Equipment and facilities

Individual force measuring devices equipped with strain gauges are used for forces of up to 10 MN. Calibration and application for forces greater than this places additional demands on the testing system.
The range of standard force measuring equipment in Germany goes up to 16 MN PTB.
One possible way of calibrating higher forces to within the required degree of measurement uncertainty is to use a combination of high-precision force transducers.
BAM has developed a modular system consisting of several force measurement components. Each component includes a force sensor, a hydraulic cylinder and two cone plates.

BAM's modular test system (bottom) during the comparative test with Empa's built-up system (top)
(enlarged picture)

Structure of force components (enlarged picture)

The hydraulic cylinders, which are needed for levelling, can offset an incline of 1.7° and carry loads up to 6.3 MN (maximum oil pressure 630 bar). Up to six components are currently available which are hydraulically connected to each other. A hand pump enables the oil pressure in the cylinders to be altered individually or jointly. When all the valves are open, a uniform force distribution can be achieved for the individual components.
Skew pressure can be measured or generated by closing certain valves in the measurement system. This feature is of great importance when the behaviour of materials testing machines is being tested. Skew pressure can be determined during force changes in the machine without using the machine's pressure control which may be unreliable.

Measurement uncertainty of the modular system

Measurement uncertainty of individual force components and BAM's modular system (enlarged picture)

Individual force transducers have been calibrated up to 5 MN using BAM's reference standard force measurement equipment. They satisfy and even exceed Class 1 per EN 10002-3. A comparative test has been performed in a 20-MN universal testing machine between a four-component system and the built-up system of Empa (3 x 7 MN). The results have proved that the modular system is also suitable for testing Class 1 materials testing machines. Measurement uncertainties calculated from the results of the comparative test and the calibration of the individual force transducers are displayed in the diagram.

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