CN113673030A - 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 PDF

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CN113673030A
CN113673030A CN202110896825.6A CN202110896825A CN113673030A CN 113673030 A CN113673030 A CN 113673030A CN 202110896825 A CN202110896825 A CN 202110896825A CN 113673030 A CN113673030 A CN 113673030A
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CN113673030B (en
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杨婷
张青
董伊康
刘需
刘天武
王佩
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HBIS Co Ltd
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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: carrying out tensile test on samples in different stress states; determining a real stress-strain curve of the simulation input: determining undetermined parameters of a fracture criterion under the large unit size; determining undetermined parameters of the initial degradation model; determining a stress degradation parameter; and carrying out the simulation of the fracture failure of the metal material. The method can effectively improve the prediction precision of the ductile fracture failure of the metal material, particularly the calculation precision of the high-strength metal material, improves the calculation efficiency, has simple acquisition of material damage parameters, can effectively solve the problem of difficulty in the engineering application of a failure model, and has wide application range.

Description

Simulation analysis method for ductile fracture coupling failure of metal material
Technical Field
The 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
The fracture is a main form causing the failure of engineering components and is a problem which is closely concerned in production and life, such as the fracture failure of materials in the automobile collision process, and the fracture analysis and simulation prediction of the materials have great significance for personal safety and property loss.
The finite element simulation analysis method is widely applied to the fields of material forming, collision and the like. The accurate establishment of the simulation model and the accurate acquisition of the material parameters are very important to the simulation prediction precision. With the improvement of finite element simulation technology and the development of computer technology, the requirement for improving the finite element simulation precision is more and more strong. Ductile fracture is a fracture mode of crack propagation formed by the combination of hole nucleation, growth and aggregation in a material. The influence of mechanical properties brought by the crack propagation process is described through a damage variable, and the influence of the material property degradation on the structural strength and the service life is well estimated. In industrial applications, especially finite element simulations, the problems of computation speed and computation accuracy need to be considered, and to solve such problems to better serve the simulation, Neukam et al in the article "On closing the coherent gate beta between forming and modeling" [ Neukam F, Feucht M, Roll K, et al].Proceedings of International
Figure BDA0003198282120000011
Users Conference.2008.]The article "connecting damage history in crashworthiness syndromes" [ Neukamm F, Feucht M, Haufe A. connecting damage history in crashworthiness syndromes [ C].Proceedings of European Ls-dyna Conference.2009.]And the like, discloses a generalized incremental stress state-related damage model, namely a GISSMO model. In the GISSMO model, the equivalent fracture stress of the material is includedThe variable curve is equivalent to a critical strain curve, damage accumulation is taken as a standard for judging material failure, the damage accumulation process is taken as a nonlinear accumulation process, and a flow stress weakening phenomenon 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 a tough material, after the loading ultimate strength is reached, the material stress is not brittle failure to 0, and the material stress is gradually weakened to 0 when the GISSMO model definition damage variable D is increased to 1, so that the material brittle failure phenomenon in the simulation process is avoided, but the stress weakening characteristics of the GISSMO model are not met with some high-strength materials, such as advanced high-strength steel and the like. Although the parameters of stress reduction index m and damage accumulation index n in the Gissmo failure model can be checked by a simulation reverse-deducing method, the stress of the material can be estimated to be too high or too low in the early stage of material damage. The GISSMO model considers that the material fails when the stress continuously decays to 0, while 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 a critical value, the material unit fails and the stress abruptly changes to 0. Therefore, the stress degradation behavior of some 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 greatly different, and the unit size required to be adopted for ensuring the simulation accuracy is 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 cell sizes, and the influence of grid size effect on fracture parameters is considered in a large cell size model, but the influence of the grid size effect on the calculation precision of the stress degradation process is not pointed out.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a simulation analysis method for ductile fracture coupling failure of a metal material, which makes up the defects of the conventional fracture failure simulation method, simultaneously considers the stress degradation characteristic and the grid size effect of a 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 technical scheme adopted by the invention is as follows:
a simulation analysis method for ductile fracture coupling failure of a metal material specifically comprises the following steps:
(1) and (3) carrying out tensile test on samples in different stress states: 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; and the third group is shear samples, each sample is subjected to a tensile test under a quasi-static condition, and displacement-force curve data is recorded.
