An Introduction to Strain Gauges
How Engineers Use Sensors to Measure Strain in Mechanical Parts
Strain gauges are the most widely used method to measure small strains in mechanical components.
A strain gauge is a resistance-based sensor used by mechanical engineers to measure strain in an object. Strain is defined as the change in length of a component divided by the length of a component. Strain, therefore, does not officially have a unit of measurement, but for reference purposes, the unit of “strain” is used. Because the changes in length are often very small, the unit of microstrain, or strain times 10 to the 6th power, is often used.
A strain gauge is the primary sensor type used to measure strain. The primary type of strain gauge is a metal foil gauge. A strain gauge consists of a long thin “wire” of metal foil that is wrapped back and forth across a grid, called a matrix. The matrix is attached to a thin flexible backing material with an adhesive, often a cyanoacrylate. The strain gauge is bonded to the part to be evaluated, and the matrix is oriented in the direction of the applied strain. The strain exerted in the part is also exerted on the strain gauges, and the wire that makes up the matrix stretches or compresses.
Strain Gauge Configurations
Strain gauges are available in a wide variety of sizes in a wide variety of sizes and configurations, depending on the material and geometry of the part to be tested and the expected strain levels. Matrix lengths can vary from a few millimeters to several inches.
While one strain gauge measures strain along a single axis, multiple strain gauge matrices can be combined into a single sensor. The most common multiple matrix configuration is the bi-axial strain gauge. In this case, two individual strain gauges are oriented at a right angle, with their axes passing through a common point. Other multiple matrix orientations include gauges for measuring shear strain, residual stresses, and hole stresses.
Strain Gauge Functionality
A strain gauge is a resistive sensor. A voltage is passed through the wire, and any variation in resistance is calculated based on a measured voltage. If the part is compressed, the wire that makes up the strain gauge matrix is compressed, and its cross-section area increases. This reduces the resistance of the wire. If the part is stretched, the wire that makes up the strain gauge matrix is compressed, and its cross-sectional area decreases. This increases the resistance of the gauge. In these terms, if tensile strain is considered positive, then resistance is proportional to strain. The measured voltage is converted to strain using a circuit called a Wheatstone Bridge.
In addition to mechanical strain, strain gauges can also be used to measure expansion or contraction in a part due to thermal strain. An example of a part where strain gauges are used to monitor thermal strain is railroad rails. Large compression forces in continuously welded rail can result in track buckling in the summer, and large tensile forces can result in rail breaks in the winter. Obviously, both conditions are undesirable, so strain gauges are often used to monitor rail conditions.
Strain gauges are a useful tool to measure small strains in mechanical parts. Strain gauges are generally inexpensive, easy to install, and provide accurate results. To learn more about strain gauge installation techniques, please read "Strain Gauge Installation Methods".
Source
Vishay-Micromeasurements website
