CN111879648A - Elastic modulus calibration method - Google Patents
Elastic modulus calibration method Download PDFInfo
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- CN111879648A CN111879648A CN202010724048.2A CN202010724048A CN111879648A CN 111879648 A CN111879648 A CN 111879648A CN 202010724048 A CN202010724048 A CN 202010724048A CN 111879648 A CN111879648 A CN 111879648A
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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
- G01N3/62—Manufacturing, calibrating, or repairing devices used in investigations covered by the preceding subgroups
Abstract
The invention provides a method for calibrating an elastic modulus, which comprises the following steps: step one, placing a standard test piece for an experiment at a chuck of a universal testing machine, and starting monitoring by the universal testing machine and a high-speed camera simultaneously until the test piece is broken to stop the experiment; determining a calibration magnification according to the data of the universal testing machine and the pictures shot by the high-speed camera; determining the elastic modulus to be calibrated according to the tensile experiment data; and step four, obtaining the calibrated elastic modulus according to the calibration multiplying power. The method calibrates the elastic modulus measured by the universal tester, and avoids system errors caused by changes of measuring equipment and measuring environment.
Description
Technical Field
The invention belongs to the field of material physical parameter measurement, and particularly relates to a method for calibrating an elastic modulus.
Background
The universal testing machine is a common device for measuring physical parameters of materials, has long service life and high use frequency, and is found to exceed a specified error range in a small range of frequent use during verification and test. The universal testing machine does not undergo professional calibration for a long time, and series errors can occur.
First, the main body of the tester: if the main body is not horizontally arranged, friction is generated between the working piston and the wall of the working cylinder, and errors are caused. Secondly, a dynamometer part: the mounting of the dynamometer component is not horizontal, which results in friction between the pendulum bearings. The side wall part which is not frequently used is easy to deposit dirt, and the dirt deposited in the groove of the inverted hanging piece connected with the force measuring piston by the shaft of the force measuring pendulum increases the friction force, so that the measured force value of the measuring range point is lower.
When a large number of experiments are carried out, the accuracy of experimental data is ensured, and experimental errors and artificial errors are avoided.
Disclosure of Invention
The invention aims to provide a method for calibrating the elastic modulus, which is used for calibrating the elastic modulus measured by a universal testing machine.
In order to solve the technical problem, the invention adopts the technical scheme that the method for calibrating the elastic modulus comprises the following steps:
step one, placing a standard test piece for an experiment at a chuck of a universal testing machine, and starting monitoring by the universal testing machine and a high-speed camera simultaneously until the test piece is broken to stop the experiment;
determining a calibration magnification according to the data of the universal testing machine and the pictures shot by the high-speed camera;
determining the elastic modulus to be calibrated according to the tensile experiment data;
and step four, obtaining the calibrated elastic modulus according to the calibration multiplying power.
In a preferred embodiment, in the second step, the data with the largest stress is located according to the data of the universal testing machine, the deformation of the tensile image shot by the high-speed camera at the time is determined according to the time t from the start to the maximum stress, and the calibration magnification k is equal to2/1，1Is the strain at t of the high speed camera,2the strain of the universal tester at the maximum stress is shown.
In a preferred embodiment, in the second step, the number n of pictures from start to maximum stress is calculated according to the shooting speed v and the time t of the high-speed camera, and the n is t/v, the deformation quantity Δ l of the nth picture is measured,1and l is the distance between the two clamping distances of the test piece, and delta l is the distance change value between the two clamping distances of the test piece before and after stretching.
In the preferred scheme, in the third step, a stress-strain curve graph is drawn according to tensile experiment data of the universal testing machine, an oblique line segment of the stress-strain curve is determined according to the stress-strain curve graph, and the elastic modulus to be calibrated is determined according to the stress and strain values of two end points of the oblique line segment.
In the preferred scheme, the slope of the point connecting line of the stress-strain curve when the total strain amount is 2 percent and 4 percent is taken to determine the elastic modulus to be calibrated,
in the formula: e' is the inaccurate modulus of elasticity, σ4％And σ2％Stress at 2% and 4% total strain respectively,4％and2％strains at 2% and 4% total strain, respectively.
In a preferred embodiment, in the fourth step, the calculation formula of the calibrated elastic modulus E is as follows:
in the formula: e is the correct modulus of elasticity and N is the number of experiments.
The method for calibrating the elastic modulus can calibrate the elastic modulus of the universal testing machine according to the calibration multiple difference, avoid system errors caused by changes of measuring equipment and measuring environment, and improve the validity and the scientificity of experimental data.
Drawings
The invention is further illustrated with reference to the accompanying drawings and examples:
FIG. 1 is a high-speed camera display image of a test piece before being stretched according to an embodiment of the present disclosure;
FIG. 2 is a high-speed camera display picture of the embodiment of the invention when the deformation of the test piece is maximum;
FIG. 3 is a stress-strain curve of a test piece according to an embodiment of the present invention.
Detailed Description
A method for calibrating an elastic modulus comprises the following steps: marking a proper clamping distance on a standard test piece of an experiment, placing the standard test piece at a chuck of a universal testing machine, simultaneously starting and monitoring the universal testing machine and a high-speed camera until the test piece is broken, and respectively connecting two devices of the universal testing machine and the high-speed camera with two different computers to reasonably set the shooting speed of the high-speed camera.
And step two, determining the calibration magnification according to the data of the universal testing machine and the pictures shot by the high-speed camera, positioning the data with the maximum stress according to the data of the universal testing machine, and determining the deformation of the tensile picture shot by the high-speed camera at the time according to the time t from starting to the maximum stress. Calculating the number n of pictures from starting to the maximum stress according to the shooting speed v and the time t of the high-speed camera, wherein n is t/v, measuring the deformation quantity delta l of the nth picture,1and l is the distance between the two clamping distances of the test piece, and delta l is the distance change value between the two clamping distances of the test piece before and after stretching.
