CN113218796B - Method for detecting relation between strain and hardness of material - Google Patents
Method for detecting relation between strain and hardness of material Download PDFInfo
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- CN113218796B CN113218796B CN202110411198.2A CN202110411198A CN113218796B CN 113218796 B CN113218796 B CN 113218796B CN 202110411198 A CN202110411198 A CN 202110411198A CN 113218796 B CN113218796 B CN 113218796B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/40—Investigating hardness or rebound hardness
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
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Abstract
A material strain-hardness relation detection method comprises the steps of utilizing a sample to be detected capable of obtaining large strain, carrying out paint spraying treatment on an axial side plane of a semi-tubular sample with a spherical pit, vertically placing the semi-tubular sample after paint spraying at the center of an upper die and a lower die, extruding from top to bottom to generate compression deformation, and obtaining strain in-situ measurement data by collecting deformation images while deforming; cutting the semi-tubular sample subjected to compression deformation, embedding the cut semi-tubular sample into phenolic resin for hardness testing, and fitting after correcting hardness data to obtain a strain-hardness relation. The invention combines the digital image technology to measure the strain distribution of the sample in real time, directly measures the surface hardness distribution of the sample after deformation, and determines the strain-hardness relation of the material based on the measured strain and hardness data.
Description
Technical Field
The invention relates to a technology in the field of material detection, in particular to a method for detecting a relation between strain and hardness of a material.
Background
For precision parts formed and manufactured by a plastic large deformation process, the hardness after forming is generally used as an important inspection index of the performance of the precision parts. The determination of the relation between the strain and the hardness of the material can predict the hardness of the deformed part, thereby guiding the design of the part and the plastic deformation process thereof. At present, the accuracy of the determination method based on finite element simulation is insufficient.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a method for detecting the relation between strain and hardness of a material, which utilizes a sample to be detected capable of obtaining large strain, combines a digital image technology to determine the strain distribution of the sample in real time, directly measures the surface hardness distribution of the sample after deformation, and determines the relation between strain and hardness of the material on the basis of the measured strain and hardness data.
The invention is realized by the following technical scheme:
the invention relates to a method for detecting a relation between strain and hardness of a material, which comprises the steps of spraying paint on an axial side plane of a semi-tubular sample with a spherical pit, vertically placing the semi-tubular sample after spraying paint in the centers of an upper die and a lower die, extruding the semi-tubular sample from top to bottom to generate compression deformation, and acquiring a deformation image while deforming to obtain strain in-situ measurement data; cutting the semi-tubular sample subjected to compression deformation, embedding the cut semi-tubular sample into phenolic resin for hardness testing, and fitting after correcting hardness data to obtain a strain-hardness relation.
The cross section of the semi-tubular sample is in an arch structure with a central angle less than 360 degrees, and the semi-tubular sample comprises: the axial side plane is provided with four hemispherical pits, and the cambered surface is connected with the axial side plane.
The four hemispherical pits are preferably symmetrically distributed at the intersection points of the diagonals of the axial side planes.
The painting treatment is as follows: and uniformly spraying white paint and black paint on the part outside the hemispherical pit on the axial side plane in sequence to serve as the surface to be measured.
The extrusion from top to bottom is realized by placing the semi-tubular sample in the middle of the lower plane die and forcing the sample to generate compression deformation downwards at a constant speed by the upper plane die.
The cutting is as follows: the square sample is cut at the intersection of the diagonals of the axial side plane.
The inlay, preferably, sets the large strain gradient part of the sample center exposed to obtain a wider range of material strain-hardness relationship.
The hardness test refers to that: placing the embedded sample in a groove of a steerable hardness-adjusting measuring workbench, adjusting a measuring position point to be opposite to a hardness tester pressure head, and measuring the hardness of the position point by using a hardness tester; this procedure was repeated to measure the hardness at each target site on the surface of the sample.
The steerable hardness measurement workstation of adjusting, include: take fixed plate, bracing piece, ball pivot, bolt, regulation pole and the backup pad of recess, wherein: the groove is positioned in the center of the fixing plate and used for placing and fixing a sample; the fixed plate is connected with the spherical hinge through the adjusting rod, and the angle of the sample can be adjusted by using the spherical hinge; the height of the sample fixing plate is adjusted through the bolts, so that the specific height of the sample is adjusted. And the three spherical hinges and three pairs of bolts are freely adjusted to ensure that the measured position of the sample is vertically contacted with a indenter of a hardness tester, so that the hardness measurement of the irregular curved surface of the sample is realized.
