A method for evaluating delamination fracture performance of a wind turbine blade sandwich structure

Abstract

The application discloses a kind of methods for evaluating the delamination fracture performance of wind power blade sandwich structure, it is related to blade sandwich structure experiment and simulation field, S1: prefabricated crack sample preparation;S2: sample processing;S3: experimental test;S4: theoretical analysis;S5: simulation modeling;S6: simulation model verification;S7: interface delamination backtracking analysis;The application can accurately obtain the delamination fracture material parameters of wind power blade sandwich structure, analyze the delamination failure load of sandwich structure by simulation model, obtain interface damage, crack propagation and other information of sandwich structure, and quantitatively evaluate the resistance of blade sandwich structure to delamination crack propagation.

Description

Technical Field

[0001] This invention relates to the field of blade testing and simulation, and in particular to a method for evaluating the delamination fracture performance of a sandwich structure in a wind turbine blade. Background Technology

[0002] Modern wind turbine blades have rapidly increased in length from tens of meters to hundreds of meters to adapt to lower wind speeds and capture more wind energy. This increase in blade size means a geometric increase in weight, resulting in significantly larger and more complex aerodynamic, gravitational, and inertial loads. Under extreme wind conditions, sudden stops, and frequent start-stop cycles, the core structure of wind turbine blades—the sandwich structure—is prone to damage at its interface, leading to debonding failure between the panel and the core material. Once the debonding area expands, it significantly reduces the local stiffness of the structure, ultimately causing catastrophic consequences such as skin wrinkling or even complete blade fracture.

[0003] Therefore, evaluating the delamination fracture performance of large deep-sea wind turbine blade sandwich structures is extremely important. Previous patents on sandwich structures have focused on evaluating panel failure and core material failure performance. The only testing method for delamination failure in sandwich structures is the roller peel test. This structural performance test cannot obtain the key interface fracture material parameters required for simulation analysis, and there is a lack of simulation analysis methods for delamination fracture of sandwich structures. This prevents the consideration of delamination failure in simulation analysis, hindering the evaluation of delamination fracture performance of sandwich structures and their efficient and reliable design, thus restricting the large-scale development of deep-sea wind turbine blades. Summary of the Invention

[0004] This patent aims to fill the gap in the evaluation method of the fracture performance of sandwich structures, so as to improve the design capability of large blade structures in terms of structural integrity.

[0005] This invention discloses a method for evaluating the delamination fracture performance of a wind turbine blade sandwich structure, the method comprising the following steps:

[0006] S1: Prepare pre-cracked samples;

[0007] The pre-cracked sample includes a panel, a pre-crack, and a core material;

[0008] S2: Process the pre-made cracked sample to obtain peel delamination test sample and peel shear test sample.

[0009] S3: Test the delamination test sample and the delamination shear test sample to obtain load-displacement data, and obtain the interface delamination crack length through a high-resolution camera.

[0010] S4: Based on experimental data, a model of compliance variation with crack length is obtained, and then compliance theory is used to obtain the evolution of energy dissipated by interfacial delamination fracture with crack length.

[0011] S5: Establish a simulation model including the panel, core material, delamination interface and pre-existing cracks;

[0012] S6: Compare and analyze the simulation results with the experimentally recorded load-displacement data, and determine whether the interface delamination and cracking process in the simulation is consistent with the experiment.

[0013] S7: Analyze the results obtained from the analysis step to confirm the interface damage index and interface fracture index; and change the panel thickness in the simulation model to evaluate the effect of increasing the panel thickness on the interface crack propagation resistance, thereby evaluating the change in crack propagation resistance when different panels are reinforced.

[0014] Furthermore, in step 1, a certain distance of Teflon film is laid on the surface of the slotted and perforated core material, and then the panel material is laid; after the laying is completed, resin is injected for curing, so that there are pre-made cracks between the panel and the core layer separated by the Teflon film.

[0015] Furthermore, step 2 also includes the following steps:

[0016] S2.1: Cut the pre-cracked plate into a cuboid sample with a pre-crack at one end, and then attach loading blocks to the upper and lower surfaces of the sample near the pre-crack side to obtain a delamination test sample.

