Method for estimating fatigue limit

Abstract

To provide a method for accurately estimating the fatigue limit of an object to be measured based on a dissipation energy distribution of the object to be measured that is measured by using an infrared imaging apparatus.SOLUTION: A method according to the present invention has: a relationship calculation step S1 of picking up an image of an object to be measured by using an infrared imaging apparatus while sequentially adding repeated loads different in load width P to the object to be measured, thereby measuring a temporal change of the temperature distribution of the object to be measured for each of the repeated loads, based on the temporal change, calculating the dissipation energy distribution of the object to be measured for each of the repeated loads, and based on the dissipation energy distribution, calculating the relationship between the load widths P and dissipation energy q; and a fatigue limit estimation step S2 of estimating, as the fatigue limit of the object to be measured, the load width P at which the maximum value of at least one parameter is obtained, the parameter of three parameters: a first-order differential value d for each of the load widths P obtained by performing first-order differentiation on the relationship at the load width P; a second-order differential value d2 for each of the load widths P obtained by performing second-order differentiation; and the product dα of the first-order differential value d and the second-order differential value d2 for each of the load widths P.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for accurately estimating the fatigue limit of a measurement object based on the dissipated energy distribution of the measurement object measured using an infrared imaging device.

Background Art

[0002] As a method for non-contact measurement of the stress distribution generated in a measurement object, a thermoelastic stress measurement method using an infrared imaging device (thermography) has been proposed (see, for example, Non-Patent Document 1). The thermoelastic stress measurement method utilizes the thermoelastic effect that a temperature change occurs when the measurement object elastically deforms adiabatically, and images the measurement object to which a repeated load is applied using an infrared imaging device to measure the temporal change of the temperature distribution of the measurement object (the change in the temperature distribution within a predetermined time), and converts this measured temporal change in the temperature distribution into a temporal change in the stress distribution of the measurement object (the change in the stress distribution within a predetermined time). If the initial value of the stress distribution is known (including not only the case where the stress distribution is actually measured and known, but also the case where it can be assumed), the stress distribution after a predetermined time can be measured by adding the temporal change in the stress distribution to this initial value.

[0003] When measuring the temporal change in the temperature distribution of a measurement object using this thermoelastic stress measurement method, for example, the heat (infrared rays) around the measurement object may be reflected on the surface of the measurement object and received by the infrared imaging device. In other words, in addition to the above-described disturbance factors, the temporal change in the temperature distribution of the measurement object measured using the infrared imaging device may include temperature changes caused by factors other than the temperature change (change in the intensity of infrared rays radiated from the measurement object) caused by the thermoelastic effect, such as heat conduction within the measurement object and heat generation due to energy dissipation described later.

[0004] Therefore, in the technique described in Non-Patent Document 1, a signal waveform corresponding to the temperature change caused by the thermoelastic effect to be measured is lock-in processed from the image signal output from the infrared imaging device. That is, only a predetermined frequency component is extracted from the image signal output from the infrared imaging device. Specifically, for example, a reference signal having the same frequency as the repetitive load applied to the object to be measured is used, which is output from a fatigue testing machine that repeatedly applies a load to the object to be measured. The image signal is synchronously detected with this reference signal, and only the image signal component in the frequency band corresponding to the reference signal (only the image signal component having the same frequency as the reference signal or only the image signal in a narrow frequency band including the same frequency as the reference signal) is extracted, thereby improving the S / N ratio of the temperature change caused by the thermoelastic effect to be measured. Then, according to the magnitude of the extracted image signal component and the correspondence between the magnitude of the image signal component stored in advance and the temperature, the temporal change of the temperature distribution of the object to be measured (the temporal change of the temperature of each pixel constituting the captured image captured by the infrared imaging device) is calculated. Next, based on the temporal change of the temperature distribution of the object to be measured and a predetermined relational expression between the temporal change of the temperature and the temporal change of the stress, the temporal change of the stress distribution of the object to be measured is calculated. Specifically, based on the temporal change of the temperature distribution of the object to be measured and the relational expression represented by the following formula (1), the temporal change of the stress distribution of the object to be measured is calculated. Δσ=-1 / K·ΔT / T ···(1) In the above formula (1), ΔT represents the temporal change of the temperature, Δσ represents the temporal change of the stress, T represents the temperature of the object to be measured, and K represents the thermoelastic coefficient. The thermoelastic coefficient K is a physical property value determined by the material of the object to be measured. For example, when the object to be measured is formed of a steel material, K = 3.5×10 -12 [Pa -1 . In this way, by using lock-in processing, it is possible to accurately calculate the temporal change of the stress distribution of the object to be measured, and thus the stress distribution of the object to be measured.

[0005] By repeatedly applying a load to the object to be measured, in addition to the temporal change of the temperature distribution caused by the above-mentioned thermoelastic effect, a temporal change of the temperature distribution caused by energy dissipation also occurs. As described in Non-Patent Document 2, the temporal change in the temperature distribution due to energy dissipation is considered to occur as heat generation components when the maximum stress and the minimum stress act on the object to be measured, respectively. The dissipated energy is defined as a frequency component twice the frequency of the repetitive load applied to the object to be measured in the temporal change of the temperature. This dissipated energy is denoted as ΔT D Let the temporal change in temperature measured using an infrared imaging device (temporal change in temperature before lock-in processing) be ΔT M and the temporal change in temperature due to the thermoelastic effect (temporal change in temperature after lock-in processing) be ΔT E . Then, when disturbance factors and heat conduction are not considered, the following equation (2) holds. ΔT D = ΔT M − ΔT E ···(2) Therefore, the dissipated energy distribution can be measured using an infrared imaging device. Specifically, for example, it can be calculated by subtracting the temporal change in the temperature distribution due to the thermoelastic effect calculated by lock-in processing as described above from the temporal change in the temperature distribution measured using an infrared imaging device.

[0006] Also, Non-Patent Document 2 proposes estimating the fatigue limit of the object to be measured based on the dissipated energy of the object to be measured measured using an infrared imaging device. Specifically, while sequentially applying repetitive loads with different load widths (= maximum load - minimum load) to the object to be measured (for example, increasing the load width of the repetitive load to be applied step by step and applying the repetitive load of each load width for about several thousand cycles), the object to be measured is imaged using an infrared imaging device to measure the temporal change in the temperature distribution of the object to be measured for each repetitive load. Then, based on the temporal change in the temperature distribution of the object to be measured measured for each repetitive load, the dissipated energy distribution of the object to be measured is calculated for each repetitive load, and based on the dissipated energy distribution of the object to be measured calculated for each repetitive load, the relationship between the load width and the dissipated energy is calculated.

