WO2022121203A1

WO2022121203A1 - Method for calculating spherical shell surface three-dimensional crack propagation fatigue life

Info

Publication number: WO2022121203A1
Application number: PCT/CN2021/087820
Authority: WO
Inventors: 李如俊; 朱永梅; 于家富; 唐文献; 杨家豪; 岳琳
Original assignee: 江苏科技大学
Priority date: 2020-12-08
Filing date: 2021-04-16
Publication date: 2022-06-16
Also published as: CN112417606B; CN112417606A

Abstract

A method for calculating spherical shell surface three-dimensional crack propagation fatigue life. The method comprises the following steps: establishing a complete spherical pressure-resistant shell initial geometric model; endowing the spherical shell model with material parameters and section attributes, dividing grids, and setting boundary and load conditions; establishing an initial crack model; endowing the crack model with material parameters, defining section attributes, dividing grids and determining the positions thereof; importing the two models to generate a complete spherical shell finite element numerical model locally containing surface crack defects; solving an initial crack leading edge stress intensity factor numerical value by using an M integral method; reading and running a fatigue life calculation model program; setting automatic crack propagation parameters; and obtaining a spherical shell surface three-dimensional crack propagation fatigue life numerical value based on a self-defined propagation program. According to the method, finite element analysis software and fracture mechanics analysis software are combined to calculate the spherical shell surface three-dimensional crack propagation fatigue life, and the applicability of the method is verified by means of numerical simulation.

Description

A calculation method of three-dimensional crack propagation fatigue life on spherical shell surface

technical field

The invention belongs to the technical field of deep-sea engineering, and relates to a calculation method for a pressure shell with crack defects, in particular to a calculation method for the fatigue life of three-dimensional crack propagation on the surface of a spherical pressure shell.

Background technique

The deep-sea submersible is an important marine engineering equipment for ocean exploration and deep-sea scientific research. The pressure hull is the key component and buoyancy unit of the submersible. As the most common basic pressure-bearing shell unit, spherical shell has irreplaceable advantages such as good bulk-to-weight ratio and bearing capacity, high strength and stability, simple structure, and convenient calculation.

However, the spherical shell itself is a medium-thickness shell with many large openings (manholes, observation windows, equipment holes, etc.), and is manufactured by the process of hemispherical stamping equatorial welding or multi-lobe group welding. Under the conditions of periodic up-and-down diving, deep-sea operations and welding residual stress, stress concentration is easily formed at weld defects, etc., fatigue cracks are initiated and expanded, which inevitably reduces the fatigue life of the manned spherical shell and becomes a serious problem. security risks. At present, domestic and foreign scholars have carried out in-depth and detailed research on the fatigue life of cracked structures using model tests, theoretical analysis, numerical simulation and other methods, but most of them are based on fatigue cumulative damage criteria, traditional fatigue expansion theory and some standard tests of two-dimensional specimens. The traditional fracture criterion is based on the two-dimensional penetrating straight crack plate specimen. The criterion for judging the fracture failure of the structure is the fracture toughness of the structural material with sufficient thickness under the condition of plane strain, which is safe and reliable, but the results It is conservative, ignoring the thickness effect and the possible influence of the three-dimensional constraints inside the structure, which not only causes the low utilization rate of materials in use, but also causes the structure to generate excess weight during design. In addition, the typical three-dimensional non-penetrating crack on the surface of the spherical shell is different from the standard specimen in the form of load, material parameters and stress state at the crack tip, and the existing model is not clear enough for applicable objects, with many parameters, complicated calculation steps, and lack of specific methods. Detailed process and calculation method description for solving three-dimensional crack propagation fatigue life of spherical shell surface.

SUMMARY OF THE INVENTION

The purpose of the present invention is to aim at the existing problems and deficiencies in the prior art, based on the contemporary fracture mechanics damage tolerance design idea and the fatigue crack propagation theory's advantages in fatigue life prediction, combined with finite element analysis software ABAQUS, fracture mechanics analysis software Franc3D and Python programming languages describe the calculation method of three-dimensional crack propagation fatigue life on spherical shell surface in detail, and verify its applicability through numerical simulation.

