JP2018141703A - Fatigue intensity estimation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fatigue intensity estimation method which can estimate the fatigue intensity of a component without performing a fatigue test.SOLUTION: A fatigue intensity estimation method comprises: a set stress ratio acquisition part for acquiring a set stress ratio; a natural crack dimension calculation step for calculating a dimension of a natural crack which is assumed on a surface of a stress concentration part on the basis of a lower-limit stress expansion coefficient range corresponding to the set stress ratio, and the fatigue intensity; a set nominal stress acquisition step for acquiring the set nominal stress of the surface of the stress concentration part; a stress distribution calculation step for calculating a stress depth distribution from the surface of the stress concentration part when the set nominal stress acts on the stress concentration part; an assumption stress expansion coefficient range calculation step for calculating an assumption stress expansion coefficient range on the basis of the dimension of the natural crack and the stress depth distribution; and an estimation fatigue intensity decision step for deciding the set nominal stress as estimation fatigue intensity when the assumption stress expansion coefficient range is determined to coincide with the lower-limit stress expansion coefficient range by an assumption stress expansion coefficient range verification step.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to a fatigue strength estimation method for estimating the fatigue strength of a material.

  As measures for improving the fatigue strength (fatigue limit) of materials, surface treatment such as surface processing such as peening and surface hardening treatment such as nitriding is known. However, due to insufficient fatigue strength of stress-concentrated parts (stress-concentrated parts) such as steps in parts (mechanical parts) that make up engines, turbines, multistory parking lots, etc. As a result, the construction conditions and effects of surface processing such as peening are not clear, which is an obstacle to applying the surface treatment described above. Therefore, it becomes important to clarify the construction conditions of the surface treatment and the effects when construction is performed under the construction conditions for each shape of the notch portion of the machine part.

  For example, in Patent Document 1, the stress concentration rate corresponding to the local shape in the stress concentration portion of the structure and the fatigue life evaluation diagram of the structure are stored in advance through experiments and the like, and obtained by these experiments and the like. It is disclosed to estimate (evaluate) the fatigue life corresponding to each local shape of the stress concentration portion by applying the stress concentration rate and the fatigue life evaluation diagram to the peak stress calculated by the finite element method. Further, Patent Document 2 discloses a method for evaluating the fatigue strength of a casting material through the creation of an SN diagram which is a relationship between the stress amplitude and the number of repetitions by a fatigue test.

JP 2003-149091 A JP 2011-58887 A

  However, in order to confirm the effect of improving fatigue strength due to construction conditions such as peening, it is necessary to conduct a fatigue test that takes a long time. For this reason, in order to obtain the optimum construction conditions for each condition of the narrow part of the stress concentration part of the machine part, it is necessary to perform a fatigue test every time the construction of the stress concentration part is changed, It takes a lot of time and effort. In response to such a problem, the present inventors have invented a new technique capable of estimating the fatigue strength of a component (stress concentration part) without conducting a fatigue test by intensive research.

  In view of the above circumstances, an object of at least one embodiment of the present invention is to provide a fatigue strength estimation method capable of estimating the fatigue strength of a component without performing a fatigue test.

(1) The fatigue strength estimation method according to at least one embodiment of the present invention includes:
A fatigue strength estimation method for estimating the fatigue strength of a stress concentration part of a component,
A set stress ratio acquisition step for acquiring a set stress ratio that is an arbitrarily set stress ratio;
Based on the lower limit stress intensity factor range and the fatigue strength of the material forming the part obtained according to the set stress ratio, the inherent crack size calculated on the surface of the stress concentration portion is calculated. Crack size calculation step;
A set nominal stress acquisition step of acquiring a set nominal stress that is a nominal stress arbitrarily set on the surface of the stress concentration portion;
A stress distribution calculation step of calculating a stress depth distribution which is a stress distribution in the depth direction from the surface of the stress concentration portion when the set nominal stress acts on the stress concentration portion;
An assumed stress intensity factor range calculating step for calculating an assumed stress intensity factor range that is a stress intensity factor range of the inherent crack based on the size of the inherent crack and the stress depth distribution;
An assumed stress intensity factor range verification step for determining whether the assumed stress intensity factor range matches the lower limit stress intensity factor range; and
An estimated fatigue strength determination step for determining the set nominal stress as an estimated fatigue strength when the assumed stress intensity factor range verification step determines that the assumed stress intensity factor range matches the lower limit stress intensity factor range; .

  According to the configuration of (1) above, for example, the estimation of the fatigue strength of a stress concentration portion, such as a notch formed in a part, is performed on the set nominal stress assumed to act on the surface (bottom surface) of the stress concentration portion. This is done by calculating whether or not a virtual intrinsic crack obtained from the set stress ratio assumed on the surface of the stress concentration portion propagates. More specifically, the stress concentration part has a stress distribution from the surface to the inside of the base metal due to the set nominal stress, and the tip of the inherent crack has a distance (dimension) from the surface of the stress concentration part. The stress intensity factor range (assumed stress intensity factor range) is calculated assuming that the corresponding stress is applied. When the assumed stress intensity factor range calculated in this way matches the lower limit stress intensity factor range of the material obtained from the set stress ratio, the lower limit value of the nominal stress at which the set nominal stress causes the propagation of the inherent crack Therefore, the estimated fatigue strength is determined. By calculating the estimated fatigue strength in this way, it is possible to estimate the fatigue strength of the stress concentration portion of the component without performing a fatigue test that requires a long time.

(2) In some embodiments, in the configuration of (1) above,
When the assumed stress intensity factor range verification step determines that the assumed stress intensity factor range does not match the lower limit stress intensity factor range, the assumed stress intensity factor range further includes a set nominal stress change step that changes the set nominal stress. ,
The set nominal stress acquisition step acquires the value changed by the set nominal stress change step as the set nominal stress,
In the fatigue strength estimation method, the set nominal stress change step, the set nominal stress acquisition, until the assumed stress intensity factor range verification step determines that the assumed stress intensity factor range matches the lower limit stress intensity factor range. The steps are repeated in the order of the stress distribution calculation step, the assumed stress intensity factor range calculation step, and the assumed stress intensity factor range verification step.
According to the configuration of (2) above, the assumed stress intensity factor range calculated based on the set nominal stress and the inherent crack size matches the lower limit stress intensity factor range determined as material properties based on the set stress ratio. If not, the set nominal stress that matches can be searched, and the estimated fatigue strength of the stress concentration portion of the part can be estimated.

