Step-by-step loading-based front and rear edge notch type foreign object damage repair-free limit verification method

Abstract

The invention discloses a step-by-step loading-based front and rear edge notch type foreign object damage maintenance-free limit verification method. The method comprises the following steps of obtaining material data and a theoretical maintenance-free limit of a blade; obtaining the inherent frequency and the vibration mode of the blade in different order modes, the coordinates of the most dangerous positions of the front and rear edges of the blade, and the stress ratio and the stress amplitude of the most dangerous positions of the front and rear edges of the blade through dynamic analog simulation; the high-cycle fatigue strength of the blade material is obtained through a high-cycle fatigue test; a high-speed ballistic impact test platform is used for impacting notch-type foreign object damage on the blade simulation piece; a complete blade simulation piece is selected to calibrate the quantitative relation between the vibration stress and the amplitude in different order modes; and carrying out a vibration fatigue assessment test on the prefabricated blade simulation piece damaged by the notch-type foreign object at different depths based on a mode of applying a vibration load step by step, and verifying a theoretical maintenance-free limit based on a judgment result. A large number of data points can be obtained by using a small number of test pieces.

Description

Technical Field

[0001] The present invention belongs to the technical field of foreign object damage tolerance design and maintenance of aeroengine blades, and particularly relates to a method for verifying the repair-free limit of foreign object damage of a leading edge and trailing edge notch type based on step-by-step loading. Background Technique

[0002] During the service process of an aeroengine, it often inhales some foreign objects due to the strong airflow pressure. When the foreign objects enter the flow path of the engine and collide with the high-speed rotating rotor components such as the fan and compressor (i.e., the blades), it will cause serious foreign object impact damage to the rotor components. The rotor components damaged by foreign objects are more likely to undergo high cycle fatigue (HCF) fracture under the combined action of aerodynamic, centrifugal force, and vibration loads, which will seriously affect the safety and economy of the aeroengine.

[0003] The design of damage tolerance and the determination of the repair-free limit are particularly important for reducing the number of blade replacements or repairs, and improving economy and combat readiness. With the understanding of the law of foreign object damage, the determination methods or prediction methods of the repair-free limit emerge in an endless stream. Although there are corresponding theoretical derivations and relatively systematic repair-free limit determination processes, the accuracy of the determined or predicted repair-free limit size has not been verified. At present, the repair-free limit size obtained through the determination method or prediction method of the repair-free limit still belongs to theoretical values or empirical values. If the determined or predicted repair-free limit size is directly applied to the actual situation, it is easy to have problems such as the repair-free limit size being too conservative or too dangerous. Therefore, the research on the verification method of the foreign object damage repair-free limit is crucial in the technical field of aeroengine blade damage tolerance design and maintenance.

[0004] The invention patent with the application publication number of CN109374449A discloses a method for determining the available limit of hard object damage of a leading edge and trailing edge crack type of a blade considering high and low cycle fatigue. First, extract the high cycle fatigue load and low cycle fatigue load from the high and low cycle composite loads borne by the blade, establish a series of crack non-propagation equivalent curves related to the stress ratio under the high cycle fatigue load and low cycle fatigue load, then determine the static stress and dynamic stress at each point position of the leading edge and trailing edge of the blade through the finite element numerical analysis method, and finally determine the crack size corresponding to the equivalent curve by comparing the positions of the static stress and dynamic stress in the crack non-propagation equivalent curve, which is the available limit of the hard object damage of the corresponding point of the leading edge and trailing edge of the blade. This technical solution is a research on the determination method of the available limit of foreign object damage (i.e., the repair-free limit), and does not involve how to verify the determined available limit. Although the theoretical available limit size is obtained, its accuracy needs to be further considered.

[0005] The invention patent with the publication number of CN115493952A discloses a test verification method for the non-repair limit of crack-type damage at the leading and trailing edges of blades, including the following steps: determining the theoretical non-repair limit; providing multiple blade simulation parts, introducing initial cracks with different lengths at different blade heights at the leading and trailing edges of the multiple blade simulation parts respectively to simulate crack-type damage; conducting vibration fatigue tests on the multiple blade simulation parts at the first three natural frequencies, and monitoring the stress levels of preset reference points on the blade simulation parts; if cracks appear on the blade simulation parts or the amplitude drops by more than the set range, and the fatigue cycle times are less than the preset number of times, record the fatigue cycle times; if no cracks appear on the blade simulation parts or no amplitude drop greater than the set range occurs, and the fatigue cycle times are greater than the preset number of times, it is considered that the damage is non-repairable. Although this technical solution can verify the non-repair limit of foreign object damage, there are still the following main technical problems: (1) Only verify the non-repair limit at different blade heights under the current service load, and there is only one test data point for one blade simulation part, which requires conducting tests on blade simulation parts with different crack depths; (2) The load level is single, that is, the obtained non-repair limit from the test only targets one load level, with a large test volume but few test data obtained; (3) Only verify the non-repair limit of crack-type foreign object damage and is not applicable to the verification of the non-repair limit of notch-type foreign object damage. Summary of the Invention

[0006] To solve the problems existing in the prior art, the present invention provides a verification method for the non-repair limit of notch-type foreign object damage at the leading and trailing edges based on step-by-step loading. The verification method includes the following steps in sequence:

[0007] Step 1: Obtain the service conditions or test conditions of the blade, the geometric shape and dimensions of the blade, and the material properties of the blade according to the design requirements; obtain the non-repair limit dimensions of notch-type foreign object damage at the most dangerous positions at the leading and trailing edges of the blade to be verified under the service conditions or test conditions according to the determination method of the non-repair limit of notch-type foreign object damage or the maintenance manual.

