Most people feel that they have an intuitive feeling for hardness in a material. However, there are different kinds of hardness that characterize many complex properties:
1. Hardness tests that measure
the resistance of a material to indentation. Examples of this test
include
the Brinell
hardness, Vickers and Knoop indenters, Barcol hardness, Shore durometer
(170) and the
ISO durometer
(7). The last two tests measure the indentation with the load applied,
which tends to
measure
the compliance. The first three tests analyze the residual indentation
after the load is
removed,
which tends to measure plastic flow.
2. Hardness
tests which analyze a material's resistance to sharp objects and other
scratching substances.
These
test also characterize plastic flow. Examples of the test are Bierbaum
hardness or scratch
resistance
tests and Moh hardness.
3. Hardness
tests that measure rebound efficiency or resilience. Examples of
this test include various
Rockwell
hardness tests as described by the standard ASTM test D 785. Because
a hardness test is
a creep
measurement, since a controlled load is applied and a deformation is observed,
the time scale
should
be rigorously monitored, especially when a viscoelastic response can be
anticipated.
Below is a table of hardness tests of plastics as measured by the methods noted:
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