Tillage tool design by the finite element method:: Part 1.: Finite element modelling of soil plastic behaviour

The finite element method calculations of draught and vertical forces, soil deformation and normal pressure distribution on subsoiler face were reported for four subsoiler types. A non-linear, three-dimensional, finite element analysis of the soil cutting process by a standard medium-deep subsoiler based upon the Drucker-Prager elastic-perfectly plastic material model was used. The mathematical construction of the Drucker-Prager model was presented. The material non-linearity of soil was dealt with using an incremental technique. Inside each step, the Newton-Raphson iteration method was utilized. The geometrical non-linearity was solved by using the small strain assumption. A comparison of subsoiler forces for calculations made with the small strain assumption and the updated Lagrange formulation of large displacement was reported for subsoiler cutting in a sandy soil. It was shown that the small strain assumption was more convenient for solving the geometrical non-linearity of a soil tilled down to relatively deep horizons. The theoretical results showed that a well coordinated angle combination of the two parts of the subsoiler made a large reduction in the draught and vertical forces of the subsoiler with a shank angle of 75 degrees and a chisel angle of 15 degrees. On the soil surface in front of the shank, the soil was deformed to produce a wedge-shaped soil upheaval. A maximum upward surface movement of 23.7 cm was calculated when soil tilling was performed with this design of subsoiler. For all the geometrical types of subsoiler studied, concentrations of normal pressure at the outer linking edges between the two parts of the subsoiler, as well as on the bottom corners of the chisel, indicated that during manufacturing these parts should be better supported against wear and deformation. The smallest chisel angle of 15 degrees reduced considerably the pressure values at these two parts, whereas changing the shank rake angle from 90 to 75 degrees only assisted in reducing the pressure values at the outer linking edges. (C) 1999 Silsoe Research Institute.