Spider DSA User’s Manual
208
Anatomy of a Strain Gage
A modern strain gage is the product of lithography, a thin etched film of metallic
foil on acetate, polyester or other flexible plastic backing. When the gage is
bonded to a stiff structure, its zigzag pattern provides a folded “wire” that can be
stretched in length in one direction and barely stretched in a perpendicular
direction. This defines the “strain direction” of the sensor. If the stiff supporting
structure is “stretched” in the intended strain direction, the wires are likewise
stretched. Attempting to stretch the structure and gage in a perpendicular
direction, produces very little change in the folded foil’s length. It merely opens
like an accordion. Hence the folded shape of the etching produces a preferential
direction for length increase/decrease.
The resistance of a wire is proportional to its length and resistivity. As the foil wire
stretches with the structure it is bonded to, its resistance increases (albeit by a
very small amount). If the structure suffers compression, the bonded sensor’s
gage length reduces minutely and its resistance decreases. Spider-80SG measures
these minute resistance changes precisely and translates them into a strain time-
history.
Figure xx3: A Typical Foil Strain Gage
The sensitivity of a strain gage is expressed by its “gage factor”, GF, defined as:
(non-dimensional)
Where R is the resistance of the un-deformed gauge, ΔR is the change in
resistance caused by the strain, ε is the strain (ΔL/L) and L is the “gage length”,
the length of each resistance wire. For metallic foil gages, the gauge factor is
usually a little over 2.
Properly installing a strain gage involves knowledge, visual acuity, manual
dexterity and patience. The gage must be installed intimately upon a smooth
surface in order to yield accurate results. If the test object has a rough surface, the
installation area must be filed down, sanded and polished. Then, the surface must
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