The correction is as follows: in consideration of the change of the roughness value of the surface to be measured after deformation, the hardness data correction is completed by using the influence coefficient of the roughness on the actually measured hardness, namely, the actually measured hardness is the roughness influence coefficient, wherein the roughness influence coefficient is preferably calibrated by the following method: grinding and polishing the surface to be tested of the sample according to the hardness test requirement, and measuring the hardness; processing the surface to be measured of the sample by using electric sparks to obtain samples with different surface roughness, and measuring the hardness; and calculating a roughness influence coefficient, and performing linear fitting on the hardness data measured in the previous two steps and the corresponding roughness data to obtain the roughness influence coefficient.
The fitting is as follows: and performing data fitting according to the strain in-situ measurement data and the corrected hardness data at the corresponding position to obtain the strain-hardness relation of the axial side plane of the semi-tubular sample.
Technical effects
The invention integrally solves the problem of hardness measurement of the irregular surface after deformation in the prior art;
compared with the prior art, the method can obtain more accurate strain and hardness data in a larger range, and determines more accurate material strain-hardness relation.
Drawings
FIG. 1 is a schematic view of a semi-tubular sample of the present invention;
FIG. 2 is a schematic view of an example surface hardness measurement and steerable adjustable hardness measurement table;
in the figure: the device comprises a sample fixing plate 1, a sample 2, a groove 3, a support rod 4, a spherical hinge 5, a bolt 6, an adjusting rod 7 and a support plate 8;
FIG. 3 is a cylindrical sample before compression;
fig. 4 is a graph of hardness-strain relationship obtained by the measurement.
Detailed Description
As shown in fig. 1(a) to (c), the method for measuring the strain-hardness relationship of the test sample according to the present embodiment specifically includes:
step one, preparing a sample to be detected: preparing an incomplete cylinder by using 20MnMoB as a material, wherein the diameter of the cylinder is 20mm, the height of the cylinder is 30mm, the axial side surface of the cylinder is a plane, and the distance between the cut plane and the central axis of the cylinder is 5 mm; four hemispherical pits are further arranged on the plane, and the four pits are symmetrically distributed around the center of the plane; the diameter of the hemispherical pits is 5mm, and the distance between the hemispherical pits in the length direction and the width direction is 10 mm.
Secondly, surface painting treatment: spraying paint on the side plane with the spherical pits of the sample obtained in the first step, firstly spraying white paint, and then spraying black paint after the white paint is dried, wherein the plane outside the spherical pits needs to be uniformly sprayed, so as to ensure that the strain in the deformation process of the plane to be detected is completely and accurately captured;
thirdly, strain in-situ measurement: the sample that awaits measuring after will spraying paint is placed perpendicularly in the middle of lower plane mould, goes up the plane mould and makes the sample take place compression deformation at the uniform velocity downwards, utilizes the compression deformation process of digital image technique capture sample, obtains the deformation in-situ measurement data that meet an emergency on the surface that awaits measuring of in-process sample, specifically does:
3.1 vertically placing the sample subjected to the second-step paint spraying treatment in the center of a mold, and adjusting digital image equipment to enable the surface to be measured of the sample to be in the center of a visual field;
3.2 the pressing speed of the upper die is set to be 0.8mm/s, and the compression stroke is 15 mm;
3.3 setting the photographing frequency of the digital image technology equipment as 20Hz, and recording the strain in-situ measurement data in the compression deformation process of the semi-tubular sample;
the strain in-situ measurement data specifically comprise: and processing the pictures shot in the sample compression process to generate a linked strain cloud picture, and obtaining the strain value of each point in the sample compression process at each moment.
Fourthly, cutting and inlaying a square sample: cutting the sample after the compression deformation, cutting a square sample at the central position, embedding the square sample into phenolic resin, and requiring exposing a region with large central strain gradient of the surface to be detected so as to obtain a material strain-hardness relation in a larger range;
fifthly, placing the embedded sample in a groove of a curved surface multidirectional adjusting device, adjusting a measuring position point to be opposite to a pressure head of a hardness tester, and measuring the hardness of the position point by using the hardness tester; this procedure was repeated to measure the hardness at each target site on the surface of the sample.
