ANALYSIS OF THE STRESS-STRAIN STATE OF AN ELASTIC WORKING TOOL OF A DISC HARROW UNDER A TWO-STEP LOADING MODEL

Abstract

The paper considers the stress-strain state of an elastic working tool of a disc harrow under a two-step loading model. The object of the study is the disc-to-frame attachment assembly, consisting of a disc, an elastic stand and an intermediate bracket connecting the stand to the disc axle; according to operational data, it is the elastic stand and the bracket that most often suffer crack initiation and failure. A three-dimensional geometric model of the assembly was created by reverse engineering from full-scale measurements of the actual part, since factory design documentation is unavailable. The elastic stand has a characteristic C-shaped curved profile (sweep length 440 mm, width 75 mm, thickness 15 mm) and is made of 60C2A steel, for which an isotropic linear-elastic model was adopted (elastic modulus 2.1·10⁵ MPa, Poisson's ratio 0.29, density 7680 kg/m³). The finite element model was built with an individually selected element size for each component: a Hex Dominant mesh with 4 mm elements was used for the stand, while the remaining parts were meshed with tetrahedral elements of 10–20 mm; in total, the model contains 137,876 nodes and 38,746 elements (excluding fastening bolts). The analysis was performed in two load steps: the first simulates the penetration of the disc into the soil (a 300 N concentrated force applied to the hub axle), and the second simulates the onset of soil resistance during travel of the implement (distributed pressure on the 140 mm edge zone of the disc, varying linearly from zero to 0.11 MPa). The maximum total deformation of the assembly is 116.7 mm and is localised at the free edge of the disc, whereas the intrinsic deformation of the disc, hub and bracket is negligible – the displacement is governed mainly by elastic bending of the stand. The zone of maximum stress (667.16 MPa) near the stand support was found to exhibit signs of finite element singularity; therefore, the local peak in this zone is not a representative estimate of the stress state of the part. On the curved section of the stand, outside the influence of the singularity, the equivalent von Mises stresses are approximately 370–445 MPa over a substantial area of the surface. The obtained results make it possible to identify characteristic stress concentration zones and can be used for further analysis of the structure; however, they remain insufficient for a final conclusion on strength or service life. They provide the necessary information for determining reliability indices such as the safety factor and the probability of failure-free operation.