JP5540448B2 - Multiaxial fatigue life evaluation method and apparatus - Google Patents

Description

  The present invention relates to a multi-axis fatigue life evaluation method and apparatus used for evaluating the fatigue life of rotating machines such as aircraft engines and superchargers, and general structures including pressure vessels.

In the conventional multiaxial fatigue life evaluation method, the stress state of an actual machine member is measured, the stress analysis is performed on the structural analysis model using the measurement result, and the life evaluation is performed. In this method, for example, the equivalent strain (equivalent strain range) Δε _{eq of} Mises or the equivalent stress (equivalent stress range) Δσ _eq of Mises is obtained from the measured stress or strain state, and the obtained equivalent strain range Δε _eq or equivalent stress range is obtained. Fatigue life is evaluated by substituting Δσ _eq into the strain range Δε or the stress range Δσ of the Manson-Coffin equation expressed by the following equation. A and B in the following equation are material constants, which can be obtained by conducting a uniaxial fatigue test and creating an SN diagram.
Nf = (Δε / A) ^{1 / B}
Or Nf = (Δσ / A) ^{1 / B}
Where Nf: number of cycles for fatigue life
A, B: Material constant
Δε: Strain range
Δσ: Stress range

  By the way, the load which an actual machine member receives is a random load which changes with time. Therefore, it is assumed that the direction of main stress or main strain (referred to as the main axis) changes. According to the results of the multiaxial fatigue test in which the main axis direction was changed, the fatigue life became uniaxial life (using equivalent strain or equivalent stress as the phase of shear strain and axial strain or the phase of shear stress and axial stress shifted. It has been reported that the fatigue life is lower than the calculated fatigue life.

  Therefore, in the multiaxial fatigue life evaluation method using the equivalent strain described above, the fatigue life can be accurately evaluated when the principal axis direction does not change or the change in the principal axis direction is small, but the change in the principal axis direction is large. However, there is a possibility that the actual fatigue life may be significantly reduced from the fatigue life obtained by equivalent strain, and the fatigue life could not be evaluated with high accuracy. In other words, depending on the load path (Path) including changes in the principal axis direction, the fatigue life may be significantly reduced. With the above-described multiaxial fatigue life evaluation method using the equivalent strain or equivalent stress, the fatigue life can be improved with high accuracy. There was a possibility that it could not be evaluated.

  In order to solve these problems, several multi-axis fatigue life evaluation methods that can cope with changes in the spindle have been proposed in consideration of load paths (load paths including changes in the spindle direction).

  In particular, in Non-Patent Documents 1 and 2, focusing on the principal stress (strain) surface that takes the largest value during the period to be evaluated, a method for quantifying the amount of change in the principal axis, and the load path (major axis direction) A multiaxial fatigue life evaluation method (IS method) based on load paths including changes has been proposed. Hereinafter, a load having a large phase difference between shear strain and axial strain is referred to as non-proportional load. In addition, multiaxial fatigue life evaluation in consideration of the load path may be referred to as life evaluation in consideration of non-proportional load.

JP 2001-329856 A JP-A-8-160035 JP 2001-330542 A

Takaki Ito, “Non-proportional multiaxial low cycle fatigue life evaluation model”, Materials, Japan Society for Materials Science, December 15, 2001, vol. 50, no. 12, pp. 1317-1322 Takamoto Itoh, Tomohiko Ozaki, Toru Amaya, and Masao Sakane, "DETERMINATION OF STRESS AND STRAIN RANGES UNDER NON-PROPORTIONAL CYCLIC LOADING", 8th International Conference on Multiaxial Fatigue & Fracture, 2007 Takamoto Itoh, Masao Sakane, Takahiro Hata, Naomi Hamada, “A design procedure for assessing low cycle fatigue life under proportional and non-proportional loading”, International Journal of Fatigue 28, 2006, pp. 459-466

  As described above, in multiaxial fatigue life evaluation using equivalent strain or equivalent stress, it may not be possible to accurately evaluate the fatigue life depending on the load path. It is necessary to use a life evaluation considering the load.

