Fracture preventing design method for metal structure

Abstract

Provided is a fracture preventing design method for a metal structure, the method comprising: establishing a stress field evolution characteristic curve of a metal structure under the action of a force load and determining a strength ratio Ib / σs of a material, calculating stress field characteristic values ω1 and ω2 under the action of a load, and, according to the calculated values, performing fracture design of the metal structure, thus enabling same to satisfy the design requirements for preventing elastic fracture and / or elastic-plastic fracture and / or fully-plastic fracture. The fracture preventing design method for a metal structure can guide structure designers in performing safety design of metal structures so as to improve the structure failure resistance, and involves simple calculations and achieves high applicability.

Description

A fracture-resistant design method for metal structures Technical Field

[0001] This invention relates to the fields of fracture mechanics of metallic materials and metal structure design technology, and in particular to a fracture-resistant design method for metal structures. Background Technology

[0002] Metallic materials are fundamental to modern civilization. Due to increasing pressure on resources, energy, and the environment, both defense and civilian industries urgently need advanced metallic materials with high strength, high toughness, and resistance to extreme service conditions. In industrial production, metal pipes and sheet metal structures need to be cut to produce the required parts. Fracture is one of the main forms of material failure, and its study has always been a major research area in materials development and application. Currently, finite element method (FEM) simulation analysis is widely used in materials forming and collision analysis. Accurate model establishment and precise acquisition of material parameters are crucial for the accuracy of simulation predictions. In 2014, the inventors proposed the I1 fracture criterion, based on the first invariant stress I1. This criterion considers I1 as the fracture parameter of metallic materials, meaning that when I1 reaches the fracture strength I of the metallic material... b Fracture occurred at that time; subsequently, experimental verification of the effectiveness of the I1 fracture criterion, consistency analysis with the brittle fracture K criterion, explanation of the intrinsic mechanism of various fracture behaviors of metallic materials based on the I1 fracture criterion, and fracture strength I were carried out. b Research on testing technologies and other related work.

[0003] However, defects often exist in actual metal structural materials. The existence of stress concentration effect makes defects a potential source of fracture in metal structures. Therefore, studying the stress field evolution characteristics at defects under force load is of practical significance for fracture design of metal structures. No relevant research papers have been published in the existing technology.

[0004] Summary of the Invention

[0005] In view of this, the present invention aims to propose a fracture-resistant design method for metal structures, which combines the I1 fracture criterion and the stress field evolution characteristics of weak locations in metal structures under force load to propose a fracture design method for metal structures.

[0006] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0007] A fracture-resistant design method for metal structures is proposed, which establishes the stress field evolution of metal structures under force loads. Characteristic curves and strength ratio of the measured material I b / σ s :

[0008] Wherein, the eigenvalue ω1 is equal time Equivalent stress concentration factor at the root of the incision The ratio of the equivalent stress at the incision root to the nominal equivalent stress. The eigenvalue ω2 is In the time structure σ s The yield strength of the material. The nominal equivalent stress in the main load-bearing unit or component during the load-bearing process of a metal structure. This represents the maximum value of the first invariant stress during the load-bearing process of a metal structure; according to The calculated values ​​are used for fracture design of metal structures to ensure that they meet the design requirements for preventing elastic fracture and / or elastoplastic fracture and / or all-plastic fracture.

[0009] Furthermore, the metal structure fracture prevention design method includes a metal structure brittle fracture prevention design method, comprising the following steps:

[0010] S11: Calculate the characteristic value ω1 of the stress field at the weak points in the metal structure. The weak points in the metal structure are the stress concentration points caused by the geometric discontinuity of the structural shape.

[0011] S12: Material selection is based on the stress field characteristic value ω1 at weak points in the metal structure. The principle is that the material's fracture strength I should be within the minimum service temperature of the metal structure design. b With yield strength σ s The ratio is greater than the characteristic value ω1 of the largest stress field at the weakest point in the metal structure, as shown in the following formula:

[0012] The fracture strength I of the material b With yield strength σ s Determined by uniaxial tensile testing of round bar specimens.

[0013] Furthermore, in step S11, for general-type metal structural steel, the method of simulation using the linear elastic finite element method is employed, including the following steps:

[0014] S111: Establish a geometric model of the metal structure, which includes stress concentration points at weak locations;

[0015] S112: Apply loads according to the actual working conditions of the metal structure or the designed force load conditions;

[0016] S113: Simulate the stress or strain components at the weak points of the metal structure, and calculate the characteristic value ω1 of the stress field at the weak points according to the basic calculation formula.

[0017] Furthermore, the formula for calculating the eigenvalue ω1 is as follows:

[0018] one of the.

[0019] Furthermore, for a center-through cut, the characteristic value ω1 of the stress field at the root of the cut under plane strain conditions is calculated using the following formula:

[0020] For a central through-crack, the characteristic value ω1 of the stress field at the root of the notch under plane strain is calculated using the following formula:

[0021] In the formula, v is the Poisson's ratio of the material.

[0022] Furthermore, the metal structure fracture prevention design method includes a metal structure fully plastic fracture design method, comprising the following steps:

[0023] S21: Considering the proposed structural form and load conditions, and taking into account potential defects, the stress field characteristic value ω2 in the structure is calculated using elastoplastic mechanics. The characteristic value ω2 is... In the time structure σ s The yield strength of the material. The nominal equivalent stress in the main load-bearing unit or component during the load-bearing process of a metal structure. This represents the maximum value of the first invariant stress during the load-bearing process of the metal structure;

[0024] S22: Material selection and design are based on the characteristic value ω2 of the stress field present in the metal structure. The principle is that the material's fracture strength I is at the lowest service temperature of the metal structure design. b With yield strength σ s The ratio is greater than the maximum stress field characteristic value ω2 at the weakest point that may exist in the metal structure, as shown in the following formula:

[0025] The fracture strength I of the material b With yield strength σ s Determined by uniaxial tensile testing of round bar specimens.

[0026] Furthermore, in step S21, the stress field characteristic value ω2 is calculated using the elastoplastic finite element method, and the material parameters are determined using the uniaxial tensile test method for round bar specimens, including the following steps:

[0027] S211: Establish a geometric model of the metal structure, which includes weak points;

[0028] S212: Apply loads according to the actual working conditions or designed force load conditions of the metal structure, including elastic modulus E, Poisson's ratio v, and yield strength σ. s Equivalent strain during the plastic deformation stage With rheological stress σ0;

[0029] S213: Calculate the stress field characteristic value ω2 at the weak location.

[0030] Furthermore, in step S212,

[0031] The equivalent plastic strain during the plastic deformation stage The correspondence between the rheological stress σ0 and the stress σ0 was determined by the uniaxial tensile test method for round bar specimens, where:

[0032] For materials that do not exhibit necking during uniaxial tensile testing of round bar specimens. A0 is the initial cross-sectional area of ​​the plastic deformation, A is the cross-sectional area of ​​the specimen at the time of measurement, and F is the force in the tensile direction of the specimen at the time of measurement.

[0033] For materials exhibiting necking during uniaxial tensile testing of round bar specimens, the initial diameter R of the round bar specimen as recorded in the test data should be considered. c The diameter of the specimen at the moment the necking deformation begins Maximum axial loading force F max The characteristic parameter r of the necked-down specimen shape at any time c r n r ip z ip , and the axial force F of the tension z The maximum equivalent plastic strain in the necked-down specimen is calculated using the following formula and denoted as: and the corresponding rheological stress Equivalent plastic strain during the plastic deformation stage of a material The correspondence with the rheological stress σ0 is equal to and The correspondence;

[0034] Among them, the tensile axial force F during the test z The minimum cross-sectional radius r of the necked-down bottom of the specimen perpendicular to the central axis c The maximum limit value r of the cross-sectional radius perpendicular to the central axis on the specimen. n The slope of the tangent at the inflection point of the generatrix of the necked-down deformation profile. The radius r of the section perpendicular to the central axis at the inflection point of the generatrix of the necked-down deformed outline. ipThe distance z between the cross section perpendicular to the central axis at the inflection point of the generatrix of the necked-down deformed outline and the minimum cross section at the bottom of the necking-down deformed shape. ip .

