Hardness Testing NDTHardness is a measure of how resistant solid matter is to various kinds of permanent shape change when a force is applied. Hardness is dependent on ductility, elastic stiffness, ;plasticity, ;strain, strength, toughness, viscoelasticity, and viscosity. There are three main types of Hardness measurements, Scratch hardness – the measure of how resistant a sample is to fracture or permanent plastic deformation due to friction from a sharp object, Indentation hardness – measures the resistance of a sample to material deformation due to a constant compression load from a sharp object and Rebound hardness – measures the resistance of a sample to material deformation due to a constant compression load from a sharp object.