Fatigue crack link-up in riveted lap joints containing multi-site damage: experiments and analyses

Xiaomo Zhang

doi:10.17918/00011145

Aircraft fuselage lap joints are critical structural elements that are subject to fatigue loading due to cyclic cabin pressurization and depressurization. During the service life of an airplane small cracks may emanate at multiple rivet holes, particularly along the lap joints. These cracks may grow rapidly, and neighboring cracks may link up, forming a long crack, leading to catastrophic failure of the entire fuselage panel and the aircraft (e.g., the 1988 Aloha Airlines Flight 243 incident). This unique fatigue crack propagation and link-up in riveted fuselage is recognized as "multiple site damage" (MSD). Since the Aloha incident, and followed by several similar cases, extensive work, both experimental and analytical, has been conducted by the aircraft industry and the Federal Aviation Administration (FAA) to address the MSD phenomenon, improve the design of the lap joints, and establish new protocols in non-destructive inspections. This dissertation focusses on the study of the fatigue crack initiation, growth, and link-up characteristics of MSD on the lap joints of a typical narrow-body aircraft. Fatigue testing was conducted on two different lap joint specimen geometries: (1) single rivet column (SRC) lap joint coupons and (2) wide flat lap-joint panels (WFP) with 19 rivet holes. Extensive testing was conducted on the former, for collecting baseline data and studying the effect of manufacturing processes on fatigue life. The testing of WFP specimens, which included initial MSD scenarios, focused on the measurements of fatigue crack initiation, growth, and link up. An anti-bend device was developed for the testing of WFP specimens to minimize the effect of secondary bending at the lap joint for closer simulating actual curved fuselage panels. A special marker band loading spectrum was applied during the fatigue testing to facilitate post-test forensic studies of the fatigue fracture surfaces. A finite element based numerical predictive framework, incorporating phase-field methodology, has been developed to simulate the fatigue crack growth and link-up in WFP specimens. This framework is built upon the prior work developed by Khalil et al. [Khalil, 2022] for modelling material degradation in ductile materials due to fatigue cycling. Modifications to Khalil's framework, including transitioning from plane-strain to plane-stress element types and decomposing of plastic strain energy, are incorporated to address the unique features in specimen configurations and loading conditions of the present study. The modified framework was applied to a simplified 3-hole model first for framework validation and then to a full-scale 19-hole model corresponding to experimental configurations. The numerical predictions showed good agreement with experimental observations, including fatigue crack growth rates, link-up behavior, and crack paths. In summary, the experimental data and numerical predictive methodologies developed in this study offer valuable resources for fatigue life prediction and structural safety assurance in aging and next-generation aircraft structures.

Fatigue crack link-up in riveted lap joints containing multi-site damage: experiments and analyses

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Fatigue crack link-up in riveted lap joints containing multi-site damage: experiments and analyses

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