3.2 Dimensional measurement of the specimen The specimen size in the test includes the length and diameter of the specimen.
(1) Length of the sample According to the formula (2), the accuracy of the measurement of the length of the sample directly affects the test result of the ring stiffness. During the test, the length measurement of the sample shall be measured in the circumferential direction, and
Pipes of the same diameter have different measurement requirements. When the nominal diameter of the pipe is ≤200mm, try
The length of the sample is measured at 3 points; when the nominal diameter of the pipe is >200mm and <500mm, the length of the sample is measured at 4 points; when the nominal diameter of the pipe is ≥500mm, the length of the sample is measured at 6 points. After the measurement is finished, calculate whether the difference between the minimum value and the maximum value meets the accuracy requirement of the sample preparation. If it is not satisfied, the sample should be re-cut and adjusted; if the requirement is met, the arithmetic mean value is calculated as the length of the sample.
(2)Sample diameter： The measurement accuracy of the sample diameter is more important than the measurement of the sample length. Through the actual test operation, it can be known that the loading rate during the test is determined according to the nominal diameter of the sample, and the test end condition is also based on the deformation of the diameter direction of the pipe. Therefore, once an error occurs in the measurement of the diameter of the sample, the result is obtained. The test data is completely wrong, so the plastic pipe diameter is a key factor affecting the test results. The sample diameter measuring portion is the middle portion in the longitudinal direction of the sample. The end of the sample is released during extrusion and cutting during the cutting process, and the local deformation is easy to occur. Therefore, the measured data at the end is not necessarily accurate. During the test, it is impossible to save the time and measure at the end of the sample. Drill down into the middle of the length of the specimen.
There are two methods for measuring the diameter of the sample: (1) A 45° interval is used at the cross section in the middle of the length of the sample, and the measurement is performed 4 times at a time, and the arithmetic mean is taken. (2) Measurement at the cross section at the middle of the length of the specimen with an inner diameter of π
When using the first method, it should be marked in strict accordance with the 45° scribe line. After 4 measurements, the measurement angle just covers the entire circumferential direction. If it is larger or smaller than 45°, it cannot be completely covered, and the error data is easy to be obtained. When using the second method, care should be taken to make the π-foot fit snugly against the inner wall of the specimen. If necessary, many people work together or use the limit magnet holder to avoid gaps between the π-foot and the specimen to obtain erroneous data.
3.3 sample state adjustment
Since the pipe is a plastic product, the mechanical properties are susceptible to temperature changes. Excessive temperature of the plastic pipe test environment will cause the sample to soften. If the temperature is too low, the brittleness of the sample will increase. The change of the mechanical properties of the material will affect the accuracy of the ring stiffness data. Therefore, the sample must be at the ambient temperature before the test. Condition adjustment at ±2 °C for at least 24 hours to ensure the accuracy of the test data.
3.4 test equipment
Ring stiffness test of plastic pipes The main test equipments are compression testers and compression plates.
The compression test machine belongs to the main hardware guarantee for the successful completion of the test. Therefore, the pressure control accuracy, stability and loading speed of the compression test machine are related to the accuracy of the ring stiffness test data. Under normal circumstances, the instrument settings used in the plastic pipe ring stiffness test meet the requirements, but the tester should also pay attention to the operating conditions options, the wrong test conditions will lead to erroneous data, during the test, the experimenter When setting the test conditions, it is important to pay attention to the test loading speed and end conditions.
The compression plate is a plate that carries the sample of the pipe during the test, one on top of the other. First of all, because of the load-bearing effect during the sample process, the compression plate must have sufficient rigidity and hardness. If the stiffness and hardness are not met, the bending or deformation may occur during the sample process to affect the experimental results. In the test of pipes of different diameters, the size of the compression plate must be matched with the diameter of the sample, and the phenomenon of small horse-drawn carts cannot be seen. This specification clearly stipulates that the length of each compression plate should not be less than the length of the specimen, and the width should be at least 25 mm wider than the contact surface of the specimen with the load plate when subjected to load.
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