CN113722957B - Equivalent stress and equivalent strain direct test method for structural element sample under unidirectional loading - Google Patents
Equivalent stress and equivalent strain direct test method for structural element sample under unidirectional loading Download PDFInfo
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Abstract
The invention discloses a direct test method for equivalent stress and equivalent strain of a component sample under unidirectional loading, which relates to the technical field of mechanical property test of materials and comprises 4 steps of obtaining a load-displacement curve, calculating strain, stress data points, obtaining a stress-strain curve and the like, wherein the whole process is a process of obtaining the equivalent stress-strain curve in real time, and the method is different from an empirical correlation method and does not need an empirical formula; secondly, unlike the finite element auxiliary test method, complex iterative computation is not needed; thirdly, unlike the database building method, a large amount of finite element calculation is not needed; and fourthly, the method is different from an analytic or semi-analytic method, the constitutive relation parameters of the material can be solved without the aid of material information contained in a complete load-displacement curve, and the stress-strain curve of the material can be obtained directly according to load-displacement data points obtained through a test. The whole method is suitable for multiple types of constituent samples with different sizes and different materials, and has universality.
Description
Technical Field
The invention relates to the technical field of material mechanical property testing, in particular to a direct test method for equivalent stress and equivalent strain of a structural element sample under unidirectional loading.
Background
The stress-strain relationship, elastic modulus, yield strength, tensile strength and the like of the material are basic mechanical relationship and performance for realizing structural integrity analysis, and the establishment of the association between the elastoplastic deformation response of different structural element samples and the mechanical performance of the material is important for structural design and safety evaluation. The common practice for obtaining the uniaxial constitutive relation of a material is to intercept a standard uniaxial tensile sample of centimeter grade from an engineering structure or a parent material and perform a tensile test in a laboratory. At present, aiming at various nano-test structural elements, 4 technical routes mainly exist for acquiring the basic mechanical properties of materials.
Empirical correlation: based on a large number of standard tensile tests and small-size component sample test results, an empirical conversion relation between the yield strength and the tensile strength of the material obtained by correlating the large-size standard tensile sample and the yield load and the limit load loaded by the small-size component sample is established, and the essence of the elastoplastic deformation behavior of the material is difficult to be revealed by the empirical formulas.
Finite element auxiliary test method: and obtaining a simulated load-displacement curve of the component sample by finite element analysis of a uniaxial equivalent stress-strain curve of a preset material in the finite element, and adjusting the preset equivalent stress-strain curve of the material by comparing the simulated load-displacement curve with the simulated load-displacement curve by taking the test load-displacement curve as an iteration target. When the load error of the simulation curve and the test curve is smaller than the set error value, the preset stress-strain curve is the real equivalent stress-strain curve of the material. The method is a method combining experiments and finite element analysis, and iterative computation is needed to be carried out by combining the finite element analysis.
Establishing a database method: and carrying out a large number of finite element analyses on materials with different constitutive relation parameters by means of finite element software to establish a constitutive element sample load-displacement relation database. And loading the structural element sample to obtain a test load-displacement curve, extracting equivalent stress under different strains by matching with the load-displacement curve in the database, and drawing a complete stress-strain curve by adopting a constitutive relation model. The method avoids the defect that the finite element iteration is carried out by a single test of the finite element auxiliary test method, but the database method can not get rid of a large amount of finite element calculation.
Resolution or semi-resolution method: for the statics problem of the linear elastic element, the method represented by the theorem of blocking, the virtual work principle and the like can simply and directly realize the analysis and the solution of the problems of the rod, the beam, the crack body and the like, and the analysis and the solution are very difficult because the elastoplastic problem relates to the nonlinearity of materials, the boundary conditions and the like. The method has universality for various typical test structural elements, but the constitutive relation parameters of the material need to be solved by means of material information contained in a complete load-displacement curve, and the stress-strain curve of the material cannot be directly obtained according to load-displacement data points obtained through the test.
Disclosure of Invention
The invention aims at: aiming at the problems, the method for directly testing the equivalent stress and the equivalent strain of the component sample under the unidirectional loading of the stress-strain curve of the material directly according to the load and the displacement data points obtained by the test without the need of an empirical formula, iteration and a large amount of calculation is provided, and has universality.
