Aero-engine blade high-cycle fatigue limit prediction method

Abstract

The application provides an aero-engine blade high-cycle fatigue limit prediction method, first, parameters of a blade subjected to impact-corrosion coupling damage are measured, a fractal dimension and a stress concentration coefficient are calculated, the equivalent stress is corrected by using the fractal dimension and the stress concentration coefficient, a corrected critical distance model is obtained, and the fatigue limit of the blade subjected to coupling damage is predicted and calculated by using the corrected critical distance model. The blade high-cycle fatigue limit prediction method provided by the application innovatively integrates the three-dimensional fractal dimension, the theoretical stress concentration coefficient and the stress gradient distribution function near the notch root into the same model, and the blade fatigue limit obtained by the prediction calculation is highly consistent with the measured value of the test.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of aero-engine blades, and particularly relates to a high-cycle fatigue limit prediction method for aero-engine blades. BACKGROUND

[0002] During the take-off, landing and low-altitude flight of an aircraft, foreign objects such as stones, sand, bolts or rivets are easily sucked into the aero-engine, thereby causing foreign object damage (FOD), which is most likely to occur in the local area of the fan blade or the compressor impeller, and more than 70% of the damage occurs at the leading edge of the blade. Common damage forms include notches, tears, protrusions and depressions, which can cause stress concentration at the impact point, residual stress and microstructure damage.

[0003] In addition, changes in environmental humidity and temperature during the service of the engine can cause corrosion damage to the blade, significantly reducing the fatigue performance of the blade and accelerating the initiation of internal cracks. Under the coupling action of corrosion and impact, the fatigue limit of the blade is significantly reduced, which can cause unpredictable fatigue fracture during operation, thereby causing serious flight accidents, which is a huge hidden danger for an aircraft.

[0004] A series of analyses of the causes of aircraft failures show that the combined action of corrosion damage and working fatigue is the "culprit" that causes material structure failure and then causes aircraft failures or even disasters. Research on the comprehensive influence of corrosion and impact on the fatigue limit of the blade is of great significance for preventing and reducing flight accidents caused by coupling damage blades, reducing maintenance costs, improving flight safety and ensuring the combat readiness of the engine. SUMMARY

[0005] The purpose of the present application is to provide a high-precision and easily interpretable high-cycle fatigue limit prediction method for aero-engine blades.

[0006] Technical scheme: A high-cycle fatigue limit prediction method for aero-engine blades, comprising the following steps:

[0007] (1) The same blades are divided into three groups, and the first group of blades is subjected to foreign object damage test, the second group of blades is subjected to foreign object damage test and then immersion corrosion test, and the third group of blades is subjected to immersion corrosion test and then foreign object damage test, and the damage notch width and depth of the three groups of blades are measured and recorded respectively;

[0008] (2) The fractal dimension of the surface of the second group and the third group of blades is calculated by using the box dimension method;

[0009] (3) The stress concentration coefficient of the damage notch is calculated;

[0010] (4) Correct the equivalent stress by using the fractal dimension and the stress concentration coefficient to obtain the corrected critical distance model;

[0011] (5) Calculate the fatigue limit of the three groups of blades after foreign object damage by using the corrected critical distance model.

[0012] Specifically, the foreign object damage test comprises: using a finite element analysis software to perform modal analysis on the blade to obtain the natural frequency and mode shape of the blade, determining the foreign object damage position, and then using foreign objects of different sizes and speeds to perform foreign object damage tests.

[0013] Specifically, the immersion corrosion test comprises: immersing the blade in a corrosion liquid, setting different immersion durations for the same group of blades, and respectively measuring and recording the surface topography of the blades.

[0014] Specifically, step (2) comprises: collecting image data of the blade surface area to be calculated, and calculating the fractal dimension by using the box dimension method:

[0015]

[0016] In the formula: F is the fractal dimension, N r is the minimum number of boxes covering the area to be calculated, and k is the box size.