  However, conventionally, the criteria for determining which evaluation method to use for any operating condition (stress or strain state), that is, the criteria for determining the necessity of life evaluation considering non-proportional load are clear. There wasn't.

  Furthermore, in multiaxial fatigue life evaluation, it is possible not only to evaluate fatigue life but also to easily determine how to modify the design (operation conditions and load path) of actual equipment based on the evaluation. Is desirable. That is, in multiaxial fatigue life evaluation, it is desirable to be able to evaluate in consideration of operation conditions and load path.

  The present invention has been made in view of the above circumstances, and can easily determine the necessity of life evaluation considering non-proportional load, and can perform evaluation taking operation conditions and load path into consideration. It is an object to provide a method and apparatus.

  The present invention was devised to achieve the above object, and based on the stress or strain state of the structure to be evaluated, the amount of change in value including the change in direction of the principal stress or principal strain relative to the load path length In addition, the equivalent non-proportional load coefficient that is the average value of the amount of change including the direction change of the main stress or main strain is calculated, the allowable life reduction rate is set in advance, and the set allowable life reduction The equivalent non-proportional load coefficient corresponding to the rate is obtained, and the equivalent non-proportional load coefficient obtained from the relationship between the change amount of the value including the direction change of the main stress or the main strain with respect to the load path length is the allowable life reduction rate. If the load coefficient is smaller than the corresponding non-proportional load coefficient, the life evaluation is performed using equivalent stress or equivalent strain, and the amount of change including the change in direction of the principal stress or principal strain with respect to the load path length When the equivalent non-proportional load coefficient obtained from the relationship is equal to or greater than the equivalent non-proportional load coefficient corresponding to the allowable life reduction rate, it is a multiaxial fatigue life evaluation method in which life evaluation is performed in consideration of the load path. .

  The life evaluation in consideration of the load path uses a test piece made of the same material as the structure, performs a uniaxial load fatigue test and a circular load fatigue test, and based on the fatigue test results, SN diagram and circular load SN diagram are created respectively, and using the created single axis load SN diagram and circular load SN diagram, the time intensity in the low cycle region of both SN diagrams The coefficient α representing the degree of additional curing under a non-proportional load or the equation (1) shown in [Equation 1] based on the stress or strain state of the structure

Thus, the non-proportional load coefficient f _NP is obtained, and based on the obtained coefficient α and non-proportional load coefficient f _NP , the following expression (2a) or
_{Nf_NP} = {(1 + αf _NP ) Δε / A} ^{1 / B} (2a)
_{Nf_NP} = {(1 + αf _NP ) Δσ / A} ^{1 / B} (2b)
Where _{Nf_NP} : number of cycles for fatigue life
A, B: Material constant
Δε: Calculated by projecting on the maximum principal strain axis in one cycle
Strain range
Δσ: Calculated by projecting to the maximum principal stress axis during one cycle
Life evaluation may be performed according to the stress range.

The equivalent non-proportional load coefficient f _{NP_a_eq} corresponding to the allowable life reduction rate β _a is _expressed by the following equation (3)
f _{NP_a} = (β _a ^B −1) / α (3)
However, f _{NP_a} : Non-proportional load coefficient corresponding to the allowable life reduction rate
β _a : Permissible life reduction rate
α: Coefficient indicating the degree of strength reduction or the degree of additional curing under non-proportional load
B: A non-proportional load coefficient f _{NP_a} corresponding to the allowable life reduction rate β _a is obtained from the material constant, and the following equation (4) is obtained based on the obtained non-proportional load coefficient f _{NP_a}
f _{NP_a_eq} = f _{NP_a} × 4 (SI _max ) ² / L (4)
However, f _{NP_a_eq} : Equivalent non-proportional load coefficient corresponding to the allowable life reduction rate
f _{NP_a} : Non-proportional load coefficient corresponding to the allowable life reduction rate
SI _max : Maximum principal stress or maximum principal strain
L: It may be obtained from the load path length.