[0008] Step 2: Establish a blade model using finite element simulation software according to the obtained geometric shape and dimensions of the blade, apply loads to the blade model for dynamic simulation, and obtain the dynamic simulation results; according to the obtained dynamic simulation results, obtain the natural frequencies, vibration modes, and coordinates of the corresponding most dangerous positions at the leading and trailing edges of the blade in different order modes through modal analysis, and apply the loads under the service conditions or test conditions to the blade model through harmonic response analysis to obtain the stress ratio and stress amplitude at the most dangerous positions at the leading and trailing edges of the blade.

[0009] Step 3: Prepare high-cycle fatigue round bar specimens using the blade material. According to the stress ratio at the most dangerous positions of the leading and trailing edges of the blade obtained, as well as the designed service temperature and number of cycles, use a high-frequency testing machine to conduct high-cycle fatigue tests on the high-cycle fatigue round bar specimens to obtain the high-cycle fatigue strength of the blade material at this service temperature and this number of cycles.

[0010] Step 4: Prepare several blade simulation parts for prefabricated notched foreign object damage according to the obtained blade geometry and dimensions; mark the corresponding positions on the blade simulation parts according to the coordinates of the most dangerous positions of the leading and trailing edges of the blade under different order modes; impact notched foreign object damage at the marked positions on the blade simulation parts through a high-speed ballistic impact test platform. The depth of the notched foreign object damage impacted at the most dangerous positions of the leading and trailing edges of the blade under different order modes on the same blade simulation part is the same, and the depth of the notched foreign object damage impacted at the most dangerous positions of the leading and trailing edges of the blade under different order modes on different blade simulation parts increases or decreases arithmetically. The depth distribution range of the impacted notched foreign object damage needs to cover the obtained free repair limit size of the notched foreign object damage.

[0011] Step 5: Calibrate the quantitative relationship between the vibration stress and amplitude under different order modes using a complete blade simulation part. Specifically, first, paste strain gauges at the corresponding positions on the complete blade simulation part according to the coordinates of the most dangerous positions of the leading and trailing edges of the blade under different order modes; secondly, select the measurement points of the amplitude, take the part near the tip of the center of the complete blade simulation part as the measurement point of the amplitude, and use a laser displacement sensor for measurement; then, apply vibration tests by controlling the vibration table to apply amplitude. During the vibration test, measure the quantitative relationship between the vibration strain and amplitude under different order modes; finally, convert the measured quantitative relationship between the vibration strain and amplitude under different order modes into the quantitative relationship between the vibration stress and amplitude under different order modes through the obtained high-temperature elastic modulus.

[0012] Step 6: Conduct vibration fatigue assessment tests on the blade simulation parts with prefabricated notched foreign object damage of different depths based on the method of gradually applying vibration loads. The specific operations are as follows.

[0013] Step 6.1: Apply a first-level vibration load to the most dangerous position in the first-order mode of the blade simulation component with a foreign object damage of a certain depth notch under the designed service temperature and number of cycles, and conduct a vibration fatigue test; if the blade simulation component does not fail, apply a first-level vibration load to the most dangerous position in the second-order mode of the same blade simulation component and conduct a vibration fatigue test; if the blade simulation component does not fail, apply a first-level vibration load to the most dangerous position in the third-order mode of the same blade simulation component and conduct a vibration fatigue test; and so on. When the blade simulation component does not fail after applying a first-level vibration load to the most dangerous positions in different-order modes of the same blade simulation component and conducting vibration fatigue tests, raise the first-level vibration load to the second-level vibration load;

[0014] Step 6.2: For the blade simulation component with the same depth notch type of foreign object damage, repeat the operation in Step 6.1, apply a second-level vibration load to the most dangerous positions in different-order modes of the blade simulation component. When the blade simulation component does not fail after applying a second-level vibration load to the most dangerous positions in different-order modes of the same blade simulation component and conducting vibration fatigue tests, raise the second-level vibration load to the third-level vibration load; and so on until the blade simulation component fails under a certain level of vibration load;

[0015] Step 6.3: For the blade simulation components with different depth notch types of foreign object damage, repeat the operations in Steps 6.1 to 6.2, that is, complete the vibration fatigue assessment test;

[0016] Step Seven: Verify the obtained repair-free limit dimensions for the foreign object damage at the leading and trailing edges of the blade based on the determination results of the vibration fatigue assessment test. The specific operations are as follows.

[0017] Step 7.1: Mark the determination results obtained in the entire vibration fatigue assessment test in a two-dimensional coordinate system with the notch depth on the abscissa and the vibration load on the ordinate; perform function fitting on the data points determined to be failed to obtain the upper limit of the test repair-free limit, and perform function fitting on the data points with the largest notch depth and determined to be passed under the same vibration load level to obtain the lower limit of the test repair-free limit; take the area between the upper limit of the test repair-free limit and the lower limit of the test repair-free limit as the reasonable range of the repair-free limit for the foreign object damage of the notch type;

[0018] Step 7.2: Mark the theoretical repair-free limit dimensions under different vibration loads in a two-dimensional coordinate system with the notch depth on the abscissa and the vibration load on the ordinate, and plot the theoretical repair-free limit;

[0019] Step 7.3: If the theoretical repair-free limit is within the region between the upper limit and the lower limit of the test repair-free limit, the theoretical repair-free limit is verified to be reasonable; if the theoretical repair-free limit is above the upper limit of the test repair-free limit, the theoretical repair-free limit is verified to be dangerous; if the theoretical repair-free limit is below the lower limit of the test repair-free limit, the theoretical repair-free limit is verified to be conservative.

[0020] Preferably, in step one, the material properties of the blade include density, Poisson's ratio, and high-temperature elastic modulus; the method for obtaining the high-temperature elastic modulus is to prepare a uniaxial tensile specimen with the blade material and conduct a uniaxial tensile test on the uniaxial tensile specimen at the designed service temperature, and then the high-temperature elastic modulus of the blade material can be obtained.

