Hardness Test of Rebound Method

Hardness testers using the Rebound method operate in a slightly different manner. Although the size of the test indentation generated is connected with the material hardness even in this case, it is indirectly measured via the loss of energy of a so-called impact body.

A mass is accelerated to the surface of the test object and impinges on it at a defined speed, i.e. kinetic energy.

The impact creates a plastic deformation of the surface, i.e. an indentation, due to which the impact body loses part of its original speed - or energy. It will lose more velocity when creating a bigger indentation on softer material.

Technically, this principle of measurement is implemented by means of an impact body which has a spherical tungsten carbide tip and which is accelerated onto the test surface by spring force. The velocities after and before the impact are each measured in a non-contact mode.

This is done by a small permanent magnet within the impact body (Fig. 2) which generates an induction voltage during its passage through a coil, with this voltage being proportional to the speed.

The inventor of this method, D. Leeb, defined “his own” hardness value, the Leeb hardness value. The Leeb hardness value, HL, is calculated from the ratio of the impact and rebound speed according to:

You might ask yourself: "Who wants to measure the hardness value in Leeb?" The answer is: as a matter of fact, anybody who uses the rebound hardness testing method does it, because the Leeb hardness value is, by definition in the equation (2), the actual physical measurement value behind this method.

However, nearly no user indicates the Leeb hardness value HL in his specifications or test reports. We mostly convert into the required hardness scales (HV, HB, HS, HRC, HRB, N/mm2). For this reason, only conversion brings the rebound hardness method to life. Empirically determined conversion tables for different material groups are stored in the Krautkramer hardness testing instruments.

To apply the principle, an impact device uses a spring to propel an impact body through a guide tube towards the test piece. As it travels towards the test piece, a magnet contained within the impact body generates a signal in a coil encircling the guide tube. After the impact, it rebounds from the surface inducing a second signal into the coil. The Krautkramer instrument calculates the hardness value using the ratio of the voltages and analyzes their phases to automatically compensate for changes in orientation.

Due to the patented signal processing, there is no need for any manual correction for the impact direction. Only Krautkramer hardness testers offer this auto balancing feature.

Application solutions are determined by the force and ball size of the impact body. The operator can select between different impact devices for the DynaMIC hardness testers (Dyna D, Dyna E and Dyna G) as well as the DynaPOCKET instrument.