[0017] S2.2: Perform a secondary cut on the cuboid sample, remove the core material portion near the pre-crack side and then re-attach it to obtain a peel-shear delamination sample; the length of the removed core material should be less than the length of the pre-crack, and there should be a gap between the re-attached core material and the unremoved core material.

[0018] Furthermore, step 3 also includes the following steps:

[0019] Step S3.1: Connect the upper and lower loading blocks with loading pins to load the peeling and delamination test sample. Keep the height direction of the lower loading block connection unchanged, and move the upper loading block connection upward to cause the panel and core material to open and the sample interface to peel and deform. Record the displacement and load through the load sensor and displacement sensor of the testing machine.

[0020] Step S3.2: The peeling and shearing delamination sample is placed on a three-point bending support fixture, which consists of two lower supports and an upper loading head. The lower supports are kept fixed, and the upper loading head moves downward to cause peeling and shearing deformation between the core material and the panel. The cracks expand as the loading displacement increases. The displacement and load are recorded by the load sensor and displacement sensor of the testing machine.

[0021] Furthermore, in step 4, the load-displacement data from the delamination test specimens are obtained to calculate the compliance at the moment before each crack propagation, and the distance and timing of crack propagation are observed by taking photos or videos. All compliance-crack length data are fitted using a cubic polynomial to obtain the compliance-crack length variation function, i.e., the cubic polynomial function of C as a changes. Then, based on the load P during crack propagation and the specimen width B, the delamination fracture material parameter GI can be obtained.

[0022] ;

[0023] Where P is the load during crack propagation, B is the sample width, and dC / da is the partial derivative of compliance with respect to crack length;

[0024] Discarding the GI values ​​of the delamination fracture material when the plastic deformation is too large, the average of the tested GI values ​​of the delamination fracture material is the delamination fracture material parameter of the sandwich structure.

[0025] The material parameters for shear fracture were obtained in the same manner.

[0026] Furthermore, step 5 also includes the following steps:

[0027] Step S5.1: Construct a geometric model consistent with the experimental scheme based on the finite element analysis software ABAQUS;

[0028] Step S5.2: Mesh the geometric model and separate the nodes at the prefabricated cracks in the panel and core layer;

[0029] Step S5.3: Insert an interfacial cohesive element in the potential interfacial cracking area, i.e. the connection between the panel and the core layer, and assign the element type "Cohesive". Establish the "Traction-separation" elastic model, the "QUADS" damage initiation model, and the "BK" damage evolution model in the material property interface, and assign the delamination fracture material parameter GI to the interfacial cohesive element to accurately simulate the potential cracking area.

[0030] Step S5.4: Set boundary conditions consistent with experimental conditions.

[0031] Furthermore, in step S5.2, the grid size is 2mm × 2mm;

[0032] In step S5.3, the panel uses a continuous shell element type and the core material uses a fully integral solid element type; interfacial cohesive elements are inserted in the potential cracking region between the panel and the core material.

[0033] Furthermore, in step S5.4, for the peel test, reference point 1 and reference point 2 are coupled and bound to the upper and lower loading surfaces respectively; the rotational degree of freedom of reference point 1 about the y direction is released, and a movement along the z direction is applied to reference point 1; the remaining 4 degrees of freedom of reference point 1 are constrained; the rotational degree of freedom of reference point 2 about the y direction is released, and the remaining 5 degrees of freedom of reference point 2 are constrained.

[0034] The loading of the shear sample is achieved through a cylindrical rigid body. Fixed constraints are applied to the six degrees of freedom of the control points of the lower supporting cylinder. The z-direction displacement is applied to the control points of the upper loading cylinder to achieve loading. The remaining five degrees of freedom of all nodes of the upper loading cylinder are constrained. The potential contact area is simulated using a general contact model.