  In addition, the present invention obtains the relationship between the change amount of the value including the direction change of the main stress or the main strain with respect to the load path length based on the stress or the strain state of the structure to be evaluated, and the main stress or the main strain. An analysis unit that obtains an equivalent non-proportional load coefficient that is an average value of a change amount of a value including a change in the direction of the load, and obtains an equivalent non-proportional load coefficient corresponding to a preset allowable life reduction rate; and the load path length If the equivalent non-proportional load coefficient obtained from the relationship between the change amount of the value including the direction change of the main stress or the main strain with respect to is smaller than the equivalent non-proportional load coefficient corresponding to the allowable life reduction rate, the equivalent stress or equivalent A life evaluation is performed using strain, and an equivalent non-proportional load coefficient obtained from a relationship between a change amount of a value including a main stress or a direction change of the main strain with respect to the load path length corresponds to the allowable life reduction rate. Equivalent when non proportional load is coefficient than that is a multi-axial fatigue life evaluation apparatus having a evaluation unit for performing life evaluation in consideration of the load path, a.

  ADVANTAGE OF THE INVENTION According to this invention, the multiaxial fatigue life evaluation method and apparatus which can judge easily the necessity of the life evaluation in consideration of the non-proportional load, and can be evaluated in consideration of the operation condition and the load path can be provided.

It is a flowchart which shows the procedure of the multiaxial fatigue life evaluation method which concerns on one embodiment of this invention. In this invention, it is a graph which shows an example of SN diagram of a uniaxial load and a circular load. (A) is a graph which shows the stress or strain state of the structure used in this invention, (b) is the direction change of the main stress or the main strain with respect to the load path length in the stress or strain state of (a). It is a graph which shows the relationship of the variation | change_quantity of the value containing. It is a schematic block diagram of the multiaxial fatigue life evaluation apparatus used for the multiaxial fatigue life evaluation method of this invention.

  Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

  First, the IS method used in the multiaxial fatigue life evaluation method according to the present embodiment will be described. Since the IS method is described in detail in Non-Patent Documents 1 and 2, only the outline will be described here.

The IS method is a criterion based on principal stress (strain) and is defined by the following equation (5).
ΔSI _NP = (1 + αf _NP ) ΔSI (5)

In Expression (5), ΔSI is a stress (strain) range calculated by projecting on the maximum principal stress (maximum principal strain) axis in one cycle. Further, α is a coefficient representing the degree of additional curing or life reduction under non-proportional load, and can be determined from the rate of decrease in time intensity due to non-proportional load as will be described in detail later. f _NP is a parameter representing a non-proportional degree of the load path, and is referred to as a non-proportional load coefficient.

The non-proportional load coefficient f _NP is obtained from Equation (1) shown in [Equation 2].

“| E ₁ × e _R SI (t) |” in the integral symbol in the equation (1) represents the amount of change of the value including the direction change of the principal stress (principal strain), and ds represents the stress (strain). ) Path, that is, a load path. The non-proportional load coefficient f _NP can be obtained by integrating the amount of change of the main stress (strain) along the entire path of the load path. The reason why the integral value is divided by 4 (SI _max ) ² in equation (1) is to standardize f _NP . In a circular load in which axial strain ε or axial stress σ and shear strain γ or shear stress τ are loaded with a phase difference of 90 degrees, f _NP is 1.

Further, e ₁ is a vector indicating the direction of the maximum principal stress (maximum principal strain), and e _R is the direction of the principal stress (principal strain) at a certain time t (time when e1 is a reference (time 0)). It is a vector which shows. Thus, the outer product of e ₁ and e _R, using e ₁ and e the angle of _R xi] a (t), it can be represented by sinξ (t). Therefore, “| e ₁ × e _R SI (t) |” in the integral symbol of Expression (1) can also be expressed as “SI (t) | sinξ (t) |”.

