A large wind turbine blade fatigue test load calculation method considering pre-bending and geometric nonlinearity

Abstract

The application discloses a large wind power blade fatigue test load calculation method considering pre-bending and geometric nonlinearity, and aims at solving the problem of significant deviation between calculation results and actual test data in traditional large wind power blade fatigue test load calculation. The method comprises the following steps: step 1, a wind power blade discretization model considering pre-bending is established, and sectional data is optimized through a piecewise cubic Hermite interpolation method; step 2, the rotation angle of a pre-bending curve and the deflection of the pre-bending curve of each discretization section of the blade are calculated based on the optimized sectional data, and a geometric nonlinearity corrected stiffness matrix is used to calculate the static equilibrium position and the average bending moment of each discretization section of the blade under the action of a static load; and step 3, based on the static equilibrium position of the blade, a multi-degree-of-freedom system dynamics method is used to solve the bending moment amplitude of each discretization section of the blade in a steady state operation of a blade fatigue test system.

Description

Technical Field

[0001] This invention relates to the field of wind power equipment testing technology, and in particular to a method for calculating fatigue test loads of large wind turbine blades that takes into account pre-bending and geometric nonlinearity. Background Technology

[0002] With the global energy structure shifting towards renewable energy, the rapid development of the wind power industry is placing higher demands on the performance of wind turbine generators. As one of the core components of a wind turbine generator, wind turbine blades must withstand complex dynamic loads throughout their service life. Under these dynamic loads, blades are highly susceptible to fatigue failure, and fatigue-induced fracture has become the primary form of wind turbine blade failure. To ensure the reliability of wind turbine blades, full-scale structural fatigue testing has become a crucial method for evaluating their fatigue resistance. Domestic and international certification standards and technical specifications such as IEC61400-23 and DNVGL-ST-0376 all require full-scale structural fatigue testing of wind turbine blades before commissioning to verify whether they meet their design service life.

[0003] Currently, with the continuous increase in the size of wind turbine blades, traditional test load techniques include forced displacement methods, resonant excitation methods, simplified single-axis loading methods, and biaxial loading methods. However, traditional test load calculation methods all neglect the combined influence of pre-bending and geometric nonlinearity on the fatigue test load calculation of blades, resulting in significant deviations between the calculated fatigue test load and actual test data, leading to low accuracy. Therefore, a new load calculation method is urgently needed that considers the influence of blade pre-bending and geometric nonlinearity on the test load calculation to improve the accuracy of fatigue testing of large wind turbine blades. Summary of the Invention

[0004] This invention provides a method for calculating fatigue test loads of large wind turbine blades that considers pre-bending and geometric nonlinearity, in order to solve the problem that the fatigue test load calculation results of traditional large wind turbine blade fatigue test load calculations do not consider pre-bending characteristics and ignore geometric nonlinear effects, resulting in significant deviations between the fatigue test load calculation results and actual test data.

[0005] To address the aforementioned technical problems, embodiments of the present invention provide a method for calculating fatigue test loads on large wind turbine blades that considers pre-bending and geometric nonlinearity. This method includes:

[0006] Step 1: Establish a discretized model of the wind turbine blade considering pre-bending, and optimize the cross-sectional data by piecewise cubic Hermite interpolation.

[0007] Among them, the discretization model of the pre-bent wind turbine blade simplifies the wind turbine blade into multiple cantilever beam models that consider pre-bending and calculates them separately. After optimization by piecewise cubic Hermite interpolation, the number of discretized cross sections of the blade increases.

[0008] Step 2: Calculate the rotation angle and deflection of the pre-bending line on each discretized section of the blade based on the optimized cross-sectional data, and use the geometrically nonlinear corrected stiffness matrix to calculate the static equilibrium position and mean bending moment of each discretized section of the blade under static load.

[0009] Among them, the first blade The angle of the pre-bent line on each discretized section is based on the blade front. The curvature of the discretized section is determined, and the blade's first... The deflection of the pre-bent line on each discretized section is based on the blade front. The rotation angle of each discretized section is determined, and the rotation angle of the pre-bent line on the first discretized section is defined. Deflection of the pre-bent line on the first discretized section , It is a positive integer greater than 1;

[0010] Leaf first Mean bending moment load of each discretized section Represented as:

[0011] ;

[0012] in, and These represent the mean bending moments of the second and third discretized sections, respectively. Indicates the first The curvature of a discretized cross section Indicates the first Bending stiffness of a discrete cross section This represents the total number of blades after the discretization cross section is increased.

[0013] Step 3: Based on the static equilibrium position of the blade, the bending moment amplitude on each discretized section of the blade is solved by using the multi-degree-of-freedom system dynamics method during steady-state operation of the blade fatigue test system.

[0014] Optionally, step 1 above can be achieved through steps 1.1 and 1.2;

[0015] Step 1.1: Simplify the wind turbine blade into multiple cantilever beam models considering pre-bending, perform calculations separately, and discretize the blade into... part.

[0016] Among them, the blade is discretized into The total number of sections Each discretized section It has two degrees of freedom, namely the deflection of the pre-bent line. and the angle of the pre-bent line For the first A discrete cross section, whose bending stiffness, mass distribution, and radius of curvature are respectively... , , ( ), greater than Positive integers.

[0017] Step 1.2: Increase the number of discretized sections in the pre-bending cantilever beam model in Step 1.1, and use piecewise cubic Hermite interpolation optimization to increase the number of section-related properties.

[0018] Among them, the cross-section related attributes include: the first Bending stiffness of a discretized section ( ), No. Mass distribution of a discretized cross section ( ), No. radius of curvature of each discretized section ( ), This represents the total number of discretized blade cross sections after the increase in discretization cross sections. Discretized blade cross sections after Hermite interpolation optimization. The quantity and cross-sectional related attributes data are given from existing data. Increase to .

[0019] Optionally, step 2 above includes steps 2.1 and 2.2;

[0020] Step 2.1: Using the cantilever beam model considering pre-bending from Step 1 above, calculate the rotation angle and deflection of the blade's pre-bending line to obtain the blade's first... The angle of the pre-bent line on the discretized section and the deflection of the pre-bent line for:

[0021] ;

[0022] ;

[0023] in, Indicates the first of the blades A discrete cross section Indicates the first The discretized section, i.e. the first discretized section. Each discretized section is the front of the blade. Any one of the discretized sections, Indicates the first blade The curvature of the pre-bent line on a discretized cross section Indicates the first blade The radius of curvature of a discretized section Indicates the first blade The curvature of the pre-bent line on a discretized cross section This represents the total number of blades after the discretization cross section is increased. and These represent the first and second blades, respectively. The and the first The angle of the pre-bent line on a discretized section and These represent the first and second leaves of the blade. and the The location of each discretized section And define the rotation angle of the pre-bent line on the first discretized section. Deflection of the pre-bent line on the first discretized section .

