The multibody 2D approach for agricultural and forestry tractors Roll Over Protective Structures design

摘要

The European directive 95/63/CE states several requirements concerning the safety of workers. With reference to the agricultural and forestry tractors, it establishes the necessity of reducing the risks for the operator related to the roll over of the tractor. This task could be fulfilled be means of two passive prevention devices: the seat belt and the roll-over protective structure (ROPS). The second one consists of a rigid or partially foldable two posts frame or a four posts structure which surrounds the operator seat in order to preserve and protect the so called clearance zone in the event of tractor overturn. The design of ROPS shall meet the safety requirements defined in the specific Organisation for Economic Cooperation and Development (OECD) codes. In particular, an established sequence of loads shall be applied on the structure and for each load a minimum amount of deformation energy shall be reached. Thus, the structure should deform continuing to protect the clearance zone of the operator. Generally, the designers of these protective structures use a Finite Element Approach (FEA) in order to fulfil this task. Applying finite elements allows to mimic the plastic deformation and the non-linear behaviour of the structure with accuracy but, on the other hand, it is necessary to have good skills for properly modelling and meshing the structure. Moreover, considering the necessity of applying a specific sequence of loads on the structure, where the next load is applied on the residual deformation of the structure due to the previous load, the finite element non-linear analysis is quite time consuming. With the aim of reducing the cpu time consumption and simplifying the ROPS modelling, the authors present in this paper an in-house developed 2D code for the design of ROPS based on multibody approach. Considering the necessity of modelling the plastic behaviour of the structure and its related large deformations, a procedure for the automatic generation of plastic hinges on the ROPS, based on Castigliano's theorem for hyperstatic structures, has been developed. Hence, the dynamic analysis of the kinematic equivalent mechanism has been performed by means of the Udwadia - Kalaba's formulation. The code automatically applies the required loads sequence and stops the dynamic analysis once the defined level of deformation energy has been reached, plotting the load - deformation curve for each load. The information needed for performing the analysis are the shape and dimensions of the main frame of the structure, the cross section of plates and tubular which build up the structure and their material. The results obtained by means of the multibody 2D code have been compared with FEA results, showing good agreement and a sensible reduction of computational time. Moreover, the experimental test on a rear two posts foldable structure mounted on a tracklaying tractor has been reproduced.