CN113673030B - Simulation analysis method for ductile fracture coupling failure of metal material - Google Patents
Simulation analysis method for ductile fracture coupling failure of metal material Download PDFInfo
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Abstract
The invention relates to a simulation analysis method for ductile fracture coupling failure of a metal material, which comprises the following steps: performing tensile tests on samples in different stress states; determining a true stress-strain curve of the simulation input: determining a fracture criterion undetermined parameter under the large cell size; determining undetermined parameters of an initial degradation model; determining a stress degradation parameter; and carrying out simulation of fracture failure of the metal material. The method can effectively improve the prediction precision of ductile fracture failure of the metal material, particularly the calculation precision of the high-strength metal material, improve the calculation efficiency, has simple acquisition of the damage parameters of the material, can effectively solve the difficult problem of engineering application of the failure model, and has wide application range.
Description
Technical Field
The patent application belongs to the field of mechanical behavior simulation prediction, and particularly relates to a simulation analysis method for ductile fracture coupling failure of a metal material.
Background
Fracture is a main form for causing the failure of engineering components, is a closely focused problem in production and life, such as the fracture failure of materials in the collision process of automobiles, and has great significance for personal safety and property loss in fracture analysis and simulation prediction of the materials.
The finite element simulation analysis method is widely applied to the fields of material forming, collision and the like. Accurate establishment of the simulation model and accurate acquisition of material parameters are very important for simulation prediction accuracy. With the improvement of finite element simulation technology and the development of computer technology, the need for improving the finite element simulation precision has become stronger. Ductile fracture is a fracture mode of crack propagation formed by the combination of nucleation, growth, and aggregation of pores in a material. The influence of mechanical properties brought by the crack propagation process is described by the damage variable, and the influence of material property degradation on structural strength and service life is estimated better. In industrial applications, particularly in finite element simulations, there is also a need to consider the problems of computational speed, computational accuracy, and to solve such problems to better serve the simulation, neukam et al, article "On closing the constitutive gap between forming and crash simulation" [ Neukam F, feucht M, roll K, et al, on closing the constitutive gap between forming and crash simulation [ C ]].Proceedings of InternationalUsers Conference.2008.]Article "Considering damage history in crashworthiness simulations" [ Neukam F, feucht M, haufe A.Considering damage history in crashworthiness simulations [ C ]].Proceedings of European Ls-dyna Conference.2009.]And discloses a generalized incremental stress state related damage model-a GISSMO model. The GISSMO model comprises an equivalent fracture strain curve and an equivalent critical strain curve of the material, wherein damage accumulation is used as a standard for judging material failure, the damage accumulation process is regarded as a nonlinear accumulation process, and meanwhile, the phenomenon of weakening of flow stress caused by damage is considered, so that the GISSMO model is widely applied to the fields of automobile collision, material failure and the like.
For the ductile material, after the loading ultimate strength is reached, the stress of the material is not brittle failure to 0, and when the damage variable D defined by the GISSMO model is increased to 1, the stress of the material is gradually weakened to 0, and the phenomenon of brittle failure of the material in the simulation process is avoided, but the stress weakening characteristics of the GISSMO model are not met for materials with high strength, such as advanced high-strength steel and the like. Although the parameters of the Gissmo failure model, namely the stress reduction index m and the damage accumulation index n, can be checked by a simulation back-pushing method, the stress of the material is estimated to be too high or too low at the initial stage of the damage of the material. The GISSMO model considers that a material fails when the stress continuously decays to 0, but for some metallic materials, the stress is not close to 0 at the critical moment before the material fails, but after the stress decays to the critical value, the material unit fails and the stress suddenly changes to 0. Therefore, the stress degradation behavior of certain high-strength materials cannot be accurately described using the GISSMO model.
In finite element simulation analysis, the accuracy of the calculation results of simulation by adopting different unit sizes is large, and the unit sizes required to be adopted for ensuring the simulation accuracy are small enough, so that the engineering application of the model is limited. At present, the parameter calibration of failure models such as GISSMO adopts extremely small unit size, and the influence of the grid size effect on fracture parameters is considered in a large unit size model, but the influence of the grid size effect on the calculation accuracy of the stress degradation process is not pointed out.
