Fatigue test method for large size composite blade transition section

Abstract

The present application belongs to the field of helicopter structure fatigue design, and relates to a large-size composite material blade transition section fatigue test method. The method comprises the following steps: defining blade load and coordinate system; determining test load of blade transition section evaluation area and evaluation section, wherein the test load comprises centrifugal force, flapping bending moment and edgewise bending moment; determining test piece length, and setting two test loading points on both sides of the transition section; modifying the test piece to obtain a blade modified piece; the blade modified piece comprises a blade root section, a transition section and a part of an airfoil section; a blade extension piece connected with the root section is designed and manufactured according to the structural size of the blade root hole, one end of the extension piece is connected with the blade root section connection hole through a bolt, and the other end is designed to be connected with a connector for connecting with a test bench restraint mechanism; the cross-sectional size of the metal extension piece should be such that the bending stiffness EI is similar to that of the blade.

Description

Technical Field

[0001] This invention belongs to the field of helicopter structural fatigue design and relates to a fatigue test method for the transition section of a large-size composite material rotor blade. Background Technology

[0002] The transition section of composite blades is a region of abrupt changes in blade structure size and stiffness, and also a high-risk area for manufacturing defects. In the past, fatigue tests on the transition section of blades were generally conducted using cantilever beam loading or resonance methods. However, for large-sized composite blades, the transition section has large structural dimensions and high stiffness, making resonance loading difficult to carry out due to power limitations of the test equipment. The bending moment distribution of cantilever beam loading is relatively steep, with only a small portion of the structure meeting the loading requirements, and the bending moment in the constrained area is large, making premature failure in the constrained area easy, thus making the test impossible. Summary of the Invention

[0003] Purpose of the invention: For the test evaluation of the transition zone of large-size composite blades, the distribution of the applied bending moment needs to be as gentle as possible to meet the load loading of the large test zone of the transition section of the large-size composite blade, and the bending moment of the joint in the constraint zone needs to be reduced to avoid premature structural failure.

[0004] Technical solution:

[0005] A fatigue testing method for the transition section of a large-size composite blade is provided, including:

[0006] Define the blade load and coordinate system;

[0007] Determine the test loads for the blade transition section test area and test profile. The test loads include centrifugal force, flapping moment, and oscillation moment.

[0008] Determine the length of the test specimen and set the two test loading points on both sides of the transition section;

[0009] The modified test piece was used to obtain the modified blade component; the modified blade component includes the blade root section, the transition section, and part of the airfoil section.

[0010] Based on the structural dimensions of the root hole of the blade, design and manufacture the blade extension that connects to the root. One end of the extension is connected to the root section of the blade by bolts, and the other end is designed with a connecting joint for connection with the test bench constraint mechanism. The cross-sectional dimensions of the metal extension should make its bending stiffness EI similar to that of the blade bending stiffness.

[0011] Strain gauges were attached to the test profile and calibrated.

[0012] After calibration, the modified blade is connected to the metal extension to form a blade transition section test piece. One end of the blade transition section test piece is constrained to prevent displacement and rotation along the x-axis, but allow rotation along the y and z axes. The other end is also constrained to allow sliding only along the x-axis, preventing rotation along the x-axis, but allowing rotation along the y and z axes. Centrifugal force is applied to the end that can slide along the x-axis. Clamps or airfoil clamps are used to hold the blade in the platform area between the blade root section and the transition section, and at the airfoil section after the transition section, applying flapping force F separately at two test loading points. 挥舞1 F 挥舞2 and pendulum force F 摆振1 F 摆振2 .

[0013] Furthermore, the method also includes:

[0014] Perform test load adjustment:

[0015] First, apply the centrifugal force Fc to the required centrifugal force, then gradually adjust the four forces at the two loading points, by adjusting the waving force F. 挥舞1 F 挥舞2 To ensure the swing moment of the test profile meets the requirements, adjust the swing force F. 摆振1 F 摆振2 The bending moment of the test section should meet the requirements.

