Discrete element modeling and shear properties of the maize stubble-soil complex

Stubble-soil complex (SSC) after crop harvest affects subsequent operations of agricultural equipment under conservation tillage system. It is necessary to identify the mechanical properties of SSC and its interaction with soil-engaging tools to develop efficient agricultural equipment. Direct shear tests of SSC samples were carried out to ascertain shear properties of soil and SSC. A discrete element model of maize SSC based on structure measurements was developed using EDEM software. The results showed that internal friction angles of soil and SSC ranged from 21.8 to 25.7° and from 20.8 to 27.9°, respectively; cohesions of soil and SSC ranged from 7.4 to 32.2 kPa and from 9.8 to 41.2 kPa respectively. Model parameters of soil, single root (SR), and SSC were calibrated by angle of repose test, shear test, and direct shear test, respectively. The calibrated surface energies of soil and soil-root ranged from 0.20 to 0.34 and from 1.72 to 3.83 J·m−2, respectively. The SR's calibrated stiffnesses and critical stress ranged from 2.5e9 to 6.51e9 N·m−3 and 6e9 to 7e9 Pa, respectively. The variations of shear stresses and cutting forces with displacement were basically consistent in DEM simulations and measurements; moreover, both simulated internal friction angle and cohesion had relative errors of less than 5%. These results indicated that the calibrated discrete element models of direct shear and cutting tests could accurately simulate the shearing and cutting processes of SSC. The current study results are beneficial to improve the reliability of numerical simulations for crop SSC and optimize tillage components.