JP2018185274A - Method and computer program for predicting fatigue limit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for predicting the fatigue limit of a metal material with fine defects.SOLUTION: The method according to the present invention includes the steps of: determining a potential crack length √(area) of a metal material (step S1); determining an apparent actual breaking stress σof the metal material (step S2); and determining the fatigue limit τ'of the metal material when the stress ratio is R, the stress amplitude is τ, the defective area protruded to the maximum principal stress surface is area, and the residual stress is σ(step S3).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method and a computer program for predicting a fatigue limit.

  A coil spring used as an element part is subjected to tension and compression. At this time, a torsional load is applied to the wire of the coil spring, but the deformation is repeatedly applied only to the tension coil spring in the tension direction and to the compression coil spring only in the compression direction. For this reason, one-way repeated torsional load is applied to the strands. This repeated torsional load becomes a driving force, which causes fatigue failure starting from minute defects such as line contact scratches generated on the surface, pits due to corrosion, and inclusions existing inside the strands. For this reason, the influence of minute defects on fatigue strength is regarded as important in the design of coil springs.

  Japanese Patent No. 5445727 discloses a method of extracting the inclusions from a sample and evaluating the fracture performance of the machine part based on the number of extracted inclusions and the number of inclusions whose dimensions exceed a predetermined threshold. .

  Japanese Patent Application Laid-Open No. 2015-1409 obtains a stress intensity factor range from a defect size, a residual stress of the defect portion, an external force applied to the defect portion, and the like, and a test piece load condition set based on the stress intensity factor range is also obtained. And a fatigue life evaluation method for a structure is disclosed.

  Koji Takahashi, Takayoshi Murakami, “Effect of Microcracks Introduced by Tension / Compression Fatigue Test on Torsional Fatigue Strength”, Transactions of the Japan Society of Mechanical Engineers (A), 68, 668, 645-652, The following formula (A) for predicting the fatigue limit of a material having a minute defect has been proposed.

ここで、τ_Ｗはせん断疲労限度［ＭＰａ］、ＨＶは材料のビッカース硬さ、√ａｒｅａは最大主応力面に投影した欠陥面積の平方根［μｍ］、Ｆは応力特異場の形状補正係数、ｂ／ａは表面欠陥のアスペクト比である。

Where τ _W is the shear fatigue limit [MPa], HV is the Vickers hardness of the material, √area is the square root [μm] of the defect area projected on the maximum principal stress surface, F is the shape correction factor of the stress singular field, b / A is the aspect ratio of the surface defect.

Japanese Patent No. 5445727 Japanese Patent Laid-Open No. 2015-1409

Koji Takahashi, Takayoshi Murakami, “Effect of microcrack introduced by tensile compression fatigue test on torsional fatigue strength”, Transactions of the Japan Society of Mechanical Engineers (A), 68, 668, 645-652 K. Tanaka, Y. Nakai, M. Yamashita, "Fatigue Growth threshold of small cracks", International Journal of Fracture, Vol. 17, No. 5, October 1981, pp. 519-533

  The above formula (A) is an empirical formula based on the results of fatigue testing of HV175 steel. In the formula (A), the fatigue strength is improved as the size of the inclusion is reduced. When the size of the inclusion is 0, the fatigue strength is infinite. For this reason, the fatigue limit cannot be correctly predicted by the formula (A) for a high-strength material extremely different from HV175 or a material having minute inclusions.

  An object of the present invention is to provide a method and a computer program for predicting the fatigue limit of a metal material having a minute defect.

ここで、τ_{Ｗ（Ｒ＝Ｃ）}は、応力比ｃにおける平滑材の疲労限度である。 A method according to an embodiment of the present invention is a method for predicting a fatigue limit of a metal material, and a fatigue limit τ ′ _{W () at} a stress ratio c of a specimen having a defect area projected onto the maximum principal stress surface of area _{1. R = c, √ (area1))} and the following equation (1) to obtain the latent crack length √ (area ₀ ) of the metal material, and the apparent true rupture stress σ _T of the metal material And the following equation (2), the stress ratio is R, the stress amplitude is τ _a , the defect area projected onto the maximum principal stress surface is area, and the residual stress is σ _res . _And a step of _{obtaining a} fatigue limit τ ′ _{W (R, √ (area), σres)} .

