-11
No. 0270 Publication or Document 1 [Boundaries, etc .: JSCE No. 50
Proceedings of the Annual Scientific Lecture Meeting, P662-663 (199
5.9)] etc.

FIG. 6 shows an example of a stress measuring method utilizing the magnetic anisotropy caused by the magnetostrictive effect.

In this method, a U-shaped yoke 11 around which an exciting coil is wound, a U-shaped yoke 12 around which a detecting coil is wound, an AC power supply 13 for excitation, and a magnetic flux flowing through the object 20 to be measured. The stress is measured on the basis of the following principle using the magnetostrictive sensor 1 which is composed of a voltmeter 14 for detecting the following, and in which the yoke 11 and the yoke 12 are arranged so as to be orthogonal to each other at the center of the yoke saddle.

When a tensile stress σx acts on the measured object 20 in the X-axis direction, the magnetic permeability μx, μy in the X- and Y-axis directions of the measured object 20, which is a magnetic material, can be expressed by the following equation due to the magnetostriction effect. The relationship of (2), that is, magnetic anisotropy occurs. μx> μy (2)

The magnetostrictive sensor 1 is brought close to the DUT 20 in such a state, and the yoke 1 of the magnetostrictive sensor 1 is moved.
When an alternating current is supplied from the alternating-current power supply 13 to the excitation coil wound around 1 to excite the object to be measured 20, most of the magnetic flux emitted from the opening end 11a of the yoke 11 goes directly to the opening end 11b of the yoke 11. Since the tensile stress σx causes the magnetic anisotropy in the object to be measured 20 as shown in the equation (2), a part of the magnetic flux passes through the yoke 12 and the opening end 1 of the yoke 11 moves.
Flow to 1b. Therefore, an electromotive force V having a waveform represented by the following formula (3) is induced in the voltmeter 14 attached to the detection coil wound around the yoke 12.

V = M ₀ · (μx−μy) · COS [2 · (θ−π / 4)] (3) where V is the rectified value of the AC electromotive force induced in the detection coil. , M ₀ is a constant determined by the excitation condition, the coil condition, the magnetic characteristics of the DUT 20, and the like, COS [2 · (θ−π /
4)] is a cosine function, and θ is the opening ends 12a and 12 of the yoke 12.
It is the angle between the straight line connecting b and the X axis.

The difference in magnetic permeability (μx-μy) is the difference in stress (σ
x-σy), the formula (3) is expressed by the following formula (4).
Can be rewritten as

V = M · (σx−σy) · COS [2 · (θ−π / 4)] (4) where M is the excitation condition, the coil condition, the magnetic field of the DUT 20. It is a constant determined by the characteristics.

From the equation (4), the stress acting on the object to be measured 20 can be obtained by measuring V.

The magnetostrictive sensor is rotated at a fixed distance (called lift-off) from an object to be measured such as a steel structure or a mechanical part, that is, θ is changed and induced in the detection coil of the magnetostrictive sensor. If the waveforms of electric power are obtained and the parameters B and C obtained when the waveforms are expressed by the equation (1) are measured on a fixed point of the object to be measured, it is clear by comparing the equations (1) and (4). Since the parameter B represents the principal stress difference and the parameter C represents the direction of the maximum principal stress, the stress state at the measurement point can be known from B and C.

Next, a method of predicting the propagation direction of a fatigue crack in a member that is repeatedly subjected to an axial load using this magnetostrictive sensor, for example, a built-up member in which a web and a flange are assembled by welding, will be described.

FIG. 2 shows an example of a built-up member in which the web and the flange are assembled by welding.

In this built-up member, the flange 3 is attached to the web 2 by fillet welding 4. When a constant amount of stress is repeatedly applied to this member, a fatigue crack is generated starting from the corner turning welded portion 5.

FIG. 3 shows measurement points of the stress state near the corner turning welded portion of FIG. The stress state is measured by the magnetostrictive sensor at a plurality of measurement points 6 near the corner turning welded portion 5.

FIG. 4 shows a stress state at the initial stage of crack initiation when a tensile stress is repeatedly applied in the X direction of the web of FIG. In FIG. 4, the size of the arrow at each measurement point 6 represents the size of the principal stress difference, and the direction represents the maximum principal stress direction.

When tensile stress is repeatedly applied in the X direction,
Although the crack 7 occurs in the Y direction, the maximum principal stress direction is not in the X direction due to the residual stress due to welding.

FIG. 5 shows a state where the maximum principal stress direction at each measurement point in FIG. 4 is extended. When the maximum principal stress direction of each measurement point 6 is extended, the extension lines of some measurement points 6 in the maximum principal stress direction converge to one point 8, and a straight line obtained by connecting the convergence points 8 of the maximum principal stress direction is drawn. It became clear that it was in agreement with the direction of crack propagation.