(3) In some embodiments, in the above configurations (1) to (2),
An assumed stress ratio calculating step of calculating an assumed stress ratio which is a stress ratio of the inherent crack calculated based on the dimension of the inherent crack and the stress depth distribution;
An assumed stress ratio verification step for determining whether or not the assumed stress ratio matches the set stress ratio;
The assumed stress intensity factor range verification step is executed when it is determined in the assumed stress ratio verification step that the assumed stress ratio matches the set stress ratio.
According to the configuration of (3) above, on the assumption that the assumed stress ratio calculated based on the set nominal stress and the inherent crack size matches the set stress ratio, the determination by the assumed stress intensity factor range verification step is performed. Made. Thus, even if the stress concentration part is formed of a material whose lower limit stress intensity factor range and fatigue strength have values corresponding to the set stress ratio, such as carbon steel, the estimated fatigue An appropriate estimate of the intensity can be made. When the stress concentration portion is made of cast iron, the assumed stress ratio calculation step and the assumed stress ratio verification step can be omitted.

(4) In some embodiments, in the configuration of (3) above,
A set stress ratio change step of changing the set stress ratio when the assumed stress ratio and the set stress ratio are determined not to match in the assumed stress ratio verification step;
The set stress ratio acquisition step acquires the value changed by the set stress ratio change step as the set stress ratio,
In the fatigue strength estimation method, the set stress ratio change step, the set stress ratio acquisition step, the inherent crack size, until the assumed stress ratio verification step determines that the assumed stress ratio matches the set stress ratio. The calculation step and the assumed stress ratio verification step are repeated in this order.
According to the configuration of (4) above, when the assumed stress ratio calculated based on the set nominal stress and the inherent crack size does not match the set stress ratio, a set stress ratio that matches is searched. Thus, it is possible to appropriately determine the estimated fatigue strength of the stress concentration portion of the component.

(5) In some embodiments, in the above configurations (1) to (4),
A residual stress distribution acquisition step of acquiring a residual stress distribution in the depth direction of the stress concentration portion generated by the surface treatment on the stress concentration portion;
In the stress distribution calculating step, the stress depth distribution is calculated in consideration of the residual stress distribution.
According to the configuration of (5) above, the residual stress distribution due to surface treatment such as peening or nitriding is obtained through numerical analysis, for example, and the estimated fatigue strength is determined using the stress depth distribution taking into account the residual stress. The fatigue strength of the stress concentration part after the surface treatment can be estimated without performing a fatigue test.

(6) In some embodiments, in the configuration of (5) above,
The estimated fatigue strength determined using the stress depth distribution taking into account the residual stress distribution and the estimated fatigue strength determined using the stress depth distribution not taking into account the residual stress distribution It further includes a surface treatment effect calculating step for calculating the difference as an effect of the surface treatment on the stress concentration portion.
According to the configuration of (6) above, the effect of the surface treatment can be obtained from the difference between the two estimated fatigue strengths according to the presence or absence of the surface treatment. Moreover, the effect according to the surface treatment construction condition to the stress concentration part can be clarified by deriving the surface treatment construction condition from the numerical analysis condition of the residual stress distribution. Therefore, the fatigue strength of the parts can be improved by applying a surface treatment to the stress concentration part under such an installation condition.

(7) The fatigue strength estimation apparatus according to at least one embodiment of the present invention is:
A fatigue strength estimation device for estimating the fatigue strength of a stress concentration part of a component,
A set stress ratio acquisition unit for acquiring a set stress ratio which is an arbitrarily set stress ratio;
Based on the lower limit stress intensity factor range and the fatigue strength of the material forming the part obtained according to the set stress ratio, the inherent crack size calculated on the surface of the stress concentration portion is calculated. Crack size calculator,
A set nominal stress acquisition unit that acquires a set nominal stress that is a nominal stress arbitrarily set on the surface of the stress concentration portion;
A stress distribution calculation unit that calculates a stress depth distribution that is a stress distribution in the depth direction from the surface of the stress concentration part when the set nominal stress acts on the stress concentration part;
An assumed stress intensity factor range calculation unit that calculates an assumed stress intensity factor range that is a stress intensity factor range of the inherent crack based on the dimensions of the inherent crack and the stress depth distribution;
An assumed stress intensity factor range verification unit that determines whether the assumed stress intensity factor range matches the lower limit stress intensity factor range; and
An estimated fatigue strength determination unit that determines the set nominal stress as an estimated fatigue strength when the assumed stress intensity factor range verification unit determines that the assumed stress intensity factor range matches the lower limit stress intensity factor range; .

  According to the configuration of (7), the same effect as (1) can be obtained.

(8) In some embodiments, in the configuration of (7) above,
When the assumed stress intensity factor range verification unit determines that the assumed stress intensity factor range does not match the lower limit stress intensity factor range, the assumed stress intensity factor range verification unit further includes a set nominal stress change unit that changes the set nominal stress. ,
The set nominal stress acquisition unit acquires the value changed by the set nominal stress change unit as the set nominal stress,
The fatigue strength estimation device is configured to obtain the set nominal stress change unit and the set nominal stress until the assumed stress intensity factor range verification unit determines that the assumed stress intensity factor range matches the lower limit stress intensity factor range. Section, the stress distribution calculation unit, the assumed stress intensity factor range calculation unit, and the assumed stress intensity factor range verification unit in this order.
According to the configuration of (8), the same effect as (2) can be achieved.

(9) In some embodiments, in the above configurations (7) to (8),
An assumed stress ratio calculation unit for calculating an assumed stress ratio that is a stress ratio of the inherent crack calculated based on the size of the inherent crack and the stress depth distribution;
An assumed stress ratio verification unit that determines whether or not the assumed stress ratio matches the set stress ratio;
The assumed stress intensity factor range verification unit is executed when the assumed stress ratio verification unit determines that the assumed stress ratio matches the set stress ratio.
According to the configuration of the above (9), the same effect as the above (3) can be obtained.

(10) In some embodiments, in the configuration of (9) above,
A set stress ratio changing unit that changes the set stress ratio when the assumed stress ratio verification unit determines that the assumed stress ratio and the set stress ratio do not match;
The set stress ratio acquisition unit acquires the value changed by the set stress ratio change unit as the set stress ratio,
The fatigue strength estimation device is configured so that the set stress ratio change unit, the set stress ratio acquisition unit, and the inherent crack size until the assumed stress ratio verification unit determines that the assumed stress ratio matches the set stress ratio. The processing is repeated in the order of the calculation unit and the assumed stress ratio verification unit.
According to the configuration of the above (10), the same effect as the above (4) can be obtained.

(11) In some embodiments, in the above configurations (7) to (10),
A residual stress distribution acquisition unit that acquires a residual stress distribution in the depth direction of the stress concentration part generated by the surface treatment on the stress concentration part;
The stress distribution calculation unit calculates the stress depth distribution in consideration of the residual stress distribution.
According to the configuration of the above (11), the same effect as the above (5) can be obtained.