[0035] Set the output of the simulation model, including: load and displacement at the reference point, load and displacement at the rigid body control point, nodal displacement, stress at the element integration point, strain at the element integration point, and interface damage index and interface fracture index at the interface element integration point.

[0036] Furthermore, in step 6, based on the load and displacement data of reference point 1, the overall deformation data of the structure, and the damage and fracture indices of the interface elements, the load-displacement data of reference point 1 in the simulation results are compared and analyzed with the load-displacement recorded by the crossbeam of the experimental machine. The simulation results are compared and analyzed to see if the damage and fracture process of the interface cohesive elements is consistent with the delamination cracking results recorded by the camera in the experiment, thus verifying the reliability of the simulation model.

[0037] Furthermore, in step 7, the initial crack length in the peeling shear simulation model is changed to obtain the peak load variation under different crack lengths. The higher the peak load, the stronger the surface crack propagation resistance. This is used to analyze the change in the crack propagation resistance of the structure under different crack lengths, and then evaluate the degree of influence of different degrees of delamination damage on the load-bearing performance of the sandwich structure.

[0038] Meanwhile, the panel thickness in the simulation model was changed to evaluate the effect of increasing panel thickness on the interface's resistance to crack propagation, so as to obtain the peak load variation under different panel thicknesses. The higher the peak load, the stronger the surface's resistance to crack propagation. This was used to analyze the changes in the resistance to crack propagation when different panels were thickened and reinforced, and then to evaluate whether the reinforcement scheme was effective.

[0039] The beneficial effects achieved by this invention are:

[0040] This invention prepares test samples of pre-cracked sandwich structures and obtains material parameters for delamination fracture at the interface of the sandwich structure based on theoretical and experimental methods. A simulation model of delamination fracture at the interface of the sandwich structure with pre-cracks is constructed. Cohesive elements simulating delamination are inserted into the potential delamination region, and the experimentally measured delamination fracture parameters are assigned to the cohesive elements to verify the effectiveness of the delamination fracture simulation model. Furthermore, damage and fracture information during the delamination fracture process is obtained based on the model, the delamination fracture resistance of the sandwich structure is evaluated, and corresponding delamination performance reinforcement schemes are proposed based on the simulation model. Attached Figure Description

[0041] Figure 1 This is a flowchart of the present invention;

[0042] Figure 2 Schematic diagram of the preparation of peeled and peeled sheared samples;

[0043] Figure 3 Schematic diagram of the peel sample and peel shear test;

[0044] Figure 4 Data for material parameter processing in interface delamination fracture;

[0045] Figure 5 Simulation models for peeled and peeled sheared samples;

[0046] Figure 6 To compare the experimental and simulation stratification processes;

[0047] Figure 7 This refers to the interface delamination fracture index in the model.

[0048] Figure 8 To change the load-displacement curves of crack length and panel thickness in the simulation. Detailed Implementation

[0049] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer as a result. However, these embodiments are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions can be made to the details and form of the technical solutions of the present invention without departing from the spirit and scope of the present invention, but all such modifications and substitutions fall within the protection scope of the present invention.

[0050] like Figure 1 As shown, a method for evaluating the delamination fracture performance of a wind turbine blade sandwich structure, taking a PET core material + glass fiber reinforced composite panel as an example, is used to conduct delamination experimental tests and simulation modeling analysis on the sandwich structure, including the following steps:

[0051] S1: Preparation of pre-cracked samples

[0052] First, a Teflon film with a certain distance is laid on the surface of the grooved and perforated core material, in this example, a 60mm Teflon film. Then, the panel material is laid. After the laying is completed, resin is injected for curing, so that there are pre-made cracks between the panel and the core layer separated by the Teflon film.

[0053] S2: Sample processing;

[0054] The pre-fabricated cracked sample is processed to obtain delamination test samples and delamination shear test samples; further, step S2 also includes:

[0055] S2.1: The prepared pre-cracked plate is cut into a cuboid sample with a pre-crack at one end. Then, a loading block is bonded to the upper and lower surfaces of a portion of the sample near the pre-crack. One side of the loading block is flush with the end of the sample. The diameter of the opening at the center of the loading block is consistent with the diameter of the loading pin of the testing machine to facilitate subsequent loading of the sample. Thus, the peeling and delamination test sample is obtained.