[0024] Step 2.2: When calculating the system stiffness, the elastic stiffness is corrected using geometric stiffness, which can characterize the nonlinear stiffness change caused by load effects. For the first... For a finite element, the overall stiffness matrix of the element after considering geometric nonlinear correction is:

[0025] ;

[0026] in,

[0027] ;

[0028] ;

[0029] Among them, the The finite element is the first The finite element corresponding to the discretized section (hereinafter referred to as: the first) (units) The first part, considering geometric nonlinear corrections, represents the... The overall stiffness matrix of each element. Indicates the first The elastic stiffness matrix of each element. The first part, considering geometric nonlinear corrections, represents the... The geometric stiffness matrix of each element. Indicates the first The bending stiffness of each element. , and They represent the first The and the first Bending stiffness of a discrete cross section Indicates the first The length of each unit , and These represent the first and second leaves of the blade. The and the first The location of each discretized section Indicates the first The axial force of each unit. The negative sign of the axial force indicates that the force is compressive, and the positive sign indicates tensile.

[0030] Optionally, step 2 above may include step 2.3 after step 2.2;

[0031] Step 2.3: Based on the deflection of the initial pre-bending line of the blade. and the global stiffness matrix considering geometric nonlinear corrections Calculate the static equilibrium position of the blade fatigue testing system.

[0032] For the first of the blades A discrete cross-section, the static equilibrium position of the blade fatigue testing system. Represented as:

[0033] ;

[0034] in,

[0035] ;

[0036] in, Indicates the first The static displacement on a discrete cross section is caused by the weight of the blade itself and the weight of the exciter's saddle. Represents the static displacements on all cross sections The matrix, Represents the global stiffness matrix The inverse matrix, This indicates that each discretized unit stiffness matrix The overall stiffness matrix obtained by assembling them together This represents the weight of the blades themselves and the weight of the exciter's saddle. The overall nodal force matrix.

[0037] Optionally, step 2 above may include steps 2.4 and 2.5 after step 2.3.

[0038] Step 2.4: Based on the static equilibrium position of the blade fatigue testing system. The central difference method is used to calculate the first... when the system is in static equilibrium. Curvature of a discretized section ;

[0039] ;

[0040] in, , and These represent the first and second leaves of the blade. The, the The and the first The static equilibrium position of each discretized section. and These represent the first and second leaves of the blade. The and the first The location of each discretized section.

[0041] Step 2.5, according to the first Curvature of a discretized section Calculate the first blade Mean bending moment load of each discretized section .

[0042] Among them, the Mean bending moment of each discretized section for:

[0043] ;

[0044] in, and These represent the mean bending moments of the second and third discretized sections, respectively. Indicates the first The curvature of a discretized cross section Indicates the first Bending stiffness of a discrete cross section.

[0045] Optionally, step 3 includes steps 3.1 to 3.4 below.

[0046] Step 3.1: Based on the static equilibrium position calculated in Step 2.3 above, the natural frequency, principal mode shape, regularization factor, and regularized mode shape matrix of the system are calculated by establishing the differential equation of undamped free vibration of the multi-degree-of-freedom system.

[0047] Step 3.2: Establish and solve the differential equation of the damped forced vibration of the multi-degree-of-freedom system to obtain the steady-state response and the system's first degree of freedom during steady-state operation. The dynamic displacement of the discretized section, the first The curvature of a discretized cross section.

[0048] Step 3.3, according to the first The curvature calculation system for the discretized cross section during steady-state operation... Dynamic bending moment of a discretized section.

[0049] Step 3.4, according to the first part of step 3.3 The dynamic bending moment of the discretized section is obtained. The bending moment amplitude of each discretized section.

[0050] Optionally, step 3.1 can be implemented through the following steps:

[0051] Step 3.1.1: Before establishing the differential equations of motion for a multi-degree-of-freedom system, the global stiffness matrix of the system is known. Then, the overall quality matrix of the system. Perform the calculation.

[0052] Among them, the overall quality matrix This indicates that each discretized unit mass matrix The result of assembling together The mass matrix for the blade's first... Discretized cross section, mass matrix It can be represented as:

[0053] ;

[0054] in, Indicates the first Mass per unit length of a unit , and They represent the first The and the first Mass distribution of a discretized cross section Indicates the first The length of each unit , and These represent the first and second leaves of the blade. The and the first The location of each discretized section.

[0055] Step 3.1.2: Obtain the overall stiffness matrix of the system in step 3.1.1. and overall quality matrix Then, boundary conditions are applied to the system to obtain the differential equation of undamped free vibration of the multi-degree-of-freedom system, and the natural frequency, principal mode shape, regularization factor and regular mode shape matrix of the system are calculated respectively.

[0056] The boundary condition is to remove the overall system stiffness matrix. and overall quality matrix The stiffness matrix of the multi-degree-of-freedom system is obtained by dividing the first row and second row and the first and second columns. and mass matrix The differential equation for the undamped free vibration of a multi-degree-of-freedom system is expressed as:

[0057] ;

[0058] in, Represents the steady-state response of the system in the geometric coordinate system. Represents the acceleration array, The matrix representing the value of 0 represents the first degree of freedom in a multi-degree-of-freedom system. The first natural frequency is The first degree of freedom system First natural frequency Substitution The first degree of freedom of the multi-degree-of-freedom system can then be calculated. First principal mode Let the mode matrix The regularization factor is ,in , This represents the modal matrix obtained by combining all the principal modes. Indicates the first Principal mass, Indicates the first Order regularization factor, for The transpose of the normal mode shape matrix is ​​then... It can be represented as:

[0059] ;

[0060] in, Represents the normal mode shape matrix. This represents the regularization factor corresponding to different degrees of freedom. This represents the principal vibration modes corresponding to different degrees of freedom.

[0061] Optionally, step 3.2 above is as follows:

[0062] Step 3.2: Establish and solve the differential equation of the damped forced vibration of the multi-degree-of-freedom system to obtain the steady-state response and the system's first degree of freedom during steady-state operation. The dynamic displacement of the discretized section, the first The curvature of a discretized cross section;

[0063] System steady-state response results for:

[0064] ;

[0065] in, Denotes the first... of the system in the natural coordinate system First steady-state response, Indicates the excitation frequency, Describing the first degree of freedom of a multi-degree-of-freedom system First natural frequency, Indicates the system uptime. Indicates frequency ratio, This represents the modal damping ratio measured by the system's free decay vibration experiment. Indicates phase difference, This represents the amplitude of the normal excitation force.

[0066] The system is in steady-state operation. Dynamic displacement of each discretized section for:

[0067] ;

[0068] in, Indicates the first of the blades The static equilibrium position of each discretized section. Indicates the first of the blades Steady-state response results for each discretized cross section.

[0069] No. Curvature of a discretized section for:

[0070] ;

[0071] in, , and These represent the first and second leaves of the blade. The, the The and the first Dynamic displacement of a discrete cross section. and These represent the first and second leaves of the blade. The and the first The location of each discretized section.