(2) Determining a real 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 meshing on the finite element model of the first group of samples according to the size of a small unit, wherein the size of the small unit is less than or equal to 0.2 mm; and (3) optimizing a plastic strain-true stress curve of the material input of the finite element model of the first group of samples by taking the experimental displacement-force curve of the first group of samples as an optimization target until the simulated displacement-force curve is completely consistent with the experimental curve, thus obtaining a true input constitutive curve of the material.
(3) Determining undetermined parameters of the fracture criterion under the large unit 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 carrying out grid division according to the size of a large unit, wherein the size of the large unit is more than or equal to 1 mm; performing constitutive reverse-pushing optimization on the finite element model under the large element size according to the constitutive reverse-pushing method in the step (2) until the area difference between the experimental displacement force curve and the simulation curve is minimum, and calculating the critical fracture strain epsilon of the maximum strain element of each samplefMean stress triaxial η and mean lode angle parameters
Figure BDA0003198282120000031
Or the average lode parameter L (average lode angle parameter)
Figure BDA0003198282120000032
The average lode parameter L may be substituted,
Figure BDA0003198282120000033
Figure BDA0003198282120000034
) Establishing a three-dimensional fracture surface model of the metal material representing different stress states, and obtaining undetermined parameters of the fracture surface model according to a surface fitting method or a method for solving an equation set.
(4) Determining the undetermined parameters of the initial degradation model: actually inputting the material in the step 2 into the constitutive curve and comparing and analyzing the constitutive curve of each sample coarse grid optimization in the step 3 to determine the initial degradation time of three groups of samples; obtaining the critical degeneration strain epsilon corresponding to the maximum strain unit at the initial degeneration moment of each group of samplescMean stress triaxial degree ηcAngle parameter with average lode
Figure BDA0003198282120000041
Or average lode parameter Lc(average lode Angle parameter)
Figure BDA0003198282120000042
The average lode parameter L can be usedcInstead of this, the user can,
Figure BDA0003198282120000043
) 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 stress degradation parameters: 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: σ' ═ σ (1-w (D-D)c)m) Where σ' is the degraded stress, σ is the non-degraded stress, and when D ═ 1, the material fails; establishing a relation between the true stress difference value delta sigma between the material true input constitutive curve in the step 2 and each sample coarse grid optimization constitutive curve in the step 3 and (D-Dc), wherein the relation is as follows: Δ σ ═ w (D-D)c)mObtaining stress degradation parameters m and w;
(6) carrying out metal material fracture failure simulation: and (3) compiling a user sub-program of the fracture failure model expressions and the model parameters determined in the step (1) to the step (5) and embedding the user sub-program into ABAQUS finite element software, carrying out finite element modeling analysis by adopting the large unit size, and substituting the real input constitutive curve in the step (2), the fracture criterion parameters in the step (3), the initial degradation model parameters in the step (4) and the stress degradation parameters in the step (5) into a material user sub-program VUMAT or a VUHARD and VUSFLD combined sub-program, so that the fracture analysis of the finite element model under the large unit size can be accurately simulated.
In another alternative implementation manner, the expression of the three-dimensional fracture surface model of the metal material is as follows:
Figure BDA0003198282120000051
in the formula C1、C2、C3The undetermined parameter of the ductile fracture criterion may be in the form of Lou-Huh, MMC, MU or other ductile fracture criterion expression.
Figure BDA0003198282120000052
Relationship to L:
Figure BDA0003198282120000053
Figure BDA0003198282120000054
and carrying out corresponding replacement.
In another alternative implementation, a parameter optimization tool such as Isight or other parameter optimization method may be used to determine the stress degradation parameters m and w in step 5.
The three groups of sample initial degradation strain points are determined by a real input constitutive curve and a coarse grid optimization constitutive curve to obtain critical degradation strain epsilon corresponding to the maximum strain unit at the initial degradation moment of each group of samplescMean stress triaxial degree ηcAngle parameter with average lode
Figure BDA0003198282120000055
Or average ofode parameter LcEstablishing a metal material initial stress degradation model representing different stress states, wherein the expression is as follows:
Figure BDA0003198282120000056
in the formula C1′、C2′、C3The undetermined parameters of the initial degradation model are obtained according to a curved surface fitting method or a method for solving an equation set, and the function expression form can also be expressed by Lou-Huh, MMC or other ductile fracture criteria.
