What do you know about the causes of slipping of tensile testing machine and the correct operation process?
Tensile testing machine, also known as material testing machine, is a mechanical testing machine used for static load, tension, compression, bending, shear, peeling and other mechanical property tests of various materials. It is suitable for various physical and mechanical property tests of plastic plates, pipes, profiles, plastic films, rubber, wires and cables, for material development, physical property tests, teaching and research Quality control and other indispensable testing equipment. The tensile testing machine will slip during use. What is the reason? Next, the technicians of our company will share with you the causes and operation process of slipping of the tensile testing machine, hoping to bring you help!
1、 Causes of slipping of tensile testing machine during use
1. Human factors cause the tensile testing machine to slip
The human factors causing the slip of the tensile testing machine are caused by the operator's failure to operate according to the correct method of the test. There are two main factors: the short clamping length of the sample and the improper selection of the jaw of the fixture.
2. The test clamping length is short
The correct use of the fixture of the tensile testing machine 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 jaw with the help of external force to generate initial friction on the clamping surface, and then load the sample through the movement of the cross beam of the testing machine. When the friction pulls the jaw (wedge-shaped mouth), the greater the axial tension due to the action of the inclined plane, The greater the clamping force, the wedge-shaped mouth with two inclined planes is designed according to the above clamping mode and receiving uniform compressive stress.
However, some operators did not operate according to the use requirements of the testing machine. The clamping length of the sample is short, or the processing time of the sample is too short, resulting in uneven stress on the inclined plane of the wedge-shaped mouth, and the local stress of the wedge-shaped mouth far exceeds the yield strength of the material, resulting in plastic deformation and serious eversion of the wedge-shaped mouth, causing collapse or wear of the inclined plane of the wedge-shaped mouth. The fixture continues to be used in this case, which reduces the angle of the wedge mouth and worsens the stress state of the fixture body.
3. Improper selection of jaw of fixture
The fixture of tensile testing machine has many different specifications and clamping surfaces. Different collets are used for different samples. Some operators use large-size jaws to clamp small cross-section samples or use flat collets to clamp large samples during the test, so that the fixture is not in close contact with the sample, and the friction coefficient is significantly reduced. It is intuitively shown that the scaly peak of the fixture is divided and flattened, The friction is greatly reduced. When the stress of the sample increases gradually and reaches large static friction, the sample will slip and produce false yield from the surface.
3. The tensile testing machine slips due to equipment reasons
The main reason for the equipment is that when the tensile machine pulls the sample, the iron oxide scale falls into the inclined plane of the wedge block, resulting in slipping. During the drawing process of the metal sample, the metal oxide scale will be generated, and the oxide scale will fall into the inclined plane combined with the wedge block and the fixture, resulting in the destruction of the flatness of the inclined plane, the serious decline of the surface roughness, and the inflexible movement of the wedge mouth (wedge block). When the tension increases, the wedge block will crawl (jump) along the dovetail inclined plane. This is how the sound of trumpet and trumpet often occurs in the process of tensile loading. This is commonly referred to as slip.