In situ X-ray imaging of heterogeneity in dynamic compaction of granular media

Dynamic compaction of granular materials is a complex process, wherein the material response at the continuum scale arises from fracture, comminution, and fragment flow at the grain scale. Granular materials have heterogeneity due to variation in grain properties as well as variation in local packing density and structure. These heterogeneities may affect the dynamic compaction response, producing a structured and time-varying compaction front. The methodology used to characterize the shock behavior in solid materials may be inappropriate for granular materials because of this unsteady behavior and interactions between the granular material and measurement surfaces. To observe the compaction front heterogeneity, in situ x-ray imaging of granular compaction is conducted at a scale between the grain- and continuum scales. To allow for sufficient x-ray transmission, a thin sample geometry is used, for which boundary affects may result in a significantly different load path than would occur in a 1-D compaction configuration. Numerical simulations of the experimental geometry support the analysis of the results so that the feature of the compaction front can be distinguished from artifacts of the experimental configuration. The results suggest that compaction front heterogeneity may have structure with a length scale of tens of particle diameters and demonstrate that finite grain size can inhibit the formation of shear-induced features that would arise in a homogenized representation of the same material.