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26 April 2017

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Microlink 770 Strain Measurement helps Compression Testing Research

Ilias Stavrakas, Assistant Professor of the Laboratory of Electronic Devices and Materials in Greece, has been investigating compression tests of Dionysos marble.

He repeatedly subjected the marble to increasing loads. To measure the stress on the marble he used Kyowa strain gauges attached to a Microlink 770 with a 120 ohm resistor bridge, and ran Windmill Streamer software to record the strain readings.

Microlink 770

The Microlink 770 is a high speed data acquisition unit which connects to the computer over USB. It is a versatile unit which, with appropriate hardware, can measure not only strain but temperature, pressure, voltage or current through 16 analogue input channels.

Windmill's Streamer software saves readings to disk and can capture pre- and post-trigger data. It measured the bridge excitation and automatically adjusted the strain results to compensate for any changes in excitation.

The initial unstrained bridge measurement was much larger than the change in measurement due to strain. Streamer balanced (zeroed) the bridge giving a precise reading.

Streamer can save data in a variety of formats. These include ASCII for loading into software like Excel, and binary for compact file storage.

The Kaiser Effect

Repetitive loadings of brittle materials like marble cause internal damages that gradually expand, leading to inevitable failures. Interestingly, a structure will only suffer further internal damage if exposed to stresses higher than previously encountered. This is known as the Kaiser effect. First investigated by Joseph Kaiser in 1950, when a material experiences a new, higher load, it emits acoustic waves - but only after the initial load or stress level is exceeded.

However, all is not that straightforward - there is the problem of the Felicity ratio. This is when the Kaiser effect doesn't apply as expected.

The Microlink 770 and Streamer software helped Professor Stavrakas discover that the Felicity ratio strongly depends on the stress range to which the material is subjected, with regard to the deformation region.

Further Reading

Microlink 770: High Speed Data Acquisition

STAVRAKAS, Ilias. Acoustic emissions and pressure stimulated currents experimental techniques used to verify Kaiser effect during compression tests of Dionysos marble. Fracture and Structural Integrity, [S.l.], n. 40, p. Pages 32-40, apr. 2017. ISSN 19718993. Date accessed: 20 Apr. 2017. doi:http://dx.doi.org/10.3221/IGF-ESIS.40.03.

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