[0072] Optionally, step 3.3 above is as follows:

[0073] Step 3.3, according to the first Curvature of a discretized section The computing system during steady-state operation Dynamic bending moment of each discretized section The system boundary conditions were processed using linear extrapolation and a beam boundary condition with zero free-end bending moment, resulting in the system's boundary condition at the [missing information - likely a specific point or time period]. Dynamic bending moment of each discretized section for:

[0074] ;

[0075] in, Indicates the first The curvature of a discretized cross section Indicates the first Bending stiffness of a discrete cross section and These represent the dynamic bending moments of the second and third discretized sections, respectively.

[0076] Optionally, step 3.4 above is as follows:

[0077] Step 3.4: After calculating the system's steady-state operation, the first... Dynamic bending moment of each discretized section After that, the first Bending moment amplitude of each discretized section for:

[0078] ;

[0079] Among them, the Maximum bending moment of each discretized section Minimum bending moment They are represented as follows:

[0080] .

[0081] The beneficial effects of this invention are:

[0082] 1. This application establishes a fatigue test load calculation model for wind turbine blades that considers pre-bending and geometric nonlinearity. By incorporating pre-bending characteristics and geometric nonlinear effects into the fatigue test load calculation, it effectively solves the error problem caused by neglecting pre-bending and geometric nonlinear effects in traditional methods. Experimental verification shows that within the blade spanwise range of 0-70%, the calculation error is controlled within ±10% (reaching ±5% in some areas), far superior to traditional methods that do not consider pre-bending and geometric nonlinear effects (errors reaching 20%-30%).

[0083] 2. This application introduces a stiffness matrix with geometric nonlinear correction to accurately reflect the stiffness change of the blade under the coupling of pre-bending and self-weight. This method effectively avoids the problem of local overload or underload of the blade caused by load distribution deviation during the test, ensuring that the fatigue test results truly reflect the actual working conditions, while optimizing the implementation efficiency of the test plan.

[0084] 3. This application utilizes a discretized cantilever beam model considering pre-bending and a piecewise cubic Hermite interpolation method to achieve a refined description of the blade cross-sectional properties while maintaining a clear calculation process and high parameter operability. Furthermore, this method can be directly extended to wind turbine blades of different sizes (such as the 90-meter blade in the embodiment), providing a standardized and reusable load calculation tool for engineering practice. Attached Figure Description

[0085] Figure 1 A flowchart illustrating the fatigue test load calculation method for large wind turbine blades provided in this application;

[0086] Figure 2 For this application, a discretized cantilever beam model of a pre-bent wind turbine blade is considered.

[0087] Figure 3 The diagrams show the distribution of pre-bending deflection and pre-bending line rotation along the blade span of the wind turbine blade in this application, where (a) is the pre-bending deflection diagram and (b) is the pre-bending rotation diagram.

[0088] Figure 4 This is a comparison diagram of the distribution along the blade span of the pre-bending deflection and the displacement of the static equilibrium position of the wind turbine blade in this application.

[0089] Figure 5 This is a diagram showing the distribution of the mean bending moment of the wind turbine blade along the blade span in this application;

[0090] Figure 6 This is a diagram showing the distribution of the maximum, minimum, and average bending moments of the wind turbine blades in this application along the blade span.

[0091] Figure 7 A comparison of the calculated bending moment amplitude and the fatigue test bending moment amplitude, considering pre-bending and geometric nonlinearity, and a relative error distribution diagram are shown for wind turbine blades. (a) is a comparison diagram of bending moment amplitude; (b) is a comparison diagram of bending moment amplitude error.

[0092] Figure 8 The diagram shows a comparison of the calculated bending moment amplitude and the fatigue test bending moment amplitude of the wind turbine blade without considering pre-bending and geometric nonlinearity, and a relative error distribution diagram. (a) is a comparison diagram of bending moment amplitude; (b) is a comparison diagram of bending moment amplitude error. Detailed Implementation

[0093] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0094] Example 1

[0095] like Figure 1 As shown in this embodiment, a method for calculating fatigue test loads of large wind turbine blades considering pre-bending and geometric nonlinearity is disclosed. This calculation method includes the following steps 1 to 3:

[0096] Step 1: Establish a discretized model of the wind turbine blade considering pre-bending, and optimize the cross-sectional data by piecewise cubic Hermite interpolation.

[0097] The discretization model of the pre-bent wind turbine blade simplifies the wind turbine blade into multiple cantilever beam models considering pre-bending, and calculates them separately. The discretized cross section of the blade is optimized by piecewise cubic Hermite interpolation. Quantity and bending stiffness of cross sections Mass distribution radius of curvature The number increased, It is a positive integer greater than 1.

[0098] Step 2: Calculate the rotation angle and deflection of the pre-bending line on each discretized section of the blade based on the optimized cross-sectional data, and use the geometrically nonlinear corrected stiffness matrix to calculate the static equilibrium position and mean bending moment of each discretized section of the blade under static load.

[0099] Among them, the first blade The angle of the pre-bent line on each discretized section is based on the blade front. The curvature of the discretized section is determined, and the blade's first... The deflection of the pre-bent line on each discretized section is based on the blade front. The rotation angle of each discretized section is determined, and the rotation angle of the pre-bent line on the first discretized section is defined. Deflection of the pre-bent line on the first discretized section .

[0100] Leaf first Mean bending moment load of each discretized section Represented as:

[0101] ;

[0102] in, and These represent the mean bending moments of the second and third discretized sections, respectively. Indicates the first The curvature of a discretized cross section Indicates the first Bending stiffness of a discrete cross section This represents the total number of blades after the discretization cross section is increased.

[0103] Step 3: Based on the static equilibrium position of the blade, the bending moment amplitude on each discretized section of the blade is solved by using the multi-degree-of-freedom system dynamics method during steady-state operation of the blade fatigue test system.

[0104] Optionally, step 1 above can be achieved through steps 1.1 and 1.2 below.

[0105] Step 1.1: Simplify the wind turbine blade into multiple cantilever beam models considering pre-bending, perform calculations separately, and discretize the blade into... part.

[0106] Among them, the blade is discretized into The total number of sections Each discretized section It has two degrees of freedom, namely the deflection of the pre-bent line. and the angle of the pre-bent line For the first There are a total of finite elements, each corresponding to a beam segment with a uniform cross-section. Adjacent elements are connected by nodes. Each node, i.e. The discretized section. For the first... A discrete cross section, whose bending stiffness, mass distribution, and radius of curvature are respectively... , , ( During fatigue testing of the blade flapping direction, each node of the finite element has two degrees of freedom: the deflection of the pre-bending line. and the angle of the pre-bent line (This describes the deflection of the pre-bent line from the perspective of the nodes.) and the angle of the pre-bent line , and each of the above discretized sections Angle description of the deflection of the pre-bent line and the angle of the pre-bent line (Not conflicting) and All are positive integers. greater than Positive integers.

