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the measured HL value may be too small or too large.
In many situations, potential problems can be checked in the following manner:
Medium-weight samples and also heavier samples with protruding parts or thin walls should be
placed on a solid support in such a manner that they do not move or flex during the test impact.
Light-weight samples should be coupled with a non-yielding support such as a heavy base plate to
guarantee that they are rigid. Clamping in a vice is of no value, since the samples become exposed
to stress and because complete rigidity is never attained, resulting in measured HL values which
would be too high and show excessive variations.
Note: Thin areas or parts can oscillate slightly upon impact, even in the case of heavy or
medium-weight workpieces. This can lead to incorrect values and a clanking noise when an
impact is carried out.
5.1.2 Coupling
The following requirements must be met for the coupling:
The contact surface of the sample and the surface of the base plate must be level, flat and ground
smooth.
The impact must be carried out perpendicular to the coupled surface.
Impact devices types
Minimum sample thickness for coupling
D, DC, DL, E
3mm
G
10mm
C
1mm
Coupling procedure:
Apply a thin layer of coupling paste to the
contact surface of the specimen.
Application of coupling paste.
Press the specimen firmly against the base
plate and spread the paste using circular
motions.
The coupling process has been carried out
properly if there is no longer any metallic contact
between the parts.
Rubbing both parts
Carry out the impact vertically on the
specimen.
The coupling ensures a rigid connection between
the two parts, the absence of surface stress on the
specimen and thus reliable test values.
Coupled specimen
Insufficiently coupled samples produce large variations of individual measurements, HL values which are
too high and the operation is characterized by a rattling noise upon impact of the test tip.