[0056] S2.2: Since it is necessary to remove the core material off-center from the center of the peel shear test sample, taking a 60mm pre-cracked sample as an example, the width of the cut is 5mm, 10mm from the tip of the pre-crack, and 45mm from the end of the crack. Taking the end as the zero point, it is necessary to cut off the core material within the range of 45-50mm. Since there is a Teflon film between the core material and the panel within the range of 0-45mm, the core material within the range of 0-45mm will fall off. It is necessary to glue the core material within the range of 0-45mm back. Thus, the peel shear delamination sample is obtained.

[0057] like Figure 2 The figures shown are schematic diagrams of the cross-sections of the delamination test sample and the delamination shear test sample.

[0058] The cross-sectional diagram of the peel delamination test specimen shows that it consists of a glass fiber reinforced composite panel, a Teflon film, and a core material. The upper part is the glass fiber reinforced composite panel, and the lower part is the core material, with a portion of the Teflon film between the core material and the panel. Since the Teflon film does not bond to the core material or the glass fiber panel before or after sample molding, a pre-existing crack can form between the core layer and the panel. In this example, the pre-existing crack length is 60 mm, and the peel specimen length is 200 mm. The material composition of the peel shear test specimen is the same as that of the peel specimen, except that a portion of the core material needs to be removed from the peel shear specimen, and then the fallen core material is adhered to one end of the specimen as a support.

[0059] S3: Experimental testing;

[0060] The delamination test samples and delamination shear test samples are tested to obtain load-displacement data, and the interface delamination crack length is obtained using a high-resolution camera; further, step S3 also includes:

[0061] Step S3.1: Connect the peeling sample loading block to the test machine crossbeam by loading pins, and control the movement of the crossbeam to cause peeling deformation at the sample interface.

[0062] The peel test process is as follows Figure 3 As shown in (a), the pre-existing crack is on the left side of the sample. Aluminum loading blocks are bonded to the upper and lower surfaces of the left side of the sample, and the loading blocks are connected to the testing machine via pins. The lower connection remains at a constant height, while the upper connection moves upwards, causing an opening displacement between the panel and the core material. Peeling deformation occurs at the interface, and this deformation gradually increases with the loading process, causing the crack to gradually expand. A scale is attached to the side of the sample to facilitate the acquisition of crack length changes during crack expansion. Displacement and load are recorded using the load sensor and displacement sensor of the testing machine.

[0063] Step S3.2: The bending deformation of the sample is achieved by using a three-point bending fixture. A rubber pad is added between the loading head and the sample to prevent the sample and the fixture from sliding against each other, and to cause peeling and shearing deformation at the sample interface.

[0064] The peel shear test process is as follows: Figure 3 As shown in (b), the three-point bending fixture consists of two lower supports and an upper loading head. The specimen is placed on the support fixture of the three-point bending fixture with a span of 100 mm. A rubber pad is bonded between the upper loading head and the specimen to prevent relative slippage between the loading head and the specimen during loading. Keeping the lower supports fixed, the upper loading head moves downward, causing peeling shear deformation between the core material and the panel. The crack expands as the loading displacement increases. The displacement and load are recorded by the load sensor and displacement sensor of the testing machine.

[0065] S4: Theoretical Analysis;

[0066] Based on experimentally obtained models of compliance variation with crack length, and then compliance theory is used to obtain the evolution of energy dissipated by interfacial delamination fracture with crack length.

[0067] For peeled samples, such as Figure 4 As shown, the compliance of the crack at the moment before each propagation is calculated using the load-displacement data obtained through S3, where compliance = displacement / load. The distance and time of crack propagation are observed by taking photos or videos. Therefore, a single load-displacement curve can obtain multiple sets of compliance-crack length data. By fitting all the compliance-crack length data, the curve of compliance versus crack length can be obtained. Using a cubic polynomial to fit all the compliance-crack length data, the function of compliance versus crack length is obtained, that is, the cubic polynomial function of C versus a. Then, based on the load P during crack propagation and the width B of the sample, the change of the delamination fracture material parameter GI (delamination fracture material parameter) with crack length can be obtained.

