This paper presents a numerical study that uses two dimensional DEM simulations to determine how strain localizes inside an idealized interphase system composed of densely-packed spherical particles in contact with rough manufactured surfaces. The manufactured surface is made up of regular or irregular triangular asperities with varying slopes. Discrete data at the micro–scale have been homogenized and transformed into stress and strain using a new discrete–continuum analysis approach. Evolution of fabric, contact force anisotropies are recorded and used to generate stress tensors for the granular media at the continuum level. The strain field is generated using a new simple method directly based on motions of individual particles. Theoretical analysis of the stress-strain behavior occurring inside the interphase zone is made possible by combining the above approach with numerical simulations. Results show that uniform pure shear deformation occurs inside the interphase zone before strain localization initiates and nonlinear stress-strain behavior begins. The computed strain field shows a distinct but discontinuous shear band above the surface early before the peak state is reached. Anisotropies of fabric, contact normal force and contact shear force increase rapidly after shearing, leading to the increase of shear stress inside the interphase zone. The principal direction of contact total force anisotropy has exclusive control over the peak interface strength behavior. It is found that the thickness of the most intense shear zone is about 8 to 10 median particle diameters above the surface.