[0007] FIG. 1 is a diagram schematically showing the relationship between the load width and the dissipated energy. In FIG. 1, the points plotted with "◆" are the dissipated energies calculated for each repeated load with a different load width. As shown in FIG. 1, in the relationship between the two, a sharp increase point where the dissipated energy rapidly increases occurs inherently at a certain load width. And the load width of the repeated load at this sharp increase point is considered to correspond to the fatigue limit obtained by the so-called S-N diagram. Therefore, if the relationship between the load width and the dissipated energy is calculated and the sharp increase point where the dissipated energy rapidly increases is detected, the load width of the repeated load at this sharp increase point can be estimated as the fatigue limit.

[0008] However, the relationship between the load width and the dissipated energy obtained from the dissipated energy distribution measured using an infrared imaging device does not necessarily become as shown in FIG. 1 in actuality. When the object to be measured is a heat-treated material or a welded material containing residual stress, or when the influence of disturbance factors is large during measurement, the variation of the dissipated energy becomes large, or the dissipated energy increases monotonically with a certain gradient as a whole, and there may be a case where a sharp increase point that can accurately estimate the fatigue limit does not clearly occur.

[0009] In Patent Documents 1 and 2, from the temperature image obtained from an infrared camera, the relationship between the temperature amplitude of the second harmonic component and the temperature amplitude of the component of the fundamental frequency of vibration with respect to the object to be measured is obtained, and the relationship is fitted by a first approximation line that is a quadratic curve and a second approximation line that is a quadratic curve, and a method for obtaining the fatigue limit stress of the object to be measured based on the intersection of the first approximation line and the second approximation line is proposed. However, the methods described in Patent Documents 1 and 2 do not detect the sharp increase point of the dissipated energy that is considered to correspond to the fatigue limit in the relationship between the load width and the dissipated energy as shown in FIG. 1.

Prior Art Documents

Non-Patent Documents

[0010]

Non-Patent Document 1

Non-Patent Document 2

Patent Document

[0011]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0012] The present invention has been made to solve the problems of the prior art as described above, and an object thereof is to provide a method for accurately estimating the fatigue limit of a measurement object based on the dissipated energy distribution of the measurement object measured using an infrared imaging device.

Means for Solving the Problems

[0013] In order to solve the above problems, as a result of intensive studies, the present inventors calculated the relationship between the load amplitude of the repeated load applied to the measurement object and the dissipated energy in the same manner as in the prior art, and then obtained the first derivative value obtained by differentiating this relationship with respect to the load amplitude, the second derivative value obtained by differentiating with respect to the load amplitude, and the product of the first derivative value and the second derivative value. It has been found that if attention is paid to any one of the three parameters, the load amplitude at which the maximum value of this parameter is obtained accurately corresponds to the fatigue limit of the measurement object. In other words, even when there is no distinct inflection point when simply plotting the relationship between the load amplitude and the dissipated energy due to the influence of the residual stress or disturbance factors of the measurement object, it has been found that the load amplitude at which the maximum value of any one of the above three parameters is obtained corresponds to the original inflection point. Further, it has been found that among the three parameters, the maximum value of the product dα of the first-order derivative value d and the second-order derivative value d2 is most likely to correspond to the original sharp increase point and often corresponds most accurately to the fatigue limit of the object to be measured.

[0014] The present invention has been completed based on the above findings of the inventors. That is, in order to solve the above problems, the present invention images the object to be measured using an infrared imaging device while sequentially applying repetitive loads with different load widths P to the object to be measured, measures the temporal change in the temperature distribution of the object to be measured for each of the repetitive loads, calculates the dissipated energy distribution of the object to be measured for each of the repetitive loads based on the temporal change in the temperature distribution of the object to be measured measured for each of the repetitive loads, calculates a relationship between the load width P and the dissipated energy q based on the dissipated energy distribution of the object to be measured calculated for each of the repetitive loads, a relationship calculation step of calculating a first derivative value d for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P by the first order, and a second derivative value d2 for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P by the second order, the maximum value dα of the product dα max and a fatigue limit estimation step of estimating the load width P for which the obtained value is obtained as the fatigue limit of the object to be measured, and provides a fatigue limit estimation method.

[0015] According to the present invention, in the relationship calculation step, the relationship between the load width P and the dissipated energy q is calculated in the same manner as in the prior art. Then, in the fatigue limit estimation step, the product dα of the first derivative value d and and the second derivative value d2 and is calculated, and the load width P for which the maximum value of is obtained is estimated as the fatigue limit of the object to be measured. Therefore, as the inventors have found, the fatigue limit of the object to be measured can be accurately estimated. dα max In the fatigue limit estimation step, the ratio d_rate of the first derivative value d to the maximum value d of the first derivative value d is calculated for each load width P, and the maximum value d

[0017] Also, in order to solve the above problems, the present invention images the object to be measured using an infrared imaging device while sequentially applying repetitive loads with different load widths P to the object to be measured, measures the temporal change in the temperature distribution of the object to be measured for each of the repetitive loads, calculates the dissipated energy distribution of the object to be measured for each of the repetitive loads based on the temporal change in the temperature distribution of the object to be measured measured for each of the repetitive loads, calculates the relationship between the load width P and the dissipated energy q based on the dissipated energy distribution of the object to be measured calculated for each of the repetitive loads, a relationship calculation step; the first-order derivative value d for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P, the second-order derivative value d2 for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P, and the product dα of the first-order derivative value d and the second-order derivative value d2 for each load width P, and a fatigue limit estimation step of estimating the load width P at which the maximum value of any one of the three parameters is obtained as the fatigue limit of the object to be measured. max for each load width P, and the maximum value d max ​When all of the ratios d_rate except for the ratio d_rate for are less than or equal to a first threshold value, the maximum value d max of the obtained load width P is estimated as the fatigue limit of the object under measurement, and when any of the ratios d_rate except for the ratio d_rate for exceeds the first threshold value, the ratio d2_rate of the second derivative value d2 to the maximum value d2 max of the second derivative value d2 is calculated for each load width P, and when all of the ratios d2_rate except for the ratio d2_rate for are less than or equal to a second threshold value, the maximum value d2 max of the obtained load width P is estimated as the fatigue limit of the object under measurement, and when any of the ratios d_rate except for the ratio d_rate for exceeds the first threshold value and any of the ratios d2_rate except for the ratio d2_rate for exceeds the second threshold value, the maximum value dα max of the product dα of the first derivative value d and the second derivative value d2 is estimated as the fatigue limit of the object under measurement for the obtained load width P max In the above method, the ratio d_rate of the first derivative value d to the maximum value d max is calculated for each load width P, and it is determined whether all of the ratios d_rate except for the ratio d_rate for are less than or equal to the first threshold value. Since the ratio d_rate for is 1, a value greater than 0 and less than 1 (for example, 0.5) is set as the first threshold value. If all of the ratios d_rate except for the ratio d_rate for are less than or equal to the first threshold value (in other words, if the maximum value d max is sufficiently larger than other first derivative values d), it can be said that the load width P at which the maximum value d max is obtained is likely to correspond to the original sharp increase point. Therefore, in the above method, the maximum value d , and is also provided as a fatigue limit estimation method .

