Notes on the tensile tester 1. It is a problem of the stretching speed. In the elastic deformation stage, the amount of deformation of the metal is small and the tensile load is rapidly increased. At this time, if the tensile test is performed by the beam displacement control, the speed is too fast, and the entire elastic section is quickly washed away. For example, a common steel with a modulus of elasticity of 200 Gpa is used. If a material with a gauge length of 50 mm is subjected to a tensile test at a speed of 10 mm/min in the elastic section, the actual stress rate is 200,000 N/mm 2 S - 1 × 10 mm / Min × 1min / 60S × 1 / 50mm = 666N / mm2S-1 The general steel yield strength is less than 600Mpa, so it takes only 1 second to pull the sample to yield, which is obviously too fast. Therefore, in the elastic section, stress rate control or load control is generally selected. After the elastic specimen has passed the elastic section, the load increases little and the deformation increases rapidly. Therefore, in order to prevent the tensile speed from being too fast, strain control or beam displacement control is generally adopted. Therefore, it is recommended in GB228-2002 that "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 (material elastic modulus E / (N / Mm2)<150000, stress rate control range is 2-20 (N/mm2)?s-1, material elastic modulus E/(N/mm2)≥150000, stress rate control range is 6-60 (N/mm2) ?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. The strain rate in the parallel length should be kept as constant as possible.
The strain rate should not exceed 0.0025/s in the plastic range and up to the specified strength (specified for non-proportional extension strength, specified total extension strength, and specified residual extension strength). "There is a very critical problem here, which is the problem of the switching point between stress velocity and strain velocity. Zui is to switch the stress velocity to the strain velocity at the end of the elastic segment. In the process of switching, there is no impact. There is no power loss. This is a very critical technology for the tensile testing machine.
2. It is the timing of clamping, tracking and taking off the extensometer. For the tensile test of steel, if it is required to take the total elongation (Agt) under the force of Zui, the extensometer must be traced to the Zui vigorously and then removed. For specimens with little impact after pulling off the sheet, the extensometer can directly track the specimen to break; but for the specimen with large tensile force, the good method is to start the beam at the test machine after stretching to the position of Zui. After the extensometer is removed, the sample is broken. Some fixtures generate an initial force when clamping the specimen. The extensometer must be clamped after the initial force is removed, so that the gauge length clamped by the extensometer is the original gauge length of the specimen in the free state.
There are not many test machines that can be tested this way. Please pay attention to these points when purchasing and using.
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