Brinell Hardness of Materials
When a ball is pressed into a piece of material, the size of the indentation made by the ball is inversely proportional to the hardness of the material. This relationship was used by the Swedish engineer Dr. Johan August Brinell to develop a standardized testing method in 1900. The Brinnel hardness (HB) is calculated by dividing the applied force by the surface of indentation. Typical Brinnel hardness values include: aluminum 15 HB, copper 35 HB, mild steel 120 HB, hardened steel 1500 HB, rhenium diboride 4600 HB.
For details see the relevant Wikipedia entry.