TW202244476A - Fatigue life estimation method, fatigue life estimation apparatus, fatigue life estimation program and storage medium - Google Patents

Abstract

A fatigue life estimation method according to the present invention comprises: acquiring fatigue test data which includes: test results of the fatigue test under various conditions relating to a target, an initial defect size of a test piece used in the fatigue test, and test conditions of the fatigue test; based on the fatigue test data, calculating a material constant which is used to calculate a developing amount of a crack to a load stress in one time, with the assumptions that the crack will develop from an initial defect when a load stress equal to or larger than a fatigue limit is applied to the target, and the fatigue limit will decrease according to the size of the detect and the crack developed from the detect; and using the material constant to estimate the fatigue life based on the developing amount of the crack when applying a certain load stress to the target.

Description

Fatigue life prediction method, fatigue life prediction device, fatigue life prediction program and storage medium

The disclosure relates to a fatigue life prediction method, a fatigue life prediction device, a fatigue life prediction program and a memory medium.

When evaluating the fatigue failure of objects such as metal materials, fatigue life and fatigue limit are often important evaluation factors, so many studies are underway. For example, Patent Document 1 shows a method based on the stress distribution inside the element without cracks, the relationship between the crack depth and the influence of creep, and the relationship between the influence of creep and the Paris equation (Paris equation) for the fatigue life. The relationship between the parameter C and the value of m can be used to estimate the progress behavior of cracks that develop in the device. Furthermore, for example, in Patent Document 2, regarding the fatigue limit, as a method of predicting the fatigue limit of a metal material having minute defects, it is shown that the tensile fatigue limit and the shear fatigue limit are predicted using the potential fatigue crack length of the metal material. method.

[Prior art literature]

[Patent Document]

Patent Document 1: International Publication No. 2014/033927

Patent Document 2: Japanese Patent Laid-Open No. 2018-185274

However, conventionally known methods for predicting the fatigue life of an object whose applied stress is assumed to fluctuate still have room for improvement in terms of accuracy.

The present disclosure was made in view of the above circumstances, and an object thereof is to provide a technology capable of predicting fatigue life with higher accuracy.

In order to achieve the above object, a type of fatigue life prediction method disclosed in the present disclosure includes the following steps: Obtaining fatigue test data, the fatigue test data includes: test results of fatigue tests on objects under multiple conditions, used in The initial defect size of the test piece for the aforementioned fatigue test, and the test conditions of the aforementioned fatigue test; under specific conditions, based on the aforementioned fatigue test data, calculate the material constant used to calculate the progress of the crack relative to one stress load , the specific premise is: when a stress load above the fatigue limit is applied to the aforementioned object, it will develop from an initial defect to a crack, and the aforementioned fatigue limit will decrease in accordance with the size of the defect and the crack that progresses from the defect; and using the aforementioned The material constant predicts the fatigue life from the amount of progress of cracks when a certain stress is applied to the aforementioned object.

According to the above-mentioned fatigue life prediction method, by specifying the material constant for calculating the amount of crack progression with respect to one stress load, the amount of crack progression with respect to one stress load can be estimated. Utilizing this point, it is possible to predict the fatigue life when a certain stress is applied to the test piece of the object. Here, when a stress load exceeding the fatigue limit is applied to the object, the initial defect will progress to It is assumed that cracks and the fatigue limit will decrease according to the size of the defect and the crack that progresses from the defect. Therefore, the progress of the defect (crack) caused by the stress load and the decrease in the fatigue limit associated with it are considered. Since the fatigue life is predicted by appropriately capturing the true driving force of crack growth in the repeatedly applied load, it is possible to realize higher-precision prediction.

Here, the step of predicting the fatigue life includes: calculating the fatigue limit based on the defect size of the object; calculating the progress of the crack with respect to one stress load when the load is larger than the calculated fatigue limit; And the number of loads of stress until the size of the defect and the crack progressing from the defect reaches the limit defect size is calculated.

With the above configuration, when a certain stress is applied, the amount of progress of cracks relative to one stress load is calculated when the load is greater than the calculated fatigue limit, and the defect and the crack progressing from the defect are calculated. The number of times the stress is loaded until the size of the crack reaches the limit defect size. Therefore, regardless of the magnitude of the stress load on the object, the fatigue life can be predicted with high accuracy.

Here, it may be as follows: the step of calculating the aforementioned material constant is to calculate C*, m*, and n* in the following formula (A), which is to calculate the cracking relative to the primary stress load The amount of progress △√area.

[In which, △√area is the amount of progress of the crack with respect to one stress load, σ is the stress amplitude of the load, σ _w is the fatigue limit when the load is applied, and √area is the size of the defect]

With the above configuration, it is possible to designate a material constant that can appropriately calculate the amount of progress of a crack with respect to one stress load, and use this material constant to predict fatigue life with high accuracy.

In the step of predicting the fatigue life, the amplitude of the stress applied to the object may be constant or two or more stages may be used.

According to the above configuration, the magnitude of the load on the object is not particularly limited. Therefore, the fatigue life can be predicted with high accuracy in any of the conditions when the stress amplitude of the load is constant or in two or more stages.

A type of fatigue life prediction device disclosed in the present disclosure includes: a fatigue test result acquisition unit, which acquires fatigue test data, and the fatigue test data includes: test results of fatigue tests on objects under multiple conditions, use The initial flaw size of the test piece in the aforementioned fatigue test, and the test conditions of the aforementioned fatigue test; the material constant calculation unit, which is used to calculate the stress load relative to one time, based on the aforementioned fatigue test data under specific conditions. The material constant for the amount of progress of cracks. The specified premise is that when a stress load above the fatigue limit is applied to the aforementioned object, it will progress from the initial defect to cracks, and the aforementioned fatigue limit will respond to the defect and the crack that progresses from the defect. and a fatigue life predicting unit that predicts the fatigue life when a certain stress is applied to the object based on the amount of progress of cracking with respect to one stress load calculated using the material constant. According to the above-mentioned fatigue life prediction device, the same effect as the above-mentioned fatigue life prediction method can be achieved.

