Fatigue test method, fatigue test device, and fatigue analysis program

Abstract

To provide a fatigue test method capable of grasping fatigue characteristics of a material in a short time.SOLUTION: A fatigue test method according to one embodiment of the present invention includes the steps of: applying a periodically varying stress to a test piece under a plurality of stress conditions, thereby obtaining a change in an index value of energy loss per unit period in the test piece with respect to the number of repetitions of the varying stress for each stress condition; setting a shift factor so that data of the plurality of stress conditions is plotted on the same master curve when a relation between the standard number obtained by multiplying the number of repetitions by the shift factor set for each stress condition and the index value is graphed in double logarithms; and calculating a correction function representing the relation between the stress conditions and the shift factor.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a fatigue testing method, a fatigue testing device, and a fatigue analysis program. [Background technology]

[0002] JIS-K7118 specifies a standard for fatigue testing methods to confirm the fatigue properties of polymeric materials, entitled "General Rules for Fatigue Testing Methods for Rigid Plastic Materials." According to this standard, a test is performed in which a cyclic stress is applied to a test piece and the number of repetitions until the test piece breaks is measured, and this is performed multiple times with different stress values, to create an SN diagram that plots the relationship between the stress value and the number of repetitions until breakage.

[0003] As a fatigue testing device for carrying out such a fatigue test, there is a device that automatically applies repeated tensile stress to a test piece using a servo motor while checking the stress value using a load cell (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 63-37233 Summary of the Invention [Problem to be solved by the invention]

[0005] In the case of resin materials, physical properties including fatigue properties can be adjusted by fine-tuning the composition, but it is difficult to accurately predict the extent to which differences in composition affect the physical properties of the material. For this reason, it is desirable to confirm whether or not desired fatigue properties can be obtained by fatigue testing. However, even if a fatigue testing device that automatically applies repeated stress as described above is used, it takes an extremely long time for the test piece to break. For this reason, it is not easy to adjust the composition of a resin material by trial and error to meet individual requirements regarding fatigue properties.

[0006] Therefore, an object of the present invention is to provide a fatigue testing method, a fatigue testing device, and a fatigue analysis program that can grasp the fatigue characteristics of a material in a short period of time. [Means for solving the problem]

[0007] A fatigue testing method according to one embodiment of the present invention comprises the steps of: applying a periodically changing fluctuating stress to a test piece under a plurality of stress conditions, thereby obtaining a change in an index value of energy loss per unit period in the test piece relative to the number of repetitions of the fluctuating stress for each of the stress conditions; setting the shift factor so that when the relationship between the index value and the standard number obtained by multiplying the number of repetitions by the shift factor set for each of the stress conditions is graphed in double logarithm, the data for the plurality of stress conditions are plotted on the same master curve; and calculating a correction function that represents the relationship between the stress condition and the shift factor.

[0008] In the above-described fatigue test method, the correction function may be calculated assuming that the stress condition and the shift factor have a linear relationship in a double logarithmic scale.

[0009] The above-mentioned fatigue testing method may further include a step of back-calculating a value of the shift factor under the desired stress conditions from the correction function, and a step of estimating fatigue characteristics under the desired stress conditions by redrawing the master curve for an assumed number of times obtained by dividing the standard number of times by the back-calculated shift factor.

[0010] In the above-described fatigue testing method, the index value may be a feature of an amount of lost energy, an energy loss rate, or a hysteresis of energy input / output.

[0011] In the fatigue testing method described above, the fluctuating stress may be varied so as to apply a sinusoidally varying stress or strain to the test piece.

[0012] A fatigue testing method according to another aspect of the present invention comprises the steps of: preparing a master curve which is a double logarithmic graph of the change in an index value of energy loss per unit period in a standard sample test piece versus the number of repetitions of a fluctuating stress when a periodically changing fluctuating stress is applied to the standard sample test piece under reference stress conditions; applying the fluctuating stress under any of the stress conditions to a target sample test piece made of the same material as the standard sample, thereby obtaining the change in the index value versus the number of repetitions of the target sample; and estimating the degree of fatigue of the target sample based on the correspondence between the index value of the target sample and the master curve.

