Method for testing thermal output of strain gauge, correction method for high-temperature strain measurement result, and stress testing device

Abstract

The application provides a strain gauge heat output testing method, a high-temperature strain measurement result correction method and a stress testing device. The strain gauge heat output testing method comprises the following steps: selecting x1 same-size to-be-tested samples, arranging k strain gauges on each to-be-tested sample; connecting each to-be-tested sample with a corresponding temperature measuring element and then placing the to-be-tested sample in a high-low temperature test box; setting x2 test temperature points, with an initial temperature T0 and a test temperature interval T3; when the temperature of the to-be-tested sample is stable at the initial temperature T0, zeroing the strain value of the strain gauge, collecting the strain value of the strain gauge at each test temperature point; and a heat output data analysis system calculating the average heat output of each to-be-tested sample at each test temperature point and performing curve fitting on the relationship between the average heat output and the temperature difference to obtain a heat output fitting curve of the strain gauge. The testing method is more accurate in the fitting result because the relationship between the temperature difference and the heat output is used for fitting.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of high-temperature strain testing, in particular to a method for testing thermal output of a strain gauge, a method for correcting high-temperature strain measurement results and a stress testing device. BACKGROUND

[0002] The thermal output of a strain gauge is the output of the strain gauge when only the temperature changes without force. The thermal output is one of the most important working characteristic parameters of a resistance strain gauge and is the basis for correcting strain gauge test data. The accuracy of the thermal output data directly affects the accuracy of high-temperature strain gauge testing. In the process of testing the strain of an air conditioner pipeline, the temperature change range of the air conditioner pipeline is about tens of degrees, and the thermal output of the strain gauge will greatly affect the experimental results.

[0003] The existing methods for removing the thermal output mostly use temperature compensation, but the temperature compensation requires additional strain gauges, which increases the testing cost, and the position of the compensation strain gauge is not easy to determine, so it is not worth popularizing.

[0004] In addition, there is also a correction method for strain gauges, which is to test the thermal output at different temperatures, fit the relationship between the temperature and the thermal output, and correct the thermal output. Since the thermal output at the initial temperature is not 0, the correction of the thermal output will produce a certain error, and the thermal output at the initial temperature needs to be processed during the correction. The correction method is relatively complex, and this correction method assumes that the strain gauge has thermal output at a certain temperature. However, the main reason for the thermal output is the difference between the resistance temperature coefficient of the sensitive grid material of the strain gauge and the linear expansion coefficient between the sensitive grid material and the test piece material. If the resistance temperature coefficient of the sensitive grid material is α and the sensitivity coefficient is K, the linear expansion coefficients of the test piece and the sensitive grid are βm and βs respectively, then the formula for the thermal output of the strain gauge is: e e = (α / K + (βm-βs))*ΔT, where α, K, βm, and βs are material property parameters. As can be seen from the formula, the size of the thermal output is related to the properties of the strain gauge itself and the temperature change, and is not related to the temperature. Therefore, the error of this correction method is large. SUMMARY

[0005] The first object of the present application is to provide a method for testing the thermal output of a strain gauge. The method uses the relationship between the temperature difference and the thermal output to fit, and the fitting result is more accurate.

[0006] The second object of the present application is to provide a method for correcting high-temperature strain measurement results. The correction method is simple, and can eliminate the influence of the thermal output of the strain gauge on the test results during the stress strain test, and solve the problem of difficult selection of the position of the compensation strain gauge.

[0007] ​The third object of the present application is to provide a stress testing device implementing the above-mentioned correction method.

[0008] To achieve the above-mentioned first object, the present application provides a method for testing thermal output of a strain gauge, comprising: selecting x1 same-size samples to be tested, and setting k strain gauges on each sample to be tested; connecting each sample to be tested to a corresponding temperature measuring element and then placing them in a high-low temperature test chamber; setting x2 testing temperature points, with an initial temperature T0 and a testing temperature interval T3; when the temperature of the sample to be tested is stabilized at the initial temperature T0, zeroing the strain value of the strain gauge, and collecting the strain value of the strain gauge at each testing temperature point; inputting each strain value into a thermal output data analysis system, and calculating the average thermal output of each sample to be tested at each testing temperature point, and performing curve fitting on the relationship between the average thermal output and the temperature difference, to obtain a thermal output fitting curve of the strain gauge, wherein the temperature difference is the difference between the testing temperature point and the initial temperature T0.

