Strain Gage Selection: Measurements on Plastics and Composites

Strain Gage Selection
Beyond the usual criteria for gage selection suitable to the strain-measurement task (i.e., single-element gage versus rosette, gage length, strain range, operating temperature, etc.), the gage resistance can be an important factor because of the characteristically low thermal conductivities of the test materials. High-resistance gages (350 ohms or greater) are generally preferable to minimize heat dissipation; but, in any case, the combination of gage resistance, grid area, and excitation voltage (grid power density) should be low enough to avoid the instability caused by a sensible temperature rise in the gage (Ref. 7 ). For any given gage installation, the threshold of instability can be established quite easily by starting with a very low excitation voltage and observing the (zero-strain) gage output while gradually increasing the voltage. When using strain gage rosettes, the single-plane rosette is preferred over the stacked variety because of better heat dissipation characteristics and less tendency for reinforcement. These recommendations for dealing with the self-generated heat in a strain gage assume continuous gage excitation since this mode of operation is employed in most contemporary instruments. Heat generation in the gage can also be minimized with pulsed gage excitation, if suitable instrumentation is available.

Although the gage length is normally selected for compatibility with the test-specimen geometry and the expected strain distribution, other considerations are also involved when making strain measurements on plastics and composites. One of these, of course, is grid power density, since the grid area commonly increases with gage length. Another is that gages of 1/8 in (3 m/n) length or greater usually offer better performance in terms of stability and strain range. Additionally, with reinforced plastics, it is imperative that the gage length be large with respect to the dimensions of the underlying inhomogeneities in order to obtain any meaningful strain measurements.

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