Ph.D. Proposal Presentation by
Monday, April 4, 2005
(Dr. W. S. Johnson, Chair)
"A Finite Element and Experimental Investigation on the Fatigue of Riveted Lap Joints in Aircraft Applications"
Aircraft fuselage skin panels are joined together by rivets. The initiation and propagation of fatigue cracks in aircraft structures at and around the rivet/skin interface is directly related to residual stress field induced during the riveting process and subsequent service loads. Variations in the manufacturing process, such as dimensions and surface finish of holes can influence the induced residual stress field. The effects of squeeze force on the initial interference in riveted lap joints and the resulting residual stress field affecting fatigue life have previously been studied using a 2-D axisymmetric finite element model and experimental testing. The 2-D finite element model cannot capture the unsymmetric residual stress state in the lap joint resulting due to rivet buckling and local geometry features representative of service hole quality at the rivet/skin interface. In this study, a quasi-static displacement controlled 3-D finite element model that takes into account material, geometric and contact non-linearities is developed using ABAQUS 6.4-1. The FE model is utilized to conduct parametric studies to observe effects of varying hole clearance, misaligned holes, presence of debris, skin defects, friction, applied rivet displacement and sealant effects on the residual stress state. The results from this model are imported to a global lap joint model to study the effect of in flight loads on fatigue crack nucleation. The application of a multi-axial fatigue theory based on previous research is proposed to determine the cycles to crack nucleation. The parametric finite element studies will be supported by experimental fatigue testing of lap joint specimens under a range of loads representative of those experienced by aircraft fuselages. The fatigue tests will aid in identifying areas of crack nucleation sites and the critical process variables that lead to crack nucleation and failure.