Testing Machines and Systems for Metals - Zwick - #4

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4 2 Metals – Use and Properties The properties of metallic materials – metals and metal alloys – cover a very broad spectrum. The distinction between function and structure also applies to materials; examples of structural and construction materials include materials for machine and plant building, precision engineering and civil engineering, while functional materials are used in areas such as electrical engineering, electronics and media technology. With construction materials the emphasis is on mechanical properties such as strength, rigidity (elasticity) and ductility (plasticity) at working temperature. These characteristics are the main influence in testing-machine design (maximum required test load for load frames and load cells) and equipment (high-resolution extensometers for Young’s Modulus determination). As an illustration, steel under mechanical loading displays very high stiffness right from the beginning of the test – unlike rubber, for example. This results in large load increases for very small deformations, often less than a hairsbreadth. These deformations are elastic and decrease again with load reduction. Only when the load is further increased is a plastic (i.e. permanent) deformation superimposed on the elastic deformation. Specimens of very brittle materials (such as cast iron) or soft steels at low temperatures fracture without warning on reaching cohesive strength (i.e. with no prior necking). Fig. 1: Load-extension curve of sheet-steel tensile specimen (upper) and enlarged view of the area until just after 0.2 % proof stress (lower) By contrast, local stiffness in tough materials decreases sharply after the elastic limit has been exceeded and can even reach values of zero or less (increase in deformation without load increase or even with load reduction). The load-extension curve of a steelsheet tensile specimen (gage length L0 = 80 mm) illustrates very well the change in stiffness as deformation increases. Table: Comparison of Specific Material Properties Material properties Metals Concrete Plastic mat. (without extreme values) (non-reinforc.) • Tensile and compression strength [MPa] 100 ... 2000 200 ... 500 20 ... 160 • Young’s Modulus [GPa] 70 ... 210 15 ... 40 0,06 ... 6 • Density (specific weigth) [g/cm³] 2,7 ... 7,8 2,1 ... 2,4 1 ... 2