(12) In some embodiments, in the configuration of (11) above,
The estimated fatigue strength determined using the stress depth distribution taking into account the residual stress distribution and the estimated fatigue strength determined using the stress depth distribution not taking into account the residual stress distribution A surface treatment effect calculation unit that calculates the difference as an effect of the surface treatment on the stress concentration portion is further provided.
According to the configuration of the above (12), the same effect as the above (6) can be obtained.

  According to at least one embodiment of the present invention, a fatigue strength estimation method capable of estimating the fatigue strength of a component without performing a fatigue test is provided.

It is a figure which shows the fatigue strength estimation method which concerns on one Embodiment of this invention, and makes object the material which a lower limit stress intensity factor range and fatigue strength do not change so much with a stress ratio especially. It is a figure for demonstrating the natural crack assumed to the stress concentration part 12 which concerns on one Embodiment of this invention. It is a figure which shows the fatigue strength estimation method which concerns on one Embodiment of this invention, and includes the flow which determines setting stress ratio. It is a figure which shows the fatigue strength estimation method which concerns on one Embodiment of this invention, and includes the step which calculates the stress depth distribution in consideration of the residual stress by surface treatment. It is a figure for demonstrating the coincidence degree of the fatigue strength by experiment, and the estimated fatigue strength in case the surface treatment is not performed to the stress concentration part which concerns on one Embodiment of this invention. It is a figure for demonstrating the coincidence of the fatigue strength by experiment, and the estimated fatigue strength in the case of performing surface treatment in the stress concentration part which concerns on one Embodiment of this invention. It is a figure which shows the residual stress distribution of the depth direction of the stress concentration part which concerns on one Embodiment of this invention. It is a figure for demonstrating the fatigue test which concerns on one Embodiment of this invention. It is a figure which shows the fatigue strength estimation method which concerns on one Embodiment of this invention, and calculates the effect by surface treatment. It is a figure which shows the structure of the fatigue strength estimation apparatus which concerns on one Embodiment of this invention, and is comprised so that execution of the fatigue strength estimation method shown in FIG. 3 or FIG. 4 is possible.

Hereinafter, some embodiments of the present invention will be described with reference to the accompanying drawings. However, the dimensions, materials, shapes, relative arrangements, etc. of the components described in the embodiments or shown in the drawings are not intended to limit the scope of the present invention, but are merely illustrative examples. Absent.
For example, expressions expressing relative or absolute arrangements such as “in a certain direction”, “along a certain direction”, “parallel”, “orthogonal”, “center”, “concentric” or “coaxial” are strictly In addition to such an arrangement, it is also possible to represent a state of relative displacement with an angle or a distance such that tolerance or the same function can be obtained.
For example, an expression indicating that things such as “identical”, “equal”, and “homogeneous” are in an equal state not only represents an exactly equal state, but also has a tolerance or a difference that can provide the same function. It also represents the existing state.
For example, expressions representing shapes such as quadrangular shapes and cylindrical shapes represent not only geometrically strict shapes such as quadrangular shapes and cylindrical shapes, but also irregularities and chamfers as long as the same effects can be obtained. A shape including a part or the like is also expressed.
On the other hand, the expressions “comprising”, “comprising”, “comprising”, “including”, or “having” one constituent element are not exclusive expressions for excluding the existence of the other constituent elements.

FIG. 1 is a diagram showing a fatigue strength estimation method according to an embodiment of the present invention, and in particular, the dimensions of the intrinsic crack C calculated from the lower limit stress intensity factor ranges ΔK _{th and R} and fatigue strength Δσ _{th and R.} a ₀ is directed to a much does not change material by stress ratio. FIG. 2 is a view for explaining the inherent crack C assumed in the stress concentration portion 12 according to the embodiment of the present invention. FIG. 3 is a diagram showing a fatigue strength estimation method according to an embodiment of the present invention, and includes a flow for determining a set stress ratio R. FIG. 4 is a diagram showing a fatigue strength estimation method according to an embodiment of the present invention, and includes a step of calculating a stress depth distribution σ (d) in consideration of a residual stress σr (d) due to surface treatment. FIG. 5 is a diagram for explaining the degree of coincidence between experimental fatigue strength and estimated fatigue strength when the stress concentration portion 12 according to an embodiment of the present invention is not subjected to surface treatment. FIG. 6 is a diagram for explaining the degree of coincidence between the experimental fatigue strength and the estimated fatigue strength when surface treatment is applied to the stress concentration portion 12 according to the embodiment of the present invention. FIG. 7 is a diagram showing a residual stress distribution σr (d) in the depth direction of the stress concentration portion 12 according to an embodiment of the present invention. Moreover, FIG. 8 is a figure for demonstrating a fatigue test.

  As shown in FIGS. 1, 3, and 4, the fatigue strength estimation method is a fatigue strength estimation method for estimating the fatigue strength of the stress concentration portion 12 included in the component 1. In this case, an estimated value of fatigue strength (estimated fatigue strength) is determined through searching for specific values of parameters (a stress intensity factor range and stress ratio described later) that can explain the phenomenon occurring in the stress concentration portion 12. Here, in fracture mechanics, the failure of the part 1 (mainly a metal part) is caused by the load 13 being repeatedly applied to the part 1 while the machine using the part 1 is in use. It can be considered that the crack C develops and breaks down. Although the fatigue test (see FIG. 8) can measure the fatigue strength of the part 1, it takes a long time because it is necessary to repeat the tensile and compression load until the part 1 breaks. According to the estimation method, the fatigue strength can be estimated without performing a fatigue test.

As shown in FIG. 1, FIG. 3, and FIG. 4, the present fatigue strength estimation method includes a set stress ratio acquisition step (S1), an inherent crack size calculation step (S2a to S2b), and a set nominal stress acquisition. A step (S3), a stress distribution calculation step (S4), an assumed stress intensity factor range calculation step (S5), an assumed stress intensity factor range verification step (S6), and an estimated fatigue strength determination step (S7). Prepare.
In addition, as the component 1 mentioned above, mechanical parts, such as a stepped shaft which comprises an engine, a turbine, a multistory parking lot, etc. are mentioned, for example, as the stress concentration part 12, a stepped formed in a mechanical part, Examples include keyways, holes, cutouts such as holes. For example, in the illustration of FIG. 8, the component 1 is a rod-shaped stepped shaft, and has a stress concentration portion 12 that forms a shape that is hollowed out along the circumferential direction of the rod.
Hereinafter, steps common to each method of the fatigue strength estimation methods shown in FIGS. 1, 3, and 4 will be described in the order of the steps in FIG.