[0035] Furthermore, in calculation When designing materials for fracture fracture, the yield strength σ s The value R is determined using existing uniaxial tensile testing standards for metallic materials. p0.2 Fracture strength I b Let I1 be the maximum value of the first invariant stress I1 inside the specimen at the instant of fracture during a uniaxial tensile test of a round bar specimen. For materials without necking deformation during a uniaxial tensile test of a round bar specimen, the value is determined based on the tensile axial force F at the instant of fracture. z Given the cross-sectional area A of the specimen, the fracture strength I is approximately calculated using the following formula. b ,

[0036] For materials exhibiting necking during uniaxial tensile testing of round bar specimens, the necking specimen shape characteristic parameter r at the moment of fracture, as recorded in the test, is used. c r n r ip z ip , and the axial force F of the tension z Calculate and determine the maximum value of the first invariant stress at this point. This value is the fracture strength Ib of the material, and the maximum value of the first invariant stress inside the specimen during necking deformation is denoted as... Calculate using the following formula:

[0037] Compared with existing technologies, the metal structure fracture prevention design method of the present invention has the following advantages:

[0038] (1) The metal structure anti-fracture design method of the present invention is based on the I1 fracture criterion proposed by the applicant. It calculates the stress field characteristic values ​​ω1 and ω2 of possible weak locations in the metal structure. Based on the calculated characteristic values ​​ω1 and ω2 of the weak locations, it conducts material selection design. Through the above settings, the potential brittle fracture, elastoplastic fracture and all-plastic fracture risks in the metal structure can be identified and assessed. This helps engineers and designers understand where there may be elastoplastic fracture risks, so as to take corresponding measures or select materials for reinforcement or design improvement. Combined with the material selection design principle, it ensures that the selected materials do not undergo elastoplastic fracture or only all-plastic fracture at the lowest service temperature of the metal structure, which helps to improve the safety of the metal structure.

[0039] (2) The metal structure anti-fracture design method of the present invention can comprehensively evaluate the safety of metal structures, provide a basis for the safety design and analysis of structures, guide structural designers to optimize or improve metal structures, so as to improve the structure's resistance to damage. It is simple to calculate and has strong applicability. Attached Figure Description

[0040] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0041] Figure 1 is a schematic diagram of the finite element model structure containing the central notch in the metal structure all-plastic fracture design method of the present invention.

[0042] Figure 2 is a schematic diagram of the plastic stress-strain constitutive relationship of a metal structure material with a central notch in the fully plastic fracture design method for metal structures according to an embodiment of the present invention.

[0043] Figure 3A is a schematic diagram showing the change of the ratio of stress to yield strength with nominal tensile strain during uniaxial tension of a metal structure with a central notch in the anti-fracture design method of the metal structure according to an embodiment of the present invention.

[0044] Figure 3B is a schematic diagram showing the change of the ratio of stress to yield strength with the ratio of nominal equivalent stress to yield strength during uniaxial tension of a metal structure with a central notch in the anti-fracture design method of the metal structure according to an embodiment of the present invention.

[0045] Figure 4 shows the evolution of the stress field of the notched body in the all-plastic fracture design method of the present invention. Characteristic curve diagram;

[0046] Figure 5 is a schematic diagram of the relationship between the characteristic value ω1 and Poisson's ratio v in the plane strain state of a metal structure with a central notch in the metal structure fully plastic fracture design method of the present invention.

[0047] Figure 6 is a schematic diagram of fracture analysis of a notched body in the fully plastic fracture design method for metal structures according to an embodiment of the present invention.

[0048] Figure 7 is a schematic diagram of the finite element model containing a central crack in the metal structure all-plastic fracture design method according to an embodiment of the present invention.

[0049] Figure 8 is a schematic diagram of the plastic stress-strain constitutive relationship of a metal structure material containing a central crack in the fully plastic fracture design method for metal structures according to an embodiment of the present invention.

[0050] Figure 9A is a schematic diagram showing the change of the ratio of stress to yield strength with nominal tensile strain during uniaxial tension of a metal structure containing a central crack in the anti-fracture design method of the metal structure according to an embodiment of the present invention.

[0051] Figure 9B is a schematic diagram showing the change of the ratio of stress to yield strength with the ratio of nominal equivalent stress to yield strength during uniaxial tension of a metal structure containing a central crack in the anti-fracture design method of the metal structure according to an embodiment of the present invention.

[0052] Figure 10A is a schematic diagram of the stress field evolution characteristics of austenitic steel under stress state in the metal structure containing a central crack as described in the embodiment of the present invention.

[0053] Figure 10B is a schematic diagram of the stress field evolution characteristics of ferritic steel under stress state in the metal structure containing a central crack as described in the embodiment of the present invention.

[0054] Figure 11 is a schematic diagram of the stress field evolution characteristics of a cracked body in the fully plastic fracture design method for metal structures according to an embodiment of the present invention.

[0055] Figure 12A is a schematic diagram of the effect of the minimum mesh size on the slope of the normal stress component at the crack tip in the plane strain state of the finite element model containing the central crack of the present invention.

[0056] Figure 12B is a schematic diagram of the effect of the minimum mesh size on the slope of the normal stress component at the crack tip in the plane stress state of the finite element model containing the central crack of the present invention.

[0057] Figure 13 illustrates the metal structure all-plastic fracture design method described in this embodiment of the invention. Schematic diagram of characteristic curves. Detailed Implementation

[0058] To make the technical means and objectives and effects of the present invention easier to understand, the embodiments of the present invention will be described in detail below with reference to specific illustrations.

[0059] Example 1

[0060] This application discloses a fracture analysis method for metal structures, based on the I1 fracture criterion hypothesis proposed by the inventors in 2014. The fracture criterion is based on the condition that the first invariant stress I1 in a local region of the material reaches a critical value I. b At this point, fracture begins to occur; the critical value I is defined. b The fracture strength I of a material can be determined by a uniaxial tensile test. b It is assumed that the first invariant stress I1 is the fracture parameter of metallic materials, that is, when I1 reaches the fracture strength I of the metallic material... bWhen fracture occurs, combining this fracture criterion with the Mises yield criterion, the four basic mechanical behavior criteria are as follows: (1) Elastic deformation σ Mises <σ y , I1 < I b (2) Plastic deformation σ Mises ≥σ y , I1 < I b (3) Brittle fracture σ Mises <σ y , I1≥I b (4) Plastic fracture σ Mises ≥σ y , I1≥I b .

[0061] The applicant subsequently conducted experimental verification of the effectiveness of the I1 fracture criterion, consistency analysis with the brittle fracture K criterion, explanation of the intrinsic mechanism of various fracture behaviors of metallic materials based on the I1 fracture criterion, and fracture strength I... b Research on testing techniques, including analysis of the stress field evolution characteristics of a body with notches under force load and analysis of the stress field evolution characteristics of a body with cracks under force load.