Disclosure of Invention
The invention aims to solve the technical problem of providing a simulation analysis method for ductile fracture coupling failure of a metal material, which overcomes the defects of the existing simulation method for fracture failure, considers the stress degradation characteristic and the grid size effect of the high-strength metal material according to experimental data of the metal material in different stress states, is suitable for engineering application, and improves the ductile fracture simulation precision of the metal high-strength material.
In order to solve the problems, the invention adopts the following technical scheme:
a simulation analysis method for ductile fracture coupling failure of a metal material specifically comprises the following steps:
(1) Tensile testing of samples in different stress states is carried out: designing and preparing a plurality of groups of samples, wherein the first group is a smooth tensile sample, and the second group is a notch or bulging sample; the third group is shear test specimens, each specimen was subjected to tensile testing under quasi-static conditions, and displacement-force curve data were recorded.
(2) Determining a true stress-strain curve of the simulation input: establishing a finite element model according to the actual size of the first group of smooth tensile samples in the step (1), and carrying out grid division on the finite element model of the first group of samples according to small unit sizes, wherein the small unit sizes are less than or equal to 0.2mm; and taking the experimental displacement-force curve of the first group of samples as an optimization target, and optimizing the plastic strain-true stress curve of the material input of the finite element model of the first group of samples until the simulated displacement-force curve is completely matched with the experimental curve, so as to obtain the true input constitutive curve of the material.
(3) Determining a fracture criterion undetermined parameter under a large cell size: establishing a finite element model according to the actual sizes of the three groups of samples in different stress states in the step (1), and dividing grids according to the large unit size, wherein the large unit size is more than or equal to 1mm; performing constitutive inverse pushing optimization on the finite element model under the large unit size according to the constitutive inverse pushing method in the step (2) until the area difference between the experimental displacement force curve and the simulated curve is minimum, and calculating the critical fracture strain epsilon of the maximum strain unit of each sample f Mean stress triaxial η and mean node angle parameterOr average code parameter L (average code angle parameter +)>The average code parameter L may be used instead of +.> ) And establishing a three-dimensional fracture curved surface model of the metal material representing different stress states, and obtaining undetermined parameters of the fracture curved surface model according to a curved surface fitting method or a method for solving an equation set.
(4) Determining undetermined parameters of an initial degradation model: the material in the step 2 is truly input into the constitutive curve and is compared and analyzed with the coarse grid optimized constitutive curve of each sample in the step 3 to determineStarting degradation time of three groups of samples; obtaining critical degradation strain epsilon corresponding to maximum strain unit at initial degradation moment of each group of samples c Triaxial degree of average stress eta c And average code angle parameterOr average code parameter L c (average code Angle parameter>Can use average code parameter L c Instead of this, the first and second heat exchangers,) And establishing a three-dimensional curved surface model representing the initial stress degradation of the metal material in different stress states, and obtaining undetermined parameters of the initial degradation model according to a curved surface fitting method or a method for solving an equation set.
(5) Determining a stress degradation parameter: defining a damage variable D, a D value (Dc) corresponding to a stress degradation starting point and stress degradation parameters m and w of the material, and defining a stress degradation form: sigma' =σ (1-w (D-D c ) m ) Wherein σ' is the stress after degradation, σ is the undegraded stress, and when d=1, the material fails; and (3) establishing a relation between a true stress difference delta sigma of the material actually input into the constitutive curve in the step (2) and the sample coarse grid optimized constitutive curve in the step (3) and (D-Dc), wherein the relation is as follows: Δσ=w (D-D c ) m Obtaining stress degradation parameters m and w;
(6) And (3) performing simulation of fracture and failure of the metal material: embedding the fracture failure model expression and the model parameter programming user subprogram determined in the steps (1) - (5) into ABAQUS finite element software, carrying out finite element modeling analysis by adopting a large unit size, substituting the real input constitutive curve in the step (2) and the fracture criterion parameter in the step (3), the initial degradation model parameter in the step (4) and the stress degradation parameter in the step (5) into a material user subprogram VUMAT or a VUHARD and VUSDFLD combined subprogram, and carrying out accurate simulation of fracture analysis on the finite element model under the large unit size.