[0013] A fatigue testing apparatus according to one embodiment of the present invention comprises a pair of gripping parts for gripping both ends of a test piece, a drive mechanism for moving at least one of the pair of gripping parts, a detection mechanism for detecting a force acting between the pair of gripping parts, a drive control part for controlling the drive mechanism so as to apply a periodically changing fluctuating stress to the test piece under a set stress condition, an index value acquisition part for acquiring a change in an index value of energy loss per unit period in the test piece relative to the number of repetitions of the fluctuating stress based on the drive amount of the drive mechanism and the detection value of the detection mechanism, a shift factor setting part for setting the shift factor so that data for the multiple stress conditions are plotted on the same master curve when the relationship between the index value and the standard number obtained by multiplying the number of repetitions by a shift factor set for each stress condition is graphed in double logarithm, and a correction function calculation part for calculating a correction function representing the relationship between the stress condition and the shift factor.

[0014] A fatigue testing program according to one embodiment of the present invention is a fatigue analysis program that applies a periodically changing fluctuating stress to a test piece under a plurality of stress conditions and analyzes fatigue characteristics based on a change in an index value of energy loss per unit period in the test piece relative to the number of repetitions of the fluctuating stress, measured for each of the stress conditions. The program includes a shift factor setting function unit that sets the shift factor so that data for the plurality of stress conditions are plotted on the same master curve when the relationship between the index value and the standard number of repetitions obtained by multiplying the shift factor set for each stress condition by the number of repetitions is graphed in double logarithm, and a correction function calculation function unit that calculates a correction function that represents the relationship between the stress condition and the shift factor. Effect of the Invention

[0015] According to the present invention, it is possible to provide a fatigue testing method, a fatigue testing device, and a fatigue analysis program that can grasp the fatigue characteristics of a material in a short period of time. [Brief description of the drawings]

[0016] [Figure 1] 1 is a schematic diagram showing a configuration of a fatigue testing device according to an embodiment of the present invention; [Diagram 2] 1 is a Lissajous diagram of stress and strain according to the present invention. [Diagram 3] 1 is a graph showing the relationship between the number of repetitions and the energy loss rate when the amplitude of strain is constant. [Figure 4] 1 is a graph showing the relationship between the number of repetitions and the energy loss rate when the amplitude of the stress is constant. [Diagram 5] 4 is a graph showing the relationship between stress conditions and shift factors in the data of FIG. 3. [Figure 6] 5 is a graph showing the relationship between stress conditions and shift factors in the data of FIG. 4. [Figure 7] 1 is a flowchart showing the procedure of a fatigue testing method according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic diagram showing the configuration of a fatigue testing device 1 according to one embodiment of the present invention.

[0018] The fatigue testing apparatus 1 is an apparatus for repeatedly applying stress to a test piece T of a material to be measured, thereby confirming the fatigue characteristics of the material. A comparatively soft resin material is assumed as a measurement target of the fatigue testing apparatus 1, but is not limited thereto. The fatigue testing apparatus 1 is also used to carry out a fatigue testing method according to the present invention, and automatically executes at least a part of the method.

[0019] The fatigue testing apparatus 1 includes a mechanical device unit 10 consisting of physical components that repeatedly apply stress to a test piece T, and a control device unit 20 that controls and calculates the mechanical device unit 10. The test piece T to be tested in the fatigue testing apparatus 1 is assumed to be, for example, a material formed into a strip shape.

[0020] The mechanical device unit 10 comprises a pair of gripping portions (first gripping portion 11 and second gripping portion 12) that grip both ends of the test piece T, a driving mechanism 13 that moves at least one of the pair of gripping portions 11, 12 (first gripping portion 11 in the illustrated example), a detection mechanism 14 that detects the force acting between the pair of gripping portions 11, 12, and a support body 15 that supports the driving mechanism 13 and the detection mechanism 14, and ultimately the gripping portions 11, 12.

[0021] The gripping parts 11 and 12 may be clamps that clamp and hold both sides of the measurement area to be fatigued of the test piece T over its entire width. By clamping the test piece T over its entire width, it is possible to prevent stress from concentrating locally and perform accurate measurement. Furthermore, the gripping parts 11 and 12 have a sufficiently large rigidity so that their distortion can be ignored.

[0022] The driving mechanism 13 moves the first gripping part 11 in the longitudinal direction of the test piece T. This allows a tensile stress to be applied to the test piece T. The driving mechanism 13 may be configured to move the first gripping part 11 by, for example, a servo motor. By using such a driving mechanism 13, a stress or strain of any magnitude can be applied to the test piece T.

[0023] The detection mechanism 14 may be configured, for example, by a load cell, and may be disposed between the second gripping portion 12 and the support body 15 .

