Advances in strain gauge measurement

Paul Boughton

There are a number of technical factors that determine the successful usage of strain gauges – and it pays to take some expert advice.

The most universal measuring device for the electrical measurement of mechanical quantities is the strain gauge. Several types of strain gauges depend on the proportional variance of electrical resistance to strain: the piezoresistive or semi-conductor gauge; the carbon-resistive gauge; the bonded metallic wire; and foil resistance gauges.

The bonded resistance strain gauge is by far the most widely used in experimental stress analysis. These gauges consist of a grid of very fine wire or foil bonded to the backing or carrier matrix. The electrical resistance of the grid varies linearly with strain. In use, the carrier matrix is bonded to the surface, force is applied and the strain is found by measuring the change in resistance. The bonded resistance strain gauge is low in cost, can be made with a short gauge length, is only moderately affected by temperature changes, has small physical size and low mass, and has fairly high sensitivity to strain. 

In a strain gauge application, the carrier matrix and the adhesive must work together to transmit the strains from the specimen to the grid. In addition, they serve as an electrical insulator and heat dissipater.

The three primary factors influencing gauge selection are operating temperature, state of strain (gradient, magnitude, and time dependence) and stability required.

Because of its outstanding sensitivity, the Wheatstone bridge circuit is the most frequently used circuit for static strain measurements. Ideally, the strain gauge is the only resistor in the circuit that varies and then only due to a change in strain on the surface.

Potential error sources

In a stress analysis application, the entire gauge installation cannot be calibrated, so it is important to examine potential error sources prior to taking data.

Some gauges may be damaged during installation. It is important to check the resistance of the gauge prior to stress.

Electrical noise and interference may alter the readings. Shielded leads and adequately insulating coatings may prevent these problems.

Thermally induced voltages are caused by thermocouple effects at the junction of dissimilar metals within the measurement circuit. Magnetically induced voltages may occur when the wiring is located in a time varying magnetic field.

Temperature effects on gauge resistance and gauge factor should be compensated for as well. This may require measurement of temperature at the gauge itself, using thermocouples, thermistors or RTDs.

Prime strain gauge selection considerations are the following:

* Gauge length;

* Number of gauges in gauge pattern;

* Arrangement of gauges in pattern;

* Grid resistance;

* Strain sensitive alloy;

* Carrier material;

* Gauge width;

* Solder tab type;

* Configuration of solder tab;

* Availability.

Strain gauge dimensions

The active grid length, in the case of foil gauges, is the net grid length without the tabs and comprises the return loops of the wire gauges. The carrier dimensions are designed by Omega for the optimum function of the strain gauge.

The resistance of a strain gauge is defined as the electrical resistance measured between the two metal ribbons or contact areas intended for the connection of measurement cables. The range comprises strain gauges with a nominal resistance of 120, 350, 600, and 700 Ohms.

The strain sensitivity k of a gauge is the proportionality factor between the relative change of the resistance. The strain sensitivity is a figure without dimension and is generally called gauge factor.

Temperature characteristic

Temperature-dependent changes of the specific strain gauge grid resistance occur in the applied gauge owing to the linear thermal expansion coefficients of the grid and specimen materials. These changes appear to be mechanical strain in the specimen. The representation of the apparent strain as a function of temperature is called the temperature characteristic of the strain gauge application. To keep apparent strain through temperature changes as small as possible, each gauge is matched during the production to a certain linear thermal expansion coefficient. Omega offers strain gauges with temperature characteristics matched to ferritic steel and aluminium.

The maximum values quoted are only permitted for appropriate application on materials with good heat conduction if room temperature is not exceeded. In other cases, temperature rise in the measuring grid area may lead to errors. Measurements on plastics and other materials with bad heat conduction require the reduction of the energising voltage or the duty cycle.

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