[0062] In the stress field evolution analysis of a body with a notch under load, the stress field evolution characteristics and influencing factors of a notch defect with a simplified root as an arc shape under tensile load are investigated. The stress field evolution process of a body with a central notch under uniaxial tension in plane strain is analyzed using the elastoplastic finite element method. Figure 1 shows the finite element model of the body with a central notch: the size W of the notch is 500 mm, the radius r of the arc at the root of the notch is 0.5 mm, and the half-length a of the notch is 2.0 mm. Due to geometric symmetry, a 1 / 4 model is used for finite element analysis, with 8-node quadrilateral elements. The mesh size near the top of the notch is set to 0.01 mm × 0.01 mm. A displacement load v is applied perpendicular to the length of the notch (i.e., the y-axis direction in the rectangular coordinate system of Figure 1). y With a load step size of 0.01 mm and a maximum displacement load of 3.0 mm, analyze the nominal value of the Mises equivalent stress (hereinafter referred to as "equivalent stress") in the body containing the notch during the loading process. (i.e., the equivalent stress at the tensile end face) and its maximum value Maximum value of the first invariant of stress Equivalent stress at the root of the incision First invariant of stress The material parameters in this study are referenced from high-strength low-alloy steel, specifically set as follows: Young's modulus E = 210 GPa, Poisson's ratio v = 0.3, and yield strength σ = 0.3. s The stress is 725 MPa, and the plastic stress-strain constitutive relation is set as shown in Figure 2.

[0063] Nominal equivalent stress of a body with a central notch during uniaxial tension Maximum equivalent stress Maximum value of the first invariant of stress Equivalent stress at the root of the incision First invariant of stress Various stresses and yield strength σ s The ratio of the nominal tensile strain ε n The evolutionary process is shown in Figure 3A, which is converted into The constant stress ratio varies with the ratio of nominal equivalent stress to yield strength. The changes are shown in Figure 3B:

[0064] According to the I1 fracture criterion, fracture is determined by the maximum value of the first invariant stress. Therefore, the stress field evolution under the bearing conditions of the cut body The curve becomes the key characteristic curve determining fracture. Based on the above finite element simulation results, the stress field evolution process of the notched body, as shown in Figure 4, can be established. Characteristic curve: This curve consists of three parts: elastic deformation segment, elastoplastic deformation segment, and fully plastic deformation segment; the ratio of the equivalent stress at the root of the notch to the nominal equivalent stress is assumed. The equivalent stress concentration factor at the root of the incision. Then when equal When yielding occurs at the root of the notch, i.e., the body containing the notch enters the elastoplastic deformation stage, let the ratio of the maximum value of the first invariant stress at this point to the yield strength be given. ω1; when When the stress ratio equals 1, the entire body including the notch yields, entering the stage of full plastic deformation. Let the ratio of the maximum value of the first invariant stress at this point to the yield strength be denoted as follows: ω2; as shown in Figure 4, the local yield point (i.e., the starting point of elastoplastic deformation) and the overall yield point (i.e., the starting point of full plastic deformation) are... The characteristic curve has two characteristic points, therefore the parameters ω1 and ω2 are The three characteristic values ​​of the characteristic curve.

[0065] The nominal equivalent stress in a rectangular coordinate system can be obtained from linear elasticity. The general forms of the stress component and strain component functions are shown in equations (1) and (2) (where... These are six nominal stress components. (representing six nominal strain components), equivalent stress at the notch root. First invariant of stress The general forms of the stress component and strain component functions are shown in equations (3) and (4) (where... These are the six stress components at the root of the incision. These are the six strain components at the root of the cut; further derivation yields the eigenvalues. The general forms of the stress component and strain component functions are shown in equations (5) and (6); as shown in Figure 6, the eigenvalue ω1 and the stress at the root of the notch have the relationship shown in equation (7), and thus the general forms of the eigenvalue ω1 with respect to the stress component and strain component functions shown in equations (8) and (9) can be derived. The slope of the elastic deformation segment of the characteristic curve With eigenvalues The relationship between ω1 and ω1 is shown in equation (10).

[0066] The fracture risk at the notch root is greatest when both the nominal stress and the principal stress at the notch root are in the coordinate axis direction, and the normal stress component in the y-axis direction is the first principal stress and is positive. In this case, the eigenvalue... The expression for ω1 degenerates into the forms shown in equations (11) and (12), which are related to the material’s Poisson’s ratio v, nominal stress-strain, and stress-strain at the root of the notch; the expression for eigenvalue ω1 degenerates into the forms shown in equations (13) and (14), which are related to the material’s Poisson’s ratio v and stress state at the root of the notch.

[0067] Under plane strain conditions, eigenvalues The expression for ω1 degenerates into the forms shown in equations (15) and (16), which are related to Poisson's ratio v, nominal stress-strain and root stress-strain; the expression for eigenvalue ω1 degenerates into the form shown in equation (17), which is only related to Poisson's ratio v. Figure 5 shows the correlation curve between eigenvalue ω1 and Poisson's ratio v under plane strain conditions, which is close to linear. The eigenvalue ω1 increases with the increase of Poisson's ratio v.

[0068] Under plane stress, eigenvalues The expression for ω1 degenerates into the forms shown in equations (18) and (19), which are related to Poisson's ratio v, nominal stress-strain and root stress-strain; the eigenvalue ω1 degenerates into the constant 1, as shown in equation (20).

[0069] ω1=1 (20);

[0070] In the axisymmetric state, eigenvalues It can be expressed in the form shown in equations (21) and (22), and is related to the material’s Poisson’s ratio v, nominal stress-strain and notch root stress-strain; the characteristic value ω1 can be expressed in the form shown in equations (23) and (24), and is related to the material’s Poisson’s ratio v and notch root stress-strain, and is not a constant value, and is different from plane strain and plane stress state.

[0071] The applicant's research shows that factors such as stress state, loading method, stress-strain concentration, and material parameters all affect the elastic deformation stage in the stress field evolution process. Characteristic curves were used to further analyze the influence of the above factors on the complete stress field evolution of the body with notches from elastic deformation to full plastic deformation. The influence of characteristic curves.

[0072] This study shows the stress field evolution of a body with a notch under force loading. The characteristic curve contains two characteristic points: the initiation point of elastic-plastic deformation and the initiation point of fully plastic deformation. Value corresponding The value is ω1, the starting point of full plastic deformation. The value is 1, corresponding to The value is ω2, so the fracture analysis diagram of the notched body can be established according to the I1 fracture criterion and the Mises yield criterion, as shown in Figure 6.

[0073] Furthermore, in the stress field evolution characteristics analysis of a cracked body under load, the applicant simplified the root of the crack defect body to a circular arc shape under tensile load and analyzed its stress field evolution characteristics and influencing factors. The elastoplastic finite element method was used to analyze the stress field evolution process of a body with a central crack under typical stress states such as plane strain, plane stress, and axisymmetry during uniaxial tension. Figure 7 shows the finite element model of the body with a central crack: the size W of the cracked body is set to 500 mm, and the crack half-length a is 2.0 mm; due to geometric symmetry, a 1 / 4 model is used for finite element analysis, with an 8-node quadrilateral element type, and the mesh size near the crack tip is set to 0.05 mm × 0.05 mm. A displacement load v is applied perpendicular to the crack length direction (i.e., the y-axis direction in the rectangular coordinate system of Figure 7). y With a load step size of 0.01 mm and a maximum displacement load of 3.0 mm, analyze the nominal value of the Mises equivalent stress (hereinafter referred to as "equivalent stress") in the cracked body during the loading process. (i.e., the equivalent stress at the tensile end face) and its maximum value Maximum value of the first invariant of stress Equivalent stress at the crack tip First invariant of stress The changes in material parameters were studied. The material parameters for both austenitic and ferritic steels were referenced, specifically set as follows: for austenitic steel, Young's modulus E was 180 GPa, Poisson's ratio v was 0.3, and yield strength σ... s The strength is 500 MPa, the Young's modulus E of the ferritic steel is 210 GPa, the Poisson's ratio v is 0.3, and the yield strength σ is... s The stress is 725 MPa. Figure 8 shows the plastic stress-strain constitutive relationship settings for the two materials.

[0074] Based on the analysis of the stress field evolution characteristics of cracked bodies, linear elastic fracture mechanics and linear elastic finite element method are used to compare and analyze the stress field characteristics at the crack tip, and the characteristic value function of the stress field at the crack tip in the plane state is explored.

