Complex structure weld toe crack stress intensity factor calculation method

Abstract

The invention provides a ship body connection joint weld toe crack stress intensity factor calculation method, which comprises the following steps: carrying out coarse mesh modeling on a ship body local structure corresponding to a ship complex structure by adopting a finite element method, and obtaining a ship body total structure stress field calculation result based on a coarse mesh model; establishing a local straight structure three-dimensional model containing weld toes and crack connection nodes; modeling and converting the local straight three-dimensional model containing the weld toe and the crack connection node into a local complex three-dimensional model containing the weld toe and the crack connection node by adopting a coordinate mapping technology; loading boundary conditions on the local complex three-dimensional model for calculation according to a ship body total structure stress field calculation result to obtain a stress field calculation result of the local complex three-dimensional model; and based on a stress field calculation result of the local complex three-dimensional model, obtaining a weld toe crack stress intensity factor at the ship body connection node. The calculation method provided by the invention can solve the problems of uneven stress distribution of an actual structure, difficulty in identifying stress components and the like.

Description

Technical Field

[0001] The invention relates to a ship structure performance analysis and evaluation technology, and in particular to a method for calculating a stress intensity factor of a weld toe crack of a complex structure. Background Art

[0002] In the fields of aircraft, chemical industry, bridges, ships, etc., fatigue cracks often occur in engineering structures under cyclic alternating loads. These fatigue cracks may continue to expand under alternating loads, and even cause component fractures, affecting structural safety. Therefore, fatigue assessment is one of the important contents of ship structure safety assessment.

[0003] After a ship has been in service for several years, cracks often appear in the structural details under the action of time-varying alternating loads such as wave-induced stress and cargo dynamic pressure. Engineering technicians need to make assessments based on the detected cracks to make maintenance decisions. Fatigue crack growth assessment based on fracture mechanics is an important method, but there are still many problems to be solved in applying this method to engineering practice. Among them, the solution of stress intensity factor (SIF), an important parameter in fracture mechanics, is a difficult problem.

[0004] At present, the more typical methods for solving stress intensity factors include analytical method, finite element method, weight function method, boundary element method and spring method. The general analytical method can only solve the problem of calculating the stress intensity factor of a simple cracked plate. Generally, slightly complex structures are modeled using the finite element method, and empirical formulas are summarized and summarized. The British standard BS7910, the International Institute of Welding (IIW) and other specifications also provide empirical formulas for calculating stress intensity factors of various typical node types.

[0005] However, there are still some problems in applying these empirical formulas to ship and marine engineering:

[0006] (1) The current authoritative empirical formula for calculating stress intensity factors is only applicable to simple structures such as flat plates, cylindrical plates, and T-nodes with cracks. It cannot calculate actual engineering structures such as intersecting welded bodies such as tube-plate connection nodes, T-tube nodes, and cone-column connection nodes. However, these nodes are locations with geometric mutations and high incidence of cracks, and are the focus of attention of engineering technicians.

[0007] (2) In the empirical formula, the tensile stress or bending stress distribution at the plate end or tube end is assumed to be uniform and perpendicular to the cross section, but the load on the actual structure is not the same;

[0008] (3) The tensile and bending stresses in the empirical formula are both nominal stresses (far-field stresses) of the crack. However, in actual structures, due to the non-uniformity of the structural stress distribution in the crack area, different points will lead to significant differences in the calculation of the stress intensity factor and the life prediction results. Therefore, the calculation of the stress intensity factor of the weld toe crack in complex structures has always been one of the hot topics studied by many scholars. Summary of the invention

[0009] The main purpose of the present invention is to provide a method for calculating the stress intensity factor of a weld toe crack in a complex structure, so as to facilitate the calculation of the stress intensity factor of a weld toe crack in a complex structure.