[0018] In the above way method, the ratio d_rate of the first derivative value d to the maximum value d max is calculated for each load width P, and it is determined whether all of the ratios d_rate except for the ratio d_rate for are less than or equal to the first threshold value. Since the ratio d_rate for is 1, a value greater than 0 and less than 1 (for example, 0.5) is set as the first threshold value. If all of the ratios d_rate except for the ratio d_rate for are less than or equal to the first threshold value (in other words, if the maximum value d max is sufficiently larger than other first derivative values d), it can be said that the load width P at which the maximum value d max is obtained is likely to correspond to the original sharp increase point. Therefore, in the above method, the maximum value d max If all of the ratios d_rate except for the ratio d_rate for are less than or equal to the first threshold value (in other words, if the maximum value d max is sufficiently larger than other first derivative values d), it can be said that the load width P at which the maximum value d max is obtained is likely to correspond to the original sharp increase point. For this reason, in the above way method, the maximum value dmax When all the ratios d_rate except the ratio d_rate for the maximum value d are less than or equal to the first threshold value, the load width P at which the maximum value d is obtained is estimated as the fatigue limit of the object to be measured. max

[0019] On the other hand, when any of the ratios d_rate except the ratio d_rate for the maximum value d exceeds the first threshold value, the load width P at which the maximum value d is obtained may not correspond to the original sharp increase point. For this reason, in the above max method, in the above case, the ratio d2_rate of the second derivative value d2 to the maximum value d2 of the second derivative value d2 is calculated for each load width P, and it is determined whether all the ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold value. Since the ratio d2_rate for the maximum value d2 is 1, as the second threshold value, a value greater than 0 and less than 1 (for example, 0.5) is set. The second threshold value may be the same as the first threshold value or a different value. If all the ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold value (in other words, if the maximum value d2 max is sufficiently larger than the other second derivative values d2), it can be said that the load width P at which the maximum value d2 is obtained is likely to correspond to the original sharp increase point. For this reason, in the above way method, when all the ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold value, the load width P at which the maximum value d2 max is obtained is estimated as the fatigue limit of the object to be measured. max max max max max way max max

[0020] And, in the above way method, when any of the ratios d_rate except the ratio d_rate for the maximum value d exceeds the first threshold value and the maximum value d2 max max ​​​​​​​​​​When any ratio d2_rate other than the ratio d2_rate for exceeds the second threshold value, as described above, the maximum value dα of the product dα of the first derivative value d and the second derivative value d2, which is most likely to correspond to the original sharp increase point, max estimates the load width P obtained as the fatigue limit of the object to be measured.

[0021] As described above, the above way According to the method, the maximum value d of the first derivative value d max , the maximum value d2 of the second derivative value d2 max , the maximum value dα of the product dα of the first derivative value d and the second derivative value d2 max In order to examine the parameters used to estimate the fatigue limit of the object to be measured in the order of, for example, the maximum value d of the first derivative value d max If it is determined to estimate the load width P obtained as the fatigue limit of the object to be measured (when all ratios d_rate other than the ratio d_rate for the maximum value d max are below the first threshold value), it is not necessary to calculate the second derivative value d2 or the product dα of the first derivative value d and the second derivative value d2, and it is possible to improve the efficiency of the estimation. Similarly, the maximum value d2 of the second derivative value d2 max If it is determined to estimate the load width P obtained as the fatigue limit of the object to be measured (when all ratios d2_rate other than the ratio d2_rate for the maximum value d2 max are below the second threshold value), it is not necessary to calculate the product dα of the first derivative value d and the second derivative value d2, and it is possible to improve the efficiency of the estimation.

[0022] Also, in order to solve the above problems, the present invention measures the temporal change of the temperature distribution of the object to be measured for each repeated load by imaging the object to be measured using an infrared imaging device while sequentially applying repeated loads with different load widths P to the object to be measured, calculates the dissipated energy distribution of the object to be measured for each repeated load based on the temporal change of the temperature distribution of the object to be measured measured for each repeated load, calculates the relationship between the load width P and the dissipated energy q based on the dissipated energy distribution of the object to be measured calculated for each repeated load, a relationship calculation step; a first derivative value d for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P by the first order, a second derivative value d2 for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P by the second order, and a fatigue limit estimation step of estimating the load width P at which the maximum value of any one of the three parameters of the product dα of the first derivative value d and the second derivative value d2 for each load width P is obtained as the fatigue limit of the object to be measured. In the fatigue limit estimation step, the ratio d_rate of the first derivative value d to the maximum value d of the first derivative value d max is calculated for each load width P, and the ratio d2_rate of the second derivative value d2 to the maximum value d2 of the second derivative value d2 max is calculated for each load width P, and all the ratios d_rate other than the ratio d_rate for the maximum value d max are below the first threshold value, and the maximum value d2 maxAll of the ratios d2_rate except for the ratio d2_rate for are less than or equal to a second threshold value, and still, the maximum value d max the load width P obtained and the maximum value d2 max When the condition that the load width P obtained and are equal is satisfied, the maximum value d max and the maximum value d2 max the load width P obtained are estimated as the fatigue limit of the object to be measured. When the condition is not satisfied, the maximum value dα of the product dα of the first derivative value d and the second derivative value d2 max the load width P obtained is estimated as the fatigue limit of the object to be measured It is also provided as a fatigue limit estimation method.