[0024] The support 15 has a sufficiently large rigidity so that the points supporting the drive mechanism 13 and the detection mechanism 14 do not move due to the stress acting on the test piece T. In addition, the support 15 may have a mechanism for applying an initial strain to the test piece T so that the stress acting on the test piece T varies within a tensile region.

[0025] The control device unit 20 may be a dedicated circuit including, for example, a control IC, or may be realized by causing one or more computer devices having, for example, a memory, a CPU, an input / output interface, etc., to execute an appropriate program, or may be realized by a combination of a dedicated circuit and a computer device. Note that the program executed in the computer device that realizes the control device unit 20 may be one embodiment of the fatigue analysis program according to the present invention. Note that the fatigue analysis program according to the present invention may be provided in a state where it is stored in a program storage medium that non-temporarily stores a program.

[0026] The control device section 20 may be configured to include a drive control section 21, an index value acquisition section 22, a shift factor setting section 23, a correction function calculation section 24, a shift factor reverse calculation section 25, a fatigue characteristic estimation section 26, and an interface section 27. Note that these components of the control device section 20 are classifications of the functions of the control device section 20, and do not necessarily have to be clearly distinguishable in terms of their physical configuration and program configuration.

[0027] The drive control unit 21 controls the drive mechanism 13 so that a periodically changing fluctuating stress acts on the test piece T under the set stress condition. The fluctuating stress is preferably changed so that a sinusoidally changing stress or strain acts on the test piece T. That is, the stress condition may be such that the amplitude of the strain acting on the test piece T is constant, or the amplitude of the stress acting on the test piece T is constant. In addition, if the drive mechanism 13 is controlled to act on the test piece T with a sinusoidal fluctuating stress, the acceleration is suppressed, so that the load on the drive mechanism 13 can be reduced. When the amplitude of the strain acting on the test piece T is constant, the waveform of the detection value of the detection mechanism 14 may change with an increase in the number of repetitions of the fluctuating stress. On the other hand, when the amplitude of the stress acting on the test piece T is constant, the drive control unit 21 feedback controls the drive mechanism 13 based on the detection value of the detection mechanism 14, so that the waveform of the drive amount of the drive mechanism 13 may change with an increase in the number of repetitions of the fluctuating stress.

[0028] In the fatigue testing device 1, it is necessary to perform a test in which variable stress is applied under different stress conditions to at least two test pieces T of the same material. For this reason, the drive control unit 21 may be configured to be able to set a plurality of stress conditions in advance. The waveform and frequency of the variable stress may affect the degree of progress of fatigue per unit period. For example, increasing the frequency increases the degree of progress of fatigue per unit period. For this reason, it is preferable that the plurality of stress conditions in a test for confirming the fatigue characteristics of at least one type of material have constant waveform and frequency, but different amplitudes of the variable stress (maximum value of stress or strain).

[0029] The index value acquiring unit 22 acquires an index value of the energy loss per unit period in the test piece T with respect to the number of repetitions of the fluctuating stress based on the drive amount of the driving mechanism 13 and the detection value of the detection mechanism 14. The index value acquired by the index value acquiring unit 22 is preferably the amount of lost energy, the energy loss rate, or a feature value of the hysteresis of the energy input / output. These index values ​​can be calculated based on the drive amount of the driving mechanism 13 and the detection value of the detection mechanism 14.

[0030] Figure 2 shows a plot of strain and stress values, which are measured values, at each time under stress conditions with a constant strain amplitude, i.e., a Lissajous figure of strain and stress. Energy is the product of the force acting on the test piece T and the amount of change, so it can be understood as the value obtained by integrating the stress acting on the test piece T with respect to the amount of change in strain. If there was no energy loss in the test piece T, the Lissajous figure would be a single straight line. However, due to energy loss in the test piece T, the actual Lissajous figure draws an upward convex curve when the test piece T expands, and a downward convex curve when the test piece T contracts. In other words, this Lissajous figure is a set of points moving clockwise, and hysteresis exists between the increase / decrease in strain, the increase / decrease in stress, and the strain.

[0031] While the test piece T is expanding, the direction of stress and the direction of change in strain are the same, so the integral value is a positive value, indicating that energy is being input to the test piece T. On the other hand, while the test piece T is contracting, the direction of stress and the direction of change in strain are different, so the integral value is a negative value, indicating that energy is being output from the test piece T. Therefore, the energy loss per unit period can be calculated as the value obtained by subtracting the integral value of the lower curved portion of the Lissajous figure from the integral value of the upper curved portion of the Lissajous figure, in other words, the area inside the Lissajous figure.

