Assessment of Residual Stresses and Hole Quality on the Fatigue Behavior of Aircraft Structural Joints. Volume 3: Finite Element Simulation of Riveting Process and Fatigue Lives

摘要

Aircraft fuselages are fatigue-critical structures, and operational damage in the lap joints have been widely recognized as a safety issue. The Federal Aviation Administration and Delta Air Lines teamed in an effort to conduct destructive evaluation, inspection, and extended fatigue testing of a retired Boeing 727 passenger aircraft near its design service goal. Inspections revealed a large number of cracks in the fuselage joints emanating from the rivet/skin interface. The presence of these cracks was attributed to the sharp stress gradients arising from contact between the installed rivet and the rivet holes. The residual-stress field generated during the rivet installation has a strong impact on the nucleation and propagation of fatigue cracks at and around the rivet/skin interface. The main objective of this research was to establish a link between critical riveting process parameters and the potential of fatigue damage in the joint. The significance of contact and plasticity in the riveting process made the problem well-suited to a finite element (FE) modeling solution. In addition, there was also a need to understand how manufacturing process variations affect the residual-stress state produced in the joint. Both of these objectives were addressed in this research. The study was conducted in four phases. First, a two-dimensional axisymmetric model of the riveting process was developed, which was deliberately designed to simulate previous experiments conducted by a team of Canadian researchers. The second phase used the FE approach and the lessons learned from the first phase to specifically focus on the development of a three-dimensional displacement controlled riveting process model for the lap joint configuration. In the third phase, the residual stress and strain generated from the riveting process models served as an input to a global three-rivet lap joint model designed to approximate the in-service loading experienced by the B727 fuselage splice. Finally, fatigue tests on riveted lap joint specimens were conducted to forge a link between rivet installation and hole-quality effects on fatigue performance of the joint. A comparison of the surface strains predicted by the FE models to the surface strains of the tested specimen captured through thermal imaging techniques offered an additional source of validation to the analysis. An observed correlation between the fatigue lifetimes of the tested specimens and controlled parameters showed underdriven rivets to be the most threatening in reducing the fatigue life of the joint.