One type of fatigue life prediction program disclosed in the present disclosure is to make the computer system perform the following steps: obtain fatigue test data, the fatigue test data includes: test results of fatigue tests on the object under multiple conditions, used for the aforementioned fatigue The initial defect size of the test piece and the test conditions of the aforementioned fatigue test; under certain conditions, based on the aforementioned fatigue test data, the material constant used to calculate the progress of the crack relative to one stress load is calculated. The specific prerequisites are: when a stress load exceeding the fatigue limit is applied to the aforementioned object, cracks will progress from the initial defect, and the aforementioned fatigue limit will decrease according to the size of the defect and the crack that progresses from the defect; and based on the use of the aforementioned material constant while The fatigue life when a certain stress is applied to the aforementioned object is predicted from the calculated amount of progress of cracking with respect to one stress load. According to the above fatigue life prediction program, the same effect as the above fatigue life prediction method can be achieved.

One type of memory medium disclosed in this disclosure is one that can be read by a computer, and it memorizes a fatigue life prediction program that causes the computer system to perform the following steps: Obtain fatigue test data, the fatigue test data includes: information about the existence of the object The test results of the fatigue test under multiple conditions, the initial defect size of the test piece used in the aforementioned fatigue test, and the test conditions of the aforementioned fatigue test; under specific conditions, based on the aforementioned fatigue test data, it is used to calculate the relative The specific premise is: when a stress load above the fatigue limit is applied to the aforementioned object, it will progress from initial defects to cracks, and the aforementioned fatigue limit will vary from defect to defect. The size of the progressing crack is reduced; and the fatigue life when the aforementioned object is loaded with a certain stress is predicted based on the amount of progress of the crack with respect to one stress load calculated using the aforementioned material constant. According to the above-mentioned memory medium, the same effect as the above-mentioned fatigue life prediction method can be achieved.

According to the present disclosure, a technique capable of predicting fatigue life with higher accuracy is provided.

1: Fatigue life prediction device

11: Fatigue test data acquisition department

12: Forecast Condition Acquisition Department

13:Material constant calculation part

14: Fatigue Life Prediction Department

15: Result output unit

16: Data memory department

100: computer

101: CPU

102: Main memory

103: Auxiliary memory department

104: Communication Control Department

105: input device

106: output device

S01~S05, S11~S22, S31~S49, S51~S60: steps

FIG. 1 is a schematic diagram of a type of fatigue life prediction device.

FIG. 2 is a flowchart illustrating an example of a fatigue life prediction method.

FIG. 3 is a flowchart illustrating an example of a fatigue life prediction method using material constants included in the fatigue life prediction method.

4 is a flowchart illustrating an example of a method of calculating a material constant included in a fatigue life prediction method.

FIG. 5 is a flowchart illustrating an example of a sub-routine for obtaining N _fpred included in FIG. 4 .

FIG. 6 is a diagram showing an example of the hardware configuration of the fatigue life prediction device.

FIG. 7 is a diagram illustrating evaluation results of fatigue life prediction performed by the fatigue life prediction device.

The modes for implementing the present disclosure are described in detail below with reference to the accompanying drawings. In the description of the drawings, the same elements are attached with the same symbols, and overlapping descriptions are omitted.

[Fatigue life prediction device]

First, a schematic configuration of a fatigue life prediction device 1 according to an embodiment will be described with reference to FIG. 1 . The fatigue life prediction device 1 shown in FIG. 1 is a device for predicting the fatigue life when a predetermined stress is applied to an object.

The types of objects to be predicted for fatigue life are not particularly limited, but industrial products, structures, etc., and industrial materials that can be used as components of industrial products, structures, etc., may be the main objects. Furthermore, among industrial materials, particularly metals, the fatigue life predictor 1 can be used to perform highly accurate life prediction. As long as it is a material that can obtain an S-N curve, the above method can also be applied to non-metals.

In addition, the fatigue life predicted by the fatigue life prediction device 1 refers to the number of repetitions of stress loading until the object is destroyed when stress is repeatedly applied to the object. In general, when a stress greater than a predetermined value is applied to an object, the object is destroyed over a certain number of times. The fatigue life prediction device 1 has a function of predicting the number of repetitions of the stress load until the failure occurs.

In general, the fatigue life is often predicted by generating an S-N curve based on the results of a fatigue test. In this case, usually, the results of multiple fatigue tests obtained at a constant stress amplitude are inserted into a known model to generate an S-N curve. However, in the case where the stress fluctuates, a stress smaller than the fatigue limit based on the above-mentioned S-N curve may also affect the progress of the crack. Therefore, it is known that the fatigue life estimation technology based on the S-N curve is not yet available. room for improvement.

