Comment on “Measurement of the elastic properties of swelling clay minerals using the digital image correlation method on a single macroscopic crystal”, by S. Hedan, F. Hubert, D. Prêt, E. Ferrage, V. Valle, P. Cosenza [Applied Clay Science 116–117 (2015), 248–256, doi: 10.1016/j.clay.2015.04.002]

Abstract Hedan et al. (2015) recently reported an interesting method for determining the elastic modulus of vermiculite, a representative swelling clay mineral. A platy natural macrocrystal of vermiculite was used in the study. The technique combining mechanical compression and digital image correlation (DIC) appears plausible. However, a fundamental issue related to the reported testing method, particularly the experimental setup and specimen geometry constraints, arises and is worth re-examination and discussion, because it may make the reported modulus inappropriate or inaccurate. In this short discussion, the stress distribution of a thin plate such as the tested vermiculite macrocrystal under uniaxial compression is analyzed, and the reported results are re-interpreted. It appears that the reported Young's modulus of 32.3 GPa is too high for the studied vermiculite.