[0107] It should be noted that the deflection of the pre-bent line and the angle of the pre-bent line The calculation formula is shown in step 2.1.

[0108] Step 1.2: Increase the number of discretized sections in the pre-bending cantilever beam model in Step 1.1, and use piecewise cubic Hermite interpolation optimization to increase the number of section-related properties.

[0109] Among them, the cross-section related attributes include: the first Bending stiffness of a discretized section ( ), No. Mass distribution of a discretized cross section ( ), No. radius of curvature of each discretized section ( ), This represents the total number of discretized blade cross sections after the increase in discretization cross sections. Discretized blade cross sections after Hermite interpolation optimization. The quantity and cross-sectional related attributes data are given from existing data. Increase to .

[0110] Optionally, step 2 above includes steps 2.1 and 2.2.

[0111] Step 2.1: Using the cantilever beam model considering pre-bending from Step 1 above, calculate the rotation angle and deflection of the blade's pre-bending line to obtain the blade's first... The angle of the pre-bent line on the discretized section and the deflection of the pre-bent line for:

[0112] ;

[0113] ;

[0114] in, Indicates the first of the blades A discrete cross section Indicates the first The discretized section, i.e. the first discretized section. Each discretized section is the front of the blade. Any one of the discretized sections, Indicates the first blade The curvature of the pre-bent line on a discretized cross section Indicates the first blade The radius of curvature of a discretized section Indicates the first blade The curvature of the pre-bent line on a discretized cross section This represents the total number of blades after the discretization cross section is increased. and These represent the first and second blades, respectively. The and the first The angle of the pre-bent line on a discretized section and These represent the first and second leaves of the blade. and the The location of each discretized section Since the blade root is fixed, and the fixed root position is discretized into the first discretized section, the deflection and rotation angle of the first discretized section are both 0. , .

[0115] Step 2.2: When calculating the system stiffness, the elastic stiffness is corrected using geometric stiffness, which can characterize the nonlinear stiffness change caused by load effects. For the first... For a finite element, the overall stiffness matrix of the element after considering geometric nonlinear correction is:

[0116] ;

[0117] in,

[0118] ;

[0119] ;

[0120] Among them, the The finite element is the first The finite element corresponding to the discretized section (referred to as: the first) (units) The first part, considering geometric nonlinear corrections, represents the... The overall stiffness matrix of each element. Indicates the first The elastic stiffness matrix of each element. The first part, considering geometric nonlinear corrections, represents the... The geometric stiffness matrix of each element. Indicates the first The bending stiffness of each element. , and They represent the first The and the first Bending stiffness of a discrete cross section Indicates the first The length of each unit , and These represent the first and second leaves of the blade. The and the first The location of each discretized section Indicates the first The axial force of each unit. The negative sign of the axial force indicates that the force is compressive, and the positive sign indicates tensile.

[0121] It should be noted that the first The finite element is the first The finite elements corresponding to the discretized sections are calculated in the following order: starting from the first discretized section at the blade root, the calculation of the first discretized section is performed. A finite number of units (see also) Figure 2 As shown, the first (Each finite element has one end as the first discretized section at the root and the other end as the second discretized section), and so on.

[0122] Optionally, step 2.3 may be included after step 2.2.

[0123] Step 2.3: Based on the deflection of the initial pre-bending line of the blade. and the global stiffness matrix considering geometric nonlinear corrections Calculate the static equilibrium position of the blade fatigue testing system.

[0124] For the first of the blades A discrete cross-section, the static equilibrium position of the blade fatigue testing system. Represented as:

[0125] ;

[0126] in,

[0127] ;

[0128] in, Indicates the first The static displacement on a discrete cross section is caused by the weight of the blade itself and the weight of the exciter's saddle. Represents the static displacements on all cross sections The matrix, Represents the global stiffness matrix The inverse matrix, This indicates that each discretized unit stiffness matrix The overall stiffness matrix obtained by assembling them together This represents the weight of the blades themselves and the weight of the exciter's saddle. The overall nodal force matrix.

[0129] Optionally, after step 2.3, step 2 may also include steps 2.4 and 2.5 as described below.

[0130] Step 2.4: Based on the static equilibrium position of the blade fatigue testing system. The central difference method is used to calculate the first blade when the system is in static equilibrium. Curvature of a discretized section ;

[0131] ;

[0132] in, , and These represent the first and second leaves of the blade. The, the The and the first The static equilibrium position of each discretized section. and These represent the first and second leaves of the blade. The and the first The location of each discretized section.

[0133] Step 2.5, according to the blade number Curvature of a discretized section Calculate the first blade Mean bending moment load of each discretized section .

[0134] Since the fixed position at the root of the blade is treated as the first discretized section, the tip of the blade is treated as the second discretized section. There are discretized sections, so for the first and second blade boundaries The discretized section is processed using linear extrapolation and beam boundary conditions with zero free-end bending moment to obtain the first discretized section. Mean bending moment of each discretized section ;

[0135] ;

[0136] in, and These represent the mean bending moments of the second and third discretized sections, respectively. Indicates the first The curvature of a discretized cross section Indicates the first Bending stiffness of a discrete cross section.

[0137] Optionally, step 3 can be implemented through steps 3.1 to 3.4 below.

[0138] Step 3.1: Based on the static equilibrium position calculated in Step 2.3 above, the natural frequency, principal mode shape, regularization factor, and regularized mode shape matrix of the system are calculated by establishing the differential equation of undamped free vibration of the multi-degree-of-freedom system.

[0139] Specifically, step 3.1 can be achieved through steps 3.1.1 and 3.1.2 below.

[0140] Step 3.1.1: Before establishing the differential equations of motion for a multi-degree-of-freedom system, the global stiffness matrix of the system is known. Then, the overall quality matrix of the system. Perform the calculation.

[0141] Among them, the overall quality matrix This indicates that each discretized unit mass matrix The result of assembling together The mass matrix, for the th A finite element, mass matrix It can be represented as:

[0142] ;

[0143] in, Indicates the first Mass per unit length of a unit , and They represent the first The and the first Mass distribution of a discretized cross section Indicates the first The length of each unit , and These represent the first and second leaves of the blade. The and the first The location of each discretized section.

[0144] It should be noted that, after calculating the mass matrix of each finite element... Then, the mass matrix of all the units is assembled together to obtain The mass matrix is ​​the overall mass matrix. For details, please refer to existing technologies; this application will not elaborate further.

[0145] Step 3.1.2: Obtain the overall stiffness matrix of the system in step 3.1.1. and overall quality matrix Then, boundary conditions are applied to the system to obtain the differential equation of undamped free vibration of the multi-degree-of-freedom system, and the natural frequency, principal mode shape, regularization factor and regular mode shape matrix of the system are calculated respectively.