[0010] To achieve the above object, the present invention provides a method for calculating the stress intensity factor of a weld toe crack at a hull connection node, comprising the following steps:

[0011] The finite element method is used to model the local structure of the hull corresponding to the complex structure of the ship in a coarse grid, and the calculation results of the total structural stress field of the hull are obtained based on the coarse grid model;

[0012] Based on the actual structure of the hull connection node to be calculated, a local straight structure 3D model of the connection node including weld toe and crack is established;

[0013] The mapping relationship between the local straight structure 3D model and the complex 3D model is determined through mathematical transformation, and the local straight 3D model containing weld toes and crack connection nodes is converted into a local complex 3D model containing weld toes and crack connection nodes by using coordinate mapping technology;

[0014] Based on the sub-model technology, the calculation results of the total structural stress field of the hull are interpolated to obtain the boundary conditions of the local complex three-dimensional model, and the boundary conditions are loaded on the local complex three-dimensional model for calculation to obtain the calculation results of the stress field of the local complex three-dimensional model;

[0015] Based on the stress field calculation results of the local complex three-dimensional model, the stress intensity factor K of the weld toe crack at the hull connection node is obtained. E .

[0016] Preferably, the stress field calculation result based on the local complex three-dimensional model obtains the stress intensity factor K of the weld toe crack at the hull connection node. E Specifically:

[0017] The stress intensity factors K of three-dimensional crack modes I, II, and III are obtained based on the stress field calculation results of the local complex three-dimensional model. I , K II , K III The stress intensity factor K of the weld toe crack at the hull connection node is calculated based on the stress intensity factor value. E .

[0018] Preferably, in the step of using the finite element method to perform coarse grid modeling on the ship structure, the coarse grid model is modeled using shell elements or volume elements, or a mixture of shell elements and volume elements.

[0019] Preferably, when establishing a three-dimensional model of a local straight structure including a weld toe and a crack, a T-node straight three-dimensional model or a cross-node straight model is established.

[0020] Preferably, when the local flat three-dimensional model containing weld toes and crack connection nodes is converted into a local complex three-dimensional model containing weld toes and crack connection nodes using coordinate mapping technology, all surfaces of the flat three-dimensional model are converted into curved surfaces to form a local complex three-dimensional model.

[0021] Preferably, the stress intensity factor K of the weld toe crack of the complex structure is calculated according to the stress intensity factor value. E The calculation is done using the following formula:

[0022]

[0023] Where μ is the Poisson’s ratio of the material.

[0024] Preferably, when the coordinate mapping technology is used to transform the local straight three-dimensional model containing the weld toe and the crack connection node into the local complex three-dimensional model containing the weld toe and the crack connection node,

[0025] When a flat plate is mapped to a cylindrical plate, in the xyz coordinate system, the coordinates of any point in the flat plate model are (x0, y0, z0). When the radius of the cylindrical plate is r i When , the coordinates of the point under x'y'z' are (x1, y1, z1) and satisfy the following formula:

[0026] x1=x0,

[0027] y0=(r i -z0)sin(y0 / r i ),

[0028] z0=-(r i -z0)cos(y0 / r i ).

[0029] Preferably, the hull connection node to be calculated is a tube-plate connection node, a T-tube node or a cone-column connection node.

[0030] Preferably, after establishing a local straight structure three-dimensional model containing the weld toe and the crack connection node, the displacement extrapolation method is used to calculate the stress intensity factor of the weld toe crack tip, and a singular unit is used to describe the singularity of the stress at the crack front.

[0031] Preferably, after establishing a three-dimensional model of a local straight structure including weld toes and crack connection nodes, the crack surface adopts an elliptical surface, and closed crack surface nodes are not coupled.

