JP7705044B2 - Fatigue evaluation method and fatigue evaluation program - Google Patents

Description

The present invention relates to a fatigue assessment method and a fatigue assessment program.

Ensuring the safety and extending the lifespan of steel products, including automobile parts, railway vehicle parts, and bridges, has become an important social issue, and there is a demand for high fatigue strength steel materials. In order to obtain design guidelines for achieving high fatigue strength, it is necessary to quantify the effects of high fatigue strength and establish technology for predicting fatigue life.

JP 2017-187472 A discloses a fatigue evaluation method for evaluating with high accuracy the fatigue damage that occurs in a structure when repeated loads act on the structure. This fatigue evaluation method compares the test results of a test specimen with the results of a simulation to determine whether the convergence conditions are met, adjusts the simulation conditions if the convergence conditions are not met, and confirms the simulation conditions if the convergence conditions are met, and evaluates the fatigue damage of the analysis target using the confirmed simulation conditions.

Patent Publication No. 2791174 discloses a method for evaluating the life of solder connections in electronic components. This evaluation method includes a first step of determining the shear strain occurring in the solder connections of electric circuit devices, a second step of determining the equivalent strain amplitude from the relationship between the equivalent strain amplitude previously determined by a three-dimensional thermo-elastic-plastic analysis using the finite element method and the shear strain, a third step of determining a crack growth rate equation showing the relationship between the crack growth rate previously determined by a fracture surface analysis after a temperature cycle test and the equivalent strain amplitude, and a fourth step of determining the life from a life evaluation criterion equation showing the relationship between the equivalent strain amplitude, the crack length, and the number of life cycles.

Non-Patent Document 1 describes microstructural modeling and crystal plasticity simulation for the evaluation of fatigue cracks in α-iron. The document discusses the relationship between average shear strain amplitude and fatigue indicator parameters.

JP 2017-187472 A Patent No. 2791174

F. Briffod, T. Shiraiwa and M. Enoki, "Microstructure modeling and crystal plasticity simulations for the evaluation of fatigue crack initiation in α-iron specimen including an elliptic defect", Material Science and Engineering A, Vol. 695(2017), pp. 165-177

To develop steel with excellent fatigue properties, it is necessary to understand the generation of stress and strain when repeated loads are applied and to elucidate the mechanism of fatigue crack initiation. When determining material design guidelines, it is necessary to understand the microscopic stress and strain that depend on the microstructural morphology, such as the crystal grain size distribution and crystal orientation distribution, and to search for the optimal microstructural morphology to achieve high fatigue strength. Furthermore, to determine whether high fatigue strength has been achieved, a fatigue evaluation method that takes into account the effect of the microstructural morphology on fatigue damage is required.

The objective of the present invention is to provide a fatigue evaluation method and a fatigue evaluation program that take into account the effect of tissue morphology on fatigue damage.

A fatigue evaluation method according to one embodiment of the present invention includes a step of calculating the shear strain range when a repeated load is applied to a part for each of a plurality of crystal slip systems, and a step of calculating the sum of the out-of-plane components of the shear strain range.

A fatigue evaluation method according to another embodiment of the present invention includes the steps of: (A) calculating the shear strain range when a repeated load is applied to a test piece for each of a plurality of crystal slip systems for each of the plurality of fatigue tests based on data from a plurality of fatigue tests conducted in advance; (B) calculating the sum of the components of the shear strain range in the out-of-plane direction of the test piece; and (C) determining a damage index that is an index of damage to the test piece based on the sum; creating a master curve that is a relationship between the damage index and fatigue life; determining the damage index of the target part by the same steps as those (A) to (C) above; and predicting the fatigue life of the target part based on the damage index of the target part and the master curve.

The fatigue evaluation program according to one embodiment of the present invention causes a computer to execute the steps of: calculating the shear strain range for each of a plurality of crystal slip systems when a repeated load is applied to a part; and calculating the sum of the out-of-plane components of the shear strain range.

A fatigue evaluation program according to another embodiment of the present invention causes a computer to execute the following steps for each of a plurality of fatigue tests based on data from a plurality of fatigue tests conducted in advance: (A) calculating the shear strain range for each of a plurality of crystal slip systems when a repeated load is applied to the test piece; (B) calculating the sum of the components of the shear strain range in the out-of-plane direction of the test piece; and (C) determining a damage index that is an index of damage to the test piece based on the sum; creating a master curve that is the relationship between the damage index and fatigue life; determining the damage index of the target part by the same steps as those (A) to (C) above; and predicting the fatigue life of the target part based on the damage index of the target part and the master curve.