Here, τ _{W (R = C)} is the fatigue limit of the smooth material at the stress ratio c.

ここで、τ_{Ｗ（Ｒ＝Ｃ）}は、応力比ｃにおける平滑材の疲労限度である。 A computer program according to an embodiment of the present invention is a computer program for predicting a fatigue limit of a metal material, and a fatigue limit τ ′ at a stress ratio c of a specimen having a defect area projected onto a maximum principal stress surface of area _{1. Based on W (R = c, √ (area1))} and the following equation (1), a step of obtaining a latent crack length √ (area ₀ ) of the metal material, and an apparent true rupture stress of the metal material The metal when the stress ratio is R, the stress amplitude is τ _a , the defect area projected onto the maximum principal stress surface is area, and the residual stress is σ _res based on the step of obtaining σ _T and the following equation (2) And causing the computer to execute a step of _obtaining a fatigue limit τ ′ _{W (R, √ (area), σres)} of the material.

Here, τ _{W (R = C)} is the fatigue limit of the smooth material at the stress ratio c.

  According to the present invention, the fatigue limit of a metal material having a minute defect can be predicted.

FIG. 1 is a flowchart of a fatigue limit prediction method according to an embodiment of the present invention. FIG. 2 is a diagram showing the relationship between the stress ratio R and the stress amplitude τ _a and the average stress. FIG. 3 is a diagram showing a relationship between the fatigue limit line L1 of the smooth material and the fatigue limit line L2 of the material having a defect size of √ (area). FIG. 4 is a diagram for explaining the first embodiment of the present invention. FIG. 5 is a diagram for explaining a second embodiment of the present invention. FIG. 6 is a diagram for explaining a third embodiment of the present invention. FIG. 7 is a block diagram illustrating an example of the configuration of a computer. FIG. 8 is a perspective view of a test piece used in the fatigue test. FIG. 9 is a plan view of a test piece used in the fatigue test. FIG. 10 is an SN diagram obtained by the fatigue test. 11 is a cross-sectional view taken along line XI-XI in FIG. FIG. 12 is a diagram showing the relationship between the fatigue limit line of a smooth material and the fatigue limit line of a material having a predetermined defect size √ (area). FIG. 13 is a graph showing the results of the fatigue test and the fatigue limit predicted by Equation (2). FIG. 14 is a graph showing the results of the fatigue test and the fatigue limit predicted by Equation (2).

  The present inventors conducted a torsional fatigue test using a test piece in which a micro defect of 10 to 1000 μm was artificially introduced. As a result, the following knowledge was obtained.

(A) The fatigue limit prediction formula based on the latent fatigue crack length proposed by Haddad et al. Can be applied not only to a bending test but also to an axial force tensile test and a shear / torsion test. When torsional stress is applied, the maximum principal stress is applied to the surface that forms an angle of 45 ° with the direction of the torsional moment, and the minimum principal stress is applied to the surface that forms an angle of −45 ° with the same direction. The approximate torsional fatigue limit can be predicted by approximating that the principal stress is applied only to the surface forming

(B) When an average stress is applied to a material having a minute defect, the ratio between the fatigue limit of a metal material having a minute defect and the fatigue limit of a smooth material works like a notch coefficient.

  By combining the above (a) and (b), it is possible to predict the tensile fatigue limit and the shear fatigue limit in the case of an arbitrary defect size, average stress, and residual stress.

  Based on the above findings, the present invention has been completed. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated. The dimensional ratio between the constituent members shown in each drawing does not necessarily indicate the actual dimensional ratio.

FIG. 1 is a flowchart of a fatigue limit prediction method according to an embodiment of the present invention. This method includes a step (step S1) of obtaining a latent crack length √ (area ₀ ) of a metal material to be measured (hereinafter referred to as “measuring material” or simply “material”), and an apparent appearance of the measuring material. A step of determining the true rupture stress σ _T (step S2) and a step of determining the fatigue limit τ ′ _{W (R, √ (area), σres)} (step S3). Hereinafter, each process is explained in full detail.

