CN112485113B - Method and device for testing tensile property of material of small-size sample - Google Patents
Method and device for testing tensile property of material of small-size sample Download PDFInfo
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Abstract
The invention discloses a material tensile property testing method of a small-size sample, which comprises the steps of obtaining stable load-displacement curves of two clamp ends through a uniaxial tensile loading test of the small-size sample under displacement control; establishing finite element simulation models of unequal straight measuring sections of two samples; extracting a test curve, and predicting uniaxial true stress-stress according with Hollomon constitutive model by using load-displacement relation of sample non-equal measurement sectionChanging the relation; simultaneously obtain the elastic modulus E and the tensile strength R of the material m The method comprises the steps of carrying out a first treatment on the surface of the Converting according to the obtained uniaxial true stress-strain relation to obtain an engineering stress-strain curve, thereby obtaining yield strength R p0.2 . The invention solves the problem of sample size limitation of the traditional tensile property test method, has reliable basic theoretical support, is not dependent on an empirical formula, has simple test operation, and can accurately acquire continuous and complete uniaxial stress-strain relation, yield strength and tensile strength of the material by using a small-size sample.
Description
Technical Field
The invention relates to the technical field of mechanical property testing, in particular to a method and a device for testing the tensile property of a material of a small-size sample.
Background
Nuclear fusion energy is considered as a novel sustainable energy source because of its safety, cleanliness, no pollution and inexhaustibility for nuclear fusion fuels. Before the fusion reactor is built, neutron irradiation test is carried out on the structural alternative materials, the mechanical properties of the materials after irradiation are tested, radiation resistance screening is carried out on the structural materials, and whether the materials meet the use requirements of the reactor is judged. However, due to the small irradiation space, high flux volumes such as IFMIF are only 0.5L and multiple samples of stretch, impact, fatigue, fracture toughness etc. need to be placed in a limited volume. Testing using conventional tensile standards (ASTM E8-16a.Standard Test Methods for Tension Testing of Metallic Materials.Annual Book of ASTM Standards.West Conshohocken,PA:American Society for Testing and Materials;2016.) has low space utilization for irradiation and the larger the individual sample volume, the longer the cooling time after irradiation. Thus, minimizing the size of the sample while achieving reliable material properties is an effective way to alleviate the above-mentioned problems. Meanwhile, small samples have larger test requirements for special fields (such as welding structures of aerospace, nuclear power engineering and the like, in-service structures and the like). Meanwhile, small samples have larger test requirements for special fields (such as welding structures of aerospace, nuclear power engineering and the like, in-service structures and the like).
Uniaxial stress-strain relationship, elastic modulus E and yield strength R of material p0.2 Tensile strength R m Is the basic mechanical property of the material, and the mechanical analysis of the engineering structureThe optimal design and the safety evaluation are of great importance, so that the tensile small sample test method of the related material is widely studied. As early as 1981, baja R (Bajaj R, shogan R P, deflash C.Tensile properties of neutron-irradiated nimonic PE [ C)]A flat-shaped tensile sample (Small Specmen-1, SS-1) is provided in EBR-2 material irradiation research projects by// ASTM STP 725.West Conshohocken,PA:ASTM Internation,1981.) and the like, the straight section of the flat-shaped tensile sample is 1.52mm wide and 20.32mm long, and a sample design and test technical thought is provided for the acquisition tensile property research of Small-size flat-shaped samples; kohno Y (Kohno Y, kohyama A, hamilton M, et al Specimen size effects on the tensile proper of JPCA and JFMS [ J)]Journal of nuclear Materials,2000,283-287:10141017.) and the like, on the basis of which the sample size is reduced, SS-J (Small specific-Japan) samples are proposed, which have straight sections of 1.2mm in width and 5mm in length, and the load-displacement curves of the straight sections of the samples and the like are measured to obtain the material tensile properties.
The existing small tensile sample testing technology has a certain problem: 1. the test method lacks an elastoplastic analysis theory, the test content is single, and the stability and accuracy of the test result are not high; 2. the test operation is more inconvenient, and the operation in the hot chamber is not facilitated; 3. the sample size is not small enough, and certain waste exists in saving irradiation space.