The stress degradation mode of the invention is as follows: σ' ═ σ (1-w (D-D)c)m) D is a damage variable and the expression is
Figure BDA0003198282120000057
Dc is equivalent plastic strain epsilonpStrain epsilon to initial point of degradationcCorresponding D value, 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 begins to degrade, wherein sigma' is the degraded stress, sigma is the non-degraded stress, and when D is 1, the material fails; establishing a relation between the true stress difference value delta sigma and (D-Dc) between the material true input constitutive curve in the step 2 and each sample coarse grid optimization constitutive curve in the step 3, wherein the relation is as follows: Δ σ ═ w (D-D)c)mThe stress degradation parameters m and w can be obtained.
Due to the adoption of the technical scheme, the invention has the beneficial effects that: by utilizing the method, the prediction accuracy of the ductile fracture failure of the metal material, particularly the calculation accuracy of the high-strength metal material, such as the high-strength automobile plate DP780 material mentioned in the embodiment, can be effectively improved, and compared with a GISSMO model and a non-coupled failure model, the improved damage failure model has better prediction accuracy on each sample. The method has the advantages of improving the calculation efficiency, reducing the experiment cost, being simple in obtaining of material damage parameters, solving the problem of difficult application of failure model engineering, being wide in application range and having extremely high application value.
Drawings
FIG. 1 is a graph of a true input constitutive curve for a material according to an embodiment of the present invention;
FIG. 2 is a fracture plot of a material under planar stress according to an embodiment of the present invention;
FIG. 3 is a graph of a stress degradation model of a material under a planar stress condition according to an embodiment of the present invention;
FIG. 4 is a graph of a parametric fit of stress degradation for a material according to an embodiment of the invention;
FIG. 5 is a graph of the results of a fracture simulation of a smooth tensile specimen of a material according to an embodiment of the present invention;
FIG. 6 is a graph of the fracture simulation results for a notched specimen of Material R8 according to an embodiment of the present invention;
fig. 7 is a graph of the fracture simulation results of the material R3 notch sample of the 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: and (3) carrying out tensile test on samples in different stress states: 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; and the third group is shear samples, each group of samples is subjected to a tensile test under a quasi-static condition, and displacement-force curve data are respectively recorded.
S2: determining a real 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 meshing the finite element model of the first group of samples according to the small element size; by using the constitutive back-pushing method, namely, the displacement-force curve of the first group of samples is used as an experimental displacement-force curve and an optimization target, a plastic strain-true stress curve input by the material of the finite element model of the first group of samples is optimized, and 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, so that a real input constitutive curve of the material is obtained. Preferably, the small unit size is less than or equal to 0.2 mm.
S3: determining undetermined parameters of fracture criterion under large unit size: establishing a finite element model according to the actual sizes of the three groups of samples in different stress states in the step S1, and carrying out meshing according to the size of a large unit; according to the constitutive backward-pushing method of the step S2, constitutive backward-pushing optimization is carried out on three groups of finite element models under the large element size until the area difference between the experimental displacement-force curve and the simulated displacement-force curve is minimum, so that a coarse grid optimization constitutive curve is formed, and the critical fracture strain epsilon of the maximum strain element of each sample is calculated under the coarse grid optimization constitutive curvefMean stress triaxial degree eta, mean lode angle parameter
Figure BDA0003198282120000071
(average lode Angle parameter)
Figure BDA0003198282120000072
The average lode parameter L may be substituted,
Figure BDA0003198282120000073
) So as to obtain a metal material three-dimensional fracture surface model representing different stress states, and obtain undetermined parameters of a ductile fracture criterion according to a surface fitting method or a method for solving an equation set.
S4: determining the undetermined parameters of the initial degradation model: comparing and analyzing the real input constitutive curve of the material in the step S2 with the coarse mesh optimization constitutive curve of each sample in the step S3, and determining the initial degradation time of each group of samples; obtaining the critical degeneration strain epsilon corresponding to the maximum strain unit at the initial degeneration moment of each group of samplescMean stress triaxial degree ηcAverage lode angle parameter
Figure BDA0003198282120000081
(average lode Angle parameter)
Figure BDA0003198282120000082
The average lode parameter L can be usedcInstead of this, the user can,
Figure BDA0003198282120000083
) To establish the characterization of different stress statesAnd obtaining undetermined parameters of the initial degradation model according to a curved surface fitting method or a method for solving an equation set by using the three-dimensional curved surface model of the initial stress degradation of the metal material in the state.