[0017] Specifically, in step (2), the image data of the blade surface area to be calculated collected is processed to obtain a gray scale surface of the image, and the fractal dimension is calculated by using the gray scale surface.

[0018] Specifically, in step (3), the stress concentration coefficient of the damage notch is:

[0019]

[0020] In the formula: K T is the stress concentration coefficient, d is the damage notch depth, p is the damage notch root radius, and l is the damage notch width.

[0021] Specifically, in step (4), the corrected critical distance model comprises:

[0022]

[0023] r=L0 / 2

[0024] N f =N A (σ A / σ eff (L0)) k

[0025] In the formula: is the stress gradient function, K TK is the stress concentration factor, F is the fractal dimension, σ eff (r) is the modified equivalent stress, σ1(r) is the maximum stress determined by the distance r, L0 is the critical distance of the material, N f is the fatigue life, N A is the high-cycle fatigue reference cycle number, σ A is the N A corresponding stress amplitude, and k is a material constant.

[0026] Specifically, step (5) comprises:

[0027] (51) Measure the fatigue limit σ0 of the blade before the foreign object damage test and the immersion corrosion test;

[0028] (52) Optionally, one of the three groups of blades after the test is selected, a blade model is established according to the damage notch width and depth data of the blade, the stress concentration factor and the fatigue limit σ s of the blade are calculated according to the modified critical distance model, and the fatigue limit σ n of the blade is predicted.

[0029] σ n = σ0 / K T

[0030] In the formula: σ n is the predicted fatigue limit value, σ0 is the fatigue limit of the blade before the test, K T is the stress concentration factor;

[0031] The fatigue limit σ n is taken as the load condition of the blade model, and the finite element analysis of the blade is carried out to obtain the distribution curve of the maximum stress with the distance at the damage notch, and the modified equivalent stress σ eff (r) is calculated by combining the modified critical distance model.

[0032] (53) The modified equivalent stress σ eff (r) is compared with the fatigue limit σ0, if the numerical values are the same, the fatigue limit σ n at this time is the fatigue limit of the blade after the test, if the numerical values are different, the load condition is reset and step (52) is repeated until the modified equivalent stress σ eff (r) is consistent with the fatigue limit σ0.

[0033] Preferably, in step (53), the load condition is reset by using the bisection method.

[0034] Preferably, the method further comprises the following steps:

[0035] (6) Measure the high-cycle fatigue limit of the three groups of blades after the foreign object damage, and compare it with the high-cycle fatigue limit calculated by the modified critical distance model for consistency.

[0036] Beneficial effects: Compared with the prior art, the significant effect of the present application is that the present application is based on the critical distance theory, that is, as the distance from the blade groove root increases, the effect of the increase of the principal stress on the fatigue damage will decrease, by combining the critical distance theory with the morphological characteristics of the corrosion area and the stress concentration effect of the damage notch, the three-dimensional fractal dimension, the theoretical stress concentration coefficient and the stress gradient distribution function near the notch root are innovatively integrated into the same model, a blade fatigue limit prediction model based on the critical distance theory of foreign object damage-corrosion damage coupling is established, which can be applied to the fan and compressor blade structures of the aero-engine under the conditions of hard object impact risk and corrosion, and the prediction results have high consistency with the test results, which provides theoretical support for evaluating the fatigue performance of the blade in actual work. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 is the flow chart of the aero-engine blade high-cycle fatigue limit prediction method of embodiment 1 of the present application.

[0038] Figure 2 is the flow chart of the fatigue limit calculation of embodiment 1 of the present application.

[0039] Figure 3 is the gray scale surface graph of the blade of embodiment 2 of the present application.

[0040] Figure 4 is the prediction result error distribution graph of embodiment 2 of the present application. DETAILED DESCRIPTION

[0041] The present application will be further illustrated below in combination with the drawings and specific embodiments.