In the fatigue life prediction device 1 of this embodiment, by focusing on the progress behavior of cracks inside the material as a process of fatigue, and formulating this behavior on the basis of mechanics, it is possible to achieve fatigue under variable stress. High-precision prediction of lifetime. The details will be described later, but in the fatigue life prediction device 1, it is said that "when the stress exceeding the fatigue limit is applied to the object, the micro cracks (initial defects) existing in the object will develop (increase in size)" As a premise, the fatigue life is predicted by evaluating the progress of cracks due to repeated loads in consideration of the decrease in the fatigue limit due to crack growth. First, three material constants, which are parameters used for fatigue life prediction, are calculated from the fatigue test data of the object under multiple conditions. Afterwards, using the calculated three material constants, calculate the progress of the crack when a certain stress is applied to the object and update the crack size (the initial defect and the size of the crack that progresses from the initial defect), based on the updated The fatigue limit of the new crack size will continue to be evaluated, and the stage where the crack size exceeds the limit value due to fatigue is identified, so as to predict the fatigue life.

(Function part of the fatigue life prediction device)

Each part of the fatigue life prediction device 1 will be described with reference to FIG. 1 . As shown in FIG. 1 , the fatigue life prediction device 1 includes a fatigue test data acquisition unit 11 (fatigue test result acquisition unit), a prediction condition acquisition unit 12, a material constant calculation unit 13, a fatigue life prediction unit 14, and a result output unit 15 ( Output unit) and data memory unit 16 and constitute.

The fatigue test data acquisition unit 11 has a function of acquiring the data of the fatigue test results of the object to be subjected to fatigue life prediction. The data of the fatigue test results acquired by the fatigue test data acquisition unit 11 is the data of the general fatigue test results, specifically, the data obtained by performing "a stress load of a certain amplitude to a test piece having a defect of a certain size." The test conditions and results during the test of the number of repetitions until the broken. Details will be described later, but a plurality of the above-mentioned test results were obtained and used for fatigue life prediction.

The prediction condition acquisition unit 12 has a function of acquiring information specifying conditions for fatigue life prediction in the fatigue life prediction device 1 . As for the information of the specified condition acquired by the prediction condition acquisition unit 12, for example, the Vickers hardness (HV) of the object, the final hardness of the object considered to be fractured, etc. Candidate numerical ranges and intervals (scales) of crack size and material constants, etc. In addition, as the information of the specified condition acquired by the prediction condition acquisition unit 12, for example, it is information necessary for fatigue life prediction after calculating the material constant, and examples include the initial value of the defect size in the object, the fatigue life Increment, upper limit, etc. of the number of repetitions of the load at the time of prediction. Although the above-mentioned information is necessary information for calculation of material constants and prediction of fatigue life by the method described later, depending on the actual calculation method, only a part of the above-mentioned information may be used, or it may not be included in The circumstances of the above information. Therefore, the type of information acquired by the prediction condition acquisition unit 12 can be changed depending on a specific method for fatigue life prediction.

The material constant calculation unit 13 has a function of calculating three kinds of material constants based on the information acquired by the fatigue test data acquisition unit 11 and the prediction condition acquisition unit 12 . The method of calculating the material constant will be described later.

The fatigue life prediction unit 14 has a function of predicting the fatigue life of the object using the material constant calculated by the material constant calculation unit 13 . The method of predicting the fatigue life will be described later.

The result output unit 15 has a function of outputting the result of the fatigue life prediction obtained by the processing of the fatigue life prediction unit 14 . The output method is not particularly limited, and well-known methods such as file output, screen output, and return value to other programs can be used. When outputting the result, for example, the material constant calculated by the material constant calculation unit 13 may also be output together.

The data storage unit 16 has a function of storing information including information acquired by the fatigue test data acquisition unit 11 and the prediction condition acquisition unit 12 and information necessary for processing performed by the above-mentioned units. Furthermore, it also has a function of memorizing the results of the processing in the material constant calculation unit 13 and the fatigue life prediction unit 14 .

[Fatigue life prediction method]

Next, the fatigue life prediction method performed by the fatigue life prediction device 1 will be described with reference to FIGS. 2 to 5 .

First, in the fatigue life prediction device 1, fatigue test data is acquired by the fatigue test data acquisition unit 11 (step S01). This processing can also be performed, for example, by an operator (user) of the fatigue life prediction device 1 operating the fatigue life prediction device 1 . In addition, the test results obtained by the fatigue test device specified in advance may be sequentially sent to the fatigue life prediction device 1, or the fatigue test data may be sequentially obtained by the fatigue life prediction device 1.

Furthermore, the fatigue test data prediction condition acquisition unit 12 acquires information specifying conditions for a series of processes of fatigue life prediction in the fatigue life prediction device 1 (step S02 ). This processing can also be performed, for example, by an operator (user) of the fatigue life prediction device 1 operating the fatigue life prediction device 1 . The sequence of step S01 and step S02 is not particularly limited, for example, step S02 may be performed first, or step S01 and step S02 may be performed simultaneously. Furthermore, specifying the conditions for fatigue life prediction The information may be acquired and held in advance by the prediction condition acquisition unit 12 of the fatigue life prediction device 1 .

Furthermore, the material constant calculation unit 13 of the fatigue life prediction device 1 first calculates the three material constants (C*, m*, n*) based on the above information (step S03). The calculation method of the material constant is described in Fig. 3 and Fig. 4, and the details will be described later.

Furthermore, the fatigue life is predicted by the fatigue life predicting unit 14 of the fatigue life predicting device 1 using the result calculated by the material constant calculating unit 13 (step S04 ). The method of predicting the fatigue life is shown in Fig. 5, and details will be described later.

Furthermore, the result of the fatigue life prediction obtained by the process of the fatigue life prediction part 14 is output by the result output part 15 of the fatigue life prediction apparatus 1 (step S05). At the time of output, conversion processing for converting to a state suitable for output, etc. may also be performed.

(basic way of thinking of fatigue life prediction device 1)

Before describing the detailed steps, the basic idea of the method described in this embodiment will be shown.