  Hereinafter, the multiaxial fatigue life evaluation method according to the present embodiment will be described with reference to FIG.

  As shown in FIG. 1, in the multiaxial fatigue life evaluation method according to the present embodiment, first, in step S1, a test piece made of the same material as the structure to be evaluated is used, and a uniaxial load (shaft tension) is used. -Perform compression test (push-pull) fatigue test and circular load (circle) fatigue test, and acquire material data.

  Thereafter, in step S2, based on the material data obtained in step S1 (results of uniaxial load and circular load fatigue tests), a uniaxial load SN diagram and a circular load SN diagram are created, respectively. .

  An example of the SN diagram created in step S2 is shown in FIG. The solid line in FIG. 2 is an SN diagram of a uniaxial load, and the broken line is an SN diagram of a circular load. In FIG. 2, the vertical axis represents the strain range Δε, the horizontal axis represents the number of cycles Nf that becomes the fatigue life, and both the vertical axis and the horizontal axis are logarithmically displayed.

As shown in FIG. 2, the SN diagram can be approximated by a straight line in the low cycle region (only the low cycle region is shown in FIG. 2). The SN diagram for uniaxial load is given by the following formula (6)
Nf = (Δε / A) ^{1 / B} (6)
Where Nf: number of cycles for fatigue life
A, B: Material constant
Δε: Can be approximated by the Manson-Coffin equation of the strain range. Material constants A and B can be obtained by creating an SN diagram of a uniaxial load. The strain range of Equation (6) may be the stress range, and Nf = (Δσ / A) ^{1 / B.} However, Δσ is a stress range.

  Also, using the SN diagram of the uniaxial load and the SN diagram of the circular load that was created, the coefficient α representing the degree of additional curing at a non-proportional load from the difference in time intensity in the low cycle region of both SN diagrams Can be requested.

  A uniaxial load fatigue test and a circular load fatigue test are performed in advance for each of various materials, and a material constant A, B and a coefficient α representing the degree of additional curing at a non-proportional load are obtained. α may be stored in association with the material. In this case, steps S1 and S2 described above can be omitted, and A, B, and α can be obtained by selecting a material.

In step S3, an equivalent non-proportional load coefficient f _{NP_a_eq} corresponding to _a preset allowable life reduction rate β _a is _obtained .

When the above equation (5) is arranged in the strain range, the following equation (7) is obtained.
Δε _NP = (1 + αf _NP ) Δε (7)
However, [Delta] [epsilon] _NP: the strain range of equivalent strain range expression (7) when considering the non-proportional load and stress range, can be a _{Δσ NP = (1 + αf NP} ) Δσ. However, Δσ _NP is an equivalent stress range when non-proportional load is considered, and Δσ is a stress range.

Therefore, when Δε in equation (6) is replaced with Δε _NP in equation (7), the number of cycles _{Nf_NP} that becomes a fatigue life when non-proportional load (f _NP ≠ 1) is considered is _expressed by the following equation (2a) or It is represented by the following formula (2b).
_{Nf_NP} = {(1 + αf _NP ) Δε / A} ^{1 / B} (2a)
_{Nf_NP} = {(1 + αf _NP ) Δσ / A} ^{1 / B} (2b)

From the formula (6) and the formula (2a), the life reduction rate β is expressed by the following formula (9)
β = _{Nf_NP} / Nf = (1 + αf _NP ) ^{1 / B} (9)
It is obtained by. By using equation (9), it is possible to determine the life reduction rate β directly from the non-proportional load coefficient f _NP . In addition, the life reduction rate β obtained here is smaller as the number of cycles _{Nf_NP} that becomes the fatigue life when considering the non-proportional load is smaller than the number of cycles Nf that becomes the fatigue life under the uniaxial load. Value. That is, if the influence of non-proportional load is large (the rate of decrease in time intensity is large), the value of life reduction rate β is small, and if the effect of non-proportional load is small (the rate of decrease in time intensity is small), the life is reduced. The value of the rate β increases.

