The Rebound or Leeb's method
Hardness testers using Leeb's 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 speed by creating a bigger indentation and, thus, at 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 fired onto the test surface by spring force.
The speeds 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 which generates an induction voltage during its passage through a coil, with this voltage being proportional to the speed.
The Swiss 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: HL = VR/VI×1000 (VI, VR = Speed before the impact / after the impact.)
You might asked 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, 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. Therefore, conversion tables, are stored in all instruments.