3. Selection criteria

initial gauge lengths L The vertical anistropy characterises the cold forming capability of fine sheet metal with reference to the deep draw behaviour of the material. The r-value identifies the resistance of sheet metal to reduction in thickness of sheet metal at single-axis tensile loading. The change in width must be measured on a dumbbell specimen for the determination of these values.An analogue or incremental reduction-in-width monitorin conjunction with the Macro extensometer is suitable for this kind of width measurement. Thus the change in width is measured in 2 or 4 cross-section levels.The biaxial incremental clip-on extensometer, designedfor length and change in width measurement, is foreseen especially for this test. In principle a decision must be taken as to whether ornot direct extension measurement is necessary, or whether indirect extension measurement via the crosshead travel monitor is sufficient (See page 4).Should the above considerations lead to direct extensionmeasurement being chosen, selection of an extensometer with the appropriate technical properties must be made. Following is a list of criteria, dependent upon the material to be tested and the test results to be determined, to enable the correct extensometer to be selected.Different
are prescribed bytest standards depending upon the shape and dimensions of the specimen to be tested. In most cases L
is relatively small for high amounts of strain andrelatively large for small strains. The initial gauge length L
is directly related to the specimen cross-section forproportional specimen when testing metals.
Tensile specimen to standard: MetalsPlastics / ElastomersTextile materials e.g.: round and flatspecimen, tapes, wirese.g.: dumbbells,strips, rings in mm 101000Initial gauge lengthL 100 Laser extLongstroke extensometerOptical extensometermultiXtensMakro extensometeroptiXtensClip-on extensometerVideo extensometer Zwick extensometers (ext) e.g.: threads, tapes,industrial yarns, strips Diagram 2: Application range of Zwick extensometers dependent upon the initial gauge length L 7