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HARDNESS TESTINGHardness is a measure of the ability of a material to resist local plastic deformation underthe action of a concentrated force. It is measured by pressing an indenter into thesurface of the test material, under a constant force for a fixed length of time.For the Vickers hardness test the indenter takes the form of a pyramidal diamond. Thesize of the indentation is a measure of the hardness of the material. For a given indenterforce, the smaller the indentation the harder the test material.Hardness is normally defined as the indenter force divided by the area of contactbetween the indenter and the indented surface of the test material, that is, the surfacearea which supports the indenter force. Hardness has units of stress (pressure). Forhistorical reasons it is normally recorded in units ofkgf.mm-2. The flow stress of thematerial (σf) can be estimated from the hardness of the material using equation 2.Whilst this is a rough estimate, hardness testing does provide an essentially non-destructive, cheap and quick estimate of a material’s flow stress.You will hardness test the materials you produced in the rolling and heattreatment labs, namely the Jominy bar, the Charpy test pieces and the rolled Alsheet.Your demonstrator will show the operation of the indentation machine. Complete thesets of tables of results (overleaf) in your groups (of 3 students) by individual use of aVickers hardness machine, and the appropriate indenting force. The demonstrators willbe able to help you with the interpretation of your results. You must ensure you havea complete set of results before leaving the laboratory.
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