Although every testing machine manufacturer is familiar with metal stretching, there are not many manufacturers who can really understand the standard and the reasons behind the standard. So now every testing machine manufacturer guides users to complete the metal tensile test. Generally, starting from the capabilities of their own equipment, the test is completed in the simplest way, for example, the entire test process is completed at the speed of the beam displacement. There are still many details in the metal tensile test, which are worthy of our attention.

The first is the issue of stretching speed.

In the elastic deformation stage, the amount of metal deformation is small and the tensile load increases rapidly. At this time, if the beam displacement control is used for the tensile test, then the speed is too fast will cause the entire elastic section to be quickly rushed past. Taking ordinary steel with a modulus of elasticity of 200Gpa as an example, if a material with a gauge length of 50mm is tested at a speed of 10mm/min in the elastic section, the actual stress rate is 200000N/mm2S-1×10mm/ min1min/60S
1/50mm=666N/mm2S-1 The yield strength of ordinary steel is less than 600Mpa, so it only takes 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 material sample with better plasticity passes the elastic section, the load increases little, but the deformation increases quickly. Therefore, in order to prevent the stretching speed from being too fast, strain control or beam displacement control is generally used. Therefore, it is recommended in GB228-2002, "In the elastic range and up to the upper yield strength, the separation rate of the chuck of the testing machine should be kept as constant as possible and within the specified stress rate range (material elastic modulus E/(N/ mm2)＜150000, the stress rate control range is 2-20 (N/mm2)?s-1, the material elastic modulus E/(N/mm2)≥150000, the 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 period 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. In the plastic range and up to the specified strength (specified non-proportional extension strength, specified total extension strength and specified residual extension strength) the strain rate should not exceed 0.0025/s. ". There is a very critical issue here, that is, the switching point between stress velocity and strain velocity. It is best to switch from stress velocity to strain velocity at the end of the elastic section. Make sure that there is no impact during the switching process. , No force loss. This is a very key technology of the tensile testing machine. The second is the timing of clamping, tracking and taking off the extensometer. For the tensile test of steel, if the total elongation under the maximum force is required (Agt), then the extensometer must be traced to the maximum force before removing. For thin plates and other specimens that have little impact after breaking, the extensometer can directly trace the fracture of the specimen; but for specimens with greater tensile force, the maximum The better way is to keep the beam position still after the testing machine is stretched to the maximum force, and then pull off the sample after removing the extensometer. Some fixtures will generate an initial force when clamping the sample, so you must Clamp the extensometer after the initial force is eliminated, so that the gauge length clamped by the extensometer is the original gauge length of the specimen in the free state.