When formula (9) is arranged by f _NP , the following formula (10)
f _NP = (β ^B −1) / α (10)
become that way. By using Expression (10), the life reduction rate β can be converted into a non-proportional load coefficient f _NP .

From equation (10), the non-proportional load coefficient f _{NP — a} that gives the allowable life reduction rate β _a is
f _{NP_a} = (β _a ^B −1) / α (3)
Can be expressed as

Since the non-proportional load coefficient f _{NP — a that} is the allowable life reduction rate β _a obtained by the equation (3) is _normalized , it is _multiplied by 4 (SI _max ) ² in order to cancel the standardization. For example, only the integral value in equation (1) can be extracted. Since this integrated value is a value obtained by integrating the amount of change of the main stress (main strain) along the load path, by dividing the integrated value by a loading path length (L-path) L, The amount of change in average principal stress (principal strain), that is, the equivalent non-proportional load coefficient f _{NP_eq} can be obtained.

That is, the equivalent non-proportional load coefficient f _{NP_a_eq} corresponding to the allowable life reduction rate β _a is _expressed by the following equation (4)
f _{NP_a_eq} = f _{NP_a} × 4 (SI _max ) ² / L (4)
Can be obtained.

  In step S4, based on the stress or strain state of the structure to be evaluated, the relationship between the change amount of the value including the main stress or the direction change of the main strain with respect to the load path length (hereinafter referred to as the main stress or the load path length). (Referred to as the relationship of change in principal strain).

  Here, as an example, as shown in FIG. 3A, a case will be described in which a load in which a torsional load (shear load) acts during one cycle of an axial load acts on a structure. In this case, the relationship between the change amount of the main stress or the main strain with respect to the load path length is as shown in FIG.

  The vertical axis in FIG. 3 (b) is the amount of change in principal stress or principal strain with respect to the load path length, and the value (here SI (t) | sinξ ( t) | is used). Further, the horizontal axis in FIG. 3B is the load path length.

As described above, the non-proportional load coefficient f _NP is obtained by integrating the change amount of SI (t) | sinξ (t) | along the load path and normalizing it. Therefore, if the area of the locus shown by the solid line in FIG. 3B is obtained and normalized, the non-proportional load coefficient f _NP can be obtained.

In step S5, an equivalent non-proportional load coefficient f _{NP_eq} that is an average value of changes in principal stress or principal strain is obtained based on the relationship between changes in principal stress or principal strain with respect to the load path length obtained in step S4. . The equivalent non-proportional load factor f _{NP_eq} is the area of the locus indicated by the solid line in FIG. 3B (that is, the integrated value obtained by integrating the change amount of the main stress or the main strain along the load path), and the load path length. It can be obtained by dividing by L. The obtained non-proportional load coefficient f _{NP_eq} is shown by a broken line in FIG. The area below the broken line in FIG. 3B is equal to the area below the locus shown by the solid line in FIG.

In step S6, it is determined whether the equivalent non-proportional load coefficient f _{NP_eq} obtained in step S5 is larger than the equivalent non-proportional load coefficient f _{NP_a_eq} corresponding to the allowable life reduction rate β _a obtained in step S3.

As an example, the equivalent non-proportional load coefficient f _{NP_a_eq} when the allowable life reduction rate β _a is 0.1 is a one-dot chain line, and the equivalent non-proportional load coefficient f _{NP_a_eq} when the allowable life reduction rate β _a is 0.25. This is shown by a two-dot chain line in FIG. In this case, when β _a = 0.1, f _{NP_eq} <f _{NP_a_eq is satisfied} , and YES is determined in step S6. When β _a = 0.25, f _{NP_eq} ≧ f _{NP_a_eq} and step S6 It will be judged as NO.

  If YES is determined in step S6, it is determined that the influence of the non-proportional load is small and it is not necessary to consider the non-proportional load. In step S7, the life evaluation is performed using the equivalent strain (or equivalent stress). Do.

