Most popular practice for analyzing the Subsystem failures in commercial vehicles is physical testing. These physical tests are carried out by three tests; Endurance testing, Accelerated Endurance Testing and Rig test simulation. All the three methods are costly and repetitive iterations of these tests is not economical. Therefore, in our organization, we established a method in virtual domain in order to reduce the repetitive iterations and also reduction in time consumed per iteration. General practice in our organization for Finite Element Analysis (FEA) calculation was inclusive of Model preparation, Transient analysis using Nastran. The results from the Transient analysis are used for performing fatigue analysis in fatigue software. In this process, Transient analysis and Model preparation are very much time consuming processes. Model preparation cannot be reduced, but to reduce the transient analysis time, we established a method in frequency domain (vibrational fatigue) [1].
For vibrational fatigue analysis we used Nastran for Frequency Response Analysis and Spectral for Fatigue Analysis. For Vibrational fatigue we needed Modal stresses, Frequency Transfer Functions and Power Spectral Density (PSD) data. The modes of the mechanical system are the fundamental possibilities of the system being excited to oscillation. The modal stresses are the structural stresses for the respective mode of oscillation appearing after the application of the loads. Modal Transfer functions specify how strong the excitation of a mode of oscillation is under the respective unit load as a function of the frequency. Combining modal stresses and transfer function defines total stress appearing on mechanical structure. Power Spectral densities defines the energy content of the signal as a function of frequency [1].
Frequency response analysis performed in Nastran gives Modal Stresses and Frequency Transfer functions. Power Spectral Density data is converted from the acceleration vs time data measured from the test rig. Using these three datasets Vibration Fatigue analysis is performed in order to evaluate the Fatigue life of the Subsystem.
The results from the vibrational fatigue analysis are correlated with the test results from Accelerated Endurance Test and Rig testing data, and we achieved 90% correlation between the vibrational fatigue analysis and the test data results.