The technical scheme adopted by the invention is as follows:
a direct test method for equivalent stress and equivalent strain of a structural element sample under unidirectional loading comprises the following steps:
(1) Selecting a component sample, carrying out quasi-static loading on the component sample to obtain test displacement h and load P data, sequentially collecting N data, taking i by data number, and recording the ith group of displacement h and load P data as (h) i ，P i )；
(2) Substituting each group of displacement h and load P data into the following formula to calculate and obtain equivalent strain epsilon eq Equivalent stress sigma eq Data (h) i ，P i ) Corresponding calculated equivalent strain epsilon eq Equivalent stress sigma eq The data are recorded as (. Epsilon.) eqi ，σ eqi )，
Wherein A is * 、h * The characteristic area and the characteristic displacement of the component sample are respectively;
is a characteristic yield strain;
k e-5 、k ep-5 、k e-6 、k ep-6 are all equivalent stress and equivalent strain model dimensionless constants, k ep-5m Is k ep-5 Is the average value of (2); the linear elastic section and the elastic plastic section are two deformation stages of the component sample;
(3) Regression (epsilon) eqi ，σ eqi ) And obtaining an equivalent stress-strain curve.
Preferably, k in step (2) e-5 、k ep-5 、k e-6 、k ep-6 The values of (2) are determined by means of simple finite element analysis, the steps being:
(2.1) carrying out finite element analysis on the component sample to obtain a linear load-displacement curve, and carrying out regression on the linear load-displacement curve to obtain the slope of the load-displacement curve, namely loading stiffness S;
(2.2) carrying out finite element analysis on the component samples by using 4 specific materials to obtain a load-displacement curve, and carrying out regression on elastoplastic segments of the 4 load-displacement curves to obtain elastoplastic segment loading curvature C and elastoplastic segment loading indexes m corresponding to different strain hardening indexes n;
(2.3) calculating according to the following formula to obtain the dimensionless constant k of the load-displacement model e-1 、k ep-1 、k ep-2 、k 3 、k 4 ，
Wherein K is a strain hardening coefficient, k=e n σ y 1-n ；h y Is the yield displacement; e is the elastic modulus;
(2.4) calculating k according to the following formula e-5 、k ep-5 、k e-6 、k ep-6 ，
Wherein k is ep-5 And mean value k ep-5m The error between the two is not more than 3%, and k is calculated ep-5 Approximately equivalent k ep-5m 。
Preferably, the regression mode in the step (3) is regression according to a hollloon model, and the hollloon model is:
in sigma y Is the nominal yield stress.
Preferably, step (1) further comprises the steps of: judging the obtained epsilon in real time eqi ，σ eqi ) Whether located at the junction of the wire elastic segment to the elastoplastic segment: pair (epsilon) eqi ，σ eqi ) Is rounded up by using the corresponding (. Epsilon.) of INT (0.3 i) to INT (0.7 i) eqi ，σ eqi ) The first temporary elastic modulus E is calculated by Hollomon model INT(0.7i) INT (0.7 i) to i corresponding (. Epsilon.) are used eqi ，σ eqi ) The second temporary elastic modulus E is obtained by Hollomon model calculation i The method comprises the steps of carrying out a first treatment on the surface of the When E is INT(0.7i) And E is i When the relative error between the two is more than 10%, the group (. Epsilon.) eqi ，σ eqi ) The corresponding equivalent stress-strain curve deviates from the linear segment, i.e. the group (ε) eqi ，σ eqi ) At the junction point between the elastic section and the elastic-plastic section, defining epsilon at this time eqi Is that
Preferably, step "real-time judgment of the obtained (. Epsilon.) eqi ，σ eqi ) Whether i is located in the line elastic segment to elastoplastic segment interface "is greater than 20.
Preferably, N in step (1) is a natural number and is greater than 100.
Preferably, in step (2), group i is said (h i ，P i ) Corresponding calculation to obtain i group (epsilon) eqi ，σ eqi )。
In summary, due to the adoption of the technical scheme, the beneficial effects of the invention are as follows: and (3) obtaining displacement and load data points through a test, substituting the displacement and load data points into the formula in the step (2) to obtain stress and strain data points, and carrying out regression to obtain an equivalent stress-strain curve. The whole process is a process of acquiring an equivalent stress-strain curve in real time, and firstly, the process is different from an empirical correlation method and does not need an empirical formula; secondly, unlike the finite element auxiliary test method, complex iterative computation is not needed; thirdly, unlike the database building method, a large amount of finite element calculation is not needed; and fourthly, the method is different from an analytic or semi-analytic method, the constitutive relation parameters of the material can be solved without the aid of material information contained in a complete load-displacement curve, and the stress-strain curve of the material can be obtained directly according to load-displacement data points obtained through a test. The whole method is suitable for multiple types of constituent samples with different sizes and different materials, and has universality.
