Hot spot stress fatigue calculation method based on consideration of nonlinearity

Abstract

The invention provides a hot spot stress fatigue calculation method based on consideration of nonlinearity, which comprises the following steps of: constructing a finite element model of a welding joint, dividing a weld toe line, a first interpolation line and a second interpolation line on the finite element model, and then respectively extracting principal stresses of all nodes on the weld toe line, the first interpolation line and the second interpolation line, the hot spot stress at the weld toe line is calculated through interpolation; setting two calculation working conditions, setting loading loads of the weld toe line, the first interpolation line and the second interpolation line on different time steps, and fitting a corresponding hot spot nonlinear load stress curve according to the hot spot stress and the loading loads; and performing interpolation calculation on the fatigue time sequence load in combination with the nonlinear load stress curve to obtain a corresponding hot spot stress fatigue time sequence, and then calculating the fatigue damage of the welding joint. According to the method, the problem that a fatigue calculation result is distorted due to the fact that a nonlinear change relation of a welding joint cannot be really considered during stress time sequence synthesis of a hot spot node is solved.

Description

Technical Field

[0001] This invention relates to the field of wind power generation technology, and in particular to a hot spot stress fatigue calculation method that takes into account nonlinearity. Background Technology

[0002] The design and development of wind turbine support structures requires simulation design for key structural components, and the accuracy of the simulation directly affects the design quality of these components. In recent years, with the rapid development of computer technology, the finite element analysis (FEA) method has been increasingly used in the design and development of wind turbine support structures. Among these methods, fatigue strength analysis of complex welded joints is mainly conducted using the hot spot stress method combined with finite element simulation, and its design quality directly determines the overall reliability of the wind turbine generator set. In the industry, the hot spot stress method recommended in the "Recommendations for Fatigue Design of Welded Joints and Components" (hereinafter referred to as the IIW standard) is mainly used for calculating hot spot areas of complex welded joints. The method consists of the following steps: 1) Selecting hot spot areas and planning weld toe lines, and screening hot spot nodes based on experience or commercial software; 2) Using the rectangular coordinate system established at each hot spot node as a reference, extracting the unit stress results at each interpolation node in the external normal direction of each hot spot node; 3) Exporting the unit stress results, and using a self-developed program or office software in conjunction with the hot spot stress extrapolation interpolation formula to calculate the unit stress at the hot spot node; 4) Using the calculated hot spot unit stress in conjunction with fatigue load and the SN curve of the welded joint to perform fatigue calculation.

[0003] Because the welded joint is affected by both the nonlinearity of the flange contact and the opening in the cylinder wall, it exhibits typical local stress concentration and nonlinear problems. If the traditional linear analysis method based on hot spot stress is used for fatigue analysis, when synthesizing the stress time sequence of the hot spot nodes, it can only assume that the relationship between stress and external load is linear. It cannot truly consider the nonlinear relationship of the welded joint at the end of the longitudinal flange, which leads to the distortion of fatigue calculation results and further affects the reliability of the overall structure. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a hot spot stress fatigue calculation method that considers nonlinearity. This method solves the problem that existing technologies cannot accurately consider the nonlinear variation of welded joints when synthesizing stress time series at hot spot nodes, thus leading to distorted fatigue calculation results.

[0005] According to an embodiment of the present invention, a method for calculating hot spot stress fatigue considering nonlinearity includes: A finite element model of the welded joint is constructed. Weld toe line, first interpolation line and second interpolation line are divided on the finite element model. The node information of weld toe line, first interpolation line and second interpolation line is obtained respectively. Then, the hot spot stress of all nodes on the weld toe line is calculated by interpolation based on the node information. Two calculation conditions are set up. Under the two conditions, the welding toe line, the first interpolation line and the second interpolation line are set with the load at different load steps. Based on the hot spot stress and the load obtained by interpolation at each load step, the corresponding hot spot nonlinear load stress curve is fitted by the multi-segment line fitting method. The fatigue time sequence load is interpolated with the nonlinear load stress curve to obtain the corresponding hot spot stress fatigue time sequence. Then, the fatigue damage of the welded joint is calculated based on the hot spot stress fatigue time sequence.

[0006] Preferably, the method for calculating hot spot stress includes: The finite element model is discretized into a mesh, and the node information on the weld toe line, the first interpolation line and the second interpolation line is determined according to the mesh to obtain the corresponding node sets C1, C2 and C3. The node information includes node coordinates and node principal stresses. Matching the node sets C1, C2, and C3 yields multiple interpolated node pairs; The hot spot stress of the nodes on the weld toe line is calculated based on the principal stresses at the first and second interpolation lines, respectively.

[0007] Preferably, after mesh discretization, the mesh network between the weld toe line and the first interpolation line, and between the first interpolation line and the second interpolation line, is a regular hexahedral mesh.

[0008] Preferably, all nodes between node sets C1 and C2, and between node sets C1 and C3, are interconnected, and the angle formed by the lines connecting the nodes of the two sets is within a certain range. The nodes within are combined into an interpolation node pair.

