MATERIALS ENGINEERING CONCEPTS - Materials for Civil and Construction Engineers - page 33

their response to the applied forces or to certain environmental conditions.
These tests require the measurement of certain parameters such as time, de-
formation, or force. Some of these parameters are measured directly, while
others are measured indirectly by relating parameters to each other. Length
and deformation can be measured directly using simple devices such as
rulers, dial gauges, and calipers. In other cases, indirect measurements are
made by measuring electric voltage and relating it to deformation, force,
stress, or strain. Examples of such devices include linear variable differen-
tial transformers (LVDTs), strain gauges, and load cells. Noncontact defor-
mation measuring devices using lasers and various optical devices are also
available. Electronic measuring devices can easily be connected to chart
recorders, digital readout devices, or computers, where the results can be
easily displayed and processed.
Each device has a certain sensitivity, which is the smallest value that can
be read on the device's scale. Sensitivity should not be mistaken for accura-
cy or precision. Magnification can be designed into a gauge to increase its
sensitivity, but wear, friction, noise, drift, and other factors may introduce
errors that limit the accuracy and precision.
Measurement accuracy cannot exceed the sensitivity of the measuring
device. For example, if a stopwatch with a sensitivity of 0.01 second is used
to measure time, the smallest time interval that can be recorded is also 0.01
second. The selection of the measuring device and its sensitivity depends on
the required accuracy of measurement. The required accuracy, on the other
hand, depends on the significance and use of the measurement. For exam-
ple, when expressing distance of travel from one city to another, an accuracy
of 1 kilometer or even 10 kilometers may be meaningful. In contrast, manu-
facturing a computer microchip may require an accuracy of one-millionth of
a meter or better. In engineering tests, the accuracy of measurement must be
determined in advance to ensure proper use of such measurements and, at
the same time, to avoid unnecessary effort and expense during testing. Many
standardized test methods, such as those of ASTM and AASHTO, state the
sensitivity of the measuring devices used in a given experiment. In any case,
care must be taken to ensure proper calibration, connections, use, and in-
terpretation of the test results of various measuring devices.
Next we will briefly describe measuring devices commonly used in ma-
terial testing, such as dial gauges, linear variable differential transformers
(LVDTs), strain gauges, proving rings, and load cells.
1.7.1 Dial Gauge
Dial gauges are used in many laboratory tests to measure deformation. The
dial gauge is attached at two points, between which the relative movement
is measured. Most of the dial gauges include two scales with two different
pointers, as depicted in Figure 1.24. The smallest division of the large scale
determines the sensitivity of the device and is usually recorded on the face
of the gauge. One division of the small pointer corresponds to one full rota-
tion of the large pointer. The full range of the small pointer determines the