[0075] Nominal equivalent stress of austenitic steel containing a central crack under plane strain during uniaxial tension Maximum equivalent stress Maximum value of the first invariant of stress Equivalent stress at crack tip First invariant of stress Various stresses and yield strength σ s The ratio of the nominal tensile strain ε n The evolutionary process is shown in Figure 9A, which is converted into The constant stress ratio varies with the ratio of nominal equivalent stress to yield strength. The changes are shown in Figure 9B. The stress field evolution characteristics of austenitic steel and ferritic steel containing a central crack under uniaxial tension in three stress states—plane stress, plane strain, and axisymmetric stress—were compared and analyzed. The results are shown in Figures 10A and 10B: both materials exhibit the same stress field evolution characteristics under different stress states. The results in Figures 10A and 10B also show that the stress-strain constitutive relationship of plastic deformation mainly affects the stress field evolution characteristic curves in the later stage of elastoplastic deformation and the fully plastic deformation stage. This characteristic is consistent with the stress field characteristic curves of the notched body mentioned above.

[0076] [Corrected according to Rule 91, 23.10.2024] As can be seen from the I1 fracture criterion, fracture is determined by the maximum value of the first invariant stress. Therefore, the stress field evolution of a cracked body under load is... The curve becomes the key characteristic curve determining fracture. Based on the above finite element simulation results, the stress field evolution process of the cracked body, as shown in Figure 11, can be established. The characteristic curve, whose shape is basically consistent with that of a notched body, consists of three parts: an elastic deformation segment, an elastoplastic deformation segment, and a fully plastic deformation segment; similarly, the ratio of the equivalent stress at the crack tip to the nominal equivalent stress is set. The equivalent stress concentration factor at the crack tip Then when equal When the crack tip yields, meaning the cracked body enters the elastoplastic deformation stage, let the ratio of the maximum value of the first invariant stress to the yield strength be given. ω1; when When the stress ratio equals 1, the cracked body yields as a whole, entering the stage of full plastic deformation. Let the ratio of the maximum value of the first invariant stress at this point to the yield strength be taken as follows. ω2; as shown in Figure 11, the local yield point (i.e., the starting point of elastoplastic deformation) and the overall yield point (i.e., the starting point of full plastic deformation) are... The characteristic curve has two characteristic points, therefore the parameters ω1 and ω2 are The three characteristic values ​​of the characteristic curve.

[0077] Based on the results of the study on the stress field evolution characteristics of the notched body and the analysis results of the stress field at the crack tip, it is inferred that the characteristic value ω1 is only related to the correlation of each stress component at the crack tip during the elastic deformation stage, and is not greatly affected by stress singularity. Moreover, from the perspective of fracture control of metal structures, the characteristic value ω1 is more practically significant for preventing brittle fracture.

[0078] Since the formula for calculating the characteristic value ω1 and the stress function derived from linear elastic fracture mechanics do not hold true at the crack tip, the applicant, based on the research of "Xue Gang. Consistency analysis of I_(1) criterion and K criterion for linear elastic fracture of plane strain type I crack under biaxial equal tensile stress [J]. Materials Development and Application, 2023, 38(1):1-8", found that the first invariant I1 of the stress at the crack tip under biaxial equal tensile stress in plane strain state obtained from linear elastic finite element simulation analysis is consistent with the stress field intensity factor K. I The consistency between the stress components at the crack tip obtained by the finite element simulation method indicates that the correlation between the stress components at the crack tip is inherently reasonable. This study also found that under biaxial tensile stress in a plane strain state, the normal stress component at the crack tip is λ... x-y (defined as the normal stress component along the x-axis at the crack tip in the coordinate system shown in Figure 7) Normal stress components in the y-axis direction The ratio of the shear stress components (λ) and the ratio of the shear stress components (λ) xy-y (defined as the shear stress component at the crack tip in the coordinate system shown in Figure 7) Normal stress components in the y-axis direction The ratios of the nominal normal stress components (k1, defined as the nominal normal stress components in the x-axis direction of the coordinate system shown in Figure 7) are all constants independent of crack length. The applicant further studied the ratio of the nominal normal stress components under plane strain and plane stress conditions (k1 is defined as the ratio of the nominal normal stress components in the x-axis direction of the coordinate system shown in Figure 7). The nominal normal stress component in the y-axis direction The ratio of the minimum mesh size l1 (defined as the side length of the minimum mesh element at the crack tip) to the stress component at the crack tip is λ. x-y and λ xy-y The effect of this result on λ under both plane strain and plane stress states is shown in the results. x-y Both maintain a linear relationship with k1, and the ratio of the normal stress component at the crack tip to the slope The variation of the minimum mesh size l1 with the stress state is shown in Figures 12A and 12B. The distribution of lnl1 all satisfy the exponential function distribution law, and the regression yields the equation (25). Regarding the distribution function of l1, this function can be used to obtain the minimum mesh size l1 tending to zero under plane strain and plane stress conditions. The limit value is 1, as shown in equation (26), that is, when l1 approaches zero, the ratio of the nominal normal stress component to the crack tip normal stress component λ under different values ​​of k1. x-y The limiting value is the ratio of the normal stress component at the crack tip under biaxial equal tensile stress conditions (i.e., k1 = 1). As shown in equation (27); according to the basic principle of the finite element method, the stress value at the crack tip obtained by finite element simulation is actually the average value over a certain region related to the minimum mesh size l1 at the crack tip. Therefore, when l1 approaches zero, λ x-y The limit value can be considered as the ratio of the actual normal stress components at the crack tip, denoted as . (Right now From the relationship shown in equation (27), the ratio of the normal stress components at the crack tip can be derived. It is independent of the ratio of the nominal normal stress components, k1.

[0079] Besides crack length and the ratio of nominal normal stress components k1, what also affects the ratio of stress components at the crack tip λ? x-y and λ xy-y Other possible factors include the magnitude of the load and the material's elastic deformation parameters (Young's modulus E and Poisson's ratio v). Therefore, a comparative analysis was conducted under both plane strain and plane stress conditions for both the load and the material's elastic deformation parameters. The study shows that the stress component at the crack tip is more pronounced under both plane strain and plane stress conditions. and It is only related to Poisson's ratio v, and is independent of other factors.

[0080] For a series of Poisson's ratio v values ​​(since v is less than 0.5 in the elastic deformation stage, and considering possible cases, we set 0.25≤v≤0.499), we conducted linear elastic finite element simulations of bodies with a central crack under biaxial constant tensile stress conditions with different minimum mesh size l1 values ​​(the finite element model is shown in Figure 7).

[0081] Based on the stress component ratio at the crack tip and The equivalent stress at the crack tip under both plane strain and plane stress conditions can be derived. First invariant of stress The expressions are shown in Equations (28) and (29) respectively. Further, the expression for the characteristic value ω1 of the stress field evolution at the crack tip is derived as shown in Equation (30): Under plane strain, the characteristic value ω1 first increases in an approximately linear relationship with the increase of Poisson's ratio v. When the Poisson's ratio v is about 0.47, the characteristic value ω1 reaches its maximum value. Subsequently, as the Poisson's ratio v continues to increase, the characteristic value ω1 decreases sharply. Under this stress state, the characteristic value ω1 can be determined by Equation (31). For most metal structural materials, the Poisson's ratio v is about 0.3. Therefore, the characteristic value ω1 under this condition can be calculated to be about 4.588. Under plane stress, the correlation curve between the characteristic value ω1 and the Poisson's ratio v is approximately a straight line with a slope of zero. The fitting function is shown in Equation (32). It can be considered that the influence of Poisson's ratio v on the characteristic value ω1 is very small and negligible. The characteristic value ω1 when the Poisson's ratio v is 0.3 is calculated to be 1.860, which is much lower than that under plane strain.

