Method for predicting high-cycle fatigue limit of aero-engine blade

Abstract

The invention provides an aero-engine blade high-cycle fatigue limit prediction method, which comprises the following steps: firstly, performing parameter measurement based on an impact-corrosion coupling damaged blade, calculating to obtain a fractal dimension and a stress concentration coefficient, and correcting equivalent stress by using the fractal dimension and the stress concentration coefficient to obtain a corrected critical distance model; and utilizing the corrected critical distance model to predict and calculate the fatigue limit of the coupling damaged blade. According to the blade high-cycle fatigue limit prediction method provided by the invention, the three-dimensional fractal dimension, the theoretical stress concentration coefficient and the stress gradient distribution function near the root of the notch are creatively integrated into the same model, and the blade fatigue limit obtained through prediction and calculation is highly consistent with the measured value of the test.

Description

Technical Field

[0001] The invention relates to the field of aeroengine blades, and in particular to a method for predicting the high cycle fatigue limit of an aeroengine blade. Background Art

[0002] When the aircraft takes off, lands, or flies at low altitude, the aircraft engine is prone to ingesting foreign objects such as stones, gravel, bolts, or rivets, which in turn causes foreign object damage (FOD). This damage is likely to occur in the local area of ​​the fan blade or compressor impeller, with more than 70% of the damage occurring at the leading edge of the blade. Common forms of damage include notches, tears, bulges, and depressions, which can lead to stress concentration at the impact point, residual stress, and microstructural damage.

[0003] In addition, changes in humidity and temperature during engine service may cause corrosion damage to the blades, significantly reducing the fatigue performance of the blades and accelerating the initiation of internal cracks. Under the coupling of corrosion and impact, the fatigue limit of the blades is significantly reduced, which may cause unpredictable fatigue fractures during operation, leading to serious flight accidents, which is a huge hidden danger for aircraft.

[0004] A series of analytical studies on the causes of aircraft failures have shown that the combined effects of corrosion damage and fatigue are the "culprits" that lead to material structure failure, which in turn causes aircraft failures and even disasters. Studying the combined effects of corrosion and impact on blade fatigue limits is of great significance for preventing and reducing flight accidents caused by coupled blade damage, reducing maintenance costs, improving flight safety, and ensuring engine combat readiness. Summary of the invention

[0005] Purpose of the invention: The purpose of the invention is to provide a method for predicting the high cycle fatigue limit of aircraft engine blades with high accuracy and good interpretability.

[0006] Technical solution: A method for predicting the high cycle fatigue limit of an aero-engine blade comprises the following steps:

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

[0008] (2) The fractal dimensions of the second and third groups of blade surfaces were calculated using the box dimension method;

[0009] (3) Calculate the stress concentration factor of the damage notch;

[0010] (4) Using fractal dimension and stress concentration factor to correct the equivalent stress, a corrected critical distance model is obtained;

[0011] (5) The fatigue limits of the three groups of blades after foreign object damage are calculated using the modified critical distance model.

[0012] Specifically, the foreign object damage test includes: using finite element analysis software to perform modal analysis on the blade to obtain the blade's natural frequency and vibration mode, determine the location of the foreign object damage, and then use foreign objects of different sizes and speeds to conduct foreign object loss tests.

[0013] Specifically, 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.

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

[0015]

[0016] Where: 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 collected image data of the area to be calculated on the blade surface is processed to obtain a grayscale surface of the image, and the fractal dimension is calculated using the grayscale surface.

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

[0019]

[0020] Where: K T is the stress concentration factor, d is the damage notch depth, ρ is the damage notch root radius, and l is the damage notch width.

[0021] Specifically, in step (4), the modified critical distance model includes:

[0022]

[0023] r=L0 / 2

[0024] N f =N A (σ A / σ eff (L0)) k

[0025] Where: is the stress gradient function, K Tis the stress concentration factor, F is the fractal dimension, σ eff (r) is the corrected equivalent stress, σ1(r) is the maximum stress determined by the distance r, L0 is the critical distance of the material, N f is fatigue life, N A is the reference number of high cycle fatigue cycles, σ A N A The corresponding stress amplitude, k is the material constant.

[0026] Specifically, step (5) includes:

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

[0028] (52) Select one of 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. s Estimate the fatigue limit σ of the blade n :

[0029] σ n =σ0 / K T

[0030] Where: n is the estimated 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 As the load condition, the blade model is assigned, and the finite element analysis of the blade is performed to obtain the distribution curve of the maximum stress at the damage notch with distance. Combined with the modified critical distance model, the modified equivalent stress σ is calculated eff (r);

[0032] (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 σ n This is the fatigue limit of the blade after the test. If the values ​​are different, reset the load conditions and repeat step (52) until the corrected equivalent stress σ eff (r) is consistent with the fatigue limit σ0.

[0033] Preferably, in step (53), the load conditions are reset using a binary method.