[0021] In the present invention, the repair-free limit dimensions of the notched foreign object damage at the most dangerous positions of the blade leading and trailing edges obtained according to the repair-free limit determination method (prediction method) or maintenance manual for notched foreign object damage are all theoretical values or empirical values. Moreover, there are many existing determination methods or prediction methods for the repair-free limit of notched foreign object damage, but these determination methods or prediction methods are all theoretical values. The present invention is applicable to verifying the accuracy of the theoretical repair-free limit provided by the existing technology.

[0022] Preferably, in any of the above solutions, in step two, the finite element simulation software is Abaqus and / or Ansys; a load is applied to the blade model, and the load is the constraints and loads of the blade under actual service conditions or test conditions, including boundary conditions, aerodynamic loads, and centrifugal force loads; at least the first six-order modes are taken for analysis, and each order of mode of the blade corresponds to a most dangerous position, and the most dangerous positions in all or part of the modes are located at the leading and trailing edges of the blade.

[0023] In the present invention, in order to comprehensively cover the vibration responses that may occur in the actual operation of the engine blade, it is recommended to use the vibration stress of the first six-order modes for the available limit assessment, and the most dangerous positions in the six-order modes are extracted respectively, and each order of mode corresponds to a most dangerous position. It is possible that the most dangerous positions in all modes are located at the leading and trailing edges of the blade, or it is possible that the most dangerous positions in some modes are located at the leading and trailing edges of the blade, and the most dangerous positions in the other part of the modes are located at the blade basin and blade back; the damage types located at the leading and trailing edges of the blade are usually notched damage, and the damage types located at the blade basin and blade back are usually pit or hole damage; the pit or hole damage does not fall within the scope of the present invention's research, and the vibration stress at the leading and trailing edges of the blade in the current mode is small, so it is recommended to only consider the modes where the most dangerous positions are located at the leading and trailing edges of the blade.

[0024] When solving the vibration stress and stress amplitude at the most dangerous positions, constraints and loads under the actual service conditions or test conditions of the blade are applied to the blade model, including boundary conditions (such as the connection method between the impeller and the disk), aerodynamic loads (aerodynamic excitation caused by the non-uniformity of the internal airflow of the compressor), and centrifugal force loads (centrifugal force generated by the high-speed rotation of the blade). Since not all load conditions can be accurately obtained, in the actual simulation process, the applied load conditions can be consistent with the constraint loads considered in the repair-free limit determination method.

[0025] Preferably, in any of the above solutions, in step three, the designed service temperature is 500 °C and the number of cycles is 10 6 or 10 7 times.

[0026] Preferably, in any of the above solutions, in step four, the method of impacting the notched foreign object damage through the high-speed ballistic impact test platform is as follows: Mark the most dangerous positions at the leading and trailing edges of the blade in different order modes obtained from the dynamic simulation on the corresponding positions of the blade simulation piece, and use the marked positions as the bull's-eye of the impact test. The impact hard object is a GCr15 bearing steel ball with a diameter of 3 mm, the impact angle is 60°, the impact speed is 300 m / s, and the impact accuracy is controlled within 0.5 mm.

[0027] Preferably, in any of the above solutions, in step four, at least six depths of notched foreign object damage are analyzed, that is, at least six blade simulation pieces for prefabricating notched foreign object damage are prepared. Among them, 80% of the notch depths are less than the obtained repair-free limit size, 20% of the notch depths are greater than the obtained repair-free limit size, the depths of the notched foreign object damage on different blade simulation pieces increase or decrease arithmetically, and the maximum notch depth is greater than the obtained repair-free limit size.

[0028] In the present invention, in order to prevent the fracture at the notch in another mode due to the excessive notch depth at the most dangerous position in other modes when conducting the vibration fatigue assessment test on the notch with a certain depth at the most dangerous position in the current mode, the notch depths at the most dangerous positions in different order modes on the same blade simulation piece should not vary too much, and the error due to the impact accuracy limitation should be controlled within 0.5 mm.

[0029] Preferably, in any of the above solutions, in step five, the position 5 mm away from the blade tip is used as the measurement point of the amplitude; the product of the vibration strain and the high-temperature elastic modulus is the vibration stress.

[0030] Preferably, in any of the above solutions, in step six, the designed service temperature is 500 °C and the number of cycles is 10 6 or 10 7 times.

[0031] Preferably, in any of the above solutions, in step six, vibration loads are applied step by step, where the first-stage vibration load, second-stage vibration load, third-stage vibration load, fourth-stage vibration load, fifth-stage vibration load, and sixth-stage vibration load are respectively 40%, 50%, 60%, 70%, 80%, and 90% of the obtained high-cycle fatigue strength.

[0032] Preferably, in any of the above solutions, in step six, the determination method for the failure of the blade simulation is that during the vibration fatigue test, when the natural frequency of the blade simulation decreases and exceeds 1% in a certain mode, it is determined that a crack appears at the most dangerous position of the blade simulation in this mode, and the blade simulation fails.

[0033] The uniaxial tensile test, high-cycle fatigue test, vibration fatigue test, high-speed ballistic impact test, etc. involved in the present invention are all carried out in accordance with relevant test specifications, and the test samples used in each test are also made in accordance with relevant test specifications. The equipment required for carrying out the tests is common and highly feasible.

[0034] The present invention is based on a verification method for the no-repair limit of foreign object damage with leading and trailing edge notches based on step-by-step loading, and has the following beneficial effects:

[0035] (1) The present invention first proposes to verify the theoretical no-repair limit of foreign object damage with leading and trailing edge notches of the blade based on the idea of step-by-step loading. By the method of increasing the notch depth and vibration load gradient, a large number of passing data points and / or failure data points can be obtained with a small number of test pieces in engineering, and more accurate no-repair limit dimensions can be obtained within a larger load range.

