Theoretical and experimental study of the fatigue behavior of large passenger transport vehicles under dynamic operating loads

Resumen

Heavy duty vehicles for passengers transport are mostly buses and coaches. The structures of these vehicles are subjected to variable loads over time during their operation, causing fatigue to be their main cause of failure, and thus the most restrictive criterion in their design. The great majority of these structures are composed of welded tubular profiles, so the inherent phenomenon of strain concentrations, associated with the weld bead created during this process, facilitates the crack initiation precisely in this zone. Therefore, an optimum design of these structures is directly linked to a precise fatigue analysis. In order analyze the fatigue behavior of any component or system accurately, it is of vital importance to have a precise and representative characterization of the loads that it undergoes during its operation, as well as to know its resistant behavior against this phenomenon. In this sense, the increasing development of finite element calculation techniques represents an important advance in fatigue oriented design and calculation, since they allow to anticipate possible sources of failure in the initial phases of design. The modeling of structures of large passenger transport vehicles is usually carried out in a simplified way by means of beam type elements, which allows to obtain satisfactory results of the overall behavior of the structure with a low computational cost. However, this type of model has certain limitations; among then the inability to reproduce the tensional behavior of the welded joints of the profiles, of vital importance in a fatigue analysis. In this scenario, the main objective of this thesis is to develop an accurate fatigue life prediction routine of these structures, capable to overcome the existing compromise between precision and complexity in the calculation. To this end, a characterization of the dynamic loads of buses and coaches was carried out. The longitudinal and lateral accelerations were obtained from the processing of position data obtained with a GPS positioning system, installed in 10 different vehicles and recording more than 600 hours of operation. In the case of vertical accelerations, an accelerometer was used to capture the higher frequencies at which these events occur. Additionally, a test plan was developed with the purpose of characterizing the structural and fatigue behavior of the welded joints that compose these structures. These studies allowed to determine the resistant characteristics of the heat affected zone, obtain a strain to life curve of typical welded joints in buses and coaches, and also to characterize the strain concentration phenomena in these joints. The process of characterization of strain concentrations in the vicinity of the welded joint was carried out by means of the Digital Image Correlation (DIC) technique, which allows obtaining a complete deformation field over the region of interest. This information was used for the construction and validation of a finite element model capable of accurately reproducing the deformation distributions as well as the maximum deformations suffered by the joints, which is where the fatigue crack is expected to start. In spite of the high precision achieved in these detailed models, the construction of a complete structure with such models becomes practically unfeasible due to its complexity. In order to overcome this limitation, a series of regressive models were configured in order to establish a correlation between the maximum deformations obtained with these models and those obtained with simplified models using beam-type elements. This correlation was carried out on a total of 2304 different welded joints corresponding to different possible joints present in structures of buses and coaches. Finally, the results obtained in the different tests and calculations were structured to develop a routine for predicting the fatigue behavior of bus and coach structures modeled using beam type elements. This routine is able to unlink the compromise between precision and computational cost: on the one hand, it achieves high precision due to the application of realistic load conditions, a characterization of the fatigue behavior of the welded joints, and the correlation algorithm of maximum deformations between detailed and simplified models. On the other hand, it maintains a low computational cost due to the use of simplified models for the simulation of the response of the structures.