Discrete-Element Quantification of the Noncoaxiality of Anisotropic Granular Materials under Orientational Rotational Shear

AbstractThe discrete-element method was employed to investigate the macro- and micromechanical behaviors of anisotropic granular specimens using orientational rotational shear (ORS) tests. The variations in the initial fabric orientation relative to the vertical direction were considered during the sample preparation stage. Several ORS tests were performed on samples with various initial fabrics, stress ratios, and densities. The contact-normal-based fabric tensor was considered as a measure of the internal structure of the granular assembly and employed in the detailed analysis. The proportional and noncoaxial components of the fabric tensor, with reference to the deviatoric stress tensor, exhibited similar sinusoidal evolution trends. The initial fabric represented by the bedding plane angle affected the phase angle of the cyclic variation waveforms of the proportional and noncoaxial components, whereas the stress ratio and initial density primarily affected their amplitude and magnitude. The decomposition of the strain increment tensor with respect to the stress indicated that the noncoaxial part was dominant, but the orthogonal component gradually vanished with an increase in the deviatoric strain, whereas the noncoaxial part decreased with an increase in the stress ratio. The new findings obtained in this study may provide useful guidance for the development of micromechanics-based constitutive models of granular materials subjected to ORS.