JP3091856B2 - How to evaluate the fatigue of structural materials - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of inspecting and evaluating the degree of damage of a structural material such as a metal due to fatigue by a nondestructive inspection technique.

[0002]

2. Description of the Related Art The soundness of the structure, material, strength, shape, etc. of a test material can be determined without destroying the test material such as various materials and structures, and the physical properties of the test material can be detected. Conventionally, various non-destructive inspection methods have been used to evaluate using the results. For example, the following non-destructive inspection methods are mainly used for material inspection.

[0003] 1. Radiation flaw detection: When a defect such as a microcrack exists in an object, the intensity of radiation (such as X-rays and γ-rays) transmitted through the defect becomes stronger than that transmitted through a healthy portion. Therefore, a radiation sensitive film is arranged behind the test material, the shape of the defect is photographed on a photographic film, and the soundness of the test material is examined from the photographed image.

[0004] 2. Ultrasonic flaw detection: (a) Pulse reflection method: An extremely short ultrasonic pulse is propagated in a test material, and a reflected echo due to a defect such as a micro crack in the test material is received to know its position and size. (b) Transmission method: A continuous wave or pulse wave ultrasonic wave is emitted from the transmitting probe, the ultrasonic wave transmitted through the test material is received by the receiving probe, and the position and size of the defect are determined based on the intensity of the received wave. Observe etc. (c) Resonance method: A high-frequency voltage whose frequency is continuously related is applied to the probe, and a standing wave that resonates with the thickness of the test material is measured to measure the thickness of the test material.

[0005] 3. Eddy current flaw detection: Detects defects such as cracks in the test material from the relationship between the strength of the AC magnetic field generated from the coil and the resulting distribution of the magnetic field due to the eddy current generated in the test material. . Generally, the detection sensitivity is lower than that of the ultrasonic method, but the sensitivity is high for a specific crack, so that it is often used as an auxiliary means for an ultrasonic inspection.

[0006]

Generally, when a metal material is subjected to, for example, a repetitive stress, a crystal grain in the material slips, and then a microcrack is generated. When the crack progresses, a fatigue fracture finally occurs. By the way, in this case, most of the period until the material reaches fatigue fracture is occupied by microscopic changes at the crystal lattice level before microcracks occur.

[0007] Non-destructive inspection techniques conventionally used for evaluating the degree of fatigue mainly aim at detecting the size, shape and position of microcracks that have already occurred. The degree of fatigue could not be evaluated.

An object of the present invention is to provide a method capable of measuring and evaluating the fatigue degree of a test material by a non-destructive inspection method before the test material such as a metal is destroyed, that is, before the occurrence of microcracks. To provide.

[0009]

The basic philosophy of the present invention is that data of an X-ray diffraction peak (amplitude height or energy) obtained by irradiating a material having a crystal structure such as a metal with X-rays is obtained. It is based on the fact that it shows a characteristic change as a precursor to fatigue fracture particularly as the fatigue of the material progresses.

According to one characteristic aspect of the invention, a method for assessing the degree of fatigue of a structural material includes, for example, bending,
A step of performing a fatigue test by applying at least one of predetermined types of repetitive stresses predicted in the use environment of the test material such as tensile compression, torsion, and impact to a reference piece of a material corresponding to the test material. And a predetermined diffraction angle of a predetermined diffraction angle generated from a portion to be irradiated by irradiating the reference piece with X-rays at a plurality of time points from when the repetitive stress is applied to the reference piece to when the reference piece is broken. A step of measuring the intensity of the diffracted X-ray, and from this measurement result, a fatigue characteristic curve corresponding to the relationship between the integrated amount of the repetitive stress (eg, represented by the number of times of action or the time of action) and the intensity of the diffracted X-ray is obtained. A step of irradiating the measurement target site of the test material with the X-ray to measure the intensity of the diffracted X-ray having the diffraction angle generated from the measurement target site, and at least two points in the use environment. Comparing the rate of change between the diffracted X-ray intensities measured for the test material with the fatigue characteristic curve to determine the degree of fatigue of the test material and, if necessary, the remaining life up to fatigue failure. And

[0011]

