Assessing mechanical properties of materials
Hardness testing enables you to evaluate the strength, ductility and wear resistance of a material, which can determine whether a material is suitable for the purpose you require. The hardness of a metallic material can be defined as its resistance to plastic deformation caused by a force applied through an indenter. It is an unusual property in that it is the result of a defined measurement procedure and not an intrinsic property which can be defined in terms of fundamental SI units.
In practice, hardness is measured in terms of the size of an impression made in a specimen by an indenter of a specified shape when a specified force is applied for a specified time, the indent being measured after the force has been removed. The three principal standard methods for expressing the relationship between hardness and the size of the impression are Brinell, Vickers, and Rockwell, each of which has a range of scales defined by a combination of applied load and indenter geometry.
Brinell hardness test: A ball indenter of diameter is pressed into the surface of the test piece using a prescribed force, and the diameter of the indentation is measured after the force has been removed. The time for the initial application of the force is 2 s to 8 s, and the test force is maintained for 10 s to 15 s.
Brinell Hardness Sensitivity Coefficients
Vickers hardness test: A diamond indenter, in the form of a square-based pyramid with an angle of 136º between the opposite faces at the vertex, is pressed into the surface of the test piece using a prescribed force and, after the force has been removed, the diagonal lengths of the indentation are measured. The time for the initial application of the force is 2 s to 8 s, and the test force is maintained for 10 s to 15 s.
Vickers Hardness Sensitivity Coefficients
Rockwell hardness test: Either a 120° diamond cone with a 0,2 mm radius spherical tip or a ball indenter of a specified diameter is pressed into the surface of the test piece using a two-step application – a preliminary test force F0 followed by an additional test force F1. The preliminary test force is applied and maintained for a duration that does not exceed 3 s and an indenter depth reading is recorded. The increase in force to the total test force F then occurs in between 1 s and 8 s. This force is maintained for a duration of 4 s ± 2 s, and the additional force is then removed. While the preliminary test force is still applied, a second indenter depth reading is made after a short stabilisation time.
Select the Rockwell scale of interest from the following choices:
The determination of a hardness value depends on many different parameters, such as force, diameter, depth, time, radius, and angle. Each of these measurements has an associated uncertainty, and these uncertainties will each contribute to the overall uncertainty of the hardness value. The sensitivity coefficient relates the uncertainty associated with the parameter and the uncertainty of the hardness value. Sensitivity coefficients can also be used to make corrections to hardness values in situations in which the measured value of an input parameter differs from the value specified within the relevant standard or, where a range of values is permitted within the standard, to correct the hardness to a nominal input parameter value.