In order to achieve the above object, the present invention adopts the following technical solutions:

A method for calculating the fatigue life of three-dimensional crack propagation on the surface of a spherical shell, comprising the following steps:

Step 1: Establish the initial geometric model of the complete spherical pressure shell in the Cartesian coordinate system;

Step 2: Assign material parameters, section properties, mesh and set boundary and load conditions to the spherical shell model;

Step 3: Establish the initial crack model in the Cartesian coordinate system;

Step 4: Assign material parameters to the crack model, define section properties, divide the mesh and determine its location;

Step 5: Import the initial geometric model and initial crack model of the complete spherical pressure shell to generate a complete spherical shell finite element numerical model with partial surface crack defects;

Step 6: Use the M integral method to obtain the value of the stress intensity factor of the initial crack front;

Step 7: Read and run the fatigue life calculation model program based on Python language;

Step 8: Set the automatic crack propagation parameters;

Step 9: Obtain the three-dimensional crack propagation fatigue life value of the spherical shell surface based on the custom expansion program.

As a further preferred solution, in the first step, in the ABAQUS/Part module, select the solid element, and create two circles with (0,0) as the center and D/2 and D/2-t ₀ as the radius Concentric circles; connect (D/2, 0) and (-D/2, 0), delete other curves, only keep two semicircles and a straight line connecting the two semicircles; rotate 360° around the straight line connecting the semicircles to form a A solid spherical shell with outer diameter D and thickness _t0 .

As a further preferred solution, the step two is divided into three steps:

(1) Set the elastic-plastic parameters of the material in the ABAQUS/Property module, create an entity mean section, and assign section properties; and use the tennis-ball division form to divide the solid element in the ABAQUS/Mesh module; select an eight-node linear hexahedral element network grid (C3D8R), the cell size is about 0.03D;

(2) Set the boundary conditions of the structure in the ABAQUS/Load module; apply the corresponding boundary constraints in the form of three-point constraints to eliminate the rigid body displacement of the structure, constrain a total of 6 displacement components, and the specific constraint form: along the x-axis in the spherical shell hemisphere Select 2 nodes on the outer surface to limit the displacement of its y and z axes (Uy=Uz=0), and select node 3 at a position 90° apart on the same longitude of these two points to limit the displacement in the x and y directions (Ux= Uy=0); and the external load is applied to the structure in the ABAQUS/Load module; the uniform load on the outer surface of the spherical shell is calculated by the formula P=0.0101×d, where P is the sea water pressure outside, and d is the diving depth;

(3) Select the spherical shell model in the ABAQUS/Job module, write the inp file and export and save.

As a further preferred solution, in the third step, surface cracks are generally described by semi-elliptical cracks in engineering practice, a represents the depth of the crack, and 2c is the length of the crack; a/c is the depth-half-length ratio of the crack; in ABAQUS Create a new Model-Crack under /Model, and create an initial crack geometric model in the Part module; select the shell element, create a semi-elliptical slice of the corresponding size, and select the crack front curve Done under ABAQUS/Tools/Create/set, set it as set1 .

As a further preferred solution, the step is divided into three steps:

(1) Set the elastoplastic parameters of the same material as the spherical shell in the ABAQUS/Property module, create the mean section of the shell, and assign section properties; since cracks are automatically redrawn when the spherical shell submodel is imported through Franc3D software, this step Meshing does not need to be considered too much;

(2) In the ABAQUS/Assembly module, select the spherical shell model in Models, and the crack is located at the center of the sphere; translate and rotate, etc., insert the crack into the shell corresponding to the position where there will be crack defects, delete the spherical shell, that is The crack model of the corresponding position can be obtained;

(3) Select the crack model in the ABAQUS/Job module, write the inp file and export and save.

