Page 88 - Instrumentation Reference Book 3E

Gauge characteristics 73
4.2.5 Residual stress measurement
The state of the surface at the time when a strain
gauge is bonded to it has, of course, to be taken
as the strain zero relative to which subsequent
changes are measured. The gauge essentially meas-
ures increments of strain with increments of
load. For many purposes of calculating stresses
and predicting life, this is the most important thing
Figure 4.5 Examples of semiconductorgauges. to do.
Courtesy, Kulite Semiconductor Products Inc.
However, during fabrication, and before a
gauge can be attached, some stresses can be
locked up in certain parts and it may be desir-
4.2.4 Rosettes able to know these. This cannot be done with
any accuracy non-destructively but if we delib-
We pointed out earlier that a full analysis of erately remove some materia! the observed
strain involves measurements in more than one
direction. In fact, three measurements are strain changes in neighboring material can tell
required on a surface because strain can be repre- us what forces were previously applied through
sented as an ellipse. for which the magnitudes and the now absent material. One technique is to
directions of the axes must be established. The strain-gauge a small area of interest, noting the
directions chosen for strain measurements are changes in the gauge readings as that area is
commonly either at 120" or at 45" and 90" to each freed by trepanning. An alternative procedure
other. is to drill a simple hole inside an array of strain
If we are dealing with large structures, it may gauges that remain attached to the main surface;
be expected that strain will only vary gradually changes in the strain they show can again indi-
cate what the residual stress was. An array for
across a surface, and three closely spaced indivi-
dual gauges can be thought of as referring to the this purpose is shown in Figure 4.7.
same point. When there is little room to spare, it
is desirable to have the three gauges constructed
integrally, which anyhow simplifies installation.
Such a unit is called a rosette. The three units may
be either close together in one plane or actually
stacked on top of each other (Figure 4.6).

I

Figure 4.7 Array of gaugesfor measuring residual stress.
Courtesy, Micro-Measurements Division, Measurements
Group Inc.

4.3 Gauge characteristics
We have discussed the gauge factor at some
length; that is what enables resistance to be used
at all to measure strain. Other features of strain
gauges are important for successful instru-
mentation. Information about the characteristics
Figure 4.6 Rosette of gauges. Courtesy, Micro- of particular gauges is available from manu-
Measurements Division, Measurements Group Inc. facturers.