In step S1, a set stress ratio acquisition step is executed. In the set stress ratio acquisition step (S1), a set stress ratio R that is an arbitrarily set stress ratio is acquired. The set stress ratio R is arbitrarily determined, and the determined value is acquired. As will be described later, the set stress ratio R is used to obtain a lower limit stress intensity factor range ΔK _{th, R} and fatigue strength Δσ _{th, R} (hereinafter collectively referred to as strength parameters as appropriate). The stress ratio is defined as minimum stress σ _min ÷ maximum stress σ _max or minimum stress intensity factor K _min ÷ maximum stress intensity factor K _max . For example, in the fatigue test, a periodic load of a tensile load and a compressive load is repeatedly applied to the component 1, and the stress ratio corresponds to the ratio of the stress due to the tensile load and the stress due to the compressive load. For example, when a constant load is applied to the rotating test piece as shown in FIG. 8 from the same direction, reverse loads of the same magnitude (σ _max = −σ _min) are applied to each part of the component 1. ) Periodically, the set stress ratio R becomes -1. The stress ratio is a parameter that represents this.

For example, when the material forming the stress concentration portion 12 is cast iron, the dimension a ₀ of the inherent crack C calculated from the strength parameter is almost the same value regardless of the value of the set stress ratio. Particularly suitable for such a case is the fatigue strength estimation method shown in FIG. On the other hand, in carbon steel, for example, the dimension a ₀ of the inherent crack C calculated from the strength parameter varies depending on the value of the set stress ratio. Particularly suitable for such a case is the fatigue strength estimation method shown in FIGS.

In step S2 (S2a to S2b), an inherent crack size calculation step is executed. The intrinsic crack size calculation step (S2) is performed based on the lower limit stress intensity factor range ΔK _{th, R} and fatigue strength Δσ _{th, R} of the material forming the component 1 acquired according to the set stress ratio R. The size of the inherent crack C assumed on the surface 13 of 1 is calculated. The inherent crack C is a virtual crack, and it is assumed that the stress concentration portion 12 is broken by the propagation of the inherent crack C. Specifically, as shown in FIG. 2, the existence of an inherent crack C is assumed in the base material of, for example, the bottom of the stress concentration portion 12, and the inherent crack C is the surface 13 of the stress concentration portion 12. _Extends a distance a0 in the depth direction of the base material. This is the dimension a ₀ of the natural crack C. In the embodiment shown in FIG. 1, the natural crack size calculation step (S2) includes two steps. In step S2a, the above-described strength parameter is obtained from a strength database described later, and in step S2b, the strength parameter is used. Then, the dimension a ₀ of the natural crack C is calculated. The stepped shaft part of FIG. 8 is formed with a stress concentrating part 12 formed of a groove in a part thereof, but the bottom part is the farthest part (the furthest away from the outer surface of the stepped shaft). Part).

More specifically, in general, the lower limit stress intensity factor range ΔK _{th, R} and fatigue strength σ _{th, R} of a material have values corresponding to the stress ratio, and are examined in advance for each material to create a database (hereinafter, It is called a strength database.) Therefore, it is possible to acquire the value of the strength parameter corresponding to the set stress ratio R from the strength database. In addition, cracks due to fatigue are thought to occur from the surface of the material. Therefore, as shown in FIG. 2, assuming the existence of an inherent crack C having a dimension a ₀ along the depth direction on the surface 13 of the stress concentration portion 12, the intrinsic crack dimension a ₀ is The shape parameter determined according to the shape of the crack C or the part 1 is set as f, and the strength parameter corresponding to the set stress ratio R obtained from the strength database is used to calculate the following equation (1).
a ₀ = (1 / π) × {ΔK _{th, R} / (f × Δσ _{th, R} )} ² (1)

In step S3, a set nominal stress acquisition step is executed. In the set nominal stress acquisition step (S3), a set nominal stress σ ₀ which is a nominal stress arbitrarily set on the surface 13 of the stress concentration portion 12 is determined. That is, the set nominal stress σ ₀ is a stress acting on the surface 13 of the stress concentration portion 12 as shown in FIG. Further, here, the magnitude of the tensile stress is + σ ₀ and the magnitude of the compressive stress is −σ ₀ .

In step S4, a stress distribution calculation step is executed. In the stress distribution calculating step (S4), a stress depth distribution σ (d), which is a stress distribution in the depth direction from the surface 13 of the stress concentrated portion 12 when the set nominal stress σ ₀ acts on the stress concentrated portion 12, is obtained. calculate. As shown in FIG. 2, the depth direction is a direction orthogonal to the surface 13 of the stress concentration portion 12 and toward the inside of the component 1 (inside the base material). The stress depth distribution σ (d) is a stress distribution in the depth direction where the surface 13 of the stress concentration portion 12 is 0, and the stress depth distribution σ (d) is a depth distance d from the surface. It is possible to determine the stress. For example, the stress depth distribution σ (d) may be calculated by numerical analysis such as FEM, and the load condition in this case is that the set nominal stress σ ₀ is in the depth direction from the surface 13 of the stress concentration portion 12. It works like that. The broken line in FIG. 2 is the nominal stress distribution, the solid line is the maximum stress distribution (σ _max ) and the minimum stress distribution (σ _min ) including stress concentration, and the stress depth distribution σ (d) is Corresponds to the solid line.

In step S5, an assumed stress intensity factor range calculation step is executed. The assumed stress intensity factor range calculating step calculates an assumed stress intensity factor range ΔK, which is a stress intensity factor range of the inherent crack C, based on the dimension a ₀ of the inherent crack C and the stress depth distribution σ (d). To do. That is, the stress intensity factor range (assumed stress expansion) when the stress depth distribution σ (d) based on the set nominal stress σ ₀ acts on the assumed crack C on the surface 13 of the stress concentration portion 12. The coefficient range ΔK) is calculated.

Here, the stress intensity factor range is calculated by subtracting the minimum stress intensity factor K _min from the maximum stress intensity factor K _max (ΔK = K _max −K _min ). Further, the maximum stress intensity factor K _max and the minimum stress intensity factor K _min are calculated from, for example, a well-known superposition formula using the stress depth distribution σ (d). Note that the superposition formula is K = 2 × √ (a ₀ / π) × ∫ [{(1 + F (x / a ₀ )) / √ (K, where K _max or K _min is K and the distance d is x. a ₀ ² −x ² )} × σ (x)] dx, and integration is performed from ₀ to a ₀ . Here, F (x / a ₀ ) is a correction coefficient due to a crack, the shape of a member, or the like.

In step S6, an assumed stress intensity factor range verification step is executed. In the assumed stress intensity factor range verification step (S6), it is determined whether or not the assumed stress intensity factor range ΔK matches the lower limit stress intensity factor range ΔK _{th, R.} In the assumed stress intensity factor range verification step (S6), if coincidence is determined, the process proceeds to the next step S7. The case of mismatch determination will be described later.