[0042] Embodiment 1

[0043] Referring to Figure 1 , the present embodiment provides an aero-engine blade high-cycle fatigue limit prediction method, which comprises the following steps:

[0044] (1) The same blade is divided into three groups, and the first group of blades is subjected to foreign object damage test, the second group of blades is subjected to foreign object damage test first and then immersion corrosion test, and the third group of blades is subjected to immersion corrosion test first and then foreign object damage test, and the damage notch width and depth of the three groups of blades are measured and recorded by using a microscope.

[0045] In the present embodiment, the modal analysis of the blade is first carried out by using the finite element analysis software absqus to obtain the natural frequency and mode shape of the blade, and the foreign object damage position is determined, and then foreign object damage test is carried out by using foreign objects of different sizes and speeds.

[0046] In the embodiment, the third group of blades is further divided into three subgroups, corresponding to different corrosion durations, 24h, 48h and 96h respectively, and the surface morphology of the blades is measured and recorded respectively.

[0047] (2) The fractal dimension of the surface of the second group and the third group of blades is calculated by using the box dimension method. The box dimension method collects a plane image of a to-be-calculated region, and then calculates the image in units of pixel points.

[0048] In the embodiment, the image data of the to-be-calculated region of the blade surface is collected, and the gray surface of the image is obtained after processing. The fractal dimension is calculated by using the gray surface, and the calculation formula is as follows:

[0049]

[0050] In the formula, F is the fractal dimension, N r is the minimum number of boxes covering the to-be-calculated region, and k is the box size.

[0051] (3) After the foreign object damage test, a macroscopic notch is generated on the leading edge of the blade, and there is a significant stress concentration effect at the root of the notch. After the corrosion test, there is also a small erosion in the corrosion area, and there is also a stress concentration effect around the erosion area. Therefore, it is necessary to calculate the stress concentration coefficient of the damage notch under the coupling condition:

[0052]

[0053] In the formula, K T is the stress concentration coefficient, d is the damage notch depth, p is the damage notch root radius, and l is the damage notch width.

[0054] (4) Since the finite element model of the damaged blade is established based on the continuous medium assumption, the traditional critical distance method cannot well simulate the local stress concentration condition, and therefore it is necessary to modify the equivalent stress calculation, that is, to modify the maximum principal stress distribution, so that the calculation result is closer to the actual situation. The fractal dimension and the stress concentration coefficient are used to modify the equivalent stress, and a modified critical distance model is obtained.

[0055] The modified critical distance model includes:

[0056]

[0057]

[0058] r = L0 / 2

[0059] N f = N A (σ A / σ eff(L0) k

[0060] wherein: is the stress gradient function, K T is the stress concentration factor, F is the fractal dimension, σ eff (r) is the modified equivalent stress, σ1(r) is the maximum stress determined by distance r, L0 is the critical distance of material, N f is the fatigue life, N A is the high cycle fatigue reference cycle number, σ A is N A corresponding stress amplitude, k is the material constant.

[0061] (5) Calculate the fatigue limit of three groups of blades after foreign object damage using the modified critical distance model, please refer to Figure 2 , which includes the following steps:

[0062] (51) Measure the fatigue limit σ0 of the blade before foreign object damage test and immersion corrosion test;

[0063] (52) Optionally, test one blade in the three groups of blades, according to the damage notch width and depth data of the blade, establish the blade model, according to the stress concentration factor and the fatigue limit σ0, estimate the fatigue limit σ n of the blade:

[0064] σ n = σ0 / K T

[0065] wherein: σ n is the estimated fatigue limit value, σ0 is the fatigue limit of the blade before test, K T is the stress concentration factor;

[0066] Take the fatigue limit σ n as the load condition to the blade model, perform finite element analysis on the blade, get the distribution curve of the maximum stress at the damage notch with distance, according to the fractal dimension and theoretical stress concentration factor calculated before, combined with the modified critical distance model, calculate the modified equivalent stress σ eff (r);

[0067] (53) Compare the modified equivalent stress σ eff (r) with the fatigue limit σ0, if the numerical value is the same, then the fatigue limit σ n at this time is the fatigue limit of the blade after test, if the size is different, then reset the load condition and repeat step (52) until the modified equivalent stress σ eff (r) is consistent with the fatigue limit σ0.

