Strain Gages

What is a strain gage?

A strain gage is a device that converts a mechanical deformation into an electronic signal. Ideally a strain gage will experience a change of resistance that is proportional to the change of strain experienced in an element. This change of resistance is the result of the expansion or contraction or wire strands within the strain gage.

History of strain gages

The first recorded mention of strain gages was in 1843 by Charles Wheatstone in his first publication of the bridge circuit he had invented, the Wheatstone bridge. William Thompson, also known as Lord Kelvin, further studied the effects of mechanical stress on the resistance of electrical conductors, however since these effects were so small his designs were never suitable for industry use or technical applications. In 1938 Arthur Claude Ruge, a faculty member at MIT, invented the strain gage while attempting to simulate stresses caused by earthquake vibrations. Through his experiments he was unable to find a satisfactory method of strain measurement until he placed a folded wire grid on a piece of tissue paper. From this the strain gage was born.

How Does a Strain Gage Work?

The strain gage is a grid of wires that will experience deformation. As the strain gage is stretched the wires in the grid will expand and their diameter will decrease. This will constrict the flow of electrons and the strain gage will increase in resistance. If the gage is compressed the wires will contract and their diameter will increase. This will encourage the flow of electrons and the strain gage will decrease in resistance.

Strain Gage Characteristics

A strain gage has multiple characteristics dependant on its size, shape and construction. For example gage of constantan, a copper nickel alloy, are much more sensitive than gages built of Nichrome V, a nickel chromium alloy. The material also holds an effect on the gage's sensitivity to temperature. The most obvious characteristic of the strain gage is its geometry. At Group Four the two most common geometries are used in gages designed for bending and gages designed for shear. The gages designed for bending typically consist one grid mounted to the measuring grid carrier and is usually placed on the outer walls of the load cell element. The grids are mounted at a 90° angle with the neutral axis, this type of gage may be considered as a rosette. Both of these gages may be seen below.