[0023] According to the above Side method, for the purpose of examining the parameters used to estimate the fatigue limit of the object to be measured in the order of the maximum value d max of the first derivative value d and the maximum value d2 max of the second derivative value d2, the maximum value dα max of the product dα of the first derivative value d and the second derivative value d2, for example, when it is determined that the load width P obtained for the maximum value d max and the maximum value d2 max of the second derivative value d2 is to be estimated as the fatigue limit of the object to be measured (when all ratios d_rate except for the ratio d_rate for the maximum value d max are less than or equal to a first threshold value, all ratios d2_rate except for the ratio d2_rate for the maximum value d2 max are less than or equal to a second threshold value, and still, the load width P obtained for the maximum value d max and the load width P obtained for the maximum value d2 max are equal), it is not necessary to calculate the product dα of the first derivative value d and the second derivative value d2, and it is possible to improve the efficiency of the estimation. Note that, in order to solve the above problems, the present invention measures the temporal change of the temperature distribution of the object to be measured for each repeated load by imaging the object to be measured using an infrared imaging device while sequentially applying repeated loads with different load widths P to the object to be measured, calculates the dissipated energy distribution of the object to be measured for each repeated load based on the temporal change of the temperature distribution of the object to be measured measured for each repeated load, calculates the relationship between the load width P and the dissipated energy q based on the dissipated energy distribution of the object to be measured calculated for each repeated load, a relationship calculation step; the maximum value d2 of the second derivative value d2 for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P by the second order max and a fatigue limit estimation step of estimating the load width P at which the maximum value d2 is obtained as the fatigue limit of the object to be measured. It is also provided as a fatigue limit estimation method.

Advantages of the Invention

[0024] According to the present invention, it is possible to accurately estimate the fatigue limit of a measurement object based on the dissipated energy distribution of the measurement object measured using an infrared imaging device.

Brief Description of Drawings

[0025]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Modes for Carrying Out the Invention

[0026] Hereinafter, with appropriate reference to the accompanying drawings, a fatigue limit estimation method according to embodiments of the present invention (the first embodiment and the second embodiment) will be described.

[0027] <First Embodiment> FIG. 2 is a flowchart schematically showing the steps of the fatigue limit estimation method according to the first embodiment. As shown in FIG. 2, the fatigue limit estimation method according to the first embodiment includes a relationship calculation step S1 and a fatigue limit estimation step S2. Hereinafter, each of steps S1 and S2 will be described in order.

[0028] [Relationship calculation step S1] In the relationship calculation step S1, while sequentially applying repetitive loads with different load widths (= maximum load - minimum load) P to the object to be measured using a fatigue testing machine or the like, the object to be measured is imaged using an infrared imaging device, thereby measuring the temporal change in the temperature distribution of the object to be measured for each repetitive load. Specifically, the load width P of the repetitive load applied to the object to be measured is gradually increased under the condition that the stress ratio (= minimum stress / maximum stress = minimum load / maximum load) is constant, and while applying the repetitive load of each load width P for about several thousand cycles, the object to be measured is imaged using an infrared imaging device, thereby measuring the temporal change in the temperature distribution of the object to be measured for each repetitive load. Then, based on the temporal change in the temperature distribution of the object to be measured measured for each repetitive load, the dissipation energy distribution of the object to be measured is calculated for each repetitive load. Specifically, from the image signal output from the infrared imaging device, a signal waveform corresponding to the temperature change caused by the thermoelastic effect is subjected to lock-in processing (synchronously detecting the image signal with a reference signal having the same frequency as the applied repetitive load and extracting only the image signal components in the frequency band corresponding to the reference signal). Then, the dissipation energy distribution of the object to be measured is calculated by subtracting the temporal change in the temperature distribution of the object to be measured caused by the thermoelastic effect obtained from the image signal components extracted by the lock-in processing from the temporal change in the temperature distribution of the object to be measured obtained from the image signal (image signal before lock-in processing) output from the infrared imaging device. Note that, as the infrared imaging device for executing the above procedure of the relationship calculation step S1, for example, the X6580 series manufactured by FLIR (cooled type, temperature resolution 0.02 °C, maximum number of pixels 640 × 512 pixels, maximum frame rate 350 Hz) can be used, and as the software for calculating the dissipation energy distribution, AltairLI manufactured by the same company can be used.

[0029] Next, in the relationship calculation step S1, based on the dissipation energy distribution of the object to be measured calculated for each repeated load as described above, the relationship between the load width P and the dissipation energy q is calculated. FIG. 3 is a diagram showing an example of the dissipation energy distribution of the object to be measured and the relationship between the load width P and the dissipation energy q calculated in the relationship calculation step S1. FIG. 3(a) shows an example of the dissipation energy distribution (an image showing the dissipation energy distribution), and FIG. 3(b) shows an example of the relationship between the load width P and the dissipation energy q. The dissipation energy distribution shown in FIG. 3(a) indicates that the darker (blacker) the pixel, the greater the dissipation energy. The relationship shown in FIG. 3(b) is obtained by calculating the dissipation energy distribution as shown in FIG. 3(a) for each repeated load (for each different load width P increased step by step), and plotting the representative value (specifically, the average value) of the dissipation energy in the stress concentration part (the pixel region of several to a dozen pixels vertically and horizontally surrounded by the broken line S shown in FIG. 3(a)) where a fracture initiation point may occur in the dissipation energy distribution for each repeated load. Note that the representative value is not limited to the average value, and for example, the maximum value can also be used.

[0030] [Fatigue limit estimation step S2] Compared with the relationship shown in FIG. 1, the relationship between the load width P and the dissipation energy q shown in FIG. 3(b) shows that the dissipation energy increases monotonically with a certain gradient as a whole, and the rapid increase point is not clear. Therefore, in the fatigue limit estimation step S2 of the fatigue limit estimation method according to the first embodiment, among the three parameters: the first derivative value d for each load width P obtained by differentiating the relationship calculated in the relationship calculation step S1 with respect to the load width P, the second derivative value d2 for each load width P obtained by differentiating the relationship calculated in the relationship calculation step S1 with respect to the load width P twice, and the product dα of the first derivative value d and the second derivative value d2 for each load width P, the load width P at which the maximum value of any one of the parameters is obtained is estimated as the fatigue limit of the object to be measured.

[0031] The first derivative value d is calculated by the following formula (3) (see FIG. 3(b)), where ΔP is the change in the load width P between adjacent plot points and Δq is the change in the dissipation energy between adjacent plot points. d = Δq / ΔP ···(3) That is, the first derivative value d of the nth plot point from the smaller one is d n Let the load width P of the nth plot point from the smaller one be P n , the dissipated energy q be q n , and the load width P of the (n + 1)th plot point from the smaller one be P n+1 , the dissipated energy q be q n+1 . Then, the first derivative value d n is calculated by, for example, the following equation (3)'. d n =(q n+1 -q n ) / (P n+1 -P n ) ···(3)'

[0032] The second derivative value d2 is calculated by the following equation (4) (see Fig. 3(b)), where Δd is the change amount of the first derivative value d of adjacent plot points. d2 = Δd / ΔP ···(4) That is, if the second derivative value d2 of the nth plot point from the smaller one is d2 n and the first derivative value d of the (n + 1)th plot point from the smaller one is d n+1 , then the second derivative value d2 n is calculated by, for example, the following equation (4)'. d2 n =(d n+1 -d n ) / (P n+1 -P n )={(q n+2 -q n+1 ) / (P n+2 -P n+1 )-(q n+1 -q n ) / (P n+1 -P n )} / (P n+1 -P n )···(4)' In the above equation (4)', q n+2 is the dissipated energy q of the (n + 2)th plot point from the smaller one, and P n+2 is the load width P of the (n + 2)th plot point from the smaller one.