[0068]

[0069] Where P is the load during crack propagation, B is the sample width, and dC / da is the partial derivative of compliance with respect to crack length. Because the panel undergoes significant plastic deformation under large deformation, the experimentally obtained GI value deviates from the true value. Therefore, GI values ​​with excessive plastic deformation are discarded; in this example, data with crack lengths exceeding 100 mm are discarded. Finally, the average of the remaining GI values ​​is taken as the material parameters for delamination fracture of the sandwich structure. The data processing procedure for the peel shear sample is the same as that for the peel sample.

[0070] S5: Simulation modeling;

[0071] Establish a simulation model including the panel, core material, delamination interface, and pre-existing cracks; further, step S5 also includes:

[0072] Step S5.1: Construct a geometric model consistent with the experimental scheme based on the finite element analysis software ABAQUS.

[0073] Step S5.2: Mesh the geometric model and separate the nodes at the prefabricated cracks in the panel / core layer.

[0074] Step S5.3: Insert an interfacial cohesive element in the potential interfacial cracking area, i.e., the panel / core layer connection, and assign the element type "Cohesive". Establish the "Traction-separation" elastic model, the "QUADS" damage initiation model, and the "BK" damage evolution model in the material property interface. Input the interfacial fracture parameter GI obtained experimentally in S4 into the "BK" damage evolution model. Establish the "Cohesive" section property and assign the established material property and section property to the cohesive element to accurately simulate the potential cracking area.

[0075] like Figure 5 As shown, geometric models of the peeled sample and the peeled shear sample are established based on the sample geometry, and then meshed with a mesh size of 2mm×2mm; the panel uses continuous shell element type and the core material uses full integral solid element type; interfacial cohesive elements are inserted in the potential cracking area between the panel and the core material.

[0076] Step S5.4: Set boundary conditions consistent with experimental conditions. For the peel test, reference point 1 and reference point 2 are coupled and bound to the upper and lower loading surfaces respectively. The length of the upper and lower loading surfaces is 10mm, and the width is the same as the sample width, which is 60mm. Specifically, the *Coupling keyword in ABAQUS is used to bind reference point 1 to the node of the upper loading surface, and the *Coupling keyword is used to bind reference point 2 to the node of the lower loading surface. Release the rotational degree of freedom of reference point 1 about the y-direction, and apply a movement along the z-direction to reference point 1. Specifically, the constraint or release of the degree of freedom is achieved through the *Boundary keyword in ABAQUS, constraining the remaining 4 degrees of freedom of reference point 1; release the rotational degree of freedom of reference point 2 about the y-direction, and constrain the remaining 5 degrees of freedom of reference point 2; the loading of the shear sample is achieved through a cylindrical rigid body. Fixed constraints are applied to the 6 degrees of freedom of the lower supporting cylinder control points, and displacement in the z-direction is applied to the upper loading cylinder control points to achieve loading. The remaining 5 degrees of freedom of all nodes of the upper loading cylinder are constrained. A general contact simulation is used to simulate the potential contact area. It should be noted that the motion of the rigid body is completely consistent with that of the control points, so the constraints of the rigid body can be directly applied to the control points; reasonably increase the simulation loading speed, and the simulation kinetic energy should not exceed 5% of the total energy; set the output, specifically including: load and displacement of the reference point, load and displacement of the rigid body control points, nodal displacement, element integration point stress, element integration point strain, interface damage index and interface fracture index of the interface element integration points.

[0077] S6: Simulation model verification;

[0078] The simulation results include: load and displacement data at reference point 1, deformation data of the overall structure, and damage and fracture indices of the interface elements. The load-displacement data at reference point 1 in the simulation results are compared and analyzed with the load-displacement recorded by the crossbeam of the experimental machine. The simulation results are also compared and analyzed to see if the damage and fracture process of the interface cohesive elements is consistent with the delamination cracking results recorded by the camera in the experiment.