Calibration magnification k ═2/1，1Is the strain at t of the high speed camera,2the strain of the universal tester at the maximum stress is shown.
And step three, determining the elastic modulus to be calibrated according to the tensile experiment data, drawing a stress-strain curve graph according to the tensile experiment data of the universal testing machine, determining an oblique line segment of the stress-strain curve according to the stress-strain curve graph, determining the elastic modulus to be calibrated according to the stress and strain values of two end points of the oblique line segment, and determining the elastic modulus to be calibrated according to the slope of a point connection line of the two end points of the oblique line segment.
Step four, obtaining the calibrated elastic modulus according to the calibration magnification, wherein the calculation formula of the calibrated elastic modulus E is as follows:
in the formula: e is the correct modulus of elasticity and N is the number of experiments.
This example was verified by a method using 9400 resin material as an experimental material, which was fabricated into a standard test piece using 3D printing technology, and marking the nip line.
Step one, two clamping distance lines are clamped at two end chucks of the universal testing machine respectively, the visual field of the high-speed camera is adjusted, the test piece is enabled to appear at the center of the screen, the shooting speed is adjusted, and the shooting speed v is 2 s/piece in the embodiment. The model of the universal testing machine is as follows: CMT4104G, model number of high speed camera: OSG030-815 UM. The picture of the specimen before the drawing was taken, and as shown in fig. 1, the nip distance l was measured, i.e., 8.1 cm. And simultaneously starting the universal testing machine and the high-speed camera, and stopping the test until the test piece is broken.
Step two, positioning the data with the maximum stress according to the data of the universal testing machine, determining the time t used for the maximum stress to be 152S according to the time t used from starting to the maximum stress, and determining the strain at the maximum stress2At 0.0731, the deformation of the stretched picture taken by the high speed camera at this time is determined. Calculating the picture number n from starting to maximum stress according to the shooting speed v and the time t of the high-speed camera, wherein n is t/v, n is 152/2 is 76, measuring the deformation quantity delta l of the 76 th picture, as shown in figure 2, and delta l is l1-l＝0.3cm，1Δ l/l is 0.037. Calibration magnification k ═2/1＝1.973。
Step three, determining the elastic modulus to be calibrated according to the tensile experiment data, drawing a stress-strain curve graph according to the tensile experiment data of the universal tester, and determining the elastic modulus to be calibrated at the moment by taking the slope of the point connecting line of the stress-strain curve when the total strain is 2% and 4%, as shown in fig. 3:
TABLE 1 Table of relevant physical parameters for 2% and 4% total strain
E′＝0.883。
And step four, calculating to obtain the elastic modulus E after calibration as 1.973 x 0.883 as 1.74 according to the calibration multiplying power and the elastic modulus to be calibrated.
In order to ensure the accuracy of experimental measurement and eliminate human and system errors, five groups of experiments are taken, and finally the correct elastic modulus is obtained according to the calibration magnification.
Five experiments, the calibration magnification is: 1.973, modulus of elasticity to be calibrated are 0.88, 0.89, 0.94, 0.91 and 0.93.
According to
The average modulus of elasticity is calculated to be 1.80, the standard modulus of elasticity provided by the supplier of the test piece material is 1.80, and the error of the calibrated modulus of elasticity is in the range of 1%, thus proving that the calibration method is feasible.
Claims (6)
1. A method for calibrating an elastic modulus, comprising the steps of:
step one, placing a standard test piece for an experiment at a chuck of a universal testing machine, and starting monitoring by the universal testing machine and a high-speed camera simultaneously until the test piece is broken to stop the experiment;
determining a calibration magnification according to the data of the universal testing machine and the pictures shot by the high-speed camera;
determining the elastic modulus to be calibrated according to the tensile experiment data;
and step four, obtaining the calibrated elastic modulus according to the calibration multiplying power.
2. The method for calibrating the modulus of elasticity according to claim 1, wherein in the second step, the data with the maximum stress is located according to the data of the universal tester, the deformation of the tensile image taken by the high-speed camera at the time is determined according to the time t from the start to the maximum stress, and the calibration magnification k is equal to2/1，1Is the strain at t of the high speed camera,2the strain of the universal tester at the maximum stress is shown.
3. The method for calibrating elastic modulus according to claim 2, wherein in the second step, the number of pictures n from start to maximum stress is calculated according to the shooting speed v and the time t of the high-speed camera, and the deformation quantity Δ l of the nth picture is measured，1And l is the distance between the two clamping distances of the test piece, and delta l is the distance change value between the two clamping distances of the test piece before and after stretching.
4. The method for calibrating the elastic modulus of claim 1, wherein in the third step, a stress-strain curve graph is drawn according to tensile experimental data of a universal testing machine, a diagonal segment of the stress-strain curve is determined according to the stress-strain curve graph, and the elastic modulus to be calibrated is determined according to the stress and strain values of two end points of the diagonal segment.
5. The method of claim 4, wherein the slope of the point-connecting line of the stress-strain curve at 2% and 4% of total strain is used to determine the elastic modulus to be calibrated,
in the formula: e' is the inaccurate modulus of elasticity, σ4％And σ2％Stress at 2% and 4% total strain respectively,4％and2％strains at 2% and 4% total strain, respectively.
6. The method for calibrating elastic modulus according to claim 1, wherein in the fourth step, the formula for calculating the elastic modulus E after calibration is:
in the formula: e is the correct modulus of elasticity and N is the number of experiments.
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