Determination of time-dependent constitutive model, parameters for microcrystalline cellulose

Material parameters of a time-dependent elasto-viscoplastic constitutive mo del, proposed by Adachi and Oka, have been determined for microcrystalline cellulose. Adachi and Oka model has eight material parameters, with five of them (i.e., consolidation index, swelling index, critical index, shear mod ulus, and Poisson's ratio) same as those in the more commonly used in rime- independent modified Cam-clay model, and remaining three parameters account for time-dependency. To measure the time-dependent parameters, constant st rain-rate conventional triaxial compression (CTC) test was designed and con ducted using Penn State's flexible-boundary cubical triaxial tester (CTT) w ith microcrystalline cellulose as the test material. To deposit the powder, the sprinkle filling method was used in a layer by layer fashion with a sm all spoon. The three time-dependent Adachi and Oka parameters were determin ed from these tests. Adachi and Oka model was verified by comparing the pre dicted values from the model with measured values from the CTT at 0.05 leve l of significance. Results showed that: 1) there were no significant differ ences (p>0.05) between measured and predicted HTC responses at all loading rates; 2) failure values were predicted accurately (p>0.05); and 3) there w ere significant differences (p<0.05) between measured and predicted CTC res ponses at all loading rates.