[0079] like Figure 6 As shown in (a) and (b), the load-displacement curves of the peeled sample and the peeled shear sample are similar in trend and magnitude, which verifies the accuracy of the simulation model. Figure 6 (c) Comparison of experimental and simulation results of crack propagation process in the peeled sample. The overall deformation degree and crack propagation distance of the two are similar, which further verifies the reliability of the simulation model.

[0080] S7: Interface layered backtracking analysis;

[0081] The results obtained from the analysis step are analyzed to confirm the interface damage index and interface fracture index; and the panel thickness in the simulation model is changed to evaluate the effect of increasing the panel thickness on the interface crack propagation resistance, thereby evaluating the change in crack propagation resistance when different panels are reinforced.

[0082] like Figure 7 As shown, there are two indicators for the interface in the simulation: the interface damage index and the interface fracture index. When the interface damage index reaches 1, it indicates that the interface has begun to be damaged, and when the interface fracture index reaches 1, it indicates that the interface crack has begun to propagate.

[0083] Furthermore, the initial crack length in the peel shear simulation model was changed. In this example, the initial crack length was set to 25mm, 29mm, 33mm, 37mm, and 41mm to obtain the peak load variation under different crack lengths. The higher the peak load, the stronger the surface crack propagation resistance. This was used to evaluate the change in the crack propagation resistance of the structure under different crack lengths. Further, the panel thickness in the simulation model was changed to evaluate the effect of increasing panel thickness on the interface crack propagation resistance. In this example, the panel thickness was adjusted by changing the number of panel layers. Specifically, the number of layers was set to 3, 6, and 9 layers to obtain the peak load variation under different panel thicknesses. The higher the peak load, the stronger the surface crack propagation resistance. This was used to evaluate the change in crack propagation resistance when different panels were reinforced.

[0084] like Figure 8 As shown, the simulation model can be used to obtain the changes in peak load with different initial crack lengths. The results show that as the initial crack length increases, the peak load gradually decreases and tends to stabilize. To enhance the structure's resistance to crack propagation, the number of panel layers was increased for simulation analysis. The results show that, for the same crack length, the peak load at which crack propagation begins increases with the increase in the number of panel layers. The above results indicate that, based on the method described in this patent, the degree of performance reduction of wind turbine blade sandwich structures with delamination damage can be effectively evaluated. Simultaneously, the effectiveness of reinforcement schemes can be evaluated based on simulation results, i.e., by assessing the magnitude or variation of peak loads in different schemes. This solves the problem of insufficient methods for evaluating the delamination performance of blade sandwich structures. In particular, the proposed simulation analysis method can significantly improve the design and evaluation efficiency of reinforcement schemes for blade sandwich structures.

[0085] Currently, the delamination failure performance of wind turbine blade sandwich structures is mostly assessed through roller peeling. There is no material parameter testing method for interfacial delamination fracture, making it impossible to obtain the key interfacial fracture parameters required for interfacial fracture simulation analysis. Furthermore, the lack of simulation analysis methods for delamination failure of blade sandwich structures prevents the consideration of delamination failure in simulation analysis, thus hindering the development of larger wind turbine blades.

[0086] The method disclosed in this embodiment can effectively solve the above-mentioned problems. It only performs delamination fracture tests on PET core material + glass fiber reinforced composite material panels. By preparing pre-cracked sandwich structure test specimens, and obtaining the delamination fracture material parameters of the sandwich structure interface based on theoretical and experimental methods, a simulation model of delamination fracture of the sandwich structure interface with pre-cracks is constructed. Cohesive units simulating delamination are inserted in the potential delamination region and assigned cohesive delamination fracture material parameters to verify the effectiveness of the sandwich structure delamination fracture simulation model. Then, based on the model, the damage fracture information of the delamination fracture process is obtained, the delamination fracture resistance performance of the sandwich structure is evaluated, and an anti-delamination performance enhancement scheme for the sandwich structure is proposed based on the simulation model.

