Role of particle morphology in monotonic and cyclic behavior of granular materials : insights from cereals

Abstract

Particle morphology, as intrinsic properties of granular materials, significantly affects the mechanical response of granular materials, but little attention has been paid to cyclic response together with monotonic behavior. In this study, six cereals with similar grain sizes but different shapes were selected as testing materials due to their minimal variation in particle morphology. First, three-dimensional particle shapes of six cereals were obtained using X-ray microcomputed tomography (μCT). Then, a series of monotonic and cyclic triaxial tests were conducted on all cereals with and without staining. The staining could effectively eliminate the surface characteristic differences among six cereals, enabling them to be employed to better investigate particle shape effects. Meanwhile, since the surface characteristics of cereals, such as surface roughness and surface-adhered starch, underwent significant changes after staining, their effects could also be analyzed by comparing the results of cereals before and after staining. Repose angle tests were conducted to investigate changes in the surface roughness of six cereals after staining, which showed that staining had no significant effect on the surface roughness of mung but significantly increased it for all other cereals. Triaxial test results showed that the peak friction angle decreased with increasing particle shape regularity and decreasing surface roughness, while accumulated volumetric strain during cyclic tests decreased under both conditions. Furthermore, the cereals with surface-adhered starch exhibited quite different mechanical behavior than conventional granular materials. The starch bond under confining pressure significantly enhanced their shear strength and reduced their compressibility. However, the specimens experienced an abrupt strength reduction and volume contraction when the deviatoric stress reached the critical threshold of starch bond rupture, which required special attention during the storage and transportation of cereals.