The three primary factors in selecting gages are operating temperature, strain state (gradient, magnitude, time dependence) and stability required. Shown here are one-axis linear, reversed bending, extra long, dual shear, and half and full-bridge gages — ranging in price from just a couple dollars to several times that for full-bridge models.

The engineering measurement called strain is the deformation per unit of length of a material when force is applied to it. In other words, strain is a ratio of a material's change in length from an initial unstressed reference length. Strain gages are elements that sense this change and convert it into an electrical signal. How? These components change resistance as they are stretched or compressed, similar to how wire behaves: Specifically, when wire is stretched, its cross-sectional area decreases, causing its resistance to increase.

To select the proper strain gage type, pattern and size, a designer must understand how the component to be monitored is loaded, and know the principal stress direction. To measure minute strains, components capable of minute resistance changes must be used.

Several strain gage designs depend on the proportional variance of electrical resistance to strain: piezoresistive or semiconductor gages, carbon-resistive gages, bonded metallic wires, and foil resistance gages. However, bonded resistance strain gages are by far the most widely used in stress analysis. These gages 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. Bonded resistance strain gages are low in cost, can be made with short gage length, are only moderately affected by temperature changes, have small physical size and low mass, and are quite sensitive to strain.

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

Potential errorsand characteristics

In a stress-analysis application, it is important to examine potential error sources prior to taking data. First of all, some gages can be damaged during installation — so strain gage resistance must be checked prior to applying stress. Electrical noise and interference can also alter readings. Shielded leads and adequately insulating coatings may prevent these problems. A value of less than 500 M Ohms (using an ohmmeter) usually indicates surface contamination.

Strain gage selection considerations include: bridge type, quarter, half, or full Wheatstone bridge; gage pattern, grid length and width; resistance; ribbon leads, solder pads (bondable terminal pads are recommended for lead wire connection) or insulated leads; foil measuring grid, constantan or karma; carrier material; availability. Active grid length in the case of foil gages is the net grid length and does not include end loops or tabs. Manufacturers often design foil and carrier dimensions for optimum strain-gage performance.

The resistance of a strain gage is defined as the electrical resistance measured between the two tinned copper leads or two solder pads. Common nominal strain gage resistance values are 120, 350, 500, and 1,000 Ohms. Gage factor or strain sensitivity GF is defined as the ratio of the fractional resistance change to the fractional change in length (strain) along the gage axis. Gage factor is a dimensionless quantity, nominally 2± tolerance, and is published in specifications for strain gages. The exact gage factor of a production lot is determined by sample measurements and is given on the package of strain gages.

Consider temperature

Bending, axial, shear, and torsional strain are the most common types measured. Output voltages from the Wheatstone bridges depend on the strain gages selected, force applied, and dimensions of the component part.

Reference temperature is the ambient temperature for which technical strain gage data are valid, unless temperature ranges are given. Data quoted for strain gages are based on a reference temperature of 23°C. Temperature characteristics vary: Temperature-dependent changes of specific gage grid resistance (in applied gages) depend on the grid's linear thermal expansion coefficient and specimen material. These resistance changes appear as mechanical strain in the specimen.

Representation of apparent strain as a function of temperature is called the temperature characteristic of the strain gage application. To keep these apparent strain changes as small as possible, each strain gage is matched during the production to a certain linear thermal expansion coefficient. Certain manufacturers offer strain gages with temperature characteristics matched to ferritic steel and aluminum.

The service temperature range is the range of ambient temperature in which strain gage use is permitted — and the range in which there is no risk of permanent changes to measurement properties. Service temperature ranges depend on whether static or dynamic values are to be sensed.

For permitted RMS bridge-energizing voltage, the maximum values quoted are only permitted for appropriate application on materials with good heat conduction (for example, steel of sufficient thickness) if room temperature is not exceeded. In other cases, temperature rise in the measuring grid area may lead to measurement errors. Measurements on plastics and other materials with bad heat conduction require the reduction of the energizing voltage or the duty cycle — pulsed operation.