[0033] The product dα of the first-order differential value d and the second-order differential value d2 is calculated by the following formula (5) (see Fig. 3(b)). dα = d · d2 ···(5) That is, if the product dα of the nth plot point from the smaller one is dα n then the product dα n is calculated by the following formula (5)'. dα n = d n · d2 n ···(5)'

[0034] As described above, in the fatigue limit estimation step S2, among the three parameters of the first-order differential value d for each load width P, the second-order differential value d2 for each load width P, and the product dα of the first-order differential value d and the second-order differential value d2 for each load width P, which have been described above, the load width P at which the maximum value of any one of the parameters is obtained is estimated as the fatigue limit of the object to be measured. Specifically, the following steps will be executed.

[0035] As shown in Fig. 2, in the fatigue limit estimation step S2, first, the relationship calculated in the relationship calculation step S1 (see Fig. 3(b)) is differentiated with respect to the load width P to calculate the first-order differential value d for each load width P, and the ratio d_rate of the first-order differential value d to the maximum value d max of the first-order differential value d is calculated for each load width P (step S21 in Fig. 2). That is, if the ratio d_rate of the first-order differential value d n of the nth plot point from the smaller one is d_rate n then the ratio d_rate n is calculated by the following formula (6). d_rate n = d n / d max ···(6) Then, it is determined whether all the ratios d_rate except the ratio d_rate for the maximum value d max are less than or equal to the first threshold Th1 (step S22 in Fig. 2). For the maximum value d maxWhen all ratios d_rate except the ratio d_rate for [ID] are less than or equal to the first threshold value Th1 (when "Yes" in step S22 of FIG. 2), the load width P at which the maximum value d max is obtained is estimated as the fatigue limit of the object under measurement (step S23 of FIG. 2). On the other hand, when any ratio d_rate except the ratio d_rate for the maximum value d max exceeds the first threshold value Th1 (when "No" in step S22 of FIG. 2), the process proceeds to step S24.

[0036] FIG. 4 is a diagram showing the relationship between the load width P and the dissipated energy q shown in FIG. 3(b), and the first derivative value d calculated from this relationship. In FIG. 4, the first derivative value d is plotted with "×". As shown in FIG. 4, at the load width P8, the first derivative value d reaches the maximum value d max . Therefore, it can be considered that the load width P8 corresponds to the original sharp increase point. However, since the first derivative value d also becomes large at the load widths P 10 and P 16 , it is difficult to conclude that the load width P8 corresponds to the original sharp increase point. Therefore, as described above, it is decided to determine whether all ratios d_rate except the ratio d_rate for the maximum value d max are less than or equal to the first threshold value Th1. In the example shown in FIG. 4, the ratio d_rate 10 for the first derivative value d 10 at the load width P 10 , the ratio d_rate 14 for the first derivative value d 14 at the load width P 14 and the ratio d_rate 16 for the first derivative value d 16 at the load width P 16 are all greater than 0.5. Therefore, if the first threshold value Th1 = 0.5, any ratio d_rate except the ratio d_rate for the maximum value d max will exceed the first threshold value Th1, and thus the process will proceed to step S24.

[0037] In step S24 of the fatigue limit estimation step S2, the relationship calculated in the relationship calculation step S1 (see Fig. 3(b)) is second-order differentiated with respect to the load width P to calculate the second-order differential value d2 for each load width P, and the ratio d2_rate of the second-order differential value d to the maximum value d2 of the second-order differential value d2 is calculated for each load width P. max That is, for the second-order differential value d2 of the nth plot point from the smaller one, the ratio d2_rate is set as d2_rate. That is, for the second-order differential value d2 of the nth plot point from the smaller one, the ratio d2_rate is set as d2_rate. n For the ratio d2_rate of the second-order differential value d2, it is set as d2_rate. n Then, the ratio d2_rate is as follows. n It is calculated by the following formula (7). d2_rate n = d2 n / d2 max ···(7) And it is determined whether all ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold Th2 (step S25 in Fig. 2). If all ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold Th2 (in the case of "Yes" in step S25 of Fig. 2), the load width P at which the maximum value d2 is obtained is estimated as the fatigue limit of the object to be measured (step S26 in Fig. 2). On the other hand, if any ratio d2_rate except the ratio d2_rate for the maximum value d2 exceeds the second threshold Th2 (in the case of "No" in step S25 of Fig. 2), it proceeds to step S27. max And it is determined whether all ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold Th2 (step S25 in Fig. 2). If all ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold Th2 (in the case of "Yes" in step S25 of Fig. 2), the load width P at which the maximum value d2 is obtained is estimated as the fatigue limit of the object to be measured (step S26 in Fig. 2). On the other hand, if any ratio d2_rate except the ratio d2_rate for the maximum value d2 exceeds the second threshold Th2 (in the case of "No" in step S25 of Fig. 2), it proceeds to step S27. max And it is determined whether all ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold Th2 (step S25 in Fig. 2). If all ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold Th2 (in the case of "Yes" in step S25 of Fig. 2), the load width P at which the maximum value d2 is obtained is estimated as the fatigue limit of the object to be measured (step S26 in Fig. 2). On the other hand, if any ratio d2_rate except the ratio d2_rate for the maximum value d2 exceeds the second threshold Th2 (in the case of "No" in step S25 of Fig. 2), it proceeds to step S27. max And it is determined whether all ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold Th2 (step S25 in Fig. 2). If all ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold Th2 (in the case of "Yes" in step S25 of Fig. 2), the load width P at which the maximum value d2 is obtained is estimated as the fatigue limit of the object to be measured (step S26 in Fig. 2). On the other hand, if any ratio d2_rate except the ratio d2_rate for the maximum value d2 exceeds the second threshold Th2 (in the case of "No" in step S25 of Fig. 2), it proceeds to step S27. max And it is determined whether all ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold Th2 (step S25 in Fig. 2). If all ratios d2_rate except the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold Th2 (in the case of "Yes" in step S25 of Fig. 2), the load width P at which the maximum value d2 is obtained is estimated as the fatigue limit of the object to be measured (step S26 in Fig. 2). On the other hand, if any ratio d2_rate except the ratio d2_rate for the maximum value d2 exceeds the second threshold Th2 (in the case of "No" in step S25 of Fig. 2), it proceeds to step S27.

