CN113722957A - Direct test method for equivalent stress and equivalent strain of unidirectional-loading lower-component sample - Google Patents

Direct test method for equivalent stress and equivalent strain of unidirectional-loading lower-component sample Download PDF

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CN113722957A
CN113722957A CN202110991624.4A CN202110991624A CN113722957A CN 113722957 A CN113722957 A CN 113722957A CN 202110991624 A CN202110991624 A CN 202110991624A CN 113722957 A CN113722957 A CN 113722957A
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蔡力勋
韩光照
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Southwest Jiaotong University
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Abstract

The invention discloses an equivalent stress and equivalent strain direct test method of a single-direction loading lower element sample, which relates to the technical field of material mechanical property test and comprises 4 steps of obtaining a load-displacement curve, calculating strain and 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 is different from an experience correlation method and does not need an experience formula; secondly, the method is different from a finite element auxiliary test method, and does not need complex iterative calculation; thirdly, different from a database establishing method, a large amount of finite element calculation is not needed; and fourthly, different from an analytic or semi-analytic method, the constitutive relation parameters of the material can be solved without the help of material information contained in the complete load-displacement curve, and the stress-strain curve of the material can be directly obtained according to load-displacement data points obtained by tests. The whole method is suitable for various structure element samples with different sizes and different materials, and has universality.

Description

Direct test method for equivalent stress and equivalent strain of unidirectional-loading lower-component sample
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 component sample under unidirectional loading.
Background
The stress-strain relationship, the elastic modulus, the yield strength, the 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 structure element samples and the mechanical properties of the material is crucial to structural design and safety evaluation. A common method for obtaining the uniaxial constitutive relation of a material is to cut a centimeter-level standard uniaxial tensile sample from an engineering structure or a parent material and perform a tensile test in a laboratory. At present, 4 technical routes mainly exist for acquiring basic mechanical properties of materials aiming at various nano-test components.
Experience correlation method: based on the test results of a large number of standard tensile tests and small-size component samples, empirical conversion relations are established between the yield strength and the tensile strength of the materials obtained by correlating the large-size standard tensile samples and the bending load and the ultimate load loaded by the small-size component samples, and the essence of the elastic-plastic deformation behavior of the materials is difficult to reveal by the empirical formulas.
Finite element auxiliary test method: and (3) obtaining a simulated load-displacement curve of the structural element sample by finite element analysis of a uniaxial equivalent stress-strain curve of a preset material in a finite element, taking the test load-displacement curve as an iteration target, and adjusting the preset material equivalent force-strain curve by comparing the simulated load-displacement curve with the test load-displacement curve. And 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 a test and finite element analysis, and iterative calculation needs to be carried out by combining the finite element analysis.
A database establishment method comprises the following steps: and (3) carrying out a large amount of finite element analysis on the materials with different constitutive relation parameters by means of finite element software to establish a structural element sample load-displacement relation database. And loading the structure 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 the constitutive relation model. The method avoids the defect that finite element iteration is carried out in a single test of a finite element auxiliary test method, but the establishment of a database method can not get rid of a large amount of finite element calculation.
Analytical or semi-analytical methods: for the statics problem of the linear elastic construction element, the method represented by the card theorem, the virtual work principle and the like can simply and directly realize the analysis and solution of the problems of the rod, the beam, the crack body and the like, and the analysis and solution is very difficult because the elastic-plastic problem relates to the nonlinearity, the boundary condition and the like of the material. The method has universality on various typical test elements, but the constitutive relation parameters of the material need to be solved by virtue 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 by tests.
Disclosure of Invention
The invention aims to: aiming at the existing problems, the direct test method for the equivalent stress and the equivalent strain of the element sample under the unidirectional loading of the stress-strain curve of the material is provided without an empirical formula, iteration and a large amount of calculation, has universality and is directly obtained according to load and displacement data points obtained by the test.
