Abstract: Geotechnical granular geomaterials exhibit complex incremental nonlinear mechanical behaviors, and whether the fundamental assumptions of classical elasto-plasticity are applicable to such materials remains a critical prerequisite for developing constitutive models capable of accurately capturing their mechanical responses. To obtain more realistic incremental behaviors-including isotropic elastic characteristics, yielding characteristics, associated and normality plastic flow characteristics and to assess the validity of the classical assumptions, this study first establishes a library of realistic particle shapes using X-ray computed tomography, image processing, and spherical harmonic analysis. Based on this, numerical specimens that incorporate realistic particle morphologies are constructed, and a series of parallel stress probing tests are conducted under different stress states and stress histories. The results reveal that, under axisymmetric stress conditions, elastic responses display pronounced anisotropy at high stress ratios and during unloading. At low stress ratios and in unloading states, the plastic flow exhibits strong dependence on the stress increment direction and a non-associated nature. Moreover, during unloading, small plastic strains are induced in all probing directions, indicating that the assumptions of a finite elastic domain and a single yield surface are not valid for granular geomaterials.