Drawings
FIG. 1 is a graph showing load-displacement data points obtained in accordance with the teachings of the present invention.
FIG. 2 is a graph showing stress-strain data points obtained from load-displacement data points in accordance with the present invention.
Fig. 3 shows a load-displacement curve obtained by the technical scheme of the invention.
Fig. 4 is a graph showing load-displacement curves of normalized three-point bent TPB component samples.
FIG. 5 is a graph showing load-displacement curves normalized to a circular compressed RC component sample.
FIG. 6 is a graph of k for different hardening indexes n ep-5 And mean value k ep-5m The error between them illustrates a curve.
FIG. 7 is a schematic diagram of the loading of compact tensile CT in numerical validation with 6 representative test element specimens.
Fig. 8 is a finite element mesh model of compact tensile CT in numerical validation with 6 representative test element specimens.
FIG. 9 is a graph showing loading of single-sided crack bending SEB in numerical verification with 6 representative test component samples.
Fig. 10 is a finite element mesh model of single-sided crack bending SEB in numerical verification with 6 typical test element specimens.
FIG. 11 is a schematic diagram of loading of small-sized C-tensile CIET with inboard side cracks in numerical verification with 6 representative test element samples.
FIG. 12 is a finite element mesh model of a small-sized C-stretched CIET with medial side cracks in numerical validation with 6 representative test element specimens.
FIG. 13 is a schematic diagram of loading of cantilever bending CB in numerical verification with 6 typical test element samples.
Fig. 14 is a finite element mesh model of cantilever bending CB in numerical verification with 6 typical test element specimens.
Fig. 15 is a graph showing loading of three-point bend TPB in numerical validation with 6 representative test element samples.
Fig. 16 is a finite element mesh model of a three-point curved TPB in numerical validation with 6 representative test element specimens.
FIG. 17 is a graph showing loading of the circular compression RC in numerical validation with 6 representative test element samples.
Fig. 18 is a finite element mesh model of circular compression RC in numerical validation with 6 representative test element specimens.
FIG. 19 is a graph comparing stress-strain curves obtained for test examples of compact tensile CT in numerical verification with 6 typical test element specimens with stress-strain curves obtained for standard examples.
FIG. 20 is a graph comparing stress-strain curves obtained in test examples of single-sided crack bending SEB with stress-strain curves obtained in standard examples in numerical verification with 6 typical test element samples.
FIG. 21 is a graph comparing stress-strain curves obtained for small-sized C-tensile CIET with internal side cracks with those obtained for standard examples for numerical verification with 6 representative test element samples.
FIG. 22 is a graph comparing stress-strain curves obtained for test examples of cantilever bending CB with stress-strain curves obtained for standard examples in numerical verification of 6 representative test element samples.
Fig. 23 is a graph comparing stress-strain curves obtained in the test examples of the three-point bending TPB in the numerical verification with 6 typical test element samples with stress-strain curves obtained in the standard examples.
FIG. 24 is a graph comparing stress-strain curves obtained for test examples of ring compression RC with stress-strain curves obtained for standard examples in numerical verification with 6 representative test element samples.
Fig. 25 is a graph showing load-displacement curves obtained by performing a quasi-static loading test on a ring-compressed element in test verification with a ring-compressed element sample.
FIG. 26 is a graph showing a comparison of stress-strain curves obtained in test examples and standard examples in test verification using a ring compressed structural element sample.
Detailed Description
The present invention will be described in detail with reference to the accompanying drawings.
