JP2010175478A - Method for evaluating lower limit value of fatigue strength of minute flaw member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for evaluating the lower limit value of the fatigue strength of a minute flaw member capable of quantitatively evaluating the lowering quantity of the fatigue strength of the minute flaw member. <P>SOLUTION: A fatigue test is performed using a sample simulating the minute flaw member, the sample is analyzed using a critical distance model to calculate the relation of the fatigue crack producing life to critical distance average stress, the relation between the notch depth in the sample and the maximum reduction ratio of fatigue strength is preliminarily calculated using the calculated relation, the maximum residual stress at the time of collision of foreign matter with the minute flaw member is estimated while calculating the lowering quantity of the fatigue strength caused by the maximum residual stress, the relation between the notch depth in the minute flaw member and the maximum reduction ratio of the fatigue strength is preliminarily calculated on the basis of the lowering quantity of the fatigue strength, the maximum reduction ratio of the fatigue strength is calculated on the basis of the notch depth of the minute flaw member to be evaluated and the lower limit value of the fatigue strength of the minute flaw member is calculated by multiplying the fatigue strength of a flaw-free member by the maximum reduction ratio of fatigue strength. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for evaluating a lower limit of fatigue strength of a micro defect member when a foreign object collides with a member such as a fan blade of an aircraft jet engine or a stationary blade of a compressor to form a micro defect. It is.

  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 (minute defects) are likely to occur on the moving blade 168 of the machine 162. The damage caused by the inhalation of foreign matter is called FOD (Foreign Object Damage). This minute defect is called a nick (a sharp scratch on the edge), a dent (a dent), or a scratch (a scratch) depending on its shape.

  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 defects and the moving blades and stator blades 168 of the compressor 162, stress concentrates on the minute defect portions and cracks are generated, which may cause the fan blades 167 and the moving blades 168 to be destroyed. is there.

  Therefore, in order to prevent such destruction, it is necessary to evaluate the fatigue strength of the fan blade 167 and the moving vane 168 having minute defects to ensure soundness.

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

D. Nowell, two others, “Prediction of fatigue performance in gas turbine blade after foreign object damage”, International Journal of Fatigue 25, 200. 963-969 Steven R. Thompson, two others, "Influence of residual stresses on high cycle fatigue of Ti-6Al-4V projected to foreligible object 23, Interfigure 23 John Warren et al., "Best Practices for the Mitigation and Control of Foreign Object Damage-Induced High Cycle Fatigue in Gas Turbine Engine Compression System Airfoils", RTO TECHNICAL REPORT TR-AVT-094, RESEARCH AND TECHNOLOGY ORGANISATION, 6 May 2005

  However, there is no conventional method for evaluating the fatigue strength of fan blades 167 and moving blades 168 (hereinafter referred to as minute defect members) that have undergone FOD.

  Therefore, at present, there is a problem that there is no choice but to evaluate the amount of decrease in fatigue strength of the minute defect member based on experience. Therefore, the allowable limit dimension of the micro defect for the micro defect member that has returned from the field (the maximum dimension that can continue to be used without repair) must be determined based on experience. In such a method based on experience, the evaluation of fatigue strength may vary depending on the evaluator, and it has been difficult to ensure soundness.

  Furthermore, the conventional method based on experience has a problem that, for example, it is not possible to appropriately evaluate the fatigue strength of a newly developed blade that has not accumulated experience.

  Accordingly, an object of the present invention is to solve the above-described problems and provide an evaluation method of a fatigue strength lower limit value of a micro defect member capable of quantitatively evaluating the amount of fatigue strength reduction of the micro defect member.