In another alternative implementation, the three-dimensional fracture surface model expression of the metal material is:c in the formula 1 、C 2 、C 3 The form of the undetermined parameter of the ductile fracture criterion can also be Lou-Huh, MMC, MU or other ductile fracture criterion expression forms. />Relationship with L: and performing corresponding replacement.
In another alternative implementation, the stress degradation parameters m and w described in step 5 may be determined using a parameter optimization tool such as Isight or other parameter optimization methods.
The initial degradation strain points of the three groups of samples are determined by truly inputting a constitutive curve and optimizing the constitutive curve through a coarse grid, and critical degradation strain epsilon corresponding to a maximum strain unit at the initial degradation moment of each group of samples is obtained c Triaxial degree of average stress eta c And average code angle parameterOr average code parameter L c And establishing a metal material initial stress degradation model representing different stress states, wherein the expression is as follows: />C in the formula 1 ′、C 2 ′、C 3 ' is a pending parameter of a material, the pending parameter of an initial degradation model is obtained according to a curved surface fitting method or a method for solving an equation set, and a function expression form can also be expressed by Lou-Huh, MMC or other toughness fracture criteria.
The invention is characterized in thatThe stress degradation mode is as follows: sigma' =σ (1-w (D-D c ) m ) D is a damage variable, and the expression isDc is the equivalent plastic strain ε p Reaching the initial degradation point strain ε c The corresponding D values, m and w are stress degradation parameters; when D is less than Dc, the strength is not degraded, when D is more than or equal to Dc, the strength starts to degrade, wherein sigma' is the stress after degradation, sigma is the undegraded stress, and when D=1, the material is invalid; according to the relation between the true stress difference delta sigma between the true input constitutive curve of the material in the step 2 and the coarse grid optimized constitutive curve of each sample in the step 3 and (D-Dc), the relation is as follows: Δσ=w (D-D c ) m Stress degradation parameters m and w can be obtained.
Due to the adoption of the technical scheme, the beneficial effects obtained by the invention are as follows: the invention can effectively improve the prediction precision of ductile fracture failure of the metal material, in particular to the calculation precision of the high-strength metal material, such as the DP780 material of the high-strength automobile plate in the embodiment, and compared with a GISSMO model and a non-coupling failure model, the improved damage failure model has better prediction precision on each sample. The calculation efficiency is improved, the experiment cost is reduced, the acquisition of the material damage parameters is simple, the problem that the engineering application of the failure model is difficult is solved, the application range is wide, and the method has extremely high application value.
Drawings
FIG. 1 is a graph of the true input of material into a mechanism in accordance with an embodiment of the present invention;
FIG. 2 is a graph of fracture in the plane of material stress for an embodiment of the present invention;
FIG. 3 is a graph of a stress degradation model in a material plane stress state according to an embodiment of the present invention;
FIG. 4 is a graph of a parametric fit for material stress degradation in accordance with an embodiment of the present invention;
FIG. 5 is a graph of fracture simulation results for a smooth tensile specimen of a material according to an embodiment of the present invention;
FIG. 6 is a graph showing the results of fracture simulation of a notched R8 specimen of the material according to an embodiment of the present invention;
FIG. 7 is a graph showing the results of fracture simulation of a notch sample of material R3 in accordance with an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to examples.
The invention discloses a simulation analysis method for ductile fracture coupling failure of a metal material, which comprises the following steps:
s1: tensile testing of samples in different stress states is carried out: designing and preparing a plurality of groups of samples, wherein the first group is a smooth tensile sample, and the second group is a notch or bulging sample; the third group is shear test samples, each group of test samples is subjected to tensile test under quasi-static conditions, and displacement-force curve data are recorded respectively.