[0036] (2) The present invention assesses the most dangerous positions in different modes, simplifies the process flow of a large number of prefabricated foreign object damages, and uses a high-speed ballistic impact test platform for impact damage, which is closer to the actual working conditions.

[0037] (3) The present invention is applicable to the verification of the no-repair limit of the most dangerous positions of the leading and trailing edges of the blade under any vibration load (the load range of step-by-step loading) and within any mode range (the first six modes).

[0038] (4) The verification method of the present invention can be used as a basis for testing the accuracy and rationality of the theoretical no-repair limit of foreign object damage with leading and trailing edge notches of the blade.

[0039] (5) The step-by-step loading idea proposed by the present invention can greatly improve the utilization rate of the blade simulation. By reasonably planning the notch depth and vibration load combination of the blade simulation, data points covering all ranges such as no-repair and repair can be obtained, and while improving the accuracy of the test no-repair limit, the no-repair limit under multiple load levels can also be obtained. Description of the Drawings

[0040] Figure 1 is a flowchart of a preferred embodiment of the method for verifying the repair-free limit of foreign object damage with leading and trailing edge notches based on step-by-step loading according to the present invention;

[0041] Figure 2 is Figure 1 the analysis diagram of the first six order modes in the shown embodiment (the most dangerous positions on the blade are shown in the figure), where: (a) is the first order mode, (b) is the second order mode, (c) is the third order mode, (d) is the fourth order mode, (e) is the fifth order mode, and (f) is the sixth order mode;

[0042] Figure 3 is Figure 1 the impact point position diagram on the blade in the shown embodiment;

[0043] Figure 4 is Figure 1 the schematic diagram of the strain gauge pasting position in the shown embodiment;

[0044] Figure 5 is Figure 1 the schematic diagram of the measuring point position of the laser displacement sensor in the shown embodiment;

[0045] Figure 6 is Figure 1 the relationship diagram between the vibration strain and the displacement of the measuring point of the laser displacement sensor in the first order mode in the shown embodiment;

[0046] Figure 7 is Figure 1 the relationship diagram between the vibration strain and the displacement of the measuring point of the laser displacement sensor in the third order mode in the shown embodiment;

[0047] Figure 8 is Figure 1 the comparison diagram between the experimental repair-free limit and the theoretical repair-free limit in the shown embodiment. Detailed implementation manners

[0048] In order to further understand the content of the present invention, the present invention will be elaborated in detail below in conjunction with specific embodiments.

[0049] As Figure 1 shown, according to a preferred embodiment of the method for verifying the repair-free limit of foreign object damage with leading and trailing edge notches based on step-by-step loading according to the present invention, the verification method includes the following steps in sequence:

[0050] Step 1: Obtain the service conditions or test conditions of the blade, the geometric shape and dimensions of the blade, and the material properties of the blade according to the design requirements; obtain the repair-free limit dimensions of the notched foreign object damage at the most dangerous positions of the leading and trailing edges of the blade to be verified under the service conditions or test conditions according to the method for determining the repair-free limit of notched foreign object damage or the maintenance manual;

[0051] Step 2: Based on the obtained blade geometry and dimensions, establish a blade model using finite element simulation software, apply loads to the blade model for dynamic simulation, and obtain the dynamic simulation results; according to the obtained dynamic simulation results, through modal analysis, obtain the natural frequencies, vibration modes, and coordinates of the most dangerous positions at the leading and trailing edges of the blade in different order modes, and apply loads under service conditions or test conditions to the blade model through harmonic response analysis to obtain the stress ratios and stress amplitudes at the most dangerous positions at the leading and trailing edges of the blade;

[0052] Step 3: Prepare high-cycle fatigue round bar specimens using the blade material. According to the stress ratios at the most dangerous positions at the leading and trailing edges of the blade obtained, as well as the designed service temperature and number of cycles, conduct high-cycle fatigue tests on the high-cycle fatigue round bar specimens using a high-frequency testing machine to obtain the high-cycle fatigue strength of the blade material at this service temperature and this number of cycles;

[0053] Step 4: According to the obtained blade geometry and dimensions, prepare several blade simulation parts for prefabricated notch-type foreign object damage; according to the coordinates of the most dangerous positions at the leading and trailing edges of the blade in different order modes obtained, mark the corresponding positions on the blade simulation parts; through a high-speed ballistic impact test platform, impact notch-type foreign object damage at the marked positions on the blade simulation parts. The depth of the notch-type foreign object damage impacted at the most dangerous positions at the leading and trailing edges of the blade in different order modes on the same blade simulation part is the same, and the depth of the notch-type foreign object damage impacted at the most dangerous positions at the leading and trailing edges of the blade in different order modes on different blade simulation parts increases or decreases arithmetically. The depth distribution range of the impacted notch-type foreign object damage needs to cover the obtained exemption limit size of the notch-type foreign object damage;

[0054] Step 5: Select a complete blade simulation part to calibrate the quantitative relationship between the vibration stress and the amplitude in different order modes. Specifically, first, according to the coordinates of the most dangerous positions at the leading and trailing edges of the blade in different order modes obtained, paste strain gauges at the corresponding positions on the complete blade simulation part; secondly, select the measurement points of the amplitude, use the part near the tip of the center of the complete blade simulation part as the measurement point of the amplitude, and measure it using a laser displacement sensor; then, apply amplitude through a vibration table to conduct a vibration test. During the vibration test, measure the quantitative relationship between the vibration strain and the amplitude in different order modes; finally, convert the measured quantitative relationship between the vibration strain and the amplitude in different order modes into the quantitative relationship between the vibration stress and the amplitude in different order modes through the obtained high-temperature elastic modulus;