[0038] Fig. 5 is a diagram showing the relationship between the load width P and the dissipated energy q shown in Fig. 3(b) and the second-order differential value d2 calculated from this relationship. In Fig. 5, the second-order differential value d2 is plotted with "+". As shown in Fig. 5, at the load width P8, the second-order differential value d2 becomes the maximum value d2. max Therefore, it can also be considered that the load width P8 corresponds to the original sharp increase point. However, for the load widths P6 and P 10Even when the second derivative value d2 is large, it is difficult to conclude that the load width P8 corresponds to the original sharp increase point. Therefore, as described above, it is determined whether all the ratios d2_rate except the ratio d2_rate for the maximum value d2 max are less than or equal to the second threshold value Th2. In the example shown in FIG. 5, the ratio d2_rate6 for the second derivative value d26 at the load width P6, and the second derivative value d2 10 at the load width P 10 for the ratio d2_rate 10 are both greater than 0.5. Therefore, if the second threshold value Th2 = 0.5, any of the ratios d2_rate except the ratio d2_rate for the maximum value d2 max will exceed the second threshold value Th2, so the process proceeds to step S27.

[0039] Step S27 of the fatigue limit estimation step S2 is, as described above, when any of the ratios d_rate except the ratio d_rate for the maximum value d max of the first derivative value d exceeds the first threshold value Th1, and any of the ratios d2_rate except the ratio d2_rate for the maximum value d2 max of the second derivative value d2 exceeds the second threshold value Th2. In step S27, the product dα of the first derivative value d and the second derivative value d2 is calculated, and the load width P at which the maximum value dα max of this product dα is obtained is estimated as the fatigue limit of the object under measurement. Note that when both the first derivative value d and the second derivative value d2 are negative values, the calculation result of the product dα of the first derivative value d and the second derivative value d2 is forcibly set to dα = 0.

[0040] FIG. 6 is a diagram showing the relationship between the load width P and the dissipated energy q shown in FIG. 3(b), and the product dα of the first derivative value d and the second derivative value d2 calculated from this relationship. In FIG. 6, the product dα is plotted with " * ". As shown in FIG. 6, at the load width P8, since the product dα is the maximum value dα max , the load width P8 is estimated as the fatigue limit of the object under measurement. The product dα at the load width P8max The difference between the product dα at the load width P and that at other load widths P is the first derivative value d at the load width P8 shown in FIG. 4 max The difference between the first derivative value d at the load width P8 shown in FIG. 5 and the first derivative values d at other load widths P, and the second derivative value d2 at the load width P8 shown in FIG. 5 max is larger than the difference between the second derivative value d2 at the load width P8 and the second derivative values d2 at other load widths P. Therefore, it can be said that there is a high possibility that the load width P8 corresponds to the original sharp increase point.

[0041] According to the fatigue limit estimation method according to the first embodiment described above, in the fatigue limit estimation step S2, among the three parameters of the first derivative value d, the second derivative value d2, and the product dα of the first derivative value d and the second derivative value d2, since the load width P at which the maximum value of any one parameter is obtained is estimated as the fatigue limit of the object to be measured, the fatigue limit of the object to be measured can be accurately estimated. In particular, according to the fatigue limit estimation method according to the first embodiment, the maximum value d of the first derivative value d max , the maximum value d2 of the second derivative value d2 max , and the maximum value dα of the product dα of the first derivative value d and the second derivative value d2 max In order to consider the parameters used to estimate the fatigue limit of the object to be measured in the order of, for example, the maximum value d of the first derivative value d max If the load width P at which the maximum value d is obtained is estimated as the fatigue limit of the object to be measured (when all ratios d_rate except the ratio d_rate for the maximum value d max are equal to or less than the first threshold Th1), there is no need to calculate the second derivative value d2 or the product dα of the first derivative value d and the second derivative value d2, and it is possible to improve the efficiency of the estimation. Similarly, when the load width P at which the maximum value d2 of the second derivative value d2 max is obtained is estimated as the fatigue limit of the object to be measured (when all ratios d2_rate except the ratio d2_rate for the maximum value d2 max are equal to or less than the second threshold Th2), there is no need to calculate the product dα of the first derivative value d and the second derivative value d2, and it is possible to improve the efficiency of the estimation.

[0042] <Second Embodiment> FIG. 7 is a flowchart schematically showing the steps of the fatigue limit estimation method according to the second embodiment. As shown in FIG. 7, the fatigue limit estimation method according to the second embodiment also has a relationship calculation step S1 and a fatigue limit estimation step S2', similar to the first embodiment. However, the content of the fatigue limit estimation step S2' is different from that of the fatigue limit estimation step S2 of the first embodiment. Hereinafter, for the fatigue limit estimation step S2', the same points as those of the fatigue limit estimation step S2 of the first embodiment will be omitted as appropriate, and mainly the differences from the first embodiment will be described.

[0043] [Fatigue limit estimation step S2'] In the fatigue limit estimation step S2' as well, similar to the fatigue limit estimation step S2 of the first embodiment, among the three parameters of the first derivative value d per load range P, the second derivative value d2 per load range P, and the product dα of the first derivative value d and the second derivative value d2 per load range P, the load range P at which the maximum value of any one of the parameters is obtained is estimated as the fatigue limit of the object to be measured. Specifically, the following steps will be executed.

[0044] As shown in FIG. 7, in the fatigue limit estimation step S2', first, the relationship calculated in the relationship calculation step S1 (see FIG. 3(b)) is differentiated with respect to the load range P to calculate the first derivative value d per load range P, and the ratio d_rate of the first derivative value d to the maximum value d of the first derivative value d is calculated for each load range P. At the same time, the relationship calculated in the relationship calculation step S1 (see FIG. 3(b)) is differentiated with respect to the load range P to calculate the second derivative value d2 per load range P, and the ratio d2_rate of the second derivative value d to the maximum value d2 of the second derivative value d2 is calculated for each load range P (step S21' in FIG. 7). max Then, it is determined whether the following conditions (a) to (c) are satisfied (step S22' in FIG. 7). max (a) All ratios d_rate except for the ratio d_rate for the maximum value d are less than or equal to the first threshold Th1. And it is determined whether the following conditions (a) to (c) are satisfied (step S22' in FIG. 7). (a) All ratios d_rate except for the ratio d_rate for the maximum value d are less than or equal to the first threshold Th1. max (b) All ratios d_rate except for the ratio d_rate for the maximum value d are less than or equal to the first threshold Th1. (b) All ratios d2_rate except for the ratio d2_rate for the maximum value d2 are less than or equal to the second threshold Th2. maxAll ratios d2_rate except the ratio d2_rate for (c) Maximum value d max The load width P obtained and the maximum value d2 max The load width P obtained are equal. When all of the above conditions (a) to (c) are satisfied (when "Yes" in step S22' of FIG. 7), the maximum value d max And the maximum value d2 max The load width P obtained is estimated as the fatigue limit of the object to be measured (step S23' in FIG. 2). On the other hand, when any of the above conditions (a) to (c) is not satisfied (when "No" in step S22' of FIG. 2), the process proceeds to step S24'.