Specifically, for example, when using the equivalent strain of Mises, the following equation (11)
^{_{Δε eq = (ε 2 + γ}} 2/3) 1/2 ··· (11)
The equivalent strain range Δε _eq is obtained by the following equation (12)
Nf = (Δε _eq / A) ^{1 / B} (12)
The life may be evaluated by

  If NO is determined in step S6, it is determined that the influence of the non-proportional load is large and it is necessary to consider the non-proportional load. In step S8, the life evaluation considering the non-proportional load (considering the load path) Life assessment).

Specifically, based on the stress state of the structure, the non-proportional load coefficient f _NP is obtained by the above equation (1), and the following equation (2a) or (2b)
_{Nf_NP} = {(1 + αf _NP ) Δε / A} ^{1 / B} (2a)
_{Nf_NP} = {(1 + αf _NP ) Δσ / A} ^{1 / B} (2b)
To evaluate the life.

In the context of the main stress or principal strain variation relative to the load path length in FIG. 3 (b), the main stress or principal strain amount of change, from the corresponding non-proportional load factor f _{NP_a_eq} corresponding to the allowable lifetime reduction rate beta _a The part where the value is higher is greatly influenced by the non-proportional load, and it can be determined that there is a design problem. Therefore, it is also possible to determine that the operation or shape (stress state and load path) of the structure is changed so as to minimize the amount of change in the main stress or main strain at this location.

  Next, a multiaxial fatigue life evaluation apparatus for realizing the multiaxial fatigue life evaluation method of the present invention will be described.

  As shown in FIG. 4, the multiaxial fatigue life evaluation apparatus 41 includes an input unit 42, a material data analysis unit 43, an analysis unit 44, an evaluation unit 45, an output unit 46, a material data storage unit 47, An analysis result storage unit 48 and an evaluation result storage unit 49 are provided. These units are realized by appropriately combining an interface, a memory, a CPU, software, and the like.

The input unit 42 includes a material selection unit 42a for selecting a material of the structure being evaluated, the allowable service life reduction rate input unit 42b for inputting an acceptable service life reduction factor beta _a, operating conditions of the structure being evaluated (stress or An operation condition input unit 42c for inputting a strain state), and a material data input unit 42d for inputting a result of a fatigue test under a uniaxial load and a circular load and a material used for the fatigue test.

  The material data analysis unit 43 creates SN diagrams of the uniaxial load and the circular load based on the fatigue test result input to the material data input unit 42d, and material constants A, B and A coefficient α representing the degree of additional curing or life reduction under non-proportional load is obtained, and the obtained A, B, α are associated with the material and stored in the material data storage unit 47.

The analysis unit 44 obtains the relationship between the change amount of the main stress or the main strain with respect to the load path length based on the operation condition (stress or strain state) of the structure to be evaluated input by the operation condition input unit 42c. The equivalent non-proportional load coefficient f _{NP — eq} , which is the average value of changes in the main stress or main strain, is obtained and stored in the analysis result storage unit 48.

The analysis unit 44 reads A, B, α of the material selected by the material selection unit 42a from the material data storage unit 47, and inputs the read A, B, α and the allowable life reduction rate input unit 42b. and acceptable lifetime reduction rate beta _a was, the operating conditions of the structure being evaluated entered in the operational condition input unit 42c (stress or strain state), on the basis of the corresponding non-proportional load corresponding to the allowable lifetime reduction rate beta _a The coefficient f _{NP_a_eq} is obtained and stored in the analysis result storage unit 48.

The evaluation unit 45 _{compares the} non-proportional load coefficient f _{NP_eq} and the allowable non-proportional load coefficient β _a corresponding to the relationship between the change amount of the main stress or the main strain with respect to the load path length stored in the analysis result storage unit 48. Based on the proportional load coefficient f _{NP_a_eq} , it is configured to determine the necessity of life considering the non-proportional load.

