Page 92 - Instrumentation Reference Book 3E

Surveys of whole surfaces 77

a multiplicity of gauges to be scanned in quick suc- ing-wire strain gauge, it is possible to make its
cession, or simultaneous recordings made of a num- effective elastic modulus the same as that of the
ber of high-speed dynamic strain measurements. concrete it is embedded in; the stress distribution
in the bulk material will not then be changed by
the presence of the gauge. A strain range up to 0.5
4.6 Vibrating wire strain gauge per cent can be covered.
It has been found that vibrating-wire strain
Although bonded-resistance strain gauges are the gauges can be very stable; the yielding that might
type that has much the widest application, one or be expected in the vibrating wire can be elimin-
two other principles are made use of in certain ated by pre-straining before assembly. With care-
situations. One of these is that of the vibrating- ful installation and use at room temperature, the
wire strain gauge. drift over months can correspond to strains of
If a wire is put under tension, then its natural less than lop6.
frequency of vibration (in its first mode) is

4.7 Capacitive strain gauges
It is possible to design a device to be fixed to a
where 1 is its length, Tits tension, and m its mass substrate so that when the latter is strained the
per unit length. electrical capacitance (rather than the resistance)
The fixing points of the wire can be bonded to of the former is changed. Figure 4.14 is a diagram
material whose strain changes are to be meas- showing the principles of such a gauge. When the
ured. As the latter stretches, 1 changes and, more feet are moved nearer together, the arched strips
significantly, T changes following Hooke's law. change curvature and the gap between the cap-
Strain can then be determined by monitoring acitor plates P changes. The greater complexity
natural frequency, easily done if the wire is mag- makes these devices several times more expensive
netic and a solenoid is placed nearby to attract it. than simple bonded-resistance strain gauges, and
Wire diameter is typically 0.25 mm, operating they are seldom used except when their unique
frequency 1 kHz. The commonest circuit arrange- characteristic of low drift at high temperature (up
ment is to excite the wire with a single current to 600 "C) is important. They are most commonly
pulse, measuring the frequency of the damped fixed to metal structures by spot-welding.
vibrations that ensue. A sketch of a typical device Although the capacitance is only about a pico-
is given in Figure 4.13. farad, it can be measured accurately using appro-
With the number of items that go to make up priate bridge circuits; because both plates are live
a vibrating-wire gauge, it is considerably larger to earth, the effects of cable capacitance can be
(typically 100 mm long) than a bonded-resistance largely eliminated.
gauge. Because of the large force needed to
stretch it, thought must be given to the gauge's
mounting. 9, insulator
In fact, the largest application of such gauges
has been to the measurement of internal strains in
concrete, where both these factors are attended
to. By embedding the gauge in the concrete when Figure 4.14 Capacitive strain gauge. Courtesy, G.V.
the concrete is cast, with electrical leads coming Planar Ltd.
out to some accessible point, good bonding to the
end-points is ensured. A large gauge-length is 4.8 Surveys of whole surfaces
desirable in order to average the properties of
the material, which is very inhomogeneous on a A strain gauge only gives irformation about what
scale up to a few centimeters. By choosing appro- is happening in a small region. Sometimes it is
priate dimensions for the components of a vibrat- desirable to take an overview of a large area.

4.8.1 Brittle lacquer
One way of surveying a large area is to use brittfe
lacquer, though the technique is much less accu-
rate than using strain gauges and does not work
at all unless the strains are large. It has particular
Figure 4.13 Vibrating-wire strain gauge. Courtesy, value for deciding where to put strain gauges for
Strainstall Ltd. more accurate measurements.