Impact of Bolt-Nut Interface Modeling Assumptions on the Calculated Low Cycle Fatigue Life of Aircraft Engine Turbine Rotor Bolted Joints

Finite element analysis is used extensively in the aircraft turbine engine industry to predict stresses to calculate low cycle fatigue (LCF) life of life-limited parts (LLP’s). A failure of an LLP can lead to a potentially catastrophic event such as a noncontainment of high energy debris. Under-predicted stress can cause the life limits to be set too high, which is a safety hazard. Over-predicted stress can cause the life limits to be set too low, which adds cost due to the need to replace expensive engine hardware more frequently. As such, high fidelity stress analysis is necessary to appropriately set LCF life limits. This study focuses on the nut-bolt interface modeling assumptions associated with a rotor bolted joint stress analysis for LCF predictions. A 3D finite element model of an actual aircraft engine rotor bolted joint is created. Different cases are analyzed and compared to investigate how the thread modeling assumptions might affect the calculated life in the mated rotor LLP hardware. Walker-adjusted alternating stress, σ_0,alt, is used to measure the affect on life impact. It is shown that elastic versus elastic-plastic nut/bolt materials properties and the inclusion of the helical thread shape have minor impact on the calculated stresses. However, inclusion of contact elements with friction at the thread interface instead of couples has a moderate impact on the calculated stresses and therefore expected life.