Finite element modelling of the interaction between flexible tines and soil for mechanical weeding

This research was carried out to obtained information about the interaction of flexible tines and soil for mechanical weeding. Due to the fact that chemical weeding has negative effects on the environment, mechanical weeding is widely used today as an alternative and more sustainable solution. The complexity of soil-tine interaction, with flexible tines, makes it highly difficult to extract the needed information from experimental works only. Therefore, the aim of this study was to obtain extended knowledge about optimal design parameters (rake angle and tine geometry) and operational conditions (working depth and bulk density) for optimal soil disturbance and least energy consumption, by the use of numerical finite element computer simulation. To achieve this, a test bench was designed to study different tine designs and operational conditions of selected flexible tines, provided by Einböck, an Austrian manufacturer of weed harrows. The results of the test bench were mainly used to validate the established finite element model, which enabled a more informed analysis of the weeding process. Furthermore, soil parameters and soil-metal properties needed as input for the FEM simulation were determined by standard laboratory tests. Results showed that FEM is an acceptable and cost effective alternative to experiments. The simulation errors for draught and upward tine tip movement were generally smaller than 15 % and 10 %, respectively. Software associated limitations were experienced to model the entire working process satisfactorily, for instance no crack propagation in soils could be taken into account so far. Nevertheless, it was possible to simulate the first soil-tine interaction contact. This was sufficient to optimise tine design parameters and operational conditions, which was considered a cost effective method for the manufacturing of prototypes. From the FEM simulation and soil bin test, it could be concluded that a stiffer tine with a higher torsion spring constant and a small rake angle should be used when less variation in working depth and more aggressive weeding is required. Otherwise, trailing positions should be used, if a shallow working depth is desired, to achieve higher soil disturbance in the surface for smaller draught requirements.