In step S7, an estimated fatigue strength determination step is executed. The estimated fatigue strength determination step (S7) is the setting acquired in step S3 when the assumed stress intensity factor range verification step determines that the assumed stress intensity factor range matches the lower limit stress intensity factor range ΔK _{th, R.} The nominal stress σ ₀ is determined as the estimated fatigue strength. As described above, the assumed stress intensity factor range ΔK is calculated based on the arbitrarily set nominal stress σ ₀ . Therefore, the assumption that the assumed stress intensity factor range ΔK coincides with the lower limit stress intensity factor range ΔK _{th, R} corresponding to the set stress ratio R. The nominal stress that causes the set stress ratio R on the surface of the stress concentration portion 12 is This means that the set nominal stress σ ₀ and that this set nominal stress σ ₀ is the lower limit value of the nominal stress that causes the propagation of the inherent crack C. Therefore, the set nominal stress σ ₀ when the assumed stress intensity factor range matches the lower limit stress intensity factor range ΔK _{th, R} can be determined as the estimated fatigue strength of the stress concentration portion 12.

  Here, the degree of coincidence between the estimated fatigue strength determined by the above-described fatigue strength estimation method and the fatigue strength by the experiment measured by actually conducting the fatigue test will be described with reference to FIGS. 5 and 6, the horizontal axis corresponds to the experimental fatigue strength, and the vertical axis corresponds to the estimated fatigue strength. Further, when the experimental fatigue strength and the estimated fatigue strength completely match, the intersection of each value is plotted on a diagonally drawn line (matching line L). Then, as shown in FIG. 5 and FIG. 6, as a result of performing a fatigue test using six different parts 1 (stress concentration portions 12) of different types as test pieces, the experimental fatigue strength and the present fatigue strength estimation method The intersection point of each value with the estimated fatigue strength determined by the above is located in the vicinity of the coincidence line L indicating the coincidence of both, and it was confirmed that the results of the both generally coincided. In addition, since the degree of coincidence is good both in the case where shot peening is not applied to the stress concentration portion 12 (FIG. 5) and in the case where shot peening is applied to the stress concentration portion 12 (FIG. 6), there is no surface treatment. Regardless of the above, it was confirmed that the estimated fatigue strength determined by this fatigue strength estimation method almost coincided with the experimental fatigue strength.

According to the above configuration, for example, the set nominal stress assumed that the estimation of the fatigue strength of the stress concentration portion 12, which is a notch portion formed in the component 1, acts on the surface 13 (bottom surface) of the stress concentration portion 12. This is performed by calculating whether or not a virtual intrinsic crack C determined by the set stress ratio R assumed on the surface 13 of the stress concentration portion 12 propagates by σ ₀ . More specifically, the stress concentration portion 12 has a stress distribution from the surface 13 toward the inside of the base metal due to the set nominal stress, and the natural crack C has a distance (from the surface 13 of the stress concentration portion 12 ( The stress intensity factor range (assumed stress intensity factor range ΔK) is calculated assuming that the stress distribution corresponding to the dimension a ₀ ) is applied. When the assumed stress intensity factor range ΔK calculated in this way matches the lower limit stress intensity factor range of the material obtained from the set stress ratio R, the set nominal stress σ0 at that time is the nominal value that causes the propagation of the inherent crack C. Since it is the lower limit of stress, it is determined as the estimated fatigue strength. By calculating the estimated fatigue strength in this way, the fatigue strength of the stress concentration portion 12 of the component can be estimated without performing a fatigue test that requires a long time.

In the above-described embodiment (FIGS. 1, 3, and 4), the set nominal stress acquisition step (S2: S2a to S2b) is performed after the set stress ratio acquisition step (S1) and the natural crack size calculation step (S2: S2a to S2b). Although S3) and the stress distribution calculation step (S4) are executed, this order may be reversed. That is, if the order in which the stress depth distribution σr (d) and the dimension a ₀ of the natural crack C can be obtained before the assumed stress intensity factor range calculation step (S5) is executed, the steps S1 to S4 are performed. The order does not matter.

In some embodiments, as shown in FIGS. 1, 3, and 4, in the fatigue strength estimation method described above, the assumed stress intensity factor range ΔK is determined by the assumed stress intensity factor range verification step (S 6). A setting nominal stress changing step (S61) is further provided for changing the setting nominal stress σ ₀ when it is determined that the lower limit stress intensity factor range ΔK _{th, R} does not match. In this case, in the set nominal stress acquisition step (S3), the value changed in the set nominal stress change step (S6) is acquired as a new set nominal stress σ ₀ . The fatigue strength estimation method also includes a step of changing the nominal stress setting step until the assumed stress intensity factor range ΔK is matched with the lower limit stress intensity factor range ΔK _{th, R} in the assumed stress intensity factor range verification step (S6). (S61), set nominal stress acquisition step (S3), stress distribution calculation step (S4), assumed stress intensity factor range calculation step (S5), and assumed stress intensity factor range verification step (S6) are repeated in this order. In other words, the calculation of the assumed stress intensity factor range ΔK based on the new set nominal stress σ ₀ is repeated until the coincidence is determined by the assumed stress intensity factor range verification step (S6).

According to the above configuration, the assumed stress intensity factor range ΔK calculated based on the set nominal stress σ ₀ and the dimension a ₀ of the natural crack C is determined as a lower limit stress intensity factor determined as a material characteristic based on the set stress ratio R. When it does not coincide with the ranges ΔK _{th and R} , the estimated fatigue strength of the stress concentration portion 12 of the part 1 can be estimated by searching for a set nominal stress σ ₀ that matches.

Next, as in the case where the material forming the stress concentration portion 12 is, for example, carbon steel, the dimension a ₀ of the inherent crack C calculated from the strength parameter changes according to the value of the set stress ratio. A fatigue strength estimation method particularly suitable for the case will be described with reference to FIGS.

In some embodiments, as shown in FIGS. 3 to 4, the fatigue strength estimation method described above is calculated based on the size a ₀ of the inherent crack C and the stress depth distribution σ (d). An assumed stress ratio calculation step (S51) for calculating an assumed stress ratio Rc, which is a stress ratio of the inherent crack C, and an assumed stress ratio verification step for determining whether or not the assumed stress ratio Rc matches the set stress ratio R ( S51). The assumed stress intensity factor range verification step (S6) is executed when it is determined by the assumed stress ratio verification step (S52) that the assumed stress ratio Rc and the set stress ratio R match. That is, in this embodiment, the inherent crack C is calculated based on the dimension a ₀ of the inherent crack C calculated through steps S1 to S2 and the stress depth distribution σ (d) calculated through steps S3 to S4. Together with the assumed stress intensity factor range ΔK, the assumed stress ratio Rc is calculated. The assumed stress ratio Rc is calculated as Rc = K _min / K _max using K _max and K _min calculated from the stress depth distribution σ (d).