[0068] In the present embodiment, the load condition is reset by dichotomy, specifically, when the equivalent stress is greater than the fatigue limit, the load condition is reduced, recalculated, if the equivalent stress at this time is equal to the fatigue limit, the calculation is ended, the result is output, if the equivalent stress at this time is still greater than the fatigue limit, the load condition is continuously reduced, the calculation is repeated, if the equivalent stress at this time is less than the fatigue limit, the average of the equivalent stress at this time and the original equivalent stress is selected for calculation, until the equivalent stress is equal to the fatigue limit value or approximately equal (such as ±5%) in the set range.

[0069] (6) Measure the high-cycle fatigue limit of the three groups of blades after foreign object damage, and compare it with the high-cycle fatigue limit calculated by the corrected critical distance model to verify the effectiveness of the method.

[0070] Embodiment 2

[0071] The present embodiment provides a specific embodiment of the high-cycle fatigue limit prediction method for an aero-engine blade described in embodiment 1.

[0072] A blade in the second group of blades is selected for illustration. The blade has undergone foreign object damage test and 96h immersion corrosion test. The damage notch width l is 1.907mm, the damage notch depth d is 0.553mm, the damage notch root radius p is 0.544mm, the stress concentration coefficient Kt is 3.02, and the surface image of the blade is collected. Please refer to FIG. 2, the gray surface of the blade image is obtained, and the fractal dimension F is calculated to be 2.2276. T Figure 3

[0073] The fatigue limit of the blade is estimated to be 190MPa using the stress concentration coefficient and the fatigue limit of the blade before the test, and this value is used as the load condition for the finite element model. The finite element analysis is performed to obtain the curve of the maximum principal stress at the notch root of the blade with respect to the distance, and the distance r uniquely determines a corresponding maximum principal stress σ1(r). Combined with the stress gradient correction function, the corrected critical distance model of the blade is obtained:

[0074] σ eff (r)=σ1(r)e -2.22r

[0075] r=L0 / 2

[0076] Substitute L0=0.07mm into the above formula to calculate the value of σ eff (r). Compare it with the fatigue limit of the blade before the test, adjust the load condition by the method of step (53) in embodiment 1, until σ eff (r) is equal to the fatigue limit of the blade before the test, and the calculated value of the fatigue limit of the blade is 103.20MPa.​​

[0077] The above process is repeated to obtain the calculated fatigue limit of each group of blade after the test, and the measured fatigue limit is compared to obtain the following Table 1.

[0078] Table 1

[0079]

[0080]

[0081] In Table 1, σ e is the measured value of the fatigue limit, and σ n is the calculated value of the fatigue limit. Visual analysis is performed on the data in Table 1 to obtain Figure 4 , that is, the error distribution diagram of the prediction result.

[0082] Please refer to Figure 4 , it can be seen from the figure that the error of all data points is within ±20%, and most of them are within ±10%. The modified critical distance model provided by the method has high precision in predicting the damage-coupling blade of foreign object damage-corrosion damage, and the overall prediction result is close to the measured result, which has high reliability.