[0146] The boundary conditions take into account the degrees of freedom at the fixed end of the cantilever beam model. , When performing dynamic analysis, only the degrees of freedom of all nodes except the fixed end need to be considered, therefore the overall system stiffness matrix can be removed. and overall quality matrix The stiffness matrix of the multi-degree-of-freedom system is obtained by dividing the first row and second row and the first and second columns. and mass matrix Both are Given the matrix, the differential equation for the undamped free vibration of a multi-degree-of-freedom system is:

[0147] ;

[0148] in, Represents the steady-state response of the system in the geometric coordinate system. Represents the acceleration array, This represents a matrix with a value of 0.

[0149] The characteristic polynomial is obtained from the existence of non-zero solutions in the differential equation of undamped free vibration of a multi-degree-of-freedom system. The first degree of freedom of a multi-degree-of-freedom system can be calculated. First natural frequency The first degree of freedom of the multi-degree-of-freedom system First natural frequency Substitution The first degree of freedom of the multi-degree-of-freedom system can then be calculated. First principal mode .

[0150] Let the mode matrix Regularization factor , ,in This represents the modal matrix obtained by combining all the principal modes. Indicates the first Principal mass, Indicates the first Order regularization factor, for The transpose of the normal mode shape matrix is ​​then... It can be represented as:

[0151] ;

[0152] in, Represents the normal mode shape matrix. This represents the regularization factor corresponding to different degrees of freedom. This represents the principal vibration modes corresponding to different degrees of freedom.

[0153] Step 3.2: Establish and solve the differential equation of the damped forced vibration of the multi-degree-of-freedom system to obtain the steady-state response and the system's first degree of freedom during steady-state operation. The dynamic displacement of the discretized section, the first The curvature of a discretized cross section.

[0154] Specifically, the differential equation for damped forced vibration of a multi-degree-of-freedom system is established and solved to calculate the steady-state response and the system's first-degree-of-freedom vibration during steady-state operation. The dynamic displacement of the discretized section, the first The method for determining the curvature of a discrete section is as follows: During the full-scale structural fatigue test of the blade, the excitation frequency is close to the natural frequency of the system. The structural damping has a significant impact on the vibration of the system. However, considering the complexity of the blade structural damping mechanism, Rayleigh damping assumption is adopted to describe the blade structural damping in order to reduce the difficulty of analysis:

[0155] ;

[0156] in, This represents the damping matrix of the blade structure. and Represents the proportionality coefficient. and They represent The mass matrix and stiffness matrix of a multi-degree-of-freedom system.

[0157] Given the blade structure damping In the case of a multi-degree-of-freedom system with damped forced vibration, the differential equation is:

[0158] ;

[0159] in, Represents the steady-state response of the system in the geometric coordinate system. and These represent the velocity matrix and the acceleration matrix, respectively. This represents the external load array.

[0160] Using the normal mode matrix Decouple the differential equation of damped forced vibration of a multi-degree-of-freedom system, and let ,in, Express the steady-state response of the system in the natural coordinate system, substitute it into the differential equation of damped forced vibration of a multi-degree-of-freedom system, and multiply by the left side. Simultaneously, normal excitation force ,in, This represents the canonical excitation force matrix. Normal mode matrix The transpose of the simplification equation yields the steady-state response. for:

[0161] ;

[0162] in, Denotes the first... of the system in the natural coordinate system First steady-state response, Indicates the excitation frequency, Describing the first degree of freedom of a multi-degree-of-freedom system First natural frequency, Indicates the system uptime. Indicates frequency ratio, This represents the modal damping ratio measured by the system's free decay vibration experiment. Indicates phase difference, This represents the amplitude of the normal excitation force.

[0163] First, transform the results from the natural coordinate system back to the geometric coordinate system to obtain the steady-state response of the system in the geometric coordinate system. for:

[0164] ;

[0165] in, Represents the steady-state response of the system in the geometric coordinate system. This represents the regularization factor corresponding to different degrees of freedom. This represents the principal modes of motion corresponding to different degrees of freedom. This represents the steady-state response results corresponding to different degrees of freedom.

[0166] Then, based on the steady-state response analysis results and the static equilibrium position of the fatigue test system, the system's first steady-state operation was obtained. Dynamic displacement of each discretized section :

[0167] ;

[0168] in, Indicates the first of the blades The static equilibrium position of each discretized section. Indicates the first of the blades Steady-state response results for each discretized cross section.

[0169] Then, the central difference method was used to determine the dynamic displacement. Calculate the first blade Curvature of a discretized section :

[0170] ;

[0171] in, , and These represent the first and second leaves of the blade. The, the The and the first Dynamic displacement of a discrete cross section. and These represent the first and second leaves of the blade. The and the first The location of each discretized section.

[0172] It should be noted that in step 2.5 of this application... Indicates the first The curvature of a discretized section, and the above formula uses Indicates the first The curvature of the discretized section is neither contradictory nor conflicting. The former focuses on characterizing the curvature of the first discretized section. The specific value of the curvature of the discretized section, the latter focusing on characterizing the first... A functional expression for the curvature of a discretized section. That is, using the latter. The curvature value calculated by the formula is Similar to the specific numerical value and formulaic expression of slope in mathematics.

[0173] Step 3.3, according to the blade number The curvature calculation system for the discretized cross section during steady-state operation... Dynamic bending moment of a discretized section.

[0174] Specifically, according to step 3.2 above, the first blade... Curvature of a discretized section The computing system during steady-state operation Dynamic bending moment of each discretized section The system boundary conditions were processed using linear extrapolation and a beam boundary condition with zero free-end bending moment, resulting in the system's boundary condition at the [missing information - likely a specific point or time period]. Dynamic bending moment of each discretized section for:

[0175] ;

[0176] in, Indicates the first The curvature of a discretized cross section Indicates the first Bending stiffness of a discrete cross section and These represent the dynamic bending moments of the second and third discretized sections, respectively.

[0177] Step 3.4, according to the above step 3.3. The dynamic bending moment of the discretized section is obtained. The bending moment amplitude of each discretized section.

[0178] In step 3.3, the system's steady-state operating time is calculated. Dynamic bending moment of each discretized section After that, the Maximum bending moment of each discretized section Minimum bending moment It can be represented as:

[0179] ;

[0180] Therefore, we can obtain the first... Bending moment amplitude of each discretized section for:

[0181] .

[0182] Understandably, this application, on the one hand, establishes a fatigue test load calculation model for large wind turbine blades that considers pre-bending and geometric nonlinearity. The calculation results and measured data are all controlled within the range of 0-70% of the blade span. Within this scope, the accuracy of load calculation is significantly improved. On the other hand, by introducing a stiffness matrix corrected for geometric nonlinearity, the stiffness change of the blade under pre-bending and self-weight coupling is accurately reflected, effectively avoiding local overload or underload phenomena and improving the efficiency of full-scale structural fatigue testing of wind turbine blades. Furthermore, the calculation process of this method is clear, and the required parameters are easy to obtain, which can directly guide the optimization of fatigue testing schemes in engineering practice, demonstrating good engineering application value.

