Hardness Testing:
Hardness refers to various properties of matter in the solid phase that gives it high resistance to various kinds of shape change when force is applied. Macroscopic hardness is generally characterized by strong intermolecular bonds. How-ever, the behaviour of solid materials under force is complex, resulting in several different scientific definitions of what might be called "hardness" in everyday usage.
Conventional hardness testers, like Rockwell, Brinell or Vickers machines, require the test piece be brought to the testing device; but this is not always possible. Portable testing devices have been developed that permit in- suit hardness measurements thus offering quick and economical supplements to conventional, stationary testing machines. There are two different physical methods particularly recognized in the practical field and which are accepted tools for many applications.

Definitions of Hardness
In materials science, there are three principal operational definitions of hardness.

Scratch hardness: Resistance to fracture or plastic (permanent) deformation due to friction from a sharp object.

Indentation hardness:
Resistance to plastic (permanent) deformation due to a constant load from a sharp object.

Rebound hardness:
Height of the bounce of an object dropped on the material, related to elasticity. The mathematical definition of hardness is the pressure applied over the projected contact area between the indenter and the material being tested. As a result, hardness values are typically reported in units of pressure, although this is only a "true" pressure if the indenter and surface interface is perfectly flat.