Rapid Fatigue Life Prediction of Nonlinear Loading and Structures

"The objective of the research detailed in this SBIR proposal is to develop the relationships between the Power Spectral Density (PSD) of the nonlinear response of a structure and the Probability Density Function (pdf) of rainflow range cycles (RRC's)within a corresponding response time signal. Given these expressions, more realistic fatigue life calculations are possible.Similar techniques for linear structures are now very well developed. For example, an expression developed by Dirlik (1985) has been fully embedded into a commercial Finite Element (FE) based system (MSC.Fatigue) developed by the MSC Software company whichis fully linked to three of the more well known solvers: Nastran, Abaqus and Ansys. Similar methods exist in other FE linked fatigue analysis programs (see for instance, FE-Safe), however, these existing frequency domain fatigue life estimation tools donot take into account nonlinear structural behaviour such as geometric nonlinearities, nor the instabilities associated with thermal buckling.The deliverable from this research will be analytical expressions for the random vibration fatigue life of structures (including composites) with simultaneous thermal and acoustic loads, using frequency domain methods.Improvements are needed for the estimation of the fatigue life due to thermal acoustic loads on current and future short take-off and landing (STOL) and hypersonic vehicle structures, as well as developing commer