In the present invention, the diffracted X-ray intensity to be measured may be either the peak height (amplitude) or the peak area (energy) of the diffracted X-ray. This measurement is performed in each of the measurement step for the reference piece and the measurement step for the test material. In the measurement process for the reference piece, a predetermined repetitive stress is applied to the reference piece by a fatigue test method, and the stress applying operation is interrupted for each predetermined amount of integrated stress. X-ray diffraction measurement is performed. On the other hand, in the measurement process for the test material, for example, when the test material is a structural material, the test material in the use condition is applied to the same test material at different time points under the use condition, for example, during a plurality of periodic inspections. X-ray diffraction measurement is performed a plurality of times. The X-ray diffraction measurement for the test material under these use conditions may be performed on the test material temporarily removed from the place of use, or a probe for the X-ray diffraction measurement may be prepared. In this case, the measurement may be performed on the test material as it is at the place of use.

As described above, in the present invention, the X-ray diffraction intensity at a predetermined diffraction angle can be measured by irradiating the test material with X-rays by using, for example, an X-ray diffraction probe. The test material does not have to be sampled and is not destroyed by the measurement. X for this test material
The line diffraction measurement is performed, for example, at an initial time as a structural material and at a time after being subjected to a certain amount of repeated stress by the subsequent use, or at two different times during use. In any case, the change rate of the diffracted X-ray intensity at the diffraction angle of the test material measured at the two time points includes information corresponding to the change in the crystal structure in the test material before the crack occurs. Therefore, this rate of change is an index for evaluating fatigue due to the effect of the repeated stress applied to the test material between the two time points.

On the other hand, a reference piece made of the same material as the test material is subjected to at least one of repetitive stresses of a type expected in the use environment of the test material, such as bending, tensile compression, torsion, and impact. Is obtained as a fatigue characteristic curve.

In the present invention, the degree of fatigue of the test material is evaluated using the fatigue characteristic curve obtained for the reference piece as an index. That is, a fatigue characteristic curve showing a change in the specific diffraction X-ray intensity with respect to an integrated amount of a certain kind of repetitive stress is obtained for a reference piece of a material corresponding to the test material, and the corresponding corresponding value measured for the test material is obtained. The rate of change of the diffracted X-ray intensity by the integrated amount of stress action is compared with this fatigue characteristic curve,
For example, by judging whether the change rate is positive or negative, the degree of fatigue of the test material at the time of measurement is evaluated without destroying the test material, and the remaining life related to the fatigue is obtained.

The above-mentioned fatigue characteristic curve can be obtained as a curve in an orthogonal coordinate system in which the horizontal axis represents the amount of integrated stress such as the number of times of repeated stress application, and the vertical axis represents the diffraction X-ray intensity. The curves are the material of the test material,
Although it differs depending on various factors such as fatigue conditions, if the fatigue characteristic curve is obtained in advance on a coordinate axis scale standardized based on necessary fatigue conditions, etc., the material of the test material, the degree of work, the fatigue conditions, etc., differ. In some cases, the fatigue characteristic curve may be represented by a single curve. To do this,
For example, in the orthogonal coordinate system of the fatigue characteristic curve, the stress integrated amount on the horizontal axis can be represented by a ratio with the stress integrated amount when the reference piece reaches fatigue failure, and the diffraction X-ray intensity on the vertical axis. Can be expressed as a ratio to the initial diffraction X-ray intensity measured for the reference piece on which the repetitive stress is not applied. With this standardization, it is possible to evaluate the fatigue degree of the test material and estimate the remaining life from one common fatigue characteristic curve even when the material of the test material, the working degree, the fatigue condition, etc. are different. it can.

As described above, since the measurement result of the diffraction X-ray intensity includes various internal information such as the crystal structure of the test material, it is necessary to measure the change in the diffraction X-ray intensity due to fatigue. Allows evaluation of fatigue. As described above, according to the present invention, fatigue damage of a test material such as a metal before a microcrack occurs can be evaluated from a measurement result by a nondestructive measurement method.

The present invention is suitable for evaluating the degree of fatigue of various structural materials such as general mechanical parts, construction, construction, aircraft, ships, automobiles, nuclear reactors and their auxiliary equipment. The material can be applied not only to metals such as stainless steel and stainless alloys but also to any other materials having a general crystal structure.