[0032] The loss energy may be understood as a characteristic value that represents the magnitude of hysteresis between the increase and decrease in strain and the increase and decrease in stress, that is, a value that represents the shape of the Lissajous figure. Specifically, the minor axis (width perpendicular to the maximum axis) of the Lissajous figure, the maximum deviation of strain between expansion and contraction under the same stress, the maximum deviation of the Lissajous figure from a straight line when there is no energy loss, the area of ​​the region enclosed by the straight line when there is no energy loss and the upper curve of the Lissajous figure, etc. may be used as index values ​​of the energy loss per unit period.

[0033] As a result of the research by the inventors, it was confirmed that the energy loss per unit period decreases as fatigue of the test piece T progresses, and that when only the amplitude of the fluctuating stress is different, when the change in the index value with an increase in the number of repetitions is graphed in double logarithmic scale, the same shape of line is drawn except for the axial position of the number of repetitions, excluding measurement error. Therefore, in the fatigue testing device 1, a shift factor for normalizing the change in the index value with respect to the number of repetitions is set for each stress condition. The device is configured to include a shift factor setting unit 23. Note that the change waveform of the index value with respect to the number of repetitions may differ depending on the selected index value, the properties of the material to be tested, the test environment (temperature and humidity), etc. Also, the change waveform of the index value may be almost linear.

[0034] The shift factor setting unit 23 calculates the amount of graph movement required to superimpose the data of each stress condition on the master curve, which is a graph under the reference stress condition, as a shift factor to be multiplied by the number of repetitions of the variable stress. In other words, the shift factor setting unit 23 converts data of the index value for each stress condition for the number of repetitions into data for the standard number of repetitions obtained by multiplying the number of repetitions by the shift factor set for each stress condition, and sets the shift factor for each stress condition so that data of a plurality of stress conditions are plotted on the same master curve when the relationship between the standard number of repetitions and the index value is graphed in double logarithm. The master curve is a graph showing the change in the index value with respect to the number of repetitions under arbitrarily selected stress conditions, and the stress conditions of the master curve may be selected from the stress conditions actually measured, or may be conditions different from the actually measured stress conditions, for example, stress conditions that are considered to be capable of accurately determining the fatigue characteristics of a part in which the material is expected to be used.

[0035] FIG. 3 shows an example of the change in the energy loss rate (index value) per unit period for test piece T versus the number of cycles of fluctuating stress under multiple stress conditions with different strain amplitudes, plotted as a double logarithmic graph. FIG. 3 also shows the change in the energy loss rate versus the standard number of cycles, which is corrected by setting an appropriate shift factor for each stress condition. In this way, by setting an appropriate shift factor, a single master curve with normalized data can be obtained. In the example shown, the shift factor is set to 1 when the strain amplitude is 0.8%, that is, the curve showing the energy loss rate versus the number of cycles (standard number) when the strain amplitude is 0.8% is used as the master curve.

[0036] Figure 3 shows an example of a double logarithmic graph of the change in energy loss rate per unit cycle for test piece T against the standard number of cycles normalized by multiplying the number of cycles of fluctuating stress under multiple stress conditions with different stress amplitudes by a shift factor. In this way, even if the stress amplitude is constant, the shape of the change in the index value against the number of cycles becomes constant, and it can be normalized onto the master curve by setting the shift factor.

[0037] The correction function calculation unit 24 calculates a correction function that represents the relationship between the stress condition and the shift factor. The correction function calculation unit 24 may calculate the relationship between the stress condition and the shift factor as being linear in double logarithm. FIG. 5 shows the relationship between the stress condition of the data in FIG. 3, that is, the strain amplitude and the shift factor. FIG. 6 shows the relationship between the amplitude of the fluctuating stress of the data in FIG. 4 and the shift factor. In this way, although the slope differs depending on the stress condition, the relationship between the stress condition and the shift factor is considered to be linear.

[0038] The shift factor reverse calculation unit 25 reverse-calculates the value of the shift factor under the desired stress condition from the correction function. In other words, the shift factor reverse calculation unit 25 estimates the value of the shift factor for obtaining a standard number of times that allows the measurement data to be plotted on the master curve when a test is performed with an amplitude that is not actually used in the test.

[0039] The fatigue characteristic estimation unit 26 estimates fatigue characteristics under desired stress conditions by redrawing a master curve for an assumed number of times obtained by dividing the standard number of times by the shift factor calculated by the shift factor reverse calculation unit 25. In other words, the fatigue characteristic estimation unit 26 estimates the curve drawn by the measured values ​​when a test is performed under the stress conditions set by the shift factor reverse calculation unit 25, or the position of a specific point on the curve.

