Impact of Bolt-Nut Interface Modeling Assumptions on the Calculated Low Cycle Fatigue Life of Aircraft Engine Turbine Rotor Bolted Joints.
element analysis is used extensively in the aircraft turbine engine industry to predict
stresses to calculate low cycle fatigue (LCF) life. An accurate prediction of stresses
is especially important to a specific subset of engine hardware that is defined by the
FAA as life-limited parts (LLP's). LLP's include rotor and major structural parts such
as disks, spacers, hubs, shaf... read morets, high pressure casings, and non-redundant mount
components. A failure of an LLP can lead to a potentially catastrophic event due to
non-containment of high energy debris, uncontrolled fire, or a complete inability to
shut the engine down. 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. High fidelity stress analysis is necessary to appropriately set LCF life
limits. One common engine feature analyzed with 3D stress analysis is a rotor bolted
joint. Geometrical features associated with bolted joints such as holes, fillets, and
scallops cause stress concentrations. Often the life limiting feature in a rotor LLP is
a geometrical feature in close proximity to the joint. Unfortunately, the detailed
stress analysis associated with accurately predicting stress in the joint is costly and
time consuming. Analysis assumptions that can simplify the effort, yet still produce
accurate results, would be valuable to the industry. The focus of this study is on the
bolt-nut 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. A series of eleven cases are analyzed and compared to
investigate how the thread modeling assumptions affect the calculated life in the mated
rotor LLP hardware. Walker-adjusted alternating stress, σ0,alt, is used to measure
the life impact. The impact is limited to the edges of the two critical features closest
to the bolt-nut interface. The results demonstrate that factors such as the thread mesh
density, elastic versus elastic-plastic bolt/nut material properties, and the inclusion
of the helical thread shape have only minor impact. The inclusion of contact elements at
the interface instead of couples has a moderate impact of 1.1 to 1.2 ksi. When couples
are used, the placement of the first couple is critical, impacting the results by 1.1 to
2.6 ksi. Also, when couples are used to join the interface, the explicit inclusion of
the thread shape has only 0.5 to 0.6 ksi
Thesis (M.S.)--Tufts University, 2012.
Submitted to the Dept. of Mechanical Engineering.
Advisor: Anil Saigal.
Committee: Thomas James, and David Kelpe.
Keyword: Mechanical engineering.read less