Thermal Output (continued)

For convenience in correcting measured strain data for thermally induced resistance changes, the thermal output of the gage is usually expressed in strain units. Thus, dividing Eq. ( 504.1 ) by the gage factor setting of the instrument,

   Eq.(504.2)

where:

= thermal output in strain units; that is, the strain magnitude registered by a strain indicator (with a gage factor setting of ), when the gage installation is subjected to a temperature change, , under conditions of free thermal expansion for the substrate.

When measuring stress-induced strains at a temperature different from the initial balance temperature, the thermal output from Eq. (504.2) is superimposed on the gage output due to mechanical strain, causing the measurement to be in error by that amount. Many factors affect the thermal output of strain gages. Some of the more important are: test specimen material and shape, grid alloy and lot, gage series and pattern, transverse sensitivity of the gage, bonding and encapsulating materials, and installation procedures. It is never possible for Micro-Measurements to predict exactly what the thermal output of any gage will be when the user has bonded it to a test structure. Even in cases where applications involve the same material as that used by Micro-Measurements in its tests, differences can be expected since structural materials vary in thermal expansion characteristics from lot to lot. The best practice is always to evaluate one or more gages under thermal conditions as nearly like those to be encountered in the testing program as possible.

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