[0082] ω1=1.86824-0.02849·v (R 2 =0.99998) (32);

[0083] The applicant studied the stress singularity at the crack tip and the location of the equivalent stress peak based on the fracture analysis diagram of a cracked body. The results show that, in the absence of internal stress, the maximum equivalent stress of the cracked body under the aforementioned force loading conditions should be located at the crack tip under both plane strain and plane stress conditions. The fundamental reason why the location of the maximum equivalent stress deviates from the crack tip under plane strain conditions is still the stress singularity at the crack tip caused by the linear elastic assumption. Therefore, in this study, the eigenvalue ω1 is approximated as the first invariant stress I1 at the crack tip during the elastic deformation stage and the equivalent stress. The ratio has its own inherent rationality.

[0084] The applicant's research shows that there is a corresponding stress field evolution in metal structures under force loads. Furthermore, the stress field evolution of the cracked body under force load Characteristic curves and stress field evolution of a body with notches under force loading The characteristic curves exhibit the same features, and the fracture analysis diagram shown in Figure 13 can also be constructed based on the I1 fracture criterion and the Mises yield criterion. This stress field evolution... The characteristic curve is the critical line between the deformation zone and the fracture zone; the area above it is the deformation zone, and the area below it is the fracture zone, meaning that within a certain range... Under certain conditions, when the ratio of the material's fracture strength to its yield strength (hereinafter referred to as "strength ratio") is I b / σ s Greater than At that time, the body containing the cut only deforms but does not break; when I b / σ s Not greater than At that time, the incision site will fracture after deformation; in the deformation zone (i.e. ),when At that time, the body containing the cut undergoes elastic deformation, when When the cut body undergoes elastoplastic deformation, When the notched body undergoes full plastic deformation, Equation (33) summarizes the criteria for the three deformation types; in the fracture zone When I b / σ s When ω<1, the fracture type of the body containing the notch is elastic fracture (i.e., brittle fracture); when ω<1≤I0 b / σ s When I < ω2, the fracture type of the notched body is elastoplastic fracture; when I b / σ s When ≥ω2, the fracture type of the body with notches is fully plastic fracture. Equation (34) shows a summary of the criteria for the three fracture types.

[0085] The aforementioned research results show that eigenvalues ω1 and ω2 must be three actual mechanical parameters. The fracture resistance of a metal structure is mainly determined by the material's strength ratio I. b / σ s Stress field evolution under force load Characteristic curve: the strength ratio I of the material b / σ s The higher, eigenvalues ​​of characteristic curves The larger the value and the smaller the eigenvalues ​​ω1 and ω2, the better the fracture resistance of the body containing the notch. Strength ratio I b / σ sThe yield strength σ of 304 stainless steel (austenitic steel), EH50 steel (ferritic steel) and 10Ni5CrMoV steel (ferritic steel) is a material property mainly determined by the material's composition and microstructure. The literature "Xue Gang, Gong Xuhui, Gao Zhenpeng, et al. Experimental verification of the validity of the I1 fracture criterion assumption under approximate two-dimensional stress state [J]. Materials Development and Application, 2018, 33(3):9.DOI:CNKI:SUN:CLKY.0.2018-03-001" gives the yield strength σ of 304 stainless steel (austenitic steel), EH50 steel (ferritic steel) and 10Ni5CrMoV steel (ferritic steel). s and fracture strength I b Data, calculated, shows the strength ratio of the three types of steel I b / σ s The values ​​are approximately 5.2, 3.5, and 2.5 respectively. Clearly, the strength of austenitic 304 stainless steel is higher than that of I... b / σ s It is much higher than the other two types of ferritic steel.

[0086] Stress field evolution The characteristic curve is mainly determined by material parameters, stress state, loading method, and stress-strain concentration at the notch, where: eigenvalues The stress field evolution is mainly determined by Poisson's ratio v, the location and size of the notch, the stress state, and the loading method; the eigenvalue ω1 is mainly determined by Poisson's ratio v and the stress state at the root of the notch; the eigenvalue ω2 is mainly determined by the plastic stress-strain constitutive relation, the location and size of the notch, the stress state, and the loading method. Characteristic curves provide new technical insights for fracture-resistant design and safety assessment of metal structures.

[0087] Example 2

[0088] This application also discloses a fracture-resistant design method for metal structures, which establishes the stress field evolution of metal structures under load. Characteristic curves and strength ratio of the measured material I b / σ s :

[0089] Wherein, the eigenvalue ω1 is equal hour The value is ω1, the equivalent stress concentration factor at the root of the cut. The ratio of the equivalent stress at the incision root to the nominal equivalent stress. The eigenvalue ω2 is In the time structure σ s The yield strength of the material. The nominal equivalent stress in the main load-bearing unit or component during the load-bearing process of a metal structure. This represents the maximum value of the first invariant stress during the load-bearing process of a metal structure; according to The calculated values ​​are used to design the fracture material selection for metal structures, so that they meet the design requirements for preventing elastic fracture and / or elastoplastic fracture and / or all-plastic fracture.

[0090] The fracture-resistant design method for metal structures disclosed in this embodiment is based on the applicant's proposed I1 fracture criterion assumption. It combines the form of the metal structure and the load-bearing conditions, especially the stress field evolution analysis for notched bodies, cracked bodies, and planar through-type central notches, to establish the strength ratio I of the metal structure's material. b / σ s Stress field evolution under force load Characteristic curve, as shown in Figure 13, is the eigenvalue in this fracture analysis diagram. ω1 and ω2 must be three actual mechanical parameters, under certain conditions. Under certain conditions, when the ratio of the material's fracture strength to its yield strength (referred to as the "strength ratio") is I b / σ s Greater than When I, the metal structure only deforms but does not break. b / σ s Not greater than When metal structures deform, they will fracture; in the deformation zone (i.e. ):when When the metal structure undergoes elastic deformation, When the metal structure undergoes elastoplastic deformation, At that time, the metal structure undergoes full plastic deformation; in the fracture zone... When I b / σ s When ω<1, the fracture type of the metallic structure is elastic fracture (i.e., brittle fracture); when ω<1 ≤ I0 b / σ s When I < ω2, the fracture type of the metal structure is elastoplastic fracture; when I < ω2, the fracture type of the metal structure is elastoplastic fracture. b / σ s When ≥ω2, the fracture type of the metal structure is fully plastic fracture.

[0091] In the example of this application, the metal structure anti-fracture design method includes a metal structure anti-brittle fracture design method, comprising the following steps:

[0092] S11: Calculate the characteristic value ω1 of the stress field at the weak points in the metal structure. The weak points in the metal structure are the stress concentration points caused by the geometric discontinuity of the structural shape.

[0093] S12: Material selection is based on the stress field characteristic value ω1 at weak points in the metal structure. The principle is that the material's fracture strength I should be within the minimum service temperature of the metal structure design. b With yield strength σ s The ratio is greater than the characteristic value ω1 of the largest stress field at the weakest point in the metal structure, as shown in the following formula:

[0094] The fracture strength I of the material b With yield strength σ s Determined by uniaxial tensile testing of round bar specimens.

[0095] The metal structure anti-brittle fracture design method described in this invention, based on the applicant's proposed I1 fracture criterion, calculates the stress field characteristic value ω1 at weak points in the metal structure. These weak points are typically stress concentration points caused by structural discontinuities, and may be stress concentration areas caused by notches, cracks, or other types of defects. Based on the calculated stress field characteristic value of the weak points, material selection is performed. The principle of material selection is to ensure that, at the lowest service temperature of the metal structure design, the ratio of the material's fracture strength to its yield strength is greater than the maximum stress field characteristic value at the potentially weak point in the metal structure. Through this approach, potential brittle fracture risks in the metal structure can be identified and assessed. This helps engineers and designers understand where brittle fracture risks may exist, allowing them to take corresponding measures or select materials for reinforcement or design improvements. Combined with the material selection design principle, this ensures that the selected materials avoid brittle fracture at the lowest service temperature of the metal structure, thus improving the safety of the metal structure and reducing the risk of brittle fracture accidents.

