Electronic tensile testing machine test slip is what is going on

The human factor that causes the equipment to slip is caused by the operator's failure to follow the correct method of the test during the test. There are two main factors: the short clamping length of the specimen and the improper selection of the jaws of the clamp.

　　 1. The test clamp length is shorter

The correct way to use the fixture of the testing machine should be that when the clamping length of the test piece is the same as the length of the tooth surface of the fixture, first push the jaws with the help of external force to generate initial friction on the clamping surface, and then pass the crossbeam of the testing machine. The movement loads the sample, and the friction force pulls the jaws (wedge-shaped mouth) due to the action of the inclined plane. The greater the axial pulling force, the greater the clamping force generated. The test machine clamp concrete has two inclined wedge-shaped openings. The above clamping method is designed to accept uniform compressive stress. However, some operators did not operate in accordance with the use requirements of the testing machine. The sample clamping length is short, or the sample processing is too short, resulting in uneven force on the inclined surface of the wedge port. The local stress of the wedge port far exceeds the material. The yield strength of the wedge-shaped mouth is plastically deformed, severely turned outward, and the inclined surface of the wedge-shaped mouth is collapsed or worn. The clamp continues to be used in this situation, reducing the angle of the wedge-shaped mouth, deteriorating the clamp body's stress state and causing slipping.

　　2. Improper selection of clamp jaws

Electronic tensile testing machine clamps have a variety of different specifications and clamping surfaces. Different chucks are used for different samples. Some operators use large-size jaws to clamp small cross-section specimens during testing, or use flat chucks. Clamping a large specimen makes the clamp and the specimen not close, and the friction coefficient is significantly reduced. The intuitive performance is that the scaly peak of the clamp is smoothed by the divisor, and the friction is greatly reduced. When the force on the specimen gradually increases to reach a large static friction force, the specimen will slip, resulting in a false yield phenomenon from the surface.

　　Of course, it is also possible that the electronic tensile machine sample slips due to the equipment.

　　 When the sample is pulled, the iron oxide scale falls into the slope of the wedge block and causes slippage. Metal oxide scale is generated during the drawing process of the metal sample. The oxide scale will fall into the inclined surface combined with the wedge block and the fixture, which will damage the flatness of the inclined surface and severely decrease the surface roughness, causing the wedge-shaped mouth (wedge block) ) The movement is not flexible. When the pulling force continues to increase, the wedge-shaped block crawls (jumps) during the sliding along the dovetail slope. This is how the sound of bangs and bangs often appear in the process of tensile loading. This is commonly referred to as skidding.

　　Electronic tensile testing machine slip solution:

　　 In response to the above-mentioned slipping caused by human factors and equipment factors, the following countermeasures have been formulated. The implementation of countermeasures can reduce or eliminate the slipping phenomenon.

　　 1. Repair the wedge-shaped inclined surface

　　 Plastic deformation of the wedge block, severe eversion, and collapse or wear of the inclined surface of the wedge block should be repaired in time. Generally, the following two methods can be used for repair.

　　A, repair welding. The collapsed or worn part of the wedge block slope should be repaired first, and then restored with a slotting machine (if the machining fails to meet the roughness requirements, it should be scraped). The hardness and strength of the repair welding part will generally be improved, and it is enough to pay attention to the correct use of the fixture in the future.

　　B, mosaic. Use a slotting machine to remove 10 mm of the inclined surface of the wedge block at the original design angle, and then inlay a polished hardened steel plate in this position.

　　C. Add a backing plate to the clamp body. This method slightly weakens the design strength of the fixture body, but the long-term use effect is better.

　　D, anti-layer device is added above the wedge block. In order to prevent the iron oxide scale from falling into the inclined surface of the wedge block during the stretching process, two rubber blocks can be installed above the wedge, which can effectively reduce the iron oxide scale falling into the inclined surface. In addition, grease can be applied to the inclined surface of the wedge block. Or grease, and regularly scrub the fixture and change the oil.

　　2, the operating procedures of the standard testing machine

In view of the short clamping length of the specimen and improper selection of the jaws of the clamp, the operating rules of the electronic tensile testing machine must stipulate: "The clamping length of the tensile specimen shall not be less than the length of the tooth surface of the clamp" and "No Use jaws for large cross-sections to clamp small cross-section specimens or use flat chucks to clamp large specimens". The operator must carry out the test in accordance with the operating procedures.

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