[0009] Preferably, the formula for calculating hot spot stress is as follows: in, Let be the hot spot stress at the i-th interpolation pair node on the weld toe line. The principal stress at the i-th interpolation pair node on the first interpolation line is... It represents the principal stress at the i-th interpolation pair node on the second interpolation line.

[0010] Preferably, the two calculation conditions include a minimum load + constant load condition and a maximum load + constant load condition; Two consecutive load steps are set. The first load step consists of n consecutive substeps, and the second load step consists of m consecutive substeps. The constant load is applied starting from the first load step and completed in n substeps. The minimum load (or maximum load) is applied starting from the second load step and completed in m substeps. After the calculation is completed, the nodal hot spot stresses under all substeps and the hot spot nonlinear load-stress curve equations of the fitted nodes for each substep are extracted.

[0011] Preferably, the equation for the hot spot nonlinear load stress curve is as follows: in, , These are the slope and intercept parameters fitted to the i-th line segment, respectively.

[0012] Preferably, the rainflow counting method and Miner's linear damage accumulation criterion are used to calculate fatigue damage.

[0013] Compared with the prior art, the present invention has the following beneficial effects: Based on the hot spot stress and corresponding loading load of all nodes of the weld toe line, the corresponding nonlinear hot spot nonlinear load stress curve is fitted in stages using the polyline fitting method. Then, fatigue damage is calculated based on the hot spot nonlinear load stress curve. This method is applicable to both linear and nonlinear analysis of fatigue calculation using the hot spot stress method, and more accurately fits the load-stress relationship curve, thus obtaining fatigue stress time series and damage calculation results that are more in line with reality. Attached Figure Description

[0014] Figure 1 This is a flowchart of the calculation method according to an embodiment of the present invention.

[0015] Figure 2 This is a model partitioning diagram of the weld toe line, the first interpolation line, and the second interpolation line in an embodiment of the present invention.

[0016] Figure 3 This is a discrete mesh diagram of the model in an embodiment of the present invention.

[0017] Figure 4 This is a multi-segment fitting diagram of the nonlinear load stress curve according to an embodiment of the present invention. Detailed Implementation

[0018] The technical solutions of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0019] like Figure 1 As shown, this embodiment of the invention proposes a method for calculating hotspot stress fatigue considering nonlinearity, including: A finite element model of the welded joint is constructed. Weld toe line, first interpolation line and second interpolation line are divided on the finite element model. The node information of weld toe line, first interpolation line and second interpolation line is obtained respectively. Then, the hot spot stress of all nodes on the weld toe line is calculated according to the node information. like Figure 2 As shown, a finite element model of the welded joint is constructed. Parallel weld toe lines, a first interpolation line (0.4t from the weld toe line, where t is the wall thickness), and a second interpolation line (1t from the weld toe line) are defined. These are then marked with segmentation marks on the model. Afterward, the welded joint undergoes mesh discretization to ensure that the meshes between the weld toe line and the first interpolation line, and between the first and second interpolation lines, are regular hexahedral meshes. Figure 3 As shown.

[0020] Then, obtain all node information (coordinates, principal stresses perpendicular to the interpolation lines) on the weld toe line, interpolation line 1, and interpolation line 2 to form a set: , , , in, C i For a set of nodes, This is coordinate information. Principal stress information.

[0021] Then, the coordinate information of sets C1, C2, and C3 is matched to form interpolation node pairs. Let C1i be the i-th node in set C1, C2i be the i-th node in set C2, and C3i be the i-th node in set C3. The matching rule is as follows: when the angle formed by the line connecting nodes C1i and C2i and the line connecting C2i and C3i in sequence is within the range of the i-th node in set C1i, the i-th node in set C2i, and the i-th node in set C3i, the i-th node in set C3i, and the i-th node in set C3i, the i-th node in set C1i, C2i, and ... C3i, and C3i, the i-th node in set C1i, C2i, C3i, and C3i, the i-th node in set C1i, C2i, C3i, C3i, C2i, C3i, C2i, C3i, C2i, C3i, C2i, C3i, C2i, C3i, C2i, C3i, C2i, C3i, C2i, C3i, C2i, C3i, C3i, C2i, C3i, C3i, C3i, C1i, C2i, and C3i are interpolation node pairs, where σ is the offset interpolation value, which is set according to the mesh quality.

[0022] Then, based on the principal stresses at the nodes of the first and second interpolation lines, the hot spot stresses at each node on the weld toe are calculated. The interpolation formula is as follows: in, Let be the hot spot stress at the i-th interpolation pair node on the weld toe line. The principal stress at the i-th interpolation pair node on the first interpolation line is... It represents the principal stress at the i-th interpolation pair node on the second interpolation line.

[0023] Two calculation conditions are set up. Under the two conditions, the welding toe line, the first interpolation line and the second interpolation line are set with loads at different load steps. Based on the hot spot stress and the load, the corresponding hot spot nonlinear load stress curve is fitted using the multi-segment line fitting method. The two calculation conditions are minimum load + constant load and maximum load + constant load. Then, as shown in Table 1, two consecutive load steps are set. The first load step includes n consecutive substeps, and the second load step includes m consecutive substeps. The constant load starts to be applied from the first load step and is applied proportionally in n substeps. The minimum load (or maximum load) starts to be applied from the second load step and is applied proportionally in m substeps.