[0009] As can be seen from the above scheme, according to the theoretical thermal output formula of the high-temperature strain gauge, the thermal output of the strain gauge is related to the temperature change, and the fitting result using the temperature difference will be more accurate. In actual application, the strain testing device needs to be zeroed and calibrated before testing, and the strain value at the initial temperature is 0. Using the temperature difference fitting method, the thermal output of the strain gauge at the initial temperature is 0, and the relationship between the temperature difference and the thermal output is directly fitted during testing, and the fitting result is more accurate. The fitting curve is used to correct the thermal output of the strain gauge, and the correction method is simpler. In addition, the method for testing thermal output of the strain gauge of the present application clearly selects the experimental equipment, and provides guidance for the placement method of the temperature measuring element, the copper pipe and the sample to be tested and the strain gauge, which is beneficial to improving the measurement accuracy of the thermal output of the strain gauge.

[0010] A preferred scheme is x1≥5; and / or k≥2; and / or x2≥5.

[0011] As can be seen, the more the number of samples to be tested and the number of strain gauges, the smaller the testing error. Moreover, the more the number of testing temperature points, the more accurate the fitted curve.

[0012] A preferred scheme is that the length of the sample to be tested is in the range of 30-80 mm.

[0013] A preferred scheme is that the corresponding temperature measuring element is connected to a position on the sample to be tested, which is within 1 cm from the strain gauge on the sample to be tested.

[0014] As can be seen, the temperature measuring element is arranged close to the strain gauge, so that the temperature value detected by the temperature measuring element is close to the actual temperature of the strain gauge, the temperature deviation is reduced, and the accuracy of the thermal output fitting is improved.

[0015] To achieve the above-mentioned second object, the application provides a high-temperature strain measurement result correction method, comprising: obtaining original strain values of each strain gauge on x1 samples to be measured, the number of strain gauges on each sample to be measured being k; obtaining strain values of each strain gauge at x2 test temperature points, the strain value of the strain gauge being zero when the temperature of the sample to be measured is stable at the test temperature point T0; calculating average thermal outputs ε of each sample to be measured at each test temperature point T , and performing curve fitting on the relationship between the average thermal outputs ε T and temperature differences , to obtain a thermal output fitting curve of the strain gauge, the temperature difference being the difference between the test temperature point and the initial temperature T0; calculating a corrected strain test value of the product to be measured according to the original strain test value of the product to be measured e , according to the formula e R = e - ε T of the product to be measured e R .

[0016] Therefore, the correction method is simple, and can eliminate the influence of the thermal output of the strain gauge on the test result during the stress-strain test, and only one strain gauge needs to be set, without the need of additionally setting a compensation strain gauge, thereby solving the problem of difficult selection of the position of the compensation strain gauge.

[0017] A preferred scheme is that the thermal output fitting curve is a second-order polynomial curve.

[0018] A preferred scheme is that x1≥5; and / or k≥2; and / or x2≥5.

[0019] A preferred scheme is that the length of the sample to be measured is in the range of 30-80 mm.

[0020] A preferred scheme is that the temperature measuring element is connected to the corresponding sample to be measured within a range of 1 cm from the strain gauge.

[0021] Therefore, the temperature measuring element is arranged close to the strain gauge, so that the temperature value detected by the temperature measuring element is close to the actual temperature of the strain gauge, the temperature deviation is reduced, and the accuracy of the thermal output fitting is improved.

[0022] To achieve the above-mentioned third object, the application provides a stress test device, comprising a device main body and a processor, the processor being used to implement the above-mentioned high-temperature strain measurement result correction method when executing a program stored in a memory. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a structural schematic diagram of the device main body in the stress test device embodiment of the application.

[0024] Figure 2 is a comparison chart of original calculation results and curve fitting results in the test method embodiment of the strain gauge heat output of the application.

[0025] The application is further described below in conjunction with the drawings and examples. DETAILED DESCRIPTION

[0026] The test method of the strain gauge heat output comprises:

[0027] First, x1 copper pipes with the same pipe diameter, length and wall thickness are selected, and the copper pipes in the air conditioner pipeline are the samples to be tested. K strain gauges are arranged on each copper pipe, wherein preferably, k≥2, and a total of k*x1 strain gauges. Preferably, x1≥5, the more the number of copper pipes and strain gauges, the smaller the test error, and preferably, the length of the copper pipe is in the range of 30-80 mm.