Chen Hui, cai Lixun (Chen H, cai L x.unified elastoplastic model based on strain energy equivalence principle [ J ]. Applied Mathematical Modelling, 2017.52:664-671.) propose C-C energy equivalent methods, namely Von Mises equivalent and effective deformation domain energy median equivalent of representative volume units (Representative Volume Element, RVE). According to the functional principle, the external force F acts equal to the total internal strain energy, and the relation between the load-displacement and the stress-strain of the unidirectional loading component is established:
a bridge between the test load-displacement relationship of unidirectional loading and the material stress-strain relationship is theoretically established.
In the measurement technology, a uniaxial tensile test is completed aiming at a small-size plate-shaped sample, and test conditions and support of test technology are provided; a load-displacement semi-analytical prediction model of the related sample geometric dimension and Hollomon model parameters is given; the method can be used for predicting the uniaxial tensile property of the material; however, the test method and the test fixture only aim at samples with specific configurations and sizes, and corresponding prediction model parameters are required to be recalibrated for the configuration sizes of other samples; the C-C energy equivalent method gives a theoretical derivation basis, and has a guiding effect on the acquisition of uniaxial tensile properties.
Disclosure of Invention
Aiming at the technical problems, the invention provides a method and a device for testing the tensile property of a material of a small-size sample, which solve the problem of sample size limitation of the traditional tensile property test method, have reliable basic theoretical support, are not dependent on an empirical formula, are simple and convenient in test operation, and can accurately acquire continuous and complete uniaxial stress-strain relation, yield strength and tensile strength of the material by using the small-size sample.
The invention is realized by the following technical scheme:
a method for testing the tensile properties of a material for a small-sized sample, comprising the steps of:
s1, taking a small-size sample for a tensile test, establishing a finite element simulation model of a non-equal measurement section of the small-size sample, and obtaining a stable load-displacement curve of two clamp ends for clamping the axial two ends of the small-size sample through a uniaxial tensile loading test of the small-size sample under displacement control;
s2, extracting a test curve, and predicting a uniaxial true stress-strain relationship conforming to the Hollomon constitutive model according to a load-displacement relationship of a sample non-equal measurement section; simultaneously obtain the elastic modulus E and the tensile strength R of the material m ；
S21, performing linear fitting on the elastic section of the load-displacement curve to obtain a slope k; performing power law fitting on the elastoplastic segment to obtain a loading coefficient C and an index m:
in the formula (1), F is a test load, and h is displacement;
s22, substituting parameters k, C and m of the formula (1) into the following formula (2):
in the formula (2), E is the elastic modulus of the sample, n is the strain hardening index, K is the strain hardening coefficient, t 0 、t 1 、t 2 、t 3 、t 4 Taking the equal straight section length L as the finite element model constant of the sample 0 The characteristic length h and the characteristic area A are respectively;
s23, substituting E, K, n obtained in the S22 into a Hollomon model to obtain a uniaxial true stress-strain relation of the material:
in the formula (3), sigma T Is true stress, epsilon T Is true strain, sigma y Is the nominal yield strength;
s3, converting the uniaxial true stress-strain relation obtained in the step S2 to obtain an engineering stress-strain curve, and obtaining the yield strength R p0.2 And tensile strength R m 。
Further preferably, in step S3, the obtained true stress-strain relationship is converted into an engineering stress-strain relationship as shown in formula (4):
in the formula (4), ε E 、σ E 、ε T 、σ T Engineering strain, engineering stress, true strain, true stress, respectively.
Further preferably, the yield strength R is determined by crossing the 0.2% bias line with the engineering stress-strain relationship curve P0.2 。
Further preferably, tensile strength R m ＝F max /A，F max For maximum load, A is the cross-sectional area of the straight section of the sample.
Further preferably, the configuration of the small-sized sample is plate-like or round bar-like.
Further preferably, the small-size sample has a size in the range of: total length h=14 mm, clamping segment width s=6 mm, transition radius r=1 mm, equal straight segment length L 0 =1mm; for the plate samples: equal straight section width b=1 mm, thickness t e (0.5 mm,1 mm); for round bar samples: equal straight section diameter d=1 mm.
Further preferably, in step S1, the surface of the small-sized sample is subjected to polishing treatment before the small-sized sample is subjected to tensile test.
