Mesoscale simulations reveal that the apparent shear strength in shock-compacted granular materials shows a path-dependent relaxation and recovery behavior due spatial heterogeneity of the material deformation. The extent of shear relaxation is shown to depend on the change in porosity, shock pressure, and material properties of the solid phase. An empirical form is proposed to describe the evolution of apparent shear strength observed in these mesoscale simulations. Importantly, the models predict that the shear stress in 1-D shock loading (which is needed to infer pressure from the measured Hugoniot stress) may be very different than the maximum shear stress that occurs in dynamic high-shear deformation. The evolving shear stress during shock compaction is difficult to measure directly, but recent efforts have attempted to infer dynamic strength of shock-loaded porous materials though observation of a Richtmyer–Meshkov instability (RMI). In the present work we use mesoscale and continuum simulations to assess the extent to which the RMI platform may be used to validate the model for relaxation and recover that has been identified in the mesoscale simulations.