Gage Installation: Measurements on Plastics and Composites

Gage Installation
Basically, the procedure for installing the strain gage on a plastic or composite is the same as that for a prewired gage (and the selected adhesive) on any other material, as recommended by the manufacturer. The procedure is sometimes modified, however, for continuously reinforced composites if premature gage failure is likely, due to local fiber breakage or to cracking of a brittle matrix. Gage life can usually be extended in such cases by interposing a thin layer of resilient material between the gage and test surface during installation. Following the usual surface preparation, a small piece of 0.001 in (0.025 mm) thick polyimide film that has been surface treated is first bonded to the test piece at the gage site. Then after cleaning and abrading the exposed surface of the polyimide, the strain gage is installed on top of the film. The presence of the film tends to filter out local peaks in the underlying strain distribution. It can also affect, of course, the heat-dissipation, reinforcement, and creep characteristics of the strain gage installation.

Gage alignment at installation sometimes requires special consideration. If the test material is at least nominally isotropic, the requirements for gage alignment are no more severe than for typical strain measurements on metals. With an orthotropic composite, in contrast, gage orientation is usually much more critical ( Ref. 14 ). In the case of a single-element gage (the use of which is particularly questionable on such a material), precise gage alignment relative to the material axes is necessary to obtain any meaningful strain data. A three-element rosette, on the other hand, can (in theory) be installed with any orientation, but the angle between one of the gage axes and a material axis must be known accurately for data-reduction purposes. When a matching dummy rosette is employed for temperature/humidity compensation, the two rosettes must, of course, be aligned identically with respect to the material axes.

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