The device for testing the material tensile property of the small-size sample is used for realizing the method for testing the material tensile property of the small-size sample, and is characterized in that two groups of clamps are symmetrically distributed, and each group of clamps comprises a bottom plate and a cover plate; one end of the base plate in the long axis direction is provided with a mounting groove which is used for placing a to-be-clamped end of a small-size sample; the cover plate is detachably covered above the mounting groove, and the end to be clamped of the small-size sample is clamped between the mounting groove and the cover plate; the other end of the bottom plate in the long axis direction is used for being arranged between an upper chuck and a lower chuck of the testing machine.
Further preferably, the bottom plate is provided with a transverse plate at the axial end far away from the mounting groove, and the transverse plate is vertically connected with the end part of the bottom plate to form a T-shaped structure.
Further preferably, the positioning plate is detachably mounted at the axial middle part of the bottom plate or at a position close to the axial middle part, and the axial direction of the positioning plate is perpendicular to the axial direction of the bottom plate.
The invention has the following advantages and beneficial effects:
1. the invention solves the problem of sample size limitation of the traditional uniaxial tensile property test method by designing the sample conception and the test calculation method, is independent of an empirical formula, can accurately acquire the continuous and complete stress-strain relation and strength of the material after a small amount of finite element calibration, and is suitable for large-range ductile metal materials;
2. the invention solves the problem that more samples are difficult to place in the space of the irradiation duct, and improves the utilization rate of the irradiation space;
3. the invention designs the small sample stretching clamp matched with the theoretical method, which is convenient and quick to operate;
the invention can be applied to the fields of small-sized components, welded joints, pipeline structures, precious materials, in-service structure monitoring and the like. The sample designed by the invention is in a stretching form, is deformed into single stretching deformation, is similar to the traditional stretching, can directly and accurately acquire the tensile strength of the material, and can also be used for acquiring the fatigue property of the material.
Drawings
The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the invention. In the drawings:
FIG. 1 is a schematic diagram of a sample configuration employed in an embodiment of the present invention; wherein, FIG. 1 (a) shows a plate-like sample, and FIG. 1 (b) shows a rod-like sample;
FIG. 2 is a finite element analysis model of a plate sample according to an embodiment of the present invention;
FIG. 3 is a graph of a Hollomon constitutive model of the present invention;
FIG. 4 is a graph showing load-displacement curves of a CLF-1 steel plate sample in an embodiment of the present invention;
FIG. 5 is a stress-strain curve of a CLF-1 steel plate sample in accordance with an embodiment of the present invention; wherein, fig. 5 (a) shows a uniaxial true stress-strain curve, and fig. 5 (b) shows an engineering stress-strain curve;
fig. 6 is a small sample test fixture provided in the present invention.
In the drawings, the reference numerals and corresponding part names: 1-bottom plate, 2-cover plate, 3-locating plate, 4-sample, 5-loading line, 6-clamping section I, 7-measuring section, 8-clamping section II.
Detailed Description
For the purpose of making apparent the objects, technical solutions and advantages of the present invention, the present invention will be further described in detail with reference to the following examples and the accompanying drawings, wherein the exemplary embodiments of the present invention and the descriptions thereof are for illustrating the present invention only and are not to be construed as limiting the present invention.
Example 1
The embodiment provides a test method for acquiring uniaxial stress-strain relation and strength of a metal material by a small-size sample, wherein the specific test method is as follows:
And 2, mounting the polished small tensile sample in the step 1 to a test fixture, and fixing the test fixture on an upper chuck and a lower chuck of a testing machine on the basis of ensuring coaxiality and fastening connection.
the sample was axially tensile loaded by the tester at a moderate loading rate (0.005 mm.s) -1 Left and right), synchronously collecting real-time load and displacement between chucks, and stretching the sample until the sample breaks.
step 4-1, performing linear fitting on the elastic section of the load-displacement curve to obtain a slope k; performing power law fitting on the elastoplastic segment to obtain a loading coefficient C and an index m:
in the formula (1), F is an external load, and h is displacement;
step 4-2, substituting parameters k, C and m of formula (1) into formula (2) below:
in the formula (2), E is the elastic modulus of the sample, n is the strain hardening index, K is the strain hardening coefficient, t 0 、t 1 、t 2 、t 3 、t 4 Taking the equal straight section length L as the finite element model constant of the sample 0 The characteristic length h and the characteristic area A are respectively;
the configuration size of the sample designed in this embodiment is shown in fig. 1, where the U end is a fixed end and the P end is a displacement loading end. For a Sheet-Type (ST) sample, a=tb, t is the sample thickness, and b is the straight section width of the sample; for Round Bar (RB) samples, A=1/4πDζ 2 D is the diameter of the straight section of the sample. The parameters E, K, n of the material Hollomon constitutive model can be obtained through the formula (2); and substituting the parameter E, K, n into the Hollomon constitutive model to obtain a uniaxial true stress-strain relationship and an elastic modulus E.