First, use the idea that "when there is a defect of a certain size in the material, the fatigue limit σ _w (stress amplitude) corresponding to the defect will be determined." The fatigue limit at this time can be predicted using the following formula (1), for example, when it is a metal material and the stress ratio R=-1. Here, the stress ratio R is defined by the ratio of the minimum value and the maximum value of repeated stress. This formula (1) is recorded in Murakami Keiyoshi "Metal Fatigue: The Influence of Small Defects and Intermediaries" published by Yokendo in 1993. The method of calculating σ _w is not limited to the formula (1). For example, when applying this method to a situation other than R=-1 or non-metallic materials, other methods can be considered to predict σ _w .

HV: Vickers hardness

：缺陷尺寸(由將缺陷投影至應力負荷方向之影像的面積的平方根所定義)

: defect size (defined by the square root of the area of the image projecting the defect onto the stress loading direction)

F: constant depending on the position of the defect (surface: 1.43, directly below the surface: 1.41, inside: 1.56)

In this idea, it is assumed that stresses below the fatigue limit do not affect fatigue. Furthermore, when a load greater than the fatigue limit is applied, fatigue is applied to the object. In other words, the σ/σ _w -1 system can be said to be a mechanical quantity affecting fatigue.

Furthermore, the above formula (1) shows that there are defects in the material, and the fatigue limit is determined according to their size. On the other hand, when a load of a stress (stress amplitude) exceeding the fatigue limit is applied to the object, cracks are generated in the object from defects that existed before. Therefore, the size of defects including cracks becomes larger as the number of repetitions of stress loading increases. It can be seen from formula (1) that the fatigue limit σ _w will gradually become smaller according to the size of the defect (defect size). In other words, even with the same stress load, the value of the mechanical quantity σ/σ _w -1 that affects fatigue will gradually increase as the internal defects become larger.

As described above, it can be said that the mechanical quantity σ/σ _w -1 is a driving force (crack growth driving force) for expanding defects (cracks). In this case, the progress state of the crack using the above-mentioned mechanical quantity can be expressed as the formula (2).

In formula (2), a corresponds to the size of the defect √area, da/dN is the progress of crack a per load cycle, and C*, m* and n* are material constants. It can also be assumed that n* is usually 1. By integrating the above formula (2), a substantial fatigue life is predicted when the value of the crack size a becomes a sufficiently large value (generally about 1 mm or more).

When the above method is used, it is possible to follow the growth and expansion of cracks even when the value of the stress applied to the object changes during repetition. Therefore, the fatigue life prediction of the object can be performed even under the condition of load fluctuation.

(Prediction method of fatigue life)

The method of predicting the fatigue life performed in step S04 will be described with reference to FIG. 3 . Here, description will be made on the premise that the above-mentioned material constants C*, m*, and n* are obtained using a method described later.

First, parameters used for prediction of fatigue life are input (step S11). As shown in Figure 3, the parameters shown in Table 1 below are prepared for the prediction of fatigue life.

[Table 1]

In the above settings, the initial defect size √area ₀ is the size of the initial defect included in the object material, and is defined by the square root of the projected area of the defect projected onto a right-angled surface. For example, it is conceivable to measure using an optical method, X-ray CT, or the like. Furthermore, it is also possible to use the method described in Keiki Murakami "Metal Fatigue: Influence of Micro Defects and Intermediaries" published by Yokendo in 1993.

Furthermore, although the limit crack size √area _crit may vary depending on the material properties of the object, it may be set to 1 mm, for example, in consideration of the process from the progress of the crack to the fracture. The number of repetitions required from 1 mm to the final breakage is relatively small compared to the overall number of repetitions, so this setting is also possible. In addition, the upper limit N _stop of the number of repetitions N can be set relatively large (for example, 10 ⁷ ) in consideration of the service life of the object and the repetition period of the stress. Furthermore, in the case where the stress is a function σ _a (N) based on N, it is possible to set the increment ΔN according to the variation interval to improve the prediction accuracy of the fatigue life and shorten the calculation time at the same time.

Furthermore, initial value setting is performed (step S12). As an initial value, N=0 is set, and an initial defect size √area ₀ is selected as √area.

Furthermore, σ in cycle N and fatigue limit σ _w in √area are calculated (step S13 to step S16). Specifically, first, in order to set the cycle N, after setting N=N+ΔN (step S13), confirm that N has not reached the upper limit N _stop (step S14) and continue the calculation (step S14-yes), the system is set After the stress amplitude σ corresponding to the cycle N (time system σ _a (N) as a function of N) (step S15 ), calculation of the fatigue limit σ _w according to the formula (1) is performed (step S16 ). In this case, F=1.43 (in the case of surface defects) or F=1.56 (in the case of internal defects) can be selected depending on the position of the defect and/or the crack that progresses from the defect, but generally, F that is on the safe side is used. =1.43.

When N is equal to or greater than _Nstop (step S14-No), the process is terminated, and a non-breaking result is output (step S17).

After the fatigue limit σ _w is calculated, it is checked whether the stress σ applied as a load is larger than the fatigue limit σ _w (step S18 ). When the stress σ given as the load is below the fatigue limit _σw (step S18-No), it is ignored as the crack does not progress due to the repetition of the stress, and the cycle N is reset to repeat the above steps (S13 to S18 ), and by doing so, the same investigation is carried out for the stress of the next load. On the other hand, when the stress σ applied as the load is larger than the fatigue limit σ _w (step S18-Yes), the crack is considered to progress due to the repetition of the stress, and the amount of progress of the crack is calculated (step S19) . Specifically, based on the above-mentioned formula (2), the amount of progress of the crack that is repeated once is calculated using the formula (3), and the new crack size is calculated using the formula (4).