[0045] In the example shown in FIG. 4 described above, the ratio d_rate of the first derivative value d 10 at the load width P 10 with respect to 10 the ratio d_rate of the first derivative value d 14 at the load width P 14 with respect to 14 and the ratio d_rate of the first derivative value d 16 at the load width P 16 with respect to 16 are all greater than 0.5. Therefore, if the first threshold value Th1 = 0.5, any ratio d_rate except the ratio d_rate for the maximum value d max will exceed the first threshold value Th1, so the above condition (a) will not be satisfied. Also, in the example shown in FIG. 5 described above, the ratio d2_rate6 of the second derivative value d26 at the load width P6, and the ratio d2_rate of the second derivative value d2 10 at the load width P 10 with respect to 10 are all greater than 0.5. Therefore, if the second threshold value Th2 = 0.5, any ratio d2_rate except the ratio d2_rate for the maximum value d2 max will exceed the second threshold value Th2, so the above condition (b) will not be satisfied. Furthermore, in the examples shown in FIGS. 4 and 5 described above, the maximum value dmax The obtained load width P and the maximum value d2 max Since both the obtained load width P and the obtained load width P are equal to the load width P8, the above condition (c) is satisfied. Therefore, in the examples shown in FIGS. 4 and 5 described above, among the above conditions (a) to (c), since conditions (a) and (b) are not satisfied, the process proceeds to step S24'.

[0046] In step S24' of the fatigue limit estimation step S2', similar to step S27 of the fatigue limit estimation step S2 of the first embodiment, the product dα of the first derivative value d and the second derivative value d2 is calculated (when both the first derivative value d and the second derivative value d2 are negative values, the calculation result of the product dα of the first derivative value d and the second derivative value d2 is forced to be dα = 0), and the maximum value dα of this product dα max The obtained load width P is estimated as the fatigue limit of the object to be measured. In the example shown in FIG. 6 described above, at the load width P8, the product dα is the maximum value dα max Therefore, the load width P8 is estimated as the fatigue limit of the object to be measured.

[0047] According to the fatigue limit estimation method according to the second embodiment described above, in the fatigue limit estimation step S2', among the three parameters of the first derivative value d, the second derivative value d2, and the product dα of the first derivative value d and the second derivative value d2, since the load width P at which the maximum value of any one parameter is obtained is estimated as the fatigue limit of the object to be measured, the fatigue limit of the object to be measured can be accurately estimated. In particular, according to the fatigue limit estimation method according to the second embodiment, the maximum value d of the first derivative value d max and the maximum value d2 of the second derivative value d2 max , the maximum value dα of the product dα of the first derivative value d and the second derivative value d2 max In order to consider the parameters used to estimate the fatigue limit of the object to be measured in order, for example, the maximum value d of the first derivative value d max and the maximum value d2 of the second derivative value d2 max If the obtained load width P is estimated as the fatigue limit of the object to be measured (the maximum value d maxAll ratios d_rate except for the ratio d_rate for [[ID=]] are equal to or less than a first threshold value Th1, and a maximum value d2 max All ratios d2_rate except for the ratio d2_rate for [[ID=]] are equal to or less than a second threshold value Th2, and moreover, a maximum value d max The obtained load width P and the maximum value d2 max If the obtained load width P and the maximum value d2

[0048] Note that the fatigue limit estimation method according to the present invention is not limited to the fatigue limit estimation method according to the first embodiment or the second embodiment described above. Among the three parameters of the first derivative value d, the second derivative value d2, and the product dα of the first derivative value d and the second derivative value d2 obtained from the relationship between the load width P and the dissipated energy q, as long as the load width P at which the maximum value of any one of the parameters is obtained is estimated as the fatigue limit of the object to be measured, various modes can be adopted.

[0049] <Example> Hereinafter, an example in which the fatigue limit estimation method according to the first embodiment is executed will be described. FIG. 8 is an explanatory diagram for explaining the outline of this example. FIG. 8(a) is a diagram schematically showing a test piece used in this example, and FIG. 8(b) is an S-N diagram obtained by performing a fatigue test on the test piece shown in FIG. 8(a). As shown in FIG. 8(a), as the test piece, a part of two steel plates was overlapped, and spot welding was performed on this overlap to join the two steel plates. In the relationship calculation step S1 of this embodiment, this test piece was attached to a fatigue testing machine that applies a repeated load, and the load amplitude P was gradually increased in the range of P = 0.8 to 2.4 kN (stress ratio: 0.05, repetition frequency: 7 Hz). The repeated load of each load amplitude P was applied in increments of 2000 cycles. In this state, by imaging the periphery of the spot weld of the test piece using an infrared imaging device, the temporal change in the temperature distribution of the test piece was measured for each repeated load. As the infrared imaging device, the X6580 series manufactured by FLIR was used, the frame rate was set to 149 Hz, and measurement was performed for 10 seconds for each repeated load (the change in the temperature distribution within 10 seconds was measured). Then, the AltairLI manufactured by FLIR was used to calculate the dissipated energy distribution. The above-mentioned FIG. 3(a) shows the dissipated energy distribution calculated by the above procedure. The vertical axis of the above-mentioned FIG. 3(b) is the average value of the dissipated energy in a pixel region of 15×15 pixels surrounded by the broken line S shown in FIG. 3(a) (a pixel region corresponding to the spot weld (stress concentration part) where a fracture origin may occur).

[0050] The first-order differential value d shown in the above-mentioned FIG. 4 is the first-order differential value d calculated in the fatigue limit estimation step S2 of this embodiment. The second-order differential value d2 shown in the above-mentioned FIG. 5 is the second-order differential value d2 calculated in the fatigue limit estimation step S2 of this embodiment. The product dα shown in the above-mentioned FIG. 6 is the product dα calculated in the fatigue limit estimation step S2 of this embodiment. Therefore, in this embodiment, the load amplitude P8 is estimated as the fatigue limit of the object to be measured.