[Step of obtaining latent crack length √ (area ₀ )]
First, the latent crack length √ (area ₀ ) of the material to be measured is obtained (step S1). Specifically, the fatigue limit τ ′ _{W (R = c, √ (area1)) at} the stress ratio c of the specimen whose area of defect projected onto the maximum principal stress surface is area ₁ and the following equation (1): Based on this, the latent crack length √ (area ₀ ) of the material is obtained.

Equation (1) is an extension of the prediction formula for bending fatigue limit proposed by Haddad et al. To shear / torsional stress. τ _{W (R = C)} is a fatigue limit of a smooth material (a material having a defect area area ₁ of 0 ₎ at a stress ratio c. The latent crack length √ (area ₀ ) is a value inherent to the material and is an index of the magnitude of the influence of the defect area area ₁ on the fatigue limit. The larger the latent crack length √ (area ₀ ), the smaller the influence of the defect area area ₁ .

As described above, the defect area area ₁ is a defect area projected on the maximum principal stress surface. Specifically, when the fatigue limit with respect to the tensile load is predicted, it is the area of the defect projected on the plane perpendicular to the tensile direction. When predicting the fatigue limit for a torsional load, it is the area of a defect projected on a surface that forms an angle of 45 ° with the direction of the moment. When a torsional load is applied to a material, the stress is actually applied to a surface that forms an angle of -45 ° with the direction of the moment. Can do.

Hereinafter referred to the square root √ defect area area ₁ a (area ₁₎ and defect size. The latent crack length √ (area ₀ ) can be obtained, for example, based on the fatigue limit obtained by measuring two or more test pieces having different defect dimensions √ (area ₁ ) at the same stress ratio. That is, when the stress ratio c and the shear fatigue limit τ ′ _{W (R = c, √ (area1)) at} the time of the defect size √ (area _{1 )} are given, the unknown variable of the equation (1) is a smoothing material. Fatigue limit τ _{W (R = c)} and latent crack length √ (area ₀ ). The fatigue limit τ _{W (R = C)} and the latent crack length √ (area ₀ ) can be obtained by substituting the two measurement results into equation (1) and solving the simultaneous equations. Further, fitting by the least square method or the like may be performed using three or more measurement results.

Here, one of the two or more kinds of test pieces having different defect dimensions √ (area ₁ ) may be a smooth material. That is, the fatigue limit τ _{W (R = c)} of the smooth material may be directly measured.

Further, the fatigue limit τ _{W (R = c)} of the smooth material can be calculated using an empirical formula based on a correlation with the tensile strength of the material. For example, the fatigue limit τ _{W (R = 0.1)} of the smoothing material when c = 0.1 can be obtained from the ultimate tensile strength σ _UTS by the following equation.

Moreover, ultimate tensile strength (sigma) _UTS can be calculated | required by the following formula from the Vickers hardness HV of material.

[Process for obtaining apparent true breaking stress σ _T ]
Next, an apparent true breaking stress σ _T of the material to be measured is obtained (step S2).

The apparent true rupture stress σ _T can be obtained, for example, based on a fatigue limit obtained by measuring a test piece having the same defect size with two or more stress ratios. Specifically, the fatigue limit obtained by measuring at two or more stress ratios is plotted as the average stress (average stress applied by external force + residual stress) on the horizontal axis and the fatigue limit on the vertical axis. That is, a fatigue limit diagram is created. Intersection of the lines and the horizontal axis connecting the plot is true rupture stress sigma _T apparent.

At this time, if a test piece having the same defect size is measured with at least two kinds of stress ratios, a straight line connecting the two points can be drawn, and the apparent true breaking stress σ _T can be obtained. In this case, the apparent true rupture stress σ _T can be obtained more accurately when the stress ratios of the two fatigue limit tests are separated from each other. Therefore, it is preferable that one of the two fatigue limit tests has a stress ratio of -1 (that is, a two-way fatigue limit test). Note that the fatigue limit may be measured at three or more stress ratios, and fitting by the least square method or the like may be performed.