The present invention makes it possible to perform fatigue evaluation that takes into account the effect of tissue morphology on fatigue damage.

FIG. 1 is a plan view of a fatigue test piece. FIG. 2 shows the crystal orientation distribution in a 0.9 mm×0.9 mm region (region A in FIG. 1) near the center of the test piece, obtained by performing crystal orientation analysis using EBSD. FIG. 3 shows a polycrystalline model created based on the information on the grain boundaries obtained by crystal orientation analysis. FIG. 4 shows the distribution of the "sum of the components of the shear strain range in the out-of-plane direction of the test specimen in the multiple slip systems present in the crystal structure" (damage parameter). FIG. 5 is a diagram showing fatigue damage locations observed in the experiment. FIG. 6 is a schematic diagram showing the slip system of a bcc crystal structure. FIG. 7 is a diagram showing a schematic diagram of the change in shear strain over time. FIG. 8 is a diagram showing the relationship between the sliding surface and the outward direction of the test piece. FIG. 9 is a schematic diagram showing a cross section near the surface of a test piece after fatigue testing. FIG. 10 is a flow diagram of a fatigue assessment method according to one embodiment of the present invention. FIG. 11 is a graph showing the relationship between the stress amplitude σ and the fatigue life N. FIG. 12 is a diagram showing a schematic diagram of a frequency distribution of the damage parameter d in a fatigue test with a stress amplitude σ of σ ₁ . FIG. 13 is a diagram showing a schematic diagram of a frequency distribution of the damage parameter d in a fatigue test in which the stress amplitude σ is σ ₁ , σ ₂ , . . . , σ _k . FIG. 14 is a diagram showing a schematic diagram of a master curve. FIG. 15 is a diagram showing a schematic frequency distribution of the damage parameter d of the target part. FIG. 16 is a diagram showing a schematic diagram of a method for predicting a fatigue life using a master curve. FIG. 17 is a diagram showing boundary conditions for the FEM analysis. FIG. 18 shows the distribution of damage parameters when the stress amplitude is 132 MPa. FIG. 19 shows the distribution of damage parameters when the stress amplitude is 150 MPa. FIG. 20 shows the distribution of damage parameters when the stress amplitude is 160 MPa. FIG. 21 shows the frequency distribution of damage parameters when the stress amplitude is 132 MPa. FIG. 22 shows the frequency distribution of damage parameters when the stress amplitude is 150 MPa. FIG. 23 shows the frequency distribution of damage parameters when the stress amplitude is 160 MPa. FIG. 24 is a diagram showing the regions from which the frequencies in FIGS. 21 to 23 are extracted. FIG. 25 is a diagram showing a master curve obtained from the results of the fatigue test. FIG. 26 shows the crystal orientation distribution of test materials having different crystal grain sizes. FIG. 27 is a scatter plot comparing experimental and predicted fatigue life values.

When it is difficult to experimentally evaluate microscopic stress and strain, numerical simulation is an effective method. Crystal plasticity analysis is a numerical analysis method that expresses the anisotropy of plastic deformation of metals based on crystallographic slip, and can evaluate microscopic stress and strain at the crystal grain level.

The inventors modeled previously conducted fatigue tests and performed crystal plasticity analysis to investigate physical quantities (parameters) that can explain the distribution of fatigue damage locations observed in the experiments.

The fatigue test to be analyzed was conducted on a test specimen of the shape shown in Figure 1 taken from an ultra-low carbon steel specimen. The dimensions in the figure are in mm. The test specimen is plate-shaped, with a dimension (thickness) perpendicular to the paper surface of 1 mm. Figure 2 shows the crystal orientation distribution in a 0.9 mm x 0.9 mm region near the center of the test specimen (region A in Figure 1), obtained by performing crystal orientation analysis using electron backscatter diffraction (EBSD). Figure 3 shows a polycrystalline model created based on the information on the crystal grain boundaries obtained by the crystal orientation analysis.

As candidates for physical quantities related to fatigue damage, we considered load direction stress, maximum principal stress, load direction plastic strain, cumulative slip, etc. However, no clear correlation was found between the locations where these stresses or strains were high and the fatigue damaged areas observed in the experiments.