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

[0035] (6) The high-cycle fatigue limits of the three groups of blades after foreign object damage were measured and compared with the high-cycle fatigue limits calculated using the modified critical distance model.

[0036] Beneficial effect: Compared with the prior art, the significant effect of the present invention is: the present invention is based on the critical distance theory, that is, as the distance from the root of the blade groove increases, the effect of the increase in principal stress on 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 damaged notch, the three-dimensional fractal dimension, the theoretical stress concentration coefficient and the stress gradient distribution function near the root of the notch are innovatively integrated into the same model, and a blade fatigue limit prediction model of foreign object damage-corrosion damage coupling based on the critical distance theory is established. The model can be applied to the fan and compressor blade structures of aircraft engines under the risk of hard object impact and corrosion conditions, and the prediction results obtained are highly consistent with the test results, which provides theoretical support for evaluating blade fatigue performance in actual work. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a flow chart of the method for predicting the high cycle fatigue limit of aircraft engine blades according to embodiment 1 of the present invention.

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

[0039] Figure 3 It is a grayscale surface diagram of the blade in Example 2 of the present invention.

[0040] Figure 4 This is a distribution diagram of the prediction results errors of Example 2 of the present invention. DETAILED DESCRIPTION

[0041] The present invention is further explained below in conjunction with the accompanying drawings and specific implementation methods.

[0042] Example 1

[0043] See also Figure 1 As shown, this embodiment provides a method for predicting the high cycle fatigue limit of an aircraft engine blade, comprising the following steps:

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

[0045] In this embodiment, the finite element analysis software absqus is first used to perform modal analysis on the blade to obtain the natural frequency and vibration mode of the blade, determine the location of the foreign object damage, and then use foreign objects of different sizes and speeds to conduct a foreign object loss test.

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

[0047] (2) The fractal dimensions of the second and third groups of blade surfaces are calculated using the box dimension method. The box dimension method collects a certain area to be calculated to obtain a plane image, and then calculates the image in units of pixels.

[0048] In this embodiment, image data of the area to be calculated on the blade surface is collected, and the grayscale surface of the image is obtained after processing. The fractal dimension is calculated using the grayscale surface, and the calculation formula is as follows:

[0049]

[0050] Where: 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.

[0051] (3) After the foreign object damage test, a macro notch is generated on the leading edge of the blade, and there is an obvious stress concentration effect at the root of the notch. After the corrosion test, there is also micro-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] Where: K T is the stress concentration factor, d is the damage notch depth, ρ 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 application of the traditional critical distance method cannot simulate the local stress concentration condition well. Therefore, it is necessary to make corrections in the equivalent stress calculation, that is, to correct the maximum principal stress distribution so that the calculation result is closer to the actual situation. The equivalent stress is corrected using the fractal dimension and stress concentration coefficient to obtain a corrected critical distance model.

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

[0061] (5) The fatigue limits of the three groups of blades after foreign object damage were calculated using the modified critical distance model. Figure 2 As shown, the following steps are included:

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

[0063] (52) Select any blade from the three groups of blades after the test, establish a blade model based on the damage notch width and depth data of the blade, and estimate the fatigue limit σ of the blade based on the stress concentration factor and fatigue limit σ0 n :

[0064] σ n =σ0 / K T

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

[0066] The fatigue limit σ n As the load condition, the blade model is assigned, and the finite element analysis of the blade is performed to obtain the distribution curve of the maximum stress at the damage notch with distance. According to the previously calculated fractal dimension and theoretical stress concentration factor, the modified critical distance model is combined to calculate the modified equivalent stress σ eff (r);

[0067] (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 σ n This is the fatigue limit of the blade after the test. If the values ​​are different, reset the load conditions and repeat step (52) until the corrected equivalent stress σ eff (r) is consistent with the fatigue limit σ0.

[0068] In this embodiment, the dichotomy method is used to reset the load conditions. Specifically, when the equivalent stress is greater than the fatigue limit, the load conditions are reduced and recalculated. If the equivalent stress is equal to the fatigue limit at this time, the calculation is terminated and the result is output. If the equivalent stress is still greater than the fatigue limit at this time, the load conditions are further reduced and the calculation is repeated. If the equivalent stress is less than the fatigue limit at this time, the average value 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 is approximately equal within a set range (such as ±5%).

[0069] (6) The high-cycle fatigue limits of the three groups of blades after foreign object damage were measured and compared with the high-cycle fatigue limits calculated using the modified critical distance model to verify the effectiveness of this method.

[0070] Example 2

[0071] This embodiment provides a specific embodiment of the method for predicting the high cycle fatigue limit of an aircraft engine blade described in Embodiment 1.

[0072] One blade in the second group of blades is selected for illustration. The blade is first subjected to the foreign object loss test and then to the 96h immersion corrosion test. The measured damage notch width l = 1.907mm, the damage notch depth d = 0.553mm, the calculated damage notch root radius ρ = 0.544mm, and the stress concentration factor K T = 3.02, collect the surface image of the blade, refer to Figure 3 As shown, the grayscale surface of the leaf image is obtained, and the fractal dimension F=2.2276 is calculated.