[0024] Table 1: Schematic diagram of load magnitude at each sub-step Taking a single hot spot node on the weld toe as an example, the corresponding hot spot stress value in each sub-step is first extracted according to step S1, as shown in Table 2 below.

[0025] Table 2 Hot spot stress at each substep in This refers to the hot spot stress at various points along the weld toe line.

[0026] After the calculation is completed, the nodal hot spot stresses and the corresponding loading loads for each substep are extracted. The nonlinear load-stress curves of the hot spot nodes are then fitted using a multi-segment approach. Figure 4 The nonlinear load-stress curve in the middle is fitted with six line segments, and the corresponding equation for the nonlinear load-stress curve is as follows: in, , Let be the slope and intercept parameters fitted to the i-th line segment, respectively. The solution method is as follows: Using the hotspot stress results corresponding to the hotspot nodes under each load substep and the applied non-constant load values ​​as input, a set of two-variable linear equations is solved for each straight segment in the multi-line segment to determine the slope and intercept parameters of the straight line equation. Figure 4 Taking the straight line segment y1 as an example, the system of two linear equations with slope a1 and intercept b1 is shown in the following equation. In the equation, and These are the values ​​for non-constant loads. and They are respectively and Nodal stress results under load: Finally, using Python and other programming languages, interpolation calculations were performed on the non-constant load time series (single-channel fatigue time series) applied in the fatigue calculation, combined with the nonlinear load stress curve, to obtain the hot spot stress fatigue time series of each hot spot. Then, the fatigue damage of the welded joint was calculated based on the rainflow counting method and the Miner linear damage accumulation criterion.

[0027] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for calculating hot-spot stress fatigue considering nonlinearity, characterized in that: include: A finite element model of the welded joint is constructed. Weld toe line, first interpolation line and second interpolation line are divided on the finite element model. The node information of weld toe line, first interpolation line and second interpolation line is obtained respectively. Then, the hot spot stress of all nodes on the weld toe line is calculated by interpolation based on the node information. Two calculation conditions are set up. Under the two conditions, the welding toe line, the first interpolation line and the second interpolation line are set with the load at different load steps. Based on the hot spot stress and the load obtained by interpolation at each load step, the corresponding hot spot nonlinear load stress curve is fitted by the multi-segment line fitting method. The fatigue time sequence load is interpolated with the nonlinear load stress curve to obtain the corresponding hot spot stress fatigue time sequence. Then, the fatigue damage of the welded joint is calculated based on the hot spot stress fatigue time sequence.

2. The method for calculating fatigue based on hot spot stress considering nonlinearity as described in claim 1, characterized in that: Methods for calculating hotspot stress include: The finite element model is discretized into a mesh, and the node information on the weld toe line, the first interpolation line and the second interpolation line is determined according to the mesh to obtain the corresponding node sets C1, C2 and C3. The node information includes node coordinates and node principal stresses. Matching the node sets C1, C2, and C3 yields multiple interpolated node pairs; The hot spot stress at the weld toe line is calculated based on the principal stresses at the nodes of the first and second interpolation lines.

3. The method for calculating hot-spot stress fatigue considering nonlinearity as described in claim 2, characterized in that: After mesh discretization, the mesh network between the weld toe line and the first interpolation line, as well as between the first interpolation line and the second interpolation line, is a regular hexahedral mesh.

4. The method for calculating hot-spot stress fatigue considering nonlinearity as described in claim 2, characterized in that: Connect all nodes between node sets C1 and C2, and between node sets C1 and C3, and set the angle between the lines connecting the nodes of the two sets to 0. The nodes within are combined into an interpolation node pair.

5. The method for calculating hot-spot stress fatigue considering nonlinearity as described in claim 2, characterized in that: The formula for calculating hot spot stress is as follows: in, Let be the hot spot stress at the i-th interpolation pair node on the weld toe line. The principal stress at the i-th interpolation pair node on the first interpolation line is... It represents the principal stress at the i-th interpolation pair node on the second interpolation line.

6. The method for calculating fatigue based on hot spot stress considering nonlinearity as described in claim 1, characterized in that: The two calculation cases include the minimum load + constant load case and the maximum load + constant load case; Two consecutive load steps are set. The first load step includes n consecutive substeps, and the second load step includes m consecutive substeps. The constant load is applied starting from the first load step and is completed in n substeps. The minimum load (or maximum load) is applied starting from the second load step and is completed in m substeps. All substeps form a time series. The load corresponding to each substep is the load of the weld toe line, the first interpolation line, or the second interpolation line in different time series.

7. The method for calculating fatigue based on hot spot stress considering nonlinearity as described in claim 6, characterized in that: The equation for the hot spot nonlinear load stress curve is as follows: in, , These are the slope and intercept parameters fitted to the i-th line segment, respectively.

8. The method for calculating fatigue based on hot spot stress considering nonlinearity as described in claim 1, characterized in that: Fatigue damage was calculated using the rainflow counting method and the Miner linear damage accumulation criterion.

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