Campbell Scientific ST350 Strain Transducer for Structural Testing - Manual (Page 41)

Appendix B. ST350 Accuracy Verification
Trying to measure the strain on a 2" wide strip of metal that is 1/8" thick
and mounted as a cantilever beam is not a good verification test for these
sensors. The primary problem with a thin bending specimen is that a large
degree of curvature is required to obtain a small level of surface strain. In
other the words, the transducer will simply be bent rather than elongated.
Furthermore, the actual location of the transducer will be relatively far
from the neutral axis compared to the surface (aggravated again by the
thickness of the tabs if they are used). Therefore, significant errors are
induced when comparing surface strains obtained by a foil strain gage and
the transducer reading.
For calibration purposes, it is highly recommended that strains be
compared at constant moment regions rather than at locations with
significant moment gradients. For the "bending beam" type of test, we
recommend a beam at least 10 to 12 long, with a shorter beam (4 ft to 6 ft)
set on top (with "pins" under each end), and the load cell above that. This
"4-point" type of setup will supply a constant moment region at mid-span.
Remember, the strain measured from the transducers is averaged over the
3" gage length. Therefore any error in gage placement or in the assumed
strain gradient will cause errors in subsequent data comparisons.
2) In almost every case we have seen, a specimen that is supposedly
undergoing tension only is actually bending as well. A popular test is to
use a "dog bone" with the transducer mounted on one side and then the
whole assembly put into tension. It is almost impossible to get pure
tension in this setup since the specimen may be slightly bent to begin with
and "straightens out" slightly. Also, since the transducers themselves have
a small amount of stiffness, they will cause a non-symmetrical system.
Another consideration is the distance of the centroid of the transducer to
the specimen's neutral axis. Since bending will most likely occur, the
output from the transducer may be reduced or amplified since its centroid
is about 1/4" away from the foil gage (further from the neutral axis), and
this might be the "compression" or "tension" side of the specimen. This
phenomenon is very critical on small laboratory specimens, but
insignificant on larger structures where the depths of the sections are
usually much bigger.
In order for the tension test to be successful, transducers should be
mounted on both sides of the specimen (on all four sides if the stiffnesses
are similar in two directions) and the output averaged to determine the
tension strain. In addition, the specimen should be relatively stiff
compared to the transducer.
3) If a compression test is being attempted, then the gages need to be at least
two member depths away from the ends, (a criteria for plane strain), with
gages mounted on both sides of the specimen and the data averaged. For
compression specimens, it may be necessary to place gages on all four
sides since it can often be difficult to know the exact orientation of the
neutral axis if the stiffness is approximately the same in both directions.
4) Using reinforced concrete as a test specimen material is a poor choice
since inaccuracies in the reinforcement locations and variations in the
concrete's elastic modulus (often up to 20%) can cause larger errors than
the accuracy range of the strain transducers. For example, more aggregate
near the surface of one gage will affect the modulus in that area. The way
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