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
Referring to fig. 1, a direct test method for equivalent stress and equivalent strain of a component sample under unidirectional loading includes the following steps:
s01: selecting a component sample, carrying out quasi-static loading on the component sample to obtain test displacement h and load P data, sequentially collecting N data, wherein N is a natural number and is larger than 100, the data number is i, and the i-th group displacement h and load P data are recorded as (h) i ，P i ) Obtaining a load-displacement curve; as shown in fig. 3, the load-displacement curve is divided into a linear elastic segment and an elastoplastic segment, the linear elastic segment load-displacement relationship is often represented as a linear relationship, and the elastoplastic segment load-displacement relationship is often represented as a power law relationship;
s02: judging the obtained epsilon in real time eqi ，σ eqi ) Whether located at the junction of the wire elastic segment to the elastoplastic segment: pair (epsilon) eqi ，σ eqi ) Is rounded up by using the corresponding (. Epsilon.) of INT (0.3 i) to INT (0.7 i) eqi ，σ eqi ) The first temporary elastic modulus E is calculated by Hollomon model INT(0.7i) INT (0.7 i) to i corresponding (. Epsilon.) are used eqi ，σ eqi ) The second temporary elastic modulus E is obtained by Hollomon model calculation i The method comprises the steps of carrying out a first treatment on the surface of the When E is INT(0.7i) And E is i When the relative error between the two is more than 10%, the group (. Epsilon.) eqi ，σ eqi ) The corresponding equivalent stress-strain curve deviates from the linear segment, i.e. the group (ε) eqi ，σ eqi ) At the junction point between the elastic section and the elastic-plastic section, defining epsilon at this time eqi Is that
Wherein i is greater than 20;
s03: calculating the equivalent strain epsilon eq Equivalent stress sigma eq Data:
s031: taking a material constant E as 200GPa, carrying out finite element analysis on a component sample to obtain a linear load-displacement curve, and carrying out regression on the linear load-displacement curve to obtain the slope of the load-displacement curve, namely loading stiffness S;
s032: material taking constants E and sigma y Carrying out finite element analysis on a structural element sample by 4 specific materials respectively with 200GPa and 500MPa and n of 0.1, 0.2, 0.3 and 0.4 to obtain a load-displacement curve, and carrying out regression on elastoplastic sections of the 4 load-displacement curves to obtain elastoplastic section loading curvature C and elastoplastic section loading indexes m corresponding to different strain hardening indexes n;
s033: the dimensionless constant k of the load-displacement model is calculated according to the following formula e-1 、k ep-1 、k ep-2 、k 3 、k 4 ，
Wherein K is a strain hardening coefficient, k=e n σ y 1-n ；h y Is the yield displacement; e is the elastic modulus;
s034: k is calculated according to the following formula e-5 、k ep-5 、k e-6 、k ep-6 ，
Wherein k is ep-5 And mean value k ep-5m The error between the two is not more than 3%, and k is calculated ep-5 Approximately equivalent k ep-5m ；
S035: substituting each group of displacement h and load P data into the following formula to calculate and obtain equivalent strain epsilon eq Equivalent stress sigma eq Data (h) i ，P i ) Corresponding calculated equivalent strain epsilon eq Equivalent stress sigma eq The data are recorded as (. Epsilon.) eqi ，σ eqi ) Group i (h i ，P i ) Corresponding calculation to obtain i group (epsilon) eqi ，σ eqi )，
Wherein A is * 、h * The characteristic area and the characteristic displacement of the component samples are respectively determined by simple finite element analysis of different component samples;
is a characteristic yield strain; k (k) e-5 、k ep-5 、k e-6 、k ep-6 Are all equivalent stress and equivalent strain model dimensionless constants, k ep-5m Is k ep-5 Is the average value of (2); the elastic wire section and the elastic plastic section are two deformation stages of the component sample, the stress-strain relationship of the elastic wire section is usually represented as a linear relationship, and the stress-strain relationship of the elastic plastic section is usually represented as a power law relationship;
feature geometry A * And h * For the characteristic area and characteristic displacement related to the geometric dimension of the component sample, a proper A is selected * And h * P/A enabling finite element analysis * ～h/h * The curve clusters overlap, at this time, the constant k in the formula of step S033 e-1 、k ep-1 、k ep-2 、k 3 、k 4 Independent of the geometry of the constituent sample. As shown in FIG. 4 and FIG. 5, the three-point bending TPB and the circular ring compression RC structural member sample, A * Respectively taken as BH 3 /4L 2 And DB (1-r) 2 )，h * Respectively taking L and D as P/A of two component samples * ～h/h * The curves are exactly coincident.
