Elastoplastic and viscoelastic behaviour of cohesionless shelled Maize en masse

Abstract

Cereal maize continues to be one of the most important crops in the world both for human and livestock use. Bulk harvesting, transportation, handling and processing of cereal maize is therefore important if the increasing global demand is to be met. In Kenya, maize is the staple food-crop for the majority of the population. This calls for the adoption of modern scientific methods in its production, storage and processing. The use of 'existing constitutive models for granular engineering materials would greatly help in accurate prediction of the behaviour of shelled maize en masse. This research project investigated the engineering properties of three varieties of shelled bulk maize pertinent to grain handling, storage and processing. In particular, it investigated the stress-strain properties of bu~ maize in relation to Lade's elastoplastic constitutive model for cohesionless sand and Maxwell-Weichert polymer viscoelastic model, respectively. The Mohr-Coulomb failure criterion was also applied to shelled maize en masse. Stress--S--t-r-ain characteristics of the three varieties of bulk maize (Kitale 614 (V1) Katumani A (V2) and Katumani B (V3)) were determined at 12% moisture content (wet basis), using the triaxial equipment and the Senstar Universal Testing (SUT) machine. Three different initial bulk densities (IBD) of 730 kg/m3 768 kg/m3 and 800 kg/m3 under three levels of confining stresses of 100 kPa, 200 kPa and 300 kPa were used with the triaxial tests while two different initial bulk densities of 730 kg/m3 and 800 kg/m3 were used with the Senstar testing machine under cyclic uniaxial isotropic loading. Both sets of tests were conducted at an axial strain rate (ASR) of 1.3 mm/min and data recorded after every 30 seconds for the SUT and after every 60 seconds for triaxial loading. The triaxial loading tests were continued until failure or until 20% deformation (whichever came first) while the SUT tests were conducted until 20% deformation. Stress relaxation tests were conducted using the SUT machine for the determination of the stress relaxation moduli and exponents. The fourteen elastoplastic parameters under Lade's model were determined for maize en masse. The results were verified and validated using data obtained from twenty seven (27) different tests of the three maize varieties. Good agreements were found between the predicted and measured values. The comparison between the measured and the calculated values under Lade's model gave coefficient of determination (R2) values of 0.95, 0.94 and 0.93 for the three maize varieties (V1, V2 and V3, respectively) indicating that en masse maize can be considered as being elastoplastic under this model. Maxwell-Weichert model gave R2 values of 0.87, 0.84 and 0.90 for V1, V2 and V3 respectively. The stress relaxation moduli E1and E2and the relaxation exponents k1.and k2 were determined under the viscoelastic model. Moreover, the Mohr-Coulomb failure envelope for shelled bulk maize was developed with successful determination of the angle of internal friction and the cohesion factor for the three maize varieties. The low cohesion factors obtained under the Mohr-Coulomb criterion justified the description of en masse shelled maize as being cohesionless. These findings agree with other results from similar studies on wheat, rice and other engineering materials and are expected to be useful in the design of bulk shelled maize handling and storage facilities. The results may also provide important data for future use in Finite Element Method (FEM), Finite Difference Method (FDM) or Boundary Element Method (BEM) analysis of the behaviour of shelled bulk maize.