The technical scheme adopted by the invention is as follows:
a direct test method for equivalent stress and equivalent strain of a component sample under unidirectional loading comprises the following steps:
(1) selecting a structure element sample, carrying out quasi-static loading on the structure element sample to obtain test displacement h and load P data, acquiring N data in sequence, numbering the data to obtain i, and recording the ith group of displacement h and load P data as (h)i，Pi)；
(2) Substituting each group of displacement h and load P data into the following formula to calculate the equivalent strain epsiloneqEquivalent stress σeqData, (h)i，Pi) Equivalent strain epsilon obtained by corresponding calculationeqEquivalent stress sigmaeqData are expressed as (ε)eqi， σeqi)，
Figure BDA0003232544770000031
In the formula, A*、h*Respectively representing the characteristic area and the characteristic displacement of the element sample;
Figure BDA0003232544770000032
is a characteristic yield strain;
ke-5、kep-5、ke-6、kep-6are all equivalent stress and equivalent strain models with dimensionless constant, kep-5mIs kep-5The mean value of (a); the linear elastic section and the elastic-plastic section are two deformation stages of the component sample;
(3) regression (ε)eqi，σeqi) And obtaining an equivalent stress-strain curve.
Preferably, k in step (2)e-5、kep-5、ke-6、kep-6The value of (a) is determined by means of simple finite element analysis, comprising the steps of:
(2.1) carrying out finite element analysis on the element sample to obtain a linear load-displacement curve, and regressing the linear load-displacement curve to obtain the slope of the load-displacement curve, namely loading rigidity S;
(2.2) taking 4 specific materials to perform finite element analysis on the component sample to obtain a load-displacement curve, and performing regression on the elastoplastic sections of the 4 load-displacement curves to obtain an elastoplastic section loading curvature C and an elastoplastic section loading index m corresponding to different strain hardening indexes n;
(2.3) calculating a dimensionless constant k of the load-displacement model according to the following formula6-1、kep-1、kep-2、 k3、k4，
Figure BDA0003232544770000033
Wherein K is the strain hardening coefficient, and K ═ Enσy 1-n；hyIs the yield displacement; e is the modulus of elasticity;
(2.4) k is calculated according to the following formulae-5、kep-5、ke-6、kep-6，
Figure BDA0003232544770000041
In the formula, kep-5And the mean value kep-5mThe error between k and k does not exceed 3%ep-5Approximately equal kep-5m。
Preferably, the regression mode in step (3) is regression according to a Hollomon model, and the Hollomon model is as follows:
Figure BDA0003232544770000042
in the formula, σyIs the nominal yield stress.
Preferably, the method further comprises the following step after the step (1): determined in real time aseqi，σeqi) Whether the line is positioned at the junction point from the linear elastic section to the elastic-plastic section: to (epsilon)eqi，σeqi) Is rounded by the corresponding (. epsilon.) from INT (0.3i) to INT (0.7i)eqi，σeqi) Calculating to obtain a first temporary elastic modulus E through a Hollomon model formulaINT(0.7i)Using (epsilon) corresponding to INT (0.7i) to ieqi，σeqi) Obtaining a second temporary elastic modulus E through Hollomon model type calculationi(ii) a When E isINT(0.7i)And EiWhen the relative error between the two exceeds 10%, the group is judged to be (epsilon)eqi，σeqi) The corresponding equivalent stress-strain curve deviates from the linear section, i.e. the set (ε)eqi，σeqi) At the boundary point between the linear elastic segment and the elastic-plastic segment, defining the epsiloneqiIs composed of
Figure BDA0003232544770000043
Preferably, step "real-time determination of (. epsilon.)eqi，σeqi) I is greater than 20 in the "line elastic to elastoplastic segment intersection".
Preferably, N in step (1) is a natural number and is greater than 100.
Preferably, i groups of said (h) in step (2)i，Pi) Corresponding calculation is carried out to obtain i groups (epsilon)eqi，σeqi)。
In summary, due to the adoption of the technical scheme, the invention has the beneficial effects that: and (3) obtaining displacement and load data points through tests, substituting the displacement and load data points into the formula in the step (2) to obtain stress and strain data points, and performing regression to obtain an equivalent stress-strain curve. The whole process is a process for acquiring an equivalent stress-strain curve in real time, and is different from an experience correlation method and does not need an experience formula; secondly, the method is different from a finite element auxiliary test method, and does not need complex iterative calculation; thirdly, different from a database establishing method, a large amount of finite element calculation is not needed; and fourthly, different from an analytic or semi-analytic method, the constitutive relation parameters of the material can be solved without the help of material information contained in the complete load-displacement curve, and the stress-strain curve of the material can be directly obtained according to load-displacement data points obtained by tests. The whole method is suitable for various structure element samples with different sizes and different materials, and has universality.
Drawings
FIG. 1 shows load-displacement data points obtained by the present invention.