S2: determining a true stress-strain curve of the simulation input: establishing a finite element model according to the actual size of the first group of smooth tensile samples in the step S1, and carrying out grid division on the finite element model of the first group of samples according to the small unit size; the method is characterized in that a inversion pushing method of the mechanism is utilized, namely, a displacement-force curve of a first group of samples is used as an experimental displacement-force curve and an optimization target, a plastic strain-true stress curve of material input of a finite element model of the first group of samples is optimized, the optimized plastic strain-true stress curve is used as a simulated displacement-force curve until the simulated displacement-force curve is completely matched with the experimental displacement-force curve, and a true input constitutive curve of the material is obtained. Preferably, the small cell size is less than or equal to 0.2mm.
S3: determining a fracture criterion undetermined parameter under a large cell size: establishing a finite element model according to the actual sizes of three groups of samples in different stress states in the step S1, and dividing grids according to the large unit size; performing constitutive inverse pushing optimization on three groups of finite element models under large unit size according to the constitutive inverse pushing method of the step S2 until the area difference between the experimental displacement-force curve and the simulated displacement-force curve is minimum, so as to form a coarse grid optimized constitutive curve, and calculating critical fracture strain epsilon of the maximum strain unit of each sample under the coarse grid optimized constitutive curve f Mean stress triaxial η, mean node angle parameter(average code Angle parameter>The average code parameter L may be used instead,) Thus, a three-dimensional fracture curved surface model of the metal material representing different stress states is obtained, and the undetermined parameters of the toughness fracture criterion are obtained according to a curved surface fitting method or a method for solving an equation set.
S4: determining undetermined parameters of an initial degradation model: performing comparative analysis between the true input constitutive curve of the material in the step S2 and the coarse grid optimized constitutive curve of each sample in the step S3, and determining the initial degradation moment of each group of samples; obtaining critical degradation strain epsilon corresponding to maximum strain unit at initial degradation moment of each group of samples c Triaxial degree of average stress eta c Average code angle parameter(average code Angle parameter>Can use average code parameter L c Instead of this, the first and second heat exchangers,) Thus, a three-dimensional curved surface model representing the initial stress degradation of the metal material in different stress states is established, and undetermined parameters of the initial degradation model are obtained according to a curved surface fitting method or a method for solving an equation set.
S5: determining a stress degradation parameter: defining a damage variable D, a D value (Dc) corresponding to a stress degradation starting point and stress degradation parameters m and w of the material; the damage variable D is expressed asEpsilon in p For equivalent plastic strain, n is nonlinear damage accumulationIndex, n=1 when linearly degenerated; dc is the equivalent plastic strain ε p Critical degradation strain epsilon at the point of onset degradation c The corresponding D value.
Establishing a relation between a true stress difference delta sigma and (D-Dc) between a true input constitutive curve of the material in the step S2 and a coarse grid optimized constitutive curve of each sample in the step S3, wherein the relation is as follows: Δσ=w (D-D c ) m And obtaining stress degradation parameters m and w.
S6: and (3) performing simulation of fracture and failure of the metal material: the fracture failure model expression and model parameters determined by S1-S5 are written and a user subroutine is embedded into ABAQUS finite element software, finite element modeling analysis is carried out by adopting a large unit size, the real input constitutive curve of the step S2 and the fracture criterion parameters of the step S3, the initial degradation model parameters of the step S4 and the stress degradation parameters of the step S5 are substituted into the material user subroutine (VUMAT or a VUHARD and VUSDFLD combined subroutine), and the accurate simulation of the fracture analysis can be carried out on the finite element model under the large unit size. Preferably, the large unit size is not less than 1mm.
Specifically, in S3, the ductile fracture criteria is the critical fracture strain ε f And the function of the triaxial degree eta of the average stress and the average node parameter L is as follows:c in the formula 1 、C 2 、C 3 And obtaining undetermined parameters of the initial degradation model according to a curved surface fitting method or a method for solving an equation set as undetermined parameters of the ductile fracture criterion. The form may also be Lou-Huh, MMC, MU or other ductile fracture criteria expression form. />Relationship with L: and performing corresponding replacement.
