JP2010175479A - Method for evaluating life of minute notched material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for evaluating the life of a minute notched material capable of precisely estimating the fatigue life of the minute notched material having minute notches. <P>SOLUTION: A fatigue test is performed using a plurality of samples different in notch depth and notch tip radius to respectively form the SN charts of the respective samples while the stress distribution σ<SB>y</SB>in the notched cross sections of the respective samples is estimated with respect to the nominal stress imparted in the fatigue test and a characteristic distance x<SB>0</SB>is calculated at every sample. On the basis of them, the characteristic distance average stress σ<SB>ave</SB>being the average stress from a notched bottom to the characteristic distance x<SB>0</SB>is calculated, the relation of the fatigue crack producing life to the characteristic distance average stress σ<SB>ave</SB>is preliminarily calculated from the calculated characteristic distance average stress σ<SB>ave</SB>and the SN charts and the characteristic distance average stress σ<SB>ave</SB>of the minute notched material estimating fatigue life is calculated using the calculated relation to estimate the fatigue life of the minute notched material. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for predicting the fatigue life of a minute notch material having a minute notch, and more particularly to a method for evaluating the life of a minute notch material using a characteristic distance model.

  In the aircraft jet engine, as shown in FIG. 16, a part of the air taken in by the fan 161 is compressed by the compressor 162 and sent to the combustor 163, and fuel is injected and ignited in the combustor 163. The high-pressure turbine 164 that drives the compressor 162 and the low-pressure turbine 165 that drives the fan 161 are sequentially driven by this gas, and then the gas is moved backward from the jet nozzle 166 at a high speed. Propulsion in the opposite direction to the gas jet is obtained by jetting.

  In such an aircraft jet engine 160, foreign objects such as birds and stones (Foreign Object Debris) may be sucked from the air intake port of the fan 161, and the suction of the foreign matters causes the fan blade 167 of the fan 161 to be compressed. Minute scratches (nick / dent, scratch) are likely to occur on the moving blade 168 of the machine 162.

  The first cause of damage to the fan blade 167 and the moving blades 168 of the compressor 162 is due to high cycle fatigue. In the fan blade 167 having minute flaws and the moving stator blade 168 of the compressor 162, stress is concentrated on the minute flaws and a crack is generated, which may cause the fan blade 167 and the moving stator blade 168 to be destroyed. There is.

  Therefore, in order to prevent such destruction, it is necessary to predict the fatigue strength (fatigue life) of the fan blade 167 and the stationary blade 168 having minute scratches, and to ensure soundness by performing an appropriate inspection. .

  Conventionally, methods for predicting the fatigue life of fan blades 167 and moving blades 168 having minute flaws (notches) include FEM (Finite Element Method) and simple calculation formulas for stress concentration portions of notches. The peak stress value of the (notch bottom) is obtained, and based on this, the fatigue life is predicted using the SN diagram of the smooth material prepared in advance, the peak stress value and the fatigue limit of the material (fatigue limit stress amplitude) ) To determine whether or not a fatigue crack occurs (whether repair or replacement is necessary).

  The prior art document information related to the invention of this application includes the following.

Luca Susmel, “The Theory of critical distances: a review of its applications in factage”, Engineering Fracture Mechanics 75 (2008). 1706-1724 D. Taylor, “The theory of critical distances”, Engineering Fracture Mechanics 75 (2008), p. 1696-1705

  However, in the conventional method, when the stress concentration factor of the stress concentration part (notch bottom) of the notch is large, if the fatigue life (fatigue strength) is predicted by the peak stress value of the notch bottom, the evaluation result on the excessively safe side There was a problem that would become.

  That is, in the conventional method, even if an extremely small notch that actually does not interfere with operation is generated, if the notch has a shape with a large stress concentration factor (for example, a V shape) The peak stress value, which is an evaluation index, becomes large, and there is a problem that the life is predicted to be too short, and the fatigue life cannot be accurately predicted.

  Accordingly, an object of the present invention is to provide a life evaluation method for a minute notch material that can accurately predict the fatigue life of a minute notch material having a minute notch.

The present invention was devised in order to achieve the above object, and is a method for predicting the fatigue life of a micro notch material having a micro notch, and includes a plurality of notches having different notch depths and notch tip radii. A fatigue test is performed using the sample, and an SN diagram of each sample is created. On the other hand, the stress distribution σ _y at the notched cross section of each sample is estimated with respect to the nominal stress applied in the fatigue test, Formula (1) shown in Formula 1

In seeking properties distance x _0, which is defined for each sample, based on the stress distribution sigma _y and characteristic distance x ₀ in these samples sectional characteristic distance mean stress is the average stress from the notch root to the characteristic distance x ₀ σ _ave is obtained, and the relationship between the obtained characteristic distance average stress σ _ave and the above SN diagram is used to obtain the relationship of the fatigue crack initiation life to the characteristic distance average stress σ _ave , and the fatigue life is predicted using this relationship This is a method for evaluating the life of a micro notch material by predicting the fatigue life of the micro notch material by obtaining the characteristic distance average stress σ _ave of the micro notch material.

