Rate-Dependent Elasto-Viscoplastic Constitutive Model for Industrial Powders. Part 1: Parameter Quantification

ABSTRACT

The rate-dependent mechanical behavior of a dry industrial powder (MZF powder) was studied using a cubical triaxial tester (CTT) within the context of a new elasto-viscoplastic model (PSU-EVP model). The compression and shear properties of the powder were quantified at compression rates of 0.62, 6.21, and 20.7 MPa/minute with pressures up to 11 MPa. Test results demonstrated that the compression and shear responses of the powder were nonlinear, consistent, and reproducible (coefficient of variation or COV ≤ 15%). Also, MZF powder exhibited varying elastic and plastic deformation at different pressure levels that were quantified using statistical correlations (R² > 0.90). For example, the average bulk modulus and shear modulus values for MZF powder increased linearly with pressure (R² > 0.90) at all compression rates. The failure stress values also increased with the increase in mean pressure. For instance, at a compression rate of 0.62 MPa/minute, failure stress increased from 5.0 to 13.3 MPa as the confining pressure increased from 2.2 to 8.5 MPa. Similar effects were noted at compression rates of 6.21 and 20.7 MPa/minute. Overall, failure stress decreased with increasing compression rate. From the data collected, it was demonstrated that compression rate does have substantial effect on the compressibility and shear behavior of powders that can be quantified using the CTT and is suitable for use in the PSU-EVP model.

Keywords:

• Dry powder compression
• PSU-EVP model
• elasto-viscoplasticity
• pore air compression
• cubical triaxial tester
• triaxial testing
• mechanical property determination

The authors are thankful to the NSF Particulate Materials Center and its corporate members and the Pennsylvania Agricultural Experiment Station (PAES) for funding and providing resources for this research.

Notes

^a Measured using Micromeritics Multivolume Helium Pycnometer 1305.

^b Measured using Micromeritics Gemini 2375 multipoint BET method.

^c Measured using the Dynamic Yield Locus Tester.

^d Particle size analysis was done using the Malvern Instruments Mastersizer light scattering equipment.

^e Determined by drying the powder under helium.

Three replications per treatment.

^a CTC test at 2.1 MPa confining pressure.

^b HTC stress path (in MPa): 0-2.4-0.3-4.5-0.3-10.3-0.3-10.3-0. The maximum pressure that was applied on the samples did not exceed 10.3 MPa. However, as part of the experimentation process, some tests were also done at pressures up to 14 MPa. The data from such tests were included in the analysis.

^a CP = Confining pressure.

^b FC = Failure criterion (% value of strain difference).

^c FS = Failure stress (COV value is given in parenthesis).

q = Cp^A

f _s  = B f _d  + β