Hardness testing technologies

In the Brinell, Vickers and Knoop hardness testing methods, the hardness value must be determined by optical evaluation of the diagonals of the hardness test indent.

There are various options for optical evaluation of the indentation:

Evaluation by measuring magnifier
Evaluation by focussing screen and calliper gauge
Evaluation by camera system – state of the art

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Exact measurement results can only be guaranteed on clearly detectable test indents with optimum contrast and best brightness conditions. It is also important to ensure that brightness conditions are always the same for all test surfaces. To guarantee this and avoid influencing the measurement result through erroneous brightness settings, the brightness is controlled fully automatically by the integrated digital camera. In this way, any operator influence is entirely precluded.

The autofocus system works in two different ways:

With the direct force measurement principle, an integrated load cell registers precisely when the indenter comes into contact with the surface. The optimum focal plane is then already identified during the test cycle based on this force measurement.

Moreover, in the case of lenses with very high magnification levels, the digital camera performs additional automatic focussing.
 

In the Brinell hardness test for metals, a clear deformation (bulge) appears around the indent. This deformation is more pronounced on soft metals than hard metals.

This in turn means that the edges of the indent are difficult to identify and evaluate if light field illumination is used. Use of a ring light for dark field illumination makes the edges of the indent easier to identify and evaluate.

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In the Rockwell hardness testing methods, plastics test, carbon test and Brinell/Vickers depth methods, measurement of the differential depth is required to determine the hardness value. Here the indentation depth caused by the indenter is measured. In modern hardness testing machines, this is done with a depth measurement gauge.

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Hardness testing machines are subject to several EU directives, with which they must comply and according to which they must be furnished with a CE label.

This means that checks are made to see which directives apply to the product and whether the product meets the safety targets specified in the relevant directives.

This is accomplished by means of a conformity assessment (risk analysis) to determine the health and safety requirements applicable to the machine. The machine is subsequently designed and built with due regard to the results of the risk analysis.

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The backbone of any hardness testing machine is the machine base. This needs to be of a robust and rigid design for all test tasks (e.g. made of aluminium or cast iron) in order to ensure absolutely consistent test conditions across the entire range of test forces. In addition, design and modularity are nowadays becoming increasingly important and therefore need to be taken into account in the design of the machine base.

Many hardness testing machines used for simple test tasks, such as single measurements, are equipped with a handwheel for specimen infeed.

Handwheels are particularly suitable for small specimens. Moreover, the specimens can be clamped and unclamped very quickly. Due to the handwheel and the associated need for manual operation, this type of hardness tester can only be used as a standalone solution and cannot be integrated into production systems.

Increasing numbers of hardness testing machines have a motor-driven Z-axis for nose cone infeed with subsequent clamping of the test specimen.

Thanks to the motorised test unit, the test area increases in size, while the operating height and compact dimensions remain the same. The resulting ergonomic benefits for the operator represent a significant advance in terms of working conditions. Moreover, hardness testers of this type are ideally suited for use with large and heavy test specimens.

The motorised Z-axis also enables optimum integration of these machines into fully automatic production systems, which means that they can be used for 100% tests. Of course, they can just as easily be deployed as standalone solutions.
 

In order to conduct curve measurements or measurements on a large number of test specimens, a fully automatic hardness testing machine with motorized cross slide is required.

The most important technical attributes of such cross slides are large travel ranges, high speeds and a high degree of repeatability and positioning accuracy. This is guaranteed by the high resolutions as well as virtually backlash-free spindles.