S5: determining stress degradation parameters: 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 expression of the damage variable D is
Figure BDA0003198282120000084
In the formula ofpThe equivalent plastic strain is obtained, n is a nonlinear damage accumulation index, and n is 1 in linear degradation; dc is the equivalent plastic strain εpCritical degeneration strain epsilon upon reaching the onset degeneration pointcThe corresponding D value.
The true stress difference Δ σ between the true input constitutive curve of the material in step S2 and the coarse mesh optimized constitutive curve of each sample in step S3 is related to (D-Dc) as follows: Δ σ ═ w (D-D)c)mAnd obtaining stress degradation parameters m and w.
S6: carrying out metal material fracture failure simulation: the fracture failure model expressions and model parameters determined in S1-S5 are compiled into a user subroutine by embedding the user subroutine into ABAQUS finite element software, finite element modeling analysis is carried out by adopting large unit size, and the real input constitutive curve in the step S2, the fracture criterion parameters in the step S3, the initial degradation model parameters in the step S4 and the stress degradation parameters in the step S5 are substituted into a material user subroutine (VUMAT or a combined subroutine of VUHARD and VUSFLD), so that the fracture analysis of the finite element model under the large unit size can be accurately simulated. Preferably, the large cell size is ≧ 1 mm.
Specifically, in S3, the ductile fracture criterion is the critical fracture strain εfAnd the function of the mean stress triaxial degree eta and the mean lode parameter L, wherein the function expression is as follows:
Figure BDA0003198282120000091
in the formula C1、C2、C3Obtaining the undetermined parameters of the initial degradation model for the ductile fracture criterion according to a curved surface fitting method or a method for solving an equation setAnd (5) determining parameters. The form may also be Lou-Huh, MMC, MU or other ductile fracture criterion expression forms.
Figure BDA0003198282120000092
Relationship to L:
Figure BDA0003198282120000093
Figure BDA0003198282120000094
and carrying out corresponding replacement.
Specifically, in S4, the initial degradation strain points of the three groups of samples are determined by the true input constitutive curve and the coarse mesh optimization constitutive curve, the initial degradation points of the first group and the third group of samples are stress drop points, and the initial degradation point of the second group of samples is the maximum tensile force point; the initial degradation model of the metal material is critical degradation strain epsiloncMean stress triaxial η corresponding theretocAnd average lode parameter LcIs expressed as follows:
Figure BDA0003198282120000095
in the formula C1′、C2′、C3The undetermined parameters of the initial degradation model are obtained according to a curved surface fitting method or a method for solving an equation set, and the function expression form can also be expressed by Lou-Huh, MMC or other ductile fracture criteria. The functional expression relates only to the average lode parameter LcWithout reference to the average lode angle parameter
Figure BDA0003198282120000096
If it relates to
Figure BDA0003198282120000097
And LcThe relationship of (1):
Figure BDA0003198282120000098
Figure BDA0003198282120000099
and carrying out corresponding replacement.
Specifically, in S5, when D < Dc, the intensity is not degraded, and when D ≧ Dc, the intensity begins to degrade, the degradation expression is: σ' ═ σ (1-w (D-D)c)m) Where σ' is the degraded stress and σ is the non-degraded stress, the material fails when D ═ 1.
The following detailed description is made by way of example with reference to fig. 1 to 7, and the process of the analysis method is as follows:
(1) designing tensile samples of the high-strength automobile plate DP780 material in different stress states to perform quasi-static tensile tests, 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 was a shear specimen with a shear angle of 0 °.
(2) Determining a real 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 meshing the finite element models of the first group of samples according to the unit size of 0.1 mm; and (3) optimizing the plastic strain-true stress curve of the material input of the finite element model of the first group of samples by taking the experimental displacement-force curve of the first group of samples as an optimization target until the simulated displacement-force curve is well matched with the experimental displacement-force curve, so as to obtain a 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 the constitutive inverse optimization method, and the black solid line is a curve obtained by fitting the determined input value by using a double Voce model.
The relevant parameters in fig. 1 are shown in table 1.