Obtaining a load-displacement curve of the elastoplastic material through finite element calculation, obtaining an elastoplastic section loading curvature C and an elastoplastic section loading index m corresponding to different hardening indexes n through regression of the load-displacement curve of the elastoplastic material in an elastoplastic stage, and determining k through regression of a C-n relation and an m-n relation ep-1 、k ep-2 、k 3 、k 4 The method comprises the steps of carrying out a first treatment on the surface of the Obtaining a load-displacement curve of the linear elastic material through finite element calculation, and returning to the load-displacement curve of the linear elastic part to obtain S, thereby determining k e-1 . The research result shows that for the structural element sample with obvious line elastic deformation stage, k ep-5 Independent of the modulus of elasticity and the yield strength of the material, only weakly related to the hardening exponent of the material, k is obtained according to the formula described in step S034 e-5 、k e-6 、k ep-6 And k corresponding to different hardening indexes n ep-5 The method comprises the steps of carrying out a first treatment on the surface of the Corresponding to different hardening indexes n, k ep-5 And mean value k ep-5m The error between them is not more than 3%, i.e. k as shown in FIG. 6 ep-5m Constant k of approximately substituted equivalent stress and strain model ep-5 . For samples of elements in which no significant linear elastic deformation stage exists, e.g. pressed elements, i.e. cone, sphere, plane elements, k corresponding to different hardening indexes n ep-5 It is difficult to use the mean value k ep-5m Instead, the material hardening index n is presupposed, the preset value is adjusted after n is obtained by the technical scheme of the invention, and finally the material equivalent stress-strain curve is accurately obtained.
Based on dimension analysis, the load and displacement of the component sample and the equivalent stress and equivalent strain of the energy density equivalent unit are directly related.
S04: regression according to Hollomon model form (ε) eqi ，σ eqi ) Obtaining an equivalent stress-strain curve, wherein the Hollomon model is as follows:
in sigma y Is the nominal yield stress. Most metals or alloys, such as aluminum alloys, magnesium alloys, titanium alloys, etc., have uniaxial stress-strain relationships that conform to a good hollloon power law hardening model, and therefore a hollloon model is used.
The invention is based on theoretical deduction, can be suitable for various configuration samples with different sizes and different materials, and can obtain the uniaxial stress-strain relation of the materials through the follow-up, high-efficiency and accurate test load and displacement data pairs.
The accuracy of the stress-strain curve obtained by the technical scheme of the invention is verified:
1. numerical verification was performed with 6 typical test element samples:
and (3) carrying out numerical verification, namely finite element verification, by assuming that some materials are input into finite element analysis software, establishing a component sample test model, then calculating, and obtaining a load-displacement curve, wherein if the test conditions are consistent with the finite element analysis boundary conditions, the finite element calculation load-displacement curve and the test load-displacement curve are consistent.
1. Numerical verification condition
6 representative test element samples were selected: compact tensile CT, single-sided crack bending SEB, small-sized C-shaped tensile CIET with inner side crack, cantilever bending CB, three-point bending TPB, and annular compression RC. FIGS. 7 to 18 show loading diagrams and finite element network models of 6 typical test element samples corresponding to Table 1, wherein Table 1 shows finite element analysis model data of 6 typical test element samples, and Table 2 shows characteristic geometry A of 6 typical test element samples * And h * Table 3 shows the constants of the two models for the 6 typical test element samples determined.
Table 1: finite element analysis model data
Table 2: feature geometry
Table 3: model constant
2. Verification method
Standard examples: inputting uniaxial stress-strain curves of 6 typical test structural elements through finite elements;
test example: by adopting the technical scheme of the invention, uniaxial stress-strain curves of 6 typical test component samples are obtained.
3. Verification result
As shown in fig. 19 to 24, the stress-strain curve obtained in the test example approximates to the stress-strain curve obtained in the standard example, i.e. the prediction accuracy of the technical scheme of the present invention is feasible.
2. Test verification is carried out by using a ring compressed structural element sample:
1. test materials
The test pieces were 10mm in size D, 6mm in size D, 1mm in size B, and the test materials were 30Cr2Ni4MoV steel and P91 steel.
2. Test method
Performing a quasi-static loading test on the ring compression element to obtain a load-displacement curve, as shown in fig. 25;
standard examples: uniaxial tensile stress-strain curve;
test example: the ring compression structural element is processed according to the technical scheme of the invention, and the obtained uniaxial stress-strain curve is obtained.
3. Test results
As shown in fig. 26, the stress-strain curve obtained in the test example is similar to the stress-strain curve obtained in the standard example, i.e. the prediction accuracy of the technical scheme of the present invention is feasible.