The steerable hardness measurement workstation that this embodiment relates to includes: take sample fixed plate 1, sample 2, bracing piece 4, ball pivot 5, bolt 6, regulation pole 7 and backup pad 8 of recess 3, wherein: the groove is positioned in the center of the sample fixing plate and used for placing and fixing a sample; the sample fixing plate is connected with the spherical hinge through the adjusting rod, and the angle of the sample can be adjusted by using the spherical hinge; the height of the sample fixing plate is adjusted through the bolts, so that the specific height of the sample is adjusted. And the three spherical hinges and three pairs of bolts are freely adjusted to ensure that the measured position of the sample is vertically contacted with a indenter of a hardness tester, so that the hardness measurement of the irregular curved surface of the sample is realized.
The surface to be measured after compression deformation is uneven, the level of the plane to be measured needs to be guaranteed in conventional hardness measurement, and the steerable hardness-adjustable measuring workbench can guarantee the accuracy of the hardness measurement value of the plane to be measured through three-dimensional movement. Meanwhile, a real-time strain value in the sample compression process can be obtained, and a strain-hardness relation is obtained by fitting after hardness data is corrected.
Sixthly, hardness data correction: in the hardness measurement process, the micro-pits on the surface of the sample can directly influence the measurement result, and the corrected actual hardness value is as follows: HV HVa/Ka, wherein: HVa is the Vickers hardness measurement value of the sample with surface roughness Ra, Ka is the hardness measurement correction coefficient for quantitatively analyzing the influence of the surface micro-pits, and the concrete steps are as follows:
6.1, grinding and polishing the surface to be tested of the sample according to the hardness test requirement, measuring the Vickers hardness, and recording the measured value as HV 0;
6.2, processing the surface to be measured of the sample by utilizing electric sparks to obtain 5 samples with different surface roughness, measuring the Vickers hardness, and recording the measured value as HV 1;
and 6.3, calculating a roughness influence coefficient, and performing linear fitting on the hardness data measured in the previous two steps and the corresponding roughness data to obtain the roughness influence coefficient Ka which is HV1/HV 0.
And seventhly, determining the strain-hardness relation: and according to the strain in-situ measurement data and the corrected hardness data of the corresponding position obtained in the sixth step, performing data fitting to obtain a strain-hardness relational expression of the 20MnMoB material, wherein the strain-hardness relational expression comprises the following steps:
as shown in fig. 3 to 4, the hardness-strain relationship between the cylindrical sample and the material before compression obtained in the present example was shown.
Through specific practical experiments, a compression test is carried out on a 100t hydraulic press, the upper die pressing speed is 0.8mm/s, the stroke is 15mm, the strain of a sample in the compression process is synchronously measured by a digital image technology, the sample after compression deformation is cut, a square sample is cut at the central position and embedded into phenolic resin, and the area with large central strain gradient of the surface to be measured is required to be exposed so as to obtain the relationship of material strain-hardness in a larger range. Root of herbaceous plantAccording to the strain in-situ measurement data and the corrected hardness data at the corresponding position obtained in the sixth step, fitting is carried out on the data points to obtain a corrected relation formula, ka0.033Ra +0.970, giving a 20MnMoB material with a strain-hardness relationship:
compared with the prior art, the method has the advantages that the large strain is obtained by digging holes in the plane to be measured, meanwhile, the strain distribution of the sample is ensured to be measured in real time by the digital image technology, the accuracy of the hardness measurement value of the plane to be measured is improved, and the strain-hardness relation is obtained by fitting after the hardness data is corrected.