Fig. 2 is a graph of stress-strain data points obtained from load-displacement data points in the present invention.
Fig. 3 is a load-displacement curve obtained by the technical scheme of the invention.
Fig. 4 is a load-displacement curve of a three-point bending TPB structural element sample after normalization processing.
FIG. 5 is a load-displacement curve after normalization with a sample of ring compression RC elements.
FIG. 6 shows k for different hardening indices nep-5And the mean value kep-5mThe error between indicates the curve.
FIG. 7 is a schematic diagram of the loading of compact tensile CT in numerical verification with 6 representative test element samples.
FIG. 8 is a finite element mesh model of compact tensile CT in numerical verification with 6 representative test element specimens.
Fig. 9 is a schematic view of the loading of single crack bend SEB in numerical verification with 6 typical test element specimens.
Fig. 10 is a finite element mesh model of single edge crack bending SEB in numerical verification with 6 typical test element samples.
Fig. 11 is a graph showing the loading of small-size C-stretch CIET with inside edge cracks in numerical verification with 6 representative test element specimens.
Fig. 12 is a finite element mesh model of small size C-tensile CIET with inside edge cracks in numerical verification with 6 representative test element specimens.
FIG. 13 is a schematic illustration of cantilever flexure CB loading during numerical verification with 6 representative test element samples.
FIG. 14 is a finite element mesh model of cantilever flexure CB in numerical verification with 6 representative test element samples.
FIG. 15 is a schematic diagram of the loading of three-point bend TPB in numerical verification with 6 representative test element specimens.
FIG. 16 is a finite element mesh model of three-point bend TPB in numerical verification with 6 representative test element specimens.
FIG. 17 is a schematic diagram of the loading of the ring compression RC in numerical verification with 6 typical test element samples.
FIG. 18 is a finite element mesh model of the ring compression RC in numerical verification with 6 representative test element samples.
FIG. 19 is a graph comparing the stress-strain curves obtained for the test example and the standard example for compact tensile CT in numerical validation with 6 representative test element specimens.
Fig. 20 is a graph comparing the stress-strain curves obtained in the test example of the one-sided crack-bending SEB in the numerical verification with those obtained in the standard example using 6 typical test member samples.
Fig. 21 is a graph comparing the stress-strain curves obtained from the test examples and the standard examples for numerically verifying the C-shaped tensile CIET with the small-size inside edge cracks using 6 representative test element specimens.
Fig. 22 is a graph showing a comparison of stress-strain curves obtained in the test example for the cantilever bending CB in the numerical verification with those obtained in the standard example using 6 typical test element samples.
Fig. 23 is a graph showing a comparison of stress-strain curves obtained in the test example of three-point bending TPB in the numerical verification using 6 typical test element samples and stress-strain curves obtained in the standard example.
FIG. 24 is a graph showing a comparison of stress-strain curves obtained in the experimental example and standard example for the ring compression RC in the numerical verification using 6 typical test element samples.
Fig. 25 is a load-displacement curve obtained by performing a quasi-static loading test on a ring compression element in a test verification performed on a ring compression element sample.
FIG. 26 is a graph showing a comparison between the stress-strain curves obtained in the test example and the stress-strain curves obtained in the standard example in the test verification using the ring compression structural element sample.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings.