  The present invention was devised to achieve the above object. When a foreign object collides with a member such as a fan blade of an aircraft jet engine or a moving blade of a compressor, a minute defect is formed. A method for evaluating the lower limit of fatigue strength of a defective member, in which a sample imitating the minute defect member is provided with minute defects having different notch depths and notch tip radii, and fatigue using these samples. An SN diagram is created by performing a test. On the other hand, the stress distribution in the cross section located at the micro defect of the sample is analyzed using a characteristic distance model. The relationship of fatigue crack initiation life to stress was obtained, and using this relationship, an SN diagram was created for each notch depth when the radius of the notch tip was 0.01 mm or less. Based on the notch While obtaining the minimum fatigue strength for each depth and taking the ratio of the fatigue strength in a smooth material without notches, the relationship between the notch depth and the maximum reduction rate of fatigue strength in the sample was obtained, and the above Estimate the maximum residual stress when a foreign object collides with a minute defect member, determine the amount of decrease in fatigue strength due to the maximum residual stress, the amount of decrease in fatigue strength due to the maximum residual stress, and the notch depth in the sample Based on the relationship with the maximum reduction rate of fatigue strength, the relationship between the notch depth and the maximum reduction rate of fatigue strength in the above minute defect member is obtained, and this relationship is used to evaluate the lower limit value of fatigue strength. From the notch depth, the maximum reduction rate of fatigue strength is obtained, and by multiplying the maximum reduction rate of fatigue strength by the fatigue strength of members without defects, the lower limit of fatigue strength of minute defect members It is a method for evaluating fatigue strength lower limit of minute defects member seeking.

  The analysis using the characteristic distance model is based on the equation (1)

In seeking properties distance x _0, which is defined for each sample, the basis of the stress distribution and characteristics distance x ₀ in a cross section which is located minute defect, characteristic distance is the average stress from the notch root to the characteristic distance x ₀ The average stress σ _ave may be determined, and the relationship between the fatigue distance at which the characteristic distance average stress is generated may be determined from the determined characteristic distance average stress σ _ave and the SN diagram created by the fatigue test.

  In the fatigue test, the relationship between the average stress and fatigue strength of the sample is obtained, and the slope in the relationship is obtained. Based on this slope, the fatigue strength when the average stress increases by the maximum residual stress. The amount of decrease may be obtained.

  By taking the ratio between the amount of decrease in fatigue strength due to the maximum residual stress and the fatigue strength in the smooth material, and subtracting this from the maximum rate of decrease in fatigue strength in the sample, the notch depth and fatigue in the microdefect member You may obtain | require the relationship with the maximum decreasing rate of intensity | strength.

  The fatigue strength may be a time strength at an arbitrary number of cycles.

  The fatigue strength may be a fatigue limit stress amplitude.

  From the ratio between the fatigue limit stress amplitude of the above-mentioned member having no defect and the maximum stress amplitude actually used, the allowable limit reduction rate of fatigue strength is obtained, and the notch depth and the maximum decrease rate of fatigue strength in the above-mentioned micro defect member Using the above relationship, the allowable limit depth of the micro defect may be obtained from the allowable limit decrease rate of the fatigue strength.

  The member may be a fan blade of an aircraft jet engine or a moving and stationary blade of a compressor.

  According to the present invention, the amount of fatigue strength reduction of a minute defect member can be quantitatively evaluated.

It is a flowchart of the evaluation method of the fatigue strength lower limit of the micro defect member 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 the portion A, and FIG. 3C is an enlarged view of the minute defect. In this invention, it is a SN diagram obtained by a fatigue test. 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 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. In this invention, it is a figure which shows the relationship between the notch depth in a sample, and the maximum decreasing rate of fatigue limit stress amplitude. FIG. 13A is a diagram showing a minute defect formed by collision of a foreign substance, and FIG. 13B is a diagram showing a residual stress distribution in the minute defect portion. In this invention, it is a figure explaining how to obtain | require the fall amount of the fatigue strength by the maximum residual stress, and is a figure which shows the relationship between an average stress and fatigue strength. It is a figure which shows the relationship between the notch depth in the minute defect member 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 evaluation apparatus of the fatigue strength lower limit of the micro defect member used for the evaluation method of the fatigue strength lower limit of the micro defect member of this invention.

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

  The method for evaluating the lower limit of fatigue strength of a minute defect member according to the present invention is that a foreign object (Foreign Object Debris) collides with a member such as a fan blade of an aircraft jet engine or a moving blade and stator blade of a compressor to cause a minute defect (FOD; This is a method for evaluating the lower limit of fatigue strength of the minute defect member when (Object Damage) is formed.