After further investigation, it was found that the locations where the "sum of the components of the shear strain range in the out-of-plane direction of the test specimen in the multiple slip systems present in the crystal structure" was high closely matched the fatigue damage locations observed in the experiment. Figure 4 shows the distribution of the "sum of the components of the shear strain range in the out-of-plane direction of the test specimen in the multiple slip systems present in the crystal structure" obtained by finite element method (FEM) analysis, and Figure 5 shows the fatigue damage locations observed in the experiment. In Figure 5, the hatched areas are the fatigue damage locations.

Here, we will explain in detail "the sum of the components of the shear strain range in the out-of-plane direction of the test specimen in the multiple slip systems present in the crystal structure."

In crystal plasticity analysis, deformation analysis is performed on the assumption that plastic deformation occurs as shear (slip) in a specific direction along the slip plane. Figure 6 is a schematic diagram of a slip system in a bcc crystal structure. In crystal plasticity analysis of a bcc crystal structure, a total of 24 slip systems, consisting of 12 slip systems of {110}<111> type and 12 slip systems of {112}<111> type, or a total of 48 slip systems, consisting of 24 slip systems of {123}<111> type, are often used. In this analysis, a total of 24 slip systems, consisting of 12 slip systems of {110}<111> type and 12 slip systems of {112}<111> type, were used.

Figure 7 is a diagram showing the change in shear strain over time. The "shear strain range" is the absolute value of the difference between the shear strain at the minimum load and the shear strain at the maximum load in the final cycle of the analysis. This shear strain range is calculated for each of the 24 slip systems mentioned above.

Figure 8 shows the relationship between the slip plane and the out-of-plane direction of the test specimen. The out-of-plane direction component of the test specimen is extracted from the shear strain range, and the sum is calculated for all slip systems considered (24 slip systems in this analysis). This value is the "sum of the out-of-plane direction components of the shear strain range of the test specimen in the multiple slip systems present in the crystal structure."

Figure 9 is a schematic diagram showing a cross section near the surface of a test piece after a fatigue test. The protrusions and intrusions (surface irregularities) observed on the surface of the test piece after a fatigue test are thought to be caused by slip and reverse slip of crystal planes, and are the cause of crack initiation. It can be assumed that the component of the shear strain range in the outward direction of the test piece's surface is the physical quantity that affects these protrusions and intrusions.

From the above, it was discovered that the fatigue of a part can be evaluated using the "sum of the out-of-plane components of the shear strain range in the multiple slip systems present in the crystal structure" as an index. The present invention was completed based on the above findings. Below, an embodiment of the present invention will be described in detail.

[Fatigue assessment method]
10 is a flow diagram of a fatigue evaluation method according to one embodiment of the present invention. This fatigue evaluation method includes a step of preparing fatigue test data (step S1), a step of analyzing each of the data (step S2), a step of creating a master curve (step S3), a step of analyzing the target component (step S4), and a step of predicting the fatigue life of the target component (step S5).

The fatigue evaluation method according to this embodiment can be applied to any polycrystalline material, but is preferably a metal material, and more preferably a steel material (iron-based alloy).

In the following description, the "sum of the components of the shear strain range in the out-of-plane direction of the test piece in the multiple slip systems present in the crystal structure" is referred to as the "damage parameter." In this embodiment, a "damage index," which is an index of damage to the test piece, is calculated based on this damage parameter. Details of the damage index will be described later. In the fatigue evaluation method according to this embodiment, first, data from multiple fatigue tests conducted in advance is prepared (step S1), a damage index is calculated for each test piece (step S2), and a master curve, which is the relationship between the damage index and fatigue life, is created (step S3). Then, a damage index is calculated for the target part (step S4), and the fatigue life of the target part is predicted by referring to the master curve (step S5). Each step will be described in detail below.

Data from multiple fatigue tests conducted in advance is prepared (step S1). Each piece of fatigue test data includes, for example, information on the test specimen shape, the crystal orientation distribution at the evaluation points, the loading conditions (stress amplitude, etc.), and the fatigue life (the number of cycles until a fatigue crack occurs, or the number of cycles until the test specimen breaks).

The crystal orientation distribution is preferably determined by measuring the test specimens used in the fatigue test using EBSD or other methods, but data on the crystal orientation distribution of the same material may be used instead. In addition, when multiple fatigue tests are performed on the same material under different loading conditions, it may be assumed that all test specimens have the same crystal orientation distribution.

To create a master curve, fatigue test data for at least two levels measured under different loading conditions is required. It is preferable to have fatigue test data for three or more levels.