[0096] In step S11, for general-type metallic structural steel, the method of linear elastic finite element method is used to determine the structure, including the following steps:

[0097] S111: Establish a geometric model of the metal structure, which includes stress concentration points at weak locations;

[0098] S112: Apply loads according to the actual working conditions of the metal structure or the designed force load conditions;

[0099] S113: Simulate the stress or strain components at the weak points of the metal structure, and calculate the characteristic value ω1 of the stress field at the weak points according to the basic calculation formula.

[0100] In the examples of this application, the weak points of the metal structure are the locations in the metal structure containing notched bodies, cracked bodies, or other stress concentration points.

[0101] This application employs the linear elastic finite element method to simulate the stress field characteristic value ω1 of a metal structure. By establishing a geometric model and applying actual working conditions or design loads, the stress distribution of the metal structure under different working conditions can be accurately simulated. The simulated stress or strain components can reflect the stress concentration at weak points in the metal structure, providing an accurate data foundation for subsequent calculations of the stress field characteristic value. During the simulation, the influence of these stress concentration locations on the structural stress distribution can be comprehensively considered, contributing to an accurate assessment of the structure's safety. The finite element method flexibly handles various structural forms and complex situations, allowing for simulation analysis of different types of metal structures, and adjustments and improvements can be made as needed. It enables efficient simulation calculations, and the analysis of simulation results guides structural design and improvement, enhancing structural safety. Furthermore, the elastic finite element method can generate visualized results, intuitively displaying the stress distribution of the structure, helping engineers and designers understand the structural behavior and make corresponding design decisions.

[0102] As a preferred example of this application, in step S11, each stress component (σ) in a rectangular coordinate system is used. x σ y σ z τ xy τ yz τ zx Perform basic formula calculations:

[0103] The first invariant expression for stress:

[0104] I1=σ x +σ y +σ z ;

[0105] Mises equivalent stress expression:

[0106] Stress field eigenvalues:

[0107] This setting discloses a method that uses stress components (σ) in a rectangular coordinate system. x σ y σ z τ xy τ yz τ zxThe method of calculating the eigenvalues ​​of the stress field can comprehensively analyze the stress distribution in metal structures, understand the stress level at various locations in the structure, including principal stresses and principal shear stresses, and help to identify areas of stress concentration and potential weak points. The calculated stress field eigenvalues, such as the first invariant stress I1 and Mises equivalent stress, are important parameters for assessing the safety of metal structures and provide important reference for structural design and analysis.

[0108] This application uses stress components in a Cartesian coordinate system to calculate the characteristic values ​​of the stress field, which helps to comprehensively analyze the stress situation, determine the characteristic values ​​of the stress field, and improve the accuracy of structural design, thereby improving the safety and reliability of metal structures.

[0109] As a preferred example of this application, in step S11, the basic formula is calculated using the principal stress components (σ1, σ2, σ3):

[0110] The first invariant expression for stress:

[0111] I1 = σ1 + σ2 + σ3;

[0112] Mises equivalent stress expression:

[0113] Stress field eigenvalues:

[0114] This method discloses a way to calculate the characteristic values ​​of the stress field using principal stress components (σ1, σ2, σ3), which simplifies the calculation process. Since the principal stresses are characteristic values ​​of the stress tensor, directly using the principal stress components avoids involving other minor stress components during the calculation, thus simplifying the calculation steps. In addition, the principal stresses are key factors affecting material deformation and failure. Therefore, when analyzing the stress situation in metal structures, using the principal stress components for calculation can directly reflect the main stress state in the metal structure. By further calculating the characteristic values ​​of the stress field, such as the first invariant stress I1 and the Mises equivalent stress, the stability and safety of the metal structure can be comprehensively evaluated. This provides an accurate basis for the design and analysis of the structure, guiding structural designers to optimize or improve the design of metal structures to enhance their resistance to damage.

[0115] This application simplifies the calculation process by using principal stress components (σ1, σ2, σ3) to calculate the characteristic values ​​of the stress field, focuses on the principal stresses, accurately assesses structural safety, and guides structural design and improvement, thereby enhancing the safety and reliability of metal structures.

[0116] As a preferred example of this application, in step S11, each strain component (ε) in a rectangular coordinate system is used. x εy ε z γ xy γ yz γ zx Perform basic formula calculations:

[0117] The first invariant expression for stress:

[0118] E is the Young's modulus (or elastic modulus) of the material, and v is the Poisson's ratio of the material.

[0119] Mises equivalent stress expression:

[0120] Stress field eigenvalues:

[0121] This invention discloses a method for calculating the characteristic values ​​of the stress field using strain components in various Cartesian coordinate systems. These strain components reflect the deformation of the metal structure in different directions, providing a comprehensive understanding of the deformation. By using strain components in various Cartesian coordinate systems, these characteristic values ​​can be accurately calculated, improving the accuracy of the assessment of the safety of the metal structure. Furthermore, this method is applicable to the analysis of various metal structures and different loading conditions. Whether subjected to uniaxial tension, biaxial tension, or torsion, this method can be used to evaluate the characteristic values ​​of the stress field of the metal structure, thereby guiding structural designers to optimize or improve the design of the metal structure to enhance its safety.

[0122] This application uses strain components in a Cartesian coordinate system to calculate the characteristic values ​​of the stress field, which can comprehensively assess the strain situation, accurately calculate the characteristic values ​​of the stress field, has wide applicability, and guide structural design and improvement, thereby improving the safety of metal structures.

[0123] As a preferred example of this application, in step S11, the principal strain components are used to perform the basic formula calculation.

[0124] The first invariant expression for stress;

[0125] Mises equivalent stress expression:

[0126] Stress field eigenvalues:

[0127] This invention discloses a method for calculating the eigenvalues ​​of the stress field using principal strain components, which simplifies the calculation process and accurately calculates the eigenvalues ​​of the stress field. The principal strain components are usually obtained by decomposing the eigenvalues ​​of the strain tensor. This decomposition method can effectively improve the calculation efficiency, especially for the calculation of large or complex structures. It is applicable to various loading conditions and metal structure types and has wide applicability.

[0128] In the examples of this application, this application also discloses a stress field characteristic value calculation under special conditions, which mainly refers to center-penetrating notches and cracks under plane stress or plane strain conditions.

[0129] As a preferred example of this application, in step S11, for a center-through cut, the stress field characteristic value ω1 at the root of the cut under plane stress is equal to 1; the stress field characteristic value ω1 at the root of the cut under plane strain is calculated according to the following formula:

[0130] In the formula, v is the Poisson's ratio of the material.

[0131] As a preferred example of this application, in step S11, for a centrally penetrating crack, the stress field characteristic value ω1 at the crack tip under plane stress is equal to 1.868; the stress field characteristic value ω1 at the root of the notch under plane strain is calculated according to the following formula:

[0132] In the formula, v is the Poisson's ratio of the material.

[0133] In special cases, namely, center-penetrating notches and cracks under plane stress or plane strain conditions, corresponding methods for calculating the characteristic values ​​of the stress field are presented. These methods are proposed for specific situations, possessing certain simplifications and particularities, and can more accurately describe the stress field characteristics of notches and cracks. For center-penetrating notches, methods for calculating the characteristic values ​​of the stress field at the root of the notch under plane stress and plane strain conditions are given. Under plane stress conditions, the characteristic value of the stress field at the root of the notch is fixed at 1, while under plane strain conditions, the calculation method for the characteristic value of the stress field at the root of the notch is based on the material's Poisson's ratio. This helps to better understand and evaluate the stress distribution of the notch under different conditions. For center-penetrating cracks, methods for calculating the characteristic values ​​of the stress field at the crack tip are also given under plane stress and plane strain conditions. Under plane stress conditions, the characteristic value of the stress field at the crack tip is fixed at 1.868, while under plane strain conditions, the calculation method for the characteristic value of the stress field at the crack tip is also based on the material's Poisson's ratio. The methods for calculating the characteristic values ​​of the stress field under these special conditions provide important references and guidance for engineering practice, and help to better analyze and evaluate the mechanical behavior of notches and cracks.

