Combined load fatigue test method for vehicle axial flow fan blade

Abstract

The invention discloses a combined load fatigue test method for a vehicle axial fan blade, which comprises the following steps: S1, cutting a fan impeller into a root-connected blade sample piece with a central angle of 60 degrees by using a water jet cutter, and keeping the integrity of a blade root, S2, immersing the blade into an aluminum powder resin mixture according to a ratio of 3: 1, and carrying out casting molding, grinding to enable the surface roughness Ra to be less than or equal to 3.2 microns so as to form a clamping plane, clamping the casting body to a Z250SN fatigue testing machine according to the 18-degree grooving datum line of the casting body, and ensuring that the load direction is aligned with the main stress direction of the blade root; the device is scientific and reasonable in structure and safe and convenient to use, direction deviation of traditional axial loading is eliminated due to the 18-degree dip angle clamp, the stress of the blade root danger area is accurately correlated in combination with the three-level stress mapping model, and the prediction error is reduced to be smaller than 3% from larger than 15%.

Description

Technical Field

[0001] The present invention relates to the technical field of composite load fatigue testing of fan blades, and in particular to a composite load fatigue testing method for axial flow fan blades for vehicles. Background Art

[0002] The existing technology is "GB / T 3075-2008 Axial Force Control Method for Fatigue Testing of Metallic Materials." This method uses a flat fixture to axially clamp the fan blade specimen, applies a constant-amplitude tensile load via a hydraulic testing machine, and records the number of fracture cycles to plot the SN curve. The device structure is a unidirectional load-applying system with no angle adjustment function.

[0003] Ignoring the angle between the actual principal stress direction at the blade root and the blade axis results in a significant deviation between the test load direction and the actual combined centrifugal load direction. Furthermore, strain gauge monitoring cannot accurately characterize the stress in the critical area of ​​the blade root, and single-stage load testing requires a large number of specimens covering the entire operating range, resulting in large life prediction errors and long test cycles.

[0004] Therefore, the present invention proposes a composite load fatigue test method for automotive axial fan blades. By accurately aligning the principal stress direction with an inclination fixture, establishing a mapping model between the critical stress of the blade root and the surface measuring points, and designing a multi-stage stepped acceleration load, accurate prediction and efficient testing of fatigue life under composite centrifugal loads can be achieved. Summary of the Invention

[0005] The present invention provides a composite load fatigue test method for an axial flow fan blade for a vehicle, which can effectively solve the problems raised in the above-mentioned background technology.

[0006] To achieve the above object, the present invention provides the following technical solution: a composite load fatigue test method for an axial flow fan blade for a vehicle, comprising the following steps:

[0007] S1. First, use a water jet to cut the fan impeller into root-connected blade samples with a central angle of 60°, preserving the integrity of the blade root;

[0008] S2. The blade is then immersed in an aluminum powder-resin mixture with a ratio of 3:1, cast and formed, and polished to a surface roughness of Ra ≤ 3.2 μm to form a clamping surface. The blade is then clamped to a Z250SN fatigue testing machine based on the 18° groove reference line of the cast body, ensuring that the load direction is aligned with the principal stress direction of the blade root.

[0009] S3. Apply six levels of stepped maximum loads in sequence: 36kN, 41kN, 45kN, 50kN, 54kN, and 59kN. At each level, a stress ratio R = 0.1, a minimum load = 10% of the maximum load, and a sinusoidal pull-pull cyclic load with a frequency of 10 Hz are used to simulate the composite centrifugal force of actual working conditions. The speed is kept stable to maintain uniform acceleration inertia force.

[0010] During the test, three strain gauges near the blade root were used to collect surface stress data in real time, with point A being 10 mm from the leading edge, point B facing the middle of the blade root section, and point C being 10 mm from the trailing edge, until the specimen broke.

[0011] S5. Finally, based on the life data under six levels of load, a quantitative mapping model of load, life, and operating speed is established through the inverse power law equation to achieve accurate prediction of fatigue life under composite centrifugal load.

[0012] According to the above technical solution, the dedicated fixture system of the test method is composed of a rectangular casting body formed by casting a mixture of aluminum powder and resin in a ratio of 3:1. The casting body completely covers the blade airfoil section, leaving only the blade root disk part exposed.

[0013] The upper surface of the casting body is machined with an 18° directional groove with a depth of 0.5mm. The centerline of the groove coincides with the extension line of the blade trailing edge, which is used to accurately align the load axis of the testing machine. The hydraulic clamping unit consists of two sets of upper and lower clamps. The upper clamp clamps the two sides of the casting body, and the lower clamp fixes the flat surface of the disk at the root of the blade.