By the fatigue test, the seek rupture life N _f of each sample, by creating an SN diagram seeking fatigue limit stress amplitude .DELTA..sigma _w respectively, while stress when cracks occurred in the notch bottom of each sample In addition to determining the expansion factor, the stress distribution σ _y at the notch cross section was determined, and the stress intensity factor when the crack occurred at the notch bottom and the stress distribution σ _y at the sample cross section were determined. In addition, based on the crack growth law, the fatigue crack growth life is analyzed, the lower limit stress intensity factor range ΔK _th is obtained, and the fatigue limit stress amplitude Δσ _w and the lower limit stress intensity factor range ΔK _th are calculated. On the basis of the characteristic distance x ₀ for each sample based on the formula (1), the distance from the notch bottom to the characteristic distance x _{0 is} calculated based on the obtained characteristic distance x ₀ and the stress distribution σ _y in the sample cross section. The characteristic distance average stress σ _ave , which is an average stress, may be obtained.

From the fracture life N _{f of} each sample obtained in the fatigue test and the fatigue crack growth life N _p obtained by the analysis of the fatigue crack growth life, the following equation (2)
N _i = N _f −N _p (2)
Need additional fatigue crack initiation life N _i of each sample, based on this, it may be obtained relation fatigue crack initiation life N _i with respect to the characteristic distance mean stress sigma _ave.

  The sample may be a scratch-type round bar test piece in which a scratch-type notch is formed on the surface of a smooth round bar.

  The stress intensity factor when a crack occurs at the notch bottom is the stress intensity factor when there is a stress concentration and the stress intensity factor when there is no stress concentration when a crack occurs at the edge of a flat plate. May be obtained by multiplying the ratio by the stress intensity factor when a crack occurs in the smooth round bar.

The characteristic distance average stress σ _ave when the radius of the notch tip is 0.01 mm or less was obtained, and from the obtained characteristic distance average stress σ _ave and the relationship of the fatigue crack initiation life to the characteristic distance average stress σ _ave , The minimum fatigue crack initiation life may be obtained.

After obtaining the relationship of fatigue crack initiation life to the above characteristic distance average stress σ _ave , using this relationship, create an SN diagram for each notch depth when the notch tip radius is 0.01 mm or less Using this, the minimum fatigue limit stress amplitude is calculated for each notch depth and the ratio of the fatigue limit stress amplitude Δσ _w in a smooth material without notches is maximized. The relationship between the notch depth and the maximum reduction rate of the fatigue limit stress amplitude is obtained, and the fatigue limit stress amplitude is calculated from the notch depth of the micro notch material that predicts the fatigue life using this relationship. obtain the maximum reduction rate, which by multiplying the fatigue limit stress amplitude .DELTA..sigma _w of smooth material, may be obtained the minimum fatigue limit stress amplitude of the fine notches material.

  Using the SN diagram when the radius of the notch tip is 0.01 mm or less, the minimum time strength at any number of cycles is obtained for each notch depth, and the ratio to the time strength in a smooth material without notches. By calculating the maximum reduction rate of time strength, the relationship between the maximum reduction rate of time strength and the notch depth is obtained, and this relationship is used to calculate the notch depth of a micro notch material that predicts fatigue life. Then, the minimum time strength at an arbitrary number of cycles of the minute notch material may be obtained by obtaining the maximum rate of decrease in time strength and multiplying this by the time strength of the smooth material.

  According to the present invention, the fatigue life of a minute notch can be accurately predicted.

It is a flowchart of the lifetime evaluation method of the micro notch material of this invention. It is a figure explaining a characteristic distance model. FIG. 3A is a plan view of a sample used in the present invention, FIG. 3B is an enlarged view of a portion A, and FIG. 3C is an enlarged view of the notch. In this invention, it is a SN diagram obtained by a fatigue test. In this invention, it is a conceptual diagram at the time of calculating | requiring the stress intensity factor when a crack generate | occur | produces in the notch bottom of the round bar in which the notch was formed. It is a figure which shows an example of the relationship of the stress intensity factor with respect to a notch depth. It is a figure explaining the relationship between the coordinate system of x, and the coordinate system of r. In this invention, it is a figure for demonstrating how to obtain | require the stress distribution in a notch cross section, and characteristic distance average stress. In this invention, it is a figure for demonstrating how to obtain | require a lower limit stress intensity factor range. In this invention, it is a figure which shows the relationship of the fatigue crack generation lifetime with respect to characteristic distance average stress. It is a figure explaining that characteristic distance average stress will be converged to a certain value when a notch tip radius is made small. It is a figure explaining that the fatigue crack initiation life converges to the minimum fatigue crack initiation life when the notch tip radius is reduced. It is SN diagram for every notch depth calculated | required using the relationship of FIG. It is SN diagram for every notch tip radius calculated | required using the relationship of FIG. It is a figure which shows the relationship between the notch depth calculated | required in this invention, and the maximum reduction rate of fatigue limit stress amplitude. 1 is a schematic sectional view of a jet engine for an aircraft and a partially enlarged view thereof. It is the schematic of the lifetime evaluation apparatus of the micro notch material used for the lifetime evaluation method of the micro notch material of this invention.