[0134] This application simplifies the calculation process and accurately calculates the characteristic values ​​of the stress field by using principal strain components, thereby improving calculation efficiency. It has wide applicability and can guide structural design and improvement, thus enhancing the safety of metal structures.

[0135] In the examples of this application, the metal structure fracture prevention design method includes a metal structure fully plastic fracture design method, characterized by comprising the following steps:

[0136] S21: Considering the proposed structural form and load conditions, and taking into account potential defects, the stress field characteristic value ω2 in the structure is calculated using elastoplastic mechanics. The characteristic value ω2 is... In the time structure The maximum value, σ s The yield strength of the material. This refers to the nominal equivalent stress of the main load-bearing unit or component during the load-bearing process of a metal structure. This represents the maximum value of the first invariant stress during the load-bearing process of the metal structure;

[0137] S22: Material selection and design are based on the characteristic value ω2 of the stress field present in the metal structure. The principle is that the material's fracture strength I is at the lowest service temperature of the metal structure design. b With yield strength σ s The ratio is greater than the maximum stress field characteristic value ω2 at the weakest point that may exist in the metal structure, as shown in the following formula:

[0138] The fracture strength I of the material b With yield strength σ s Determined by uniaxial tensile testing of round bar specimens.

[0139] Based on this research, the applicant combined elastoplastic mechanics to calculate the stress field characteristic value ω2 in the structure. Then, based on this calculated stress field characteristic value ω2, the applicant selected materials to ensure they met the required fracture strength I. b With yield strength σ s If the ratio is greater than the maximum stress field characteristic value ω2 at the weakest point in the metal structure, the structure can achieve full plastic fracture when it fractures, which improves the fracture resistance and safety of the structure, while optimizing material selection and reducing design costs.

[0140] The metal structure fully plastic fracture design method described in this invention combines the form of the metal structure, the load-bearing conditions, and possible defects. It uses elastoplastic mechanics to calculate the stress field characteristic values ​​in the structure for material selection and design, thereby realizing fully plastic fracture of the structure at the time of fracture, improving the safety and reliability of the structure, and reducing the risk of failure and cost.

[0141] In the example of this application, in step S21, the stress field characteristic value ω2 is calculated using the elastoplastic finite element method, and the material parameters are determined using the uniaxial tensile test method for round bar specimens, including the following steps:

[0142] S211: Establish a geometric model of the metal structure, which includes weak points;

[0143] S212: Apply loads according to the actual working conditions or designed force load conditions of the metal structure, including elastic modulus E, Poisson's ratio v, and yield strength σ. s Equivalent plastic strain during the plastic deformation stage The correspondence between the rheological stress σ0 and the rheological stress σ0;

[0144] S213: Calculate the stress field characteristic value ω2 at the weak location.

[0145] This setup employs the elastoplastic finite element method combined with measured material parameters to calculate the stress field eigenvalue ω², enabling more accurate simulation of the mechanical behavior of metal structures under actual working conditions. This includes stress distribution, stress concentration, and potential fracture risks, improving the reliability and accuracy of the analysis results. It provides a reliable analytical basis for material selection and design, reducing the likelihood of structural failure. The finite element method flexibly handles various structural forms and complex situations, allowing for simulation analysis of different types of metal structures. Adjustments and improvements can be made as needed, enabling efficient simulation calculations. Analysis of simulation results guides structural design and improvement, enhancing structural safety. Furthermore, the elastic finite element method can generate visualized results, intuitively displaying the stress distribution of the structure, helping engineers and designers understand the structural behavior and make appropriate design decisions.

[0146] In step S111 or step S211, the weak point in the metal structure is a stress concentration point caused by geometric discontinuities in the structural shape. The weak point in the metal structure is a stress concentration area caused by a notched body, a cracked body, or other forms of defect.

[0147] In the example of this application, in step S212, the elastic modulus E, Poisson's ratio v, and yield strength σ of the metal structure are... s The calculation of parameters is based on the corresponding standard tests, while the equivalent plastic strain during the plastic deformation stage is... The correspondence between the rheological stress σ0 and the rheological stress σ0 was determined by uniaxial tensile testing of round bar specimens.

[0148] The elastic modulus E, Poisson's ratio v, and yield strength σ were obtained using standard tests. s The parameters, as well as the method of determining the equivalent plastic strain and flow stress during the plastic deformation stage using uniaxial tensile testing of round bar specimens, are all verified and widely used methods, demonstrating high feasibility and practicality. Therefore, this setup can effectively support the design and analysis of metal structures.

[0149] As a preferred example of this application, in step S213, the equivalent plastic strain of the plastic deformation stage The correspondence between the rheological stress σ0 and the stress σ0 was determined by the uniaxial tensile test method for round bar specimens, where:

[0150] For materials that do not exhibit necking during uniaxial tensile testing of round bar specimens. A0 is the initial cross-sectional area of ​​the plastic deformation, A is the cross-sectional area of ​​the specimen at the measurement time, and F is the force in the tensile direction of the specimen at the measurement time.

[0151] For materials exhibiting necking during uniaxial tensile testing of round bar specimens, the initial diameter R of the round bar specimen as recorded in the test data should be considered. c The diameter of the specimen at the moment the necking deformation begins Maximum axial loading force F max The characteristic parameter r of the necked-down specimen shape at any time c r n r ip z ip , and the axial force F of the tension z The maximum equivalent plastic strain in the necked-down specimen is calculated using the following formula and denoted as: and the corresponding rheological stress Equivalent plastic strain during the plastic deformation stage of a material The correspondence with the rheological stress σ0 is equal to and The correspondence.

[0152] Among them, the tensile axial force F during the test z The minimum cross-sectional radius r of the necked-down bottom of the specimen perpendicular to the central axis c The maximum limit value r of the cross-sectional radius perpendicular to the central axis on the specimen. n The slope of the tangent at the inflection point of the generatrix of the necked-down deformation profile. The radius r of the section perpendicular to the central axis at the inflection point of the generatrix of the necked-down deformed outline.ip The distance z between the cross section perpendicular to the central axis at the inflection point of the generatrix of the necked-down deformed outline and the minimum cross section at the bottom of the necking-down deformed shape. ip .

[0153] As a preferred example of this application, the calculation is performed in step S12 or step S22. When designing materials for fracture fracture, the yield strength σ s The value R is determined using existing uniaxial tensile testing standards for metallic materials. p0.2 Fracture strength I b I1 is the maximum value of the first invariant stress inside the specimen at the moment of fracture during the uniaxial tensile test of the round bar specimen.

[0154] For materials without necking deformation during a uniaxial tensile test of a round bar specimen, the tensile axial force F at the moment of fracture is considered. z Given the cross-sectional area A of the specimen, the fracture strength I is approximately calculated using the following formula. b ,

[0155] For materials exhibiting necking during uniaxial tensile testing of round bar specimens, the necking specimen shape characteristic parameter r at the moment of fracture, as recorded in the test, is used. c r n r ip z ip , and the axial force F of the tension z Calculate and determine the maximum value of the first invariant stress at this point. This value is the material's fracture strength I. b The maximum value of the first invariant stress inside the specimen during necking deformation is denoted as... Calculate using the following formula:

[0156] The setup ensures that accurate and reliable experimental data are used for the strength parameters of the material when performing full plastic fracture analysis of metal structures, which helps to improve the accuracy and credibility of the analysis.