[0014] Three strain gauges are pasted on the front of the blade at a distance of 10±1mm from the root edge, and the stress data of the actual failure point is captured through point B.

[0015] According to the above technical solution, the method for determining the maximum load value is:

[0016] Finite element simulation is used to analyze the fan blades at a steady speed (7000-14000 rpm) and uniform acceleration (angular acceleration 10.5 rad / s 2 ) stress distribution under combined loads, and the middle of the blade root section is determined to be the dangerous area;

[0017] A quadratic relationship between the critical blade root stress σ and the operating speed φ was established. Taking the stress of 62.32 MPa at the extreme operating condition (11,000 rpm) as the benchmark, six step stress levels (49.86 MPa to 81.02 MPa) were generated with a gradient of ±10%.

[0018] Combined with the load-stress mapping relationship calibrated in the pre-test, the load values ​​of the testing machine were finally determined to be 36kN, 41kN, 45kN, 50kN, 54kN, and 59kN, covering the extreme working stress range of 80%-130%.

[0019] According to the above technical solution, the test method for determining the loading direction is as follows:

[0020] Based on extreme working conditions (11000rpm+10.5rad / s 2 ) Finite element simulation results, extract the maximum principal stress direction vector of the blade root section;

[0021] Through mesh geometry analysis, the angle between the principal stress direction and the blade trailing edge is calculated to be 18°;

[0022] The angle stability under rated operating conditions (7000 rpm) and fracture operating conditions (14000 rpm) was further verified, and it was finally determined that the load direction of the testing machine needed to form a constant angle of 18° with the trailing edge of the blade.

[0023] According to the above technical solution, the method for determining the position of the strain gauge measuring point in the test method is as follows:

[0024] Based on the simulated stress cloud map, the center of the blade root section is identified as the point of maximum stress. However, strain gauges cannot be directly attached to this area. Three measuring points are selected on the front of the blade close to the edge of the blade root:

[0025] Point A: 10±1mm from the leading edge of the blade;

[0026] Point B: facing the middle of the blade root section;

[0027] Point C: 10±1mm from the trailing edge of the blade;

[0028] Verified by stress data from multiple working conditions, the stress at point B is in a fixed ratio to the stress in the dangerous area of ​​the blade root, which can accurately characterize the state of the failure point.

[0029] According to the above technical solution, the sample preparation method of the test method is:

[0030] The water jet cutting technology is used to divide the complete impeller into root-connected blade samples with a central angle of 60°, retaining the blade root and part of the disc structure; to solve the problem of clamping the airfoil blades, an aluminum powder and resin mixture is used to cast and wrap the airfoil section of the blade to form an 80mm×50mm×40mm rectangular casting body, which is polished to a surface roughness of Ra≤3.2μm.

[0031] According to the above technical solution, the clamping and load control of the test method are as follows:

[0032] The casting body is clamped to the Z250SN testing machine according to the 18° groove reference line. The upper and lower clamps fix the casting body plane and the blade root disk respectively; the load control adopts the pull-pull cycle mode:

[0033] Stress ratio: R = 0.1 (minimum load = 10% of maximum load);

[0034] Waveform: 10Hz sine curve;

[0035] Gradient loading: 6 levels of maximum load are applied in sequence, and each level is continued until the sample breaks.

[0036] According to the above technical solution, the data collection and processing of the test method are as follows:

[0037] Strain data from three measurement points were recorded in real time using a KYOWA KFEM-5-120 strain gauge and an NI 9237 acquisition board with a sampling rate of 50 kS / s.

[0038] Based on the material mechanics formula σ=E·εσ=E·ε (elastic modulus E=75GPa), it is converted into stress value; the testing machine simultaneously records the load, frequency and number of cycles.

[0039] Compared with the prior art, the present invention has the following beneficial effects: the present invention has a scientific and reasonable structure and is safe and convenient to use:

[0040] 1. Life prediction accuracy is improved to over 97%: The 18° inclination fixture eliminates the directional deviation of traditional axial loading, and combined with a three-level stress mapping model, accurately correlates the stress in the blade root danger zone, reducing the prediction error from >15% to <3%.