  In addition, the evaluation method of the fatigue strength lower limit value of the minute defect member of the present invention uses a characteristic distance model to obtain the relationship between the notch depth and the maximum reduction rate of the fatigue strength in advance. Furthermore, considering the residual stress when a foreign object collides with the above member, a fatigue design diagram (relation between notch depth and maximum reduction rate of fatigue strength) for a minute defect member is created, and based on this This is a method for evaluating the lower limit of fatigue strength of minute defect members.

  First, the characteristic distance model will be briefly described.

As shown in FIG. 2, the stress concentrates on the notch bottom B in the minute defect member 21 made of a metal material and having the minute defect 22. Therefore, the stress distribution σ _y at the cross section (notch cross section) where the micro defect 22 of the micro defect member 21 is located becomes the largest at the notch bottom B, and from the notch bottom B toward the inside of the micro defect member 21. Decrease gradually.

Therefore, for example, even if 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 defect member 21 and the case where the stress distribution σ _y gradually decreases. It can be seen that there is a difference in the burden received by the defective member 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 as the stress distribution σ _y decreases more rapidly toward the inside of the minute defect member 21. The burden on 21 is reduced and the fatigue life is increased.

As described above, in the minute defect member 21, the fatigue strength is not determined by the stress at one point on the notched bottom B, and how the stress distribution σ _y changes toward the inside of the minute defect member 21 is an important factor. Become.

In the present invention, the average stress from the notch bottom B to the characteristic distance (Critical Distance) x ₀ , that is, the characteristic distance average stress (Critical Distance Stress) σ _ave is used as an evaluation index of the fatigue strength of the minute defect member 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 The lower limit stress intensity factor range ΔK _th and the fatigue limit stress amplitude Δσ _w in the equation (1) are values determined by the material.

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).

  Now, FIG. 1 is a flowchart of the evaluation method of the fatigue strength lower limit value of the minute defect member according to the present embodiment.

  As shown in FIG. 1, the evaluation method of the fatigue strength lower limit value of the microdefect member according to the present embodiment can be roughly divided into two steps S101 and S102.

  In step S101, a fatigue test is performed using a sample simulating a minute defect member, and an analysis is performed using a characteristic distance model to obtain a relationship between the notch depth and the maximum reduction rate of fatigue strength in the sample.

  In step S102, the maximum residual stress when a foreign object collides with the minute defect member is estimated, the amount of decrease in fatigue strength due to the maximum residual stress is obtained, and the notch depth and fatigue strength in the minute defect member are taken into consideration. The relationship with the maximum reduction rate is obtained, and the fatigue strength lower limit value of the minute defect member is evaluated using this relationship.

  Hereinafter, each step will be described in detail.

First, a fatigue test is performed using a sample imitating a minute defect member, and an analysis is performed using a characteristic distance model to obtain a relationship between the fatigue crack initiation life and the characteristic distance average stress σ _ave (step S1).

  Samples used in this embodiment are shown in FIGS.

  As shown in FIGS. 3A to 3C, the sample 31 is a scratch type round bar test piece in which scratch type micro defects 22 are formed on the surface of a smooth round bar. The minute defect 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 micro defect 22 of the sample 31 is 1 mm or less, desirably 0.5 mm or less. This is because when a large notch with a depth of 1 mm or more is actually generated in a minute defect member such as a fan blade, it is dangerously 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.

In step S <b> 1, first, a fatigue test is performed on each sample 31, and an SN diagram is created for each sample 31. An example of the created SN diagram is shown in FIG. 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 distribution σ _y at the notch 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. The stress distribution σ _y at the notched cross section is expressed by equation (5)

It is represented by Here, r in Equation (5) indicates a coordinate system moved in the x direction by −ρ / 2 from the coordinate system of x, as shown in FIG. 5, 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. 6, 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.

The stress distribution σ _y at the notched cross section and the formula (1) shown in Equation 8

The characteristic distance average stress σ _ave is obtained from the characteristic distance x ₀ defined by 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, Equation (10)

(See FIG. 6). When the stress distribution σ _y at the notched cross section represented by the formula (5) is substituted into the formula (10) and calculated, the characteristic distance average stress σ _ave is expressed by the formula (11) shown in the formula (10).

It becomes.