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

  The method for evaluating the life of a micro notch material according to the present invention is a micro notch as a pre-stage for predicting the fatigue life of a fan blade of an aircraft jet engine in which micro scratches have occurred due to suction of foreign object debris, etc. This is a method for evaluating the fatigue life of a micro-notch material having a.

  Moreover, the life evaluation method of the micro notch material of this invention uses the characteristic distance model (Critical Distance Model), the average stress from a notch bottom to a characteristic distance (Critical Distance), ie, characteristic distance average stress (Critical Distance Stress) Is a method for evaluating the fatigue life of a micro-notch material.

  First, the characteristic distance model will be briefly described.

As shown in FIG. 2, stress is concentrated on the notch bottom B in the notch material 21 made of a metal material and having the notch 22. Therefore, the stress distribution σ _y at the notch cross section of the minute notch material 21 becomes the largest at the notch bottom B, and gradually decreases from the notch bottom B toward the inside of the minute notch material 21.

Therefore, for example, even when the peak stress value of the notch bottom B is the same, the stress distribution σ _y gradually decreases toward the inside of the minute notch material 21 and the case where the stress distribution σ _y decreases sharply. It can be seen that there is a difference in the burden received by the notch material 21 and a difference in fatigue life. Specifically, even if the peak stress value of the notch bottom B is the same, the average stress becomes smaller when the stress distribution σ _y decreases more rapidly toward the inside of the minute notch material 21. The burden on 21 is reduced and the fatigue life is increased.

Thus, in the minute notch material 21, the fatigue strength is not determined by the stress at one point of the notch bottom B, and how the stress distribution σ _y changes toward the inside of the minute notch material 21 is an important factor. Become. In the conventional method using the peak stress value of the notch bottom as an evaluation index, only the stress at one point on the notch bottom B is considered and the stress distribution σ _y at the notch cross section is not considered. It is considered that 21 was not able to evaluate the fatigue life accurately.

In the present invention, the average stress from the notch bottom B to the characteristic distance x ₀ , that is, the characteristic distance average stress σ _ave is used as an evaluation index of the fatigue life of the minute notch material 21.

In a metal material, fatigue failure occurs due to impurities contained in the metal material. That is, in a metal material, a minute crack can occur in high cycle fatigue. The micro Without maximum length of crack (maximum can裂深of a smooth metallic material may comprise) is a characteristic distance x _0. In other words, the characteristic distance x ₀ is that the crack propagates with respect to the fatigue limit stress amplitude Δσ _w of the smooth material (a limit stress that does not generate a fatigue crack no matter how many times the load is repeated with a stress less than this). Means limit length not.

Therefore, the characteristic distance x ₀ can be obtained from the fatigue limit stress amplitude Δσ _w in the smooth material and the lower limit stress intensity factor range ΔK _{th at} which the crack does not propagate.

Defined by A specific method for obtaining the lower limit stress intensity factor range ΔK _th and the fatigue limit stress amplitude Δσ _{w in} the equation (1) will be described later.

The average stress when the crack having the maximum length (characteristic distance x ₀ ) that can be included in the metal material is generated in the notch bottom B is the characteristic distance average stress σ _ave . Since the characteristic distance average stress σ _ave is an average stress from the notch bottom B to the characteristic distance x ₀ , the equation (3)

(See FIG. 2).

As a result of repeated studies on the fatigue life evaluation method of the micro notch material 21 using the characteristic distance model, the present inventor found that the relationship between the fatigue crack initiation life and the characteristic distance average stress σ _ave is the notch depth and the notch tip. The inventors have found that a single SN diagram can be created regardless of the radius, and have reached the present invention.

  FIG. 1 is a flowchart of a method for evaluating the lifetime of a minute notch material according to the present embodiment.

  As shown in FIG. 1, the method for evaluating the lifetime of a micro notch material according to this embodiment can be roughly divided into two steps S101 and S102.

In step S101, a fatigue test is performed and a characteristic distance average stress σ _ave is calculated to obtain a relationship between the fatigue crack initiation life and the characteristic distance average stress σ _ave .

In step S102, based on the relationship between the fatigue crack initiation life for characteristic distance mean stress sigma _ave obtained in step S101, it obtains the relationship between the maximum rate of decrease of the notch depth and the fatigue limit stress amplitude .DELTA..sigma _w, the relationship The minimum fatigue limit stress amplitude is obtained from the notch depth d of the minute notch 21 for predicting the fatigue life.

  Hereinafter, each step will be described in detail.