Claims

1. A method for predicting the high-cycle fatigue limit of aero-engine blades, characterized in that, Includes the following steps: (1) Divide the same blades into three groups, and conduct an external damage test on the first group of blades, conduct an external damage test on the second group of blades and then conduct an immersion corrosion test, and conduct an immersion corrosion test on the third group of blades and then conduct an external damage test. Measure and record the width and depth of the damage notch of the three groups of blades respectively. (2) The fractal dimensions of the second and third groups of blade surfaces were calculated using the box dimension method; (3) Calculate the stress concentration factor of the damage notch; (4) The equivalent stress is corrected by using the fractal dimension and stress concentration coefficient to obtain the corrected critical distance model; The modified critical distance model includes: r＝L0 / 2 N f =N A (s A / s eff (L0)) k In the formula: K is the stress gradient function. T σ is the stress concentration factor, F is the fractal dimension, and σ is the stress concentration factor. eff (r) is the corrected equivalent stress, σ1(r) is the maximum stress determined by the distance r, L0 is the material critical distance, and N f For fatigue life, N A σ is the reference number of cycles for high-cycle fatigue. A For N A The corresponding stress amplitude, where k is a material constant; (5) Calculate the fatigue limit of the three sets of blades after external damage using the modified critical distance model, including the following sub-steps: (51) Measure the fatigue limit σ0 of the blade before the external damage test and immersion corrosion test; (52) Select one blade from the three groups of blades after the test, and establish a blade model based on the damage notch width and depth data of the blade. Based on the stress concentration factor and fatigue limit σ s Estimate the fatigue limit σ of this blade n : s n =σ0 / K T Where: σ n The estimated fatigue limit value is given by σ0, where σ0 is the fatigue limit of the blade before the test, and K is the value of K. T The stress concentration factor; fatigue limit σ n As a load condition, the blade model was subjected to finite element analysis to obtain the distribution curve of the maximum stress at the damage notch as a function of distance. The corrected equivalent stress σ was then calculated using the modified critical distance model. eff (r); (53) The corrected equivalent stress σ eff (r) is compared with the fatigue limit σ0. If the values ​​are the same, then the fatigue limit σ0 is... n This is the fatigue limit of the blade after the test. If the value is different, the load conditions are reset and step (52) is repeated until the corrected equivalent stress σ is obtained. eff (r) is consistent with the fatigue limit σ0.

2. The method for predicting the high-cycle fatigue limit of aero-engine blades according to claim 1, characterized in that: The external object damage test includes: using finite element analysis software to perform modal analysis on the blade to obtain the blade's natural frequency and mode shape, determining the location of external object damage, and then using external objects of different sizes and speeds to conduct external object loss tests.

3. The method for predicting the high-cycle fatigue limit of aero-engine blades according to claim 1, characterized in that: The immersion corrosion test includes immersing the blades in a corrosive solution, setting different immersion times for the same group of blades, and measuring and recording the surface morphology of the blades respectively.

4. The method for predicting the high-cycle fatigue limit of aero-engine blades according to claim 1, characterized in that: Step (2) includes: acquiring image data of the area to be calculated on the blade surface, and calculating the fractal dimension using the box-counting method. In the formula: F is the fractal dimension, N r The minimum number of boxes to cover the area to be calculated, where k is the size of the boxes.

5. The method for predicting the high-cycle fatigue limit of aero-engine blades according to claim 4, characterized in that: The image data of the area to be calculated on the blade surface is processed to obtain the grayscale surface of the image, and the fractal dimension is calculated using the grayscale surface.

6. The method for predicting the high-cycle fatigue limit of aero-engine blades according to claim 1, characterized in that: In step (3), the stress concentration factor of the damage notch is: Where: K T ρ is the stress concentration factor, d is the depth of the damage notch, ρ is the radius of the root of the damage notch, and l is the width of the damage notch.

7. The method for predicting the high-cycle fatigue limit of aero-engine blades according to claim 1, characterized in that: In step (53), the load conditions are reset using the dichotomy method.

8. The method for predicting the high-cycle fatigue limit of aero-engine blades according to claim 1, characterized in that: It also includes the following steps: (6) Measure the high-cycle fatigue limit of the three sets of blades after external damage and compare the consistency with the high-cycle fatigue limit calculated by the modified critical distance model.

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