As a further preferred solution, the step five is divided into three steps:

(1) Import the complete spherical shell model file; open the Franc3D software, set the working path in English, select the inp file of the spherical shell model under the File/Import menu, import and divide it into global and local models; retain the local model;

(2) Click User mesh under Cracks/Multiple Flaw Insert to select the user-defined model, and import the crack model file in step 4 from the file, select the set1 of the crack front, and insert; Franc3D software will automatically divide the mesh, and Do geometric intersecting surface meshing, surface meshing, volume meshing, smooth meshing;

(3) The distribution of the welding residual stress perpendicular to the direction of the spherical shell weld along the wall thickness direction is simplified to a linear distribution form, and if the plate thickness is t, the distribution expression of the residual stress σR along the thickness direction is (x = 0: Weld toe outer surface):

From this, the specific value of the linear distribution of the crack surface along the thickness direction can be calculated, and the residual stress is applied to the crack through the Franc3D/Load module.

As a further preferred solution, in the sixth step, after the finite element model is established, the ABAQUS static analysis solver is called to perform finite element calculation; after the solution is completed, the M integral method is selected to calculate the stress intensity factor and output Three-type stress intensity factor (KI, KII, KIII) numerical curve of the crack front.

As a further preferred solution, in the step 7, the result model of the step 6 is retained, and the user-defined extended model program file written based on the python language is read in the Franc3D secondary development port; the software reads and displays that the program contains Valid function list (which includes various initialization functions, custom extensions, custom kink angles, custom cyclic growth rates, custom time growth rates and other function modules written by the user; including static load, fatigue loading, and load-holding loading) mode and other modules; including three-dimensional fracture parameters, environmental parameters, structural parameters and other modules that need to be defined and assigned in the user model); this paper provides a new type of fatigue crack growth rate function as an example:

(1) The equivalent thickness is introduced into the fracture criterion and extended, and the three-dimensional fracture toughness suitable for spherical shells is obtained; the three-dimensional fracture criterion of the structure with type I semi-elliptical surface crack is: K _IZ,maxi =K _IZC , where K _{IZ, maxi} is the maximum three-dimensional stress intensity factor point i of the crack front point concentration on the semi-elliptical surface, and K _IZC is the three-dimensional fracture toughness of the shell material;

(2) For the semi-elliptical surface crack K _IZ,maxi , as the maximum three-dimensional stress intensity factor of the crack front, it can be obtained by the following formula:

where K _I,maxi can be obtained by the finite element numerical method,

is a function of the material Poisson's ratio v and the three-dimensional out-of-plane stress constraint factor T _Z

B _eq,i is the equivalent thickness after the thickness of B is equivalent to the semi-elliptical surface crack after the three-dimensional stress confinement of the structure with penetration cracks, by

beg,

where t=a/c is the crack length-diameter ratio,

is the angle of the semi-elliptical crack front; at this time, the three-dimensional constraint factor is

For K _ZC =const is the material constant, which is independent of thickness, the three-dimensional fracture toughness of the material can be obtained from the plane fracture toughness obtained under the thickness of the standard penetrating specimen and the simultaneous equations of a certain structural thickness;

(3) The effective stress intensity factor is used as the real driving force of crack propagation ΔK _eff,i =K _max,i -K _open,i , which is also affected by factors such as thickness and stress ratio under three-dimensional conditions; at this time, before the crack The crack opening ratio at any point i of the edge is

in

The cyclic stress ratio R and the combined constraint factor α _g,i are considered here, where

Considering the thickness effect and the effect of plastic closure, its

is the calculation method of the size of the open plastic zone at the crack tip; here, the three-dimensional effective stress intensity factor ΔK _IZeff,i is obtained, and σ _o is the flow stress;

(4) At this point, we can consider simplifying the complex unified fatigue life prediction model to make it both reliable and widely applicable; the revised formula is as follows:

Among them, A is the influence factor of material environmental factors, m is the stable slope of the fatigue crack growth rate curve of the standard sample, n is the structural instability growth coefficient which can be obtained from the material tensile test, ΔK _effth,i =f(R _i )ΔK _th0 is threshold value of effective stress intensity factor amplitude, which is a function of stress ratio R and the threshold value of stress intensity factor when the stress ratio is 0; other parameter values have been detailed in steps (1)(2)(3);

(5) Arrange the formula and the parameters involved, and use the Python language to write a complete program script, and read it through the secondary development port of the Franc3D software.