The apparent true breaking stress σ _T can also be calculated using an empirical formula based on the correlation with the tensile strength of the material. The apparent true breaking stress σ _T can be obtained from the ultimate tensile strength σ _UTS by the following equation, for example.

[Step of _obtaining fatigue limit τ ′ _{W (R, √ (area), σres)} ]
Next, based on the equation (2), the fatigue limit τ ′ of the measurement target material when the stress ratio is R, the stress amplitude is τ _a , the defect area projected onto the maximum principal stress surface is area, and the residual stress is σ _{res W (R, √ (area), σres)} is obtained (step S3).

In addition, “(1 + R) / (1−R) τ _a ” in the formula (2) is an average stress (see FIG. 2).

  FIG. 3 is a diagram showing a relationship between the fatigue limit line L1 of the smooth material and the fatigue limit line L2 of the material having a defect size of √ (area). When average stress is applied to a material having microdefects, the ratio between the fatigue limit of the material having microdefects and the fatigue limit of the smoothing material works like a notch coefficient. The slope a of the fatigue limit line L2 can be calculated from the points P1 and P2 in FIG. 2, and the above equation (2) is obtained.

According to the equation (2), the fatigue at the time of arbitrary defect size, average stress, and residual stress for the material for which the latent crack length √ (area ₀ ) and the apparent true fracture stress σ _T were obtained in steps S1 and S2. The limit τ ′ _{W (R, √ (area), σres)} can be predicted. That is, if τa / τ′W _{(R, √ (area), σres)} is less than 1, it does not break, and if τa / τ′W _{(R, √ (area), σres)} is 1 or more, it breaks. Can be predicted.

  The fatigue limit prediction method according to the embodiment of the present invention has been described above. In the present embodiment, the material to be measured is not particularly limited as long as it is a metal material, but is particularly suitable for predicting the fatigue limit of a steel material, and predicting the fatigue limit of a high-strength steel having a hardness of 500 to 700 HV. It is particularly suitable for the case.

The present embodiment is particularly suitable for predicting the fatigue limit when a shear / torsion load is applied to the test piece. However, the present embodiment is not limited to this, and any fatigue test (axial tension test, shear / torsion) The fatigue limit of the test and bending test can be predicted. The latent crack length √ (area ₀ ) varies depending on the type of fatigue test (axial tension test, shear / torsion test, bending test). Therefore, in the step of obtaining the latent crack length √ (area ₀ ) (step S1), the fatigue limit τ ′ _{W (R = c, √)} measured by the same type of fatigue test as the type of fatigue test for which the fatigue limit is to be predicted. _(Area1)) is used.

[Specific examples of methods for predicting fatigue limits]
Hereinafter, a more specific example of the method for predicting the fatigue limit according to the present embodiment will be described.

[First Embodiment]
In the first embodiment, the fatigue limit is measured under three conditions to determine the latent crack length √ (area ₀ ) and the apparent true fracture stress σ _T. Specifically, first, a fatigue test is performed at a stress ratio c using a smoothing material and a test piece having a defect size of √ (area ₁ ), and each fatigue limit is obtained. Furthermore, a fatigue test is performed by swinging (R = −1) using a smooth material to determine the fatigue limit.

FIG. 4 is a plot of fatigue limits under the three conditions described above on a fatigue limit diagram. Point P3 is a fatigue limit when a fatigue test is performed with a stress ratio c using a smooth material, and point P4 is a fatigue test with a stress ratio c using a test piece having a defect size of √ (area ₁ ). The point P5 is a fatigue limit when a fatigue test is performed by using a smooth material with a double swing (R = -1). The latent crack length √ (area ₀ ) can be obtained from the points P3 and P4, and the apparent true rupture stress σ _T can be obtained from the intersection of the straight line connecting the points P3 and P5 and the horizontal axis. it can.

By substituting these values into equation (2), it is possible to predict the fatigue limit τ ′ _{W (R, √ (area), σres)} at an arbitrary defect size, average stress, and residual stress.