When the material of the stress concentration portion 12 is, for example, carbon steel, the strength parameter has a value corresponding to the stress ratio. If the assumed stress ratio Rc calculated based on the set nominal stress σ ₀ and the inherent crack size a ₀ is different from the arbitrarily set stress ratio R, the stress concentration portion is calculated in both calculations. Therefore, the estimated fatigue strength cannot be determined appropriately. Therefore, in the present embodiment, assuming that the assumed stress intensity factor range ΔK and the lower limit stress intensity factor range ΔK _{th, R} in the assumed stress intensity factor range verification step (S6) match, the assumed stress ratio calculation By the step (S51) and the assumed stress ratio verification step (S52), the assumed stress ratio Rc and the set stress ratio R are made to coincide.

According to the above configuration, the assumed stress intensity factor range verification step on the assumption that the assumed stress ratio Rc calculated based on the set nominal stress σ ₀ and the inherent crack size a ₀ matches the set stress ratio R. Determination by (S6) is made. Accordingly, the dimension a ₀ of the inherent crack C calculated from the lower limit stress intensity factor range ΔK _{th, R} and fatigue strength σ _{th, R} , such as carbon steel, is a value corresponding to the set stress ratio R. Even if the stress concentration portion 12 is formed of such a material, it is possible to appropriately estimate the estimated fatigue strength. When the stress concentration portion 12 is made of cast iron, the assumed stress ratio calculation step (S51) and the assumed stress ratio verification step (S52) can be omitted.

  In the above-described embodiment (FIGS. 3 and 4), the assumed stress ratio calculation step (S51) and the assumed stress ratio verification step (S52) are executed after the assumed stress intensity factor range calculating step (S5). However, the order may be changed as long as the assumed stress ratio Rc can be calculated. Further, the assumed stress intensity factor range calculation step (S5) may be executed after the coincidence determination is made in the assumed stress ratio verification step (S52).

  In some embodiments, as shown in FIGS. 3 to 4, in the fatigue strength estimation method described above, the assumed stress ratio Rc does not match the set stress ratio R in the assumed stress ratio verification step (S 52). When it is determined that, a set stress ratio changing step (S53) for changing the set stress ratio R is further provided. In this case, in the set stress ratio acquisition step (S1), the value changed in the set stress ratio change step (S53) is acquired as the set stress ratio. In the fatigue strength estimation method, the set stress ratio change step (S53) and the set stress ratio acquisition step (S1) until it is determined in the assumed stress ratio verification step (S52) that the assumed stress ratio matches the set stress ratio. The inherent crack size calculation step (S2) and the assumed stress ratio verification step (S52) are repeated in this order.

More specifically, for example, in the set stress ratio changing step (S53), the assumed stress ratio Rc calculated immediately before may be determined as the set stress ratio R. In this case, a determination of coincidence in the assumed stress ratio verification step (S52) is obtained. However, if it is determined that there is a mismatch in the assumed stress intensity factor range verification step (S6) executed thereafter, the set nominal stress σ ₀ is updated (S61). When the set nominal stress σ ₀ is updated, the stress depth distribution σ (d) changes accordingly. Therefore, the assumed stress ratio Rc also changes. Therefore, when the assumed stress ratio Rc and the set stress ratio R do not coincide with each other, the set stress ratio changing step (S53) is performed again. That is, until the assumed stress intensity factor range verification step (S7) determines a match, the set stress ratio change step (S52), the set stress ratio acquisition step (S1), the inherent crack size calculation step (S2), and the assumed stress extension The coefficient range calculation step (S5), the assumed stress ratio calculation step (S51), and the assumed stress ratio verification step (S52) are repeated in this order.

  According to the above configuration, when the assumed stress ratio Rc determined from the stress depth distribution σ (d) does not match the set stress ratio R, the set stress ratio R that matches is searched for, and thus the component 1 The estimated fatigue strength of the stress concentration part 12 can be appropriately determined.

Next, calculation of fatigue strength when surface treatment is performed on the stress concentration portion 12 will be described with reference to FIG.
In some embodiments, as shown in FIG. 4, the above-described fatigue strength estimation method uses the residual stress distribution σr (d) in the depth direction of the stress concentration portion 12 generated by the surface treatment on the stress concentration portion 12. Is further provided with a residual stress distribution acquisition step (S31). In the stress distribution calculating step (S31), the stress depth distribution σ (d) is calculated in consideration of the residual stress distribution σr (d). For example, in shot peening, an infinite number of iron balls are collided with a metal surface at a high speed, which causes compressive residual stress in the vicinity of the processed surface. The fatigue strength is increased by canceling out the tensile stress acting on the stress concentration portion 12 by the compressive residual stress. Therefore, the residual stress distribution σr (d) generated by the surface treatment such as shot peening is obtained by numerical analysis such as FEM and reflected on the stress depth distribution σ (d) with the set nominal stress σ ₀ as a reference. By obtaining the parameters, the estimated fatigue strength after the surface treatment can be obtained.

  For example, when the residual stress distribution shown by the broken line in FIG. 7 is generated by performing shot peening on the stress concentration portion 12, the stress distribution shown by the solid line in FIG. In the stress distribution calculation step (S31), the stress depth distribution σ (d) taking into account the residual stress distribution σr (d) is obtained by setting the distribution obtained by subtracting the broken line from the solid line as the stress depth distribution σ (d). ) Is calculated.

  According to the above configuration, the residual stress distribution σr (d) due to the surface treatment such as peening or nitriding is obtained through numerical analysis, for example, and the estimated fatigue strength is calculated using the stress depth distribution σ (d) in consideration of the residual stress. By determining the fatigue strength of the stress concentration portion after the surface treatment can be estimated without performing a fatigue test.

Next, calculation of effects when surface treatment is performed on the stress concentration portion 12 will be described with reference to FIG.
In some embodiments, as shown in FIG. 9, the above-described fatigue strength estimation method uses the estimated fatigue strength determined using the stress depth distribution σ (d) taking into account the residual stress distribution σr (d). And the difference between the estimated fatigue strength determined using the stress depth distribution σ (d) not taking into account the residual stress distribution σr (d) as a result of the surface treatment on the stress concentration portion 12 A processing effect calculation step (S8) is further provided. Specifically, FIG. 6 shows the estimated fatigue strength when shopping is applied, but is plotted in FIG. 5 showing the case where there is no surface treatment from the values indicated by the respective symbols plotted in FIG. By calculating the difference between the values indicated by the same symbols, the difference is used as the effect of the surface treatment.