After calculating the new crack size √area, it is checked whether the new crack size √area is larger than the limit crack size √area _crit (step S20). When the new crack size √area is below the limit crack size √area _crit (step S20-No), it is determined that the object has not been broken, and the cycle N is reset and the above steps are repeated (steps S13 to S20). On the other hand, when the new crack size √area is larger than the limit crack size √area _crit (step S20-Yes), it is determined that the object has been broken, and the number of repetitions of breaking Nf is set to the current cycle Number N (step S21). Then, the number of repetitions of fracture _Nf is output as a fatigue life prediction result (step S22). With this, a series of processes for predicting the fatigue life ends.

(Calculation method of material factor)

The calculation method of the material coefficient performed in step S03 is demonstrated, referring FIG.4 and FIG.5. FIG. 4 is a flowchart of a step included in the flowchart shown in FIG. 2 . As described above, the three material constants to be calculated are C*, m*, and n*.

When calculating the material factor, the formula used in the above-mentioned method of predicting the fatigue life is used. Specifically, using arbitrary C*, m*, and n*, the predicted value N _fpred of the number of repetitions of breaking N _f is calculated based on the measurement conditions included in the results of the fatigue test. Calculate the difference between this predicted value N _fpred and the actual measured value N _fexp of the number of breaking repetitions N _f obtained from the fatigue test, and find the combination of C*, m*, and n* that minimizes the difference, thereby specifying C* , m* and n*. The method of specifying the most suitable material constants C*, m*, and n* to minimize the difference between the predicted value N _fpred and the measured value N _fexp is not particularly limited, but in the following implementation forms, it is explained that the settings are as follows: (5) The objective function O shown in (5), and specify the methods of C*, m* and n* to minimize the objective function O. In formula (5), M is the number of groups of experimental data, and E _i is the logarithmic difference E between N _fpred and N _fexp in the i-th experimental data group. E _i is defined by formula (6).

E ( i )=log N _fpred ( i ) - log N _{f exp} ( i ) (6)

As a method of designating C*, m*, and n* that minimize the objective function O, iterative calculation can be used, for example. Specifically, a method of cyclically calculating C*, m*, and n* within a preset range may be mentioned. In addition, various optimization algorithms generally used may also be used.

4 and 5 , an example of the procedure when the above-mentioned method is specifically executed will be described.

First, parameters used for calculation of material constants are input (step S31). As shown in Fig. 4, the parameters shown in Table 2 below are prepared for the calculation of the material constant.

[Table 2]

The number of fatigue test data is set from 1 to M, but at least three test data (i≧3) different from each other (conditions different from each other) are required in order to obtain three material constants. However, in practice, it is also possible to assume that the material constant n* is 1, so in this case, two pieces of test data may also be used.

Furthermore, initial value setting is performed (step S32). As initial values, minimum values C* _min , m* _min and n* _min are respectively set for the C*, m* and n* systems. As an example of the initial value of the objective function O, O _update =1000 is set. As described above, since this is a step of specifying the material constant that can minimize the objective function, a sufficiently large value is set as the initial value. Furthermore, O _update refers to the latest minimum value of the objective function, which is used as a reference for repeated calculations in the steps described below.

In addition, first, i=1 (step S33), and the predicted value N _fpred of the number N _f of breaking repetitions is calculated (step S34).

The details of step S34, that is, the calculation method of the predicted value N _fpred of the number of breaking repetitions N _f is shown in FIG. 5 , and since it is roughly the same as the steps shown in FIG. 3 , it will be briefly described.

First, parameters used for prediction of fatigue life are input (step S51). Part of the parameters prepared in step S31 is input here.

Furthermore, the fatigue limit σ _w based on the formula (1) is calculated using the conditions for the fatigue test (the above-mentioned input parameters) (step S52 ). Thereby, the predicted fatigue limit based on the fatigue test data is calculated. After the fatigue limit σ _w is calculated, it is checked whether the load stress σ _exp (i) used in the fatigue test is larger than the fatigue limit σ _w (step S53 ). When the load stress σ _exp (i) is below the fatigue limit σ _w (step S53-No), since it is predicted that the crack will not progress due to the load stress σ _exp (i), the output display corresponding to the σ _exp (i ) is notified that the experimental data cannot be used for the calculation of fatigue life prediction (step S54). In this case, since the predicted value N _fpred cannot be calculated, an error message or a warning may be output.

On the other hand, when the load σ _exp (i) used for the fatigue test is larger than the fatigue limit σ _w (step S53-Yes), the amount of progress of the crack is calculated (steps S55 to S57). Specifically, set N=0 as the initial value, select the initial defect size √area ₀ as √area (step S55), set N=N+ΔN (step S56), and calculate the amount of crack progress Δ √area and the new crack size √area (step S57). These calculation methods are the same as steps S16 and S18 shown in FIG. 3 . However, the calculation of the amount of crack progress △√area uses the following formula (7). Although Equation (7) corresponds to Equation (3), it is different from Equation (3) in that the load σ _exp (i) used in the test is used as the stress σ.

After calculating the new crack size √area, it is confirmed whether the new crack size √area is larger than the limit crack size √area _crit (step S58). When the new crack size √area is below the limit crack size √area _crit (step S58-No), it is determined that the object has not been broken, and the cycle N is reset and the above steps are repeated (steps S56 to S57). On the other hand, when the new crack size √area is larger than the limit crack size √area _crit (step S58-Yes), it is determined that the object is broken, and the predicted value N _fpred of the number of times of breaking repetition Nf is determined. Set as the current cycle number N (step S59). Then, the predicted value N _fpred is output as the predicted result of the fatigue life (step S60 ). Through this series of steps, the predicted value N _fpred based on the fatigue test data is obtained.

