Failure Analysis and Redesign of Tubular Cross Members in a Bus Chassis Prototype

An analysis of the failure which was observed during field endurance tests of a special purpose personnel carrier bus chassis is reported. Failure occurred in the connection region of a tubular type of cross-members and channel section side members. Firstly, in order to determine the cause of the failure, a simplified load model which stands for the case of driving over a bump is built. Afterwards, a comprehensive Finite Element (FE) model of the vehicle which includes the frame, the superstructure and the suspension system elements is created. Symmetrical and asymmetrical vertical wheel loads which simulate various idealised service conditions are applied to the model. The results showed that, depending on the magnitude of impact forces, not only considerable stress concentrations emerge within the cross-member weld beads but also safety limits could be exceeded from the aspects of maximum permissible stress. Moreover, it was observed that the system is more sensitive in terms of torsional loading. Secondly, structural suitability of an alternative rectangular section cross-member which is mounted to side member in a more flexible way, is evaluated by using FE analysis. In addition, effects of various design improvement solutions on stress concentration are discussed. The analysis results show that, in cross-member weld beads, by using the new design, it is possible to reduce the stress concentration to 90%. In field tests in which enhanced cross-member prototypes are used, no damages are reported in either chassis frame or superstructure under the service loads.