[0032] The method for calculating the stress intensity factor of the weld toe crack at the hull connection node proposed in the present invention has the following characteristics:

[0033] Beneficial effects:

[0034] 1. The combination of finite element method, sub-model technology and coordinate mapping technology can solve the problems faced by simplified empirical formulas of stress intensity factors, such as uneven stress distribution in actual structures and difficult identification of stress components. It can calculate the stress intensity factor at the tip of the weld toe crack of complex connection nodes such as cone-column connection nodes, pipe-to-pipe connection nodes, and pipe-to-plate connection nodes in the shipbuilding field under complex loads, providing a way of thinking for the calculation of stress intensity factors of weld toe cracks in complex structures and providing support for fatigue assessment technology based on fracture mechanics.

[0035] 2. The results calculated by this calculation method are accurate and reliable. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic flow chart of a first embodiment of a method for calculating a stress intensity factor of a weld toe crack at a hull connection node of the present invention;

[0037] Figure 2 It is a schematic diagram of a coarse grid of a pipe column bracket connection in the first embodiment of the method for calculating the stress intensity factor of a weld toe crack at a hull connection node of the present invention;

[0038] Figure 3 It is a schematic diagram of a shell unit coarse mesh model in the first embodiment of the method for calculating the stress intensity factor of a weld toe crack at a hull connection node of the present invention;

[0039] Figure 4 Schematic diagram of the coarse mesh of the shell-body hybrid unit in the first embodiment of the method for calculating the stress intensity factor of the weld toe crack at the hull connection node of the present invention;

[0040] Figure 5 It is a schematic diagram of a straight three-dimensional model of a T-node in the first embodiment of the method for calculating the stress intensity factor of a weld toe crack at a hull connection node of the present invention;

[0041] Figure 6a It is a schematic diagram of the position of the target T-tube node in the first embodiment of the method for calculating the stress intensity factor of the weld toe crack at the hull connection node of the present invention;

[0042] Figure 6b It is a schematic diagram of the complex weld toe position in the first embodiment of the method for calculating the stress intensity factor of the weld toe crack at the hull connection node of the present invention;

[0043] Figure 7a It is a schematic diagram of the first step of welding crack complex body model mapping of the first embodiment of the method for calculating the stress intensity factor of the weld toe crack at the hull connection node of the present invention;

[0044] Figure 7b It is a schematic diagram of the second step of welding crack complex body model mapping of the first embodiment of the method for calculating the stress intensity factor of the weld toe crack at the hull connection node of the present invention;

[0045] Figure 7c It is a schematic diagram of the third step of welding crack complex body model mapping of the first embodiment of the method for calculating the stress intensity factor of the weld toe crack at the hull connection node of the present invention;

[0046] Figure 8 This is a comparison diagram between the stress intensity factor of a complex weld toe crack obtained by the first embodiment of the method for calculating the stress intensity factor of a weld toe crack at a hull connection node of the present invention and the result of an empirical formula.

[0047] In the figure, 1-column, 2-fatigue hot spot area, 3-bracket.

[0048] The realization of the purpose, functional features and advantages of the present invention will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0049] It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention.

[0050] It should be noted that in the description of the present invention, the terms "transverse", "longitudinal", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate the orientation or position relationship based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second" and the like are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0051] Reference Figures 1 to 8 The present invention proposes a method for calculating the stress intensity factor of a weld toe crack at a hull connection node. In a first embodiment of the present invention, a method for calculating the stress intensity factor of a weld toe crack at a hull connection node comprises the following steps:

[0052] Step S10, using the finite element method to perform coarse mesh modeling on the local structure of the hull corresponding to the complex structure of the ship, and obtaining the calculation result of the total structural stress field of the hull based on the coarse mesh model;

[0053] Step S20, based on the actual structure of the hull connection node to be calculated, a local straight structure three-dimensional model including the weld toe and the crack connection node is established;

[0054] Step S30, determining the mapping relationship between the local straight structure three-dimensional model and the complex three-dimensional model through mathematical transformation relationship, and using coordinate mapping technology to transform the local straight three-dimensional model containing weld toes and crack connection nodes into the local complex three-dimensional model containing weld toes and crack connection nodes;

[0055] Step S40, based on the sub-model technology, interpolate the calculation result of the stress field of the total hull structure to obtain the boundary conditions of the local complex three-dimensional model, load the boundary conditions on the local complex three-dimensional model for calculation, and obtain the calculation result of the stress field of the local complex three-dimensional model;

[0056] Step S50, based on the stress field calculation results of the local complex three-dimensional model, obtain the stress intensity factor K of the weld toe crack at the hull connection node E .