Each of the data from the multiple fatigue tests is analyzed (step S2). Specifically, for each of the data from the multiple fatigue tests, (A) a process of calculating the shear strain range when repeated loads are applied to the test specimen for each of the multiple crystal slip systems (step S2-A), (B) a process of calculating the sum of the components of the shear strain range in the out-of-plane direction of the test specimen (i.e., the damage parameter) (step S2-B), and (C) a process of calculating a damage index based on the damage parameter (step S2-C) are performed. These analyses can be performed, for example, by FEM analysis.

The slip systems used in this analysis are not limited to these, but for example, 12 slip systems of {111}<110> type can be used in an fcc crystal structure. As mentioned above, in crystal plasticity analysis of a bcc crystal structure, a total of 24 slip systems, consisting of 12 slip systems of {110}<111> type and 12 slip systems of {112}<111> type, or a total of 48 slip systems, consisting of 24 slip systems of {123}<111> type, are often used. Instead of these, the analysis may be performed using, for example, 12 slip systems of {110}<111> type.

In addition, in the hcp crystal structure, for example, the following slip systems can be used:

For each of the target crystal slip systems, the shear strain range is calculated when a cyclic load is applied to the test piece (step S2-A). The shear strain range changes with each cycle due to work hardening. The shear strain range taking work hardening into account can be calculated, for example, using the formula described in D. Pierce, R.J. Asaro, A. Needleman, Acta metall., vol.30(1982), pp.1087-1119. In fatigue tests, cyclic loads may be applied for several thousand to several hundred thousand cycles, but when calculating the shear strain range using FEM analysis, it is practical to terminate the calculation after several cycles and calculate the shear strain range at the final cycle from the viewpoint of calculation cost.

The sum of the components of the shear strain range in the out-of-plane direction of the test specimen is calculated (step S2-B). More specifically, the components of the shear strain range in the out-of-plane direction of the test specimen are extracted from the relationship between the slip plane and the global coordinate system, and the sum for all crystal slip systems considered in step S2-A is calculated. In this embodiment, this sum is called the "damage parameter." Areas with a large damage parameter can be evaluated as areas where cracks are likely to occur due to repeated loading.

Based on the damage parameters, a "damage index" is calculated (step S2-C), as described below. The above-mentioned damage parameters are values calculated for each element of the model used in the analysis. The "damage index" is calculated as an index for evaluating the degree of damage in the entire analysis area.

The damage index is calculated based on the damage parameters. For example, the damage index may be the sum of the damage parameters of all elements of the analytical model. The damage index may be the sum of the damage parameters of all elements of the analytical model that are equal to or greater than a predetermined magnitude.

Alternatively, a predetermined percentage of elements may be extracted in descending order of damage parameters, and the sum of these may be used as the damage index. That is, the total number of elements may be N, the predetermined percentage may be s (0<s<1), and the kth largest damage parameter may be d _k (k=1, 2, 3, ..., N), and the damage index may be D expressed by the following formula.

In addition, from all the elements of the analysis model, only the parts related to the crack, more specifically, the elements corresponding to the outermost layer of the test piece, may be extracted, and the damage index may be calculated based on the damage parameters of the extracted elements. For example, the elements corresponding to the outermost layer of the test piece may be extracted, and from these, a predetermined percentage of elements may be extracted in descending order of damage parameters, and the sum of these may be used as the damage index.

The damage index may be calculated by a method other than the above. The damage index may be calculated based on the damage parameter. For example, the maximum value of the damage parameter or the number of elements whose damage parameter is equal to or greater than a predetermined value may be used as the damage index.

By using the above steps, a damage index is calculated for each of the multiple fatigue test data.

A master curve is created, which shows the relationship between the damage index and fatigue life, from the damage index obtained in step S2 and the fatigue life obtained in the fatigue test (step S3).

Next, a crystal plasticity analysis is similarly performed on the target component to be subjected to fatigue evaluation (Step S4). Specifically, the following steps are performed: (A) calculating the shear strain range when repeated loads are applied to the target component for each of multiple crystal slip systems (Step S4-A); (B) calculating the sum of the out-of-plane components of the shear strain range (i.e., the damage parameter) (Step S4-B); and (C) calculating a damage index based on the damage parameter (Step S4-C).

The target part is preferably made of the same material as the test piece used to create the master curve, and more preferably made of a material with the same chemical composition. On the other hand, the crystal orientation distribution of the test piece used to create the master curve and the crystal orientation distribution of the target part may be the same or different.