First, prior to obtaining the relationship between the fatigue crack initiation life and the characteristic distance average stress σ _ave (step S101), a plurality of samples having different notch depths d and notch tip radii ρ are prepared (step S1). Samples used in this embodiment are shown in FIGS.

  As shown in FIGS. 3A to 3C, a sample 31 is a scratch type round bar test piece in which a scratch type notch 22 is formed on the surface of a smooth round bar. The notch 22 is formed at the center of the sample 31 in the axial direction, and is uniformly formed on the outer periphery of the sample 31.

  The depth d of the notch 22 of the sample 31 is 1 mm or less, preferably 0.5 mm or less. This is because when a large notch with a depth of 1 mm or more actually occurs in a fan blade or the like, it is dangerous and is replaced and repaired unconditionally. In the present embodiment, the notch depth d is 0.1 mm, 0.3 mm, and 0.5 mm, respectively, and the notch tip radius ρ is 0.05 mm. For the sample 31 with the notch depth d = 0.3 mm, a sample 31 with a notch tip radius ρ of 0.2 mm was also prepared.

  The sample 31 is made of a metal material. In the present embodiment, the sample 31 is made of a titanium alloy. As shown in FIG. 3A, in this embodiment, the sample 31 formed so that the diameters at both ends are increased so that the fatigue test can be easily performed. However, the diameter of the sample 31 is constant over the entire length. There is no problem even if it exists.

Thereafter, the fatigue test for each sample 31, along with determining the rupture life N _f of each sample 31, to create the SN diagram for each sample 31 (step S2). An example of the created SN diagram is shown in FIG.

After the SN diagram of FIG. 4 is created, the fatigue limit stress amplitude Δσ _w of each sample 31 is determined from this SN diagram (step S3). In the present embodiment, the maximum stress amplitude at the number of repetitions N (rupture life N _f ) = 10 ⁷ cycles is defined as the fatigue limit stress amplitude Δσ _w .

On the other hand, the stress intensity factor _{Knotch_semicircle_roundbar} of the crack generated at the notch bottom B of the sample 31 is _obtained (step S4). FIG. 5 shows a conceptual diagram when _obtaining this stress intensity factor K _{notch_semicircle_roundbar} .

As shown in FIG. 5, when _obtaining the K value (K _{notch_semicircle_roundbar} ) of the crack generated at the notch bottom B of the sample 31, first, there is stress concentration when a crack is generated at the edge of the flat plate. and the stress intensity factor K _{Notch_throughcrack} of time, determined stress intensity factor K _Throughcrack when there is no stress concentration, taking these ratios _(K notch_throughcrack / K throughcrack). Furthermore, by multiplying this ratio by the stress intensity factor K _{semicircle_roundbar} when a crack occurs in a smooth round bar, the stress intensity factor when a stress occurs, that is, when a crack occurs in the notch bottom B _Find K _{notch_semicircle_roundbar} . That is, the stress intensity factor K _{notch_semicircle_roundbar} when a crack occurs in the notch bottom B is _expressed by the following equation (4)
K _{notch_semicircle_roundbar} = (K _{notch_throughcrack} / K _throughcrack ) × K _{semicircle_roundbar} (4)
It is represented by An example of a stress intensity factor K _{semicircle_roundbar} when a crack occurs in a smooth round bar and an example of a stress intensity factor K _{notch_semicircle_roundbar} when a crack occurs in a notch bottom B are shown in FIG.

Further, the stress distribution σ _y at the notched cross section of each sample 31 is estimated with respect to the nominal stress (net cross section average stress) σ _{m applied} in the fatigue test (step S5).

The stress distribution σ _y at the notched cross section is expressed by equation (5)

It is represented by Here, r in Expression (5) indicates a coordinate system moved in the x direction by −ρ / 2 from the coordinate system of x, as shown in FIG. 7, and is represented by r = x + ρ / 2. A and B are unknown numbers, and it is necessary to obtain these unknown numbers A and B.

  In order to obtain the unknowns A and B, a force balance equation for the sample 31 (because the sample 31 is a round bar test piece, an axial force balance equation) is derived.

As shown in FIG. 8, the axial force F due to the nominal stress σ _m is F = σ _m · πR ² . On the other hand, the axial force F due to the stress distribution σ _y in the notched cross section is expressed by Equation (6)

It becomes. Therefore, the balance formula of the axial force in the sample 31 is the formula (7) shown in Equation 6.

It is represented by When the unknowns A and B are obtained using the equations (5) and (7), the unknown A is expressed by the equation (8) shown in the equation (7).

The unknown B is given by the following formula (9)
B = K _t −A (9)
It becomes. By substituting the obtained unknowns A and B into Equation (5), the stress distribution σ _y at the notched section can be obtained.