Specifically, in S4, the initial degradation strain points of the three groups of samples are determined by actually inputting a constitutive curve and optimizing the constitutive curve by a coarse grid, the initial degradation points of the first group of samples and the third group of samples are stress reduction points, and the initial degradation points of the second group of samples are tensile maximum force points; the initial degradation model of the metal material is critical degradation strain epsilon c The corresponding average stress triaxial degree eta c And average code parameter L c The expression of which is as follows:c in the formula 1 ′、C 2 ′、C 3 ' is a pending parameter of a material, the pending parameter of an initial degradation model is obtained according to a curved surface fitting method or a method for solving an equation set, and a function expression form can also be expressed by Lou-Huh, MMC or other toughness fracture criteria. This functional expression relates only to the average node parameter L c There is no reference to the average code angle parameter +.>If involve->And L is equal to c Is the relation of: and performing corresponding replacement.
Specifically, in S5, when D is smaller than Dc, the strength is not degraded, and when D is larger than or equal to Dc, the strength starts to degrade, and the degradation expression is: sigma' =σ (1-w (D-D c ) m ) Where σ' is the stress after degradation, σ is the undegraded stress, and when d=1, the material fails.
The following is a detailed description of an embodiment with reference to fig. 1-7, and the analysis method includes the following steps:
(1) Designing tensile samples of the high-strength automobile plate DP780 material in different stress states, performing a quasi-static tensile test, and recording displacement-force curve data; the first group is a smooth tensile sample, the second group is a notch sample, and the notch radius is R3, R8 and R15; the third group is shear samples, with a shear angle of 0 °.
(2) Determining a true stress-strain curve of the simulation input: establishing a finite element model according to the actual size of each stress state sample in the step 1, and carrying out grid division on the finite element model of the first group of samples according to the unit size of 0.1 mm; and taking the experimental displacement-force curve of the first group of samples as an optimization target, and optimizing the plastic strain-true stress curve of the material input of the finite element model of the first group of samples until the simulated displacement-force curve is well matched with the experimental displacement-force curve, so as to obtain the true input constitutive curve of the material, as shown in figure 1. In fig. 1, the black node is an input value of the plastic strain and the corresponding true stress determined by adopting the constitutive inverse push optimization method, and the black solid line is a curve for fitting the determined input value by adopting the double Voce model.
The relevant parameters in fig. 1 are shown in table 1.
TABLE 1
(3) Determining a fracture criterion undetermined parameter under a large cell size: performing grid division on the finite element model of the third group of samples according to the unit size of 1mm; performing constitutive inverse pushing optimization on the model according to the constitutive inverse pushing method in the step 2 until the area difference between the experimental displacement-force curve and the simulated displacement-force curve is minimum, obtaining a constitutive curve under a coarse grid, and calculating critical fracture strain epsilon f, average stress triaxial degree eta and average code angle parameters of the maximum strain unit of each sampleOr average node parameter L, and establishing a three-dimensional fracture curved surface model of the metal material representing different stress states, wherein the expression is as followsAnd obtaining undetermined material parameters C1, C2 and C3 of the fracture surface model according to a surface fitting method or a method for solving an equation set, as shown in figure 2.
In fig. 2, black nodes are stress axises and fracture strain values corresponding to critical fracture points of three groups of samples calibrated by adopting the constitutive inverse optimization method, and black solid lines are curves for fitting the critical fracture values of the three groups of samples by adopting a fracture criterion formula.
(4) Determining undetermined parameters of an initial degradation model: the material in the step 2 is truly input into a constitutive curve, and comparison analysis is carried out between the constitutive curve of each sample coarse mesh in the step 3 to determine the initial degradation time of three groups of samples, wherein the initial degradation points of the first group of samples and the third group of samples are stress reduction points, and the initial degradation point of the second group of samples is a tensile maximum force point; obtaining critical degradation strain epsilon c, average stress triaxial degree eta c and average node parameter Lc corresponding to the maximum strain unit at the initial degradation moment of each group of samples, establishing a three-dimensional curved surface model representing the initial stress degradation of the metal materials in different stress states,the undetermined parameters C1', C2', C3' of the initial degradation model are obtained according to a surface fitting method or a method for solving an equation set, as shown in FIG. 3.
In fig. 3, black nodes are stress axises and fracture strain values corresponding to critical degeneration points of three groups of samples calibrated by adopting the constitutive inverse optimization method, and black solid lines are curves for fitting the critical fracture values of the three groups of samples by adopting a fracture criterion formula.