  In the embodiment shown in FIG. 9, in step S8, the estimated fatigue strength without reference surface treatment is determined by the method described using FIG. 1 and FIG. 3 described above, and in FIG. The estimated fatigue strength with surface treatment is determined by the method described above. Thereafter, in step S10, the difference between the estimated fatigue strength calculated in step S8 and the estimated fatigue strength calculated in step S9 is calculated as the effect of the surface treatment.

  According to said structure, the effect by surface treatment can be acquired from the difference of the two estimated fatigue strength according to the presence or absence of surface treatment. In addition, by deriving the surface treatment conditions from the numerical analysis conditions of the residual stress distribution σr (d), the effects according to the surface treatment conditions on the stress concentration portion 12 can be clarified. Therefore, the fatigue strength of the component 1 can be improved by applying a surface treatment to the stress concentration portion 12 under such an installation condition.

  Hereinafter, the fatigue strength estimation apparatus 3 for executing the above-described fatigue strength estimation method will be described with reference to FIG. FIG. 10 is a diagram showing a configuration of the fatigue strength estimation apparatus 3 according to an embodiment of the present invention, and executes the fatigue strength estimation method shown in FIG. 3 or FIG. In addition, since the function part of the fatigue strength estimation apparatus 3 demonstrated below is a function part which performs the same content as the step of the fatigue strength estimation method of the same name mentioned above, detailed description is abbreviate | omitted.

  The fatigue strength estimation device 3 is a device that estimates the fatigue strength of the stress concentration portion 12 of the component 1, and as shown in FIG. 10, a set stress ratio acquisition portion 31, an inherent crack size calculation portion 32, and , A set nominal stress acquisition unit (stress information acquisition unit 33), a stress distribution calculation unit 34, an assumed stress intensity factor range calculation unit (assumed strength parameter calculation unit 35), and an assumed stress intensity factor range verification unit (assumed strength parameter) A verification unit 36) and an estimated fatigue strength determination unit 37. The fatigue strength estimation device 3 is configured by a computer, and includes a CPU (processor) (not shown), a memory such as ROM and RAM, an auxiliary storage device (storage device), and an external communication interface. The CPU operates (data calculation, etc.) according to the instructions of the program (state monitoring program) loaded in the main storage device, thereby realizing each of the above functional units.

In the embodiment shown in FIG. 10, the set stress ratio acquisition unit 31 is configured to receive the set stress ratio R and is connected to the natural crack size calculation unit 32. The intrinsic crack size calculation unit 32 is connected to the strength database, and outputs the dimension a ₀ of the intrinsic crack C to the assumed strength parameter calculation unit 35. On the other hand, the stress information acquisition unit 33 is configured to receive the set nominal stress σ _{0 and} is connected to the stress distribution calculation unit 34. The stress distribution calculation unit 34 outputs the calculated stress depth distribution σr (d) to the assumed strength parameter calculation unit 35. The assumed strength parameter calculation unit 35 uses the size a ₀ of the natural crack C input from the natural crack size calculation unit 32 and the stress depth distribution σr (d) input from the stress distribution calculation unit 34. The assumed stress intensity factor range ΔK and / or the assumed stress ratio Rc (assumed strength parameter) are calculated. The assumed strength parameter calculation unit 35 is connected to the assumed strength parameter verification unit 36 and outputs a calculation result. The assumed strength parameter verification unit 36 is connected to the estimated fatigue strength determination unit 37, and outputs a set nominal stress σ ₀ at that time to the estimated fatigue strength determination unit 37 when a match determination is made. 37 is determined as the estimated fatigue strength, and is output to a display device, for example.

  In some embodiments, as illustrated in FIG. 10, the fatigue strength estimation device 3 may further include an assumed stress ratio calculation unit and an assumed stress ratio verification unit. In FIG. 10, the assumed stress intensity factor range calculation unit and the assumed stress ratio calculation unit are included in the assumed strength parameter calculation unit 35.

In some embodiments, as illustrated in FIG. 10, the fatigue strength estimation device 3 may further include a set stress ratio changing unit.
In some embodiments, as illustrated in FIG. 10, the fatigue strength estimation device 3 may further include a set nominal stress changing unit.
In the embodiment shown in FIG. 10, a set stress ratio change unit and a set nominal stress change unit are included in the assumed strength parameter calculation unit 35. Then, assuming the intensity parameter calculating unit 35 is connected to a setting stress ratio acquisition unit 31 and the stress information acquisition unit 33, and changes the setting stress ratio R and set nominal stress sigma _0. For example, the changed value may be output from the assumed strength parameter calculation unit 35. Alternatively, the stress information acquisition unit 33 may receive a command to generate a changed value, or may request a new value from the operator and acquire a value input by the operator.

  In some embodiments, as illustrated in FIG. 10, the fatigue strength estimation device 3 may further include a residual stress distribution acquisition unit. In the embodiment shown in FIG. 10, the stress information acquisition unit 33 includes a residual stress distribution acquisition unit.

  In some embodiments, as illustrated in FIG. 10, the fatigue strength estimation device 3 may further include a surface treatment effect calculation unit 38. In the embodiment shown in FIG. 10, the estimated fatigue strength determination unit 37 is connected to the surface treatment effect calculation unit 38 and uses the output of the estimated fatigue strength determination unit 37 as an effect of the surface treatment on the stress concentration portion. calculate.

  The present invention is not limited to the above-described embodiments, and includes forms obtained by modifying the above-described embodiments and forms obtained by appropriately combining these forms.

DESCRIPTION OF SYMBOLS 1 Parts 12 Stress concentration part 13 Surface 3 of stress concentration part Fatigue strength estimation apparatus 31 Set stress ratio acquisition part 32 Intrinsic crack size calculation part 33 Stress information acquisition part 34 Stress distribution calculation part 35 Assumed strength parameter calculation part 36 Assumed strength parameter Verification unit 37 Estimated fatigue strength determination unit 38 Surface treatment effect calculation unit C Inherent crack a ₀ Inherent crack size ΔK _{th, R} lower limit stress intensity factor range (strength database)
Δσ _{th, R} fatigue strength (strength database)
ΔK Assumed stress intensity factor range K _max Maximum stress intensity factor K _min Minimum stress intensity factor L Match line R Set stress ratio Rc Assumed stress ratio d Depth distance from the surface of the stress concentration part

Claims (12)

