Stress, strain, and bulk microstructure in a cohesive powder.

Spin-echo small-angle neutron scattering is able to characterize powders in terms of their density-density correlation function. Here we present a microstructural study on a fine cohesive powder undergoing uniaxial compression. As a function of compression, we measure the autocorrelation function of the density distribution. From these measurements we quantify the typical sizes of the heterogeneities as well as the fractal nature of the powder packing. The fractal dimension increases with increasing stress, creating a more space-filling structure with rougher phase boundaries. The microscopic stress-strain relation showed the same nonlinear behavior as the macroscopic relation. In this way it was possible to link the macroscopic mechanical response with the evolution of microstructure inside the bulk of the cohesive powder. The total macroscopic compressive strain is in agreement with a corresponding decrease in microstructural length scales.