What are the 9 factors that affect the test results of metal tensile machines?
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The tensile test of the metal tensile machine is the most common test in the mechanical properties test of the metal material. The tensile test reflects the basic properties of the material, but the same material does not necessarily have the same measurement result through different tensile test processes. What factors influence the test results?
Shape, Dimensions and Accuracy of Specimens Metallic materials with the same state of the same material of the metal tensile machine, if the cross-sectional shape is different, the measured results have a large upper yield strength, a small influence on the lower yield strength, and a large cross-sectional area (large size). The tensile strength of the sample is small and the size is low, and the plasticity index is also lowered. The parallelism and dimensional accuracy in the parallel length of the sample are also very likely to affect the test results. Because the size value measured by the sample may not be the minimum position size of the actual sample, this will result in a lower test result. Therefore, the shape and size of the test piece need to be carried out in accordance with the standard.
In terms of measuring instruments, the accuracy of the dimension measuring instrument and the gage must meet the test requirements, so it is necessary to verify the various measuring instruments before the test, while keeping the gage clean.
The choice of gripper and the grip of the specimen are extremely critical. Improper loading and unloading of the gripper, sample grip and extensometer will affect the test results. The shape of the clamp does not match the shape of the test specimen and the shape of the surface of the clamp is unsuitable. This will result in insufficient clamping area between the clamp and the specimen. The static friction is insufficient to cause the clamp and the specimen to be opposite during the stretching process. Sliding, which affects the stretching result.
Determination of the cross-section of the tensile specimen, the cross-section of the tensile specimen of the metal tensile machine is determined by two methods: one is the metal tensile test method GB/T 228, and the other is the corresponding product standard of the material. Some product standards clearly stipulate the cross-sectional area of ​​the tensile specimen according to the cross-sectional area of ​​the nominal size. If there is no such special provision in the product standard, the GB/T 228 standard should be followed for a long time. .
The measurement method of the sample size and the measurement of human error. For tensile specimens, the outer diameter micrometer, vernier caliper or rectangular vernier caliper should be selected according to the diameter. If the measurement method is inaccurate, the artificial size measurement will be too large. Due to subjective factors and operational techniques, errors can also be brought to the measurement results.
Sampling site and method. The difference in sampling position directly affects the performance indexes of elongation, yield strength and tensile strength of the tensile test of metal materials. The uneven distribution of composition, structure, structure, defects, processing deformation, etc. in metal materials makes the mechanical properties of the same batch or even different parts of the same product appear different. Therefore, sampling should be carried out in strict accordance with the provisions of the GB/T 228-2002 appendix. In addition, when cutting the blank, it is necessary to prevent the mechanical properties from being affected by heat, work hardening and deformation.
The metal tensile machine and extensometer in the test equipment are two kinds of test equipment commonly used in the tensile test of metal materials, which directly affect the accuracy and authenticity of the test results. The former is used to measure the force value; the latter is mainly used to measure the displacement or extension. Therefore, it is necessary to ensure that the test machine and the extensometer are valid within the validity period of the test and are regularly checked during the test.
Test ambient temperature. Some metal materials have high temperature sensitivity. Even for ordinary metal materials, if the test temperature is too large, the test results may be inconsistent. In general, the body-centered cubic metal decreases sharply with temperature, and the change in face-centered cubic metal is less pronounced. As the temperature rises, the yield strength of the metal material generally decreases. Therefore GB/T 228-2002 In the provisions on the tensile test method for metallic materials at room temperature, the test room temperature must be carried out in the range of 10 ° C -35 ° C. The test temperature for which temperature is critical shall be 23 °C ± 5 °C.
Stretch rate. The rate of stretching directly affects the stress-strain relationship of the metal material. Different materials have different sensitivity to speed, and the stretching rate has different effects on different materials, and has a great influence on materials with low strength and good plasticity. GB/T 228-2002 In the provisions on the tensile test method for metallic materials at room temperature: "In the elastic range and up to the upper yield strength, the separation rate of the test machine chuck should be kept as constant as possible and within the specified stress rate range. (The material elastic modulus is E<150000N/mm2, the stress rate is in the range of (2-20)N/mm2•s-1, the elastic modulus is E≥150000N/mm2, and the stress rate is (6-60)N/mm2• In the range of s-1.) If only the lower yield strength is measured, the strain rate during the yield of the parallel length of the specimen should be between 0.00025/s and 0.0025/s, and the strain rate in the parallel length should be kept as constant as possible. Range and strain rate up to the specified strength should not exceed 0.0025/s"
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