Search Results

This study presents a method to achieve a concise description of multidimensional loading histories for fatigue analysis using the stochastic process theory. For purposes of this study, the load history is considered to have stationary random and non-stationary mean and variance content. The stationary variations are represented by a vector Autoregressive Moving Average (ARMA) model while Fourier series are used to model the non-stationary variations. Justification for this method is provided by comparing the dynamic characteristics of the original loading and reconstruction through their power spectral densities. Further justification is obtained by comparing histograms of principal strain and the corresponding orientation for original loading and reconstruction. Final justification is provided using the resulting fatigue lives of original and simulated loading.

There is a trend in the automotive industry to reduce the number of physical prototypes and to rely more on Computer Aided Engineering (CAE) for sizing and final design of vehicle structures. The traditional deterministic approach does not necessarily clarify the degree of variability and conservatism. With small variability in influence parameters and a design factor for final design, the approach may be over conservative resulting in weight and cost penalty. On the other hand, with large variability and the same design factor, the deterministic approach may not satisfy durability requirements. It is important to identify the variability of all factors including road loads and sensitivities of the control parameters, and to minimize their effects on durability so that fatigue life distribution meets the durability requirements.

Automotive product development requires higher degree of quality upfront engineering, faster CAE turn-around, and integration with other functional requirements. Prediction of potential durability concerns using analytical methods for sheet metal structures subjected to road loads and other customer uses has become very important. A process has been developed to provide design direction based upon peak loads, simultaneous peak loads, and vehicle program analytical or measured loads. It identifies critical loads at each input location and load sets for multiple input locations, filters load time histories, selects critical areas and analyzes for fatigue life. Several case studies have been completed. The results show that the variations are consistent with the accuracies in finite element analysis, road load data acquisition, and fatigue calculation methods.