[0057] In step S10, in the step of using the finite element method to perform coarse mesh modeling on the ship structure, the coarse mesh model is modeled using shell elements or volume elements, or a mixture of shell elements and volume elements.

[0058] Solving the stress intensity factor at the crack tip requires obtaining the stress field at the crack tip, otherwise the calculation accuracy of the stress field at the crack tip is low, so a finer mesh division is required. However, the ship structure is often larger than the crack. If the entire ship structure is modeled with a fine mesh, the number of finite element meshes will be huge, far exceeding the computing power of current conventional computers, and it is not practical for engineering. Therefore, only the local structure of the hull corresponding to the complex structure of the ship needs to be modeled with a coarse mesh. At the same time, the ship structure is complex in load, and the simplification of a smaller local structure will inevitably lead to errors on the boundary.

[0059] The ship structure is modeled with a coarse grid, and the relevant requirements for finite element direct calculation in existing specifications (such as CCS "Rules for Classification of Sea-Going Steel Ships") can be used to model, load and calculate the ship engineering structure.

[0060] The coarse mesh model can use shell elements (such as Figure 2 As shown) or solid elements can be used for modeling, or a mixture of shell elements and solid elements can be used for modeling (as shown Figure 3 shown).

[0061] In step S20, when establishing a local straight structure three-dimensional model including weld toe and crack, a T-node straight three-dimensional model (such as Figure 4 as shown) or a cross-node straight model.

[0062] Since the J integral loses the independence of the integral path when the crack meets the weld toe, the displacement extrapolation method is often used to calculate the stress intensity factor at the weld toe crack tip. When using the displacement interpolation method, in order to ensure the accuracy of the calculation, it is generally necessary to use singular units to describe the singularity of the stress at the crack front. In this embodiment, a stable and reliable 20-node singular unit is used for calculation. The crack surface adopts an elliptical surface, and the closed crack surface nodes are not coupled and can be opened under tension.

[0063] For complex structures containing weld toes and cracks, there is currently no method to calculate the stress intensity factor at the crack tip. However, for simple straight structures containing weld toes and cracks, a relatively mature finite element calculation method has been basically formed.

[0064] In step S30, when the local flat three-dimensional model containing weld toes and crack connection nodes is converted into a local complex three-dimensional model containing weld toes and crack connection nodes using coordinate mapping technology, all surfaces of the flat three-dimensional model are converted into curved surfaces to form a local complex three-dimensional model.

[0065] When the coordinate mapping technology is used to transform the local flat three-dimensional model containing weld toe and crack connection node into the local complex three-dimensional model containing weld toe and crack connection node,

[0066] When a flat plate is mapped to a cylindrical plate, in the xyz coordinate system, the coordinates of any point in the flat plate model are (x0, y0, z0). When the radius of the cylindrical plate is r i When , the coordinates of the point under x'y'z' are (x1, y1, z1) and satisfy the following formula:

[0067] x1=x0,

[0068] y0=(r i -z0)sin(y0 / r i ),

[0069] z0=-(r i -z0)cos(y0 / r i ).

[0070] The mapping of a flat plate to a spherical plate is similar. On this basis, the local flat 3D model with weld toe and crack is transformed into a local complex 3D model with weld toe and crack.