In order to clarify the above and other objects, features and advantages of the present invention, preferred embodiments which are not intended to be limited are described in detail below with reference to the accompanying drawings.

[0019]

In FIG. 1, an object 11 to be measured is a test material or a reference piece, and an X-ray is applied to a portion of the surface of the object 11 to be measured.
An incident X-ray 13 from a radiation source 12 is emitted at an incident angle θ.
A diffracted X-ray 14 having a diffraction angle θ is generated from the irradiated portion, and is incident on an X-ray detector 15. The detector 15 supplies a detection signal to a measurement signal processing device (not shown), whereby the peak value of the diffracted X-ray at the diffraction angle θ from the irradiation site is obtained together with the measurement position data.

The specific operations of the main measurement performed in accordance with the present embodiment include the following processes 1 to 6.

Process 1: Prepare a reference piece of a material corresponding to the test material to be subjected to the fatigue damage evaluation, and use the test piece such as bending, tensile compression, torsion, and impact on the reference piece. At least one kind of repetitive stress of the kind expected in the environment is applied, and at a certain integrated amount of the applied repetitive stress, the peak value of the diffraction X-ray intensity is measured by X-ray diffraction measurement at a specific diffraction angle. The integrated amount is represented by, for example, the number of times of stress such as the number of times of bending.

Process 2: The peak intensity (peak height or peak area) of the obtained diffracted X-ray and the number of times of the stress at that time are plotted on a rectangular coordinate system. This operation may be performed by, for example, data processing in a computer attached to the measuring device.

Process 3: By repeating the above processes 1 and 2 for each different number of times of stress action, a characteristic graph (fatigue damage characteristic curve) showing the relationship between the number of times of stress action and the intensity of diffracted X-rays is obtained. FIG. 2 shows an example of the obtained fatigue characteristic curve, and FIG. 3 shows an example of the shape and size of the reference piece.

Process 4: The same X-ray diffraction measurement as in Process 1 is performed on the test material to obtain a first X-ray diffraction value.

Process 5: X-ray diffraction measurement of the test material after subjecting it to repeated stress for a certain period of time in a use environment in the same manner as in Process 4 to obtain a second diffraction X-ray intensity value Get.

Process 6: From the ratio between the first value obtained in Process 4 and the second value obtained in Process 5, the rate of change of the diffraction X-ray intensity for the test sample is determined.

Based on the rate of change obtained in the process 6 and the fatigue characteristic curve obtained in the process 3, it is possible to grasp the degree of fatigue of the test material.

The degree of fatigue is determined, for example, as follows.

In FIG. 2, the horizontal axis N represents the number of times of stress acting on the reference piece, and the vertical axis I represents the diffraction X-ray intensity. Stress acting number N _A, N respectively I _A respectively measured diffraction X-ray intensity at the time of _B, and the I _B, the change rate of the diffracted X-ray intensity to stress action times of between (ΔI / ΔN) is of the formula Become like

The _{ΔI / ΔN = (I B -I} A) / (N B -N A)

For example, in FIG. 2, the rate of change is negative before the number of times of action N _C showing the lowest diffraction X-ray intensity, and the number of times of action N
_It is 0 at _C and positive after the number of actions N _C.

Therefore, the rate of change (ΔI /
ΔN), if at least positive or negative is determined, the fatigue degree of the test material is either before or after the number of times of action N _C indicating the lowest diffraction X-ray intensity of the fatigue characteristic curve obtained for the reference piece. It knows whether the found from the fatigue curve, remaining life (fatigue) up to the stress acting time for fatigue failure N _f is predictable with considerable precision. Of course, the precursor to the fatigue failure point of the fatigue characteristic curve according to the material of the individual structural material as the test material also appears in, for example, the magnitude of the change rate, so that the remaining life of the material can be reduced. It is also possible to evaluate.

As for the interval (ΔN) at which the evaluation of the degree of fatigue is actually performed for the test material, it is preferable to determine the required interval from the fatigue characteristic curve obtained in advance for the corresponding reference piece.