[0087] The above are merely preferred embodiments of the present invention and do not constitute any limitation on the scope of protection of the present invention; all technical solutions formed by equivalent transformations or equivalent substitutions fall within the scope of protection of the present invention; the parts of the present invention not described in detail are well-known technologies to those skilled in the art.

Claims

1. A method for evaluating the delamination fracture performance of a sandwich structure for wind turbine blades, characterized in that, The method for evaluating the delamination fracture performance of wind turbine blade sandwich structures includes the following steps: S1: Prepare pre-cracked samples; The pre-cracked sample includes a panel, a pre-crack, and a core material; S2: Process the pre-made cracked sample to obtain peel delamination test sample and peel shear test sample. S3: Test the delamination test sample and the delamination shear test sample to obtain load-displacement data, and obtain the interface delamination crack length through a high-resolution camera. S4: Based on experimental data, a model of compliance variation with crack length is obtained, and then compliance theory is used to obtain the evolution of energy dissipated by interfacial delamination fracture with crack length. S5: Establish a simulation model including the panel, core material, delamination interface and pre-existing cracks; S6: Compare and analyze the simulation results with the experimentally recorded load-displacement data, and determine whether the interface delamination and cracking process in the simulation is consistent with the experiment. S7: Analyze the results obtained from the analysis step to confirm the interface damage index and interface fracture index; and change the panel thickness in the simulation model to evaluate the effect of increasing the panel thickness on the interface crack propagation resistance, thereby evaluating the change in crack propagation resistance when different panels are reinforced.

2. The method for evaluating the delamination fracture performance of a wind turbine blade sandwich structure according to claim 1, characterized in that, In step 1, a certain distance of Teflon film is laid on the surface of the slotted and perforated core material, and then the panel material is laid; after the laying is completed, resin is injected for curing, so that there are pre-made cracks between the panel and the core layer separated by the Teflon film.

3. The method for evaluating the delamination fracture performance of a wind turbine blade sandwich structure according to claim 1, characterized in that, Step 2 also includes the following steps: S2.1: Cut the pre-cracked plate into a cuboid sample with a pre-crack at one end, and then attach loading blocks to the upper and lower surfaces of the sample near the pre-crack side to obtain a delamination test sample. S2.2: Perform a secondary cut on the cuboid sample, remove the core material portion near the pre-crack side and then re-attach it to obtain a peel-shear delamination sample; the length of the removed core material should be less than the length of the pre-crack, and there should be a gap between the re-attached core material and the unremoved core material.

4. The method for evaluating the delamination fracture performance of a wind turbine blade sandwich structure according to claim 1, characterized in that, Step 3 also includes the following steps: Step S3.1: Connect the upper and lower loading blocks with loading pins to load the peeling and delamination test sample. Keep the height direction of the lower loading block connection unchanged, and move the upper loading block connection upward to cause the panel and core material to open and the sample interface to peel and deform. Record the displacement and load through the load sensor and displacement sensor of the testing machine. Step S3.2: The peeling and shearing delamination sample is placed on a three-point bending support fixture, which consists of two lower supports and an upper loading head. The lower supports are kept fixed, and the upper loading head moves downward to cause peeling and shearing deformation between the core material and the panel. The cracks expand as the loading displacement increases. The displacement and load are recorded by the load sensor and displacement sensor of the testing machine.

5. The method for evaluating the delamination fracture performance of a wind turbine blade sandwich structure according to claim 1, characterized in that, In step 4, the load-displacement data from the delamination test specimens are obtained to calculate the compliance at the moment before each crack propagation. The distance and timing of crack propagation are observed by taking photographs or videos. All compliance-crack length data are fitted using a cubic polynomial to obtain the compliance-crack length variation function, i.e., a cubic polynomial function of C as a changes. Then, based on the load P during crack propagation and the specimen width B, the delamination fracture material parameter GI can be obtained. ； Where P is the load during crack propagation, B is the sample width, and dC / da is the partial derivative of compliance with respect to crack length; Discarding the GI values ​​of the delamination fracture material when the plastic deformation is too large, the average of the tested GI values ​​of the delamination fracture material is the delamination fracture material parameter of the sandwich structure. The material parameters for shear fracture were obtained in the same manner.

