In the Brinell method, a carbide ball penetrator (ball diameter is expressed in mm) is pressed on the test surface (flat and smooth) with a certain load and for a certain time (normally 15 seconds). The indentation diameter is measured with an optical system (microscope or profile projector); in case the indentation is not perfectly round, the medium value will be considered.
In practice, given the load, the diameter of the ball and the diameter of the impression, with the use of tables the Brinell hardness number is obtained. The Brinell hardness (HB) is given by the relation between the applied load and the rounded shape given from indentation Normally, the Brinell method employs balls having the following diameters: 10, 5 and 2.5 mm.
Advantages of the Brinell method
The main advantage offered by the Brinell method consists in the possibility of employing heavy loads using rugged and easy-to-use instruments.
Furthermore, the indentation can be read by a microscope or an eyepiece.
Testing is possible even if positioning of the piece is not perfect, as required by Rockwell method. Brinell testing is not sensitive to the deflection of the testpiece.
It is possible to derive the tensile strength value simply by multiplying the Brinell hardness number by a certain factor, changing according to the test material.
Deformation of the indentation may show existing stress in the test material.
Limitations of the Brinell method
The main limitation of this method is due to the optical reading of the indentation that may cause measurement errors due to the operator.
Furthermore, an accurate surface preparation is needed to obtain reliable results.
To overcome this problem, the choice is often to work according to the Rockwell method but using Brinell penetrators and loads.
In case of testing of cylindrical surface, it is necessary to create a flat area on the test surface