Hardness tests, with hardness defined as resistance to the elasto-plastic deformation (i.e. the resistance that a material opposes to penetration), are widely used among mechanical tests because of their many advantages: they allow localized analysis, they don’t destroy nor significantly alter the piece on which they take place, and they allow to indirectly infer other fundamental mechanical properties.

These tests are performed with the use of an instrument called durometer. The principle of operation of durometers is almost identical for the various types: a tip (called indenter) of various shape is pushed with a known force against the material to be tested; depending on the hardness of the material, it will penetrate to a certain depth. By measuring the penetration depth or the size of the impression left on the surface, you can have an indication of the material’s hardness.

On the principle of resistance that a material opposes the penetration are based the most widely used methods in the metallurgical field; they are classified, by the shape of the indenter and the test conditions, in:

• BRINELL HARNDESS (HB) (indenter is a tempered steel ball)
• VICKERS HARDNESS (HV) (indenter is a square-based pyramid with 136° vertex angle)
• ROCKWELL HARDNESS (HRC) (indenter is a diamond cone with a 120° opening)