Page 156 - Mechanical Behavior of Materials

Section 4.7 Hardness Tests 157

4.7 HARDNESS TESTS

In engineering, hardness is most commonly defined as the resistance of a material to indentation.
Indentation is the pressing of a hard round ball or point against the material sample with a known
force so that a depression is made. The depression, or indentation, results from plastic deformation
beneath the indenter, as shown in Fig. 4.28. Some specific characteristic of the indentation, such as
its size or depth, is then taken as a measure of hardness.
Other principles are also used to measure hardness. For example, the Scleroscope hardness test
is a rebound test that employs a hammer with a rounded diamond tip. This hammer is dropped from
a fixed height onto the surface of the material being tested. The hardness number is proportional to
the height of rebound of the hammer, with the scale for metals being set so that fully hardened tool
steel has a value of 100. A modified version of this test is also used for polymers.
In mineralogy, the Mohs hardness scale is used. Diamond, the hardest known material, is
assigned a value of 10. Decreasing values are assigned to other minerals, down to 1 for the soft
mineral talc. Decimal fractions, such as 9.7 for tungsten carbide, are used for materials intermediate
between the standard ones. Where a material lies on the Mohs scale is determined by a simple
manual scratch test. If two materials are compared, the harder one is capable of scratching the
softer one, but not vice versa. This allows materials to be ranked as to hardness, and decimal values
between the standard ones are assigned as a matter of judgment.
Very hard steels have a Mohs hardness around 7, and lower strength steels and other relatively
hard metal alloys are generally in the range 4 to 5. Soft metals may be below 1, so their Mohs hard-
ness is difficult to specify. Various materials are compared as to their Mohs hardness in Fig. 4.29.
Also shown are values for two of the indentation hardness scales that will be discussed shortly.
Indentation hardness has an advantage over Mohs hardness in that the values obtained are less a
matter of interpretation and judgment. There are a number of different standard indentation hardness
tests. They differ from one another as to the geometry of the indenter, the amount of force used, etc.
As time-dependent deformations may occur that affect the indentation, loading rates and/or times
of application are fixed for each standard test.

P

D
d

Plastic

Elastic

Figure 4.28 Plastic deformation under a Brinell hardness indenter.