Examining the failure modes of wet granular materials using dynamic diametrical compression

In this work, the deformation properties of single wet granular compacts are studied using diametrical compression, with the objective of examining in detail the mechanical properties of the pellets and the mode of deformation.Cylindrical pellets of wet granular material, 20 mm in diameter and 10 mm wide, were compressed diametrically in a load frame at speeds ranging from 0.1 to 180 mm/s. The deformation of these pellets was filmed using a high speed camera, with the aim of connecting pellet deformation mode with the pellet mechanical properties. Powder size, powder shape, liquid binder viscosity and granule saturation were varied. Two modes of granule failure were observed: semi-brittle, and plastic. Semi-brittle pellets failed via a catastrophic crack propagating through the centre of the pellet, with some small plastic deformation at the platens. Plastic pellets showed no major crack, and flowed between the platens like a paste. Pellet plasticity was strongly strain-rate dependant, increased with increasing strain rate and viscosity. However, there was no clear capillary number transition for pellet deformation behaviour. Observation based criteria were developed to distinguish between plastic and semi-brittle pellets. Pellets with a peak stress value at a strain less than 5% were classified as brittle; pellets with no peak flow stress, or with a peak stress value at a strain above 5%, were classified as plastic. The implications of the failure mode to breakage of wet granules in a mixer granulator are discussed.The effect of formulation properties and strain rate on tensile strength and fracture energy are similar to literature data observed for dimensionless peak flow stress in axial compression experiments. Unlike diametrical experiment results, the axial experiments gave peak flow stress data for all experimental conditions, with better reproducibility than for the diametrical experiments. The diametrical test is an excellent tool for studying the mode of failure for wet compacts. However, the axial compression test is superior for the measurement of pellet mechanical properties.

Graphical abstractThe failure mechanism of wet powder compacts has been investigated via diametrical compression. Two modes of granule failure have been observed; semi-brittle compacts fail via a single catastrophic crack though or close to the centre of the pellet, while plastically failing pellets demonstrate no major crack, and flow between the platens like a paste.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Failure of wet granular pellets studied under diametrical compression. ► Two modes of granule failure were observed: semi-brittle and plastic. ► Pellet plasticity was strongly strain rate dependant. ► Peak stress at strains under 5% defined as criterion for semi-brittle failure. ► Axial compression is superior over diametrical mechanical property measurement.