Measurement of anisotropic behavior of dry cohesive and cohesionless powders using a cubical triaxial tester

Anisotropy of dry cohesive and cohesionless powders and the effect of particle shape and sample deposition method on the anisotropy were investigated using a cubical triaxial tester. Hydrostatic triaxial compression (HTC) tests and conventional triaxial compression (CTC) tests were conducted on four test powders: two cohesive, wheat flour (irregular-shaped particles) and potato starch (rounded); and two cohesionless, glass beads (spherical) and milled glass fibers (cylindrical). The loading-unloading cycle for HTC tests was from 0-190-0 kPa. The CTC tests were conducted at 34.5 kPa confining (σ₂ = σ₃) pressure, where σ₂ and σ₃ are intermediate and minor principal stresses, respectively. Two different deposition methods, plunging and tapping, were used to prepare cubical (50 mm) test samples. Test results showed that wheat flour and glass beads were basically isotropic materials in the stress range used in this study. Potato starch showed some anisotropy which tended to increase with applied stress. Significant anisotropy was observed for milled glass fibers (0.05 level of significance). The sample deposition method influenced the anisotropy of powder depending on the particle shape and cohesion. The anisotropy of powder was strongly dependent upon the particle shape. Cohesion was found to minimize the effect of sample deposition method. The material parameter values for the linear elastic isotropic and anisotropic constitutive equations were determined based on test data. No large difference between material parameter values was found for wheat flour, potato starch and glass beads. However, the percent difference between elastic modulus for milled glass fibers reached 37.2%, whereas for the Poisson ratio this difference was 800%.