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Referring to fig. 1, a direct test method for equivalent stress and equivalent strain of a unidirectional-loading lower-element sample includes the following steps:
s01: selecting a structure element sample, carrying out quasi-static loading on the structure element sample to obtain test displacement h and load P data, sequentially collecting N data, wherein N is a natural number and is more than 100, the data number is i, and the ith group of displacement h and load P data are recorded as (h)i，Pi) Obtaining a load-displacement curve; as shown in fig. 3, the load-displacement curve is divided into a linear elastic section and an elastic-plastic section, the load-displacement relationship of the linear elastic section is usually expressed as a linear relationship, and the load-displacement relationship of the elastic-plastic section is usually expressed as a power law relationship;
S02: determined in real time aseqi，σeqi) Whether the line is positioned at the junction point from the linear elastic section to the elastic-plastic section: to (epsilon)eqi，σeqi) Is rounded by the corresponding (. epsilon.) from INT (0.3i) to INT (0.7i)eqi， σeqi) Calculating to obtain a first temporary elastic modulus E through a Hollomon model formulaINT(0.7i)Using (epsilon) corresponding to INT (0.7i) to ieqi，σeqi) Obtaining a second temporary elastic modulus E through calculation of a Hollomon model formulai(ii) a When E isINT(0.7i)And EiWhen the relative error between the two exceeds 10%, the group is judged to be (epsilon)eqi，σeqi) The corresponding equivalent stress-strain curve deviates from the linear section, i.e. the set (ε)eqi，σeqi) At the boundary point between the linear elastic segment and the elastic-plastic segment, defining the epsiloneqiIs composed of
Figure BDA0003232544770000081
Wherein i is greater than 20;
s03: calculating the equivalent strain εeqEquivalent stress sigmaeqData:
s031: taking the material constant E as 200GPa, carrying out finite element analysis on the component sample to obtain a linear load-displacement curve, and regressing the linear load-displacement curve to obtain the slope of the load-displacement curve, namely the loading rigidity S;
s032: taking the material constants E and sigmayCarrying out finite element analysis on the component sample by 4 specific materials respectively at 200GPa and 500MPa, wherein n is 0.1, 0.2, 0.3 and 0.4 respectively to obtain a load-displacement curve, and regressing the elastoplastic sections of the 4 load-displacement curves to obtain the elastoplastic section loading curvature C and the elastoplastic section loading index m corresponding to different strain hardening indexes n;
s033: the dimensionless constant k of the load-displacement model is calculated according to the following formulae-1、kep-1、kep-2、 k3、k4，
Figure BDA0003232544770000082
Wherein K is the strain hardening coefficient, and K ═ Enσy 1-n；hyIs the yield displacement; e is the modulus of elasticity;
s034: k is calculated according to the following formulae-5、kep-5、ke-6、kep-6，
Figure BDA0003232544770000091
In the formula, kep-5And the mean value kep-5mThe error between k and k does not exceed 3%ep-5Approximately equal kep-5m；
S035: substituting each group of displacement h and load P data into the following formula to calculate the equivalent strain epsiloneqEquivalent stress sigmaeqData, (h)i，Pi) Equivalent strain epsilon obtained by corresponding calculationeqEquivalent stress sigmaeqData are expressed as (ε)eqi， σeqi) Group i (h)i，Pi) Corresponding calculation is carried out to obtain i groups (epsilon)eqi，σeqi)，
Figure BDA0003232544770000092
In the formula, A*、h*Respectively determining the characteristic area and the characteristic displacement of the element sample by simply performing finite element analysis on different element samples;
Figure BDA0003232544770000093
is a characteristic yield strain; k is a radical ofe-5、kep-5、ke-6、kep-6Are all equal effective stress and equivalent strain model dimensionless constant, kep-5mIs kep-5The mean value of (a); the linear elastic section and the elastic-plastic section are two deformation stages of the structure element sample, the stress-strain relation of the linear elastic section is usually expressed as a linear relation, and the stress-strain relation of the elastic-plastic section is usually expressed as a power law relation;
characteristic geometric quantity A*And h*Selecting proper A for the characteristic area and characteristic displacement related to the geometric dimension of the element sample*And h*Making P/A of finite element analysis*～h/h*The curve clusters coincide, and the constant k in the formula in step S033e-1、kep-1、kep-2、k3、k4Are independent of the geometry of the construction sample. As shown in fig. 4 and 5, the three-point bending TPB and ring compression RC structural element samples, a*Are respectively taken as BH3/4L2And DB (1-r)2)， h*Taking the P/A of two constitutional element samples as L and D respectively*～h/h*The curves are precisely matched.
Obtaining a load-displacement curve of the elastic-plastic material through finite element calculation, obtaining an elastic-plastic section loading curvature C and an elastic-plastic section loading index m corresponding to different hardening indexes n by regressing the load-displacement curve at the elastic-plastic stage, and determining k through the regression C-n relation and the m-n relationep-1、kep-2、k3、k4(ii) a Obtaining a load-displacement curve of the linear elastic material through finite element calculation, obtaining S by regressing the load-displacement curve of the elastic part of the line, and further determining ke-1. The research result shows that for the component sample with obvious linear elastic deformation stage, kep-5K is obtained according to the formula of step S034, regardless of the elastic modulus and yield strength of the material, and only weakly related to the hardening index of the materiale-5、 ke-6、kep-6And k corresponding to different hardening indexes nep-5(ii) a Corresponding to different hardening indexes n, kep-5And the mean value kep-5mThe error between them does not exceed 3%, as shown in FIG. 6, i.e., kep-5mConstant k approximately replacing equivalent stress, strain modelep-5. For the structural element samples without obvious linear elastic deformation stage, such as pressed structural elements, i.e. cone, sphere and plane structural elements, corresponding to k of different hardening indexes nep-5Difficulty in adopting the mean value kep-5mAnd therefore, assuming a material hardening index n in advance, adjusting a preset value after obtaining n by the technical scheme of the invention, and finally accurately obtaining the equivalent stress-strain curve of the material.