[0040] The master curve can indicate, as its end point, the standard number of times at which the test piece T breaks. In other words, in the fatigue testing apparatus 1, by performing a test until the test piece T breaks under at least one stress condition among the tests under a plurality of stress conditions, it is possible to calculate the expected number of times at which the test piece T is expected to break under any stress condition. Therefore, in the fatigue testing apparatus 1, if the test piece T can be brought to break with a relatively small number of repetitions under stress conditions with a large amplitude of the fluctuating stress, it is not necessary to continue applying the fluctuating stress until the test piece T breaks under stress conditions with a relatively small amplitude.

[0041] The interface unit 27 allows the user to set stress conditions, and displays to the user the fatigue characteristics estimated by the fatigue characteristic estimation unit 26. For this reason, the fatigue testing apparatus 1 may have an input / output device such as a display or a touch panel, and may be configured to be connected to an external input / output device.

[0042] The display of fatigue characteristics by the interface unit 27 may be a graph of a master curve redrawn for the expected number of times, or may be a numerical display of the expected number of times at which breakage is predicted, the ratio of the expected number of times at which breakage is predicted to occur to the expected number of times specified by the user (for example, the margin of a part against the product's usable load number of times), etc.

[0043] By using the fatigue testing apparatus 1 as described above, it is possible to prepare a master curve in which the number of repetitions of the variable stress, i.e., the standard number of repetitions, of the variable stress applied to the standard specimen T under reference stress conditions is plotted in double logarithm, cut out a specimen T from a target specimen of unknown fatigue degree made of the same material as the standard specimen, apply the variable stress a minimum number of times under any stress conditions to obtain an index value of the energy loss per unit period, and estimate the degree of fatigue of the target specimen based on the correspondence between the index value of the target specimen and the master curve, specifically, from the position on the master curve when the index value of the target specimen is translated in the axial direction of the number of repetitions. For this reason, the interface unit 27 may have a function of displaying the position on the master curve where specific test data is plotted, the numerical value of the fatigue degree of the specimen T estimated from the position, etc.

[0044] As is clear from the above description, a fatigue testing method according to one embodiment of the present invention, which is performed using the fatigue testing apparatus 1, includes an index value acquisition process (S1), a shift factor setting process (S2), a correction function calculation process (S3), a shift factor back-calculation process (S4), and a fatigue characteristic estimation process (S5), as shown in FIG. 5.

[0045] In the index value acquisition process of step S1, the drive control unit 21 applies a periodically changing fluctuating stress to T, and the index value acquisition unit 22 calculates the change in the index value of energy loss per unit period in the test piece T relative to the number of repetitions of the fluctuating stress for each stress condition.

[0046] In the shift factor setting step S2, the shift factor setting unit 23 sets a shift factor for each stress condition so that data for all stress conditions applied by the drive control unit 21 can be plotted on the same master curve. In other words, the shift factor is set for each stress condition so that a line matching the master curve is drawn when the index value for the standard number of times obtained by multiplying the number of repetitions by the shift factor is plotted.

[0047] In the correction function calculation step S3, the correction function calculation unit 24 calculates a correction function that represents the relationship between the stress condition, that is, the amplitude of the strain or stress, and the shift factor.

[0048] In the shift factor inverse calculation step S4, the shift factor inverse calculation unit 25 inversely calculates the shift factor value under desired stress conditions from the correction function.

[0049] In the fatigue characteristic estimation step of S5, the fatigue characteristic estimation unit 26 estimates the fatigue characteristics under the desired stress conditions by redrawing the master curve for the assumed number of times obtained by dividing the standard number of times by the shift factor calculated in the shift factor inverse calculation step.

[0050] In addition, a fatigue analysis program according to one embodiment of the present invention that realizes the fatigue testing apparatus 1 may include a drive control function unit that realizes the drive control unit 21, an index acquisition function unit that realizes the index value acquisition unit 22, a shift factor setting function unit that realizes the shift factor setting unit 23, a correction function calculation function unit that realizes the correction function calculation unit 24, a shift factor inverse calculation function unit that realizes the shift factor inverse calculation unit 25, a fatigue characteristic estimation function unit that realizes the fatigue characteristic estimation unit 26, and an interface function unit that realizes the interface unit 27.

[0051] Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and can be modified as appropriate.

