This presentation was made at the NAFEMS Americas Seminar - Confidence in Engineering Simulation: The Next 10 Years of CAE in Mexico.

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Resource Abstract

The finite element analysis (FEA) can be used not only to calculate the stress and strain distribution in a structure or a mechanical part, it can also be a computational lab when experimental results are not available due to cost, sustainability or schedule conditions. It is a suitable method for static, thermo-mechanics or modal analysis and it also helps to solve inverse problems where boundary conditions are not previously known, in particular when there is a lack of information of some mechanical parts and also when the structure is only at the stage of design. Limitations of budget and time defined the need of the computational lab to study the structure of a passenger bus from a Mexican factory.



The finite elements library of a commercial software permits to simulate beams and plates of the structure, as well as the elastic properties of the materials. Despite string and dampers are available in the library, the properties of the shock absorbers were unknown, in fact, the catalogue of the producer does not provide technical data of the shock absorbers to be used as input for FEA and the commercial representatives do not have this kind of information.



The computational lab helps to solve this problem, simulating the behaviour of the vehicle under several boundary conditions of modal damping and elastic properties for the shock absorbers, prescribed as properties of the elements in the FEA software. The computational lab is used when no experimental results are available, then it is a theoretical analysis based on the designer’s previous experiences from other projects where experimental results were available. A detailed explanation will be available in the final presentation. A convergent series of more than 50 iterations of modal simulations did provide a final reasonable set of values for the damping and string elements. With these parameters it was possible to calculate modal shapes and frequencies as well as the worse stress conditions for the vehicle structure under static and dynamic conditions.



The FEA modal dynamics simulation of the new vehicle provided the response under various dynamic conditions. It is possible to compare the results of the new structure to those from previous vehicles where experimental modal analysis were available. Some differences between the new vehicle and previous ones were found, in particular in the range of frequencies. To reduce these discrepancies some more iterations were needed.



The final model was used to simulate impact dynamics. The resultant principal stress state was used as input in fatigue analysis according to a cumulative damage model. The structure was redesigned for the beams and structural supports if: (a) Von Mises criterion exceeded yield stress or (b) fatigue simulation provided a finite life.



This project is based on the experience in modal analysis and fatigue since 1989, both in experimental and computational techniques. The buses are actually in use in Mexico.