(5) Determining a stress degradation parameter: defining a damage variable D, a D value Dc corresponding to a stress degradation starting point and stress degradation parameters m and w of the material, and defining a stress degradation form: sigma' =σ (1-w (D-D c ) m ) Wherein σ' is the stress after degradation, σ is the undegraded stress, and when d=1, the material fails; and (3) establishing a relation between a true stress difference delta sigma and (D-Dc) between the material actually input into the constitutive curve in the step (2) and the sample coarse grid optimized constitutive curve in the step (3), wherein the relation is as follows: Δσ=w(D-D c ) m Obtaining stress degradation parameters m and w; as shown in fig. 4, the relevant parameters in fig. 4 are shown in table 2 (x=d-D in the table c )。
In fig. 4, black nodes are true stress difference points between the input constitutive curve and the coarse grid optimized constitutive curve in the calculation result, and black solid lines are fitting curves of the difference points by adopting a stress degradation formula.
TABLE 2
(6) And (3) performing simulation of fracture and failure of the metal material: the model is used for compiling a user subprogram VUHARD and VUSDFLD combined subprogram by using the FORTRAN language, embedded into ABAQUS finite element software, finite element modeling analysis is carried out by adopting a 1mm unit size, the real input constitutive curve in the step 2, the fracture criterion parameter in the step 3, the initial degradation model parameter in the step 4 and the stress degradation parameter in the step 5 are substituted into the subprogram, and the accurate simulation of fracture analysis can be carried out on the finite element model established under the 1mm grid size, and the simulation result is shown in figures 5-7.
In fig. 5, a smooth sample is adopted, the experimental curve is represented by a straight line, the three curves below are a GISSMO model, an invention model and a non-coupling model in sequence from left to right, and the invention model is positioned above the non-coupling model at the lower right.
In fig. 6, using an R8 sample, the experimental curve is represented by a straight line, and the three curves below are, in order from left to right, a GISSMO model, an invention model, and a non-coupling model, and the invention model extends to the non-coupling model at the lower right.
In fig. 7, an R3 sample is adopted, an experimental curve is represented by a straight line, three curves below are a GISSMO model, an invention model and a non-coupling model in sequence from left to right, and the invention model extends to the non-coupling model after oscillation at the lower right, so that the non-coupling model oscillates more severely.
As can be seen from fig. 5 to 7, the calculation result of the stress and strain values of the failure point (d=1) of the GISSMO model unit is lower, and the calculation result of the stress and strain values of the failure point of the uncoupled failure model unit is higher, compared with the improved damage failure model, the prediction accuracy of each sample is better.
Claims (5)
1. A simulation analysis method for ductile fracture coupling failure of a metal material is characterized by comprising the following steps:
s1: tensile testing of samples in different stress states is carried out: designing and preparing a plurality of groups of samples, wherein the first group is a smooth tensile sample, and the second group is a notch or bulging sample; the third group is a shear sample, each group of samples is subjected to tensile test under quasi-static conditions, and displacement-force curve data are recorded respectively;
s2: determining a true stress-strain curve of the simulation input: establishing a finite element model according to the actual size of the first group of smooth tensile samples in the step S1, and carrying out grid division on the finite element model of the first group of samples according to the small unit size; the method comprises the steps of utilizing a constitutive reverse pushing method, namely taking a displacement-force curve of a first group of samples as an experimental displacement-force curve and an optimization target, optimizing a plastic strain-true stress curve of material input of a finite element model of the first group of samples, taking the optimized plastic strain-true stress curve as a simulated displacement-force curve until the simulated displacement-force curve is completely matched with the experimental displacement-force curve, and obtaining a true input constitutive curve of the material;
s3: determining a fracture criterion undetermined parameter under a large cell size: establishing a finite element model according to the actual sizes of three groups of samples in different stress states in the step S1, and dividing grids according to the large unit size; performing constitutive inverse pushing optimization on three groups of finite element models under large unit size according to the constitutive inverse pushing method of the step S2 until the area difference between the experimental displacement-force curve and the simulated displacement-force curve is minimum, so as to form a coarse grid optimized constitutive curve, and calculating critical fracture strain epsilon of the maximum strain unit of each sample under the coarse grid optimized constitutive curve f Mean stress triaxial η, mean node angle parameterObtaining a three-dimensional fracture curved surface model of the metal material representing different stress states, and obtaining undetermined parameters of the ductile fracture criterion according to a curved surface fitting method or a method for solving an equation set;