[0028] Next, each copper pipe is connected with a corresponding thermocouple as a temperature measuring element and then placed in a high-low temperature test chamber. The position of the thermocouple is selected to ensure that the temperature change on the copper pipe can be accurately detected. In addition, when the copper pipe with the strain gauge is placed in the high-low temperature test chamber during the experiment, it should be completely unrestrained to ensure that the test result is completely the heat output of the strain gauge on the air conditioner pipeline. At the same time, in order to reduce the influence of the temperature change of the thermocouple wire on the test result, the length of the wire in the high-low temperature test chamber should be as short as possible during the experiment.

[0029] Next, x2 test temperature points are set, preferably x2≥5, the test temperature point refers to the environmental temperature setting value of the strain gauge heat output test (i.e. the temperature setting value of the high-low temperature test chamber), the initial temperature is T0, the test temperature interval is T3, and each test temperature point is T0, (T0+T3), (T0+2*T3), (T0+3*T3)……[T0+(x2-1)*T3], wherein the value range of T0 is: strain gauge use temperature lower limit value < T0 < strain gauge use temperature upper limit value-(x2-1)*T3, since the use temperature range of different types and different materials of strain gauges is different, the use temperature range can be found in the strain gauge specification book as a reference for setting the test temperature point in the application, to ensure that the value of T0 is in the above range, i.e. the value range of T0 should be within the use temperature range of the strain gauge, and the values of all test temperature points should be within the use temperature range of the strain gauge. After determining the initial temperature T0, the temperature of the high-low temperature test chamber is set to T0, and the temperature is maintained for more than 30 min after stabilization to check whether the copper pipe temperature display is stable near T0. When the temperature of the copper pipe is stable at the test temperature point T0, the strain value of the strain gauge is zeroed.

[0030] Then, the strain values of the strain gauges at each test temperature point are collected. The temperature of the high-low temperature test chamber is set to T0+T3. After the temperature of the high-low temperature test chamber is displayed from T0 to T0+T3, the temperature is kept for more than 30 minutes, and the strain value at this temperature point is collected. The test and collection methods of each subsequent test temperature point are the same.

[0031] Then, each strain value of the test is introduced into the heat output data analysis system for processing and analysis. The heat output data analysis system calculates the average heat output ε T of each copper pipe at each test temperature point. T The calculation formula of the average heat output ε

[0032]

[0033] Finally, the heat output data analysis system performs curve fitting on the relationship between the average heat output and the temperature difference to obtain the heat output fitting curve of the strain gauge. The heat output fitting curve is a second-order polynomial curve, and the temperature difference is the difference between the test temperature point and the initial temperature T0. The least squares method is used to perform polynomial fitting on the temperature difference and the average heat output value. A second-order polynomial is used for fitting, which can be assumed as After fitting by the least squares method, the values of the polynomial coefficients A, B, and C are obtained, and thus the relationship equation between the heat output and the temperature difference is obtained. According to the curve fitting equation, the heat output at each temperature difference point can be obtained.

[0034] Strain gauge heat output test method embodiment:

[0035] Referring to Figure 1 , first, five copper pipes 1 with the same pipe diameter, length, and wall thickness are selected. The length of each copper pipe 1 is 50 mm. Two one-way strain gauges 2 are attached to each copper pipe 1, for a total of 10 strain gauges 2. A thermocouple 3 is attached to each copper pipe 1 within a one-centimeter range of one of the strain gauges 21 on the copper pipe 1. The thermocouple 3 is arranged close to the strain gauge 2. In this way, the temperature value detected by the thermocouple 3 is close to the actual temperature of the strain gauge 2, reducing temperature deviation and improving the accuracy of heat output fitting.

[0036] Then, during the experiment, the copper pipe 1 with the strain gauges 2 and thermocouples 3 attached is placed in the high-low temperature test chamber, allowing it to be completely unrestrained, i.e., not subject to external forces. The length of the wire inside the high-low temperature test chamber is as short as possible to avoid the influence of the heat output generated by the wire on the experimental results.