Finally, the fatigue life of the target part is predicted based on the damage index of the target part obtained in step S4 and the master curve obtained in step S3 (step S5).

An example of the above-mentioned fatigue evaluation method will be described with reference to Figures 11 to 15. As an example of data from a plurality of fatigue tests, consider data from fatigue tests performed on the same material with stress amplitude σ changed to σ ₁ , σ ₂ , ..., σ _k . Figure 11 is a diagram (S-N curve) showing the relationship between stress amplitude σ and fatigue life N (the number of cycles until a fatigue crack occurs, or the number of cycles until the test piece breaks).

First, the above-mentioned crystal plasticity analysis is performed on the data of the fatigue test with stress amplitude σ of σ ₁ , and the damage parameter d is calculated for each element of the analysis model. Fig. 12 is a diagram showing a typical frequency distribution of the damage parameter d in the fatigue test with stress amplitude σ of σ _1. Based on the damage parameter d, a damage index _D1 is calculated for the data of the fatigue test with stress amplitude σ of σ _1. In Fig. 13, as an example, the damage parameters d are extracted in a predetermined ratio in descending order and the sum is taken as the damage index _D1 .

Similarly, crystal plasticity analysis is performed on fatigue test data with stress amplitudes σ of σ ₂ , ..., σ _k to calculate damage indices D ₂ , ..., D _k (Fig. 13). A master curve for predicting fatigue life is obtained by matching the fatigue lives N ₁ , N ₂ , ..., N _k obtained from the fatigue tests with the damage indices D ₁ , D ₂ , ..., D _k (Fig. 14). In the master curve, D is expressed as a function f of N as follows:
D = f(N)

A similar crystal plasticity analysis is performed on the target part for fatigue evaluation to determine the damage index D _X (FIG. 15). The fatigue life of the part is calculated by calculating N corresponding to D _X from the inverse function of f as shown below (FIG. 16).
N = f ^-1 (D)

The above describes a fatigue evaluation method according to one embodiment of the present invention. This embodiment enables fatigue evaluation that takes into account the effect of tissue morphology on fatigue damage.

The above-mentioned fatigue evaluation method can also be realized as a computer program. A fatigue evaluation program according to one embodiment of the present invention causes a computer to execute the steps of accepting fatigue test data, analyzing each piece of data, creating a master curve, analyzing the target component, and predicting the fatigue life of the target component. This embodiment also enables fatigue evaluation that takes into account the effect of tissue morphology on fatigue damage.

The present invention will be explained in more detail below with reference to examples (analysis examples). The present invention is not limited to these examples.

A crystal plasticity analysis was performed by modeling fatigue tests that had been conducted in the past. The test material used in the fatigue tests to be analyzed was ultra-low carbon steel with the chemical composition shown in Table 1.

Fatigue tests were conducted by taking test pieces of the shape shown in Figure 1 from this test material. The dimensions in the figure are in mm. The test pieces were plate-shaped, and the dimension (thickness) in the direction perpendicular to the paper surface was 1 mm. The results of the fatigue tests are shown in Table 2.

The stress amplitudes actually used in the tests were 132 MPa and 160 MPa, and the fatigue crack initiation life for a stress amplitude of 150 MPa is an estimated value obtained by interpolating the test results for the two conditions mentioned above.

Measuring the crystal grain shape of the entire test piece and performing crystal plasticity analysis is not reasonable in terms of measurement work and calculation costs. Therefore, only the center of the test piece was analyzed. Figure 2 shows the crystal orientation distribution in a 0.9 mm x 0.9 mm area (area A in Figure 1) near the center of the test piece, obtained by performing crystal orientation analysis by EBSD on the test piece used in the fatigue test with a stress amplitude of 132 MPa. Figure 3 shows a polycrystalline model created based on the information on the crystal grain boundaries obtained by the crystal orientation analysis. Note that there was no EBSD analysis information for the test pieces used in the fatigue tests with stress amplitudes of 150 MPa and 160 MPa, so it was decided to use the same polycrystalline model as in Figure 3.

Figure 17 shows the boundary conditions for the FEM analysis. In order to estimate fatigue damage on the surface of the test piece, only a portion 0.05 mm from the surface of the 1 mm thickness of the test piece was taken in the thickness direction of the test piece, and the analysis was performed under the assumption that crystal grains of the same shape and crystal orientation continue in the thickness direction. The total number of elements was 49,575. A symmetrical boundary was set at the left end of the analysis region, and a 5-cycle load was applied to the right end under the stress amplitude conditions for the fatigue test.