[0055] Step 6: Conduct a vibration fatigue assessment test on the blade simulation parts with prefabricated notch-type foreign object damage of different depths based on the method of gradually applying vibration loads. The specific operations are as follows,

[0056] Step 6.1: Apply a first-level vibration load to the most dangerous position in the first-order mode of the blade simulation component with a foreign object damage of a certain depth notch under the designed service temperature and number of cycles, and conduct a vibration fatigue test; if the blade simulation component does not fail, apply a first-level vibration load to the most dangerous position in the second-order mode of the same blade simulation component, and conduct a vibration fatigue test; if the blade simulation component does not fail, apply a first-level vibration load to the most dangerous position in the third-order mode of the same blade simulation component, and conduct a vibration fatigue test; and so on. When the blade simulation component does not fail after applying a first-level vibration load and conducting a vibration fatigue test at the most dangerous positions in different-order modes of the same blade simulation component, raise the first-level vibration load to the second-level vibration load;

[0057] Step 6.2: For the blade simulation component with a foreign object damage of the same depth notch, repeat the operation in Step 6.1, apply a second-level vibration load to the most dangerous positions in different-order modes of the blade simulation component. When the blade simulation component does not fail after applying a second-level vibration load and conducting a vibration fatigue test at the most dangerous positions in different-order modes of the same blade simulation component, raise the second-level vibration load to the third-level vibration load; and so on until the blade simulation component fails under a certain level of vibration load;

[0058] Step 6.3: For the blade simulation components with foreign object damages of different depths notches, repeat the operations in Step 6.1 to Step 6.2, that is, complete the vibration fatigue assessment test;

[0059] Step Seven: Verify the obtained repair-free limit dimensions for the foreign object damages at the leading and trailing edges of the blade based on the determination results of the vibration fatigue assessment test. The specific operations are as follows:

[0060] Step 7.1: Mark the determination results obtained in the entire vibration fatigue assessment test in a two-dimensional coordinate system with the notch depth on the abscissa and the vibration load on the ordinate; perform function fitting on the data points determined to be failed to obtain the upper limit of the test repair-free limit, and perform function fitting on the data points with the maximum notch depth and determined to be passed under the same vibration load level to obtain the lower limit of the test repair-free limit; take the area between the upper limit of the test repair-free limit and the lower limit of the test repair-free limit as the reasonable range of the repair-free limit for the foreign object damage of the notch;

[0061] Step 7.2: Mark the theoretical repair-free limit dimensions under different vibration loads in a two-dimensional coordinate system with the notch depth on the abscissa and the vibration load on the ordinate, and plot the theoretical repair-free limit;

[0062] Step 7.3: If the theoretical exemption limit is located in the region between the upper limit and the lower limit of the experimental exemption limit, the theoretical exemption limit is verified to be reasonable; if the theoretical exemption limit is located in the region above the upper limit of the experimental exemption limit, the theoretical exemption limit is verified to be dangerous; if the theoretical exemption limit is located in the region below the lower limit of the experimental exemption limit, the theoretical exemption limit is verified to be conservative.

[0063] In Step 1, the material properties of the blade include density, Poisson's ratio, and high-temperature elastic modulus; the method for obtaining the high-temperature elastic modulus is to prepare a uniaxial tensile specimen with the blade material and conduct a uniaxial tensile test on the uniaxial tensile specimen at the designed service temperature, and then the high-temperature elastic modulus of the blade material can be obtained.

[0064] In this embodiment, the theoretical exemption limit size of the notched foreign object damage is obtained by the fatigue strength reduction method based on the worst-notch theory. The main steps include: obtaining the stress ratio and stress amplitude at the target position; obtaining the high-cycle fatigue strength and crack threshold value of the blade material; inserting notched foreign object damages with different depths at the target position to simulate the stress distribution of the blade; inserting cracks with different lengths at the maximum stress of the notched foreign object damage to calculate the maximum stress intensity factor at the crack tip; fitting the maximum stress intensity factor at the crack tip and the crack length, and substituting the fitting function into the worst-notch method model to obtain the variation relationship between the non-expansion threshold load and the crack length; fitting the maximum value of the non-expansion threshold load and the depth of the notched foreign object damage, and the fitting function is the allowable load boundary at the target position, and then the exemption limit size corresponding to different load conditions can be determined.

[0065] The key parameters involved include: elastic modulus of 162.7 GPa, high-cycle fatigue strength of 300 MPa, crack threshold value of 8.216. Finally, the theoretical exemption limit sizes of the most dangerous points under different vibration load levels are obtained as shown in Table 1.

[0066] Table 1 Theoretical exemption limit sizes under different vibration loads

[0067] Theoretical limit dimension for exemption from repair (mm) 0 0.1 0.2 0.3 0.4 Vibration stress (MPa) 327.74 259.65 233.58 216.8 204.44 Theoretical limit dimension for exemption from repair (mm) 0.5 0.6 0.8 1.0 1.2 Vibration stress (MPa) 196.46 189.04 179.26 170.35 163.02

[0068] The blade material is obtained as GH4169 according to the experiment and relevant literature review, with an elastic modulus of 162.7 GPa and a density of 8293 kg / m 3 .

[0069] In Step 2, the finite element simulation software is Abaqus and / or Ansys; a load is applied to the blade model, and the load is the constraints and loads of the blade under actual service conditions or test conditions, including boundary conditions, aerodynamic loads, and centrifugal force loads; at least the first six-order modes are taken for analysis, and each order of mode of the blade corresponds to a most dangerous position, and the most dangerous positions in all or part of the modes are located at the leading and trailing edges of the blade.