In this embodiment, in order to obtain the latent crack length √ (area ₀ ), the fatigue limit (point P3) when a fatigue test is performed with a stress ratio c using a smoothing material and the defect size is √ (area ₁ ). The fatigue limit (point P4) when a fatigue test is performed with the stress ratio c using the above test piece is used. However, in order to obtain the latent crack length √ (area ₀ ), it is sufficient to use the fatigue limit obtained by measuring two or more types of test pieces having different defect dimensions at the same stress ratio. It is not essential that is a smooth material.

In this embodiment, in order to obtain the apparent true breaking stress σ _T , the fatigue limit (point P3) when a fatigue test is performed with a stress ratio c using a smoothing material, and the fatigue test with both swings using a smoothing material. The fatigue limit (point P5) when used is used. However, in order to obtain the apparent true rupture stress σ _T , the fatigue limit obtained by measuring a test piece having the same defect size with two or more stress ratios may be used. Therefore, it is not essential to use a smooth specimen. Also, it is not essential that one side of the fatigue test is a double swing.

[Second Embodiment]
In the second embodiment, the fatigue limit is measured under two conditions (see FIG. 5). Specifically, similarly to the first embodiment, a fatigue test is performed at a stress ratio c using a smoothing material and a test piece having a defect size of √ (area ₁ ), and each fatigue limit is obtained. On the other hand, in the second embodiment, the measurement of the fatigue limit for determining the true fracture stress sigma _T The apparent not performed, the true stress at break sigma _T apparent obtained by equation (5).

Also according to the present embodiment, the fatigue limit τ ′ _{W (R, √ (area), σres)} at an arbitrary defect size, average stress, and residual stress can be predicted.

[Third Embodiment]
In the third embodiment, the fatigue limit is measured under one condition (see FIG. 6). Specifically, the fatigue limit is obtained by conducting a fatigue test at a stress ratio of 0.1 using a test piece having a defect size of √ (area ₁ ). In the third embodiment, the fatigue limit τ _{W (R = 0.1)} at a stress ratio of 0.1 in the test of the smooth material is obtained by the equation (3), and the latent crack length √ is obtained from this value and the measured fatigue limit. Find (area ₀ ). Further, the apparent true breaking stress σ _{T is obtained} by the equation (5).

Also according to the present embodiment, the fatigue limit τ ′ _{W (R, √ (area), σres)} at an arbitrary defect size, average stress, and residual stress can be predicted.

[Computer program to predict fatigue limit]
A computer program according to an embodiment of the present invention causes a computer to execute the above-described steps. FIG. 7 is a block diagram illustrating a configuration of a computer 10 that is an example of a computer that executes the computer program according to the present embodiment. The computer 10 includes an input device 11, an arithmetic device 12, a storage device 13, and an output device 14. The computer program is stored in the storage device 13.

First, through the input device 11, the fatigue limit defect area projected in the maximum principal stress surface in stress ratio c of the specimen is _{area 1 τ 'W (R =} c, √ (area1)) is, the arithmetic unit 12 Is input. Based on the fatigue limit τ ′ _{W (R = c, √ (area1))} and the equation (1) stored in the storage device 13, the arithmetic unit 12 calculates the latent crack length √ (area) of the material to be measured. ₀ ). The obtained latent crack length √ (area ₀ ) is stored in the storage device 13.

The latent crack length √ (area ₀ ) may be obtained based on this value by inputting a fatigue limit obtained by measuring a test piece having the same defect size with two or more stress ratios. . The latent crack length √ (area ₀ ) or the fatigue limit τ ′ _{W (R = c, √ (} area ₁ )) at the stress ratio c of the specimen having a defect area of area ₁ and the tensile strength of the material to be measured It is also possible to input the thickness or hardness and obtain these values based on these values and the equations (3) and (4) stored in the storage device 13.

Next, the arithmetic unit 12 calculates an apparent true breaking stress σ _T. The obtained apparent true rupture stress σ _T is stored in the storage device 13.

The apparent true breaking stress σ _T may be obtained based on this value by inputting a fatigue limit obtained by measuring a test piece having the same defect size at two or more stress ratios. The apparent true rupture stress σ _T is input based on the tensile strength or hardness of the material to be measured and this value and the equations (5) and (4) stored in the storage device 13. You may make it ask.