As a further preferred solution, in the step 8, under the Franc3D/Cracks menu, enter the crack propagation option, select the M integral method to calculate the stress intensity factor of the crack front in each step; and add the stress ratio R to the external pressure static load Step1 Or time t, get the loading method, select the number of user-defined expansion steps, set the expansion step size, and select a fixed order to connect multi-order front edge points to fit a new crack front line; realize the automatic expansion of structural cracks under the user-defined model .

As a further preferred solution, the step 9 is divided into two steps:

(1) After reading the summary graph of the stress intensity factor curve formed by each expansion in step 8; set the finite element model unit under the Franc3D/Fatigue menu, also select the fatigue loading method in step 8, read the user expansion model, and get the crack extension path;

(2) Enter the path, by selecting the crack tip or the midpoint of the front edge respectively (along the crack front line connecting the two crack tips, the displacement normalization constant is set to 0-1, and the midpoint of the front edge is 0.5), and set The initial crack length c is compared with the length-life (Path Length vs Cycles) curve, and the lowest life value at three points is selected as the three-dimensional crack propagation fatigue life of the spherical shell surface.

beneficial effect

1. Through the interaction between the finite element analysis software ABAQUS and the fracture mechanics analysis software Franc3D, this method realizes the introduction of cracks in the corresponding positions of the spherical shell and the automatic division of crack meshes, which simplifies the establishment of the CAE model of the cracked structure.

2. This method comprehensively considers the seawater external pressure and welding residual stress, and restores the actual working conditions of the spherical shell in service, which ensures the reliability of the three-dimensional fatigue crack growth analysis on the spherical shell surface.

3. This method can comprehensively consider the influence of environmental factors, material parameters, structural parameters, etc., and convert the material parameters, external environment, structure and other factors suitable for the two-dimensional penetration crack standard test piece into three-dimensional fracture parameters and apply them to the spherical shell. In the surface crack fatigue life calculation, the accuracy of the three-dimensional crack propagation fatigue life calculation model on the spherical shell surface is improved.

4. This method is based on Python language for secondary development of Franc3D software. The three-dimensional crack propagation fatigue life calculation model of spherical shell surface is established by programming language. By modifying the corresponding material parameters, environmental parameters and structural parameters in the program, the purpose of calculating the fatigue life of cracked structures under different conditions is achieved, and the calculation efficiency is improved.

Description of drawings

Fig. 1 is the flow chart of the calculation method of three-dimensional crack propagation fatigue life on spherical shell surface;

Figure 2 is the flow chart of the establishment of the spherical shell with crack defects and the calculation of the initial stress intensity factor;

Figure 3 is a flow chart of parameterized programming of the life calculation model;

Figure 4 shows the finite element model and boundary conditions of a spherical shell with surface cracks;

Fig. 5 is the stress intensity factor variation curve of the finite element model of the present invention under corresponding conditions;

Fig. 6 is the expansion path diagram of the finite element model of the present invention under corresponding conditions;

FIG. 7 is a graph of the fatigue crack growth rate of the finite element model of the present invention under corresponding conditions.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments.

The present invention is further elaborated according to the flow chart of the method for calculating the fatigue life of the three-dimensional crack propagation on the surface of the spherical shell shown in FIG. 1 . The shell material is martensitic nickel steel, and the pressure shell of the embodiment is a full-sea deep manned pressure-resistant spherical shell, which needs to be able to operate in a 7km deep sea. The dimensions and material parameters of the pressure spherical shell are shown in Table 1.

Table 1 Example spherical pressure shell size and material parameters

In the first step (S1), the initial geometric model of the complete spherical pressure hull is established in the Cartesian coordinate system.

In the ABAQUS/Part module, select the solid element and create two concentric circles with (0,0) as the center and D/2 and D/2-t ₀ as the radii; connect (D/2,0) with ( -D/2, 0), delete other curves, only keep the two semicircles and the straight line connecting the two semicircles; take the straight line connecting the semicircles as the axis to rotate 360° to form a solid spherical shell with an outer diameter of D and a thickness of t ₀ .

In the second step (S2), the spherical shell model is given material parameters, section properties, meshed, and boundary and load conditions are set.