When characteristic distance mean stress sigma _ave is obtained, determine its properties distance mean stress sigma _ave, and a SN diagram obtained by a fatigue test, the fatigue crack initiation life of relationship characteristics distance mean stress sigma _ave. The relationship between the properties obtained distance mean stress σ Fatigue against _ave crack initiation life N _i shown in FIG.

In Figure 7, and the vertical axis represents the fatigue crack initiation life N _i. The fatigue crack initiation life N _i is, the following equation (12)
N _i = N _f −N _p (12)
However, N _i: Fatigue Crack Initiation life
N _f : Fracture life
N _p : It can be obtained from the fatigue crack growth life. Rupture life N _f can be obtained by performing an analysis using a can be calculated by the fatigue test as shown in SN diagram of FIG. 4, the fatigue crack growth life N _p wear crack growth law.

FIG. 7 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.

  Next, using the relationship shown in FIG. 7, an SN diagram is created for each notch depth d when the notch tip radius ρ is 0.01 mm or less, preferably 0.001 mm or less (step S2).

As shown in FIG. 8, 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. 9, 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.

As an example, FIG. 10 shows an SN diagram in the case where the notch tip radius ρ is 0.05 mm. In FIG. 10, plots (□, Δ, ○: ◇ are smooth materials) are experimental values in the fatigue test in step S1, and solid lines and broken lines are SN diagrams created in step S2. As shown in FIG. 10, 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. 10 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. 7 to the fatigue crack growth life N _p. .

  FIG. 11 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. 11, the created SN diagram (solid line) is in good agreement with the experimental values (□, Δ: ◇ are smooth materials).

  FIG. 11 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 fatigue strength is obtained based on the SN diagram (notch tip radius ρ ≦ 0.01 mm) created in step S2 (step S3).

Here, the time strength in the cycle number N = 10 ⁷ cycles, that is, the fatigue limit stress amplitude Δσ _w is examined. The time strength is the fatigue strength at an arbitrary number of cycles, that is, the maximum stress amplitude.

First, from the SN diagram (notch tip radius ρ ≦ 0.01 mm) created in step S2, the maximum stress amplitude in the cycle number N = 10 ⁷ cycles, that is, the minimum fatigue limit stress amplitude (minimum time strength) is notched. It calculates | requires for every depth d.

Thereafter, the ratio between the obtained minimum fatigue limit stress amplitude for each notch depth d and the fatigue limit stress amplitude Δσ _w in the smooth material without the microdefect 22 (minimum fatigue limit stress amplitude of the microdefect member / smooth material Taking the fatigue limit stress amplitude Δσ _w ), the maximum reduction rate (dimensionless fatigue limit or fatigue strength ratio) 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 fatigue strength (fatigue limit stress amplitude) is shown by a solid line in FIG. In FIG. 12, 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. 10) is shown by a solid line.

  If the fatigue strength (fatigue limit stress amplitude) of the smoothing material is known by using the relationship (solid line) obtained in FIG. 12, from the notch depth d of the minute defect portion (FOD damaged portion) of the sample 31. The fatigue strength (fatigue limit stress amplitude) of the sample 31 can be obtained. Therefore, FIG. 12 can be used as a fatigue design diagram for evaluating the fatigue strength from the notch depth d of the sample 31.

  As described above, the relationship between the notch depth d and the maximum reduction rate of the fatigue strength in the sample 31 is obtained.

  Next, based on the relationship obtained in step S101, the step of obtaining the relationship between the notch depth d in the minute defect member and the maximum reduction rate of fatigue strength (step S102) will be described.

  First, the maximum residual stress when a foreign object collides with a minute defect member is estimated (step S4).

  As an example, the maximum residual stress in the case where a foreign substance collides with a minute defect member and a minute defect as shown in FIG. FIG. 13B shows the residual stress distribution at the site where the foreign matter collides (the site that has received FOD) in this case.

  As shown in FIG. 13B, the residual stress (tensile) generated in the minute defect member is the highest at the center of the notch and is a little less than 200 MPa at the maximum.

  Similarly, as a result of various studies on the maximum residual stress generated in the minute defect member, it has been found that the maximum residual stress when a foreign object collides with the minute defect member is about 200 MPa at most. Therefore, in this embodiment, the maximum residual stress is set to 200 MPa.