Figure BDA0003198282120000101
TABLE 1
(3) Determining undetermined parameters of the fracture criterion under the large unit size: carrying out meshing on the finite element models of the third group of samples according to the unit size of 1 mm; performing constitutive inverse push optimization on the model according to the constitutive inverse push method in the step 2 until an experimental displacement-force curveThe area difference value between the simulated displacement-force curve and the simulated displacement-force curve is minimum, the constitutive curve under the coarse grid is obtained, and the critical fracture strain Epsilon f, the mean stress triaxial degree eta and the mean lode angle parameter of the maximum strain unit of each sample are calculated
Figure BDA0003198282120000111
Or average lode parameter L, establishing a metal material three-dimensional fracture surface model representing different stress states, wherein the expression is
Figure BDA0003198282120000112
And obtaining the parameters C1, C2 and C3 of the undetermined materials of the fracture surface model according to a surface fitting method or a method for solving an equation set, as shown in FIG. 2.
In fig. 2, the black nodes are stress axiality and fracture strain values corresponding to the critical fracture points of the three groups of samples calibrated by the constitutive back-push optimization method, and the black solid line is a curve fitting the critical fracture values of the three groups of samples by a fracture criterion formula.
(4) Determining the undetermined parameters of the initial degradation model: comparing and analyzing the material really input into the constitutive curve of the step 2 and the crude grid constitutive curves of the samples in the step 3 to determine the initial degradation time of the 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 maximum tensile force point; obtaining critical degradation strain epsilon c, average stress triaxial degree eta c and average lode 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 material in different stress states,
Figure BDA0003198282120000113
the undetermined parameters C1 ', C2 ' and C3 ' of the initial degradation model are obtained according to a surface fitting method or a method of solving an equation set, as shown in fig. 3.
In fig. 3, the black nodes are stress axiality and fracture strain values corresponding to the critical degeneration points of the three groups of samples calibrated by the constitutive back-push optimization method, and the black solid line is a curve fitting the critical fracture values of the three groups of samples by a fracture criterion formula.
(5) Determining stress degradation parameters: 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: σ' ═ σ (1-w (D-D)c)m) Where σ' is the degraded stress, σ is the non-degraded stress, and when D ═ 1, the material fails; establishing a relation between the true stress difference value delta sigma between the material true input constitutive curve in the step 2 and each sample coarse grid optimization constitutive curve in the step 3 and (D-Dc), wherein the relation is as follows: Δ σ ═ w (D-D)c)mObtaining stress degradation parameters m and w; as shown in fig. 4, the relevant parameters in fig. 4 are shown in table 2 (where x is D-D)c)。
In fig. 4, the black nodes are true stress difference points between the input constitutive curve and the coarse grid optimized constitutive curve in the calculation result, and the black solid line is a fitting curve of the stress degradation formula to the difference points.
Figure BDA0003198282120000121
TABLE 2
(6) Carrying out metal material fracture failure simulation: the model is applied to FORTRAN language to write a user subprogram VUHARD and VUSFLD combined subprogram, the subprogram is embedded into ABAQUS finite element software, finite element modeling analysis is carried out by adopting the unit size of 1mm, the actually 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, accurate simulation of fracture analysis can be carried out on the finite element model established under the grid size of 1mm, and the simulation result is shown in FIGS. 5-7.
In fig. 5, a smooth sample is used, the experimental curves are represented by straight lines, the three lower curves are the GISSMO model, the invention model and the non-coupling model from left to right, and the invention model is located above the non-coupling model at the lower right.
In fig. 6, the R8 sample is used, the experimental curve is represented by a straight line, the lower three curves are the GISSMO model, the invention model and the uncoupled model from left to right, and the invention model extends to the uncoupled model at the lower right.
In fig. 7, an R3 sample is used, an experimental curve is represented by a straight line, the three lower curves are a GISSMO model, an invention model and a non-coupled model from left to right, at the lower right, the invention model extends to the non-coupled model after oscillating, and the non-coupled model oscillates more intensely.
As can be seen from fig. 5-7, the calculation results of the stress and strain values of the GISSMO model cell failure point (D ═ 1) are lower, and the calculation results of the stress and strain values of the cell failure point of the uncoupled failure model are higher.