[0052] The shift factor back-calculation unit and fatigue characteristic estimation unit in the fatigue analysis device according to the present invention, the shift factor back-calculation step and fatigue characteristic estimation step in the fatigue analysis method according to the present invention, and the shift factor back-calculation function unit and fatigue characteristic estimation function unit in the fatigue analysis program according to the present invention may be configured arbitrarily. That is, the fatigue analysis device, fatigue analysis method, and fatigue analysis program according to the present invention may provide a master curve and a correction function to a user, and leave the back-calculation of the shift factor and the estimation of the fatigue characteristic to the user. Also, the fatigue analysis program according to the present invention may not include a drive control function unit and an index value acquisition function unit. That is, the fatigue analysis program according to the present invention may calculate a master curve and a correction function from measurement data of a conventional fatigue testing device, for example. [Explanation of symbols]

[0053] 1. Fatigue test equipment 10 Mechanical equipment department 11,12 Grip part 13 Drive mechanism 14 Detection mechanism 15 Support 20 Control device section 21 Drive control unit 22 Index value acquisition section 23 Shift factor setting section 24 Correction function calculation section 25 Shift factor reverse calculation section 26 Fatigue property prediction section 27 Interface section T test piece

Claims

1. A step of applying a periodically changing fluctuating stress to a test piece under a plurality of stress conditions, thereby acquiring a change in an index value of energy loss per unit period in the test piece with respect to the number of repetitions of the fluctuating stress for each of the stress conditions; a step of setting the shift factor so that when a relationship between the standard number of repetitions obtained by multiplying the shift factor set for each stress condition by the number of repetitions and the index value is graphed in double logarithm, data for the plurality of stress conditions are plotted on the same master curve; calculating a correction function that represents a relationship between the stress condition and the shift factor; A fatigue testing method comprising:

2. 2. The fatigue test method according to claim 1, wherein the correction function is calculated assuming that the stress condition and the shift factor have a linear relationship in a double logarithmic scale.

3. back-calculating the value of the shift factor at the desired stress condition from the correction function; a step of estimating fatigue characteristics under the desired stress conditions by redrawing the master curve for an assumed number of times obtained by dividing the standard number of times by the back-calculated shift factor; The fatigue testing method according to claim 1 or 2, further comprising:

4. 4. The fatigue testing method according to claim 1, wherein the index value is a feature value of an amount of lost energy, an energy loss rate, or a hysteresis of energy input / output.

5. 5. The fatigue testing method according to claim 1, wherein the fluctuating stress is varied so as to apply a sinusoidally varying stress or strain to the test piece.

6. preparing a master curve which is a double logarithmic graph of the change in index value of energy loss per unit period in a standard sample test piece against the number of repetitions of a periodically changing fluctuating stress applied to the test piece under reference stress conditions; A step of applying the variable stress under any of the stress conditions to a test piece of a target sample made of the same material as the standard sample, thereby obtaining the index value; estimating a degree of fatigue of the target sample based on a correspondence relationship between the index value of the target sample and the master curve; A fatigue testing method comprising:

7. A pair of gripping parts for gripping both ends of the test piece; A drive mechanism that moves at least one of the pair of gripping portions; A detection mechanism for detecting a force acting between the pair of gripping portions; a drive control unit that controls the drive mechanism so as to apply a periodically changing fluctuating stress to the test piece under a set stress condition; an index value acquiring unit that acquires a change in an index value of energy loss per unit period in the test piece with respect to the number of repetitions of the fluctuating stress based on the drive amount of the drive mechanism and the detection value of the detection mechanism; a shift factor setting unit that sets the shift factor so that data for a plurality of stress conditions are plotted on the same master curve when a relationship between a standard number of times obtained by multiplying the number of repetitions by the shift factor set for each stress condition and the index value is graphed in double logarithm; a correction function calculation unit that calculates a correction function that represents a relationship between the stress condition and the shift factor; A fatigue testing apparatus comprising:

8. A fatigue analysis program for analyzing fatigue characteristics based on a change in an index value of energy loss per unit period in a test piece with respect to the number of repetitions of the variable stress, the change being measured for each of the stress conditions by applying a variable stress that changes periodically to the test piece under a plurality of stress conditions, the program comprising: a shift factor setting function unit that sets the shift factor so that data for the plurality of stress conditions are plotted on the same master curve when a relationship between a standard number of times obtained by multiplying the number of repetitions by the shift factor set for each stress condition and the index value is graphed in double logarithm; a correction function calculation function unit that calculates a correction function that represents a relationship between the stress condition and the shift factor; A fatigue analysis program that provides:

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