6. The method for evaluating the delamination fracture performance of a wind turbine blade sandwich structure according to claim 1, characterized in that, Step 5 also includes the following steps: Step S5.1: Construct a geometric model consistent with the experimental scheme based on the finite element analysis software ABAQUS; Step S5.2: Mesh the geometric model and separate the nodes at the prefabricated cracks in the panel and core layer; Step S5.3: Insert an interfacial cohesive element in the potential interfacial cracking area, i.e. the connection between the panel and the core layer, and assign the element type "Cohesive". Establish the "Traction-separation" elastic model, the "QUADS" damage initiation model, and the "BK" damage evolution model in the material property interface, and assign the delamination fracture material parameter GI to the interfacial cohesive element to accurately simulate the potential cracking area. Step S5.4: Set boundary conditions consistent with experimental conditions.

7. The method for evaluating the delamination fracture performance of a wind turbine blade sandwich structure according to claim 6, characterized in that, In step S5.2, the grid size is 2mm × 2mm; In step S5.3, the panel uses a continuous shell element type and the core material uses a fully integral solid element type; interfacial cohesive elements are inserted in the potential cracking region between the panel and the core material.

8. The method for evaluating the delamination fracture performance of a wind turbine blade sandwich structure according to claim 6, characterized in that, In step S5.4, for the peel test, reference point 1 and reference point 2 are coupled and bound to the upper and lower loading surfaces respectively; the rotational degree of freedom of reference point 1 about the y direction is released, and a movement along the z direction is applied to reference point 1; the remaining 4 degrees of freedom of reference point 1 are constrained; the rotational degree of freedom of reference point 2 about the y direction is released, and the remaining 5 degrees of freedom of reference point 2 are constrained. The loading of the shear sample is achieved through a cylindrical rigid body. Fixed constraints are applied to the six degrees of freedom of the control points of the lower supporting cylinder. The z-direction displacement is applied to the control points of the upper loading cylinder to achieve loading. The remaining five degrees of freedom of all nodes of the upper loading cylinder are constrained. The potential contact area is simulated using a general contact model. Set the output of the simulation model, including: load and displacement at the reference point, load and displacement at the rigid body control point, nodal displacement, stress at the element integration point, strain at the element integration point, and interface damage index and interface fracture index at the interface element integration point.

9. The method for evaluating the delamination fracture performance of a wind turbine blade sandwich structure according to claim 1, characterized in that, In step 6, based on the load and displacement data of reference point 1, the overall deformation data of the structure, and the damage and fracture indices of the interface elements, the load-displacement data of reference point 1 in the simulation results are compared and analyzed with the load-displacement recorded by the crossbeam of the experimental machine. The simulation results are compared and analyzed to see if the damage and fracture process of the interface cohesive elements is consistent with the delamination cracking results recorded by the camera in the experiment, thus verifying the reliability of the simulation model.

10. The method for evaluating the delamination fracture performance of a wind turbine blade sandwich structure according to claim 1, characterized in that, In step 7, the initial crack length in the peeling shear simulation model is changed to obtain the peak load variation under different crack lengths. The higher the peak load, the stronger the surface crack propagation resistance. This is used to analyze the change in the crack propagation resistance of the structure under different crack lengths, and then evaluate the influence of different degrees of delamination damage on the load-bearing performance of the sandwich structure. Meanwhile, the panel thickness in the simulation model was changed to evaluate the effect of increasing panel thickness on the interface's resistance to crack propagation, so as to obtain the peak load variation under different panel thicknesses. The higher the peak load, the stronger the surface's resistance to crack propagation. This was used to analyze the changes in the resistance to crack propagation when different panels were thickened and reinforced, and then to evaluate whether the reinforcement scheme was effective.

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