Wide-view and accurate deformation measurement at microscales by phase extraction of scanning moiré pattern with a spatial phase-shifting technique
Full-field optical measurement techniques play a pivotal role in experimental mechanics providing accurate and non-invasive means for material testing (e.g., external loads, crack prediction and failure analysis). Incoherent moiré methods are especially popular in measuring displacement and strain spatial distributions due to robustness and straightforward operation of periodic patterns superimposition analysis. They do, however, suffer from the trade-off between the available field-of-view (FOV) and overall measurement accuracy. In the paper by Wang, Ri and Xia, a second-order moiré method is proposed, merging the advantages of scanning and sampling moiré techniques and thus bypassing the FOV/accuracy limitation. This method has a larger FOV than the sampling moiré (grating pitch can be as small as a single pixel) and higher measurement accuracy than the scanning moiré (thanks to the spatial-phase-shifting single-shot phase demodulation). Numerical results confirm high quality of strain distribution determination at various strain and noise levels, validating the theoretical moiré feature that smallest measurement error is to be obtained with very similar scanning and specimen pitches (they can differ up to 10%). The authors performed successful experimental verification of the proposed hybrid scanning-sampling moiré technique studying aluminum specimens under different tensile loads. Strain measurement accuracy was corroborated in comparison with the strain gauge results.