[0041] 2) Test cycle shortened by 83.3%: Due to the 6-stage stepped load design (36-59kN gradient) covering the full stress range, only 6 specimens are needed to complete the task of traditional single-stage testing of 20+ specimens, shortening the test cycle from 3 months to 2 weeks;

[0042] 3. 100% weak point identification rate: Because the strain gauge at point B faces the middle of the blade root section and the casting fixture ensures consistent load direction, all failures in the Level 6 test occurred in this area, completely eliminating the blind spot problem of traditional surface monitoring;

[0043] 4. Cost reduction: The aluminum powder resin casting process replaces the universal joint adjustment solution, and the fixture is reusable, and the overall cost is lower than that of traditional custom fixtures. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0045] In the attached figure:

[0046] Figure 1 It is a schematic diagram of the blade sample and casting body fixture structure of the present invention;

[0047] Figure 2 It is a schematic diagram of the structure of the blade sample and the casting body fixture model of the present invention;

[0048] Figure 3 2. It is a schematic diagram of the front structure of the fan blade of the present invention, showing the stress simulation result under rated working conditions;

[0049] Figure 4 2. It is a schematic diagram of the back structure of the fan blade of the present invention, showing the stress simulation result under rated working conditions;

[0050] Figure 5 2. It is a schematic diagram of the cross-sectional structure of the blade root of the fan blade of the present invention under rated working conditions according to the stress simulation result;

[0051] Figure 6 This is a schematic diagram of the front structure of the blade in the direction of the principal stress under the extreme working conditions of the present invention;

[0052] Figure 7 Schematic diagram of the back structure of the blade in the direction of the principal stress under the extreme working conditions of the present invention;

[0053] Figure 8 This is a schematic diagram of the attachment position of the strain gauge on the blade surface of the present invention;

[0054] Figure 9 This is a schematic structural diagram of the finite element model of the ZL390 fan impeller of the present invention;

[0055] Figure 10 is a schematic diagram of a load application curve in a test of the present invention, taking a maximum load of 50 kN as an example;

[0056] Figure 11 Schematic diagram of the method of marking a blade sample of the present invention;

[0057] Figure 12 is a schematic diagram of a blade sample after being marked with lines according to the present invention;

[0058] Figure 13 It is a schematic diagram of the experimental SN curve of the present invention;

[0059] Figure 14 Schematic diagram of the experimental SN curve of the modified polynomial fitting of the present invention;

[0060] Figure 15 Schematic diagram of the experimental SN curve fitted by the modified inverse power law model of the present invention;

[0061] Figure 16 This is a schematic diagram of the blade reliability enhancement test principle of the present invention;

[0062] Figure 17 is a schematic diagram of the strain measurement point area extracted from the blade simulation results of the present invention;

[0063] Figure 18 It is a schematic diagram of the principal stress directions of the strain measurement points extracted from the blade simulation results of the present invention;

[0064] Figure 19 It is a schematic diagram of the pasting position of the strain gauge on the test bench of the present invention. DETAILED DESCRIPTION

[0065] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0066] Example:

[0067] The present invention provides a technical solution, a composite load fatigue test method for an axial flow fan blade for a vehicle, comprising the following steps:

[0068] S1. First, use a water jet to cut the fan impeller into root-connected blade samples with a central angle of 60°, preserving the integrity of the blade root;

[0069] S2. The blade is then immersed in an aluminum powder-resin mixture with a ratio of 3:1, cast and formed, and polished to a surface roughness of Ra ≤ 3.2 μm to form a clamping surface. The blade is then clamped to a Z250SN fatigue testing machine based on the 18° groove reference line of the cast body, ensuring that the load direction is aligned with the principal stress direction of the blade root.

[0070] S3. Apply six levels of stepped maximum loads in sequence: 36kN, 41kN, 45kN, 50kN, 54kN, and 59kN. At each level, a stress ratio R = 0.1, a minimum load = 10% of the maximum load, and a sinusoidal pull-pull cyclic load with a frequency of 10 Hz are used to simulate the composite centrifugal force of actual working conditions. The speed is kept stable to maintain uniform acceleration inertia force.

[0071] During the test, three strain gauges near the blade root were used to collect surface stress data in real time, with point A being 10 mm from the leading edge, point B facing the middle of the blade root section, and point C being 10 mm from the trailing edge, until the specimen broke.

[0072] S5. Finally, based on the life data under six levels of load, a quantitative mapping model of load, life, and operating speed is established through the inverse power law equation to achieve accurate prediction of fatigue life under composite centrifugal load.

[0073] According to the above technical solution, if Figure 1-2 As shown in the figure, the dedicated fixture system of the test method consists of a rectangular casting body (80 mm long × 50 mm wide × 40 mm high) formed by casting a mixture of aluminum powder and resin in a ratio of 3:1. It completely covers the blade airfoil section (from the blade surface to the blade tip), leaving only the blade root disk part exposed.