The equation is based on dimensional analysis, and directly relates the load and displacement of the structural element sample and the equivalent stress and equivalent strain of the energy density equivalent unit.
S04: regression (epsilon) according to Hollomon modeleqi，σeqi) And obtaining an equivalent stress-strain curve, wherein the Hollomon model formula is as follows:
Figure BDA0003232544770000101
in the formula, σyIs 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 Hollomon power-law hardening model, and thus, Hollomon model types are used.
The method is based on theoretical derivation, can be suitable for various types of configuration samples with different sizes and different materials, and can efficiently and accurately obtain the uniaxial stress-strain relation of the material through test load and displacement data pairs in a follow-up manner.
The accuracy of the stress-strain curve obtained by the technical scheme of the invention is verified:
first, the numerical verification was performed with 6 typical test element samples:
and (3) performing numerical verification, namely finite element verification, establishing a structural element sample test model by assuming that some materials are input into finite element analysis software, then calculating to obtain a load-displacement curve, and if the test conditions are consistent with the boundary conditions of the finite element analysis, the load-displacement curve of the finite element calculation and the test load-displacement curve are consistent.
1. Condition for verifying numerical value
6 typical test element samples were selected: compact tensile CT, unilateral crack bending SEB, small-size C-shaped tensile CIET with an internal side crack, cantilever bending CB, three-point bending TPB and circular ring compression RC. Fig. 7 to 18 are schematic diagrams showing loading and finite element network models of 6 typical test element samples corresponding to table 1, where table 1 is finite element analysis model data of the 6 typical test element samples, and table 2 is characteristic geometric quantity a of the 6 typical test element samples*And h*Table 3 shows the constants of the two models determined for the 6 typical test element samples.
Table 1: finite element analysis model data
Figure BDA0003232544770000111
Figure BDA0003232544770000121
Table 2: characteristic geometric quantity
Figure BDA0003232544770000122
Table 3: model constants
Figure BDA0003232544770000123
2. Verification method
The standard example: inputting uniaxial stress-strain curves of 6 typical test structural elements through finite elements;
test example: the technical scheme of the invention is adopted to obtain the uniaxial stress-strain curves of the 6 typical test element samples.
3. Verification result
As shown in fig. 19 to 24, the stress-strain curve obtained in the experimental example is similar to the stress-strain curve obtained in the standard example, that is, the prediction accuracy of the technical solution of the present invention is feasible.
Secondly, carrying out test verification by using a circular ring compression component sample:
1. test materials
The specimen dimensions D were 10mm, D6 mm, B1 mm, and the test materials were 30Cr2Ni4MoV steel and P91 steel.
2. Test method
Performing a quasi-static loading test on the annular compression elements to obtain a load-displacement curve, as shown in fig. 25;
the standard example: uniaxial tensile stress-strain curves;
test example: the annular compression structural element is processed according to the technical scheme of the invention to obtain a uniaxial stress-strain curve.
3. Test results
As shown in fig. 26, the stress-strain curve obtained in the experimental example is similar to the stress-strain curve obtained in the standard example, that is, the prediction accuracy of the technical solution of the present invention is feasible.
The principles and embodiments of the present invention are explained herein using specific examples, which are set forth only to help understand the method and its core ideas of the present invention. It should be noted that, for those skilled in the art, without departing from the principle of the present invention, the present invention can be subject to several improvements and modifications, which also fall into the protection scope of the present claims.

