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Mechanical property testing of metal materials: 10 points to pay attention to in tensile testing

October 31, 2023

Tensile testing is the most common experiment in mechanical property testing of metal materials. The steps of metal tensile testing can be found in the ASTM E-8 standard. It is said that tensile testing reflects the basic properties of materials, but the measurement results of the same material through different tensile testing processes are not necessarily the same. So what factors affect tensile testing?


1. Sampling parts and methods

The difference in the sampling location will directly affect various performance indicators such as post-break elongation, yield strength, and tensile strength in the tensile test of metal materials. Due to uneven distribution of components, organization, structure, defects, processing deformation, etc, in metal materials, the mechanical properties of different parts of the same batch or even the same product vary. Therefore, sampling should be carried out strictly in accordance with the provisions in the appendix of GB/T 228-2002. In addition, when cutting the sample blank, it is necessary to prevent its mechanical properties from being affected by heat, work hardening and deformation.


2. Shape, size and accuracy of specimen

For metal materials of the same material and in the same state, if the cross-sectional shapes are different, the measured results will have a greater impact on the upper yield strength and less impact on the lower yield strength; the tensile strength of a sample with a large cross-sectional area (large size) will be smaller than that of a larger size. Low, and the plasticity index also decreases the parallelism and dimensional accuracy within the parallel length of the sample can also easily affect the test results. Because the measured size value of the sample may not be the minimum position size of the actual sample, this will cause the test results to be lower. Therefore, the shape and size of the test piece must be in accordance with the standard.


3. Measuring instruments

The accuracy of dimensional measuring instruments and measuring tools must meet the test requirements. Therefore, before conducting tests, various measuring instruments must be calibrated and the measuring tools must be kept clean.


4. Test equipment

Testing machines and extensometers are two types of testing equipment commonly used in tensile testing of metal materials, which directly affect the accuracy and authenticity of the test results. The former is used to measure force values; the latter is mainly used to measure displacement or extension. Therefore, during testing, it is necessary to ensure that the testing machine and extensometer are within the validity period of the calibration and are calibrated regularly.


5. Test environment temperature

Some metal materials have high temperature sensitivity. Even for ordinary metal materials, if the test temperature difference is too large, it may lead to inconsistent test measurement results. Generally speaking, as the temperature decreases, the yield strength of body-centered cubic metals increases sharply, while the change of face-centered cubic metals is not so obvious. As the temperature rises, the yield strength of metal materials generally decreases. Therefore, in the provisions of GB/T 228-2002 on the room temperature tensile test method for metal materials, the test must generally be conducted at room temperature within the range of 10°C-35°C. The test temperature that has strict temperature requirements should be 23℃±5℃.


6. The influence of clamp selection and specimen clamping

Improper gripper selection, specimen clamping, and extensometer loading and unloading can affect test results. If the shape of the clamp does not match the test sample and the surface pattern shape of the clamp is inappropriate, it will cause insufficient clamping area between the clamp and the sample, and the static friction force will not be enough, resulting in the clamp and the sample being stretched during the stretching process. Relative sliding occurs, which affects the stretching results.


7. Clamping method

The clamping method of the specimen is crucial to the success of the tensile test. If the specimen cannot be clamped, the test will inevitably fail. If the clamping method is unreasonable, it may easily cause the specimen to slip or break in the jaws, resulting in the test data is inaccurate or low.


8. Stretching rate

The stretching rate directly affects the stress-strain relationship of metallic materials. Different materials have different sensitivity to speed. The impact of stretching rate on different materials is different, and the impact on materials with low strength and good plasticity is greater. GB/T 228-2002 In the regulations on room temperature tensile testing methods for metallic materials: “In the elastic range and up to the upper yield strength, the separation rate of the testing machine chuck should be kept as constant as possible and within the specified stress rate range. (The elastic modulus of the material E<150000N/mm2, the stress rate is within the range of (2-20) N/mm2·s- 1, the elastic modulus E≥150000N/mm2, the stress rate is within the range of (6-60) N/mm2· s-1 range). If only the lower yield strength is measured, the strain rate during the yield period of the parallel length of the specimen should be between 0.00025/s-0.0025/s, and the strain rate within the parallel length should be kept as constant as possible. In plasticity The strain rate over the range and up to the specified strength shall not exceed 0.0025/s.


9. Determination of cross-sectional area of tensile specimens

There are two methods for determining the cross-sectional area of tensile specimens: one is the metal tensile test method GB/T 228, and the other is the corresponding product standard for the material. Some product standards explicitly stipulate that the cross-sectional area of tensile test specimens should be measured according to the nominal size of the cross-sectional area. If there is no such special provision in the product standard, the GB/T 228 standard should be followed, and the actual size should be measured to measure the cross-sectional area.


10. Measurement method of sample size and human errors in measurement

For tensile specimens, an outer diameter micrometer, vernier caliper or vernier caliper for rectangular specimens should be used depending on the diameter. If the measurement method is inaccurate, the size measurement will be artificially large. Due to differences in subjective factors and operating techniques, errors will also be brought to the measurement results.