[0071] In step S40, the sub-model technology is a step-by-step calculation method. To achieve local fine analysis, the detailed structure is first ignored, a global model analysis is established, and then the concerned part is selected to establish a fine sub-model analysis separately. The sub-model technology is a relatively mature technology in the finite element calculation method. In this embodiment, the boundary of the complex local three-dimensional model containing weld toes and cracks is extracted, and then the calculation results of the coarse mesh model of the complex structure without weld toes and cracks are used to interpolate and obtain the boundary conditions of the sub-model, and then loaded on the sub-model for calculation.

[0072] In step S50, based on the stress field calculation results of the local complex three-dimensional model, the stress intensity factor K of the weld toe crack at the hull connection node is obtained. E Specifically:

[0073] The stress intensity factors K of three-dimensional crack modes I, II, and III are obtained based on the stress field calculation results of the local complex three-dimensional model. I , K II , K III The stress intensity factor K of the weld toe crack at the hull connection node is calculated based on the stress intensity factor value. E .

[0074] The stress intensity factor K of the weld toe crack of complex structure is calculated based on the stress intensity factor value. E The calculation is done using the following formula:

[0075]

[0076] Where μ is the Poisson’s ratio of the material.

[0077] Specifically, the hull connection node to be calculated is a tube-plate connection node, a T-tube node or a cone-column connection node.

[0078] The following is a specific description taking a T-type pipe node as an example.

[0079] T-tube joints are typical joints in marine structures and are also the locations where fatigue failure is prone to occur. However, since the crack surface is a three-dimensional surface, especially the saddle point crack surface, it is difficult to solve the stress intensity factor of the surface crack. At present, some scholars have obtained the empirical formula through numerical calculation and induction. This intellectual achievement takes this as the research object for verification calculation.

[0080] The location and size of the surface crack at the saddle point of the T-tube node are shown in Figure 6.

[0081] Using sub-modeling technology, a coarse mesh model of the T-type tube node is established. Figure 3 As shown in Figure 2. The unit axial tensile surface force is added to the branch pipe for calculation. The sub-model area is as follows Figures 7a to 7cThe coordinate transformation mapping of the T-shaped weld joint shown in the figure is carried out in two steps: (1) transforming the T-shaped weld joint into a curved model (from Figure 7a to Figure 7b ); (2) then become a sub-model with cracks at the saddle point of the T-tube node (from Figure 7b to Figure 7b ). Map the coarse model calculation results to the sub-model boundary and solve the stress intensity factor K I , K II , K III , and the stress intensity factor is equivalent using the principle of equal strain energy release rate, and the calculated results are compared with those calculated by empirical formula, such as Figure 8 As shown in Figure 2. So far, the calculation of the surface crack stress intensity factor at the saddle point of the T-tube node has been completed. Figure 8 The comparison results show that the calculation method of stress intensity factor of weld toe crack at hull connection node is accurate and reliable.

[0082] The method for calculating the stress intensity factor of the weld toe crack at the hull connection node proposed in the present invention has the following characteristics:

[0083] Beneficial effects:

[0084] 1. The combination of finite element method, sub-model technology and coordinate mapping technology can solve the problems faced by simplified empirical formulas of stress intensity factors, such as uneven stress distribution in actual structures and difficult identification of stress components. It can calculate the stress intensity factor at the tip of the weld toe crack of complex connection nodes such as cone-column connection nodes, pipe-to-pipe connection nodes, and pipe-to-plate connection nodes in the shipbuilding field under complex loads, providing a way of thinking for the calculation of stress intensity factors of weld toe cracks in complex structures and providing support for fatigue assessment technology based on fracture mechanics.

[0085] 2. The results calculated by this calculation method are accurate and reliable.