The fatigue characteristic curve depends on the material of the test material, the degree of working, the fatigue condition, and the like. If the fatigue characteristic curve is standardized under necessary fatigue conditions in advance, the standardized fatigue characteristic curve is obtained. Based on the above, the degree of fatigue and the remaining life can be obtained even when the material of the test material, the degree of processing, the fatigue conditions, and the like are different. That is, for example, the ratio of the number of times of stress action on the horizontal axis in FIG. 2 divided by the number of times of stress action at the time of fatigue fracture, or the diffraction X-ray intensity on the vertical axis is the diffraction X-ray intensity (initial value) when no stress is applied. By using the divided ratio, the fatigue characteristic curve can be adjusted to one standardized characteristic curve even when the applied stress to the test material is different.

Next, an example in which the degree of fatigue of a reactor structural material was evaluated according to the present invention will be described.

The degree of fatigue of a reactor structural material made of a high nickel heat resistant alloy material (trade name: Inconel 718, FCC crystal) was evaluated by X-ray diffraction measurement.

Two reference pieces 1 having the same dimensions as those shown in FIG. 3 were prepared from the same alloy material. For these reference pieces 1,
A repetitive rotational bending stress was applied by the fatigue test apparatus shown in FIG. That is, this fatigue test apparatus
As shown in FIG. 4, bearings 4a, 4 supported tiltably at fulcrums 6a, 6b, respectively, are arranged facing each other.
b, two rotating chucks 2a, 2b rotatably supported by these bearings, a motor 5 for rotating one of the rotating chucks 2a, and a bending stress acting on the reference piece 1 gripped between the two bearings. And a weight 3 for applying a load to the vehicle.

The reference piece 1 is held by the chucks 2a and 2b.
By applying a load to the reference piece 1 by rotating the reference piece 1 by the motor 5 while applying a load to the reference piece 1 by rotating the reference piece 1 by the motor 5, a so-called rotary bending fatigue test is performed. Was done. The load applied by the weight to one reference piece is 75 kg / mm ² , and the load applied to the other reference piece is 80 kg / mm ² (the maximum evaluation stress value), and the rotation frequency is 30 Hz. .

The X-ray diffraction measurement was performed as follows.
That is, the X-ray tube used for the X-ray source of the X-ray diffractometer is a Cu tube (with a monochromator), and the excitation electron beam is
The wavelength of X-rays generated by irradiating a (copper) target was aligned by screening a graphite single crystal with a monochromator. Specific tube voltage and tube current are 40 kV and 4
The sample was irradiated with X-rays by a focused beam method using a slit at 0 mA. On the detection side of the X-ray diffractometer, a few degrees centered on the peak of the diffracted X-ray generated from the irradiated site
X-rays were counted while scanning with an X-ray counter for the width of. The scanning speed at this time is 1.
0 deg / min and the sampling step were 0.002 deg. In addition, the X-ray diffraction measurement was performed by removing the test piece from the fatigue tester at appropriate times during the fatigue test.

FIG. 5 shows the F of the reference piece 1 with respect to the number of times of rotation bending.
FIG. 4 is a fatigue characteristic diagram in which the peak height of a diffracted X-ray from the CC crystal (111) plane is plotted. In the figure, the symbol △ indicates the measurement result at a load of 75 kg / mm ² , and the symbol □ indicates a load of 80 kg / mm ^2.
4 shows the measurement results at / mm ² . I ₀ is an initial value, and F is a fatigue fracture point. As shown in FIG. 5, in any case of the applied load, the peak height showed a characteristic change such that it suddenly decreased and increased suddenly before the fatigue fracture.

[0041] FIG. 6 is a vertical axis and the horizontal axis was normalized (i.e. the horizontal axis scale divided by the rotating bending number N _F breaking point, peak height I ₀ of the initial values the scale of the vertical axis in FIG. 5 FIG. 3 is a diagram that has been calibrated on a scale (divided).

In FIG. 5, the initial peak height and the number of bending at the break point are differently shown for each applied load, but by standardizing them as described above, a single line as shown in FIG. Can be represented by a fatigue characteristic curve of That is, in FIG. 6, when the rate of change of the peak height ratio (I / I ₀ ) with respect to the number of bending times (N / Nf) is 0, for example, the fatigue life ratio is 0.75, that is, the life of the fatigue failure fracture. It can be seen that it has reached 75%. In this example, the peak height of the diffracted X-ray was measured, but a curve having the same tendency can be obtained by measuring the peak area.