[0074] The upper surface of the casting body is machined with an 18° directional groove with a depth of 0.5mm (based on the constant angle between the principal stress direction and the blade trailing edge determined by finite element simulation of the blade root cross-section). The centerline of the groove coincides with the extension line of the blade trailing edge, which is used to accurately align the load axis of the testing machine. The hydraulic clamping unit consists of two sets of upper and lower clamps. The upper clamp clamps the two sides of the casting body, and the lower clamp fixes the flat surface of the blade root wheel, forming a closed force transmission chain of "testing machine piston → upper clamp → casting body → blade airfoil section → blade root → lower clamp".

[0075] Three strain gauges are attached to the front of the blade at 10±1mm from the root edge (point A is 10mm from the leading edge, point B is facing the dangerous area in the middle of the root section, and point C is 10mm from the trailing edge). The stress data of the actual failure point is captured through point B.

[0076] Function and position relationship: The casting body transforms the special-shaped blade into a standard clamping plane, eliminating the line contact stress concentration;

[0077] 18° notches ensure that the load direction is parallel to the principal stress direction of the blade root (deviation < 1°);

[0078] The strain gauge layout covers the stress gradient change area in the dangerous area of ​​the blade root, realizing accurate monitoring of stress under complex loads.

[0079] According to the above technical solution, if Figure 3-5 As shown in the figure, the method for determining the maximum load value is:

[0080] Finite element simulation is used to analyze the fan blades at a steady speed (7000-14000 rpm) and uniform acceleration (angular acceleration 10.5 rad / s 2 ) stress distribution under combined loads, and the middle of the blade root section is determined to be the dangerous area;

[0081] A quadratic relationship between the critical blade root stress σ and the operating speed φ was established. Taking the stress of 62.32 MPa at the extreme operating condition (11,000 rpm) as the benchmark, six step stress levels (49.86 MPa to 81.02 MPa) were generated with a gradient of ±10%.

[0082] Combined with the load-stress mapping relationship calibrated in the pre-test, the load values ​​of the testing machine were finally determined to be 36kN, 41kN, 45kN, 50kN, 54kN, and 59kN, covering the extreme working stress range of 80%-130%.

[0083] According to the above technical solution, if Figure 6-7 As shown in the figure, the test method loading direction determination method is:

[0084] Based on extreme working conditions (11000rpm+10.5rad / s 2 ) Finite element simulation results, extract the maximum principal stress direction vector of the blade root section;

[0085] Through mesh geometry analysis, the angle between the principal stress direction and the blade trailing edge is calculated to be 18°;

[0086] The angle stability under rated operating conditions (7000 rpm) and fracture operating conditions (14000 rpm) was further verified, and it was finally determined that the load direction of the testing machine needed to form a constant angle of 18° with the trailing edge of the blade.

[0087] According to the above technical solution, if Figure 8 As shown in the figure, the method for determining the position of the strain gauge measuring point of the test method is:

[0088] Based on the simulated stress cloud map, the center of the blade root section is identified as the point of maximum stress. However, strain gauges cannot be directly attached to this area. Three measuring points are selected on the front of the blade close to the edge of the blade root:

[0089] Point A: 10±1mm from the leading edge of the blade;

[0090] Point B: Opposite the middle of the blade root section (directly above the danger zone);

[0091] Point C: 10±1mm from the trailing edge of the blade;

[0092] Verified by stress data from multiple working conditions, the stress at point B is in a fixed ratio to the stress in the dangerous area of ​​the blade root, which can accurately characterize the state of the failure point.

[0093] According to the above technical solution, if Figure 9 As shown in the figure, the sample preparation method of the test method is:

[0094] The water jet cutting technology is used to split the complete impeller into root-connected blade samples with a central angle of 60°, retaining the blade root and part of the disc structure; to solve the problem of clamping the airfoil blades, an aluminum powder and resin mixture (mass ratio 3:1) is used to cast and wrap the airfoil section of the blade to form an 80mm×50mm×40mm rectangular casting body, which is polished to a surface roughness Ra≤3.2μm to ensure surface contact force transmission of the hydraulic chuck.

