Effects of Frequency Response Spacing on Vibration Fatigue Analysis

Abstract

Vibration fatigue analysis, which is established by Dirlik at 1985 for the random load PSD, is widely used to develop automotive components or parts. For the vibration fatigue analysis, normal mode analysis and frequency response analysis precede in order to obtain the system transfer function. The estimated fatigue life by vibration fatigue analysis is highly affected by the frequency request method in the frequency response analysis. In this study, we proposed the most effective frequency request method, which is requiring small number of frequency response points, producing less output file size and giving reasonable results. For this study, we made a primitive analysis model and compared 18 frequency request methods to find out the best method. 18 methods can be categorized by four classes—equally spaced, natural frequency included, half power bandwidth included, and transmissibility frequency included. For each methods, frequency response analysis and vibration fatigue analysis is performed and fatigue life and output file size of each models are compared. As a result, natural frequency peaks play an essential part in fatigue life estimation, so the peaks should not be omitted. Half power bandwidth highly related with damping ratio can provide criteria for the frequency response spacing and transmissibility frequency can set the limit of influence area. The proposed method, which needs only 11 points per natural frequency, reduces output file size by 1/20 and gives 0.69 times of fatigue life compare to 1 Hz equally spaced frequency request method. Current study provides the guide post to frequency response spacing, but the number of request points and the positions may not be the optimal values. We are finding out the optimal values by using optimization process. Most of the vibration fatigue analysis studies overlook the frequency request methods, because their models are not so large. For their works, only using equally spaced frequency request method could be sufficient. But nowadays there is a growing need to deal with large size models, so we consider the new method that can deal with large size models. With the proposed method to determine frequency response spacing for the vibration fatigue analysis, criteria of spacing and limit of influence area are set by half power bandwidth and transmissibility. This method could be utilized for the large sized models like as head lamp, front end module or automotive battery system, etc.

F2012-E03-020