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Measuring Stress and Strain

Strain gauges are metallic foil grids on a flexible backing material which are glued to a test item‘s surface to measure tension or compression. Tension and compression are represented by very small changes in the gauges resistance. The small changes in resistance are translated into measurable voltages using a Wheatstone bridge. Using multiple gauges the Wheatstone bridge can be configured to measure only the strain of interest, such as bending, torsion or axial components. The gauge configuration can also eliminate apparent strains which result from temperature changes, a phenomenon to which strain gauges are particularly sensitive.

Strain (change in length/length) is a very small number so results are generally multiplied by 106 and expressed as microstrain, or me.

Did you know that m+p international's measurement systems can be used for all your strain measurement requirements?

For channel counts from 4 to 32 we support National Instruments 9237 USB and CompactDAQ Simultaneous Sampling Bridge Module.

National Instruments instrument

The system has a 24-bit dynamic range with simultaneous sampling rates up to 50,000 samples per second.
¼, ½ and full-bridge conditioning are available with a range of excitation voltages.  Hence, transducers, e.g. load cells, are supported as well as traditional foil strain gauges. The Bridge Module can be mixed with other modules in a CompactDAQ chassis for simultaneous measurement of acceleration, temperature, voltage, rpm, etc.

SO Analyzer automatically detects the bridge module(s) and adds the Strains page to the Setup Wizard, as shown below:

Screenshot numbers

All strain gauge settings are entered on the Strain page of the Setup Wizard including Bridge Configuration, Excitation Voltage, Gauge Factor, Gauge Resistance and Lead Wire Resistance.

In parallel with collection of raw strain data at high sampling rates SO Analyzer provides the ability to save one computed value every, for example, 2048 samples.  This computed value can be any combination of RMS, +ve Peak, -ve Peak, Peak to Peak and Mean.  This can be an invaluable summary which identifies significant events in the raw data, thereby avoiding unnecessary manipulation of files whose size may run into GBytes. 

The following plot shows an example of this feature; the strain induced by increasing temperature has been plotted over a 15 minute period whilst the raw dynamic data is also available:

Screenshot curve blue


Traditionally, a shunt resistor was used in one arm of the Wheatstone bridge to change the gauge resistance, thereby simulating a known strain.  The simulated strain and the change in output voltage were used to provide the calibration in microstrain per volt.  Because SO Analyzer has fully integrated software and hardware the shunt calibration is unnecessary, but it is still useful to confirm that everything is working as it should.  SO Analyzer includes an ‘app’ or user macro to perform the calculations, as shown below:

Screenshot numbers

Need more than 32 channels?

For higher channels counts SO Analyzer supports VXI Technology's EX1629 Remote Strain Gauge Measurement Unit.  This provides conditioning in multiples of 48 channels.

1629 instrument


This module is also supported by m+p international’s Coda system which provides continuous acquisition of <1000’s of channels, typically for large-scale fatigue testing and process monitoring.

Or a very rugged, standalone solution?

New from m+p international for portable and standalone data acquisition of strain, temperature and vibration signals is the compact and rugged cube I/O chassis together with our Coda (Continuous Data Acquisition) software product family.

The Ethernet-based cube I/O chassis from United Electronic Industries is ideally suited for a wide variety of industrial, aerospace and laboratory data acquisition and control applications. The small 3-slot cube only measures 4” x 4.1” x 4”, its “big” brother provides slots for 6 I/O boards and is 5.8” high. For larger applications compact, rugged I/O racks are available. Cubes and racks support standalone applications by storing the data on SD, SSD or other HDDs for later visualization and post-processing by the Coda software.

In summary, m+p has a range of strain gauge solutions which have been proven time and again to meet the needs of industry.

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