A fatigue strength estimation method for estimating the fatigue strength of a stress concentration part of a component,
A set stress ratio acquisition step for acquiring a set stress ratio that is an arbitrarily set stress ratio;
Based on the lower limit stress intensity factor range and the fatigue strength of the material forming the part obtained according to the set stress ratio, the inherent crack size calculated on the surface of the stress concentration portion is calculated. Crack size calculation step;
A set nominal stress acquisition step of acquiring a set nominal stress that is a nominal stress arbitrarily set on the surface of the stress concentration portion;
A stress distribution calculation step of calculating a stress depth distribution which is a stress distribution in the depth direction from the surface of the stress concentration portion when the set nominal stress acts on the stress concentration portion;
An assumed stress intensity factor range calculating step for calculating an assumed stress intensity factor range that is a stress intensity factor range of the inherent crack based on the size of the inherent crack and the stress depth distribution;
An assumed stress intensity factor range verification step for determining whether the assumed stress intensity factor range matches the lower limit stress intensity factor range; and
An estimated fatigue strength determination step for determining the set nominal stress as an estimated fatigue strength when the assumed stress intensity factor range verification step determines that the assumed stress intensity factor range matches the lower limit stress intensity factor range; Fatigue strength estimation method comprising: When the assumed stress intensity factor range verification step determines that the assumed stress intensity factor range does not match the lower limit stress intensity factor range, the assumed stress intensity factor range further includes a set nominal stress change step that changes the set nominal stress. ,
The set nominal stress acquisition step acquires the value changed by the set nominal stress change step as the set nominal stress,
In the fatigue strength estimation method, the set nominal stress change step, the set nominal stress acquisition, until the assumed stress intensity factor range verification step determines that the assumed stress intensity factor range matches the lower limit stress intensity factor range. The fatigue strength estimation method according to claim 1, wherein the fatigue strength estimation method is repeated in the order of a step, the stress distribution calculation step, the assumed stress intensity factor range calculation step, and the assumed stress intensity factor range verification step. An assumed stress ratio calculating step of calculating an assumed stress ratio which is a stress ratio of the inherent crack calculated based on the dimension of the inherent crack and the stress depth distribution;
An assumed stress ratio verification step for determining whether or not the assumed stress ratio matches the set stress ratio;
3. The assumed stress intensity factor range verification step is executed when it is determined by the assumed stress ratio verification step that the assumed stress ratio matches the set stress ratio. Fatigue strength estimation method. A set stress ratio change step of changing the set stress ratio when the assumed stress ratio and the set stress ratio are determined not to match in the assumed stress ratio verification step;
The set stress ratio acquisition step acquires the value changed by the set stress ratio change step as the set stress ratio,
In the fatigue strength estimation method, the set stress ratio change step, the set stress ratio acquisition step, the inherent crack size, until the assumed stress ratio verification step determines that the assumed stress ratio matches the set stress ratio. 4. The fatigue strength estimation method according to claim 3, wherein the calculation step and the assumed stress ratio verification step are repeated in this order. A residual stress distribution acquisition step of acquiring a residual stress distribution in the depth direction of the stress concentration portion generated by the surface treatment on the stress concentration portion;
The fatigue strength estimation method according to any one of claims 1 to 4, wherein the stress distribution calculation step calculates the stress depth distribution in consideration of the residual stress distribution.   The estimated fatigue strength determined using the stress depth distribution taking into account the residual stress distribution and the estimated fatigue strength determined using the stress depth distribution not taking into account the residual stress distribution The fatigue strength estimation method according to claim 5, further comprising a surface treatment effect calculation step of calculating a difference as an effect of the surface treatment on the stress concentration portion. A fatigue strength estimation device for estimating the fatigue strength of a stress concentration part of a component,
A set stress ratio acquisition unit for acquiring a set stress ratio which is an arbitrarily set stress ratio;
Based on the lower limit stress intensity factor range and the fatigue strength of the material forming the part obtained according to the set stress ratio, the inherent crack size calculated on the surface of the stress concentration portion is calculated. Crack size calculator,
A set nominal stress acquisition unit that acquires a set nominal stress that is a nominal stress arbitrarily set on the surface of the stress concentration portion;
A stress distribution calculation unit that calculates a stress depth distribution that is a stress distribution in the depth direction from the surface of the stress concentration part when the set nominal stress acts on the stress concentration part;
An assumed stress intensity factor range calculation unit that calculates an assumed stress intensity factor range that is a stress intensity factor range of the inherent crack based on the dimensions of the inherent crack and the stress depth distribution;
An assumed stress intensity factor range verification unit that determines whether the assumed stress intensity factor range matches the lower limit stress intensity factor range; and
An estimated fatigue strength determination unit that determines the set nominal stress as an estimated fatigue strength when the assumed stress intensity factor range verification unit determines that the assumed stress intensity factor range matches the lower limit stress intensity factor range; A fatigue strength estimating device comprising: When the assumed stress intensity factor range verification unit determines that the assumed stress intensity factor range does not match the lower limit stress intensity factor range, the assumed stress intensity factor range verification unit further includes a set nominal stress change unit that changes the set nominal stress. ,
The set nominal stress acquisition unit acquires the value changed by the set nominal stress change unit as the set nominal stress,
The fatigue strength estimation device is configured to obtain the set nominal stress change unit and the set nominal stress until the assumed stress intensity factor range verification unit determines that the assumed stress intensity factor range matches the lower limit stress intensity factor range. The fatigue strength estimation apparatus according to claim 7, wherein the process is repeated in the order of a part, the stress distribution calculation part, the assumed stress intensity factor range calculation part, and the assumed stress intensity factor range verification part. An assumed stress ratio calculation unit for calculating an assumed stress ratio that is a stress ratio of the inherent crack calculated based on the size of the inherent crack and the stress depth distribution;
An assumed stress ratio verification unit that determines whether or not the assumed stress ratio matches the set stress ratio;
The said assumed stress intensity factor range verification part is performed when it is determined by the said assumed stress ratio verification part that the said assumed stress ratio and the said set stress ratio are in agreement. Fatigue strength estimation device. A set stress ratio changing unit that changes the set stress ratio when the assumed stress ratio verification unit determines that the assumed stress ratio and the set stress ratio do not match;
The set stress ratio acquisition unit acquires the value changed by the set stress ratio change unit as the set stress ratio,
The fatigue strength estimation device is configured so that the set stress ratio change unit, the set stress ratio acquisition unit, and the inherent crack size until the assumed stress ratio verification unit determines that the assumed stress ratio matches the set stress ratio. The fatigue strength estimation apparatus according to claim 9, wherein the processing is repeated in the order of a calculation unit and the assumed stress ratio verification unit. A residual stress distribution acquisition unit that acquires a residual stress distribution in the depth direction of the stress concentration part generated by the surface treatment on the stress concentration part;
The fatigue strength estimation apparatus according to claim 7, wherein the stress distribution calculation unit calculates the stress depth distribution in consideration of the residual stress distribution.   The estimated fatigue strength determined using the stress depth distribution taking into account the residual stress distribution and the estimated fatigue strength determined using the stress depth distribution not taking into account the residual stress distribution The fatigue strength estimation apparatus according to claim 11, further comprising a surface treatment effect calculation unit that calculates a difference as an effect of the surface treatment on the stress concentration portion.

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