according to the stress strain curve, they should be differentiated quite easily
the area under the curve is tough, which means the energy the material could absorb up to fracture.
hardness is the measure of a material's resistance to deformation by surface indentation or by abrasion. If you see from the curve, high hardness means low strain, however, the material may be hard but not tough, since strain is low, the area should be little
rigid, not very sure, seems quite similar to hardness :p
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