History
The Leeb rebound hardness test method was developed in 1975 by Leeb and Brandestini at Proceq SA to provide a portable hardness test for metals. It was developed as an alternative to the unwieldy and sometimes intricate traditional hardness measuring equipment. The first Leeb rebound product on the market was named 揈quotip? a phrase which still is used synonymously to 揕eeb rebound?due to the wide circulation of the 揈quotip?product.
Traditional hardness measurements, e.g. according to Rockwell, Vickers and Brinell, are stationary, i.e. fixed workstations are set up in segregated testing areas or laboratories of plants. Most of the times, these methods are used selectively in destructive tests. Samples are cut off from selected parts and measured in the laboratory. From the individual results, statistical conclusions are drawn for the entire batch. The portability of Leeb testers can sometimes help to achieve higher testing rates without destruction of samples (test is considered as non-destructive), which in turn simplifies processes and saves costs.
Method
The traditional methods are based on well-defined physical indentation hardness tests. Very hard indenters of defined geometries and sizes are continuously pressed into the material under a particular force. Deformation parameters, such as the indentation depth in the Rockwell method, are recorded to give measures of hardness.
When using the dynamic Leeb principle, the hardness value is derived from the energy loss of a defined impact body after impacting on a metal sample, similarly to the Shore scleroscope. The Leeb quotient (vi,vr)is taken as measure of the energy loss by plastic deformation: the impact body rebounds faster from harder test samples than it does from softer ones, resulting in a greater value 1000?I>vr/vi.When using a magnetic impact body, the velocities can be deduced from the voltage induced by the body as it moves through the measuring coil. The quotient 1000譾r/vi is quoted in the Leeb rebound hardness unit HL.
While in the traditional static tests the test force is applied uniformly with increasing magnitude, dynamic testing methods apply an instantaneous load. A test takes a mere 2 seconds and, using the standard probe D, leaves an indentation of just ~0.5 mm in diameter on steel / steel casting with a Leeb hardness of 600 HLD. By comparison, a Brinell indentation on the same material is ~3 mm (hardness value ~400 HBW 10/3000), with a standard-compliant measuring time of ~15 seconds plus the time for measuring the indentation.
Scales
Depending on the probe (搃mpact device? and indenter (搃mpact body? types that vary by geometry, size, weight, material and spring force, diverse impact devices and hardness units are distinguished, e.g.:
Equotip impact device D with hardness unit HLD
Equotip impact device G with hardness unit HLG
Generally, impact device types are optimized for certain application fields. This is similar to using various indenter geometries and test loads in Rockwell (e.g. HRA, HRB, HRC), Brinell and Vickers. Leeb hardness results in HL are often converted to the traditional hardness scales HRC, HB and HV mainly for convention reasons between supplier and customer.
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