Next, conditions (stress ratio R, stress amplitude τ _a , residual stress σ _res , defect size √ (area)) for which a fatigue limit is to be predicted are input to the arithmetic device 12 via the input device 11. Based on these values, the latent crack length √ (area ₀ ), the apparent true fracture stress σ _T , and the equation (2) stored in the storage device 13, the arithmetic unit 12 determines the fatigue of the measurement target material. The limit τ ′ _{W (R, √ (area), σres)} is obtained.

The computing device 12 can store the obtained fatigue limit τ ′ _{W (R, √ (area), σres)} in the storage device 13 or output it to the output device 14 as necessary.

The arithmetic device 12 may further perform a determination process. When the determination process is performed, if the value of τ _a / τ ′ _{W (R, √ (area), σres)} is less than 1, it is determined not to break, and if it is 1 or more, it is determined to break. The arithmetic unit 12 can store the determination result in the storage unit 13 or output it to the output unit 14 as necessary.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited to these examples.

  A φ4 mm wire having a chemical composition in which V was added to valve spring steel SAE (Society of Automotive Engineers) 9254 was prepared. This wire was heat-treated to obtain a tempered martensite structure, and specimens having mechanical properties shown in Table 1 were produced.

  A test piece 20 having the shape shown in FIGS. 8 and 9 was collected from this test material. The test piece 20 had an outer diameter φ = 3.5 mm, a length L = 60 mm, a score portion cross-sectional diameter φ1 = 2.3 mm, and a score portion length L1 = 10 mm. After the graded part was polished to diamond buff # 3000, micro holes with different dimensions were drilled to form artificial defects 21 having the dimensions shown in Table 2.

Using the test piece 20, a one-way torsional fatigue test was performed. Specifically, one end of the test piece was fixed to the torque cell, the other end was connected to the motor, and a torsional load was repeatedly applied by torque control. The stress ratio R = 0.1, the input waveform was a sine wave, the frequency was 10 to 20 Hz, and the number of repetitions was 10 ⁷ times. The test results were arranged by the maximum shear stress amplitude τa generated on the outer surface of the score part and the number of repetitions N, and the fatigue limit was determined.

  FIG. 10 is an SN diagram obtained by the fatigue test. There was a tendency for the fatigue limit to decrease as the size of the artificial defect increased. Each test piece broke with an artificial defect as a starting point, and cracks propagated in the 45 ° direction (XI-XI line direction in FIG. 9), which is the maximum principal stress surface.

11 is a cross-sectional view taken along line XI-XI in FIG. By approximating the shape of the artificial defect 21 to a semi-elliptical shape, the area area of the artificial defect 21 projected onto the maximum principal stress surface was obtained from the following equation. The square root of the area area was taken as the defect size √ (area) of each test piece. The value of defect size √ (area) of each test piece is shown in Table 2 above.
area = 1/2 · rdπ

By substituting the test result of the smoothing material and the test result of φ150 into the equation (1), the latent crack length √ (area ₀ ) was obtained. The obtained latent crack length √ (area ₀ ) was 151 μm.

Next, a torsional fatigue test with a swing (R = -1) was performed using a smooth material to determine the fatigue limit. The test results of R = −1 and R = 0.1 of the smooth material were plotted on the fatigue limit diagram as shown in FIG. 12, and the apparent true rupture stress σ _T was determined from the intersection with the horizontal axis. The apparent true breaking stress σ _{T obtained} was 3774 MPa. Furthermore, the inclination a (smooth material) and the y-axis intercept b (smooth material) of the fatigue limit diagram of the smooth material were obtained by connecting the intersection and the test result of the smooth material. Further, by connecting the intersection and the test result of φ150, the inclination a (φ150) and the y-axis intercept b (φ150) of the fatigue limit diagram of φ150 were obtained.

  As described above, all parameters necessary for predicting the fatigue limit were obtained. Hereinafter, it was verified whether each test result was predictable using Formula (2).

  FIG. 13 is a graph showing the fatigue test results (open marks) for φ30, φ45, and φ80 described above and the fatigue limit (solid line) predicted by Equation (2). FIG. 13 also shows the smoothing material used for deriving the parameters of the equation (2) and the test result (solid mark) of φ150. The defect size of the smoothing material is 0, but is 10 μm for convenience in order to display it on a logarithmic graph. FIG. 13 also shows the fatigue limit (broken line) based on the prediction formula (formula (A)) of the prior art.