[0037] Next, 5 test temperature points are selected, the initial temperature of the strain gauge 2 is 20℃, the temperature interval is 20℃, and the 5 test temperature points are 20℃, 40℃, 60℃, 80℃ and 100℃ respectively. The temperature of the high-low temperature test chamber is set to 20℃, and after the temperature is stabilized for 30 minutes, the strain value displayed on the stress testing equipment is zeroed, and the strain data of 10 strain gauges 2 at this time is collected. The temperature of the high-low temperature test chamber is set to 40℃, and after the temperature of the high-low temperature test chamber rises from 20℃ to 40℃, the temperature is maintained for 30 minutes, and the strain value at this test temperature point is collected. The test and collection methods of subsequent test temperature points are the same, and the strain data of 60℃, 80℃ and 100℃ are collected in turn. The specific data of the test is shown in Table 1.

[0038] Table 1 Relationship between temperature difference and original heat output value

[0039]

[0040] Next, the test value is imported into the heat output data analysis processing system for processing and analysis. The heat output data analysis system calculates the average heat output ε of each copper pipe 1 at each test temperature point T (as shown in Table 1),

[0041] Next, the heat output data analysis system performs polynomial fitting on the curve of the average heat output ε T and the temperature difference . The least square method is used to obtain the polynomial coefficients A, B and C, and thus the relationship between the strain gauge heat output and the temperature difference is obtained: The original calculation result and the curve fitting result are shown in Figure 2 .

[0042] Stress testing equipment and correction method of high-temperature strain measurement result

[0043] The stress testing equipment includes an equipment main body and a heat output data analysis system. The heat output data analysis system processor includes a processor for implementing the following high-temperature strain measurement result correction method when executing a program stored in a memory.

[0044] The equipment main body includes a high-low temperature test chamber, a thermocouple 3 and a data collection device. The high-low temperature test chamber has a good stability and can be stabilized at a set temperature within ±1℃. The data collection device is used to collect temperature values and strain values. The heat output data analysis system is used for test value statistics, calculation, heat output curve and formula fitting, and automatic correction of heat output strain values.

[0045] The high-temperature strain measurement result correction method includes:

[0046] First, perform step 1 to obtain the original strain values ​​of each strain gauge 2 on the five copper tubes 1.

[0047] Next, step 2 is performed to obtain the strain values ​​of each strain gauge 2 at 5 test temperature points.

[0048] Next, proceed to step 3 to calculate the average heat output ε of each copper tube 1 at each test temperature point. T and the average heat output ε T and temperature difference Curve fitting was performed on the relationship between the two elements to obtain the thermal output fitting curve of strain gauge 2.

[0049] Next, in step 4, when the copper tube 1, the product under test, undergoes strain testing, the temperature change of the strain gauge 2 will generate heat output. This part of the strain is spurious strain. To improve the accuracy of the strain test, this part of the strain needs to be removed. Based on the relationship between heat output and temperature difference obtained in the previous step, the program automatically corrects the heat output to obtain the true strain value of the pipe. In the process of testing the strain value of copper tube 1, the initial temperature of copper tube 1 before the experiment is the same as the laboratory ambient temperature, T1. The real-time temperature of copper tube 1 during the strain test is T2, and the real-time displayed original strain is... e That is, the original strain test value e ,but During this process, the heat output ε of strain gauge 2 on copper tube 1 T It can be represented as: .

[0050] Next, after executing step 5 and obtaining the heat output of strain gauge 2 on copper pipe 1, the system automatically uses the original strain test value while testing the strain of the air conditioning pipeline. e Remove the thermal output value ε of strain gauge 2 under the corresponding temperature difference T The actual strain value can then be obtained. e R That is, according to the formula e R = e - ε T Calculate the corrected strain test value for copper tube 1 e R Then, the temperature change, the heat output of the real-time strain gauge 2, the original strain test value, and the corrected strain test value can be displayed on the screen of the stress testing equipment.