[0016] Furthermore, the bending moment of the transition section test specimen exhibits a roughly linear increasing or decreasing distribution with the change of the section. The non-uniform bending moment distribution of the two test sections is achieved by adjusting the numerical difference of the swing force at the two loading points; the non-uniform bending moment distribution of the two test sections is achieved by adjusting the numerical difference of the swing force at the two loading points.

[0017] Furthermore, the total length of the test specimen shall not be less than 3 (ba);

[0018] Where a and b are the distances from the start and end positions of the transition section to the center of the propeller root connection hole.

[0019] Furthermore, the length of the metal extension is b-2a.

[0020] Furthermore, define the coordinate system, including:

[0021] With the direction pointing towards the blade tip as the x-axis, the direction of the maximum chord length of the blade profile pointing towards the trailing edge as the y-axis, and the z-axis obtained according to the right-hand rule; the centrifugal force Fc pointing towards the blade tip is positive, the flapping moment My is positive because it causes tension on the upper surface of the blade, the oscillation moment Mz is positive because it causes compression on the trailing edge of the blade, and the torsional moment Mx is positive because it causes the leading edge of the blade to bend upwards.

[0022] Furthermore, when the centrifugal force Fc is zero, the bending moments generated by the loads at the two test loading points are as follows:

[0023]

[0024] Where F1 and M1 are the forces and bending moments generated at test loading point 1; F2 and M2 are the forces and bending moments generated at test loading point 2; L1 is the distance from test loading point 1 to the nearest constraint point; L2 is the distance between the two test loading points; and L3 is the distance from test loading point 2 to the nearest constraint point.

[0025] Beneficial effects:

[0026] This invention provides a fatigue testing method for the transition section of a large-size composite blade by extending the test specimen with a dummy piece and using four-point bending loading. This method can meet the load requirements for varying load distribution in the larger test area of ​​the transition section of the large-size composite blade, and also reduces the bending moment at the joint in the constraint area to avoid premature failure of the joint structure. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of four-point bending moment loading;

[0028] Figure 2 This is a schematic diagram of the constraint loading during the blade transition section test;

[0029] Figure 3 This is a schematic diagram of the blade transition section structure and patch;

[0030] Figure 4 This is a schematic diagram illustrating the principle of non-uniform bending moment loading at four points in the blade transition section. Detailed Implementation

[0031] This invention proposes a fatigue testing method for the transition section of a large-size composite blade by extending the test specimen with a dummy piece and using four-point bending loading. This method can effectively meet the fatigue test loading requirements of the transition section of a large-size composite blade and solve the problem that the transition zone cannot be fully tested due to premature failure of the joint constraint area.

[0032] This invention provides a fatigue testing method for the transition section of a large-size composite blade. The method involves extending the test specimen by adding a metal dummy and employing a four-point bending loading method. The specific steps of the testing method are as follows:

[0033] [1] Define blade load and coordinate system

[0034] With the direction pointing towards the blade tip as the x-axis, the direction of the maximum chord length of the blade section pointing towards the trailing edge as the y-axis, and the z-axis obtained according to the right-hand rule, the centrifugal force (Fc) pointing towards the blade tip is positive, the flapping moment (My) is positive when the upper surface of the blade is under tension, the oscillation moment (Mz) is positive when the trailing edge of the blade is under compression, and the torsional moment (Mx) is positive when the leading edge of the blade bends upward.

[0035] [2] Determine the test area and test section load of the blade transition section; According to the blade structure size, the area where the chord length changes between the blade root section and the airfoil section is the transition section area, and the entire transition section area is the test area. Define the distances from the start and end positions of the transition section to the center of the blade root connection hole as amm and bmm, respectively. Then define a+50mm and b-50mm as the test sections.

[0036] Based on the calculated load spectrum of the two test profiles, and according to the helicopter blade strength design method, the fatigue loads required for the two profiles are determined respectively. The blade fatigue test loads generally include centrifugal force Fc, flapping moment ±My, and oscillation moment ±Mz.

