This paper was produced for the 2019 NAFEMS World Congress in Quebec Canada
The public transportation industry benefits from extensive legacy data, in terms of design criteria and in-operation performance data. However, like any other industry, vehicles used in public transportation are not immune to in-service failure. Mass transportation vehicles, such as city buses, need to be robust and offer extensive service life without defects or major failures. Manufacturers have to provide warranties covering several hundred thousand kilometres (> 500,000 - 800,000 km) over a period of up to twelve (12) or even sixteen (16) years. City buses have the particularity of integrating multiple systems and personalized accessories to fulfil operator and user needs using modern technologies. City buses also have the particularity of repeatedly traveling the same routes, allowing to establish cyclic loading conditions and damage mechanisms (fatigue life) specific to routes and vehicle configurations. These factors, combined, can increase the risk of in-service failures of structural elements and secondary systems with a sensitivity to a specific component of the cyclic loading. This paper focuses on the process used to identify the root cause of in-service city bus failures and how these situations can be resolved. This paper also shows how numerical simulation played a central role in finding solutions to the structural problems by highlighting several cases.
Finding the best compromise between a low failure rate and reduced conservatism leads to increased performance and requires a thorough understanding of failure mechanisms and in-service loading of the structures. While data acquisition is essential to gather precise knowledge of the loading conditions, structural finite element analyses allows a mathematical description of the failure mechanism difficult to obtain using any other means. Such analyses allow for a more robust design as well as a reduction of conservatism, leading to an optimization which is often not permitted when using legacy guidelines and design criteria. Legacy guidelines and design criteria remain critical since they have evolved through past success and failures. Using the structural finite element analysis in the study of in-service failures and reducing uncertainties using appropriate data acquisition allows the design of robust countermeasures without unnecessary conservatism, weight and costs.
Many in-service failures are due to specific loading conditions. Properly representing effects of such loading conditions requires advanced simulation techniques such as dynamic analysis for complex vibration environments, transient analysis for impact events and time-dependent events, and advanced fatigue life calculation to evaluate the consequences of cyclic loading. In addition to fatigue damage cumulation, vibrations induced by the complex road environment produce other detrimental effects such as modal dynamic amplification. To account for the contribution of the different structural modes over the appropriate frequency range, a detailed finite element model which accurately represents the modal behaviour of the structure is required. Vibration excitation is specific to the road type, vehicle type and equipment mounted to the structure and must be based on data acquisition. Model correlation with the measured response can be carried out, and the root cause identification of in-service failures can be performed, and effective countermeasures can be designed.