Thereafter, based on the stress intensity factor K _{notch_semicircle_roundbar} obtained in step S4 and the stress distribution σ _y in the notch cross section obtained in step S5, the stress intensity factor range ΔK is obtained, and the fatigue crack growth life N _p is analyzed. It performs (step S6). The analysis of fatigue crack growth life N _p is based on the following equation (10)
da / dN = C (ΔK) ^m (10)
Where C and m are material constants.
a: Crack depth
N: Number of cycles
da / dN: Crack elongation per cycle
ΔK: The crack growth law expressed by the stress intensity factor range is used. In the formula (10), since the material constants C and m use a titanium alloy for the sample 31, C = 4.625 × 10 ⁻¹² (MPa) and m = 3.295 (m) so as to correspond to this. It was. An example of the relationship between the crack elongation per cycle da / dN and the stress intensity factor range ΔK is shown in FIG.

From FIG. 9, the lower limit stress intensity factor range ΔK _{th at} which the crack does not propagate is obtained. Fatigue Crack Propagation lifetime N _p is numerically ordinary differential equation of Formula (10), sequentially, to calculate using the method for calculating while updating the裂寸method when the stress intensity factor range. The initial condition is that the initial crack depth is the characteristic distance X0, and the time when the crack depth becomes larger than the diameter of the round bar test piece (when the crack penetrates) is the time when the analysis is completed (when the fracture occurs). ) and to calculate the fatigue crack growth life N _p by.

After analysis of the fatigue crack propagation life N _p, fatigue and crack growth life N _p obtained in step S6, and a rupture life N _f obtained in step S2, the fatigue crack generation in each sample 31 The service life _Ni is obtained (step S7).

The sum of the rupture life N _f is Fatigue and fatigue crack initiation life N _i crack growth life N _p, namely the following equation (11)
N _f = N _i + N _p (11)
It is represented by Therefore, the fatigue crack initiation life N _i is obtained by modifying equation (11)
N _i = N _f −N _p (2)
Can be obtained.

After that, using the fatigue limit stress amplitude Δσ _w obtained in step S3 and the lower limit stress intensity factor ΔK _th obtained in step S6, equation (1)

Thus, the characteristic distance x ₀ is obtained for each sample, and the characteristic distance average stress σ _ave is obtained based on the obtained characteristic distance x ₀ (step S8). In the formula (1), the material constant F is set to a value corresponding to the titanium alloy (F = 1.215 × 2).

Since the characteristic distance average stress σ _ave is obtained by dividing the total stress P in the range from the notch bottom B to the characteristic distance x ₀ by the cross-sectional area S, the equation (12)

Is required. When the stress distribution σ _y at the notched cross section represented by the formula (5) is substituted into the formula (12) and calculated, the characteristic distance average stress σ _ave is expressed by the formula (13) shown in Formula 10.

It becomes.

When characteristic distance mean stress sigma _ave is obtained, and its properties distance mean stress sigma _ave, and a fatigue crack initiation life N _i of each sample obtained in step S7, Fatigue on the properties distance mean stress sigma _ave Crack Initiation life determining the relationship N _i (step S9). The relationship between the properties obtained distance mean stress σ Fatigue against _ave crack initiation life N _i shown in FIG. 10.

FIG. 10 is a plot of all samples 31 (notch depth d = 0.1 mm, 0.3 mm, 0.5 mm, notch tip radius ρ = 0.05 mm, 0.2 mm) created in step S1. It is. Thus, the relationship between the characteristic distance fatigue to the average stress sigma _ave crack initiation life N _i, regardless of the tip radius ρ-out d and the notch cut-out depth, can be created as a single SN diagram.

In FIG. 10, the data when the fatigue crack initiation life N _i is less than 10 ⁵ cycles and when the fatigue crack initiation life N _i is 10 ⁵ cycles or more are separated. _{This is because i} = 10 ⁵ cycles to indicate that the slope changes.

Once you have created in advance a characteristic distance mean relationship stress sigma fatigue against _ave crack initiation life N _i in FIG. 10, by obtaining the characteristic distance mean stress sigma _ave of micro notch member 21 for predicting the fatigue life, small notches material it is possible to predict 21 of fatigue life (the fatigue crack initiation life N _i).

Further, the characteristic distance average stress σ _ave is determined when the notch tip radius ρ is 0.01 mm or less, preferably 0.001 mm or less. The obtained characteristic distance average stress σ _ave and the characteristic distance average stress σ of FIG. and a relationship between the fatigue crack initiation life N _i for _ave, obtain the minimum fatigue crack initiation life (step S10).

As a result of repeatedly examining the relationship between the notch tip radius ρ and the fatigue life, the present inventor has a region where the characteristic distance average stress σ _ave does not increase no matter how small the notch tip radius ρ is. That is, it has been found that when the notch tip radius ρ is reduced, the characteristic distance average stress σ _ave converges to a certain value.

As shown in FIG. 11, when the notch tip radius ρ becomes a small value of 0.01 mm or less, the characteristic distance average stress σ _ave converges to a certain value. Therefore, as shown in FIG. 12, the notch tip radius ρ is below a small value 0.01 mm, it will converge to a number of cycles there is fatigue crack initiation life N _i. This cycle number is the minimum fatigue crack initiation life.