  From FIG. 13, it can be seen that the prediction formula of the prior art predicts a fatigue limit on the safe side that is larger than the actual fatigue limit. On the other hand, according to the present invention, the fatigue limit can be predicted with high accuracy.

  Subsequently, after forming an artificial defect by drilling, a test piece in which a compressive residual stress of about 850 MPa was applied to the surface by shot processing was produced. Furthermore, the test piece which formed the artificial defect by electric discharge machining was produced. This test piece was presumed that a tensile residual stress of about 560 MPa was applied to the surface by heating and cooling during electric discharge machining. Using these test pieces, the torsional fatigue test was performed under the same conditions as described above to determine the fatigue limit.

FIG. 14 is a graph showing the results of these fatigue tests and the fatigue limit predicted by equation (2). In FIG. 14, the mark is an actual measurement value of the fatigue limit, and the solid line, the broken line, and the chain line are values obtained by substituting σ _res = 0, σ _res = −850, and σ _res = 560, respectively, into Equation (2).

  As shown in FIG. 14, the prediction formula of the present invention reproduced the measured values well.

  From the above results, it was confirmed that the fatigue limit at the time of arbitrary defect size, average stress, and residual stress can be predicted by the present invention.

  The embodiment of the present invention has been described above. The above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

10 Computer 11 Input Device 12 Arithmetic Device 13 Storage Device 14 Output Device 20 Specimen 21 Artificial Defect

Claims (6)

A method for predicting the fatigue limit of a metallic material,
Based on the fatigue limit τ ′ _{W (R = c, √ (area1)) at} the stress ratio c of the test piece whose area of defect projected onto the maximum principal stress surface is area ₁ and the following equation (1): Obtaining a latent crack length √ (area ₀ ) of the material;
Obtaining an apparent true breaking stress σ _T of the metal material;
Based on the following equation (2), the fatigue limit τ ′ _{W of the} metal material when the stress ratio is R, the stress amplitude is τ _a , the defect area projected onto the maximum principal stress surface is area, and the residual stress is σ _{res. Determining (R, √ (area), σres)} .

Here, τ _{W (R = C)} is the fatigue limit of the smooth material at the stress ratio c. The method of claim 1, comprising:
The step of obtaining the latent crack length √ (area ₀ ) is based on a fatigue limit obtained by measuring two or more types of test pieces having different defect dimensions at the same stress ratio, and the latent crack length √ ( area ₀ ). The method according to claim 1 or 2, wherein
Step of determining the true fracture stress sigma _T are based on specimens of the same defect size in fatigue limit obtained by measuring at two or more stress ratio, determine the true fracture stress sigma _T of the apparent method . A computer program for predicting the fatigue limit of a metal material,
Based on the fatigue limit τ ′ _{W (R = c, √ (area1)) at} the stress ratio c of the test piece whose area of defect projected onto the maximum principal stress surface is area ₁ and the following equation (1): Obtaining a latent crack length √ (area ₀ ) of the material;
Obtaining an apparent true breaking stress σ _T of the metal material;
Based on the following equation (2), the fatigue limit τ ′ _{W of the} metal material when the stress ratio is R, the stress amplitude is τ _a , the defect area projected onto the maximum principal stress surface is area, and the residual stress is σ _{res. The} computer program which makes a computer perform the process of _{calculating | requiring (R, (root), (sigma) res)} .

Here, τ _{W (R = C)} is the fatigue limit of the smooth material at the stress ratio c. A computer program according to claim 4,
The step of obtaining the latent crack length √ (area ₀ ) is based on a fatigue limit obtained by measuring two or more types of test pieces having different defect dimensions at the same stress ratio, and the latent crack length √ ( area ₀ ). A computer program according to claim 4 or 5,
Step of determining the true fracture stress sigma _T are based on specimens of the same defect size in fatigue limit obtained by measuring at two or more stress ratio, determine the true fracture stress sigma _T of the apparent, computer program.

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