Page 89 - Instrumentation Reference Book 3E

74 Measurement of strain
4.3.1 Range Self-temperature compensated gauges are
made in which the conductor material is heat
The materials that strain gauges are made from treated to make its resistivity change with tem-
cannot be expected to stretch by more than a few perature in such a way as to balance out the
percent at most and still retain their properties in resistance change from thermal expansion.
linear relationships; generally non-linearity is Because the expansion coefficient of the substrate
introduced before permanent damage occurs. has an important effect, these gauges are specified
Metals vary in the strain range over which they for use on a particular material. The commonly
can be used; semiconductors have an appreciably matched materials are ferritic steel (coefficient
shorter range. Although their limited range is an 11 x 10-6K-'), austenitic steel (16 x 10@K-')
obvious theoretical restriction on the use of strain and aluminum (23 x K-').
gauges, they can in fact cover most of the com-
mon field of interest for metals and other hard
structural materials. Strain gauges are not gener- 4.3.4 Response times
ally suitable for use on rubber.
In practice, there are few fields of study where
strain gauges do not respond quickly enough to
4.3.2 Cross-sensitivity follow the strain that has been imposed. An
ultimate limit to usefulness is set by the finite
We have so far described the action of a strain time taken for stress waves to travel through the
gauge in terms of strain in the direction of the substrate, which means that different parts of a
length of its conductor: this is the strain it is strain gauge could be measuring different phases
intended to measure. But, as explained above, of a high-frequency stress cycle. But with stress-
some strain is generally present in the substrate wave velocities (in metals) of the order 5000 mls,
also in a direction at right angles to this, and a lOmm gauge can be thought of as giving
gauges are liable to respond in some degree to a point measurement at frequencies up to
this. For one thing, part of the conducting path 10-20kHz. Of course it is necessary that the
may be in that direction; for another the variation measuring circuits used should be able to handle
of resistivity with strain is a complex phenom- high-frequency signals.
enon. The cross-sensitivity of a gauge is seldom It must be noted that strain gauges essentially
more than a few percent of its direct sensitivity, measure the change in strain from the moment
and for foil gauges can be very small, but it when they are fixed on. They do not give absolute
should be taken into account for the most accur- readings.
ate work. Very slowly varying strains present particular
measurement problems. If a strain gauge is to be
4.3.3 Temperature sensitivity used over periods of months or years without
an opportunity to check back to its zero reading,
The resistance of a strain gauge, as of most then errors will be introduced if the zero has
things, varies with temperature. The magnitude drifted. Several factors can contribute to this:
of the effect may be comparable with the vari- creep in the cement or the conductor, corrosion,
ations from the strain to be measured, and a lot of or other structural changes. Drift performance
strain gauge technology has been devoted to depends on the quality of the installation; pro-
ensuring that results are not falsified in this way. vided that it has been carried out to high stand-
Several effects must be taken account of. Not ards, gauges used at room temperature should
only does the resistance of an unstrained con- have their zero constant to a strain of about
ductor vary with temperature but the expansion over months. At high temperatures it is a differ-
coefficients of the gauge material and of the sub- ent matter; gauges using ceramic bonding can be
strate it is bonded to mean that temperature used with difficulty up to 500/600"C, but high-
changes cause dimensional changes apart from temperature operation is a specialized matter.
those, resulting from stress, that it is desired to
measure.
It is possible to eliminate these errors by com- 4.4 Installation
pensation. Gauge resistance is commonly meas-
ured in a bridge circuit (see below), and if one of Sometimes strain gauges are incorporated into
the adjacent bridge arms consists of a similar some measuring device from the design stage.
strain gauge (called a dummy) mounted on simi- More often they are used for a stress survey of a
lar but unstressed material whose temperature pre-existing structure. In either case it is most
follows that of the surface being strained, then important to pay very close attention to correct
thermal, but not strain effects will cancel and be mounting of the gauges and other details of
eliminated from the output. installation. The whole operation depends on a