The principles and embodiments of the present invention have been described herein with reference to specific examples, which are intended to be merely illustrative of the methods of the present invention and their core ideas. It should be noted that it will be apparent to those skilled in the art that various modifications and adaptations of the invention can be made without departing from the principles of the invention and these modifications and adaptations are intended to be within the scope of the invention as defined in the following claims.
Claims (2)
1. The direct test method for equivalent stress and equivalent strain of the structural element sample under unidirectional loading is characterized by comprising the following steps:
(1) Selecting a component sample, carrying out quasi-static loading on the component sample to obtain test displacement h and load P data, sequentially collecting N data, wherein N is a natural number and is larger than 100, the data number is i, and the i-th group displacement h and load P data are recorded as (h) i ，P i )；
(2) Substituting each group of displacement h and load P data into the following formula to calculate and obtain equivalent strain epsilon eq Equivalent stress sigma eq Data (h) i ，P i ) Corresponding calculated equivalent strain epsilon eq Equivalent stress sigma eq The data are recorded as (. Epsilon.) eqi ，σ eqi )，
Wherein A is * 、h * The characteristic area and the characteristic displacement of the component sample are respectively;
is a characteristic yield strain; k (k) e-5 、k ep-5 、k e-6 、k ep-6 Are all equivalent stress and equivalent strain model dimensionless constants, k ep-5m Is k ep-5 Is the average value of (2); the linear elastic section and the elastic plastic section are two deformation stages of the component sample;
k in the step (2) e-5 、k ep-5 、k e-6 、k ep-6 By means of the value of (2)The simple finite element analysis determines the steps as follows:
(2.1) carrying out finite element analysis on the component sample to obtain a linear load-displacement curve, and carrying out regression on the linear load-displacement curve to obtain the slope of the load-displacement curve, namely loading stiffness S;
(2.2) Material constants E and sigma y Carrying out finite element analysis on a structural element sample by 4 specific materials respectively with 200GPa and 500MPa and n of 0.1, 0.2, 0.3 and 0.4 to obtain a load-displacement curve, and carrying out regression on elastoplastic sections of the 4 load-displacement curves to obtain elastoplastic section loading curvature C and elastoplastic section loading indexes m corresponding to different strain hardening indexes n;
(2.3) calculating according to the following formula to obtain the dimensionless constant k of the load-displacement model e-1 、k ep-1 、k ep-2 、k 3 、k 4 ，
Wherein K is a strain hardening coefficient, k=e n σ y 1-n ；h y Is the yield displacement; e is the elastic modulus;
(2.4) calculating k according to the following formula e-5 、k ep-5 、k e-6 、k ep-6 ，
Wherein k is ep-5 And mean value k ep-5m The error between the two is not more than 3%, and k is calculated ep-5 Approximately equivalent k ep-5m ；
Further comprises: judging the obtained epsilon in real time eqi ，σ eqi ) Whether located at the junction of the wire elastic segment to the elastoplastic segment: pair (epsilon) eqi ，σ eqi ) Is rounded up by using the corresponding (. Epsilon.) of INT (0.3 i) to INT (0.7 i) eqi ，σ eqi ) The first temporary elastic modulus E is calculated by Hollomon model INT(0.7i) By usingINT (0.7 i) to i (ε) eqi ，σ eqi ) The second temporary elastic modulus E is obtained by Hollomon model calculation i The method comprises the steps of carrying out a first treatment on the surface of the When E is INT(0.7i) And E is i When the relative error between the two is more than 10%, the group (. Epsilon.) eqi ，σ eqi ) The corresponding equivalent stress-strain curve deviates from the linear segment, i.e. the group (ε) eqi ，σ eqi ) At the junction point between the elastic section and the elastic-plastic section, defining epsilon at this time eqi Is that
(3) Regression (epsilon) eqi ，σ eqi ) Obtaining an equivalent stress-strain curve;
the regression mode in the step (3) is regression according to a Hollomon model, and the Hollomon model is as follows:
in sigma y Is the nominal yield stress.
2. The method for direct test of equivalent stress and strain of a unidirectionally loaded constituent element sample according to claim 1, wherein the step of determining the obtained (. Epsilon.) in real time eqi ，σ eqi ) Whether i is located in the line elastic segment to elastoplastic segment interface "is greater than 20.
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