Effect of muffler mounting bracket designs on durability

摘要

Automotive industries perform durability tests on vehicles in the end-user environment to reduce failures and warranty costs in the end-user hands. In this paper we present the failure analysis of muffler mounting brackets of three-wheeler vehicles observed during the durability test. Cracks at the weld location between the engine cradle and brackets were observed in all the vehicles at an average distance of 10,000 km. Many possible causes of the failures are identified using fishbone diagram. The fishbone diagram is a graphical analysis tool that provides a systematic way of looking at effects and the causes that contribute to those effects. Statistical analysis of the failure data was conducted using Weibull distribution for durability life prediction. Further investigations were carried out on the design using finite element method (FEM). A FEM model was developed for the engine cradle assembly in which engine and muffler were modeled as point mass. Vertical forces were applied on the assembly using '4g' criterion, where g is the acceleration due to gravity. The applied force accounts for the high impact forces that act on the structure during durability testing. Results show high magnitude of stresses and strain energy at the weld location. Analysis of the design suggests that bracket was acting as a cantilever beam with one-plane welding mounted on the engine cradle. Modified design, though eliminated the above failure, shifted the failure mode to the bush-bracket region. Various design modifications were carried out and its effect on durability has been discussed. FEM analysis on the final design shows significant reduction in stresses at the critical locations. The new design of the bush-bracket system passed the durability target of 1,00,000 km.