Plate-shaped samples or round bar samples can be selected according to test conditions and requirements, and the configuration related parameters of the two samples are shown in table 1:
TABLE 1 list of parameters
For other similar configuration dimensions, only t is recalibrated 0 、t 1 、t 2 、t 3 、t 4 The method can be used, and ANSYS software is used for setting the elastic modulus E=200GPa (can be any certain value of 60 GPa-250 GPa) of the material and the nominal yield strength sigma y =500 MPa (may be any constant value of 200MPa to 1000 MPa), and the transformation strain hardening indexes n are 5 values of 0.1, 0.15, 0.2, 0.25, and 0.3 in this order, respectivelyCalculating to obtain 5 load-displacement curves, and combining the formulas (1) and (2) and knowing E, K, n to obtain the parameter t 0 、t 1 、t 2 、t 3 、t 4 。
Converting the obtained true stress-strain relationship into an engineering stress-strain relationship:
in the formula (3), ε E 、σ E 、ε T 、σ T Engineering strain, engineering stress, true strain, true stress, respectively.
Further, the yield strength R is determined by the intersection of the 0.2% bias line and the engineering stress-strain relationship curve P0.2 。
Further, tensile strength R m ＝F max /A，F max For maximum load, A is the cross-sectional area of the straight section of the sample.
Example 2
The embodiment provides a test device for acquiring the uniaxial stress-strain relationship and the strength of a metal material by using a small-size sample, which is used for the test method for acquiring the uniaxial stress-strain relationship and the strength of the metal material by using the small-size sample provided by the embodiment 1. Two groups of lenses are symmetrically distributed clamps, and each group of clamps comprises a bottom plate 1 and a cover plate 2; one end of the base plate 1 in the long axis direction is provided with a mounting groove which is used for placing a to-be-clamped end of a small-size sample; the cover plate 2 is detachably covered above the mounting groove, the end to be clamped of the small-size sample is clamped between the mounting groove and the cover plate 2, the detachable mounting structure can be connected by threads, a through hole is formed in the cover plate 2, a threaded hole is formed in one end surface (near the mounting groove) of the bottom plate 1, and the threaded hole is screwed into the threaded hole for fixation after the threaded hole is penetrated by a bolt; the other end of the base plate 1 in the long axis direction is used for being installed between an upper chuck and a lower chuck of the testing machine. The bottom plate 1 is provided with a transverse plate at the axial end far away from the mounting groove, and the transverse plate is vertically connected with the end part of the bottom plate 1 to form a T-shaped structure. The positioning plate 3 is detachably arranged at the axial middle part of the bottom plate 1 or near the axial middle part, and the axial direction of the positioning plate 3 is perpendicular to the axial direction of the bottom plate 1; the detachable installation structure of the locating plate 3 and the bottom plate 1 can adopt threaded connection, a through hole is formed in the locating plate 3, a threaded hole is formed in the bottom plate 1, and the locating plate 3 is fixed in the threaded hole through screwing after passing through the through hole by a bolt, so that the locating plate 3 can be detachably fixed on the bottom plate 1.
