Loose sandy soil subject to undrained shearing manifests deviatoric softening, and such a behavior has been referred to collapse of static liquefaction. Feedback analysis of some slope failures reveals that the static liquefaction induced by underground water may be one of the implicit mechanisms of the slope failure. Densely packed assemblies exhibit similar phenomenon when sheared along certain proportional strain loading paths. For granular materials, such as soils, which are non-associated materials, diffusion failure can occur for stress states strictly included within the plastic limit condition. The expression “diffusion failure” is used in contrast to “localized failure”, to highlight the failure mode characterized by the lack of localization patterns. The mechanical response of dense assembly of irregular shaped particles subjected to proportional strain loading paths was studied using the combined finite-discrete element method(FDEM). According to Hill’s instability theory, a stress-strain state is stable if the stress and strain increments linked by the constitutive behavior of the material produce a strictly positive second-order work. The second-order work criterion is linked to the occurrence of a diffusing failure mode. In the meridian plane, the stress space is divided into dilation and compaction as distinguished by the controlling parameter of proportional strain loading path. The instability line is built by joining the points marking the onset of diffusion failure from each confining pressure.