Stress distribution in a powder column under uniaxial compression

We investigate the distribution of stresses transmitted by a compressed powder onto the containing vessel base and side walls. We make high sensitivity and resolution measurements using a pressure-sensitive film (Fujifilm Prescale) and a thin flexible capacitive sensor (SingleTact) to visualise pressure distributions and to quantify stresses during and after powder compression in an FT4 Powder Rheometer®, (Freeman Technology, UK). We find that a uniform pressure on the base is only achieved when the compressed powder column is taller than a critical value, and that for shorter samples the pressure on the base is uneven and highly localised. In the region immediately below the piston we measure very high radial stresses on the vessel sidewall, implying a radial distribution of radial stresses near the surface where the external pressure is applied. Through the method developed in this work, we differentiate five different powders according to their compression behaviour, stress gradient in depth, and stress retention after the external pressure is removed. A comparison of the sample stress profiles, and their powder flow properties allows us to make predictions about their suitability for compaction processes.