1, the test clamping length is shorter
The correct use of the electronic tensile machine test machine fixture should be that when the clamping length of the test piece is the same as the length of the tooth surface of the clamp, the external force is used to push the jaws to generate initial friction on the clamping surface, and then pass the test. The movement of the machine beam is applied to the sample. When the friction force pulls the jaw (wedge shape), the larger the axial pulling force is, the larger the clamping force is due to the action of the inclined surface. The test machine clamp has two inclined wedge-shaped openings. It is according to the above clamping method, designed according to the acceptance of uniform compressive stress. However, some operators did not operate according to the requirements of the testing machine. The length of the sample clamping was short, or the sample processing was too short, resulting in uneven stress on the wedge-shaped bevel. The local stress of the wedge-shaped opening far exceeded. The yield strength of the material causes plastic deformation and severe valgus of the wedge-shaped opening to collapse or wear the wedge-shaped slope. The jig continues to be used in this case, and the angle of the wedge-shaped opening is reduced, so that the state of the clamp body is deteriorated and the slip phenomenon occurs.
2, the jaws of the fixture are not properly selected
The electronic tensile tester clips are available in a variety of different sizes and grips. Different chucks are used for different specimens. Some operators use large-size jaws to hold small-section specimens or flat collets when testing. The large sample is clamped, so that the contact between the clamp and the sample is not tight, and the friction coefficient is obviously reduced. The most intuitive performance is that the scaly peak of the clamp is smoothed by the divisor and the friction is greatly reduced. When the force of the sample is gradually increased to reach the maximum static friction force, the sample will slip and produce a false yield phenomenon from the surface.
Second, the equipment causes the electronic tensile tester sample to slip
The reason for the equipment is mainly that the tensile machine causes slippage due to the falling of the iron oxide skin into the wedge-shaped slope when the sample is pulled. The metal sample produces a metal oxide scale during the drawing process, and the iron oxide scale falls into the inclined surface of the wedge block and the clamp, so that the flatness of the slope is broken, the surface roughness is seriously reduced, and the wedge-shaped opening (wedge block) The movement is inflexible, and as the pulling force continues to increase, the wedge block generates creep (jumping travel) along the sliding of the dovetail slope. This is the result of the sounds of the horns and horns that often occur during the stretching process. This is what is commonly referred to as slipping.
Electronic tensile testing machine fixture slipping solution
In view of the above reasons caused by human factors and equipment factors, Shanghai Tiji Instrument & Meter Technology Co., Ltd. has formulated the following countermeasures, which can reduce or eliminate the phenomenon of pulling machine slip through the implementation of countermeasures.
1. Specification of the operating procedures of the testing machine
For the short clamping length of the specimen and the improper selection of the clamp of the clamp, in the operating procedure of the electronic tensile testing machine, it must be specified: "The clamping length of the tensile specimen shall not be less than the length of the tooth surface of the clamp" and "No Use a jaw for a large section to hold a small section specimen or a flat jaw to hold a large specimen." The operator must test according to the operating procedures.
2. Repair the wedge bevel
Plastic deformation and severe eversion of the wedge block, so that the wedge block slope collapse or wear should be repaired in time, the repair can generally take the following two methods
2.1, repair welding
The trapped or worn parts of the wedge block are first repaired and then restored by the inserting machine (should be scraped if the machining does not meet the roughness requirements). The hardness and strength of the repaired parts are generally improved, and it is ok to use the jig correctly later.
2.2, mosaic
The wedge-shaped block slope is removed by the insertion bed at the original design angle by 10 mm, and then a polished hardened steel plate is placed in this part. The design strength of the clamp body is slightly weakened, but the long-term use effect is better.
2.3 Adding a layer protection device above the wedge block
In order to prevent the metal oxide sample from falling into the wedge-shaped slope during the stretching process, two rubber blocks can be placed on the wedge shape, so that the iron oxide skin can be effectively reduced to fall into the slope, and the bevel of the wedge block can be coated with butter. Or grease, and scrub and change oil on the fixture regularly.
Electronic tensile testing machine fixture slippage summary
The probability of slippage of the electronic tensile testing machine during the tensile test is still relatively high, mainly due to factors such as slippage of the clamping part of the specimen or large gap between the transmission points of certain force values ​​of the testing machine, if the test is used before. The test yield value of the machine is normal, and the test yield value will be significantly lower now, and the phenomenon is especially obvious when testing some hard or brittle materials. Generally, the reason why the test machine slips should be considered first. At this time, it is necessary to analyze the cause of the slippage in time, and take corresponding measures for the reasons to ensure the accuracy of our test data.
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