All materials will deform when a stress or load is applied. Strain is a measure of the amount of deformation that occurs when an object is under stress. A strain gauge is an implement that is used to accurately measure this change in a material that is under load. Understanding the dimensional changes that occur when an object experiences strain is important in many mechanical design applications.
Elastic strain occurs when a material is subjected to a low level of stress. It will disappear after the stress is removed, and the material will return to its original state. At high levels of stress, a substance will deform to the point where it will not recover and return to its original dimensions. This is called plastic strain. A strain gauge can be used to determine when a material undergoes plastic strain and permanent deformation has occurred.
The metal-foil strain gauge is commonly used when a very small amount of strain and dimensional change occurs. This type works by measuring the change in electrical resistance that occurs when an electrically conductive material deforms. The metal-foil strain gauge typically consists of a series of very thin parallel lines of conductive metal applied to a thin foil. The foil is attached to a backing material that is electrically insulating, and will not conduct voltage or current.
The metal-foil strain gauge is glued to an object to be measured. As stress is applied and the object deforms, the metallic foil will also deform and its electrical resistance will change. The change in electrical resistance is measured and can be related to the strain in the material. The numerical relationship between the change in electrical resistance and corresponding mechanical strain is known as the gauge factor.
Foil strain gauges require careful calibration to ensure the measurement of strain is accurate. Calibration is completed through the use of an electrical circuit known as a Wheatstone bridge. This is used to determine the gauge factor and also compensates for temperature changes that cause expansion and contraction of the metal foil material, which could generate incorrect measurement results.
Strain gauges are available for a variety of measurement applications. These are used to measure strain due to elongation caused by a pulling force, compression caused by a pushing force, or even pressure applied to a surface area by a weight or other load. Strain gauges are often used for experimental testing in a laboratory and can be permanently mounted to an object to allow ongoing measurement.