The instrumental resolution of a moire extensometer in light of its recent automatisation

•The resolution of a moire extensometer is assessed in light of its automatisation.•An analytical solution is given for the interpretation of a generic moire pattern.•Instrumental resolution is defined according to realistic, not optimal, settings.•The automated instrument has a rotation resolution in the order of 8.7 ∗ 10−5 rad.•The automated instrument has a displacement resolution of better than 5 μm.

This paper assesses the instrumental resolution of a mechanical extensometer in light of its recent automatisation. The instrument takes advantage of the moire phenomenon of optical interference to measure angular rotation in two perpendicular planes and displacement in three dimensions. Our assessment systematically defines an analytical solution for the complete interpretation of a generic moire pattern and a set of mathematical approximations for the moire patterns used to measure rotation and displacement. The ultimate sensitivity of the automated instrument is determined on the basis of a generic least square differences fitting procedure while the instrumental resolution is defined on the basis of realistic, rather than optimal, scenarios: the resolution of the rotation measurements are in the order of 8.7 ∗ 10−5 rad while the resolution of the displacement measurements are better than 5 μm. This assessment represents the first step towards a global numerical repository for processed data recorded by the automated extensometers.