(1) Set the elastic-plastic parameters of the material in the ABAQUS/Property module, create an entity mean section, and assign section properties; and use the tennis ball division form to divide the solid element in the ABAQUS/Mesh module. An eight-node linear hexahedral mesh (C3D8R) is used, and the element size is about 0.03D.

(2) Set the boundary conditions of the structure in the ABAQUS/Load module. The corresponding boundary constraints are imposed in the form of three-point constraint to eliminate the rigid body displacement of the structure, and a total of 6 displacement components are constrained. , the displacement of the z-axis (Uy=Uz=0), take node 3 at a position 90° apart on the same longitude of these two points, limit its displacement in the x, y direction (Ux=Uy=0); and in ABAQUS/Load The module imposes an external load on the structure. The uniform load on the outer surface of the spherical shell is calculated by the formula P=0.0101×d, where P is the sea water pressure, and d is the diving depth;

In the third step (S3), an initial crack model is established in the Cartesian coordinate system.

In engineering practice, surface cracks are generally described by semi-elliptical cracks, where a represents the depth of the crack and 2c is the length of the crack. a/c is the depth-half-length ratio of the crack. Create a new Model-Crack under ABAQUS/Model, and create an initial crack geometric model in the Part module. Select the shell element, create a semi-elliptical slice of the corresponding size, and select the crack front curve Done under ABAQUS/Tools/Create/set and set it to set1.

In the fourth step (S4), material parameters are assigned to the crack model, section properties are defined, meshes are divided, and its position is determined.

(1) Set the elastic-plastic parameters of the same material as the spherical shell in the ABAQUS/Property module, create a shell mean section, and assign section properties. Since cracks are automatically redrawn when the spherical shell sub-model is imported through Franc3D software, meshing in this step does not need to be considered too much.

(2) In the ABAQUS/Assembly module, select the spherical shell model in Models, and the crack is located at the center of the sphere. Translate and rotate, etc., insert the crack into the position of the shell corresponding to the crack defect, delete the spherical shell, and then the crack model of the corresponding position can be obtained.

In the fifth step (S5), import the two models to generate a complete spherical shell finite element numerical model with partial surface crack defects.

(1) Import the complete spherical shell model file. Open Franc3D software, set the working path in English, select the inp file of the spherical shell model under the File/Import menu, import and divide it into global and local models. Keep local models.

(2) Click User mesh under Cracks/Multiple Flaw Insert to select the user-defined model, and import the crack model file (S4) from the file, select the crack front set set1, and insert it. Franc3D software will automatically divide the mesh, and make geometric intersecting surface meshing, surface meshing, volume meshing, and smooth meshing.

In the sixth step (S6), the M integral method is used to obtain the value of the stress intensity factor of the initial crack front.

After the finite element model is established, the ABAQUS static analysis solver is called for finite element calculation. After the solution is completed, through the results of the Franc3D software, the M integral method is selected to calculate the stress intensity factor, and the three-type stress intensity factor (KI, KII, KIII) numerical curve of the crack front is output.

The seventh step (S7) is to read and run the fatigue life calculation model program written based on the Python language.

Retain the result model of (S6), and read the user-defined extended model program file written based on python language in the Franc3D secondary development port. The software reads and displays a list of valid functions contained in the program (including various initialization functions, self-defined extensions, user-defined kink angles, user-defined cycle growth rates, user-defined time growth rates and other function modules written by the user; including Modules such as static load, fatigue loading, and load retention mode; including modules such as 3D fracture parameters, environmental parameters, and structural parameters that need to be defined and assigned in the user model). This article provides a novel fatigue crack growth rate function as an example:

(1) The equivalent thickness is introduced into the fracture criterion and generalized to obtain the three-dimensional fracture toughness suitable for spherical shells. The three-dimensional fracture criterion of the structure with type I semi-elliptical surface crack is: K _IZ,maxi =K _IZC , where K _IZ,maxi is the maximum three-dimensional stress intensity factor point i at the front edge of the semi-elliptical surface crack, and K _IZC is the shell material 3D fracture toughness.

where K _I,maxi can be obtained by the finite element numerical method,

is the equivalent thickness after the three-dimensional stress constraint of the structure containing the penetration crack is equivalent to the semi-elliptical surface crack, and is given by

beg,

where t=a/c is the crack length-diameter ratio,

is the angle of the semi-elliptical crack front. At this time, the three-dimensional constraint factor is

For K _ZC =const is a material constant, which is independent of thickness, the three-dimensional fracture toughness of the material can be obtained from the plane fracture toughness obtained under the thickness of the standard penetrating specimen and the simultaneous equations of a certain structural thickness.