[0183] Example 2

[0184] To better understand the impact of pre-bending and geometric nonlinearity on the fatigue test load calculation of large wind turbine blades, based on Embodiment 1 above, Embodiment 2 is described in detail below with reference to the accompanying drawings, illustrating the discretization modeling of wind turbine blades, the calculation of the average load, and the calculation of the load amplitude. Embodiment 2 focuses on a full-size structural fatigue test of a 90-meter wind turbine blade, but the fatigue test load calculation method proposed in Embodiment 1 can be applied to the fatigue test load calculation of wind turbine blades of different sizes. Equivalent substitutions or simple variations made by those skilled in the art based on the technical solution of this invention are all within the scope of protection of this invention.

[0185] This embodiment provides a method for calculating fatigue test loads of large wind turbine blades considering pre-bending and geometric nonlinearity, specifically including:

[0186] Step 1: Establish a discretized model of the wind turbine blade considering pre-bending, and optimize the cross-sectional data by piecewise cubic Hermite interpolation.

[0187] Specifically, the discretization modeling and data processing of wind turbine blades are as follows:

[0188] This embodiment selects a 90-meter wind turbine blade as the research object. The blade root is fixed, and the wind turbine blade is simplified into a cantilever beam model with pre-bending and discretized. See [link to relevant documentation]. Figure 2 .in, Figure 2 This application illustrates a discretized cantilever beam model of a wind turbine blade considering pre-bending. Figure 2 The left side represents the blade root, and the right side represents the blade tip (the blade tip can be appropriately trimmed during actual analysis). The x-direction is the opposite of vertical, y represents the axis of the wind turbine (wind turbine blades are mounted on the wind turbine; typically, the axis of the wind turbine in a horizontal wind turbine is parallel to the ground), and z represents the blade's extension direction. The distance from the blade root to the blade tip is defined as follows: There are nodes, totaling The discretized finite elements are: (the first discretized section is at the blade root, the (1)th finite element is at the blade root, and the (2)th to (3)th finite elements are distributed sequentially along the blade extension direction). (Finite element), each finite element corresponds to two discretized sections, each discretized section has two degrees of freedom, each corresponding to a set of deflection and rotation angles. For example, in the figure, the first... The finite element corresponds to the th finite element The node, the The left side of the nth node is the nth The discretized section, the right side is the first... The discretized cross section, wherein the first discretization section... Each discretized section represents the deflection of the corresponding pre-bending line. The angle of the pre-bent line is , No. The same applies to each discretized section (specifically, defining the angle of the pre-bent line on the first discretized section). Deflection of the pre-bent line on the first discretized section , Figure 2 For other nodes, see section [number]. The node, the (The relevant description of each discretized section), and then the cumulative algorithm can be used to solve for each of the first to the last discretized sections in turn. The deflection and rotation angle of the pre-bending line corresponding to each discretized section.

[0189] It should be noted that this application selects a 90-meter wind turbine blade as the research object. During the fatigue test in the flapping direction, the blade was trimmed at 93% of its span, removing the tip portion. Based on the relevant data of this 90-meter blade, the blade was simplified into a cantilever beam model, and the number of discretized finite element models was [number missing]. The model has 85 nodes. To improve computational accuracy while ensuring efficiency, four points are inserted between every two nodes in the original discretization. After interpolation optimization, the total number of nodes is [number missing]. .

[0190] Step 2: Calculate the rotation angle and deflection of the pre-bending line on each discretized section of the blade based on the optimized cross-sectional data, and use the geometrically nonlinear corrected stiffness matrix to calculate the static equilibrium position and mean bending moment of each discretized section of the blade under static load.

[0191] After discretizing the wind turbine blades in step 1 and interpolating and optimizing the original data, the blade pre-bending deflection and the rotation angle of the pre-bending line are calculated. The calculation results are shown in [reference needed]. Figure 3 .from Figure 3 It can be seen that the maximum deflection and the angle of the pre-bending line both occur at the blade tip, with a maximum deflection of 5.6m and a maximum angle of 8.5°. These results are consistent with the maximum deflection and angle of the pre-bending line values ​​given in the test report for this 90m blade. Based on the pre-bending, a vibrator saddle is installed at 53.3% of the wind turbine blade's span. When the wind turbine blade reaches static equilibrium under the combined action of its own weight and the weight of the vibrator saddle, the displacement of the wind turbine blade at its static equilibrium position is shown in [reference needed]. Figure 4 .from Figure 4 As can be seen, after considering the effects of pre-bending and geometric nonlinearity, the blade axis is no longer horizontal when the wind turbine blade fatigue test system is in static equilibrium. After calculating the displacement of the blade in static equilibrium position, the discretized blade can be further calculated. The mean bending moment of each discretized section, see [reference]. Figure 5 .from Figure 5 As can be seen, the average bending moment is the largest at the leaf root, and gradually decreases from the leaf root to the leaf tip along the blade span until the average bending moment is zero at the leaf tip. This trend of the average bending moment is the same as the trend of the average bending moment described in the standard.

[0192] Step 3: Based on the static equilibrium position of the blade, the bending moment amplitude on each discretized section of the blade is solved by using the multi-degree-of-freedom system dynamics method during steady-state operation of the blade fatigue test system.

[0193] Based on the static equilibrium position and mean bending moment of the wind turbine blade calculated in step 2, the blade was excited at the first-order flapping natural frequency of 0.34Hz during full-scale structural fatigue testing. The multi-degree-of-freedom system vibrated back and forth at the static equilibrium position. When the system reached steady-state vibration, 10 cycles of steady-state operation were selected, with 400 points selected for each cycle and the first five modes selected to solve for the steady-state response of the system. Then, the discretized blade... The motion of each discretized section was analyzed. Then, the dynamic bending moment of each discretized section of the blade at 4000 time points was calculated, and the maximum and minimum values ​​of the dynamic bending moment at each discretized section were obtained. (See [link to relevant documentation]). Figure 6 . Figure 6 The solid black line represents the calculated mean bending moment of each section of the blade. The dashed black line and the dotted black line represent the maximum and minimum bending moments of each section, respectively. The gray area represents the range of dynamic bending moment variation. The figure shows that the maximum bending moment occurs at the blade root, decreasing towards the blade tip. A slight fluctuation in bending moment occurs at the 53.3% mark where the exciter saddle is installed on the blade.

[0194] After calculating the maximum and minimum bending moments of each discretized section of the wind turbine blade, the bending moment amplitude can be obtained. This amplitude is then compared with the bending moment amplitude converted from the actual strain data obtained by full-scale structural fatigue testing of the wind turbine blade. The results are shown in [reference needed]. Figure 7 . Figure 7 The solid line represents the bending moment amplitude of each section of the blade, calculated considering pre-bending and geometric nonlinearity. The scatter plot represents the bending moment amplitude converted from the measured strain data of the full-size structural fatigue test of the wind turbine blade. It can be seen that the bending moment amplitudes of the two are in good agreement within the range of 0-70% of the blade span, and the errors are all controlled within a certain range. Within this range, both the verification area and the error results meet the requirements of industry standards. Furthermore, the bending moment amplitudes of both methods match very well within the blade spanwise range of 2% to 44.4%, with the error controlled within [specific range]. Within.

