JPH0792139A - Method for evaluating fatigue damage of material - Google Patents

Abstract

PURPOSE:To make it possible to evaluate fatigue damage not only in the final stage but also the initial stage by conducting fatigue test of a reference piece to determine a fatigue characteristic curve and then collating the field strength measured for a material to be inspected with the fatigue characteristic curve. CONSTITUTION:A flux meter 3 and a superconducting coil 4 for exciting alternating magnetic field are disposed in a Dewar vessel 2 filled with liquid helium. Assuming the instantaneous strength of alternating magnetic field generated by the coil 4 is H and the instantaneous strength of field when a test piece 5 is placed in the alternating magnetic field is H', the ratio theta=H'/H is constant. The magnitude of theta corresponds to the accumulated value of repetitive stress. The value of theta is determined for the number of acting times of repetitive stress assuming it is equal to theta0 under unfatigued state. Subsequently, a variation DELTAtheta=theta-theta0 is calculated and a fatigue characteristic curve is prepared from the ratio DELTAtheta/theta0 for the number of acting times N. The field strength measured for a member to be inspected is then collated with the fatigue characteristic curve thus determining and evaluating the fatigue damage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating the degree of fatigue damage of materials by a nondestructive inspection method.

[0002]

2. Description of the Related Art Conventionally, various methods have been provided for evaluating the degree of fatigue damage of various materials without destroying them. For example, the fact that when a defect such as a macrocrack occurs in the material, the intensity of the radiation (X-rays, γ-rays, etc.) transmitted through the defective portion becomes larger than that transmitted through a healthy portion, Radiation-sensitive film is placed behind the to detect the defect of the material to be detected from the captured image, or a very short ultrasonic pulse is emitted inside the material to detect its reflection echo and transmitted ultrasonic waves. An ultrasonic flaw detection method or the like is known in which the position and the size of a defect such as a macrocrack in the material to be inspected are received.

[0003]

However, the above-mentioned conventional nondestructive inspection method has the following problems. In other words, before a material undergoes fatigue failure, first a microscopic change in physical properties occurs at the crystal lattice level,
After that, it is known that macrocracks occur and lead to rupture, but most of the period until such fatigue fracture occurs due to microscopic physical property changes at the crystal lattice level before macrocrack occurrence. Occupied.

On the other hand, in the conventional nondestructive inspection method, the fatigue damage degree of the material is evaluated from the position and shape of the macrocracks generated in the material as described above. Only the end-of-life assessment leading to fatigue failure could be done.

The present invention has been made in order to solve such a conventional problem, and provides a method capable of evaluating the degree of fatigue damage not only in the final stage of fatigue fracture but also in the early stages. The purpose is to

[0006]

[Means for Solving the Problems] To achieve the above object,
The invention of claim 1 is a method for evaluating the degree of fatigue damage of a material, comprising a step of performing a fatigue test on a reference piece of a material corresponding to a test material, and a plurality of time points during the fatigue test. Positioning the reference piece in an alternating magnetic field whose instantaneous strength is known and measuring the magnetic field strength when the reference piece is located, a step of obtaining a fatigue characteristic curve from the measured magnetic field strength, and The step of locating the test material in an alternating magnetic field whose instantaneous strength is known and measuring the strength of the magnetic field, and the magnetic field strength measured for the test material are compared with the fatigue characteristic curve to determine the degree of fatigue damage of the test material. And a method of evaluating the degree of fatigue damage of a material including the step of:

According to a second aspect of the invention, in the first aspect of the invention, the strength of the magnetic field is measured by a superconducting quantum interference device, and the instantaneous strength of the alternating magnetic field is H, the reference piece or the test object. When the instantaneous value of the magnetic field strength when the material is positioned in the alternating magnetic field is H ′, and the ratio of H ′ to H (H ′ / H) is θ, the θ of the θ at a plurality of time points during the fatigue test is set. The present invention relates to a method of evaluating the fatigue characteristic curve from a value and obtaining the fatigue damage degree of the test material from the value of θ for the test material to evaluate the fatigue damage degree of the material.

[0008]

The principle of the evaluation method according to the present invention will be described below.

A. First, the fatigue characteristic curve of the test material is obtained, and this "fatigue characteristic curve" is the reference of the evaluation when evaluating the fatigue damage degree of the test material, It is possible to determine the remaining life of the material to be tested, that is, how much life is left until it breaks, by referring to the position on the fatigue characteristic curve at which is. The fatigue characteristic curve is obtained as follows.