  When the maximum residual stress is obtained, the amount of decrease in fatigue strength due to the maximum residual stress is obtained (step S5).

In order to obtain the amount of decrease in fatigue strength due to the maximum residual stress, first, the relationship between the average stress σ _mean and the fatigue strength is obtained by a fatigue test. FIG. 14 shows the relationship between the obtained average stress σ _mean and fatigue strength (stress amplitude σ _alt ). In FIG. 14, together with the result of the fatigue test using the above-described sample 31 (◯), the fatigue test was performed using the smooth material (◇) having no microdefects 22 and the dent type sample (□). The results are also shown.

In FIG. 14, the average stress σ _{mean on} the horizontal axis represents the thermal stress and centrifugal force of the minute defect member, and the following equation (13)
σ _mean = (σ _max + σ _min ) / 2 (13)
Where σ _max : Maximum stress
σ _min : Expressed by minimum stress. When the residual stress is present in the minute defect member, the average stress σ _mean is increased by the amount of the residual stress at a portion where the residual stress exists (minute defect portion).

The stress amplitude σ _{alt on the} vertical axis represents the stress amplitude due to vibration, and here represents the maximum stress amplitude in 10 ⁷ cycles, that is, the fatigue limit stress amplitude Δσ _w . Moreover, in FIG. 14, R is a stress ratio, and the following formula (14)
R = σ _min / σ _max (14)
It is represented by

As shown in FIG. 14, the relationship between the average stress σ _mean and the fatigue strength (here, fatigue limit stress amplitude Δσ _w ) can be expressed by a linear relationship.

Therefore, by obtaining the inclination, the amount of decrease in fatigue strength when the mean stress sigma _mean rises by up to residual stress (here, the amount of decrease in the fatigue limit stress amplitude .DELTA..sigma _w) can be obtained.

  In the present embodiment, from the viewpoint of evaluation on the safety side, the inclination in the scratch type (◯; sample 31 described above) having the largest inclination is employed among the relationships in FIG.

  When the amount of decrease in fatigue strength due to the maximum residual stress is obtained, this is applied to the relationship of FIG. 12 obtained in step S101, and the relationship between the notch depth d and the maximum reduction rate of fatigue strength in the minute defect member is obtained. (Step S6).

  Specifically, by taking a ratio between the amount of decrease in fatigue strength due to the maximum residual stress and the fatigue strength in the smooth material, and subtracting this from the maximum reduction rate of fatigue strength in FIG. The relationship between the height d and the maximum reduction rate of fatigue strength is obtained. The relationship between the notch depth d and the maximum reduction rate of fatigue strength in the obtained minute defect member is shown by a solid line in FIG.

As shown in FIG. 15, the relationship between the notch depth d in the minute defect member and the maximum reduction rate of the fatigue strength is the relationship in the sample indicated by the broken line (reduction amount due to residual stress / fatigue strength of the smooth material (here Then, it is shifted downward by the fatigue limit stress amplitude Δσ _w )).

  In FIG. 15, hatching is a population representing experimental data obtained in a fatigue test and literature data published in literature. As shown in FIG. 15, the relationship between the notch depth d and the maximum reduction rate of fatigue strength in the minute defect member obtained in this embodiment is the lower limit value of the fatigue strength in the experimental data and literature data. I understand that.

  Next, the fatigue strength lower limit value is obtained from the notch depth d of the minute defect member whose fatigue strength lower limit value is evaluated (step S7).

  15 is obtained from the notch depth (FOD depth) d of a minute defect member (for example, a blade such as a fan blade returned from the field) whose fatigue strength lower limit value is evaluated. Using the relationship, the maximum reduction rate of fatigue strength can be obtained. Furthermore, the lower limit value of the fatigue strength of the minute defect member can be obtained by multiplying the maximum reduction rate of the obtained fatigue strength by the fatigue strength of the member having no defect. That is, FIG. 15 can be used as a fatigue design diagram for evaluating the fatigue strength from the notch depth d of the minute defect member.

  Further, by obtaining the relationship shown in FIG. 15, it is possible to obtain the permissible limit depth of minute defects.

