Page 87 - Mechanical Engineers' Handbook (Volume 2)

76 Bridge Transducers

verse sensitivity, distance from the surface, bonding and installation, and gage frequency
response.

Temperature
A qualitative discussion of temperature effects on bonded strain gages indicates the effects
to be attributable to three principal causes: (1) The transducer flexure to which the gage is
attached expands or contracts, (2) the strain gage resistance changes with temperature, and
(3) the strain gage grid expands or contracts. With some gages (particularly semiconductors),
the change of gage factor with temperature is also extremely significant. These temperature
effects are accounted for by temperature–strain calibration; self-temperature-compensated
gages where combined effects 1, 2, and 3 above are minimized over a given temperature
range for a given combination of grid and flexure material; and a dummy gage integrated
into a bridge circuit (discussed later) to electrically subtract temperature-induced strain.

Backing Material
The purpose of the backing material used in constructing strain gages is to provide support,
dimensional stability, and mechanical protection for the grid element. The backing material
of the gage element(s) acts as a spring in parallel with the flexure to which it is attached
and can potentially modify flexure mechanical behavior. In addition, the temperature oper-
ating range of the gage can be constrained by its backing material. Most backings are epoxies
or glass fiber–reinforced epoxies. Some gages are encapsulated for chemical and mechanical
protection as well as extended fatigue life. For high-temperature applications, some gages
have strippable backings for mounting with ceramic adhesives. Still other metal gages can
be welded. The frequency response of welded gages, due to uncertainties in dynamic re-
sponse, is a subject area still requiring investigation.

Size
The major factors to be considered in determining the size of strain gage to use are available
space for gage mounting, strain gradient at the test location, and character of the material
under test. The strain gage must be small enough to be compatible with mounting location
and concentrated strain field. It must be large enough so that, on metals with large grain
size, it measures average strain as opposed to local effects. Grid elements greater than 3 mm
generally have greater fatigue resistance.

Transverse Sensitivity and Orientation
Strain gage transverse sensitivity and mounting orientation are concurrent considerations.
Transverse sensitivity in strain gages is important due to the fact that part of the geometry
of the gage grid is oriented in directions other than parallel to the principal gage sensing
direction. Values of transverse sensitivities are provided with individual gages but typically
vary between fractional and several percent. The position of the strain gage axis relative to
the numerically larger principal strain on the surface to which it is mounted will have an
influence on indicated strain.

Distance from the Surface
The grid element of a strain gage is separated from the transducer flexure by its backing
material and cement. The grid then responds to strain at a location removed from the flexure
surface. The strain on flexures such as thin plates in bending can vary considerably from
that measured by the strain gage.