Surface hardening definition

The Brinell test uses an indentor of 10 mm diameter hardened steel ball, and applies a load which is usually 3000 kg. The Brinell hardness number (BHN) is defined as the load, F (kilogrammes), divided by the surface area of the indentation. The expression given below describes the definition. 

The mechanism of the formation of such stripes can be the following. Under plastic deformation the material can reach some definite limit of hardening (here this problem is not discussed in detail). Then this material behaves as an ideally plastic material. From the theory of ideal plasticity, it is known that such a state is the necessary condition for the loss of the stability of homogeneous plastic flow, that is, layers of large plastic shear appear. Such layers are modeled as the surfaces of the shear. That is to say, the parts of the material separated by these surfaces slip relative to each other with a resistance to the plastic flow that is equal to the plastic shear stress for this material. The disposition of the shear surface for die-forming is shown in Fig. 20. 

The unit of the hardness is that of a pressure. Since the total surface of the indentation was used, the hardness does not correspond to the average pressure between indenter and material. This can be corrected by using only the projected area of the indentation. If such a definition is used, the hardness is almost independent of the testing force, provided the material does not harden and the testing force was sufficient to cause significant plastic deformation. If... 

---