The foregoing embodiments may be modified in many different ways by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
Claims (8)
1. A method for detecting a relation between strain and hardness of a material is characterized in that a semi-tubular sample is painted on an axial side plane with a spherical pit, the painted semi-tubular sample is vertically placed in the center of an upper die and a lower die of a steerable hardness-adjustable workbench, compression deformation is generated by extrusion from top to bottom, and strain in-situ measurement data are obtained by acquiring deformation images while deformation is performed; cutting the semi-tubular sample subjected to compression deformation, embedding the cut semi-tubular sample into phenolic resin for hardness testing, and fitting after correcting hardness data to obtain a strain-hardness relation;
the cross section of the semi-tubular sample is in an arch structure with a central angle less than 360 degrees, and the semi-tubular sample comprises: the axial side plane is provided with four hemispherical pits;
the four hemispherical pits are symmetrically distributed at the diagonal intersection point of the axial side plane;
the painting treatment is as follows: uniformly spraying white paint and black paint on the part outside the hemispherical pit on the axial side plane in sequence to serve as a surface to be tested for strain;
the steerable hardness measurement workstation of adjusting, include: take fixed plate, bracing piece, ball pivot, bolt, regulation pole and the backup pad of recess, wherein: the groove is positioned in the center of the fixing plate and used for placing and fixing a sample; the fixed plate is connected with the spherical hinge through the adjusting rod, and the angle of the sample can be adjusted by using the spherical hinge; the height of the sample fixing plate and the height of the sample are adjusted through the bolts, and the position of the sample to be measured is guaranteed to be vertically contacted with a pressure head of a hardness tester by utilizing three spherical hinges and three pairs of bolts, so that the hardness measurement of the irregular curved surface of the sample is realized.
2. The method for detecting the relationship between the strain and the hardness of the material as claimed in claim 1, wherein the extrusion from top to bottom is carried out by placing the semi-tubular sample in the middle of a lower plane die, and forcing the sample to generate compression deformation by an upper plane die at a constant speed downwards.
3. The method for detecting the relationship between the strain and the hardness of the material as claimed in claim 1, wherein the cutting is performed by: the square sample is cut at the intersection of the diagonals of the axial side plane.
4. The method for detecting the relationship between strain and hardness of a material as claimed in claim 1, wherein the inlaid structure is configured such that a portion of the sample having a large strain gradient at the center is exposed to obtain a larger range of relationship between strain and hardness of the material.
5. The method for detecting the relationship between the strain and the hardness of the material as claimed in claim 1, wherein the hardness test is as follows: placing the embedded sample in a groove of a steerable hardness-adjusting measuring workbench, adjusting a measuring position point to be opposite to a hardness tester pressure head, and measuring the hardness of the position point by using a hardness tester; this procedure was repeated to measure the hardness at each target site on the surface of the sample.
6. The method for detecting the relationship between strain and hardness of a material as claimed in claim 1, wherein the correction is: considering that the roughness value of the surface to be measured changes after deformation, the influence coefficient of roughness on the actually measured hardness is utilized to complete hardness data correction, namely the actually measured hardness is the roughness influence coefficient, and the roughness influence coefficient is calibrated in the following mode: grinding and polishing the surface to be tested of the sample according to the hardness test requirement, and measuring the hardness; processing the surface to be measured of the sample by using electric sparks to obtain samples with different surface roughness, and measuring the hardness; and calculating a roughness influence coefficient, and performing linear fitting on the hardness data measured in the previous two steps and the corresponding roughness data to obtain the roughness influence coefficient.
7. The method for detecting the relationship between the strain and the hardness of the material as claimed in claim 1 or 6, wherein the corrected actual hardness value is as follows: HV HVa/Ka, wherein: HVa is the Vickers hardness measurement value of the sample with surface roughness Ra, Ka is the hardness measurement correction coefficient for quantitatively analyzing the influence of the surface micro-pits, and the concrete steps are as follows: grinding and polishing the surface to be tested of the sample according to the hardness test requirement, measuring the Vickers hardness, and recording the measured value as HV 0; processing the surface to be measured of the sample by using electric spark to obtain 5 samples with different surface roughness, measuring the Vickers hardness, and recording the measured value as HV 1; and calculating a roughness influence coefficient, and performing linear fitting on the hardness data measured in the previous two steps and the corresponding roughness data to obtain the roughness influence coefficient Ka which is HV1/HV 0.
8. The method for detecting the relationship between the strain and the hardness of the material as claimed in claim 1, wherein the fitting is performed by: and performing data fitting according to the strain in-situ measurement data and the corrected hardness data at the corresponding position to obtain the strain-hardness relation of the axial side plane of the semi-tubular sample.
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