  Therefore, the minimum fatigue crack initiation life depending only on the notch depth d can be obtained. This minimum fatigue crack generation life is the fatigue crack generation life when the notch tip radius ρ is 0.01 mm or less, that is, when the notch 22 is substantially V-shaped. It can be said that this is the most safe evaluation.

Next, the step of using the relationship between the fatigue crack initiation life N _i, obtains the relationship between the maximum rate of decrease of the notch depth d fatigue limit stress amplitude .DELTA..sigma _w on the properties distance mean stress sigma _ave obtained in step S9 (Step S102) will be described.

After obtaining the relationship characteristic distance fatigue to the average stress sigma _ave crack initiation life N _i in step S9, and used to create a SN diagram for each notch depth d (step S11).

First, as an example, FIG. 13 shows an SN diagram when the notch tip radius ρ is 0.05 mm. In FIG. 13, plots (□, Δ, ○: ◇ are smooth materials) are experimental values in the fatigue test of Step S2, and solid lines and broken lines are SN diagrams created in Step S11. As shown in FIG. 13, the created SN diagram is in good agreement with the experimental value, and it can be seen that the SN diagram can be created with high accuracy from the relationship of FIG. When the SN diagram of FIG. 13 is created, the fracture life N _f (repetition number N) may be obtained by adding the fatigue crack initiation life N _i obtained from FIG. 10 to the fatigue crack growth life N _p. .

  FIG. 14 shows an SN diagram when the notch tip radius ρ is 0.2 mm and 0.05 mm at the notch depth d = 0.3 mm. As shown in FIG. 14, the created SN diagram (solid line) is in good agreement with the experimental values (□, Δ: ◇ are smooth materials).

  FIG. 14 also shows an SN diagram when the notch tip radius ρ is set to a small value of 0.01 mm or less. This SN diagram is an SN diagram showing the minimum strength at the notch depth d = 0.3 mm.

  Similarly, for each notch depth d, an SN diagram is created when the notch tip radius ρ is set to a small value of 0.01 mm or less.

  Thereafter, the relationship between the notch depth d and the maximum reduction rate of the fatigue limit stress amplitude is obtained based on the SN diagram (notch tip radius ρ ≦ 0.01 mm) created in step S11 (step S12).

Here, the time strength at the number of cycles N = 10 ⁷ , that is, the fatigue limit stress amplitude Δσ _w is examined. Time strength is the maximum stress amplitude at any number of cycles.

As described in step S10, when the notch tip radius ρ is reduced to 0.01 mm or less, the characteristic distance average stress σ _ave converges to a certain value, and the minimum fatigue crack initiation life is obtained based on this convergence value. Can do. Therefore, if an SN diagram is created for each notch depth d when the notch tip radius ρ is 0.01 mm or less, the minimum fatigue limit is determined for each notch depth d based on the created SN diagram. The stress amplitude can be obtained, and by taking the ratio of this to the fatigue limit stress amplitude Δσ _w of the smooth material, the notch depth d and the maximum reduction rate of the fatigue limit stress amplitude (dimensionless fatigue limit) A relationship can be sought.

More specifically, first, from the SN diagram (notch tip radius ρ ≦ 0.01 mm) created in step S11, the maximum stress amplitude at the cycle number N = 10 ⁷ times, that is, the minimum fatigue limit stress amplitude (minimum time). Strength) is obtained for each notch depth d.

Thereafter, the ratio of the obtained minimum fatigue limit stress amplitude for each notch depth d to the fatigue limit stress amplitude Δσ _w in the smooth material without the notch 22 (minimum fatigue limit stress amplitude of the minute notch material 21 / smooth material Of the fatigue limit stress amplitude Δσ _w ), the maximum reduction rate (dimensionless fatigue limit) of the fatigue limit stress amplitude is obtained for each notch depth d.

  The relationship between the obtained notch depth d and the maximum reduction rate of the fatigue limit stress amplitude is shown by a broken line in FIG. In FIG. 15, for reference, the relationship between the notch depth d and the reduction rate of the fatigue limit stress amplitude when ρ = 0.05 mm (corresponding to the SN diagram of FIG. 13) is shown by a solid line.

  By obtaining the relationship (broken line) in FIG. 15 in advance, if only the notch depth d is measured without measuring the notch tip radius ρ of the minute notch material 21 that predicts the fatigue life, the fatigue limit stress It is possible to obtain the maximum reduction rate of the amplitude.

Furthermore, Multiplying the maximum reduction of the resulting fatigue limit stress amplitude fatigue limit stress amplitude .DELTA..sigma _w of smooth material, it is possible to obtain the minimum fatigue limit stress amplitude of the micro notch member 21 (step S13).

  Thereby, when the value of the obtained minimum fatigue limit stress amplitude becomes equal to or smaller than a predetermined value (allowable limit value), it can be determined that replacement or repair is necessary.

