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The elastic moduli of powdered materials undergoing uniaxial compaction was investigated, paying particular attention to effects of solid phase material properties and initial particle shape. Elastic properties were characterised by the isotropic elastic moduli Poisson’s ratio and Young’s modulus, calculated from elastic wave speeds measured in the axial (pressing direction). To isolate material property effects, three different ductile metal powders (copper, stainless steel, and aluminium) with equivalent particle shape (spheroidal) were tested. Comparison with similar measurements for a brittle spheroidal powder (glass) illustrated that solid phase yield mechanism affects the evolution of pore character, and hence bulk elastic properties of the powder compact. Pore character was also studied separately by comparing copper powders with differing particle shapes (spheroidal, irregular, and dendritic). For all powders, Young’s modulus increased monotonically with compaction (reducing porosity). For the ductile spheroidal powders, differences in evolution of Young’s modulus with compaction were accounted for by solid phase elastic properties. The different morphology copper powders showed an increase in compact compliance as particle (pore) ruggedness increased. Poisson’s ratio followed a concave porosity dependence: decreasing in the initial stages of compaction, then increasing as porosity approached zero. Comparison between powders indicated the initial decrease in Poisson’s ratio was insensitive to solid phase material properties. However, as the compact approached solid phase density, the Poisson’s ratio—porosity locus diverged towards corresponding solid phase values for each particle material, indicating an influence of solid phase elastic properties. [ABSTRACT FROM AUTHOR]

Isotropy in the elastic properties of powders undergoing uniaxial compaction in a cylindrical die was evaluated from in situ measurements of elastic wave speed. Shear and bulk longitudinal wave speeds were measured in both the axial (pressing) and radial directions. For the five different metal powders studied, wave speeds were generally higher in the axial direction. As such, the powder body was best described as a transversely isotropic material; complete isotropy was approached only when the powder was close to the loose packed state, or completely solid. Transversely isotropic elastic moduli analogous to the common isotropic ‘engineering’ moduli (Young’s modulus, Poisson’s ratio, etc.) were calculated by combining elastic wave speed measurements with the Saint-Venant approximation. Pseudo-isotropic elastic moduli (calculated from axial wave speed measurements and assuming elastic isotropy) were found to be only qualitatively similar to transversely isotropic elastic moduli for the axial plane. [ABSTRACT FROM AUTHOR]

General approaches for developing models to describe the elastic properties of granular and porous materials are discussed, with emphasis on their application to predicting the elastic properties of powders undergoing uniaxial compaction. Both particle-based, and pore-based models were considered so as to reflect the transition in compact response with decreasing porosity; being particle-dominated at high porosity, then pore-dominated at low porosity. Pore-based models were further subdivided into: mechanistic models, which consider the effects of porosity on internal mechanical fields; and geometric models, for which the elastic response is assumed to correlate with a microstructural feature (e.g. load-bearing area). A selection of models suggested in the literature, considered representative of these approaches, was applied to experimental measurements of the elastic moduli of powders during compaction. In general, the geometric pore-based models show most promise, as these are able to approximate the transition in pore character during compaction. However, further developments are required for application to uniaxially compacted powders. In particular, it is necessary to develop the ability to predict more than one elastic modulus, handle irregular powder particles, and accommodate powders comprised of brittle solid phase materials. [ABSTRACT FROM AUTHOR]