The shear strain range (the difference between the maximum and minimum values at the fifth cycle) of each slip system was calculated using FEM analysis. From the relationship between the slip plane and the global coordinate system, the component of the shear strain range in the out-of-plane direction of the test specimen (here, the z-direction) was extracted, and the sum (damage parameter) for these 24 slip systems (a total of 24 slip systems, including 12 {110}<111> type slip systems and 12 {112}<111> type slip systems) was calculated.

Figures 18, 19, and 20 show the distribution of damage parameters when the stress amplitude is 132 MPa, 150 MPa, and 160 MPa, respectively. From Figures 18 to 20, it can be seen that the areas where the damage parameters increase expand as the stress amplitude increases.

Figures 21, 22, and 23 show frequency distributions of damage parameters when the stress amplitude is 132 MPa, 150 MPa, and 160 MPa, respectively. The frequency distributions in Figures 21 to 23 were obtained for the elements corresponding to the surface of the test piece (see Figure 24) among all the elements of the analysis model. The sum of the top 0.5% of damage parameters from the frequency distribution under each stress amplitude condition was obtained as a damage index D, and the master curve in Figure 25 was obtained by correlating it with the fatigue life N under each stress amplitude condition. This master curve is expressed by the following equation.
D=0.739e ^-0.000891N
In this embodiment, the master curve is expressed as an exponential function, but the master curve is not limited to an exponential function.

The fatigue life under other test conditions can be calculated by determining the damage index D from crystal plasticity analysis and substituting the corresponding fatigue life into the following formula (A).
N=-(1/0.000891N)ln(D/0.739)...Formula (A)

To verify the validity of this prediction formula (A), fatigue tests of other steel materials were analyzed. The test material was an ultra-low carbon steel with the same chemical composition as in Table 1 but different crystal grain size. The crystal orientation distribution of this test material is shown in Figure 26. The average crystal grain size of this test material was 100 μm, while the average crystal grain size of the test material shown in Figure 2 was 400 μm. A test piece with the same shape as in Figure 1 was taken from this test material and a fatigue test was conducted. The test conditions and test results are shown in Table 3.

A polycrystalline model was created based on the grain boundary information obtained from the crystal orientation analysis, and a crystal plasticity analysis was performed under the stress amplitude conditions in Table 3. The damage index D was calculated and substituted into formula (A) to predict the fatigue life N. The results are shown in Figure 27. The error in the predicted value compared to the experimental value was approximately -30%. The prediction was made within an error of 1/2 to 2 times, which is generally considered to be accurate in fatigue life prediction, confirming the validity of this fatigue evaluation method.

Although the embodiment of the present invention has been described above, the above-mentioned embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-mentioned embodiment, and the above-mentioned embodiment can be appropriately modified and carried out without departing from the spirit of the present invention.

Claims (4)

calculating a shear strain range for each of a plurality of crystal slip systems when the component is subjected to cyclic loading;
calculating a sum of the out-of-plane components of the shear strain range. Based on data from a plurality of fatigue tests previously conducted, for each of the plurality of fatigue tests,
(A) calculating the shear strain range for each of a plurality of crystal slip systems when a test specimen is subjected to cyclic loading;
(B) calculating a sum of the components of the shear strain range in the out-of-plane direction of the test piece; and (C) calculating a damage index, which is an index of damage to the test piece, based on the sum.
A step of carrying out
creating a master curve representing the relationship between the damage index and fatigue life;
determining a damage index of the target part by the same steps as those (A) to (C) above;
and predicting a fatigue life of the target component based on the damage index of the target component and the master curve. calculating a shear strain range for each of a plurality of crystal slip systems when the component is subjected to cyclic loading;
and calculating a sum of the out-of-plane direction components of the shear strain range of the component. Based on data from a plurality of fatigue tests previously conducted, for each of the plurality of fatigue tests,
(A) calculating the shear strain range for each of a plurality of crystal slip systems when a test specimen is subjected to cyclic loading;
(B) calculating a sum of the components of the shear strain range in the out-of-plane direction of the test piece; and (C) calculating a damage index, which is an index of damage to the test piece, based on the sum.
A step of carrying out
creating a master curve representing the relationship between the damage index and fatigue life;
determining a damage index of the target part by the same steps as those (A) to (C) above;
and predicting a fatigue life of the target component based on the damage index of the target component and the master curve.

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