Hardness testing technologies

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Hardness testing technologies

HARDNESS KNOW-HOW

Hardness testing technologies

Here you will find useful information on the load application variants, the possibilities of measured value evaluation in hardness testing (optical vs. depth difference measurement), the electronics of a hardness tester (PC, PLC) as well as design topics from CE conformity to equipment variants (e.g. handwheel vs. motorized probe adjustment).

Load application

Interesting facts about the variants.

Load application

Measurement evaluation

Optical vs. Differential depth measurement.

Measurement evaluation

Electronics

Electronics of a hardness tester (PC, PLC).

Electronics

Design

From CE conformity to equipment options.

Design

Load application

Dead load with weights

In the past, the test force in hardness testing machines was applied by dead loads using weights. The test force is transferred directly to the test piece via a specific mass. The test force is usually adjusted using a lever system or by replacing the weights.

In depth difference methods such as Rockwell hardness testing, the preload and main force are usually generated by coupling additional weights for the main force.

Dead load systems of this type are often equipped with damping elements to apply the test forces as evenly and shock-free as possible. Nevertheless, overshooting of the test force cannot be completely avoided due to the design. In addition, vibrations and external shocks can noticeably influence the measurement result.

Dead load with springs

In some hardness testing machines, especially portable models, the test force is applied by dead weights with spring assemblies. In this case, the test force is not generated by a directly acting mass, but by a spring. The force remains constant and cannot usually be changed. The main advantage of this principle is its lower sensitivity to vibrations compared to systems that work with directly acting weights.

Closed-loop control system

A closed control loop has the task of adjusting a physical control variable (r) to a defined setpoint (s) and keeping it constant by continuously measuring and readjusting the actual value (i). The functions of measuring, comparing and adjusting are performed continuously.

All information about closed control loops

Dead load with closed-loop control system

When force is applied using dead load in combination with a closed control loop, both techniques are combined. Part of the test force is generated by directly acting masses, while the remaining portion is controlled via a closed control loop. Dead loads are used particularly for very small test forces in order to precisely and reliably comply with the low tolerance limits.

All information about dead load with closed control loop

Measurement Evaluation

Brinell SmartLight

The Brinell hardness test has always represented a challenge with soft metals and difficult surfaces. Particularly with soft materials, the edges are not always perfectly recognisable due to considerable deformation (bulging) around the indentation.

The use of Brinell SmartLight makes the edges of the indent easier to identify and evaluate.

The SmartLight technology developed by EMCO-TEST combines a lens with collimated light. With this lighting, parallel light beams are directed by a mirror system onto the test indentation. The light therefore strikes the test indentation perpendicularly from above and prevents any shadowing in the area of the bulge. The contour is clearly recognisable and the indentation can be precisely evaluated.

The SmartLight technology is permanently integrated into the lens and requires no further settings by the operator.

Infos zu Brinell SmartLight

Optical evaluation

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

Brightness control

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.

Autofocus

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.

Automatic evaluation

It is hard to imagine present-day hardness testing machines without automatic evaluation of hardness indents. It dramatically reduces operator influence in hardness tests and improves the comparability of results.

A hardness indent is evaluated automatically in depth measurement methods by means of a depth measurement sensor in combination with the test software. The depth measurement gauge determines a depth differential during the indentation process with the indenter. Then the test software uses this differential to immediately calculate the hardness value automatically. Following indentation, the determined hardness value is shown on the display of the hardness testing machine.

Light/ dark field illumination

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.

All information about bright field and dark field illumination

Camera system

Modern hardness testing machines use a camera system for performing optical hardness tests. In the majority of cases, this consists of a digital camera with 1.3 megapixel resolution, a light source and lenses with various magnification strengths.

The camera system is used for two different processes. Firstly, it is used to determine the position of the hardness test, but the main purpose of the camera system is to manually or automatically evaluate the hardness test indents. In order to keep operator influence during hardness testing to a minimum, the camera system carries out focusing, brightness control and evaluation of the indent fully automatically. This guarantees a maximum level of repeat accuracy.

Differential depth measurement

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.

Electronics

Programmable logic controller (PLC)

A programmable logic controller (PLC) is a device that is employed and programmed for controlling or regulating a machine or system. These controllers can be found in application areas in the widest variety of industries, e.g. in fully automated production systems, injection moulding machines and likewise in hardness testing machines.

Further information on programmable logic controllers (PLCs)

Embedded-PC

An embedded PC is a modularly structured, compact industrial PC. It is an embedded system with a computer architecture based on that of a standard PC. In this case, however, only those components required for the relevant application are included in the system. Due to their smaller memory requirement, such PCs are frequently employed without a hard disk (using, e.g., a CF card instead). Embedded PCs can be found in products of the automotive industry, traffic technology, production and manufacturing technology, telecommunication and many more.

External PC

An external PC used to control a hardness testing machine refers to a standard commercially available PC that meets all the technical requirements (interfaces, memory, processing power, etc.) for the hardness tester. The standard PC is loaded with a standard operating system (e.g. WIN 7, WIN 8), potentially other software packages as well as the operating software for the hardness testing machine.

External PCs are operated using standard input devices, such as a mouse and keyboard, and equipped with a standard monitor for display purposes.

External PCs are primarily used in conjunction with fully automatic hardness testing machines. This is due to the fact that a large number of very complex and laborious hardness testing functions need to be implemented, which require a high storage capacity and plenty of processing power.

Design

CE conformity/ labeling

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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Cast-iron base

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.

Handwheel

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.

Motor-driven Z-axis

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.

Motorized cross slide

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.