(3) The effective stress intensity factor is used as the real driving force of crack propagation ΔK _eff,i =K _max,i -K _open,i , which is also affected by factors such as thickness and stress ratio under three-dimensional conditions. At this time, the crack opening ratio at any point i of the crack front is

in

Considering the thickness effect and the effect of plastic closure, its

It is the calculation method of the size of the open plastic zone at the crack tip. The three-dimensional effective stress intensity factor ΔK _IZeff,i is obtained here.

(4) At this time, it is possible to consider simplifying the complex unified fatigue life prediction model to make it both reliable and widely applicable. The revised formula is as follows:

Among them, A is the influence factor of material environmental factors, m is the stable slope of the fatigue crack growth rate curve of the standard sample, n is the structural instability growth coefficient which can be obtained from the material tensile test, ΔK _effth,i =f(R _i )ΔK _th0 is The effective stress intensity factor amplitude threshold, which is a function of the stress ratio R and the stress intensity factor threshold at a stress ratio of 0. Other parameter values are detailed in steps (1)(2)(3).

In the eighth step (S8), the parameters of automatic crack propagation are set.

Under the Franc3D/Cracks menu, enter the crack propagation option and select the M integral method to calculate the stress intensity factor of the crack front at each step. Add the stress ratio R or time t to the external pressure static load Step1 to obtain the loading method, select the user-defined expansion step number, set the expansion step size, and select a fixed sequence to connect multi-order front edge points to fit a new crack front line. Realize the automatic expansion of structural cracks under the user-defined model.

In the ninth step (S9), the three-dimensional crack propagation fatigue life value of the spherical shell surface based on the self-defined expansion program is obtained.

(1) After reading the stress intensity factor curves of all steps after expansion, as shown in Figure 6. Set the finite element model unit under the Franc3D/Fatigue menu, also select (S8) fatigue loading mode, read the user expansion model, and obtain the crack growth path as shown in Figure 7.

The above description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited to this. The equivalent replacement or change of the inventive concept thereof shall be included within the protection scope of the present invention.

Claims

A method for calculating the fatigue life of three-dimensional crack propagation on the surface of a spherical shell, characterized by comprising the following steps:

Step 1: Establish the initial geometric model of the complete spherical pressure shell in the Cartesian coordinate system;

Step 2: Assign material parameters, section properties, mesh and set boundary and load conditions to the spherical shell model;

Step 3: Establish the initial crack model in the Cartesian coordinate system;

Step 4: Assign material parameters to the crack model, define section properties, divide the mesh and determine its location;

Step 5: Import the initial geometric model and initial crack model of the complete spherical pressure shell to generate a complete spherical shell finite element numerical model with partial surface crack defects;

Step 6: Use the M integral method to obtain the value of the stress intensity factor of the initial crack front;

Step 7: Read and run the fatigue life calculation model program based on Python language;

Step 8: Set the automatic crack propagation parameters;

Step 9: Obtain the three-dimensional crack propagation fatigue life value of the spherical shell surface based on the custom expansion program.
The method for calculating the fatigue life of three-dimensional crack propagation on the surface of a spherical shell according to claim 1, characterized in that: in the first step, in the ABAQUS/Part module, a solid element is selected, and the creation takes (0,0) as the Center, two concentric circles with D/2 and D/2-t 0 as radii; connect (D/2, 0) and (-D/2, 0), delete other curves, only keep the two semicircles and the connection A straight line of two semicircles; a solid spherical shell with an outer diameter of D and a thickness of t 0 is formed by rotating 360° with the line connecting the semi-circles as the axis.
The method for calculating the fatigue life of three-dimensional crack propagation on the surface of a spherical shell according to claim 2, wherein the step is divided into three steps:

(1) Set the elastic-plastic parameters of the material in the ABAQUS/Property module, create an entity mean section, and assign section properties; and use the tennis-ball division form to divide the solid element in the ABAQUS/Mesh module; select an eight-node linear hexahedral element network grid (C3D8R), the cell size is about 0.03D;

(2) Set the boundary conditions of the structure in the ABAQUS/Load module; apply the corresponding boundary constraints in the form of three-point constraints to eliminate the rigid body displacement of the structure, constrain a total of 6 displacement components, and the specific constraint form: along the x-axis in the spherical shell hemisphere Select 2 nodes on the outer surface to limit the displacement of its y and z axes (Uy=Uz=0), and select node 3 at a position 90° apart on the same longitude of these two points to limit the displacement in the x and y directions (Ux= Uy=0); and the external load is applied to the structure in the ABAQUS/Load module; the uniform load on the outer surface of the spherical shell is calculated by the formula P=0.0101×d, where P is the sea water pressure outside, and d is the diving depth;

(3) Select the spherical shell model in the ABAQUS/Job module, write the inp file and export and save.
The method for calculating the fatigue life of three-dimensional crack propagation on the surface of a spherical shell according to claim 3, characterized in that: in the step 3, in practical engineering, surface cracks are generally described by semi-elliptical cracks, where a represents the crack depth, and 2c is the length of the crack; a/c is the depth-half-length ratio of the crack; create a Model-Crack under ABAQUS/Model, and create an initial crack geometric model in the Part module; Select the crack front curve Done in ABAQUS/Tools/Create/set and set it to set1.
The method for calculating the fatigue life of three-dimensional crack propagation on the surface of a spherical shell according to claim 4, wherein the step is divided into three steps:

(1) Set the elastoplastic parameters of the same material as the spherical shell in the ABAQUS/Property module, create the mean section of the shell, and assign section properties; since cracks are automatically redrawn when the spherical shell submodel is imported through Franc3D software, this step Meshing does not need to be considered too much;

(2) In the ABAQUS/Assembly module, select the spherical shell model in Models, and the crack is located at the center of the sphere; translate and rotate, etc., insert the crack into the shell corresponding to the position where there will be crack defects, delete the spherical shell, that is The crack model of the corresponding position can be obtained;

(3) Select the crack model in the ABAQUS/Job module, write the inp file and export and save.
The method for calculating the fatigue life of three-dimensional crack propagation on a spherical shell surface according to claim 5, wherein the step five is divided into three steps:

(1) Import the complete spherical shell model file; open the Franc3D software, set the full English working path, select the inp file of the spherical shell model under the File/Import menu, import and divide it into global and local models; retain the local model;

(2) Click User mesh under Cracks/Multiple Flaw Insert to select the user-defined model, and import the crack model file in step 4 from the file, select the set1 of the crack front, and insert it; Franc3D software will automatically divide the mesh, and Do geometric intersecting surface meshing, surface meshing, volume meshing, smooth meshing;

(3) The distribution of the welding residual stress perpendicular to the direction of the spherical shell weld along the wall thickness direction is simplified to a linear distribution form, and if the plate thickness is t, the distribution expression of the residual stress σR along the thickness direction is (x = 0: Weld toe outer surface):

From this, the specific value of the linear distribution of the crack surface along the thickness direction can be calculated, and the residual stress is applied to the crack through the Franc3D/Load module.
The method for calculating the fatigue life of three-dimensional crack propagation on the surface of a spherical shell according to claim 6, wherein: in the step 6, after the finite element model is established, the ABAQUS static analysis solver is called to perform the finite element calculation; After the solution is completed, through the results of the Franc3D software, the M integral method is selected to calculate the stress intensity factor, and the three-type stress intensity factor (KI, KII, KIII) numerical curve of the crack front is output.
The method for calculating the fatigue life of three-dimensional crack propagation on the surface of a spherical shell according to claim 7, wherein: in the step 7, the result model of the step 6 is retained, and the Franc3D secondary development port is read and written based on the python language. The user-defined extension model program file; the software reads and displays a list of valid functions included in the program (including various initialization functions written by the user, user-defined extension, user-defined kink angle, user-defined cycle growth rate, automatic Define function modules such as time growth rate; including modules such as static load, fatigue loading, and load retention mode; including three-dimensional fracture parameters, environmental parameters, structural parameters and other modules that need to be defined and assigned in the user model); this paper provides a new type of fatigue Crack growth rate function as an example:

(1) The equivalent thickness is introduced into the fracture criterion and extended, and the three-dimensional fracture toughness suitable for spherical shells is obtained; the three-dimensional fracture criterion of the structure with type I semi-elliptical surface crack is: K IZ,maxi =K IZC , where K IZ, maxi is the maximum three-dimensional stress intensity factor point i of the crack front point concentration on the semi-elliptical surface, and K IZC is the three-dimensional fracture toughness of the shell material;

(2) For the semi-elliptical surface crack K IZ,maxi , as the maximum three-dimensional stress intensity factor of the crack front, it can be obtained by the following formula:
where K I,maxi can be obtained by the finite element numerical method,
is a function of the material Poisson's ratio v and the three-dimensional out-of-plane stress constraint factor T Z
B eq,i is the equivalent thickness after the thickness of B is equivalent to the semi-elliptical surface crack after the three-dimensional stress confinement of the structure with penetration cracks, by
beg,
where t=a/c is the crack length-diameter ratio,
is the angle of the semi-elliptical crack front; at this time, the three-dimensional constraint factor is
For K ZC =const is the material constant, which is independent of thickness, the three-dimensional fracture toughness of the material can be obtained from the plane fracture toughness obtained under the thickness of the standard penetrating specimen and the simultaneous equations of a certain structural thickness;

(3) The effective stress intensity factor is used as the real driving force of crack propagation ΔK eff,i =K max,i -K open,i , which is also affected by factors such as thickness and stress ratio under three-dimensional conditions; at this time, before the crack The crack opening ratio at any point i of the edge is
in
The cyclic stress ratio R and the combined constraint factor α g,i are considered here, where
Considering the thickness effect and the effect of plastic closure, its
is the calculation method of the size of the open plastic zone at the crack tip; here, the three-dimensional effective stress intensity factor ΔK IZeff,i is obtained, and σ o is the flow stress;

(4) At this point, we can consider simplifying the complex unified fatigue life prediction model to make it both reliable and widely applicable; the revised formula is as follows:
Among them, A is the influence factor of material environmental factors, m is the stable slope of the fatigue crack growth rate curve of the standard sample, n is the structural instability growth coefficient which can be obtained from the material tensile test, ΔK effth,i =f(R i )ΔK th0 is threshold value of effective stress intensity factor amplitude, which is a function of stress ratio R and the threshold value of stress intensity factor when the stress ratio is 0; other parameter values have been detailed in steps (1)(2)(3);

(5) Arrange the formula and the parameters involved, and use the Python language to write a complete program script, and read it through the secondary development port of the Franc3D software.
The method for calculating the fatigue life of three-dimensional crack propagation on the surface of a spherical shell according to claim 8, wherein in the step 8, in the Franc3D/Cracks menu, enter the crack propagation option, and select the M integral method to calculate each step Crack front stress intensity factor; and add the stress ratio R or time t to the external pressure static load Step1 to obtain the loading method, select the user-defined expansion step number, set the expansion step size, and select a fixed order to connect multi-order front edge point fitting New crack front line; enables automatic propagation of structural cracks under user-defined models.
The method for calculating the fatigue life of three-dimensional crack propagation on the surface of a spherical shell according to claim 9, wherein the step is divided into two steps:

(1) After reading the summary graph of the stress intensity factor curve formed by each expansion in step 8; set the finite element model unit under the Franc3D/Fatigue menu, also select the fatigue loading method in step 8, read the user expansion model, and get the crack extension path;

(2) Enter the path, by selecting the crack tip or the midpoint of the front edge respectively (along the crack front line connecting the two crack tips, the displacement normalization constant is set to 0-1, and the midpoint of the front edge is 0.5), and set The initial crack length c is compared with the length-life (Path Length vs Cycles) curve, and the lowest life value at three points is selected as the three-dimensional crack propagation fatigue life of the spherical shell surface.