Returning to FIG. 4 , when the predicted value N _fpred is obtained, the difference E(i) between the test result and the predicted value is calculated according to the above formula (6) (step S35 ). From i=1 to M, each time i+1 is gradually increased, and this series of processing is repeated (steps S36, S37). When the calculation of the difference E(i) between the test result and the predicted value is completed for all fatigue test results (i=1 to M) (step S36-yes), then substitute into the objective function O shown in formula (5) to calculate The result is calculated (step S38). Furthermore, compare the calculation result of the objective function 0 with O _update (step S39). When the calculation result of the current objective function O is smaller than the minimum value O _update of the previous objective function O (step S39-yes), then O _update is changed (updated) to the calculation result of the current objective function O, and The current C*, m*, and n* used in the calculation of the objective function O are changed (updated) as C** _update , m* _update , and n* _update (step S40). When the calculation result of the current objective function 0 is above the minimum value O _update of the previous objective function O (step S39-No), update such as O _update is not performed (step S40).

Thereafter, the above-described processing is repeated while changing C*, m*, and n* (steps S33 to S40). Specifically, at first, in the state that m* and n* are fixed at the minimum values m* _min and n* _min , C* is changed by increments ΔC each time, and the calculation is repeated until it becomes C* _max (step S41, S42). As a result, it is possible to designate C* _update used when calculating the minimum value _Oupdate in a state where m* and n* are fixed. Thereafter, m* _update and n* _update , which are used for calculating the minimum value O _update in a state where other material constants are fixed at minimum values, can be calculated by performing the same calculations for m* and n* respectively.

By performing this series of iterative calculations, the minimum value assigned can be obtained within the range of m* _min ≦m*≦m* _max , C* _min ≦C*≦C* _max , n* _min ≦n*≦n* _max The most suitable C* _update , m* _update , and n* _update of the objective function O are output as calculation results of C*, m*, and n* (step S49).

Through the above steps, C*, m*, and n* can be calculated using the fatigue test results.

[hardware configuration]

The hardware configuration of the fatigue life prediction device 1 will be described with reference to FIG. 6 . FIG. 6 is a diagram showing an example of the hardware configuration of the fatigue life prediction device. The fatigue life prediction device 1 includes one or a plurality of computers 100 . The computer 100 includes a CPU (Central Processing Unit) 101 , a main memory 102 , an auxiliary memory 103 , a communication control 104 , an input device 105 and an output device 106 . The fatigue life prediction device 1 is composed of one or a plurality of computers 100, and the computers 100 are composed of such hardware and software such as programs.

When the fatigue life prediction device 1 is constituted by a plurality of computers 100, these computers 100 may be connected locally or via a communication network such as the Internet or an intranet. One fatigue life prediction device 1 is logically constructed by this connection.

The CPU 101 executes an operating system and/or application programs and the like. The main memory unit 102 is composed of ROM (Read only Memory, read only memory) and RAM (Random Access Memory, random access memory). The auxiliary storage unit 103 is a storage medium including a hard disk, a flash memory, and the like. Generally speaking, the auxiliary memory unit 103 stores a larger amount of data than the main memory unit 102 . At least a part of the material constant calculation unit 13 and the fatigue life prediction unit 14 are realized by the auxiliary memory unit 103 . The communication control unit 104 is composed of a network card or a wireless communication module. Fatigue test data acquisition department 11. Acquisition of prediction conditions At least part of the unit 12 and the result output unit 15 can be realized by the communication control unit 104 . The input device 105 is composed of a keyboard, a mouse, a touch panel, and a sound input microphone. For example, at least a part of the prediction condition acquisition unit 12 can be realized by the input device 105 . The output device 106 is composed of a display, a printer, and the like. At least a part of the result output unit 15 is realized by the output device 106 . For example, the output device 106 can display the result of fatigue life prediction on a display or the like.

The auxiliary memory unit 103 stores the program 110 (fatigue life prediction program) and data necessary for processing in advance. The program 110 causes the computer 100 to execute each functional element of the fatigue life prediction device 1 . By means of the program 110 , for example, the processing of the above-mentioned step S01 to step S04 is executed in the computer 100 . For example, the program 110 is read by the CPU 101 or the main storage unit 102 to cause at least one of the CPU 101 , the main storage unit 102 , the auxiliary storage unit 103 , the communication control unit 104 , the input device 105 and the output device 106 to operate. For example, the program 110 reads and writes data in the main memory unit 102 and the auxiliary memory unit 103 .

The program 110 can be provided after being recorded on a tangible storage medium such as a CD-ROM, DVD-ROM, or semiconductor memory, for example. The program 110 may also be provided as a data signal via a communication network.

[Validity of Fatigue Life Prediction Results]

The results of verifying the validity of the fatigue life prediction results obtained by the above-mentioned fatigue life prediction device 1 are shown below.

First, a test piece of carbon steel S45C (HV=176) was prepared as an object, and two experimental data were obtained by a fatigue test. The experimental data are shown below.

Experimental data 1: σ=270MPa, N _f =30, 407

Experimental data 2: σ=235MPa, N _f =120, 264

Experimental data 1 and 2 are shown in the graph shown in Figure 7, which shows the corresponding relationship between the number of fracture repetitions N _f and the stress amplitude σ (SN curve graph). The initial defect size √area ₀ in the test piece of the object was 92 μm.

Next, the material constants C*, m*, and n* were calculated by the calculation method of the above-mentioned material constants C*, m*, and n* using the above-mentioned two experimental data. However, it is assumed that n*=1 based on past experimental data and dimensional analysis, so C* and m* are actually calculated as follows.