[0070] In this embodiment, the first six-order modes of the simulated blade are analyzed by using the finite element simulation software, and the analysis results under different order modes are as Figure 2 shown. The figure shows the most dangerous positions on the blade, where: (a) is the first-order mode, (b) is the second-order mode, (c) is the third-order mode, (d) is the fourth-order mode, (e) is the fifth-order mode, and (f) is the sixth-order mode. By comparing the simulation results of the first six-order modes, it is found that the most dangerous positions in the first-order, third-order, and fourth-order modes are all located at the leading and trailing edges of the blade, while the most dangerous positions in the second-order, fifth-order, and sixth-order modes are located at the blade basin and blade back. Also considering that the most dangerous positions in the first-order mode and the fourth-order mode are similar, so in this example, the first-order (first bending) and the third-order (second bending) are selected as the verification modes for the repair-free limit. It is necessary to impact two points on a blade model. The most dangerous position in the first-order mode is 25 mm from the blade root, and the most dangerous position in the second-order mode is 62.4 mm from the blade root. The impact points on the blade are as Figure 3 shown.

[0071] Since the simulated blade is a straight blade, and during the laboratory vibration test, there is no complex stress action such as centrifugal load and aerodynamic load, the key notch fatigue part can be approximated as uniaxial fatigue with a stress ratio of -1. Therefore, when determining the repair-free limit of the simulated blade, the vibration fatigue of the simulated blade is equivalently simplified to uniaxial fatigue with a stress ratio of -1 and the profile leaf type of the simulated blade section. During the simplification, the maximum stress of the undamaged leading edge opposite to the foreign object damage notch in the vibration fatigue of the simulated blade is used as the fatigue strength of the simulated blade, and it is considered that when the maximum stress value is equal to the nominal fatigue load of the tensile model with the profile leaf type, the vibration fatigue of the simulated blade is equivalent to the tensile fatigue.

[0072] In Step 3, the designed service temperature is 500 °C, the number of cycles is 10 6 or 10 7 times, and the stress ratio used in the high-cycle fatigue test is -1. The high-cycle fatigue strength of the GH4169 material at 500 °C is obtained through the test, and its maximum cyclic stress value is 300 MPa. This result provides key material property data support for the subsequent verification of the repair-free limit.

[0073] In Step 4, the method of impacting the notched foreign object damage through the high-speed ballistic impact test platform is as follows: Mark the most dangerous positions at the leading and trailing edges of the blade in different order modes obtained from dynamic simulation on the corresponding positions of the blade simulation part, and use the marked positions as the bull's-eyes for the impact test. The hard object for impact is a GCr15 bearing steel ball with a diameter of 3 mm, the impact angle is 60°, the impact speed is 300 m / s, and the impact accuracy is controlled within 0.5 mm.

[0074] Analyze at least six depths of notched foreign object damage, that is, at least prepare six blade simulation parts for prefabricating notched foreign object damage. Among them, 80% of the notch depths are less than the obtained repair-free limit size, 20% of the notch depths are greater than the obtained repair-free limit size. The depths of the notched foreign object damage on different blade simulation parts increase or decrease arithmetically, and the maximum notch depth is greater than the obtained repair-free limit size.

[0075] In this embodiment, 6 blade simulation parts were analyzed, with a total of 12 impact points, and 12 different depths of impact notches were obtained simultaneously. The maximum depth of the impact notches is distributed between 0 - 1.2 mm. The specific impact results are shown in Table 2.

[0076] Table 2 Test results of foreign object impact damage on blade simulation parts

[0077]

[0078] In Step 5, the part 5 mm away from the blade tip is used as the measurement point for the amplitude; the product of the vibration strain and the high-temperature elastic modulus is the vibration stress. In this embodiment, strain gauges are pasted at the most dangerous positions of the first and second bends for detection. The blade amplitude is measured by a laser displacement sensor. The measurement point is at the position 5 mm away from the blade tip at the blade center. The paste positions of the strain gauges are as Figure 4 shown, and the measurement point positions of the laser displacement sensor are as Figure 5 shown. Finally, the relationships between the vibration strains and the displacements of the measurement points of the laser displacement sensor in the first-order (first bend) and third-order (second bend) modes are as Figure 6 and Figure 7 shown. The relationships between the vibration strains and the amplitudes in different modes are converted into the relationships between the vibration stresses and the amplitudes through the high-temperature elastic modulus of the material at 500 °C. The specific expressions are as follows:

[0079] First bend: σ = 47.907x - 3.5989

[0080] Second bend: σ = 437.92x - 4.8052

[0081] In Step 6, the designed service temperature is 500 °C, and the number of cycles is 10 6 or 10 7times. The vibration load is applied step by step, where the first-level vibration load, the second-level vibration load, the third-level vibration load, the fourth-level vibration load, the fifth-level vibration load, and the sixth-level vibration load are 40%, 50%, 60%, 70%, 80%, and 90% of the obtained high-cycle fatigue strength respectively. The determination method for the failure of the blade simulation is that during the vibration fatigue test, when the natural frequency of the blade simulation in a certain mode drops and exceeds 1%, it is determined that a crack appears at the most dangerous position of the blade simulation in this mode, and the blade simulation fails.

[0082] In step seven, the assessment situations (pass, fail) of each blade simulation under each level of vibration load are statistically plotted, and the results are as Figure 8 shown. If the theoretical exemption limit is located in the area between the upper limit and the lower limit of the test exemption limit, the theoretical exemption limit is verified to be reasonable; if the theoretical exemption limit is located in the area above the upper limit of the test exemption limit, the theoretical exemption limit is verified to be dangerous; if the theoretical exemption limit is located in the area below the lower limit of the test exemption limit, the theoretical exemption limit is verified to be conservative.

