<p>A unified localized damage model is proposed to describe both short- and long-term mechanical behaviors of sandy mudstone under hydro-mechanical coupling. The model considers microcrack evolution, shear band formation, and the interplay between plasticity and damage. A time-dependent variable is introduced to capture progressive degradation under sustained loading. Model validation was carried out using triaxial and rheological tests. The model shows good agreement with experimental data. It accurately replicates the transient, steady-state, and accelerated creep phases. Strain localization under varying pore and confining pressures is also well captured. The results confirm that the model can predict the onset and evolution of localized failure in semi-brittle rocks. This approach offers a practical tool for assessing stability and failure mechanisms.</p>