A strain gauge is a sensor that measures how much a material deforms under load by converting that deformation into a change in electrical resistance. A fine metallic foil grid, bonded to the surface of the test object, stretches and compresses with the object, and the resulting resistance change is directly proportional to strain. Because stress cannot be measured directly, this small resistance change is the starting point for most stress measurement in engineering, from prototype validation to long-term structural monitoring.
Strain, and why engineers measure it
When an external force acts on an elastic material, it generates stress, and the stress deforms the material. If a bar of original length L extends by an amount ΔL under tension, the ratio ΔL/L is the strain. A 100 mm specimen that extends by 0.1 mm has a strain of 0.001, more usually written as 1,000 × 10⁻⁶ or 1,000 microstrain. Compression works the same way with a negative sign.
Engineers measure strain for three practical reasons. The first is to quantify deformation itself, for example how much a component deflects in service. The second is to determine stress and the degree of safety of a material or a structural element, since stress follows from measured strain and the material’s elastic modulus. And many transducers, including load cells, pressure sensors and torque sensors, work by converting a physical quantity into strain, so strain gauges sit inside instruments engineers use every day without thinking about them.
Strain can be measured mechanically or optically, but the vast majority of stress measurement is carried out with electrical resistance strain gauges. They are small, add negligible mass to the specimen, and connect naturally to multi-channel data acquisition systems, which makes them practical at the scale a real test program demands.
How a strain gauge works
A foil strain gauge consists of a grid of photographically etched metallic resistance foil, or fine resistance wire, formed on an electrically insulating backing, with gauge leads attached and a cover film protecting the grid. The gauge is bonded to the specimen with an adhesive so that strain in the specimen transfers through the backing into the foil.
The working principle rests on one property of metals. The electrical resistance of a metal changes in proportion to the mechanical deformation applied to it. When the specimen strains, the foil strains with it, and its resistance R changes by ΔR. The relationship is ΔR/R = K × ε, where ε is the strain and K is the gauge factor, a calibration constant shown on every gauge package.
That resistance change is far too small to read directly. A Wheatstone bridge circuit converts it into a measurable voltage. With the gauge forming one arm of the bridge and an excitation voltage E applied, the quarter-bridge output is approximately (E/4) × K × ε when ΔR is much smaller than R. In practice the gauge connects to a strainmeter, which configures the bridge, supplies the excitation voltage and shows the measured strain on a digital display or outputs it as an analogue signal to a data acquisition system.
Matching the gauge to the measurement
There is no universal strain gauge. The measurement dictates the gauge, and four choices carry most of the weight.
Gauge length comes first. Short gauge lengths measure local strain, at a fillet, in the valley of a gear tooth or on a printed circuit board where installation space is limited. Long gauge lengths average strain over a larger area, which matters on heterogeneous materials. Concrete is the classic case, because it is composed of cement and aggregate, so a gauge needs enough length to average out the irregular strain between the two.
Gauge resistance is 120 Ω in general use, with 60 Ω, 350 Ω and 1,000 Ω versions also produced. Higher-resistance gauges yield a higher bridge output when higher excitation voltages are applied, but they are also more susceptible to noise. Most gauges used in transducer production are 350 Ω.
Temperature affects the reading even when nothing about the load changes. A change in ambient temperature alters gauge resistance through thermal expansion of both the gauge and the specimen, together with the thermal coefficient of resistance of the grid alloy. Self-temperature-compensated gauges are manufactured to suit a specimen with a specific coefficient of linear thermal expansion, such as mild steel at 11 × 10⁻⁶/°C, and hold thermal output within approximately ±1.8 × 10⁻⁶ strain/°C across the compensated range. The grid alloy also sets the operating temperature limits. Copper-nickel alloys cover most work, nickel-chromium alloys extend the usable range, and platinum-tungsten alloys serve environments from 500 °C to 800 °C.
Direction matters as well. A single gauge measures strain along its own axis only. Where the direction of principal strain is unknown, a three-element rosette with grids at 0°, 45° and 90° provides three readings from which the principal strains, their directions and the maximum shearing strain can all be calculated.
Where strain gauges are used
Structural and materials testing is the core territory, and the range of installations is wider than most first-time users expect. Miniature gauges fit on printed circuit boards and surface-mount components. Long gauges measure strain on concrete surfaces, and purpose-made gauges cast into concrete measure internal strain. Weldable gauges suit steel structures where adhesive bonding is impractical on site. Bolt gauges insert into a pre-drilled hole in a bolt head to measure tensile force, with the bolt calibrated after installation for accurate readings. High-endurance gauges bonded to composite materials have been demonstrated in fatigue testing beyond ten million cycles at strain levels of ±3,000 microstrain. Dedicated three-element gauges support residual stress measurement by the centre-hole drilling method, and waterproof, high-temperature, cryogenic and post-yield series cover environments a standard gauge cannot survive. Long-term versions support construction monitoring and structural maintenance over years rather than hours.
Selecting a strain gauge for your test
These are the questions we field most often when engineers specify gauges for a test.
What gauge length should I choose? As short as the strain gradient demands and as long as the material allows. Local strain concentrations call for short gauges. Heterogeneous materials such as concrete call for long gauges that average across the aggregate.
Which gauge resistance do I need? 120 Ω suits general stress measurement. 350 Ω is the usual choice where higher bridge output is wanted or a transducer is being built, accepting the greater noise susceptibility at high excitation.
Do I need a self-temperature-compensated gauge? If temperature will change during the measurement, yes, matched to the thermal expansion coefficient of your specimen material. For higher accuracy, the apparent strain versus temperature curve supplied with each gauge package can be used to correct the residual thermal output.
What about the leadwires? Published gauge data do not include the influence of the leadwire, so leadwire effects must be corrected, and three-wire connections are the standard way to compensate for leadwire temperature effects in quarter-bridge measurements.
We stock foil, weldable, waterproof, high-temperature, concrete-embedment and post-yield strain gauge series, together with bonding adhesives, coating materials and the data acquisition systems to read them. Our application engineers assist with gauge selection, installation method and bridge configuration, and we run strain gauge application training for teams doing their own installations.
Getting the measurement right starts before the first gauge is bonded. If you are working through gauge selection, installation or data acquisition for a test program, have a chat with us about your application.