[0157] When analyzing the influence of stress-strain concentration, the applicant compared and analyzed the stress field evolution characteristics under two notch positions—central notch and surface notch—and a series of notch half-lengths 'a' (set values ​​ranging from 0.5 mm to 5 mm). Regarding material factors, two typical materials were considered: one was the material used in the aforementioned study (as shown in Figure 2), a ferritic steel exhibiting necking in uniaxial tensile tests of round bar specimens (hereinafter referred to as "round tensile tests"); the other was austenitic steel not exhibiting necking in round tensile tests. The Young's modulus E was set to 180 GPa, Poisson's ratio v to 0.3, and yield strength σ to be... sIt is 500 MPa; considering the Poisson ratio reflected in the aforementioned studies The effect of the characteristic curve, with other parameters remaining unchanged, is that when the Poisson's ratio v is increased by 0.35 for each of the two materials.

[0158] Table 1. Parameter settings for various finite element simulations

[0159] Table 2 Characteristic values ​​of uniaxial tensile stress field evolution under typical stress states.

[0160] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for designing fracture-resistant metal structures, characterized in that, Establish the stress field evolution of metal structures under force loads Characteristic curves and strength ratio of the measured material I b / σ s : Wherein, the eigenvalue ω1 is equal time Equivalent stress concentration factor at the root of the incision The ratio of the equivalent stress at the incision root to the nominal equivalent stress. The eigenvalue ω2 is In the time structure σ s The yield strength of the material. The nominal equivalent stress in the main load-bearing unit or component during the load-bearing process of a metal structure. This represents the maximum value of the first invariant stress during the load-bearing process of a metal structure; according to The calculated values ​​are used for fracture design of metal structures to ensure that they meet the design requirements for preventing elastic fracture and / or elastoplastic fracture and / or all-plastic fracture.

2. The metal structure fracture prevention design method according to claim 1, characterized in that, The metal structure fracture prevention design method includes a metal structure brittle fracture prevention design method, comprising the following steps: S11: Calculate the characteristic value ω1 of the stress field at the weak points in the metal structure. The weak points in the metal structure are the stress concentration points caused by the geometric discontinuity of the structural shape. S12: Material selection is based on the stress field characteristic value ω1 at weak points in the metal structure. The principle is that the material's fracture strength I should be within the minimum service temperature of the metal structure design. b With yield strength σ s The ratio is greater than the characteristic value ω1 of the largest stress field at the weakest point in the metal structure, as shown in the following formula: The fracture strength I of the material b With yield strength σ s Determined by uniaxial tensile testing of round bar specimens.

3. The metal structure fracture prevention design method according to claim 2, characterized in that, In step S11, for general-type metallic structural steel, the method of linear elastic finite element method is used to determine the structure, including the following steps: S111: Establish a geometric model of the metal structure, which includes stress concentration points at weak locations; S112: Apply loads according to the actual working conditions of the metal structure or the designed force load conditions; S113: Simulate the stress or strain components at the weak points of the metal structure, and calculate the characteristic value ω1 of the stress field at the weak points according to the basic calculation formula.

4. The metal structure fracture prevention design method according to claim 2, characterized in that, In step S11, the formula for calculating the eigenvalue ω1 is as follows: one of the.

5. The metal structure fracture prevention design method according to claim 2, characterized in that, In step S11, for a center-through cut, the stress field characteristic value ω1 at the root of the cut under plane strain is calculated according to the following formula: For a central through-crack, the characteristic value ω1 of the stress field at the root of the notch under plane strain is calculated using the following formula: In the formula, v is the Poisson's ratio of the material.

6. The metal structure fracture prevention design method according to any one of claims 1 to 5, characterized in that, The metal structure fracture prevention design method includes a metal structure fully plastic fracture design method, characterized by comprising the following steps: S21: Considering the proposed structural form and load conditions, and taking into account potential defects, the stress field characteristic value ω2 in the structure is calculated using elastoplastic mechanics. The characteristic value ω2 is... In the time structure σ s The yield strength of the material. The nominal equivalent stress in the main load-bearing unit or component during the load-bearing process of a metal structure. This represents the maximum value of the first invariant stress during the load-bearing process of the metal structure; S22: Material selection and design are based on the characteristic value ω2 of the stress field present in the metal structure. The principle is that the material's fracture strength I is at the lowest service temperature of the metal structure design. b With yield strength σ s The ratio is greater than the maximum stress field characteristic value ω2 at the weakest point that may exist in the metal structure, as shown in the following formula: The fracture strength I of the material b With yield strength σ s Determined by uniaxial tensile testing of round bar specimens.

7. The metal structure fracture prevention design method according to claim 6, characterized in that, In step S21, the stress field characteristic value ω2 is calculated using the elastoplastic finite element method, and the material parameters are determined using the uniaxial tensile test method for round bar specimens, including the following steps: S211: Establish a geometric model of the metal structure, which includes weak points; S212: Apply loads according to the actual working conditions or designed force load conditions of the metal structure, including elastic modulus E, Poisson's ratio v, and yield strength σ. s Equivalent plastic strain during the plastic deformation stage The correspondence between the rheological stress σ0 and the rheological stress σ0; S213: Calculate the stress field characteristic value ω2 at the weak location.

8. The metal structure fracture-resistant design method according to claim 7, characterized in that, In step S212, the equivalent plastic strain during the plastic deformation stage The correspondence between the rheological stress σ0 and the stress σ0 was determined by the uniaxial tensile test method for round bar specimens, where: For materials that do not exhibit necking during uniaxial tensile testing of round bar specimens. A0 is the initial cross-sectional area of ​​the plastic deformation, A is the cross-sectional area of ​​the specimen at the time of measurement, and F is the force in the tensile direction of the specimen at the time of measurement. For materials exhibiting necking during uniaxial tensile testing of round bar specimens, the initial diameter R of the round bar specimen as recorded in the test data should be considered. c The diameter of the specimen at the moment the necking deformation begins Maximum axial loading force F max The characteristic parameter r of the necked-down specimen shape at any time c r n r ip z ip , and the axial force F of the tension z The maximum equivalent plastic strain in the necked-down specimen is calculated using the following formula and denoted as: and the corresponding rheological stress Equivalent plastic strain during the plastic deformation stage of a material The correspondence with the rheological stress σ0 is equal to and The correspondence; Among them, the tensile axial force F during the test z The minimum cross-sectional radius r of the necked-down bottom of the specimen perpendicular to the central axis c The maximum limit value r of the cross-sectional radius perpendicular to the central axis on the specimen. n The slope of the tangent at the inflection point of the generatrix of the necked-down deformation profile. The radius r of the section perpendicular to the central axis at the inflection point of the generatrix of the necked-down deformed outline. ip The distance z between the cross section perpendicular to the central axis at the inflection point of the generatrix of the necked-down deformed outline and the minimum cross section at the bottom of the necking-down deformed shape. ip .

9. The metal structure fracture prevention design method according to claim 2, characterized in that, In calculation When designing materials for fracture fracture, the yield strength σ s The value R is determined using existing uniaxial tensile testing standards for metallic materials. p0.2 Fracture strength I b Let I1 be the maximum value of the first invariant stress I1 inside the specimen at the instant of fracture during a uniaxial tensile test of a round bar specimen. For materials without necking deformation during a uniaxial tensile test of a round bar specimen, this is determined based on the tensile axial force F at the instant of fracture. z Given the cross-sectional area A of the specimen, the fracture strength I is approximately calculated using the following formula. b , For materials exhibiting necking during uniaxial tensile testing of round bar specimens, the necking specimen shape characteristic parameter r at the moment of fracture, as recorded in the test, is used. c r n r ip z ip , and the axial force F of the tension z Calculate and determine the maximum value of the first invariant stress at this point. This value is the material's fracture strength I. b The maximum value of the first invariant stress inside the specimen during necking deformation is denoted as... Calculate using the following formula:

Images