In fact, in the case of FIG. 4, the straight line obtained by connecting the convergence points in the direction of the maximum principal stress is the propagation direction of the crack 7,
That is, it can be seen that they match in the Y direction.

Therefore, even when the tensile stress repeatedly acts in any direction, the maximum principal stress direction is obtained at a plurality of points as described above, and the propagation direction of the fatigue crack can be predicted by connecting the convergence points.

The method of predicting the direction of fatigue crack growth according to the present invention is applicable not only to a built-up member in which a web and a flange are assembled by welding, but also to a structure having a fillet weld which is repeatedly subjected to an axial load. The same applies.

[0029]

1 is a flow chart showing an embodiment of the method of the present invention.

First, a plurality of measurement points are set in the vicinity of the corner-turned welded portion with the flange on the web, and the magnetostrictive sensor shown in FIG. 6 is installed at a certain measurement point at a lift-off of, for example, 1 mm ( S1). Then, the magnetostrictive sensor is rotated and the electromotive force is measured to obtain the waveform (S
2) Obtain the parameter C when the obtained waveform is expressed by the above equation (1) (S3). Similarly, the parameter C at another measurement point is also obtained (S3).

Next, the maximum principal stress direction of each measurement point is obtained from the parameter C of each measurement point (S4), its convergence point is obtained (S5), the convergence point is connected and the crack propagation direction is determined.

[0032]

Since the present invention is configured as described above, it is possible to provide a method for predicting the propagation direction of a fatigue crack that occurs in a member that is repeatedly subjected to an axial load.

If a drill hole is provided in the fatigue crack propagation direction predicted by the method of the present invention, the crack propagation can be prevented and a serious accident can be prevented.

[Brief description of drawings]

FIG. 1 is a flow chart showing an embodiment of a method of the present invention.

FIG. 2 is a view showing an example of a built-up member in which a web and a flange are assembled by welding.

FIG. 3 is a diagram showing measurement points of a stress state in the vicinity of the corner turning welded portion of FIG. 2;

FIG. 4 is a diagram showing a stress state in the initial stage of crack generation when a tensile stress is repeatedly applied to the web of FIG. 2 in the X direction.

FIG. 5 is a diagram showing a state in which the maximum principal stress direction at each measurement point in FIG. 4 is extended.

FIG. 6 is a diagram showing an example of a stress measuring method utilizing magnetic anisotropy generated by a magnetostrictive effect.

[Explanation of symbols]

1 Magnetostrictive sensor 2 web 3 flange 4 fillet welds 5 corner turning weld 6 measurement points 7 cracks 8 Convergence point in the direction of maximum principal stress 11 U-shaped yoke with a coil for excitation 11a Opening end of yoke 11 11b Opening end of yoke 11 12 U-shaped yoke with a coil for detection 12a Opening end of yoke 12 12b Opening end of yoke 12 13 AC power supply 14 Voltmeter 20 DUT 30 Direction of magnetic flux

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP 2000-2601 (JP, A) JP 55-46143 (JP, A) JP 11-23537 (JP, A) JP 10-332643 (JP, A) JP-A-6-265525 (JP, A) Special table: 8-508343 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/72-27 / 90 JISST file (JOIS) Practical file (PATOLIS) Patent file (PATOLIS)

Claims (2)

(57) [Claims] 1. A U-shaped yoke around which an exciting coil is wound and a U-shaped yoke around which a detecting coil is wound are arranged so as to be orthogonal to each other at the center of the yoke saddle, and the exciting coil is connected to the exciting coil. A magnetostrictive sensor is excited on an object to be measured by exciting an object to be measured by applying an alternating current and measuring the electromotive force induced in the detection coil to obtain a stress state acting on the object to be measured. When the waveform of the electromotive force induced in the detection coil by rotating the coil is represented by the following formula (1), the maximum principal stress direction obtained from the parameter C is measured at a plurality of points of the measured object, A method of predicting a fatigue crack propagation direction obtained by connecting the convergence points of the plurality of points in the maximum principal stress direction. V = A + B · COS [2 · (θ−C)] (1) where V is the rectified value of the AC electromotive force induced in the detection coil, and θ is the coil wound around the detection coil. The angle between the straight line connecting the open ends of the U-shaped yoke and the direction of maximum principal stress, CO
S [2 · (θ−C)] is a cosine function, and A, B and C are parameters. 2. The method for predicting the direction of propagation of a fatigue crack according to claim 1, wherein the structure of the object to be measured has a fillet weld that is repeatedly subjected to an axial load.