Claims (5)

1. A simulation analysis method for ductile fracture coupling failure of a metal material is characterized by comprising the following steps:
s1: and (3) carrying out tensile test on samples in different stress states: 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 shearing sample, each group of samples is subjected to a tensile test under a quasi-static condition, and displacement-force curve data are respectively recorded;
s2: determining a real 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 meshing the finite element model of the first group of samples according to the small element size; by using the constitutive backward-pushing method, namely, taking the displacement-force curve of the first group of samples as an experimental displacement-force curve and an optimization target, optimizing a plastic strain-true stress curve input by the material of the finite element model of the first group of samples, and 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, so as to obtain a true input constitutive curve of the material;
s3: determining undetermined parameters of the fracture criterion under the large unit size: establishing a finite element model according to the actual sizes of the three groups of samples with different stress states in the step S1, and according to the size of the large elementCarrying out grid division; according to the constitutive backward-pushing method of the step S2, constitutive backward-pushing optimization is carried out on three groups of finite element models under the large element size until the area difference between the experimental displacement-force curve and the simulated displacement-force curve is minimum, so that a coarse grid optimization constitutive curve is formed, and the critical fracture strain epsilon of the maximum strain element of each sample is calculated under the coarse grid optimization constitutive curvefMean stress triaxial degree eta, mean lode angle parameter
Figure FDA0003198282110000011
Thereby obtaining a metal material three-dimensional fracture surface model representing different stress states, and obtaining undetermined parameters of a ductile fracture criterion according to a surface fitting method or a method for solving an equation set;
s4: determining the undetermined parameters of the initial degradation model: comparing and analyzing the real input constitutive curve of the material in the step S2 with the coarse mesh optimization constitutive curve of each sample in the step S3, and determining the initial degradation time of each group of samples; obtaining the critical degeneration strain epsilon corresponding to the maximum strain unit at the initial degeneration moment of each group of samplescMean stress triaxial degree ηcAverage lode angle parameter
Figure FDA0003198282110000021
Thereby 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;
s5: determining stress degradation parameters: 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 expression of the damage variable D is
Figure FDA0003198282110000022
In the formula ofpThe equivalent plastic strain is obtained, n is a nonlinear damage accumulation index, and n is 1 in linear degradation; dc is the equivalent plastic strain εpCritical degeneration strain epsilon upon reaching the onset degeneration pointcThe corresponding D value;
will be described in detailThe true stress difference Δ σ between the true input constitutive curve for the material in S2 and the coarse mesh optimized constitutive curve for each sample in step S3 is related to (D-Dc) as follows: Δ σ ═ w (D-D)c)mObtaining stress degradation parameters m and w;
s6: carrying out metal material fracture failure simulation: and compiling a user subprogram of the fracture failure model expression and the model parameters determined in S1-S5 into ABAQUS finite element software, performing finite element modeling analysis by adopting a large unit size, and substituting the real input constitutive curve in the step S2, the fracture criterion parameters in the step S3, the initial degradation model parameters in the step S4 and the stress degradation parameters in the step S5 into the material user subprogram, so that the finite element model under the large unit size can be subjected to accurate simulation of fracture analysis.
2. The simulation analysis method for ductile fracture coupling failure of metal material according to claim 1, wherein: in S3, the ductile fracture criterion is the critical fracture strain εfAnd the function of the mean stress triaxial degree eta and the mean lode parameter L, wherein the function expression is as follows:
Figure FDA0003198282110000031
in the formula C1、C2、C3And obtaining undetermined parameters of the initial degradation model for undetermined parameters of the material according to a curved surface fitting method or a method for solving an equation set.
3. The simulation analysis method for ductile fracture coupling failure of metal material according to claim 2, wherein: in S4, the initial degradation strain points of the three groups of samples are determined by a real input constitutive curve and a coarse grid optimization constitutive curve, the initial degradation points of the first group and the third group of samples are stress drop points, and the initial degradation point of the second group of samples is a maximum tensile force point; the initial degradation model of the metal material is critical degradation strain epsiloncMean stress triaxial η corresponding theretocAnd average lode parameter LcIs expressed as follows:
Figure FDA0003198282110000032
in the formula C1′、C2′、C3The undetermined parameters of the initial degradation model are 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: in S5, when D < Dc, the intensity is not degraded, and when D is more than or equal to Dc, the intensity begins to degrade, and the degradation expression is: σ' ═ σ (1-w (D-D)c)m) Where σ' is the degraded stress and σ is the non-degraded stress, the material fails when D ═ 1.
5. The simulation analysis method for the ductile fracture coupling failure of the metal material according to any one of claims 1 to 4, wherein: the size of the small unit is less than or equal to 0.2mm, and the size of the large unit is more than or equal to 1 mm.
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