[0095] According to the above technical solution, if Figure 10-12 As shown, the clamping and load control of the test method are:

[0096] The casting body is clamped to the Z250SN testing machine according to the 18° groove reference line. The upper and lower clamps fix the casting body plane and the blade root disk respectively; the load control adopts the pull-pull cycle mode:

[0097] Stress ratio: R = 0.1 (minimum load = 10% of maximum load);

[0098] Waveform: 10Hz sine curve;

[0099] Gradient loading: 6 levels of maximum load are applied in sequence, and each level is continued until the sample breaks.

[0100] According to the above technical solution, if Figure 13-15 As shown, the data collection and processing of the test method are:

[0101] Strain data from three measurement points were recorded in real time using a KYOWA KFEM-5-120 strain gauge and an NI 9237 acquisition board with a sampling rate of 50 kS / s.

[0102] Based on the material mechanics formula σ=E·εσ=E·ε (elastic modulus E=75GPa), it is converted into stress value; the testing machine simultaneously records the load, frequency and number of cycles.

[0103] Fatigue life-operating condition mapping relationship

[0104] After obtaining the results shown in Table 1, a stress-life mapping model was established.

[0105] Table 1 Fatigue life data of fan blade strengthening test

[0106]

[0107]

[0108] According to the data in Table 1, the maximum value of each load level, i.e., the maximum load Fmax, is used as the ordinate, and the logarithmic fatigue life lgN is used as the abscissa. The six data points are fitted with a curve to obtain the following test SN curve: Figure 13 shown.

[0109] according to Figure 13 As shown in Table 1, at the 6th load level, i.e., 59kN, the blade life is already lower than 105, which is considered to have entered the low-cycle fatigue life zone. The blade fatigue failure at this time belongs to low-cycle fatigue, while the fatigue failure of fan blades in actual operation belongs to high-cycle fatigue, that is, the blade life is often higher than 105 cycles. Therefore, the test results at the 7th load level are removed, and the test results at the 1st to 5th load levels are used to redraw the SN curve. The SN curves of the maximum load Fmax and the logarithmic fatigue life lgN are drawn using the polynomial fitting method and the inverse power law model fitting method respectively. Figure 14 and Figure 15 shown.

[0110] contrast Figure 14 and Figure 15 It can be found that the SN curve fitted by the inverse power law model is more consistent with the trend of the original experimental data points. From a quantitative point of view, the calculation Figure 14 The relative variance of the polynomial fitting curve relative to the original data is R = 0.041794. Figure 15 The relative variance of the inverse power law model fitting curve relative to the original data is R = 0.029107. Obviously, 0.029107 < 0.041794. The inverse power law model has a better fitting effect on the fan blade SN curve. Figure 15 The inverse power law model curve is used to obtain the SN curve function expression of the fan life and load in the fan blade strengthening test:

[0111] F max=611.5058×N -0.209 (1)

[0112]

[0113] Combining equations (1) and (2), the relationship between the fan life and the average stress σ in the middle of the blade root section in the fan blade strengthening test is obtained as follows:

[0114]

[0115] Then, the empirical formula of the average stress σ in the middle of the root section and the stable speed φ of the fan under working condition is substituted into formula (3) to obtain the relationship between the fan life and the stable speed under corresponding working condition in the strengthening test:

[0116]

[0117] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A composite load fatigue test method for automotive axial flow fan blades, characterized by: The steps include: S1. First, use a water jet to cut the fan impeller into root-connected blade samples with a central angle of 60°, preserving the integrity of the blade root; S2. The blade is then immersed in an aluminum powder-resin mixture with a ratio of 3:1, cast and formed, and polished to a surface roughness of Ra ≤ 3.2 μm to form a clamping surface. The blade is then clamped to a Z250SN fatigue testing machine based on the 18° groove reference line of the cast body, ensuring that the load direction is aligned with the principal stress direction of the blade root. S3. Apply six levels of stepped maximum loads in sequence: 36kN, 41kN, 45kN, 50kN, 54kN, and 59kN. At each level, a stress ratio R = 0.1, a minimum load = 10% of the maximum load, and a sinusoidal pull-pull cyclic load with a frequency of 10 Hz are used to simulate the composite centrifugal force of actual working conditions. The speed is kept stable to maintain uniform acceleration inertia force. During the test, three strain gauges near the blade root were used to collect surface stress data in real time, with point A being 10 mm from the leading edge, point B facing the middle of the blade root section, and point C being 10 mm from the trailing edge, until the specimen broke. S5. Finally, based on the life data under six levels of load, a quantitative mapping model of load, life, and operating speed is established through the inverse power law equation to achieve accurate prediction of fatigue life under composite centrifugal load.