C*=10 ^-3.4

m*=2.8

Next, using the above-mentioned material constants, the fatigue life of the object (test piece) in the following simulated variable load experiment was predicted. Let the initial defect size √area ₀ be 92 μm.

(Test conditions)

Step 1: Stress load at σ=270MPa until N=23400

Step 2: After completing Step 1, carry out stress loading at σ=185MPa until it breaks

Based on the calculation results of the above-mentioned fatigue life prediction method, it is predicted that the size √area of the crack generated from the initial defect is 251 μm at the point when step 1 is completed. Furthermore, it is estimated that the fatigue limit at this time is 169 MPa. From the results, it is speculated that the load stress of step 2 is lower than the fatigue limit of the initial state, and higher than the fatigue limit of the stress load of step 1 at the end. In addition, the predicted value of the number of times of stress loading until fracture in step 2 (the predicted value of the number of times of stress loading in steps 1 and 2), that is, the predicted value of fatigue life is N _fpred =564700.

On the other hand, as a result of measuring the fatigue life under the above-mentioned test conditions using the test piece actually, the fatigue life was N _fexp =585830. Where step 1 is 23400 (experimental conditions), step 2 is 562430.

Comparing the above predicted value N _fpred with the measured value N _fexp , the relative error is 4%. As a result, it can be said that the fatigue life can be predicted with considerably high accuracy.

Fig. 7 shows the above test results. In FIG. 7 , T1 shows the estimated value of fatigue limit σ _w =199 MPa calculated from the conditions of the initial state, and T2 shows the estimated value of fatigue limit σ _w =169 MPa at the time of switching from step 1 to step 2 .

[effect]

According to the fatigue life prediction method performed by the above-mentioned fatigue life prediction device 1, by designating a material constant for calculating the amount of progress of cracks with respect to one stress load, the progress of cracks with respect to one stress load can be estimated. quantity. Utilizing this point, it is possible to predict the fatigue life when a certain stress is applied to the object. Here, it is assumed that when a stress load exceeding the fatigue limit is applied to the object, the initial defect will develop into a crack, and the fatigue limit will decrease according to the size of the defect and the crack that progresses from the defect, so the stress load is considered. The progress of the resulting defects (cracks) and the accompanying decrease in the fatigue limit. Therefore, since the fatigue life is predicted by properly capturing the true driving force of crack growth in the stress of repeated loads, higher accuracy prediction can be realized.

As mentioned above, it has been pointed out in the past that there is still room for improvement in the estimation of fatigue limit and fatigue life based on the S-N curve. Regarding this point, for example, it has been discussed to add a new interpretation to the S-N curve such as the modified linear damage law (Modified Miner's rule). curve. However, these methods are only theoretical constructions for assigning corresponding relationships between experimental results and S-N curves, and cannot be said to have sufficient mechanical basis, and there is still room for improvement in this regard.

On the other hand, in the above-mentioned fatigue life prediction method, when a stress load exceeding the fatigue limit is applied to the object, the initial defect will develop into a crack, and the fatigue limit will vary depending on the defect and Based on the premise that the size of cracks in which defects progress decreases, the amount of progress of cracks with respect to one stress load is calculated, and the fatigue life is predicted based on the results. In this way, the fatigue life prediction is performed by capturing the change of the object when the stress is applied to the object, and it is possible to predict the fatigue life with high accuracy, unlike conventional methods. In particular, the above-mentioned fatigue life prediction method predicts the fatigue life on the premise that the object will increase the size of the crack due to repeated stress loading, and as a result, the fatigue limit of the object will decrease. This point is different from the conventional Knowing methods are very different. In conventional techniques, growth of cracks during repeated stress loads, etc. are not taken into consideration. On the other hand, the fatigue life prediction method described above constantly grasps the reduction of the fatigue limit due to the increase in the size of the cracks, and reflects the result in the prediction of the fatigue life. Therefore, in the fatigue life prediction method described above, not only the fatigue life can be predicted with higher accuracy, but also the fatigue life can be predicted even in the case where the magnitude of the stress changes midway.

Furthermore, with the above method, the number of fatigue tests required for fatigue life prediction can be significantly reduced. According to the method described in the above embodiment, since the material constant can be set to three (or two by adding assumptions), the number of fatigue tests only needs to be at least three (or two). In the past, it was considered to perform multiple fatigue tests to generate an S-N curve that is more in line with the actual state. If the accuracy is considered, it is desirable to increase the number of fatigue tests. Also, in order to calculate the fatigue life more accurately, it is conceivable to perform a fatigue test by actually applying a stress to a test piece, but it is necessary to repeat the fatigue test every time the stress of the load is changed. On the other hand, in the method described in the above-mentioned embodiment, the fatigue life can be predicted with high accuracy with less fatigue test data, which is more advantageous than the conventional method. Furthermore, even when the applied stress is changed as a condition for predicting fatigue life prediction, the above-mentioned method can be easily recalculated, so it can be applied to various conditions, which is also more advantageous than conventional methods. .

Furthermore, when predicting the fatigue life, the system performs the following operations: calculate the fatigue limit based on the size of the defect from the object and the crack progressing from the defect; When it is large, the amount of progress of the crack relative to one stress load; and calculate the number of stress loads until the size of the defect and the crack progressing from the defect reaches the limit defect size. Specifically, when a certain stress is applied, when the load is greater than the calculated fatigue limit, the amount of progress of the crack with respect to one stress load is calculated; on the other hand, when the load stress is greater than the calculated fatigue limit The number of stress loads until the size of the flaw and the crack progressing from the flaw reaches the limit flaw size is calculated by setting the amount of progress of the crack with respect to one stress load as 0. Therefore, regardless of the magnitude of the stress load, the fatigue life can be predicted with high accuracy.