[0083] The verification method for the exemption limit of foreign object damage with leading and trailing edge notches based on step-by-step loading in this embodiment has the following beneficial effects:

[0084] (1) For the first time, an idea based on step-by-step loading is proposed to verify the theoretical exemption limit of foreign object damage with leading and trailing edge notches of the blade, and a large number of passing data points and / or failure data points can be obtained by using a small number of test pieces. (2) It is applicable to the verification of the exemption limit at the most dangerous position of the leading and trailing edges of the blade under any vibration load (the load range of step-by-step loading) and within any modal range (the first six modes). (3) The proposed step-by-step loading idea can greatly improve the utilization rate of the blade simulation. By reasonably planning the notch depth and vibration load combination of the blade simulation, data points covering all ranges such as exemption and repair can be obtained, and while improving the accuracy of the test exemption limit, the exemption limits at multiple load levels can also be obtained.

[0085] Special note: Many parameters are involved in the technical solution of the present invention. It is necessary to comprehensively consider the synergistic effect between each parameter to obtain the beneficial effects and remarkable progress of the present invention. Moreover, the value ranges of each parameter in the technical solution are obtained through a large number of tests. For each parameter and the combination of each parameter, the inventor has recorded a large amount of test data. Due to space limitations, the specific test data are not disclosed here.

[0086] It is not difficult for those skilled in the art to understand that the method for verifying the limit of exemption from repair of foreign object damage with leading and trailing edge notches based on step-by-step loading according to the present invention includes any combination of the above-mentioned content of the invention, specific implementation manners in the specification of the present invention, and various parts shown in the drawings. Due to space limitations and to make the specification concise, the various solutions formed by these combinations are not described one by one. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for verifying the repair-free limit of leading and trailing edge notch-type foreign object damage based on step-by-step loading, characterized in that: The verification method comprises the following steps in order: Step 1: Obtain the service condition or test condition of the blade, the geometric shape and size of the blade, and the material properties of the blade according to the design requirements; obtain the notch-type foreign object damage exemption limit size at the most dangerous position of the leading and trailing edges of the blade to be verified under the service condition or test condition according to the notch-type foreign object damage exemption limit determination method or maintenance manual; Step 2: Based on the obtained blade geometry and size, a blade model is established using finite element simulation software, and a load is applied to the blade model for dynamic simulation to obtain dynamic simulation results; based on the obtained dynamic simulation results, the natural frequency, vibration mode, and coordinates of the most dangerous positions of the leading and trailing edges of the blade under different modes are obtained through modal analysis, and loads under service conditions or test conditions are applied to the blade model through harmonic response analysis to obtain the stress ratio and stress amplitude at the most dangerous positions of the leading and trailing edges of the blade; Step 3: Use blade material to prepare high-cycle fatigue round bar specimens, and use a high-frequency testing machine to perform high-cycle fatigue tests on the high-cycle fatigue round bar specimens based on the obtained stress ratio at the most dangerous positions of the leading and trailing edges of the blades and the designed service temperature and cycle number, to obtain the high-cycle fatigue strength of the blade material at the service temperature and cycle number; Step 4: According to the obtained blade geometry and size, prepare several blade simulation parts for prefabricating notch-type foreign object damage; according to the coordinates of the most dangerous positions of the leading and trailing edges of the blade under different modes, mark the corresponding positions of the blade simulation parts; impact the notch-type foreign object damage at the marked positions on the blade simulation parts through the high-speed ballistic impact test platform, the depth of the notch-type foreign object damage impacted at the most dangerous positions of the leading and trailing edges of the blades under different modes on the same blade simulation part is the same, the depth of the notch-type foreign object damage impacted at the most dangerous positions of the leading and trailing edges of the blades under different modes on different blade simulation parts increases or decreases arithmetically, and the depth distribution range of the impacted notch-type foreign object damage must cover the obtained notch-type foreign object damage free-of-repair limit size; Step 5: Use a complete blade simulation to calibrate the quantitative relationship between vibration stress and amplitude under different modes, that is, first, according to the coordinates of the most dangerous positions of the leading and trailing edges of the blade under different modes, paste strain gauges at the corresponding positions of the complete blade simulation; secondly, select the amplitude measurement point, and use the center of the complete blade simulation close to the blade tip as the amplitude measurement point, and use a laser displacement sensor to measure; then, control the vibration table to apply the amplitude to carry out a vibration test, and during the vibration test, measure the quantitative relationship between vibration strain and amplitude under different modes; finally, convert the quantitative relationship between vibration strain and amplitude under different modes into the quantitative relationship between vibration stress and amplitude under different modes through the obtained high-temperature elastic modulus; Step 6: Perform vibration fatigue test on prefabricated blade simulation parts with different depths of notch-type foreign object damage by applying vibration load step by step. The specific operation is as follows: Step 6.1: Under the designed service temperature and cycle number, apply a primary vibration load to the most dangerous position of the first-order mode of the blade simulation component with a certain depth of notch-type foreign object damage, and conduct a vibration fatigue test; If the blade simulation component does not fail, a first-order vibration load is applied to the most dangerous position of the second-order mode of the same blade simulation component, and a vibration fatigue test is performed; If the blade simulation component does not fail, a first-order vibration load is applied to the most dangerous position of the third-order mode of the same blade simulation component, and a vibration fatigue test is performed; By analogy, when the first-order vibration load is applied to the most dangerous position of the same blade simulation under different modes and a vibration fatigue test is carried out, the blade simulation does not fail. At this time, the first-order vibration load is increased to the second-order vibration load; Step 6.2: For the blade simulation piece with the same depth notch type foreign object damage, repeat the operation of step 6.1, and apply the secondary vibration load to the most dangerous position of the blade simulation piece under different order modes. When the secondary vibration load is applied to the most dangerous position of the same blade simulation piece under different order modes and the vibration fatigue test is carried out, the blade simulation piece does not fail. At this time, the secondary vibration load is increased to the third level vibration load; and so on, until the blade simulation piece fails under a certain level of vibration load; Step 6.3: Repeat the operations from step 6.1 to step 6.2 for the blade simulation parts with different depths of notch-type foreign object damage, thus completing the vibration fatigue assessment test; Step 7: Based on the results of the vibration fatigue assessment test, verify the obtained limit size of the blade leading and trailing edge notch type foreign object damage free of repair. The specific operation is as follows: Step 7.1: Mark the judgment results obtained in the entire vibration fatigue assessment test in a two-dimensional coordinate system with the notch depth as the horizontal axis and the vibration load as the vertical axis; perform function fitting on the data points judged as failures to obtain the upper limit of the test exemption limit; perform function fitting on the data points with the largest notch depth and judged as passed under the same vibration load level to obtain the lower limit of the test exemption limit; take the area between the upper limit of the test exemption limit and the lower limit of the test exemption limit as the reasonable range of the notch-type foreign object damage exemption limit; Step 7.2: Mark the theoretical maintenance-free limit size under different vibration loads in a two-dimensional coordinate system with the abscissa being the notch depth and the ordinate being the vibration load, and draw the theoretical maintenance-free limit; Step 7.3: If the theoretical exemption limit is located in the area between the upper limit of the experimental exemption limit and the lower limit of the experimental exemption limit, then the theoretical exemption limit is verified to be reasonable; if the theoretical exemption limit is located in the area above the upper limit of the experimental exemption limit, then the theoretical exemption limit is verified to be dangerous; if the theoretical exemption limit is located in the area below the lower limit of the experimental exemption limit, then the theoretical exemption limit is verified to be conservative.