2. A composite load fatigue test method for an axial flow fan blade for a vehicle according to claim 1, characterized in that: The special fixture system of the test method is composed of a rectangular casting body formed by casting a mixture of aluminum powder and resin in a ratio of 3:

1. It completely covers the blade airfoil section and only exposes the blade root disk part; The upper surface of the casting body is machined with an 18° directional groove with a depth of 0.5mm. The centerline of the groove coincides with the extension line of the blade trailing edge, which is used to accurately align the load axis of the testing machine. The hydraulic clamping unit consists of two sets of upper and lower clamps. The upper clamp clamps the two sides of the casting body, and the lower clamp fixes the flat surface of the disk at the root of the blade. Three strain gauges are pasted on the front of the blade at a distance of 10±1mm from the root edge, and the stress data of the actual failure point is captured through point B.

3. A composite load fatigue test method for an axial flow fan blade for a vehicle according to claim 1, characterized in that: Method for determining the maximum load value: Finite element simulation is used to analyze the fan blades at a steady speed (7000-14000 rpm) and uniform acceleration (angular acceleration 10.5 rad / s 2 ) stress distribution under combined loads, and the middle of the blade root section is determined to be the dangerous area; A quadratic relationship between the critical blade root stress σ and the operating speed φ was established. Taking the stress of 62.32 MPa at the extreme operating condition (11,000 rpm) as the benchmark, six step stress levels (49.86 MPa to 81.02 MPa) were generated with a gradient of ±10%. Combined with the load-stress mapping relationship calibrated in the pre-test, the load values ​​of the testing machine were finally determined to be 36kN, 41kN, 45kN, 50kN, 54kN, and 59kN, covering the extreme working stress range of 80%-130%.

4. A composite load fatigue test method for an axial flow fan blade for a vehicle according to claim 1, characterized in that: Method for determining the loading direction of the test method: Based on extreme working conditions (11000rpm+10.5rad / s 2 ) Finite element simulation results, extract the maximum principal stress direction vector of the blade root section; Through mesh geometry analysis, the angle between the principal stress direction and the blade trailing edge is calculated to be 18°; The angle stability under rated operating conditions (7000 rpm) and fracture operating conditions (14000 rpm) was further verified, and it was finally determined that the load direction of the testing machine needed to form a constant angle of 18° with the trailing edge of the blade.

5. The composite load fatigue test method for an axial flow fan blade for a vehicle according to claim 1, characterized in that: Method for determining the position of strain gauge measuring points in the test method: Based on the simulated stress cloud map, the center of the blade root section is identified as the point of maximum stress. However, strain gauges cannot be directly attached to this area. Three measuring points are selected on the front of the blade close to the edge of the blade root: Point A: 10±1mm from the leading edge of the blade; Point B: facing the middle of the blade root section; Point C: 10±1mm from the trailing edge of the blade; Verified by stress data from multiple working conditions, the stress at point B is in a fixed ratio to the stress in the dangerous area of ​​the blade root, which can accurately characterize the state of the failure point.

6. A composite load fatigue test method for an axial flow fan blade for a vehicle according to claim 1, characterized in that: Sample preparation method for the test method: The water jet cutting technology is used to divide the complete impeller into root-connected blade samples with a central angle of 60°, retaining the blade root and part of the disc structure; to solve the problem of clamping the airfoil blades, an aluminum powder and resin mixture is used to cast and wrap the airfoil section of the blade to form an 80mm×50mm×40mm rectangular casting body, which is polished to a surface roughness of Ra≤3.2μm.

7. A composite load fatigue test method for an axial flow fan blade for a vehicle according to claim 1, characterized in that: Clamping and load control for the test method described: The casting body is clamped to the Z250SN testing machine according to the 18° groove reference line. The upper and lower clamps fix the casting body plane and the blade root disk respectively; the load control adopts the pull-pull cycle mode: Stress ratio: R = 0.1 (minimum load = 10% of maximum load); Waveform: 10Hz sine curve; Gradient loading: 6 levels of maximum load are applied in sequence, and each level is continued until the sample breaks.

8. The composite load fatigue test method for an axial flow fan blade for a vehicle according to claim 1, characterized in that: Data collection and processing of the test method: Strain data from three measurement points were recorded in real time using a KYOWA KFEM-5-120 strain gauge and an NI 9237 acquisition board with a sampling rate of 50 kS / s. Based on the material mechanics formula σ=E·εσ=E·ε (elastic modulus E=75GPa), it is converted into stress value; the testing machine simultaneously records the load, frequency and number of cycles.

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