The using method comprises the following steps: the polished small sample of example 1 was mounted on the apparatus as shown in fig. 6 and fixed to the upper and lower chucks of the tester on the basis of ensuring coaxiality and fastening connection; as shown in fig. 6, the small sample stretching device has two sets of clamps with mirror pairs distributed in a similar manner and identical in structure, and the whole clamp has 6 parts (namely, two bases 1, two cover plates 2 and two positioning plates 3). The clamping section of the small-size sample 4 is placed in the mounting groove of the clamp bottom plate 1, the small-size sample 4 is preliminarily fixed by using the bolts on the cover plate 2, and the other end to be clamped of the small-size sample 4 performs the same operation. Finally, placing the clamp into an upper plate clamp and a lower plate clamp of the testing machine, and ensuring that the clamp is coaxial with a clamping head of the testing machine through a positioning plate 3; if the depth of the clamp of the test machine plate is smaller, the positioning plate 3 can be removed, the small sample clamp and the loading shaft of the test machine can be guaranteed to have good coaxiality through the T-shaped design of the end part of the clamp bottom plate 1, the test machine plate clamp is clamped, then a tensile load of 2N-5N is applied, and then the bolts on the cover plate 2 are screwed, so that the sample installation is completed.
Example 3
The test was performed using the method provided in example 1 and the test jig provided in example 2, and the test method is summarized as follows:
taking a small-sized plate sample and a round bar sample as examples. As shown in fig. 1, the sample is divided into a non-uniform measuring section and a clamping section. As shown in FIG. 2, two types of finite element simulation models of unequal straight measuring sections of samples are established, one end of each finite element simulation model is fixedly hinged, and the other end of each finite element simulation model is axially stretched and loaded. The Hollomon constitutive model (shown in figure 3) is adopted as a constitutive relation of a simulation material, the model comprises an elastic modulus E, a strain hardening index n and a strain hardening coefficient K, various working conditions can be calculated by changing the three parameters, load-displacement curves of different imaginary materials are obtained, and the calibration of constants in a formula is completed.
Uniaxial tensile test under displacement control was completed using a plate-shaped sample of CLF-1 (Chinese Low activation Ferritic) steel, with a transitional circular arc radius r=1 mm, a sample thickness t=0.5 mm, and an equal straight length L 0 1mm, width b=1 mm, non-equidistant measurement segment l=6 mm; the uniaxial tensile test was performed on 3 parallel samples, the relative displacement h of the clamps at both ends was collected, and a continuous and stable load-displacement test curve was collected in fig. 5.
Fitting a linear segment of a curve by using a linear function, fitting an elastoplastic curve segment by using a power law function to obtain a slope K, a loading coefficient C and an index m, combining a formula to obtain a parameter elastic modulus E, a strain hardening coefficient K and a strain hardening index n of the Hollomon constitutive model, and substituting E, K, n into the Hollomon model to obtain a uniaxial true stress-strain relation prediction result of the material; by the formula R m ＝F max Calculated by A to obtain the tensile strength R m The method comprises the steps of carrying out a first treatment on the surface of the Simultaneously carrying out a uniaxial tensile test on a CLF-1 steel Standard Round Bar (SRB) sample to obtain a uniaxial true stress-strain relationship of the CLF-1 steel; as shown in fig. 5 (a), the uniaxial true stress-strain curve of 3 parallel samples (referring to 3 plate-like small samples of the same configuration and size) substantially coincides with the results of the standard round bar; as shown in FIG. 5 (b), the resulting true stress-strain relationship is converted into an engineering stress-strain relationship to obtain yield strength R p0.2 . In practical engineering use, the sample size can be adjusted by simply calculating parameters of the calibration prediction model again. The results of the CLF-1 steel test are shown in Table 2.
TABLE 2 list of predicted results
The invention designs a small-size plate-shaped and round bar stretching sample and a special fixture, which can carry out the small sampleAccording to the uniaxial tensile test of the product and the load-displacement curve of the unequal straight measuring section, the uniaxial true stress-strain curve corresponding to the material can be predicted, and meanwhile, the yield strength R of common engineering parameters can be obtained p0.2 And tensile strength R m The method comprises the steps of carrying out a first treatment on the surface of the The invention solves the problem of sample size limitation, has reliable basic theoretical support, and can acquire continuous and complete stress-strain relation without depending on an empirical formula. The method has great engineering application value for obtaining the uniaxial tensile property of materials in key fields such as small-sized components, welded joints, pipeline structures, precious materials, in-service structure monitoring and the like.
The foregoing description of the embodiments has been provided for the purpose of illustrating the general principles of the invention, and is not meant to limit the scope of the invention, but to limit the invention to the particular embodiments, and any modifications, equivalents, improvements, etc. that fall within the spirit and principles of the invention are intended to be included within the scope of the invention.
