Hardness has been defined simply as the resistance to penetration. In order to measure this resistance to penetration a testing machine is used to push a specific indenter into the test material with a specified force, and either the depth of penetration or the area of the "footprint" left by the indenter is used to derive a hardness "number". Hardness as measured by indentation is not a single fundamental property but a combination of properties, and the contribution of each to the hardness number varies with the material and the type of test.
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