[0086] The above are only preferred embodiments of the present invention, and are not intended to limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention's specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for calculating the stress intensity factor of weld toe crack at a hull connection node, characterized in that: The following steps are involved: The finite element method is used to model the local structure of the hull corresponding to the complex structure of the ship in a coarse grid, and the calculation results of the total structural stress field of the hull are obtained based on the coarse grid model; Based on the actual structure of the hull connection node to be calculated, a local straight structure 3D model of the connection node including weld toe and crack is established; The mapping relationship between the local straight structure 3D model and the complex 3D model is determined through mathematical transformation, and the local straight 3D model containing weld toes and crack connection nodes is converted into a local complex 3D model containing weld toes and crack connection nodes by using coordinate mapping technology; Based on the sub-model technology, the calculation results of the total structural stress field of the hull are interpolated to obtain the boundary conditions of the local complex three-dimensional model, and the boundary conditions are loaded on the local complex three-dimensional model for calculation to obtain the calculation results of the stress field of the local complex three-dimensional model; Based on the stress field calculation results of the local complex three-dimensional model, the stress intensity factor K of the weld toe crack at the hull connection node is obtained. E .

2. The method for calculating stress intensity factor of weld toe crack of complex structure according to claim 1, characterized in that: The stress field calculation results based on the local complex three-dimensional model are used to obtain the stress intensity factor K of the weld toe crack at the hull connection node. E Specifically: The stress intensity factors K of three-dimensional crack modes I, II, and III are obtained based on the stress field calculation results of the local complex three-dimensional model. I , K II , K III The stress intensity factor K of the weld toe crack at the hull connection node is calculated based on the stress intensity factor value. E .

3. The method for calculating stress intensity factor of weld toe crack of complex structure according to claim 1, characterized in that: In the step of using the finite element method to perform coarse grid modeling on the ship structure, the coarse grid model is modeled using shell elements or volume elements, or a mixture of shell elements and volume elements.

4. The method for calculating stress intensity factor of weld toe crack of complex structure according to claim 1, characterized in that: When establishing a three-dimensional model of a local straight structure including weld toes and cracks, a T-node straight three-dimensional model or a cross-node straight model is established.

5. The method for calculating stress intensity factor of weld toe crack of complex structure according to claim 1, characterized in that: When the local flat 3D model including weld toe and crack connection node is converted into the local complex 3D model including weld toe and crack connection node by using coordinate mapping technology, all the faces of the flat 3D model are converted into curved surfaces to form the local complex 3D model.

6. The method for calculating stress intensity factor of weld toe crack of complex structure according to claim 1, characterized in that: The stress intensity factor K of the weld toe crack of complex structure is calculated based on the stress intensity factor value. E The calculation is done using the following formula: Where μ is the Poisson’s ratio of the material.

7. The method for calculating stress intensity factor of weld toe crack of complex structure according to claim 1, characterized in that: When the coordinate mapping technology is used to transform the local flat three-dimensional model containing weld toe and crack connection node into the local complex three-dimensional model containing weld toe and crack connection node, When a flat plate is mapped to a cylindrical plate, in the xyz coordinate system, the coordinates of any point in the flat plate model are (x0, y0, z0). When the radius of the cylindrical plate is r i When , the coordinates of the point under x'y'z' are (x1, y1, z1) and satisfy the following formula: x1=x0, y0=(r i -z0)sin(y0 / r i ), z0=-(r i -z0)cos(y0 / r i )。 8. The method for calculating stress intensity factor of weld toe crack of complex structure according to claim 1, characterized in that: The hull connection nodes to be calculated are tube-plate connection nodes, T-tube nodes or cone-column connection nodes.

9. The method for calculating stress intensity factor of weld toe crack of complex structure according to claim 1, characterized in that: After establishing a three-dimensional model of the local straight structure containing the weld toe and the crack connection node, the displacement extrapolation method is used to calculate the stress intensity factor at the weld toe crack tip, and the singular element is used to describe the singularity of the stress at the crack front.

10. The method for calculating stress intensity factor of weld toe crack of complex structure according to any one of claims 1 to 9, characterized in that: After establishing a local straight structure 3D model containing weld toe and crack connection nodes, the crack surface adopts an elliptical surface and the closed crack surface nodes are not coupled.

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