[0195] To further verify the effectiveness and advantages of the method for calculating test loads considering pre-bending and geometric nonlinearity, the method proposed in this application is compared and analyzed with existing test load calculation methods that do not consider pre-bending and geometric nonlinearity. The results are shown in [reference needed]. Figure 8 . Figure 8 The solid line represents the bending moment amplitude of each section of the blade calculated without considering pre-bending and geometric nonlinearity. The scatter points represent the bending moment amplitude converted from the measured strain data of the full-size structural fatigue test of the wind turbine blade. It can be seen that the error in bending moment amplitude between the two is controlled within 20% in the blade spanwise range of 0-33%, and within -30% in the blade spanwise range of 35.6%-70%. Furthermore, the bending moment amplitudes of the two methods match well in the blade spanwise range of 2%-42.2%, with the error controlled within [missing information]. Within. Combined Figure 7 , Figure 8 It can be seen that for the calculation of test loads for large wind turbine blades, the results calculated considering pre-bending and geometric nonlinearity are better than those calculated without considering pre-bending and geometric nonlinearity in the blade spanwise range of 0-70%. This shows that considering pre-bending and geometric nonlinearity is very important for the calculation of test loads for large wind turbine blades. The comparative analysis further illustrates the effectiveness and advantages of the method of calculating test loads considering pre-bending and geometric nonlinearity.

[0196] This application uses specific examples to illustrate the principles and implementation methods of the present invention. The above description of the embodiments is only for the purpose of helping to understand the core ideas of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for calculating fatigue test loads for large wind turbine blades taking into account pre-bending and geometric nonlinearity, characterized in that, The method comprises: Step 1, establishing a pre-bending wind turbine blade discretization model, optimizing the section data by piecewise cubic Hermite interpolation method; Wherein, the pre-bending wind turbine blade discretization model is to simplify the wind turbine blade into a plurality of cantilever beam models considering pre-bending for calculation, and the number of discrete sections of the blade is increased after optimization by piecewise cubic Hermite interpolation method; Step 2, calculating the turning angle of the pre-bending line and the deflection of the pre-bending line on each discrete section of the blade based on the optimized section data, and calculating the static equilibrium position and bending moment average of each discrete section of the blade under the action of static load by using the geometric nonlinear correction stiffness matrix; Among them, the first blade The angle of the pre-bent line on each discretized section is based on the blade front. The curvature of the discretized section is determined, and the blade's first... The deflection of the pre-bent line on each discretized section is based on the blade front. The rotation angle of each discretized section is determined, and the rotation angle of the pre-bent line on the first discretized section is defined. Deflection of the pre-bent line on the first discretized section , is a positive integer greater than 1; among which, the stiffness matrix of the geometric nonlinear correction can characterize the change of geometric stiffness with respect to elastic stiffness caused by load effects; The blade section Moment of inertia of each discretized section Is represented as: ； in, and These represent the mean bending moments of the second and third discretized sections, respectively. Indicates the first The curvature of a discretized cross section Indicates the first Bending stiffness of a discrete cross section This represents the total number of blades after the discretization cross section is increased; Step 3, based on the static equilibrium position of the blade, using multi-degree-of-freedom system dynamics method, solving the bending moment amplitude of each discrete section of the blade in the steady state operation of the blade fatigue test system.

2. The method for large wind turbine blade fatigue test load calculation considering pre-bending and geometric nonlinearity according to claim 1, characterized in that, Step 1 includes: Step 1.1, the wind power blade is simplified into a plurality of cantilever beam models considering pre-bending, respectively calculated, and the blade is discretely divided into paragraphs; wherein the blade is discretely divided into segments in common each of the discretized segments has two degrees of freedom, respectively, a deflection of the pre-bending line and a rotation angle of the pre-bending line , for the i-th discretized segment, its bending stiffness, mass distribution and radius of curvature are respectively , , , , ; a positive integer greater than ; Step 1.2, increasing the number of discrete sections of the pre-bending cantilever beam model in step 1.1, and using piecewise cubic Hermite interpolation optimization to increase the number of section related attributes; The cross-section related attributes include: bending stiffness of the first discrete cross-section , ( ) mass distribution of the first discrete cross-section , ( ) radius of curvature of the first discrete cross-section , ( ) and the like. The total number of the blade discrete cross-sections after the increase is represented.

3. The method for large wind turbine blade fatigue test load calculation considering pre-bending and geometric nonlinearity according to claim 1, characterized in that, Step 2 includes: Step 2.

1. Calculate the turning angle of the pre-bending line and the deflection of the pre-bending line on the discretized section of the blade using the pre-bending considered cantilever beam model of Step 1 to get the pre-bending line turning angle and the deflection of the pre-bending line on the first discretized section of the blade as: ​ ； ； in, Indicates the first of the blades A discrete cross section Indicates the first The discretized section, i.e. the first discretized section. Each discretized section is the front of the blade. Any one of the discretized sections, Indicates the first blade The curvature of the pre-bent line on a discretized cross section Indicates the first blade The radius of curvature of a discretized section Indicates the first blade The curvature of the pre-bent line on a discretized cross section This represents the total number of blades after the discretization cross section is increased. and These represent the first and second blades, respectively. The and the first The angle of the pre-bent line on a discretized section and These represent the first and second leaves of the blade. and the The location of each discretized section And define the rotation angle of the pre-bent line on the first discretized section. Deflection of the pre-bent line on the first discretized section ; Step 2.

2. In calculating the stiffness of the computing system, the geometric stiffness is used to modify the elastic stiffness to account for the nonlinear stiffness change due to the load effect. For the first element, the total stiffness matrix of the element after the geometric nonlinear modification is: K = K + K ； Wherein, ； ； wherein the first finite element is the finite element corresponding to the first discretization section, Ktotai represents the total stiffness matrix of the first element taking into account the geometric nonlinearity correction, Kelast represents the elastic stiffness matrix of the first element, Kgeom represents the geometric stiffness matrix of the first element taking into account the geometric nonlinearity correction, Kbend represents the bending stiffness of the first element, , and respectively represent the bending stiffness of the first and the first discretization section, L represents the length of the first element, , and respectively represent the position of the first and the first discretization section of the blade, F represents the axial force of the first element, the negative sign of the axial force indicating that the axial force is a compression force, and vice versa.

4. The method for large wind turbine blade fatigue test load calculation considering pre-bending and geometric nonlinearity according to claim 3, characterized in that, After step 2.2, it further includes: Step 2.