As described above, when the fatigue life of the minute notch material 21 is predicted using the characteristic distance model, it is necessary to measure the notch depth d and the notch tip radius ρ of the minute notch material 21. However, in practice, the notch depth d can be easily measured, but it is very difficult to measure the notch tip radius ρ. In the present invention, since it is possible to predict the minimum fatigue limit stress amplitude Δσ _w of the minute notch material 21 only by the notch depth d, when actually predicting the fatigue life of an aircraft jet engine fan blade or the like. It is very effective.

In the present embodiment, the minimum time strength (minimum fatigue limit stress amplitude) at the cycle number N = 10 ⁷ cycles has been described, but the minimum time strength (for example, 80,000 times strength, etc.) at any cycle number is obtained in the same manner. be able to.

As described above, in the method for evaluating the life of a micro notch material according to the present embodiment, a fatigue test is performed using a plurality of samples 31 having different notch depths d and notch tip radii ρ. On the other hand, the stress distribution σ _y at the notch cross section of each sample 31 is estimated and the characteristic distance x ₀ is obtained with respect to the nominal stress σ _{m applied} in the fatigue test. determined distance mean stress sigma _ave respectively, from the determined characteristic distance mean stress sigma _ave and the SN diagram, to previously obtain a relation between the fatigue crack initiation life N _i on the properties distance mean stress sigma _ave, using this relationship , by obtaining the characteristic distance mean stress sigma _ave of micro notch member 21 for predicting the fatigue life, I predict the fatigue crack initiation life N _i of the small notch member 31.

This makes it possible to predict the life (fatigue crack initiation life N _i ) of the minute notch material 21 in consideration of the shape of the notch 22 and the stress distribution σ _y of the notch cross section. Therefore, the life is not excessively evaluated on the safe side, and the life of the minute notch material 21 can be accurately predicted.

  In this embodiment, a scratch-type round bar test piece in which a scratch-type notch is formed on the surface of a smooth round bar is used as the sample 31. Scratch-type samples have the lowest fatigue strength compared to nick-type samples with sharp flaws and dent-type samples with dents. The safety side can be evaluated.

  Further, in the present embodiment, an SN diagram when the notch tip radius ρ is 0.01 mm or less is created for each notch depth d, and the notch depth d and fatigue limit stress amplitude are used. The relationship with the maximum rate of decrease is demanded.

  In this embodiment, since the relationship between the notch depth d and the maximum reduction rate of the fatigue limit stress amplitude is obtained in advance, the notch tip radius ρ of the minute notch material 21 that predicts the fatigue life cannot be measured. However, the minimum fatigue limit stress amplitude of the minute notch material 21 can be obtained from the notch depth d, and the necessity for replacement or repair can be known.

  The micro notch material life evaluation method according to the present embodiment is realized by, for example, a micro notch material life evaluation apparatus 170 shown in FIG.

  The micro notch material life evaluation apparatus 170 includes an analysis data input unit 171 for inputting analysis data such as a notch depth d, a notch tip radius ρ, and a fatigue test result of the sample 31, and material data for storing material data. Based on the storage unit 172, the analysis condition storage unit 173 for storing the analysis conditions, the analysis data input to the input unit 171 and the material data stored in the material data storage unit 173, the analysis condition storage unit 173 stores the analysis data. The analysis unit 174 that performs the analysis described in steps S2 to S12 according to the analysis conditions described above, and the analysis that stores the relationship between the notch depth d obtained by the analysis unit 174 and the maximum reduction rate of the fatigue limit stress amplitude A result storage unit 175.

  Further, the micro notch material life evaluation apparatus 170 includes an evaluation object data input unit 176 for inputting the notch depth d of the micro notch material to be evaluated, and fatigue required when the micro notch material is actually used. Based on the notch depth d input by the evaluation object data input unit 176 and the notch depth d stored in the analysis result storage unit 175, An evaluation unit 178 that obtains the minimum fatigue limit stress amplitude of the minute notch material to be evaluated using the relationship with the maximum reduction rate of the fatigue limit stress amplitude and outputs whether or not this satisfies the standard data.

  The input unit 171, the material data storage unit 172, the analysis condition storage unit 173, the analysis unit 174, the analysis result storage unit 175, the evaluation object data input unit 176, the standard storage unit 177, and the evaluation unit 178 are an interface, memory, CPU, software It implement | achieves combining suitably.

  When performing the evaluation using the micro notch material life evaluation device 170, first, analysis data is input from the input unit 171 and analyzed by the analysis unit 174, and the analysis result (notch depth) is stored in the analysis result storage unit 175 in advance. (Relationship between d and maximum reduction rate of fatigue limit stress amplitude) is stored.

  After that, when data of the notch material to be evaluated (notch depth d) is input from the evaluation object data input unit 176, the fatigue limit corresponding to the input notch depth d is entered in the evaluation unit 178. The stress amplitude is determined, and it is determined whether or not the determined fatigue limit stress amplitude satisfies preset standard data, and the result is output to the outside.