2. The method for verifying the repair-free limit of leading and trailing edge notch-type foreign object damage based on step-by-step loading according to claim 1 is characterized in that: In step one, the material properties of the blade include density, Poisson's ratio and high-temperature elastic modulus; the method for obtaining the high-temperature elastic modulus is to prepare a uniaxial tensile specimen using the blade material, and perform a uniaxial tensile test on the uniaxial tensile specimen at a designed service temperature to obtain the high-temperature elastic modulus of the blade material.

3. The method for verifying the repair-free limit of leading and trailing edge notch-type foreign object damage based on step-by-step loading according to claim 2 is characterized in that: In step 2, the finite element simulation software is Abaqus and / or Ansys; a load is applied to the blade model, and the load is the constraint and load of the blade under actual service conditions or test conditions, including boundary conditions, aerodynamic loads and centrifugal loads; At least the first six modes are taken for analysis. Each mode of the blade corresponds to a most dangerous position. The most dangerous position in all or part of the modes is located at the leading and trailing edges of the blade.

4. The method for verifying the repair-free limit of leading and trailing edge notch-type foreign object damage based on step-by-step loading according to claim 3 is characterized in that: In step 3, the designed service temperature is 500℃ and the number of cycles is 10 6 or 10 7 Second-rate.

5. The method for verifying the repair-free limit of leading and trailing edge notch-type foreign object damage based on step-by-step loading according to claim 4 is characterized in that: In step 4, the method of impacting notch-type foreign object damage through a high-speed ballistic impact test platform is to mark the most dangerous positions of the leading and trailing edges of the blades under different modes obtained by dynamic simulation at the corresponding positions of the blade simulation parts, and use the marked positions as the bull's eye of the impact test. A GCr15 bearing steel ball with a diameter of 3 mm is selected as the impact hard object, the impact angle is 60°, the impact speed is 300 m / s, and the impact accuracy is controlled within 0.5 mm.

6. The method for verifying the repair-free limit of leading and trailing edge notch-type foreign object damage based on step-by-step loading according to claim 5 is characterized in that: In step four, at least six depths of notch-type foreign object damage are impacted for analysis, that is, at least six blade simulation parts for prefabricated notch-type foreign object damage are prepared, of which 80% of the notch depths are smaller than the obtained repair-free limit size, and 20% of the notch depths are larger than the obtained repair-free limit size, and the depths of notch-type foreign object damage on different blade simulation parts increase or decrease arbitrarily, and the maximum notch depth is larger than the obtained repair-free limit size.

7. The method for verifying the repair-free limit of leading and trailing edge notch-type foreign object damage based on step-by-step loading according to claim 6 is characterized in that: In step 5, the position 5 mm away from the blade tip is used as the measurement point of the amplitude; the product of the vibration strain and the high-temperature elastic modulus is the vibration stress.

8. The method for verifying the repair-free limit of leading and trailing edge notch-type foreign object damage based on step-by-step loading according to claim 7 is characterized in that: In step 6, the designed service temperature is 500℃ and the number of cycles is 10 6 or 10 7 Second-rate.

9. The method for verifying the repair-free limit of leading and trailing edge notch-type foreign object damage based on step-by-step loading according to claim 8, characterized in that: In step six, vibration loads are applied step by step, wherein the first vibration load, the second vibration load, the third vibration load, the fourth vibration load, the fifth vibration load, and the sixth vibration load are 40%, 50%, 60%, 70%, 80%, and 90% of the obtained high cycle fatigue strength, respectively.

10. The method for verifying the repair-free limit of leading and trailing edge notch-type foreign object damage based on step-by-step loading according to claim 9, characterized in that: In step six, the method for determining whether the blade simulation component has failed is that, during the vibration fatigue test, when the natural frequency of the blade simulation component in a certain mode decreases and exceeds 1%, it is determined that a crack has appeared at the most dangerous position of the blade simulation component in the mode, and the blade simulation component has failed.

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