[0051] From the above, according to the theoretical heat output formula of the high-temperature strain gauge, the heat output of the strain gauge is related to the temperature change, and the fitting result of the temperature difference is more accurate. In practical application, the strain testing equipment needs to be zeroed and calibrated before testing, and the strain value at the initial temperature is 0. Using the temperature difference fitting method, the heat output of the strain gauge at the initial temperature is 0, and the relationship between the temperature difference and the heat output is directly used for fitting during testing, and the fitting result is more accurate. The fitting curve is used to correct the heat output of the strain gauge, and the correction method is simpler. In addition, the strain gauge heat output testing method of the present application clearly selects the experimental equipment, and gives guidance on the position of the thermocouple, the copper pipe and the placement method of the strain gauge, which is beneficial to improve the measurement accuracy of the strain gauge heat output. The correction method of the present application is simple, which can eliminate the influence of the heat output of the strain gauge on the test results during the stress and strain test. At the same time, when the copper pipe of the product to be tested is tested, only one strain gauge is needed, and no additional compensation strain gauge is needed, which solves the problem of difficult selection of the compensation strain gauge position.

[0052] In addition, the number of copper pipes can also be more than one. The number of strain gauges on each copper pipe can also be more than one. The total number of strain gauges arranged on the sample to be tested is greater than or equal to two. The number, size of the copper pipe and the number of strain gauges, as well as the number of test temperature points arranged, can be changed as needed. The position of the thermocouple connected to the copper pipe can also be changed as needed. The sample to be tested and the product to be tested can also be selected as needed. The above changes can also achieve the purpose of the present application.

[0053] Finally, it should be emphasized that the above is only a preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various changes and modifications, and any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method of testing the thermal output of a strain gauge, characterised in that, The method comprises the following steps: selecting x1 samples to be measured, each of which has k strain gauges arranged thereon; connecting each of the samples to be measured with a corresponding temperature measuring element, and then placing them in a high-low temperature test chamber, and connecting a corresponding temperature measuring element to each sample to be measured near the strain gauge arranged on the sample to be measured; setting x2 test temperature points, the initial temperature of the plurality of test temperature points being T0, and the test temperature interval being T3, and when the temperature of the sample to be measured is stabilized at the initial temperature T0, the strain value of the strain gauge is zeroed, and the strain value of the strain gauge at each test temperature point is collected; introducing each strain value into a heat output data analysis system, the heat output data analysis system calculating the average heat output of each sample to be measured at each test temperature point, and performing curve fitting on the relationship between the average heat output and the temperature difference to obtain a heat output fitting curve of the strain gauge, the heat output fitting curve being a second-order polynomial curve; wherein the temperature difference is the difference between the test temperature point and the initial temperature T0.

2. The method according to claim 1, wherein: x1≥5; and / or k≥2; and / or x2≥5.

3. The method according to claim 1 or 2, wherein: the length of the sample to be measured is in the range of 30 mm to 80 mm.

4. The method according to claim 1 or 2, wherein: the temperature measuring element is connected to the sample to be measured within 1 cm of the strain gauge arranged on the sample to be measured.

5. A method of correcting high temperature strain measurements, characterised in that, The method comprises the following steps: obtaining the original strain value of each strain gauge on x1 samples to be measured, each of which has k strain gauges arranged thereon; obtaining the strain value of each strain gauge at x2 test temperature points, the strain value of the strain gauge being zero when the temperature of the sample to be measured is stabilized at the initial temperature T0 in the plurality of test temperature points; calculate the average heat output ε of each sample under each test temperature point T , and perform curve fitting on the relationship between the average heat output ε T and the temperature difference , to obtain a heat output fitting curve of the strain gauge, the heat output fitting curve being a second-order polynomial curve, wherein the temperature difference is the difference between the test temperature point and the initial temperature T0; wherein the temperature measuring element is connected to the corresponding sample near the strain gauge; According to the original strain test value of the sample to be measured ε , according to the formula ε R =ε - ε T Calculate the corrected strain test value of the sample to be measured ε R .

6. The method according to claim 5, wherein: x1≥5; and / or k≥2; and / or x2≥5.

7. The method according to claim 5, wherein: the length of the sample to be measured is in the range of 30 mm to 80 mm.

8. The method according to any one of claims 5 to 7, wherein: the temperature measuring element is connected to the sample to be measured within 1 cm of the strain gauge arranged on the sample to be measured.

9. Stress testing apparatus, characterized in that The device comprises a device body and a processor, and the processor is used to implement the method for correcting high-temperature strain measurement results according to any one of claims 5 to 8 when executing a program stored in a memory.

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