21 Micro notch material 22 Notch 31 Sample B Notch bottom

Claims (8)

A method for predicting the fatigue life of a minute notch material having a minute notch,
A fatigue test is performed using a plurality of samples having different notch depths and notch tip radii, and an SN diagram is prepared for each sample. On the other hand, each sample is cut against the nominal stress applied in the fatigue test. Estimating the stress distribution σ _y at the notched cross section, and using the equation (1)

In seeking properties distance x _0, which is defined for each sample, based on the stress distribution sigma _y and characteristic distance x ₀ in these samples sectional characteristic distance mean stress is the average stress from the notch root to the characteristic distance x ₀ σ _ave is obtained, and the relationship between the obtained characteristic distance average stress σ _ave and the above SN diagram is used to obtain the relationship of the fatigue crack initiation life to the characteristic distance average stress σ _ave ,
A life evaluation method for a micro notch material, wherein the fatigue life of the micro notch material is predicted by obtaining the characteristic distance average stress σ _ave of the micro notch material for predicting the fatigue life using this relationship. Through the fatigue test, the fracture life N _f of each sample is obtained, and an SN diagram is created to obtain the fatigue limit stress amplitude Δσ _w .
On the other hand, the stress intensity factor when a crack occurred at the notch bottom of each sample was determined, and the stress distribution σ _y at the notch cross section was determined,
Based on the stress intensity factor when the crack occurred at the notch bottom and the stress distribution σ _y at the sample cross section, the fatigue crack growth life was analyzed based on the crack growth law. Obtain the limit stress intensity factor range ΔK _th ,
Based on the fatigue limit stress amplitude Δσ _w and the lower limit stress intensity factor range ΔK _th , the characteristic distance x ₀ is obtained for each sample from the equation (1), and the obtained characteristic distance x ₀ and the sample cross section The life evaluation method for a micro notch material according to claim 1, wherein a characteristic distance average stress σ _ave , which is an average stress from the notch bottom to the characteristic distance x ₀ , is obtained based on the stress distribution σ _y of the notch. From the fracture life N _{f of} each sample obtained in the fatigue test and the fatigue crack growth life N _p obtained by the analysis of the fatigue crack growth life, the following equation (2)
N _i = N _f −N _p (2)
Need additional fatigue crack initiation life N _i of each sample, based on this, the characteristic distance mean stress life evaluation method for a micro notch material according to claim 2, wherein determining the relationship between the fatigue crack initiation life N _i for sigma _ave .   The method for evaluating the life of a minute notch material according to any one of claims 1 to 3, wherein the sample is a scratch-type round bar test piece in which a scratch-type notch is formed on the surface of a smooth round bar.   The stress intensity factor when a crack occurs at the notch bottom is the stress intensity factor when there is a stress concentration and the stress intensity factor when there is no stress concentration when a crack occurs at the edge of a flat plate. 5. The method for evaluating the life of a micro notch material according to claim 4, wherein the ratio is obtained by multiplying the ratio by a stress intensity factor when a crack occurs in the smooth round bar. The characteristic distance average stress σ _ave when the radius of the notch tip is 0.01 mm or less was obtained, and from the obtained characteristic distance average stress σ _ave and the relationship of the fatigue crack initiation life to the characteristic distance average stress σ _ave , The lifetime evaluation method of the micro notch material in any one of Claims 1-5 which calculates | requires the minimum fatigue crack generation lifetime. After obtaining the relationship of fatigue crack initiation life to the above characteristic distance average stress σ _ave , using this relationship, create an SN diagram for each notch depth when the notch tip radius is 0.01 mm or less Using this, the minimum fatigue limit stress amplitude is calculated for each notch depth and the ratio of the fatigue limit stress amplitude Δσ _w in a smooth material without notches is maximized. Rate, and find the relationship between the notch depth and the maximum reduction rate of fatigue limit stress amplitude,
Using this relationship, the maximum reduction rate of the fatigue limit stress amplitude is obtained from the notch depth of the micro notch that predicts the fatigue life, and this is multiplied by the fatigue limit stress amplitude Δσ _w of the smooth material to obtain the above-mentioned micro The method for evaluating the life of a minute notch material according to any one of claims 1 to 6, wherein a minimum fatigue limit stress amplitude of the notch material is obtained. Using the SN diagram when the radius of the notch tip is 0.01 mm or less, the minimum time strength at any number of cycles is obtained for each notch depth and the ratio to the time strength in a smooth material without notches. By taking each, the maximum rate of decrease in time strength is determined, and the relationship between the maximum rate of decrease in time strength with respect to the notch depth is determined,
Using this relationship, the maximum reduction rate of time strength is obtained from the notch depth of the minute notch material that predicts the fatigue life, and this is multiplied by the time strength of the smooth material. The lifetime evaluation method of the micro notch material of Claim 7 which calculates | requires the minimum time intensity | strength in a number.

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