Image brightness analysis as a tool for strain measurement in photo-elastic compression tests on PDMS granulate

Abstract The paper presents the behaviour of samples made of self-manufactured polydimethylsiloxane (PDMS) grains in a photo-elastic confined uniaxial compression test. PDMS particles were produced using the water-based method, resulting in grain shapes that are close to spherical, with diameters ranging from 2.0 to 16.0 mm. The samples tested were of the same size; however, they consisted of grains of different fractions, diameter spans, and aspect ratios relative to the dimensions of the test box. They were subjected to a load-unload cycle according to several scenarios, first to determine the load that induces a measurable photo-elastic effect. To study the potential relationship between the uniaxial strain calculated based on the change in a sample’s height and the corresponding optical parameters, a parallel analysis was conducted using the average image brightness. It led to the proposal of standardised strain and brightness parameters, which reduced the influence of loading scenarios and camera settings on the test results, allowing for the study of the other factors. On this basis, a unique relationship has been found between normalised uniaxial strain and normalised relative average image brightness, encompassing both loading and unloading regimes. In contrast, two calibration curves are required to calculate stress from brightness. The most interesting finding is that, despite the different force network structures observed in the uniaxial compression test, they do not appear to affect the behaviour of the granular samples on a macro-scale. Graphical abstract