The Development of Electrical Strain Gages Page: 31 of 66
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IT.A.C.A. TechnicA,-l 1ote 1To. 744
found to be reasonably true. The theory and data are giv-
en in the following para.rrhs
Consider a fine wire emboidi.d in synthetic resin, in
the form of an experimental :'e. Then such a gage is
hcated, it expands linearly .t a rat-te given by the cocf-
ficient of expansion of the rosin, siuce compared to that
of the resin the cross-scctional area of the wire is
eligible The cofficient of expansion of resins is
much greater than that of metAllic wires. Some values
are ;iven in tablc IX.
TABLE IX
Coefficient of
Resin lina,.r exprmsion
per x. , x 106
akeclite 30
( i n t e d pa r )
E]iaelite (cast) 60
Lucite o70
Hence on heating the c0, the wire is str etched olasti-
cllyi by the resin. It would thus be expected that the
temper .ture, coefficient of resistance of the wire when
embedded in rosin would be greater than that of the free
wire. Furthermore, if the gage be firmly cemented to a
netal bar, and so constrained to expand thermally at the
same rate as the bar, the temperature coefficient of re-
sista:,co of the cemented gagc would be expected to be do-
pendent on the coefficient of expansion of the bar.
Asunme that the lateral strain of the gage has no
effect on the resistance of the wire. Consider two iden-
tical g~a,:es, one firmly comonted to a stuel bar and the
other free. Let both be heated through 10 C. Lot the
steel bar be now stretched so that the total increase in
length of its gago, due to temperature and strain, is
the same as the increase in length of the free specimen.
If the later-, restraint of the cementcd gage can be ne-
glected, the <ages ar under identical conditions, hence
the incr:: se s in resistance must be the same. The in-
crosc in resistance of the cemented gage is then R S_ _~___
_~__
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de Forest, A. V. & Leaderman, H. The Development of Electrical Strain Gages, report, January 1940; (https://digital.library.unt.edu/ark:/67531/metadc54544/m1/31/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.