MATERIALS ENGINEERING CONCEPTS - Materials for Civil and Construction Engineers - page 37

FIGURE 1.29 Extensometer with an LVDT.

LVDT that can be used to measure the deformation of a metal rod during the
tensile test.

1.7.3 Strain Gauge
Strain gauges are used to measure small deformations within a certain
gauge length. There are several types of strain gauge, but the most domi-
nant type is the electrical strain gauge, which consists of a foil or wire
bonded to a thin base of plastic or paper (Figure 1.30). An electric current
is passed through the element (foil or wire). As the element is strained, its
electrical resistance changes proportionally. The strain gauge is bonded via
an adhesive to the surface on which the strain measurement is desired. As
the surface deforms, the strain gauge also deforms and, consequently, the
resistance changes. Since the amount of resistance change is very small, an
ordinary ohmmeter cannot be used. Therefore, special electric circuits,
such as the Wheatstone bridge, are used to detect the change in resistance
(Dally and Riley 1991).
Strain gauges are manufactured with different sizes, but the most conve-
nient strain gauges have a gauge length of about 5 mm to 15 mm (1/4 in. to
1/2 in.). Larger strain gauges can also be made and used in some applications.
A wire gauge consists of a length of very fine wire (about 0.025 mm di-
ameter) that is looped into a pattern. A foil gauge is made by etching a pattern
on a very thin metal foil (about 0.0025 mm thick). Foils or wires are made in
a great variety of shapes, sizes, and types and are bonded to a plastic or paper