Furthermore, the step of calculating the material constant is to calculate C*, m* and n* in the above formula (3), and the formula (3) is to calculate the progress amount Δ√area of the crack relative to one stress load. With such a configuration, it is possible to designate a material constant that can appropriately calculate the amount of progress of cracks with respect to one stress load, and use this material constant to predict the fatigue life with high accuracy.

In addition, in the operation of predicting the aforementioned fatigue life, the stress amplitude applied to the object is constant or has two or more stages. As described above, in the technique described in this embodiment, the magnitude of the load is not particularly limited. Therefore, the fatigue life can be predicted with high accuracy in any of the conditions when the stress amplitude of the load is constant or when it is two or more stages. In particular, the fact that the fatigue life can be predicted with high accuracy even when the stress amplitude applied to the object has two or more stages is evident from the aforementioned effectiveness evaluation.

[modified example]

As described above, the present disclosure is not necessarily limited to the above-described embodiment, and various changes can be made without departing from the spirit.

The above-mentioned embodiment described the case where the fatigue life prediction device 1 is constituted by one computer system, but it may also be constituted by plural computer systems.

The above-mentioned embodiment assumes and explains that the operator directly operates the fatigue life prediction device 1 to perform the above-mentioned processing. However, the fatigue life prediction device 1 and the fatigue life prediction method of the above-mentioned embodiment may be provided as a service such as the Web that can be accessed by an unspecified number of users, for example.

S01~S05: Steps

Claims (7)

A fatigue life prediction method comprises the following steps: Obtain fatigue test data, the fatigue test data includes: the test results of the fatigue test on the object under multiple conditions, the initial defect size of the test piece used in the aforementioned fatigue test, and the test conditions of the aforementioned fatigue test; On the basis of the aforementioned fatigue test data, the material constant used to calculate the amount of progress of the crack with respect to one stress load is calculated. The specific premise is: when a stress load exceeding the fatigue limit is applied to the aforementioned object will progress from the initial defect to a crack, and the aforementioned fatigue limit will decrease according to the size of the defect and the crack progressing from the defect; and The fatigue life is predicted from the amount of progress of cracks when a certain stress is applied to the object using the material constant. The fatigue life prediction method as described in Claim 1, wherein the step of predicting the aforementioned fatigue life includes: Calculate the fatigue limit based on the defect size of the aforementioned object; Calculate the progress of cracking with respect to one stress load when the load is greater than the fatigue limit calculated above; and The number of loads of the stress until the size of the defect and the crack progressing from the defect reaches the limit defect size is calculated. The fatigue life prediction method as described in claim 1 or 2, wherein the step of calculating the aforementioned material constant is to calculate C*, m* and n* in the following formula (A), the formula (A) is calculated relative to a The progress of the crack under the stress load △√area,

Here, Δ√area is the amount of progress of the crack with respect to one stress load, σ is the stress amplitude of the load, σ _w is the fatigue limit when the load is applied, and √area is the size of the defect. The fatigue life prediction method according to any one of claims 1 to 3, wherein, in the step of predicting the fatigue life, the amplitude of the stress applied to the object is made constant or in two or more stages. A fatigue life prediction device, comprising: The Fatigue Test Result Acquisition Department acquires fatigue test data including: test results of fatigue tests on objects under multiple conditions, initial defect sizes of test pieces used in the aforementioned fatigue tests, and the aforementioned fatigue test results. the test conditions of the test; The material constant calculation unit calculates the material constant for calculating the amount of crack progress with respect to one stress load based on the aforementioned fatigue test data under a specific premise. The initial defect progresses to a crack when subjected to stress loading above the fatigue limit, and the aforementioned fatigue limit decreases with the size of the defect and the crack progressing from the defect; and The fatigue life predicting unit predicts the fatigue life when a certain stress is applied to the object based on the amount of progress of cracks with respect to one stress load calculated using the material constant. A fatigue life prediction program that causes a computer system to perform the following steps: Obtain fatigue test data, the fatigue test data includes: the test results of the fatigue test on the object under multiple conditions, the initial defect size of the test piece used in the aforementioned fatigue test, and the test conditions of the aforementioned fatigue test; On the basis of the aforementioned fatigue test data, the material constant used to calculate the amount of progress of cracks with respect to one stress load is calculated on the basis of a specific premise. Cracks will progress from initial defects when subjected to a stress load above 100 degrees, and the aforementioned fatigue limit will decrease according to the size of the defects and the cracks progressing from the defects; and The fatigue life when a certain stress is applied to the aforementioned object is predicted from the amount of progress of cracks with respect to one stress load calculated using the aforementioned material constants. A memory medium that can be read by a computer and that stores a fatigue life prediction program that enables the computer system to perform the following steps: Obtain fatigue test data, the fatigue test data includes: the test results of the fatigue test on the object under multiple conditions, the initial defect size of the test piece used in the aforementioned fatigue test, and the test conditions of the aforementioned fatigue test; On the basis of the aforementioned fatigue test data, the material constant used to calculate the amount of progress of the crack with respect to one stress load is calculated. The specific premise is: when a stress load exceeding the fatigue limit is applied to the aforementioned object will progress from the initial defect to a crack, and the aforementioned fatigue limit will decrease according to the size of the defect and the crack progressing from the defect; and The fatigue life when a certain stress is applied to the aforementioned object is predicted from the amount of progress of cracks with respect to one stress load calculated using the aforementioned material constants.

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