Testing Machines and Systems for Metals - Zwick - #5

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5 Fig. 1: Stress-strain/crosshead travel curve of a tensile specimen made of steel sheet (tested with a Zwick Z100 materials testing machine with hydraulic specimen grips) The purely elastic strain up to the elasticity limit is only approx. 0.06 % (approx. 50 ìm); by contrast the plastic strain up to failure is almost 32 % (approx. 26 mm) – more than 500 times greater! This imposes particular requirements on the extensometer used in the same test to determine the slope of the straight line in Hooke’s Law (Young’s Modulus), the proof stress, the strain at maximum force and the strain at break - high resolution and large displacement measurement are essential. Additionally, the measurement system must not be damaged by a sudden specimen break. It is not only the specimen that deforms under the influence of the test load. All components located in the load train of the testing machine (load frame, load cell, specimen grips and the parts of the specimen outside the gage length) also deform – but only elastically. For specimens with high stiffness, the elastic deformation of the testing machine tends to be significantly greater (up to ten times) than that of the specimen. Indirect strain measurement via crosshead or piston travel is therefore much too imprecise to measure the slope of Hooke’s lines and the corresponding proof stress. The crosshead or piston travel of the testing machine is distributed between specimen and machine deformation according to the ratio of specimen stiffness to machine stiffness. The deformation rate is thus not only dependent on crosshead or piston speed, but also on the specimen (stiffness, geometry) and on machine configuration. Test results, particularly for softer materials, are affected by deformation rate, so comparability of test results from specimens of different dimensions or between different machines can only be guaranteed if the crosshead or piston speed is set or regulated (in accordance with the measured load or deformation) to allow comparable deformation rates. Testing Standards – Essential for Comparable Results A fundamental role of testing standards lies in creating the same conditions for specimen and test sequence, regardless of when, where and what is being tested. The international standards, which are increasingly replacing national standards, are an essential step to further improving the comparability of test results. The tables from page 67 onwards summarize the principal national and international standards used today for testing metals. As a rule the standards are reviewed every five years and also amended if applications to do so have been received and a majority of the standards committee are in favor. Zwick Roell employees are active on several standards committees, bringing the specialist knowledge and experience of a testing machine manufacturer to the table and at the same time keeping their knowledge up to date for product development and specialist advice to customers.