3. Deflection according to the initial pre-bending line of the blade and the overall stiffness matrix considering the correction of geometric nonlinearity calculating the static equilibrium position of the blade fatigue test system; For the first discrete section of the blade the static equilibrium position in which the blade fatigue test system is placed is represented as: ； ； wherein, represents static displacement on the i-th discretized section due to the blade self-weight and the exciter's saddle weight, the exciter's saddle being a fixing device when the exciter is fixed to the blade surface, represents a matrix of static displacement on all sections, represents a matrix of static displacement on all sections, represents an inverse matrix of the overall stiffness matrix represents an overall stiffness matrix obtained by assembling the stiffness matrix of each discretized unit, represents an overall stiffness matrix obtained by assembling the stiffness matrix of each discretized unit, represents an overall stiffness matrix obtained by assembling the stiffness matrix of each discretized unit, represents an overall nodal force matrix of the matrix 5. The method for large wind turbine blade fatigue test load calculation considering pre-bending and geometric nonlinearity according to claim 4, characterized in that, After step 2.3, it further includes step 2.4 and step 2.5: Step 2.

4. Calculate the curvature of the discrete section at which the blade fatigue test system is in static equilibrium position , using central difference method ; and ; ； wherein, , and respectively indicate the static equilibrium position of the first , second and third discretized section of the blade, and respectively indicate the position of the first and second discretized section of the blade; Step 2.

5. Calculate the curvature of the discretized section according to the first Step 2.

6. Calculate the mean load of the bending moment of the discretized section according to the second ;​​ ； in, and These represent the mean bending moments of the second and third discretized sections, respectively. Indicates the first The curvature of a discretized cross section Indicates the first Bending stiffness of a discrete cross section.

6. The method for large wind turbine blade fatigue test load calculation considering pre-bending and geometric nonlinearity according to claim 4, characterized in that, Step 3 includes: Step 3.1, based on the static equilibrium position calculated in step 2.3, by establishing the differential equation of multi-degree-of-freedom system undamped free vibration, the natural frequency, main vibration mode, regularization factor and regular vibration mode matrix are calculated respectively; Step 3.2, establish the forced vibration differential equation of the multi-degree of freedom system with damping and solve it, calculate the steady-state response results of the system, the dynamic displacement of the first discrete section of the system when the system is in steady-state operation, the curvature of the first discrete section . ​ Step 3.3, calculating the curvature of the discretized section according to the first dynamic moment of the discretized section at steady state operation; and the second Step 3.

4. Calculate the dynamic moment of the first discretized section according to the dynamic moment of the first discretized section of step 3.3, resulting in a moment amplitude of the first discretized section. Step 3.

4. Calculate the dynamic moment of the first discretized section according to the dynamic moment of the first discretized section of step 3.3, resulting in a moment amplitude of the first discretized section. Step 3.

4. Calculate the dynamic moment of the 7. The method for large wind turbine blade fatigue test load calculation considering pre-bending and geometric nonlinearity according to claim 6, characterized in that, Step 3.1 includes step 3.1.1 and step 3.1.2; Step 3.1.

1. Before establishing the motion differential equations of the multi-degree-of-freedom system, the overall stiffness matrix of the system is known After, the overall mass matrix of the system is calculated ; where the global mass matrix represents the mass matrix of each discretized element assembled together to obtain the mass matrix of the global system for the first finite element, the mass matrix may be expressed as: ； wherein, denotes the unit length mass of the th element, , and denote the mass distribution of the th and the th discretized section, respectively, denotes the length of the th element, , and denote the position of the th and the th discretized section of the blade, respectively; Step 3.1.

2. Obtain the overall stiffness matrix of the system from step 3.1.1 and the overall mass matrix After that, the boundary conditions are applied to the system to obtain the undamped free vibration differential equation of the multi-degree-of-freedom system, and the natural frequency, the principal mode shape, the regularization factor and the regularized mode shape matrix of the system are calculated respectively; where the boundary conditions are the first and second rows of the global stiffness matrix and the first and second columns of the global mass matrix resulting in the stiffness matrix and the mass matrix of the multi-degree-of-freedom system, the undamped free vibration differential equation of the multi-degree-of-freedom system is represented as ； wherein, represents the steady-state response of the system in the geometric coordinate system, represents the acceleration array, represents a matrix with numerical values of 0, the natural frequency of the order of the multi-degree-of-freedom system is the natural frequency of the order of the multi-degree-of-freedom system is the mode shape of the order of the multi-degree-of-freedom system can be calculated by substituting into , the regularization factor is , the modal matrix obtained by combining all the mode shapes together is the mode mass is the order regularization factor is , and the transpose of is , then the regular modal shape matrix ； wherein, denotes a regularized mode shape matrix, denotes a regularization factor corresponding to different degrees of freedom, denotes a principal mode shape corresponding to different degrees of freedom.

8. The large wind turbine blade fatigue test load calculation method considering pre-bending and geometric nonlinearity according to claim 6, characterized in that, Step 3.2, establish the forced vibration differential equation of the multi-degree of freedom system with damping and solve it, calculate the steady-state response results of the system, the dynamic displacement of the first discrete section of the system when the system is in steady-state operation, the curvature of the first discrete section . ​ System steady state response results are: ； wherein, represents the first order steady-state response of the system in the natural coordinate system, represents the excitation frequency, represents the first order natural frequency of the multi-degree-of-freedom system, represents the time at which the system is operating, represents the frequency ratio, represents the modal damping ratio measured from the free decay vibration experiment of the system, represents the phase difference, represents the regular excitation force amplitude; The system has a dynamic displacement of the discretized section at steady state operation is: is: ； wherein, represents a static equilibrium position of the first discretized section of the blade, represents a steady state response result of the first discretized section of the blade. The curvature of the discretized section is given by: is given by:​ ； wherein, , and denote the dynamic displacement of the first , second and third discretized section of the blade, respectively, and denote the position of the first and second discretized section of the blade, respectively.

9. The large wind turbine blade fatigue test load calculation method considering pre-bending and geometric nonlinearity according to claim 6, characterized in that, Step 3.3, according to the first curvature of the discretized section the dynamic bending moment of the first discretized section , and the boundary conditions of the system are processed by setting the boundary conditions of the beam as 0 by using the linear extrapolation method and the free end bending moment, to obtain the dynamic bending moment of the first discretized section : ； wherein, denotes the curvature of the discretized section, denotes the bending stiffness of the discretized section, and denotes the dynamic bending moment of the 2nd and 3rd discretized section, respectively.

10. The large wind turbine blade fatigue test load calculation method considering pre-bending and geometric nonlinearity according to claim 6, characterized in that, Step 3.4, after calculating the dynamic bending moment of the first discretized section of the system at steady state operation Step 3.5, after calculating the dynamic bending moment of the first discretized section of the system at steady state operation Step 3.6, after calculating the dynamic bending moment of the first discretized section of the system at steady state operation Step 3.7, after calculating the dynamic bending moment of the first discretized section of the system at steady state operation​ ； wherein the maximum bending moment of the discretized section , the minimum bending moment are respectively expressed as: 。

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