First, for example, bending, tension, compression, twisting,
A fatigue test is performed in which at least one kind of repetitive stress of a predetermined type predicted in the use environment of the test material such as impact is applied to a reference piece of a material corresponding to the test material.

Then, the reference piece is positioned in an alternating magnetic field of known strength and the reference piece is positioned at a plurality of times from the time when the repeated stress is applied to the reference piece until the reference piece is destroyed. Measure the magnetic field strength.

[0012] Here, the inventor has found that the magnetic field strength when the reference piece is positioned with respect to the strength of the alternating magnetic field is
It has been found that a constant value is taken according to the cumulative amount of repeated stress (for example, the number of actions or the action time). Also,
This constant value varies depending on the type of material (material), workability, etc., and fatigue conditions, but if the material, workability, fatigue conditions, etc. are the same, it takes the same value.

Therefore, a reference piece having the same material, workability, etc. as the test material is prepared, and the relationship between the integrated amount of repeated stress with respect to the reference piece and the constant value corresponding to the integrated amount is obtained, and this is calculated. Fatigue characteristic curve.

B. Next, the fatigue damage degree of the test material is evaluated, but first, the magnetic field strength is measured when the test material is positioned in an alternating magnetic field whose strength is known, and when the test material is positioned, The magnitude of this magnetic field strength with respect to the strength of the alternating magnetic field (the strength when the test material is not in the magnetic field) is calculated.

Then, the remaining life of the material to be tested can be judged depending on the position of the calculated value on the fatigue characteristic curve. This is because, as described above, the magnetic field strength when the material is positioned with respect to the magnetic field strength when there is no material takes a constant value according to the cumulative amount of repeated stress (for example, the number of times of action or the time of action). Moreover, this value is the same if the type of material and the degree of processing are the same.

Further, according to the invention of claim 2,
A superconducting quantum interference device is used to measure the magnetic field strength. This superconducting quantum interference device (SQUID: Superconductive Quan
tumInterference Device) is widely used in various fields such as measurement of biomagnetic field because its magnetic detection sensitivity is very high as a magnetic sensor.
By using this SQUID, 10 ^-14 T (Wb / m
² ) It becomes possible to measure the magnetic field strength with high accuracy as below.
Therefore, accurate fatigue evaluation is possible.

[0017]

EXAMPLES The present invention will be described in more detail by way of examples. As one embodiment of the present invention, a steam generator (S
(G) Using a test piece made of Inconel 600 material, which is a heat transfer tube material, a fatigue test is performed on this to obtain a fatigue characteristic curve, and an AC magnetic field is applied by a superconducting coil and a SQUID magnetometer is used. Fatigue damage was evaluated by measuring the magnetic field strength.

As shown in FIG. 2, the test piece has a shape in which the width of the intermediate portion is narrowed, and the dimension thereof is the total length 2
The width is 20 mm, the width is 60 mm, the thickness is 5 mm, and the width of the middle portion is 20 mm. Further, in the fatigue test, compression is not performed by perfect swinging in which pulling and returning are repeated in the longitudinal direction of the test piece. The applied load has a maximum stress of 50 kgf / mm ² and a frequency of 10 Hz.

FIG. 1 shows an apparatus (hereinafter, simply referred to as "measuring apparatus") for generating an alternating magnetic field and measuring the magnetic field strength, which is used in this embodiment. As shown in the figure, this measuring apparatus 1 is provided with a SQUID magnetometer 3 and a superconducting coil 4 for exciting an AC magnetic field in a dewar 2 filled with liquid helium.
Although it depends on the size of the distance d between the detection coil 3a of the magnetometer 3 and the test piece 5, it is about 1 A or less, and the frequency of the excited AC magnetic field is 0.5 Hz. Further, the sampling rate of the SQUID magnetometer 3 is 100 Hz (measurement is performed 100 times per second).

FIG. 3 shows the strength of the AC magnetic field generated by the coil 4 of the measuring apparatus 1 and the output of the SQUID magnetometer 3 (the magnetic field strength when the test piece is placed in the magnetic field generated by the coil). And FIG. 4 is a diagram showing the relationship between the two. Now, let H be the instantaneous value of the AC magnetic field strength generated by the coil 4 of the measuring apparatus 1 and H ′ be the instantaneous value of the magnetic field strength when the test piece 5 is positioned in the AC magnetic field. Ratio θ (θ = H '/ H)
Takes a constant value as is clear from FIG.

