Development of an efficient approach for fatigue crack initiation and propagation analysis of bridge critical details using the modal superposition technique

Abstract

The fatigue damage assessment of large bridges is highly conditioned by the required computational high demands. Generally, in order to overcome the multi-scale problem, global and local models are needed to properly account for both global structural behaviour and the local nature of the fatigue damage. The analysis of such structural problems using direct time-integration algorithms is impracticable in most of the cases, which leads to the necessity of developing alternative methodologies in order to increase the computational efficiency and the accuracy of fatigue cracking assessments. In this respect, effective computational algorithms based on the modal superposition technique have been proposed and implemented in previous works. Overall, such workflow considers the interaction between the global and local models combined with the application of the modal stress intensity factor concept. Aiming at performing an efficient and accurate assessment of the fatigue damage, firstly, combining the Fracture Mechanics principles and crack propagation laws, the crack propagation phase in a complex bridge detail is analysed. In this regard, the present paper aims at proposing relevant improvements to the above-mentioned methodology, namely: i) the refinement of the implemented submodelling techniques in order to increase the accuracy of stress and strain fields computation and allow to account for smaller initial crack lengths; ii) the analysis and limitation of the considered number of vibration modes to the relevant ones for the local dynamic response; and iii) the implementation of a parallel computing approach for the calculation of the modal stress intensity factors related to the vibration modes defined in ii). The fatigue assessment procedures were applied to an assumed cracked welded detail of a recent railway composite bowstring bridge located in Portugal. Also, since the assumption of a pre-existing crack may lead to very conservative predictions, the modal superposition technique is further extended to evaluate the fatigue crack initiation phase, demonstrating the safety of the analysed case study in the absence of existing defects.