MEASUREMENT OF POWDER FLOW CONSTITUTIVE MODEL PARAMETERS USING A CUBICAL TRIAXIAL TESTER

A true (cubical) triaxial tester was built to study the three-dimensio nal load response of dry cohesionless and cohesive food powders at low pressures. The cubical triaxial tester was calibrated and used to loa d samples of wheat flour along isotropic, deviatoric and mean effectiv e stress paths. Using results from isotropic, i.e., hydrostatic triaxi al compression (HTC), tests, the critical state parameters were evalua ted, namely, the modified Cam clay constitutive parameters lambda (loa ding slope) and kappa (unloading-reloading slope). Deviatoric and mean effective stress test results were used to evaluate the slope of the critical state line, M. Test results yielded lambda = 0.046, kappa = 0 .011 and M = 0.98. The modified Cam clay constitutive model was verifi ed by comparing the model predicted stress-strain behavior of powder w ith the cubical triaxial tester experimental data. The mean average re lative difference (ARD) and mean absolute difference between measured and predicted curves for: (i) deviatoric stresses under conventional t riaxial compression (CTC) stress path were 20.7% and 6.1 kPa, respecti vely; (ii) volumetric strains under CTC stress path were 50.3% and 1.2 %, respectively; (iii) deviatoric stresses under mean effective stress (MES) test path were 15.7% and 5.6 kPa, respectively; (iv) volumetric strain under MES test path were 22.4% and 0.4%, respectively; and (v) voids ratio under HTC test path were 0.07% and 7 X 10(-4), respective ly.