[0037] [3] Length of test specimen and loading point

[0038] According to such Figure 1 The four-point moment loading method shown places the test loading points on both sides of the transition section. The test loading lever arm L1 = L3, and it is recommended that it be no less than the distance L2 between the two loading clamping points. Final loading scheme. Figure 2 As shown, the total length of the test piece is not less than 3 (ba) mm.

[0039] Note: The final design length should take into account the size of the airfoil clamp at the loading clamping point. Steps [3][4][5] ignore this length.

[0040] [4] Modification of test specimens

[0041] According to the principle of step [3], the length of the blade part of the test piece is determined to be 2b-amm. The blade is cut off at this point, the foam within the specified range of the blade is hollowed out from the cut surface, and short-cut glass fiber mixture is filled in and heated and cured. A reinforcing beam belt is laid on the outer surface of the skin, and the cloth is wrapped and then heat-cured. The machined parts are assembled with holes and metal clamps are installed.

[0042] [5] Design of propeller metal extension component

[0043] According to the structural dimensions of the root hole of the blade, the blade extension is designed and manufactured to connect with the root. One end of the extension is connected to the root hole of the blade by bolts, and the other end is designed with a connecting joint for connection with the test bench constraint mechanism. According to the lever arm requirements of the four-point bending moment loading method in step [3], the length of the metal extension is determined to be b-2amm. The cross-sectional dimensions of the metal extension should make its bending stiffness EI in the flapping and oscillating directions similar to the flapping and oscillating bending stiffness of the blade. The preferred flapping stiffness is the average value of the actual flapping stiffness of the blade modification part; the preferred oscillating stiffness is the average value of the actual oscillating stiffness of the blade modification part.

[0044] [6] Assessment of cross-sectional patch calibration

[0045] In step [2], strain gauges for flapping and oscillating bending moments were applied to the test specimen profile and load calibration was performed. The purpose of calibration was to find the correspondence between strain output and bending moment. The strain gauges were calibrated using the overall calibration method, that is, the measurement system and strain gauges were calibrated as a whole. During calibration, the test specimen and the root end fixed joint were installed on a special calibration platform, and a standard load was applied to the process joint end of the test specimen through an actuator. The entire calibration process was strictly carried out in accordance with the requirements of the test outline. Finally, the influence of the blade's self-weight was eliminated by vertically placing the test specimen and then uniformly clearing the zero point.

[0046] [7] Test specimen installation and loading

[0047] After connecting the blade modification part of step [4] to the metal extension part, a constraint loading is performed in the form of 4-point bending loading. One end is constrained by ux = uy = uz = urx = 0, which can rotate along the y and z axes. The other end is constrained by uy = uz = urx = 0, which can slide along the x axis and rotate along the y and z axes. Centrifugal force of the blade is applied at the end that can slide along the x axis. Clamping plates or airfoil clamps are used to hold the blade in the platform area between the blade root section and the transition section and the airfoil section after the transition section, and to apply flapping force F separately. 挥舞1 F 挥舞2 and pendulum force F 摆振1 F 摆振2 The loading diagram is as follows. Figure 2 As shown.

[0048] [8] Test load adjustment

[0049] The entire experiment was controlled by a control system, and a dynamic signal testing and analysis system was used for data acquisition, processing, and load monitoring. First, the centrifugal force Fc was applied to the required level. Then, the four forces at the two loading points were gradually adjusted by adjusting the waving force F. 挥舞1 F 挥舞2 To ensure the swing moment of the test profile meets the requirements, adjust the swing force F. 摆振1 F 摆振2 The bending moment of the test section is made to meet the requirements. The bending moment of the test section of the transition section specimen increases or decreases approximately linearly with the change of the section. The non-uniform bending moment distribution of the swinging and oscillating bending moment loads on the two test sections is achieved by adjusting the numerical difference between the two loading points of the swinging force and the oscillating force. The schematic diagram of the non-uniform bending moment loading principle is shown in [see diagram]. Figure 4 .

