Hardness Testing
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SCIENCE &
Biomimetic Materials, Sensors and Systems C A L L
F O R P A P E R S
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P. Calvert
D. De Rossi
T. Tateishi
Tucson, AZ, USA
Pisa, Italy
Tsukuba, Japan
For scientists and engineers with an interest in the respective interfaces of biology / biotechnology and materials ana structures, information processes, sensors and actuators, robotics This international and interdisciplinary journal reports on scientific and technical contributions dealing with all aspects of conceiving, designing, constructing and testing man-made materials, structures, devices and systems which replicate or are based on biological entities and processes. These include, but are not limited to: • Materials and Structures • Sensors and Information Processes • Dynamics and Control Systems For customers in the USA and Canada:
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Hardness Testing To create effective tools, people need to find materials harder than the substance they wish to work on. The direct measurement of hardness has been attempted since before the Machine Age. Evaluations of a material's relative resistance to deformation were noted in early treatises, such as the description of a file test performed in 1640, a test which is still used today. Over time, a variety of hardness tests have been developed to quickly evaluate and compare materials. Hardness, however, is not a fundamental property like tensile strength or elastic modulus, but rather a derived quantity based on the complicated response of a material to a specific test. Each hardness system, therefore, measures a slightly different set of material traits. The earliest method for measuring hardness involved the use of a scratch test, in which a tester determines a material's resistance to scratching or abrasion by a sequence of standard objects. This system was first codified in Germany by the mineralogist Friedrich Mohs (1773-1839). In 1812 Mohs assigned a numerical value from 1 to 10 to a series of common minerals and published his scale in 1822 in Grundriss der Minemlogie.
For the softest mineral in his scale, talc, Mohs assigned the number 1. Gypsum received a 2; calcite, 3; fluorite, 4; apatite, 5; orthoclase feldspar, 6; quartz, 7; topaz, 8; corundum, 9; and diamond, 10. Mohs was most interested in using his scale to identify unknown minerals he found in the field. Since harder minerals scratch softer ones, a mineralogist, by bringing along samples of his 10 standard substances, could readily position unknown samples between minerals on the standard scale. As useful additions to the hardness scale, Mohs included the human fingernail (slightly greater than 2),
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