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Fatigue Life Estimation For Preloaded Bolted Joint Under Vibration Environment

Prashant Sangule, Suhas Dhakate, Doug Scott
Eaton Corporation

One of the failure modes for aerospace structures is fatigue failure under vibration environment at the bolted joint region. Fatigue life is sensitive to mean stress, stress ratio and frequency of alternating stress. The traditional approach is to assume vibration load completely reversible (stress ratio = -1) in fatigue life calculation. The assumption of completely reversible load in some cases leads to
high conservatism in fatigue life which results in over-design (in terms of weight) of component. In reality, stresses may not be completely reversible as lugs are bolted to foundation. The movement of mounting lug in one direction is restricted due to mounting foundation while in opposite direction it is free to move. Simulating such support condition in dynamic analysis is a challenge as traditional dynamic analysis does not simulate contact non-linearites. Equivalent static approach is proposed to calculate realistic stress ratio for dynamic analysis. This approach can be used on rigid systems like manifolds, pumps, motors etc. The advantage of proposed approach is realistic stress ratio
evaluation to reduce fatigue life conservatism. Weight of component is critical CTQ for aerospace applications. Fatigue life conservatism reduction would help to minimize weight by removing un-necessary material. Validation of proposed approach is done with qualification test results to gain more confidence. The proposed approach yields accurate results for first mode which is dominant in one direction only. The limitation of this approach is that it cannot be used for mode having participation in two or more directions as well as for higher order modes.