Claims (7)

1. A direct test method for equivalent stress and equivalent strain of a component sample under unidirectional loading is characterized by comprising the following steps:
(1) selecting a structure element sample, carrying out quasi-static loading on the structure element sample to obtain test displacement h and load P data, acquiring N data in sequence, numbering the data to obtain i, and recording the ith group of displacement h and load P data as (h)i，Pi)；
(2) Substituting each group of displacement h and load P data into the following formula to calculate the equivalent strain epsiloneqEquivalent stress sigmaeqData, (h)i，Pi) Equivalent strain epsilon obtained by corresponding calculationeqEquivalent stress sigmaeqData are expressed as (ε)eqi，σeqi)，
Figure FDA0003232544760000011
In the formula, A*、h*Respectively representing the characteristic area and the characteristic displacement of the element sample;
Figure FDA0003232544760000012
is a characteristic yield strain; k is a radical ofe-5、kep-5、ke-6、kep-6Are all equivalent stress and equivalent strain models with dimensionless constant, kep-5mIs kep-5The mean value of (a); the linear elastic section and the elastic-plastic section are two deformation stages of the component sample;
(3) regression (ε)eqi，σeqi) And obtaining an equivalent stress-strain curve.
2. The method for directly testing the equivalent stress and the equivalent strain of the unidirectional-loading lower structural element sample as claimed in claim 1, wherein k in the step (2)e-5、kep-5、ke-6、kep-6The values of (a) are determined by means of simple finite element analysis, comprising the steps of:
(2.1) carrying out finite element analysis on the element sample to obtain a linear load-displacement curve, and regressing the linear load-displacement curve to obtain the slope of the load-displacement curve, namely the loading rigidity S;
(2.2) taking 4 specific materials to perform finite element analysis on the component sample to obtain a load-displacement curve, and performing regression on the elastoplastic sections of the 4 load-displacement curves to obtain an elastoplastic section loading curvature C and an elastoplastic section loading index m corresponding to different strain hardening indexes n;
(2.3) calculating a dimensionless constant k of the load-displacement model according to the following formulae-1、kep-1、kep-2、k3、k4，
Figure FDA0003232544760000021
Wherein K is the strain hardening coefficient, and K ═ Enσy 1-n；hyIs the yield displacement; e is the modulus of elasticity;
(2.4) k is calculated according to the following formulae-5、kep-5、ke-6、kep-6，
Figure FDA0003232544760000022
In the formula, kep-5And the mean value kep-5mThe error between k and k does not exceed 3%ep-5Approximately equal kep-5m。
3. The method for directly testing the equivalent stress and the equivalent strain of the one-way loaded lower-component test sample according to claim 1, wherein the regression manner in the step (3) is according to a Hollomon model formula:
Figure FDA0003232544760000023
in the formula, σyIs the nominal yield stress.
4. The method for directly testing the equivalent stress and the equivalent strain of the unidirectional-loading lower-component sample according to claim 1, wherein the step (1) is followed by the steps of: determined in real time aseqi，σeqi) Whether the line is positioned at the junction point from the linear elastic section to the elastic-plastic section: to (epsilon)eqi，σeqi) Is rounded by the corresponding (. epsilon.) from INT (0.3i) to INT (0.7i)eqi，σeqi) Calculating to obtain a first temporary elastic modulus E through a Hollomon model formulaINT(0.7i)Using (epsilon) corresponding to INT (0.7i) to ieqi，σeqi) Obtaining a second temporary elastic modulus E through calculation of a Hollomon model formulai(ii) a When E isINT(0.7i)And EiWhen the relative error between the two exceeds 10%, the group is judged to be (epsilon)eqi，σeqi) The corresponding equivalent stress-strain curve deviates from the linear section, i.e. the set (ε)eqi，σeqi) At the boundary point between the linear elastic segment and the elastic-plastic segment, defining the epsiloneqiIs composed of
Figure FDA0003232544760000031
5. The method for directly testing equivalent stress and equivalent strain of a unidirectional-loading lower-element sample as claimed in claim 4, wherein the step of "real-time judging to obtain (epsilon)eqi，σeqi) I is greater than 20 in the "line elastic to elastoplastic segment intersection".
6. The method for directly testing the equivalent stress and the equivalent strain of the unidirectional-loading lower structural element sample as claimed in claim 1, wherein N in the step (1) is a natural number and is more than 100.
7. The method for directly testing the equivalent stress and the equivalent strain of the unidirectional-loading lower structural element sample as claimed in claim 1, wherein in the step (2), the i group and the h group in the step (2) arei，Pi) Corresponding calculation is carried out to obtain i groups (epsilon)eqi，σeqi)。
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