Identification of the critical angle of repose of cohesive granular materials by continuous heap measurements

The angle of repose (AR) is a fundamental parameter in the characterization of granular material, crucial for both terrestrial and extraterrestrial applications. This angle is defined as the maximum angle that a steady pile of granular material can form with the horizontal surface on which it lies. Traditional methods identify the angle from the analysis of one or more images of the formed pile. However, because of the highly non-linear behavior of the pile formation process, these methods often suffer from significant uncertainty in the measurement, particularly when the granular material possesses cohesive properties. This study introduces a novel measurement technique that addresses this problem and mitigates it. A custom-designed test stand, equipped with synchronized cameras, records the entire pile formation process. Then, a multi-step analytical procedure creates time series of the angle of the pile by processing all frames of the recordings, detects instances when the pile is in steady conditions, and identifies the angle of repose. Through this method, we can effectively isolate critical steady states of the pile occurring during the pile formation process, and precisely estimate the angle through a few test replicates. Experiments carried out on several lunar regolith simulants indicate that the proposed method can improve the accuracy of the AR estimate and reduce the measurement uncertainty.