Constitutive Model for Dry Cohesive Powders with Application to Powder Compaction

Continuum theory-based constitutive models suitable for simulating the load response of dry cohesive powders have been critically reviewed. Based on a set of criteria, three candidate models for further investigation have been identified: Cam clay, Adachi and Oka model, and Krizek et al.'s endochronic theory based model. Since the modified Cam clay model is the foundation of many advanced constitutive models, it was chosen and applied to compaction load response of a dry cohesive powder (wheat flour). The constitutive equation parameters were determined using four types of conventional triaxial tests: drained, undrained, mean effective stress and isotropic tests. Based on these tests, the three Cam clay parameters for wheat flour were: 2.1 (slope of critical state line), 0.130 (loading index) and 0.022 (unloading index). Low pressure uniaxial compaction tests were performed with the powder in a thin-walled aluminum column. A finite element model (FEM), with the modified Cam clay constitutive model for dry cohesive powder’s response, was used to predict the compression behavior. The FEM calculated values compared favorably with measured wall strains. The FEM predicted stress distribution in the powder mass identified troublesome regions with large shear stresses and tensile stresses.