In general, members such as fan blades and compressor blades are used in a range where the fatigue limit stress amplitude Δσ _w is larger than the maximum stress amplitude σ _a in actual operation. If the fatigue limit stress amplitude .DELTA..sigma _w is used within an amount of less than the maximum stress amplitude sigma _a, because the fatigue fracture occurs.

Therefore, if the fatigue limit stress amplitude Δσ _w in a member having no defect and the maximum stress amplitude (vibration stress amplitude) σ _a in actual operation are known, how much the fatigue limit stress amplitude Δσ _w is reduced by FOD. A permissible limit reduction rate of the fatigue strength that is below the stress amplitude σ _a is obtained. If the notch depth d corresponding to the allowable limit reduction rate of the fatigue strength is obtained from the relationship shown in FIG. 15, the allowable limit depth of the minute defect can be obtained.

In the present embodiment, the fatigue limit stress amplitude Δσ _w that is the time strength in 10 ⁷ cycles is used as the fatigue strength, but the present invention is not limited to this, and the time strength at any number of cycles (for example, 80,000 times strength). Can be similarly evaluated.

  As described above, in the evaluation method of the fatigue strength lower limit value of the micro defect member according to the present embodiment, the fatigue test is performed using the sample 31 imitating the micro defect member, and the analysis is performed using the characteristic distance model. The relationship between the notch depth d at 31 and the maximum reduction rate of fatigue strength is obtained, and on the other hand, the maximum residual stress when a foreign object collides with a minute defect member is estimated, and the fatigue strength is reduced by the maximum residual stress. The amount of decrease in fatigue strength due to the maximum residual stress is applied to the relationship between the notch depth d in the sample 31 and the maximum reduction rate of fatigue strength, so that the notch depth d in the minute defect member The relationship with the maximum reduction rate of fatigue strength is obtained.

Thus, the maximum reduction rate of fatigue strength is obtained from the notch depth d of the minute defect member for which the lower limit of fatigue strength is evaluated, and the obtained maximum reduction rate of fatigue strength is multiplied by the fatigue strength of the member having no defect. Thus, the lower limit value of the fatigue strength of the minute defect member can be obtained. That is, it becomes possible to quantitatively evaluate the lower limit value of the fatigue strength (fatigue limit stress amplitude Δσ _w , or time strength at an arbitrary number of cycles) of a minute defect member that has not been evaluated in the past. It is possible to ensure the soundness of the.

Further, since a fatigue strength design diagram (a relation between the notch depth d and the maximum reduction rate of the fatigue limit stress amplitude Δσ _{w in} the minute defect member) as shown in FIG. 15 is obtained, for example, a newly developed blade However, it is possible to obtain an allowable limit depth of minute defects.

  The evaluation method of the fatigue strength lower limit value of the minute defect member according to the present embodiment is realized by, for example, the evaluation device 170 for the fatigue strength lower limit value of the minute defect member shown in FIG.

  The evaluation device 170 for the fatigue strength lower limit value of the minute defect member 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; Material data storage unit 172 for storing preset maximum residual stress, analysis condition storage unit 173 for storing analysis conditions, analysis data input to input unit 171 and material data stored in material data storage unit 173 Based on the above, according to the analysis conditions stored in the analysis condition storage unit 173, the analysis unit 174 that performs the analysis described in steps S1 to S6 described above, and the notch depth in the minute defect member obtained by the analysis unit 174 an analysis result storage unit 175 for storing the relationship between d and the maximum reduction rate of fatigue strength.

  Further, the evaluation device 170 for the fatigue strength lower limit value of the minute defect member is necessary for actually using the evaluation object data input unit 176 for inputting the notch depth d of the minute defect member to be evaluated and the minute defect member. Based on the notch depth d input by the evaluation object data input unit 176, the notch depth d stored in the analysis result storage unit 175, An evaluation unit 178 is provided that obtains the minimum fatigue limit stress amplitude of the minute defect member to be evaluated using the relationship with the maximum reduction rate of the fatigue strength 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 the evaluation is performed using the evaluation device 170 for the fatigue strength lower limit value of the minute defect member, first, analysis data is input from the input unit 171 and analyzed by the analysis unit 174, and the analysis result ( The relationship between the notch depth d and the maximum reduction rate of the fatigue limit stress amplitude in the minute defect member) 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 evaluation unit 178 causes the fatigue strength corresponding to the input notch depth d. A lower limit value is obtained, and it is determined whether or not the obtained lower limit value of fatigue strength satisfies standard data set in advance, and the result is output to the outside.

