There are still many details about metal tensile testing machine, which deserve our attention. The first is the problem of the speed of tension. In the elastic deformation stage, the deformation of metal is very small and the tensile load increases rapidly. At this time, if the beam displacement control is used to do the tensile test, the speed will cause the whole elastic segment to rush quickly.
For an ordinary steel with a modulus of 200Gpa as an example, if the material with a standard distance of 50mm and a tensile test at the speed of 10mm/min in the elastic section, the actual yield strength of the actual stress rate of 200000N/mm2S-1 * 10mm/min * 1min/60S x 1/50mm=666N/mm2S-1 is smaller than that of 600Mpa, so only 1 seconds will be required. The specimen has been pulled to yield. This speed is obviously too fast. Therefore, in elastic section, stress rate control or load control are generally adopted.
When the plastic material has passed the elastic section, the load increases little, and the deformation increases very quickly. So in order to prevent the stretching speed from being too fast, the strain control or the beam displacement control are generally adopted. Therefore, it is suggested in GB228-2002 that "in the elastic range and straight first yield strength, the separation rate of the test chuck should be kept constant and in the range of the specified stress rate (material elastic modulus E/ (N/mm2) < 150000, the stress rate control range is 2 - 20 (N/mm2) s-1, and the elastic modulus E/ (N/m) (N/m)." M2 = 150000), stress rate control range of 6 - 60 (S-1 = N/mm2). If the yield strength is measured only, the strain rate should be between 0.00025/s and 0.0025/s when the specimen is of parallel length.
The strain rate within the parallel length should be kept as constant as possible. The strain rate should not exceed 0.0025/s in the plastic range and until the specified strength (the non proportional extension strength, the prescribed total extension strength and the prescribed residual elongation strength). A key problem is the switching point between stress velocity and strain velocity. It is best to switch the stress velocity to the strain speed at the end of the elastic section. During the handover process, we must ensure that there is no impact and no drop. This is a key technology of the tensile testing machine. Next is the timing of clamping, tracking and taking off of extensometer. For the tensile test of steel, if we want to take the most vigorous total elongation (Agt), then the extensometer must track the maximum force and take it later. The extensometer can be directly traced to the specimen fracture for the specimen with small impact after the plate is broken, but for the tensile test machine, the best way is to maintain the beam position after the test machine is stretched to the maximum force, and then take the extensometer to break the specimen. Some jig will produce an initial force when the specimen is clamped, so the initial force must be removed and then the extensometer is clamped, so the standard distance of the extensometer is the original standard distance under the free state.
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