s4: determining undetermined parameters of an initial degradation model: performing comparative analysis between the true input constitutive curve of the material in the step S2 and the coarse grid optimized constitutive curve of each sample in the step S3, and determining the initial degradation moment of each group of samples; obtaining critical degradation strain epsilon corresponding to maximum strain unit at initial degradation moment of each group of samples c Triaxial degree of average stress eta c Average code angle parameterThus, a three-dimensional curved surface model representing the initial stress degradation of the metal material in different stress states is established, and undetermined parameters of the initial degradation model are obtained according to a curved surface fitting method or a method for solving an equation set;
s5: determining a stress degradation parameter: defining a damage variable D, a D value (Dc) corresponding to a stress degradation starting point and stress degradation parameters m and w of the material; the damage variable D is expressed asEpsilon in p N is the nonlinear damage accumulation index, n=1 when linearly degenerated; dc is the equivalent plastic strain ε p Critical degradation strain epsilon at the point of onset degradation c The corresponding D value; wherein L is the average code parameter, +.>
Establishing a relation between a true stress difference delta sigma and (D-Dc) between a true input constitutive curve of the material in the step S2 and a coarse grid optimized constitutive curve of each sample in the step S3, wherein the relation is as follows: Δσ=w (D-D c ) m Obtaining stress degradation parameters m and w;
s6: and (3) performing simulation of fracture and failure of the metal material: the fracture failure model expression and model parameters determined by the steps S1-S5 are written into ABAQUS finite element software, finite element modeling analysis is carried out by adopting large unit size, and the real input constitutive curve of the step S2, the fracture criterion parameters of the step S3, the initial degradation model parameters of the step S4 and the stress degradation parameters of the step S5 are substituted into the material user subroutine, so that the accurate simulation of the fracture analysis can be carried out on the finite element model under the large unit size.
2. The simulation analysis method for ductile fracture coupling failure of a metal material according to claim 1, wherein the simulation analysis method is characterized by comprising the following steps: in S3, the ductile fracture criterion is critical fracture strain ε f And the function of the triaxial degree eta of the average stress and the average node parameter L is as follows:c in the formula 1 、C 2 、C 3 And obtaining undetermined parameters of the initial degradation model according to a curved surface fitting method or a method for solving an equation set as undetermined parameters of the material.
3. The simulation analysis method for ductile fracture coupling failure of a metal material according to claim 2, wherein the simulation analysis method is characterized by comprising the following steps: s4, determining initial degradation strain points of the three groups of samples by truly inputting a constitutive curve and a coarse grid optimized constitutive curve, wherein the initial degradation points of the first group of samples and the third group of samples are stress reduction points, and the initial degradation points of the second group of samples are tensile maximum force points; the initial degradation model of the metal material is critical degradation strain epsilon c The corresponding average stress triaxial degree eta c And average code parameter L c The expression of which is as follows:c in the formula 1 ′、C 2 ′、C 3 ' is a pending parameter of a material, and the pending parameter of an initial degradation model is obtained according to a curved surface fitting method or a method for solving an equation set.
4. The simulation analysis method for ductile fracture coupling failure of metal material according to claim 3, wherein the method comprises the following steps: in S5, when D is smaller than Dc, the strength is not degraded, and when D is larger than or equal to Dc, the strength starts to degrade, and the degradation expression is: sigma' =σ (1-w (D-D c ) m ) Where σ' is the stress after degradation, σ is the undegraded stress, and when d=1, the material fails.
5. The simulation analysis method for ductile fracture coupling failure of a metal material according to any one of claims 1 to 4, wherein: the small unit size is less than or equal to 0.2mm, and the large unit size is more than or equal to 1mm.
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