This study investigates the role of particle morphology in the strength and dilatancy behaviour of Namibian dune sand under direct shear test (DST) conditions. For this purpose, a series of DSTs were performed under three normal stresses (50, 100, and 200 kPa) and three relative densities (0.25, 0.5, and 0.75). To understand the role of particle morphology, the particle shape descriptors, including aspect ratio, roundness, convexity, sphericity, and overall regularity, were quantified from two-dimensional binary images derived from scanning electron microscopy using a computational geometry approach considering the major plane of orientation. Results indicate that the stress ratio exhibits strain-hardening followed by softening in dense assemblies, while loose assemblies display continuous hardening. The vertical strain and dilatancy angle decrease with increasing normal stress, whereas peak friction angles rise with relative density due to enhanced particle interlocking; critical state friction angles remain largely insensitive to density, reflecting stabilisation at ultimate shearing. Bolton's empirical model underestimates dilatancy angles, which are highly sensitive to normal stress (empirical constant, Q approximate to 10.47 at 200 kPa). The critical state parameters in the void's ratio - logarithm of normal stress normalised by atmospheric pressure plane vary systematically with density and particle regularity, aligning with literature and confirming the robustness of the proposed framework. These findings validate constitutive predictions and underscore the pivotal role of particle morphology in controlling sand strength and dilatancy, offering insights for physically grounded constitutive modelling of granular materials.