X-ray diffraction measurement was carried out twice on a structural part made of the alloy actually used in a nuclear reactor plant under a similar measuring condition with an X-ray diffraction probe at a time interval, and diffraction between them was performed. When the rate of change of the X-ray peak height was determined, a negative value was obtained. This indicates that the fatigue life of this structural component has not yet reached 75%.

The present invention is useful mainly for evaluating the fatigue of a structural material due to the action of repeated stress. However, it is considered that a method for evaluating the life of the structural material by a similar method can be developed if the reference hysteresis characteristics can be obtained by X-ray diffraction measurement for creep, thermal embrittlement, radiation embrittlement, and the like, for example.

[0045]

As described above, according to the present invention, diffraction X
Since the measurement results of the line intensity include various internal information such as the crystal structure of the test material, it is possible to evaluate the fatigue by measuring the change in the diffraction X-ray intensity accompanying the fatigue. As described above, according to the present invention, fatigue damage of a test material such as a metal before a microcrack occurs can be evaluated from a measurement result by a nondestructive measurement method.

[Brief description of the drawings]

FIG. 1 is an explanatory view schematically showing a state of an X-ray diffraction measurement performed on a test material and a reference piece according to a preferred embodiment of the present invention.

FIG. 2 is a diagram showing an example of a fatigue characteristic curve obtained for a reference piece.

FIG. 3 is an explanatory diagram showing an example of the external shape and dimensions (unit: mm) of a reference piece.

FIG. 4 is an explanatory view showing an outline of an apparatus for repeatedly applying a rotating bending stress as a repetitive stress to a reference piece.

FIG. 5 shows the reference piece (1
FIG. 11 is a diagram plotting the diffraction X-ray peak height of the 11) plane.

FIG. 6 is a diagram showing the scale of FIG. 5 in which the scale on the horizontal axis is normalized by the number of times of stress applied to breakage, and the scale on the vertical axis is also normalized by the initial value of the diffraction X-ray peak height.

[Explanation of symbols]

 11 Measurement object (test material or reference piece) 12 X-ray source 13 Incident X-ray 14 Diffracted X-ray 15 X-ray detector

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-56-87849 (JP, A) JP-A-51-20894 (JP, A) JP-A-2-1655041 (JP, A) JP-A-4- 66852 (JP, A) JP-A-2-136737 (JP, A) JP-A-56-162039 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 23/20-23 / 207 JICST file (JOIS)

Claims (6)

(57) [Claims] 1. A method for evaluating the degree of fatigue of a structural material, comprising: performing a fatigue test on a reference piece of a material corresponding to a test material; and performing a fatigue test at a plurality of time points during the fatigue test. A step of irradiating the reference piece with X-rays and measuring the intensity of the diffracted X-rays having a predetermined diffraction angle generated from the irradiated portion; and from the measurement results, the integrated amount of the repetitive stress by the fatigue test and the diffracted X-rays A step of obtaining a fatigue characteristic curve corresponding to a relationship with the intensity; and a step of irradiating the measurement target site of the test material with the X-ray and measuring the intensity of the diffraction X-ray having the diffraction angle generated from the measurement target site. And comparing the rate of change between the diffracted X-ray intensities measured for the test material at at least two points in the middle of the action of the type of cyclic stress in the use environment on the test material with the damage characteristic curve. I Method characterized by including the step of determining a degree of fatigue of the material being tested. 2. The method according to claim 1, wherein in the fatigue test, at least one of predetermined types of repetitive stresses, such as bending, tensile compression, torsion, and impact, which are expected in an environment in which the test material is used. A method comprising applying a reference piece of a material corresponding to an inspection material. 3. The method according to claim 2, wherein the number of applied stresses is used as the integrated amount of the repeated stress. 4. The method according to claim 1, wherein the reference piece is irradiated with X-rays at a plurality of times from when the repetitive stress is applied to the reference piece to when the reference piece is broken. Measuring the intensity of diffracted X-rays from the site at a predetermined diffraction angle. 5. The method according to claim 1, wherein the X-ray diffraction measurement on either the reference piece or the test material is performed by measuring the peak height or amplitude of the diffracted X-ray. 6. The method according to claim 1, wherein the X-ray diffraction measurement on either the reference piece or the test material is performed by measuring the peak area of the diffracted X-ray, that is, the energy.