Punchability tests during skew rolling. The classic punchability test is characterized by the varying compression of the cross-section of the originally conical sample while passing between the rollers without the use of a piercing mandrel. A description of the mechanism of cavity formation based on the analysis of the state of stress in deformation between punching rollers, mainly on the basis of works by Russian authors, was published by O. Pejčoch . Briefly, punching is a special state of stress caused by a combination of pressure applied to the material by punching rollers and deformation unevenness. A three-axis stress diagram with two main tensile stresses acts in the centre of the rolled material. They, in a given cross-section. act perpendicular to the main compressive stress caused by the pressure of the cylinders and cause an essentially brittle failure of the material in the axis of the rolled material in the absence of plastic deformation . Compared to other formability tests, or even to punchability tests with purely cylindrical specimens, conical specimens allow for the evaluation of amounts of critical removals when a cavity is formed, and thus basic parameters for punch setting adjustment and piercing mandrel adjustment are obtained. However, the evaluation of hollow specimens is extremely difficult as it requires highly professional equipment and experience in ultrasonic investigation, or the use of costly X-ray or metallographic methods to determine where the failure occurred. Punchability tests on LP are currently performed in a manner analogous to the operational punching process , with full or centrally pre-drilled samples with circular cross-section. Versatility of the laboratory piercer means the possibility of its classical configuration with two punching rollers and one support roller or its modification as an elongating bench with three conical cylinders. The bench allows for the adjustm[...]
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