Towards strain gauge 2.0: Substituting the electric resistance routinely deposited on polyimide film by the optimal pattern for full-field strain measurement
Résumé
The checkerboard constitutes the best pattern for full-field strain measurement because it maximizes image gradient. In the experimental mechanics community, employing this pattern is currently strongly limited because depositing it on the surface of specimens raises practical difficulties. A recent study shows that it is technically possible by using a laser engraver. The present paper aims to push this solution forward by printing the same pattern on a thin polymeric film, and then gluing the resulting laser-engraved film on the specimen surface. The underlying idea is to separate the manufacturing process of this optical strain gage on the one hand, and its use on the other hand, to help spread this strain measuring tool in the experimental mechanics community. The polymeric film employed here is the same as that used in the manufacturing process of classic electric gages, so one can rely on the know-how of classic strain gage bonding to glue this optical strain gage on the specimen surface. The main difference between the proposed tool and classic electric gages is that the strain field beneath the polymeric support is measured instead of pointwise strain values. The paper is a proof of concept for this strain field measuring tool. The manufacturing and bonding processes are described in the paper. The Localized Spectrum Analysis, a spectral technique developed for processing images of periodic patterns, is used to retrieve the strain fields from checkerboard images. Through two complementary examples, we show the ability of this new type of strain gage to detect and quantify local details in the strain field under it. A simplified 1D model is also proposed to assess the minimum width of the strain peak that can reliably be measured with this technique.