The magnitude of θ depends on the cumulative amount of repeated stress (for example, the number of actions). Figure 5
FIG. 4 is a diagram obtained by plotting the value of θ against the number of times (N) the repeated stress is applied. In addition,
In the figure, θ ₀ is a value of θ in an unfatigue state where repetitive stress is not applied.

Furthermore, the amount of change in θ due to fatigue Δθ (Δθ
= Θ−θ ₀ ) and calculate Δθ / with respect to the number of actions (N)
The plot shown in FIG. 6 was obtained by plotting θ ₀ . This is the fatigue characteristic curve. The fatigue characteristic curve may change depending on the type of material, the degree of working, the fatigue condition, etc. as described above, but the fatigue characteristic becomes a calibration curve under the fatigue condition necessary for the object to be evaluated for fatigue damage. If the curve is obtained, the degree of fatigue damage (remaining fatigue life) can be obtained based on this calibration curve.

As described above, when the fatigue conditions (material, workability, maximum stress, number of cycles, operating temperature, atmosphere, etc.) of the target material are known, the fatigue characteristics of the material under that condition If the curve (FIG. 6) is obtained in advance, the degree of fatigue damage in an actual machine can be easily known by measuring Δθ / θ ₀ with the measuring device 1. Note that FIG.
In the example of, Δθ / θ ₀ ≈−0.05 is the standard of fracture.

When performing the measurement by the measuring device 1 of FIG. 1, the measuring device 1 may be arranged close to the fatigue testing machine for performing the fatigue test, or the test may be performed from the fatigue testing machine. It is also possible to remove the piece and put it on the measuring device 1. Further, the AC magnetic field strength H does not have to be measured every time, and if the current flowing through the coil 4 is kept constant during measurement, the value set at the beginning can be used.

[0025]

As described above, according to the present invention,
It is possible to evaluate a wide range of fatigue damage not only after the occurrence of macrocracks but also before that.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing the configuration of a measuring device used in an evaluation method according to an embodiment of the present invention.

FIG. 2 is a diagram showing a test piece used in an evaluation method according to an example of the present invention.

FIG. 3 is a diagram showing the strength of an AC magnetic field generated by a coil of the measuring device and the magnetic field strength (output of a SQUID magnetometer) when a test piece is placed in the magnetic field.

FIG. 4 is a diagram showing a relationship between the strength of an alternating magnetic field generated by a coil of the measuring device and the magnetic field strength (output of a SQUID magnetometer) when a test piece is placed in the magnetic field. .

FIG. 5 is a diagram showing a relationship between the number of times (N) the repeated stress is applied and θ.

FIG. 6 is the number of times (N) the repeated stress is applied and Δθ / θ ₀
It is a diagram which shows the relationship with.

[Explanation of symbols]

1: Measuring device, 2: Dewar filled with liquid helium, 3: SQUID magnetometer 4: Superconducting coil for AC magnetic field excitation, 5: Test piece

Claims (2)

[Claims] 1. A method for evaluating a degree of fatigue damage of a material, comprising a step of performing a fatigue test on a reference piece of a material corresponding to a material to be tested, and the reference at a plurality of points during the fatigue test. A step of locating the piece in an alternating magnetic field whose instantaneous strength is known and a step of measuring the magnetic field strength when the reference piece is positioned; a step of obtaining a fatigue characteristic curve from the measured magnetic field strength; And the step of measuring the strength of the magnetic field by locating the instantaneous strength in a known AC magnetic field, and determining the fatigue damage degree of the test material by collating the magnetic field strength measured for the test material with the fatigue characteristic curve. And a method of evaluating the degree of fatigue damage of a material including the steps. 2. The strength of the magnetic field is measured by a superconducting quantum interference device, the instantaneous strength of the AC magnetic field is H, and the strength of the magnetic field when the reference piece or the test material is positioned in the AC magnetic field is measured. The instantaneous value is H ', the ratio of H'to H (H' / H) is θ
The material according to claim 1, wherein the fatigue characteristic curve is obtained from the value of the θ at a plurality of times during the fatigue test, and the degree of fatigue damage of the material is obtained from the value of the θ for the material to be inspected. Method for assessing fatigue damage in humans.