Claims

1. A method of fatigue testing a large scale composite blade transition section, characterized by, The method comprises the following steps: Defining the blade load and coordinate system; Determining the test load of the blade transition section examination area and examination section, the test load comprising centrifugal force, flap bending moment and edgewise bending moment; Determining the length of the test piece, and setting two test loading points on the two sides of the transition section; Reforming the test piece to obtain a blade reforming piece; the blade reforming piece comprises a blade root section, a transition section and a part of airfoil section; Designing and manufacturing a blade extension piece connected with the root according to the structural size of the blade root hole; one end of the extension piece is connected with the connecting hole of the blade root section through a bolt, and the other end is designed with a connecting joint for connecting with the restraint mechanism of the test bed; the sectional size of the metal extension piece should be such that the bending stiffness EI thereof is similar to the bending stiffness of the blade; Pasting strain gauges on the examination section and calibrating; After calibration, the blade modification piece and the metal extension piece are connected to form a blade transition section test piece. One end of the blade transition section test piece is constrained so that it cannot displace and cannot rotate along the x-axis, but can rotate along the y-axis and the z-axis. The other end of the blade transition section test piece is also constrained so that it can only slide along the x-axis, cannot rotate along the x-axis, but can rotate along the y-axis and the z-axis. The centrifugal force of the blade is applied to the end that can slide along the x-axis. A clamp plate or an airfoil clamp is used to clamp the platform area between the blade root section and the transition section and the airfoil section after the transition section, and is used to separate the flapping force F 挥舞1 , F 挥舞2 and the pitch force F 摆振1 , F 摆振2 .

2. The method of claim 1, wherein, The method further comprises the following steps: Performing test load debugging: First, the centrifugal force Fc is loaded to the required centrifugal force, the four forces of the two loading points are gradually adjusted, the flap force F 挥舞1 , F 挥舞2 The flap bending moment of the test section is adjusted to the required value, and the flap force F 摆振1 , F 摆振2 The flap bending moment of the test section is adjusted to the required value.

3. The method of claim 2, wherein, The bending moment of the examination section of the transition section test piece changes linearly with the section; the non-equal flap bending moment distribution of the two examination sections is realized by adjusting the numerical difference of the flap force at the two loading points; the non-equal edgewise bending moment distribution of the two examination sections is realized by adjusting the numerical difference of the edgewise force at the two loading points.

4. The method of claim 3, wherein, The total length of the test piece is not less than 3(b-a); Wherein, a and b are the distances from the start and end positions of the transition section to the center of the blade root connecting hole.

5. The method of claim 3, wherein, The length of the metal extension piece is b-2a.

6. The method of claim 3, wherein, Defining the coordinate system, comprising: The x-axis points to the blade tip direction, the y-axis points to the trailing edge direction along the longest chord of the blade section, and the z-axis is obtained according to the right-hand rule; the centrifugal force Fc points to the blade tip and is positive, the flap bending moment My is positive when the upper surface of the blade is in tension, the edgewise bending moment Mz is positive when the trailing edge of the blade is in compression, and the torsional bending moment Mx is positive when the leading edge of the blade is upward.

7. The method of claim 6, wherein, When the centrifugal force Fc is zero, the bending moments generated by the loads of the two test loading points are respectively: Wherein, F1 and M1 are the force and bending moment of the test loading point 1; F2 and M2 are the force and bending moment of the test loading point 2; L1 is the distance from the test loading point 1 to the nearest restraint point, L2 is the distance between the two test loading points, and L3 is the distance from the test loading point 2 to the nearest restraint point.

8. A computer-readable storage medium storing a computer program, the computer program comprising instructions that, when executed by a computer, cause the computer to perform the method of any one of claims 1 to 7. The computer program is executed by a processor to realize the method of any one of claims 1-7.

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