Using laser marking to engrave optimal patterns for in-plane displacement and strain measurement

The checkerboard constitutes the best pattern for in-plane displacement and strain measurement because it maximizes image gradient. The use of laser marking to deposit such a pattern on flat surfaces is investigated in this paper. Optimal settings for the parameters influencing the quality of the pattern are given. This pattern being periodic, the images are processed with the localized spectrum analysis, which is one of the spectral techniques used to process such images. It is shown that the metrological performance in terms of measurement resolution is equivalent to the one obtained with transferred checkerboards. Compared to the classic transfer technique generally used to deposit such patterns, the benefit of using laser marking is its ease of use, a much higher print speed and the fact that a thin coat of white paint instead of a thicker layer of white adhesive is employed to ensure a good contrast in the images. Various examples of strain measurements are given, in particular the measurement of the strain field around a knot embedded in a wood specimen subjected to a tensile test.