Specifically, the evaluation unit 45 performs life evaluation using equivalent stress or equivalent strain when f _{NP_eq} <f _{NP_a_eq} , and performs life evaluation considering a non-proportional load when f _{NP_eq} ≧ f _{NP_a_eq.} Configured to do. The evaluation unit 45 stores the evaluation result in the evaluation result storage unit 49.

The output unit 46 graphs the relationship of the change amount of the main stress or the main strain with respect to the load path length stored in the analysis result storage unit 48, and the corresponding non-proportional load coefficient f _{NP_eq} and the allowable life reduction are _{displayed on} the graph. corresponding non-proportional load factor f _{NP_a_eq} graph displaying the equivalent to the rate beta _a create a (see FIG. 3 (b)), evaluation evaluation with the results stored in the result storage unit 49, output to a display device such as a display To be done.

As described above, in the multiaxial fatigue life evaluation method according to the present embodiment, based on the stress or strain state of the structure to be evaluated, a value including the direction change of the main stress or main strain with respect to the load path length. _{Is calculated} , and an equivalent non-proportional load coefficient f _{NP_eq} which is an average value of the amount of change including the direction change of the main stress or the main strain is obtained, and the allowable life reduction rate β _a is set in advance. together, determine the corresponding non-proportional load factor f _{NP_a_eq} corresponding to the allowable life reduction factor beta _a set, when a f _{NP_eq} <f _{NP_a_eq} performs life evaluation using the equivalent stress or equivalent strain, f _{NP_eq} ≧ f _When it is _{NP_a_eq} , life evaluation considering non-proportional load is performed.

  As a result, the criteria for determining the necessity of life evaluation considering the non-proportional load, which has been unclear in the past, are clarified, and it becomes possible to easily determine the necessity of life evaluation considering the non-proportional load. .

Further, in the relationship of the change amount of the main stress or the main strain with respect to the load path length, the change amount of the main stress or the main strain is higher than the equivalent non-proportional load coefficient f _{NP_a_eq} corresponding to the allowable life reduction rate β _a. It is possible to determine that there is a problem in design because the influence of non-proportional load is large. In other words, according to the present invention, it is possible not only to simply evaluate the fatigue life but also to easily determine how to modify the design of the actual machine member in consideration of the operation conditions and the load path.

  The present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made without departing from the spirit of the present invention.

For example, in the above-described embodiment, the substantial non-proportional load coefficient f _{NP_eq} obtained from the relationship between the change amount of the main stress or the main strain with respect to the load path length, and the substantial non-proportional load coefficient f _{corresponding} to the allowable life reduction rate β _a. Compared with _{NP_a_eq} , the necessity of life evaluation considering non-proportional load was judged. Based on the relationship between changes in main stress or main strain, the non-proportional load coefficient f _NP And calculate the life reduction rate β by the above formula (9) based on the calculated non-proportional load coefficient f _NP and compare the determined life reduction rate β with the set allowable life reduction rate β _a. Accordingly, it may be determined whether or not the life evaluation is necessary in consideration of the non-proportional load. The life reduction rate β corresponds to the equivalent non-proportional load coefficient f _{NP_eq} , and the allowable life reduction rate β _a corresponds to the equivalent non-proportional load coefficient f _{NP_a_eq.} It is possible to determine the necessity of life evaluation in consideration.

41 Multiaxial fatigue life evaluation device 42 Input unit 43 Material data analysis unit 44 Analysis unit 45 Evaluation unit 46 Output unit

Claims (4)

