Testing of plastics and rubber - Zwick - #29

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29 Specimen Grips and Tooling Specimen grips for tensile, creep and cyclic tests Zwick offers a large product range of specimen grips in various designs, test load ranges and test temperatures to cover the wide range of applications for the plastics and rubber testing (see table “selection criteria for specimen grips”). The specific range of application of a specimen grip depends on the operating principle and the maximum permissible test load. For tests inside a temperature or climatic chamber, the temperature range is another important factor. Force transfer between specimen and specimen grips The clamping principle defines the type of force transfer between specimen and specimen grips. Most grips are named according to their clamping principle. For the majority of specimens, the test load can only be transferred by a force-holding gripping principle, i.e. friction. The frictional force between specimen ends and gripping jaws of the specimen grips must always be greater than the test load. The required gripping forces acting vertically to the test load are generated externally (e.g. by means of pneumatic pressure) or are generated mechanically from the test load (e.g. by means of sliding wedges). In order to avoid specimen break within the grips – particularly for gripping-sensitive and flexible specimens (plastic films, strips, monofilaments) – the test load is slightly reduced by a frictional force applied by wrapping around prior to gripping. For this purpose, the specimen ends are wound onto cam plates and are subsequently clamped. For rubber ring specimens, the force is transferred according to the formfitting gripping principle. They are wound over pulleys. Gripping force For specimen grips with an external gripping force application, e.g. hydraulic, motorized, pneumatic, the set gripping force is effective during the entire test. When testing thick and soft specimens, specimen material may flow out of the gripping range (slip) during load application causing a reduction of the specimen thickness. When using hydraulic or pneumatic specimen grips, the gripping force remains constant because the pressure generator maintains the oil or compressed air pressure. When using screw grips, the gripping pressure is reduced in accordance to the rigidity and the resilience of the specimen grip. Due to the high gripping force of these specimen grips, specimen material is “pushed” out of the gripping range while closing the grips. The specimen is compressed and may be damaged. This effect can be avoided by activating the machine drive during the closure of the clamps. (“Zero-Force- Regulation”). In case of self-clamping grips, the initially low gripping force increases in relation to the acting tensile force and the function principle of the grips (Wedge, pincer, etc.). Gripping surfaces The frictional force depends not only on the gripping force, but also on the coefficient of friction of the contacting surfaces. For this reason, exchangeable gripping jaws or jaw faces with different surface types (shape, surface structure, material etc.) are availalable for many specimen grips. Gripping travel and opening width Specimen grips with an external gripping force application have a long gripping travel and consequently a large opening width. This means an easy specimen feed even when testing thicker specimens. Exchangeable gripping jaws for different specimen thicknesses are not required. Clamping force Tensile force Hydr. and pneum. grips Screw grips Wedge-screw grips Wedge grips Pincer grips Dependence of the clamping force on the test load for different types of specimen grips