Spider DSA User’s Manual
209
be physically and chemically cleaned to remove all debris, dirt and especially
grease and oil. Next, the selected gage is carefully bonded to the surface using a
low viscosity epoxy (or cyanoacrylate “super glue” for temporary installations in a
room temperature environment). After curing and cleaning, the lead wires are
carefully soldered to the gage’s tabs. The lead wires are then carefully routed away
from the gage site and are secured along the structure to prevent stressing and
fatigue. Then the installation is chemically cleaned, and a protective coating is
applied. Lastly, the wires are led to the analysis equipment and integrated with it.
Once installed, the gage will serve for years. Strain gages are simple and rugged
devices. They require very little in the way of post-installation maintenance.
Strain gages are made by dozens of manufacturers in a blinding array of designs.
Most of these provide a nominal gage resistance, R
G
, of either 120 or 350 ohms.
Spider-80SG will work with all of these. Gages are available in a wide variety of
sizes and shapes. They are also available etched from various alloys to match
thermal characteristics of specific materials.
Figure xx4: Examples of strain gage rosettes
Gage groups are also packaged with two, three or four gages on a common
mounting substrate. Such groups of gages are called “rosettes”. The gages in a
rosette are invariable oriented at different angles from one another (45°, 60°, 90°
and 120° being most common). Rosettes are commonly used on static stress
analysis models to determine the direction and magnitude of principal strains.
These can be determined from three strain measurements simultaneously made
in different directions and the rosette provides these. In addition to differences in
designed angular separation, rosettes come in “planar” and “stacked” designs, two
different approaches to focusing the multiple gages on a common location.
Rosettes are not commonly used in dynamics work. Note that each gage in a
rosette requires a separate processing channel.
Strain Gage Processing
The resistance changes experienced by a strain gage are very small. After all, we
are talking about near-molecular displacements of the instrumented substrate!
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