Measuring material strength with portable hardness testers

Hardness testing instruments have been in use for more than a century to
measure the strength of metallic materials and hardness of their surface
finish.

The earlier models of these machines were bench mounted and measured
strength by applying a given force to a probe and making an indentation to cause
plastic deformation. The material hardness was determined by measuring the
depth or width of the indentation. However, different materials required different
shaped indenters and different methods of measuring the size of the indent. Additionally,
these machines were not suitable for fixed structures or very large or very
small and thin items, resulting in the development of less accurate portable instruments
using different methods of measurement.

The two common methods for this application include the dynamic Leeb
method, used for large items, where a spring loaded probe impacts the surface
and its bounce back distance is measured; and the ultrasonic contact impedance method,
where different forces are used for different materials, as well as for small
items and for the thin surface finish on an item.

Advances in digital technology have led to the development of an
instrument by a European manufacturer that not only offers extreme accuracy using
both Leeb and ultrasonic contact impedance methods but also tests to either US
or European standards.

Key features and advantages of the new instrument include a camera to
record the position of the probe on the item and any identifying number on the
test item; digital readouts of hardness and statistics provided on the tests;
ability to detect and reject inaccurate Leeb readings; and facility to store all
the information from the tests in the instrument for downloading later to a PC.

The accuracy and calibration of this instrument can be checked in the
same way as it’s done for bench testers by performing hardness tests on a range
of certified test blocks of known hardness.

Available from Hylec Controls, the new portable
hardness testers equip users with greater capabilities, previously only
available on sophisticated bench mount testers, at a price similar to much
simpler portable instruments.

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