[0073] The fatigue limit of the blade is estimated to be 190 MPa using the stress concentration factor and the fatigue limit of the blade before the test. This value is assigned to the finite element model as a load condition, and a finite element analysis is performed to obtain the variation curve of the maximum principal stress at the root of the blade notch with distance. The distance r uniquely determines a corresponding maximum principal stress σ1(r). Combined with the stress gradient correction function, the modified 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 σ eff The value of (r) is compared with the fatigue limit of the blade before the test, and the load conditions are adjusted using the method of step (53) of Example 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 was repeated to obtain the calculated value of the fatigue limit of each group of blades after the test, and compared with the measured fatigue limit to obtain the following Table 1.

[0078] Table 1

[0079]

[0080]

[0081] In Table 1, σ e is the measured value of fatigue limit, σ n is the calculated value of fatigue limit. Visual analysis of the data in Table 1 yields Figure 4 , which is the error distribution diagram of the prediction results.

[0082] See also Figure 4 As shown in the figure, it can be seen that the errors of all data points are within the range of ±20%, and most of them are within the range of ±10%. The modified critical distance model provided by this method has a high prediction accuracy for the foreign object damage-corrosion damage coupled blade, and the overall prediction results are close to the measured results and have a high degree of credibility.

Claims

1. A method for predicting the high cycle fatigue limit of an aero-engine blade, characterized in that: The following steps are involved: (1) The same blades are divided into three groups. The first group of blades is subjected to a foreign object damage test. The second group of blades is subjected to a foreign object damage test followed by an immersion corrosion test. The third group of blades is subjected to an immersion corrosion test followed by a foreign object damage test. The width and depth of the damage notch of the three groups of blades are measured and recorded 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) Using fractal dimension and stress concentration factor to correct the equivalent stress, a corrected critical distance model is obtained; (5) The fatigue limits of the three groups of blades after foreign object damage are calculated using the modified critical distance model.

2. The method for predicting the high cycle fatigue limit of an aero-engine blade according to claim 1, characterized in that: The foreign object damage test includes: using finite element analysis software to perform modal analysis on the blade to obtain the natural frequency and vibration mode of the blade, determine the location of the foreign object damage, and then use foreign objects of different sizes and speeds to perform a foreign object loss test.

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

4. The method for predicting the high cycle fatigue limit of an aero-engine blade according to claim 1, characterized in that: The step (2) comprises: collecting image data of the area to be calculated on the blade surface, and calculating the fractal dimension using the box dimension method: Where: 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.

5. The method for predicting the high cycle fatigue limit of an aero-engine blade according to claim 4, characterized in that: The collected 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 an aero-engine blade according to claim 1, characterized in that: In the step (3), the stress concentration factor of the damage notch is: Where: K T is the stress concentration factor, d is the damage notch depth, ρ is the damage notch root radius, and l is the damage notch width.

7. The method for predicting the high cycle fatigue limit of an aero-engine blade according to claim 1, characterized in that: In the step (4), the modified critical distance model includes: r＝L0 / 2 N f =N A (s A / s eff (L0)) k Where: is the stress gradient function, K T is the stress concentration factor, F is the fractal dimension, σ eff (r) is the corrected equivalent stress, σ1(r) is the maximum stress determined by the distance r, L0 is the critical distance of the material, N f is fatigue life, N A is the reference number of high cycle fatigue cycles, σ A N A The corresponding stress amplitude, k is the material constant.

8. The method for predicting the high cycle fatigue limit of an aircraft engine blade according to claim 1, characterized in that: The step (5) comprises: (51) Measure the fatigue limit σ0 of the blade before the foreign object damage test and the immersion corrosion test; (52) Select one of 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. s Estimate the fatigue limit σ of the blade n : s n =σ0 / K T Where: n is the estimated fatigue limit value, σ0 is the fatigue limit of the blade before the test, K T is the stress concentration factor; The fatigue limit σ n As the load condition, the blade model is assigned, and the finite element analysis of the blade is performed to obtain the distribution curve of the maximum stress at the damage notch with distance. Combined with the modified critical distance model, the modified equivalent stress σ is calculated 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 σ n This is the fatigue limit of the blade after the test. If the values ​​are different, reset the load conditions and repeat step (52) until the corrected equivalent stress σ eff (r) is consistent with the fatigue limit σ0.

9. The method for predicting the high cycle fatigue limit of an aircraft engine blade according to claim 8, characterized in that: In the step (53), the load conditions are reset using a binary method.

10. The method for predicting the high cycle fatigue limit of an aircraft engine blade according to claim 1, characterized in that: The following steps are also included: (6) The high-cycle fatigue limits of the three groups of blades after foreign object damage were measured and compared with the high-cycle fatigue limits calculated using the modified critical distance model.

Images