- Strain Gauge Definition: A strain gauge is a device that measures the deformation (strain) of an object when force is applied, through changes in electrical resistance.
- Working Principle: The strain gauge detects minute geometrical changes as resistance changes, which indicate the level of stress on the material.
- Bridge Circuit: The strain gauge is part of a bridge circuit, where it helps detect imbalances in resistance that correspond to stress, measured by a central voltmeter.
- Temperature Compensation: To address temperature-induced resistance changes, strain gauges often include compensation techniques using materials like constantan alloy or dummy gauges.
- Common Uses: Employed extensively in mechanical engineering, strain gauges analyze stress on machinery and structural components, crucial for safety and functionality.
What Is a Strain Gauge?
A bonded resistance strain gauge measures deformation at the surface of a component. Strain is the change in length divided by the original length, so it is dimensionless and is often reported in microstrain. When the component stretches or compresses along the gauge axis, the bonded measuring grid deforms with it. The grid’s end-to-end resistance changes by a small fraction.
The gauge factor relates fractional resistance change to strain. A strain gauge measures strain directly, not stress. Calculating stress from that strain also requires a valid material model, elastic properties, loading direction and knowledge of the stress state.
A common foil gauge uses a thin metallic grid on an insulating carrier. The folded grid places a long conductor within a small area and makes the gauge most sensitive along one axis; it does not mechanically amplify stress. A specified adhesive bonds the carrier to a prepared surface so strain can transfer into the grid.
Because the resistance change is usually very small, the measuring circuit needs stable excitation, bridge completion, amplification and calibration. Installation quality, gauge alignment, lead resistance and environmental protection all affect the result.
Strain Gauge Bridge Circuit
A strain gauge bridge circuit uses the gauge in a Wheatstone bridge. Excitation is applied across one diagonal, and a differential voltmeter or instrumentation amplifier reads the other. The bridge converts a small resistance ratio change into a measurable voltage:
In the illustrated circuit, R1 and R3 form two bridge arms, while R2 is a completion or balance element. When the arm ratios are equal, the ideal differential output is zero. Strain changes the active gauge resistance, unbalances the bridge and creates an output whose polarity shows tension or compression for the chosen wiring. A differential amplifier can amplify this output before digitisation.
Temperature Effects on a Strain Gauge
Temperature can change grid resistance, gauge factor, lead resistance, adhesive behaviour and the dimensions of both the gauge and specimen. The resulting bridge output can look like mechanical strain. The sign and size of each effect depend on the materials and installation, so conductor resistance does not always follow one universal temperature rule.
Many foil gauges use constantan because it provides useful strain sensitivity and can be processed for self-temperature compensation. This compensation is matched to a specified specimen expansion coefficient and does not remove every temperature error. In the depicted quarter-bridge approach, a dummy gauge can replace R2 as a compensating arm.
The dummy gauge must experience the same temperature as the active gauge but little or none of the measured mechanical strain. Similar thermal changes in each arm then reduce the bridge’s apparent-strain output, although cancellation is not perfect. Lead-wire compensation and suitable resistance measurement wiring may also be needed. Use excitation within the manufacturer’s limit because electrical power can heat the strain gauge; allowable excitation depends on gauge area, resistance, mounting material and heat transfer.
Quarter-, half- and full-bridge configurations describe the number and placement of active gauges. Half- and full-bridge layouts can increase sensitivity or compensate unwanted strain components when their orientations match the measurement.
Uses of Strain Gauges
- Experimental stress analysis and structural testing
- Load cells, force transducers, torque sensors and pressure transducers
- Monitoring strain in machinery, vehicles, aircraft components and civil structures