[0051] As shown in FIG. 8(b), as a result of the fatigue test, the test piece did not break at the load amplitude P7, but broke when the load amplitude increased to P9. For this reason, it can be seen that the load amplitude P8, which is an intermediate value between the load amplitude P7 and the load amplitude P9, is the fatigue limit. Therefore, it was confirmed that the fatigue limit estimated by the fatigue limit estimation method according to the first embodiment coincides with the fatigue limit obtained from the S-N diagram and can be accurately estimated.

Explanation of symbols

[0052] d ··· First-order differential value d2 ··· Second-order differential value dα ··· Product of the first-order differential value and the second-order differential value P ··· Load width q ··· Dissipated energy S1 ··· Relationship calculation step S2, S2’ ··· Fatigue limit estimation step

Claims

1. While sequentially applying repetitive loads with different load widths P to the object to be measured, imaging the object to be measured using an infrared imaging device, measuring the temporal change in the temperature distribution of the object to be measured for each repetitive load, calculating the dissipation energy distribution of the object to be measured for each repetitive load based on the temporal change in the temperature distribution of the object to be measured measured for each repetitive load, and calculating the relationship between the load width P and the dissipation energy q based on the dissipation energy distribution of the object to be measured calculated for each repetitive load; a relationship calculation step; A fatigue limit estimation step of estimating, as the fatigue limit of the object to be measured, the load width P at which the maximum value dα max of the product dα of the first derivative value d for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P and the second derivative value d2 for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P is obtained; A fatigue limit estimation method.

2. While sequentially applying repetitive loads with different load widths P to the object to be measured, imaging the object to be measured using an infrared imaging device, measuring the temporal change in the temperature distribution of the object to be measured for each repetitive load, calculating the dissipation energy distribution of the object to be measured for each repetitive load based on the temporal change in the temperature distribution of the object to be measured measured for each repetitive load, and calculating the relationship between the load width P and the dissipation energy q based on the dissipation energy distribution of the object to be measured calculated for each repetitive load; a relationship calculation step; A fatigue limit estimation step of estimating, as the fatigue limit of the object to be measured, the load width P at which the maximum value of any one of the three parameters, namely, the first derivative value d for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P, the second derivative value d2 for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P, and the product dα of the first derivative value d and the second derivative value d2 for each load width P, is obtained; In the fatigue limit estimation step, The maximum value d of the first derivative value d ｍａｘCalculate the ratio d_rate of the first-order differential value d with respect to each load width P, and when all the ratios d_rate except the ratio d_rate for the maximum value d ｍａｘ are less than or equal to the first threshold value, estimate the load width P at which the maximum value d ｍａｘ is obtained as the fatigue limit of the object to be measured, and for the maximum value d ｍａｘ when any of the ratios d_rate except the ratio d_rate for the maximum value d exceeds the first threshold value, calculate the ratio d2_rate of the second-order differential value d2 with respect to the second-order differential value d2 for each load width P, and when all the ratios d2_rate except the ratio d2_rate for the maximum value d2 ｍａｘ are less than or equal to the second threshold value, estimate the load width P at which the maximum value d2 ｍａｘ is obtained as the fatigue limit of the object to be measured, ｍａｘ and for the maximum value d when any of the ratios d_rate except the ratio d_rate for the maximum value d exceeds the first threshold value and any of the ratios d2_rate except the ratio d2_rate for the maximum value d2 exceeds the second threshold value, estimate the load width P at which the maximum value dα of the product dα of the first-order differential value d and the second-order differential value d2 ｍａｘ is obtained as the fatigue limit of the object to be measured. ｍａｘ ｍａｘ A method for estimating the fatigue limit.

3. While sequentially applying repetitive loads with different load widths P to the object to be measured, imaging the object to be measured using an infrared imaging device to measure the temporal change in the temperature distribution of the object to be measured for each repetitive load, calculating the dissipated energy distribution of the object to be measured for each repetitive load based on the temporal change in the temperature distribution of the object to be measured measured for each repetitive load, and calculating the relationship between the load width P and the dissipated energy q based on the dissipated energy distribution of the object to be measured calculated for each repetitive load, a relationship calculation step; A fatigue limit estimation step of estimating, as the fatigue limit of the object under measurement, the load width P at which the maximum value of any one of the three parameters, namely, the first derivative value d for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P, the second derivative value d2 for each load width P obtained by differentiating the relationship calculated in the relationship calculation step with respect to the load width P twice, and the product dα of the first derivative value d and the second derivative value d2 for each load width P, is obtained, In the fatigue limit estimation step, The ratio d_rate of the first derivative value d to the maximum value d of the first derivative value d ｍａｘ is calculated for each load width P, and the ratio d2_rate of the second derivative value d2 to the maximum value d2 ｍａｘ of the second derivative value d2 is calculated for each load width P. When all the ratios d_rate except the ratio d_rate for the maximum value d ｍａｘ are less than or equal to a first threshold value, all the ratios d2_rate except the ratio d2_rate for the maximum value d2 ｍａｘ are less than or equal to a second threshold value, and moreover, when the condition that the load width P at which the maximum value d ｍａｘ is obtained is equal to the load width P at which the maximum value d2 ｍａｘ is obtained is satisfied, the load width P at which the maximum value d ｍａｘ and the maximum value d2 ｍａｘ are obtained is estimated as the fatigue limit of the object under measurement. When the condition is not satisfied, the load width P at which the maximum value dα ｍａｘ of the product dα of the first derivative value d and the second derivative value d2 is obtained is estimated as the fatigue limit of the object under measurement. A fatigue limit estimation method.

4. A relationship calculation step of calculating the relationship between the load width P and the dissipated energy q based on the dissipated energy distribution of the object to be measured calculated for each repeated load, by imaging the object to be measured using an infrared imaging device while sequentially applying repeated loads with different load widths P to the object to be measured, measuring the temporal change in the temperature distribution of the object to be measured for each repeated load, and calculating the dissipated energy distribution of the object to be measured for each repeated load based on the temporal change in the temperature distribution of the object to be measured measured for each repeated load. A fatigue limit estimation step of estimating, as the fatigue limit of the object to be measured, the load width P at which the maximum value d2 max of the second derivative value d2 for each load width P obtained by second-order differentiating the relationship calculated in the relationship calculation step with respect to the load width P is obtained. A fatigue limit estimation method.

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