Claims (10)
1. A method for testing the tensile properties of a material for a small-sized sample, comprising the steps of:
s1, taking a small-size sample for a tensile test, establishing a finite element simulation model of a non-equal measurement section of the small-size sample, and obtaining a stable load-displacement curve of two clamp ends for clamping the axial two ends of the small-size sample through a uniaxial tensile loading test of the small-size sample under displacement control;
s2, extracting a test curve, and predicting a uniaxial true stress-strain relationship conforming to the Hollomon constitutive model according to a load-displacement relationship of a sample non-equal measurement section; simultaneously obtain the elastic modulus E and the tensile strength R of the material m ；
S21, performing linear fitting on the elastic section of the load-displacement curve to obtain a slope k; performing power law fitting on the elastoplastic segment to obtain a loading coefficient C and an index m:
in the formula (1), F is a test load, and h is displacement;
s22, substituting parameters k, C and m of the formula (1) into the following formula (2):
in the formula (2), E is the elastic modulus of the sample, n is the strain hardening index, K is the strain hardening coefficient, t 0 、t 1 、t 2 、t 3 、t 4 Taking the equal straight section length L as the finite element model constant of the sample 0 The characteristic length h and the characteristic area A are respectively;
s23, substituting E, K, n obtained in the S22 into a Hollomon model to obtain a uniaxial true stress-strain relation of the material:
in the formula (3), sigma T Is true stress, epsilon T Is true strain, sigma y Is the nominal yield strength;
s3, converting the uniaxial true stress-strain relation obtained in the step S2 to obtain an engineering stress-strain curve, and obtaining the yield strength R p0.2 And tensile strength R m 。
2. The method for testing the tensile properties of a material for a small-sized sample according to claim 1, wherein in step S3, the obtained true stress-strain relationship is converted into an engineering stress-strain relationship as shown in formula (4):
in the formula (4), ε E 、σ E 、ε T 、σ T Engineering strain, engineering stress, true strain, true stress, respectively.
3. The method for testing the tensile properties of a material for a small-sized sample according to claim 2, wherein the yield strength R is determined by crossing a 0.2% bias line with an engineering stress-strain relationship curve P0.2 。
4. The method for testing the tensile properties of a material for a small-sized sample according to claim 1, wherein the tensile strength R m ＝F max /A，F max For maximum load, A is the cross-sectional area of the straight section of the sample.
5. The method for testing the tensile properties of a material of a small-sized sample according to claim 1, wherein the small-sized sample has a plate-like or round bar-like configuration.
6. The method for testing the tensile properties of a material of a small-sized sample according to claim 1 or 5, wherein the small-sized sample has a size ranging from: total length h=14 mm, clamping segment width s=6 mm, transition radius r=1 mm, equal straight segment length L 0 =1mm; for the plate samples: equal straight section width b=1 mm, thickness t e (0.5 mm,1 mm); for round bar samples: equal straight section diameter d=1 mm.
7. The method for testing the tensile properties of a material for a small-sized sample according to claim 1, wherein in step S1, the surface of the small-sized sample is polished before the small-sized sample is subjected to the tensile test.
8. A material tensile properties testing device for small-sized samples for realizing a material tensile properties testing method for small-sized samples according to any one of claims 1 to 7, characterized in that two sets of clamps are arranged in pairs, each set of clamps comprising a bottom plate (1) and a cover plate (2); one end of the base plate (1) in the long axis direction is provided with a mounting groove which is used for placing a to-be-clamped end of a small-size sample; the cover plate (2) is detachably covered above the mounting groove, and the end to be clamped of the small-size sample is clamped between the mounting groove and the cover plate (2); the other end of the base plate (1) in the long axis direction is used for being installed between an upper chuck and a lower chuck of the testing machine.
9. The device for testing the tensile properties of materials of small-sized samples according to claim 8, wherein the bottom plate (1) is provided with a transverse plate at the axial end far away from the mounting groove, and the transverse plate is vertically connected with the end part of the bottom plate (1) to form a T-shaped structure.
10. The device for testing the tensile properties of materials of small-sized samples according to claim 1, further comprising a positioning plate (3), wherein the positioning plate (3) is detachably mounted at the axial middle part of the bottom plate (1) or at a position close to the axial middle part, and the axial direction of the positioning plate (3) is perpendicular to the axial direction of the bottom plate (1).
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