21 Minute Defect Member 22 Minute Defect 31 Sample B Notch Bottom

Claims (8)

When a foreign object collides with a member and a micro defect is formed, a method for evaluating a fatigue strength lower limit value of the micro defect member,
The sample imitating the microdefect member is provided with microdefects having different notch depths and notch tip radii, and a fatigue test is performed using these samples to create an SN diagram. The stress distribution at the cross section located at the micro defect is analyzed using the characteristic distance model, and the relationship between the fatigue crack initiation life and the characteristic distance average stress is obtained from the analysis result and the SN diagram,
Using this relationship, create an SN diagram for each notch depth when the notch tip radius is 0.01 mm or less, and based on the created SN diagram, minimum fatigue strength for each notch depth And obtaining the relationship between the notch depth and the maximum reduction rate of fatigue strength in the above sample by taking the ratio with the fatigue strength in the smooth material without notches,
Estimate the maximum residual stress when a foreign object collides with the minute defect member, determine the amount of decrease in fatigue strength due to the maximum residual stress, and determine the amount of decrease in fatigue strength due to the maximum residual stress and the notch depth in the sample. Based on the relationship between the maximum decrease rate of fatigue strength and the relationship between the notch depth and the maximum decrease rate of fatigue strength in the above minute defect member,
Using this relationship, the maximum reduction rate of fatigue strength is obtained from the notch depth of the minute defect member for which the lower limit of fatigue strength is evaluated. A method for evaluating a lower limit value of fatigue strength of a minute defect member, wherein the lower limit value of fatigue strength of the minute defect member is obtained by combining them. The analysis using the characteristic distance model is based on the equation (1)

In seeking properties distance x _0, which is defined for each sample, the basis of the stress distribution and characteristics distance x ₀ in a cross section which is located minute defect, characteristic distance is the average stress from the notch root to the characteristic distance x ₀ The average stress σ _ave is obtained, and the relationship between the obtained characteristic distance average stress σ _ave and the SN diagram created in the fatigue test is used to determine the relationship between the fatigue crack initiation life and the characteristic distance average stress. Evaluation method of fatigue strength lower limit value of minute defect member.   In the fatigue test, the relationship between the average stress and fatigue strength of the sample is obtained, and the slope in the relationship is obtained, and the fatigue strength when the mean stress increases by the maximum residual stress based on this slope. The evaluation method of the fatigue strength lower limit of the micro defect member of Claim 1 or 2 which calculates | requires the fall amount of.   By taking the ratio between the amount of decrease in fatigue strength due to the maximum residual stress and the fatigue strength in the smooth material, and subtracting this from the maximum rate of decrease in fatigue strength in the sample, the notch depth and fatigue in the microdefect member The evaluation method of the fatigue strength lower limit of the micro defect member in any one of Claims 1-3 which calculates | requires the relationship with the maximum reduction | decrease rate of intensity | strength.   The fatigue strength is a time strength at an arbitrary number of cycles. The method for evaluating a fatigue strength lower limit value of a minute defect member according to any one of claims 1 to 4.   The method for evaluating a lower limit value of fatigue strength of a minute defect member according to claim 5, wherein the fatigue strength is a fatigue limit stress amplitude.   From the ratio between the fatigue limit stress amplitude of the above-mentioned member having no defect and the maximum stress amplitude actually used, the allowable limit reduction rate of fatigue strength is obtained, and the notch depth and the maximum reduction rate of fatigue strength in the above-mentioned micro defect member The evaluation method of the fatigue strength lower limit value of the micro defect member of Claim 6 which calculates | requires the permissible limit depth of a micro defect from the permissible limit fall rate of the said fatigue strength using this relationship.   The method for evaluating a lower limit of fatigue strength of a minute defect member according to any one of claims 1 to 7, wherein the member is a fan blade of an aircraft jet engine or a moving and stationary blade of a compressor.

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