Based on the stress or strain state of the structure to be evaluated, obtain the relationship of the change amount of the value including the main stress or main strain direction change to the load path length, and the value including the main stress or main strain direction change The equivalent non-proportional load coefficient that is the average value of the change amount of
In addition, the allowable life reduction rate is set in advance, and an equivalent non-proportional load coefficient corresponding to the set allowable life reduction rate is obtained,
When the substantial non-proportional load coefficient obtained from the relationship between the change amount of the value including the direction change of the main stress or the main strain with respect to the load path length is smaller than the substantial non-proportional load coefficient corresponding to the allowable life reduction rate , Perform life evaluation using equivalent stress or equivalent strain,
When the substantial non-proportional load coefficient obtained from the relationship between the change amount of the value including the direction change of the main stress or the main strain with respect to the load path length is equal to or greater than the substantial non-proportional load coefficient corresponding to the allowable life reduction rate A multiaxial fatigue life evaluation method characterized in that life evaluation is performed in consideration of the load path. Life evaluation considering the load path is:
Using a test piece made of the same material as the structure, a uniaxial load fatigue test and a circular load fatigue test are performed. Based on the results of the fatigue test, a uniaxial load SN diagram and a circular load SN line are obtained. Create a figure and each
Using the SN diagram of the uniaxial load and the SN diagram of the circular load that was created, a coefficient α representing the difference in time intensity in the low cycle region of both SN diagrams or the degree of additional curing in a non-proportional load Seeking
And, based on the stress or strain state of the structure, the formula (1) shown in [Equation 1]
To obtain the non-proportional load coefficient f _NP ,
Based on the obtained coefficient α and non-proportional load coefficient f _NP , the following expression (2a) or the following expression (2b)
_{Nf_NP} = {(1 + αf _NP ) Δε / A} ^{1 / B} (2a)
_{Nf_NP} = {(1 + αf _NP ) Δσ / A} ^{1 / B} (2b)
Where _{Nf_NP} : number of cycles for fatigue life
A, B: Material constant
Δε: Calculated by projecting on the maximum principal strain axis in one cycle
Strain range
Δσ: Calculated by projecting to the maximum principal stress axis during one cycle
The multiaxial fatigue life evaluation method according to claim 1, wherein life evaluation is performed based on a stress range. The equivalent non-proportional load coefficient f _{NP_a_eq} corresponding to the allowable life reduction rate β _a is
The following formula (3)
f _{NP_a} = (β _a ^B −1) / α (3)
However, f _{NP_a} : Non-proportional load coefficient corresponding to the allowable life reduction rate
β _a : Permissible life reduction rate
α: Coefficient indicating the degree of strength reduction or the degree of additional curing under non-proportional load
B: A non-proportional load coefficient f _{NP_a} corresponding to the allowable life reduction rate β _a is obtained from the material constant,
_Based on the _calculated non-proportional load coefficient f _{NP_a} , the following formula (4)
f _{NP_a_eq} = f _{NP_a} × 4 (SI _max ) ² / L (4)
However, f _{NP_a_eq} : Equivalent non-proportional load coefficient corresponding to the allowable life reduction rate
f _{NP_a} : Non-proportional load coefficient corresponding to the allowable life reduction rate
SI _max : Maximum principal stress or maximum principal strain
L: The multiaxial fatigue life evaluation method according to claim 1 or 2, which is obtained from a load path length. Based on the stress or strain state of the structure to be evaluated, obtain the relationship of the change amount of the value including the main stress or main strain direction change to the load path length, and the value including the main stress or main strain direction change The equivalent non-proportional load coefficient that is the average value of the change amount of
In addition, an analysis unit that obtains a substantially non-proportional load coefficient corresponding to a preset allowable life reduction rate, and a non-proportional value obtained from a relationship between a change amount of a value including a main stress or a main strain with respect to the load path length. When the proportional load coefficient is smaller than the equivalent non-proportional load coefficient corresponding to the allowable life reduction rate, life evaluation is performed using equivalent stress or equivalent strain,
When the substantial non-proportional load coefficient obtained from the relationship between the change amount of the value including the direction change of the main stress or the main strain with respect to the load path length is equal to or greater than the substantial non-proportional load coefficient corresponding to the allowable life reduction rate An evaluation unit that performs life evaluation considering the load path;
A multiaxial fatigue life evaluation apparatus characterized by comprising: