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On the mechanical performance of carbon fiber-reinforced polymer (CFRP) panels containing adhesively bonded repair patches
Dissertation   Open access

On the mechanical performance of carbon fiber-reinforced polymer (CFRP) panels containing adhesively bonded repair patches

Ryan James Neel
Doctor of Philosophy (Ph.D.), Drexel University
Mar 2024
DOI:
https://doi.org/10.17918/00001900
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Abstract

Adhesively bonded repair patches Airplane wingbox structure Carbon fiber-reinforced plastics Full-depth scarf cutouts Half-depth scarf cutouts Non-destructive inspections (NDI) Quasi-static fatigue testing Residual strength testing
Over the last few decades, the increased use of composite materials in transport-category aircraft and the development of sophisticated, adhesive-reliant methodologies to repair them has led to the issuance of a number of safety-centric guidelines, including a policy statement issued by the Federal Aviation Administration (FAA) in November 2014, which requires that critical structures (i.e., primary structural elements of transport-category aircraft) must, in the event of a failed repair patch, maintain the ability to satisfy limit-load strength requirements. While a significant number of investigations conducted throughout the last few decades have focused on the efficacy of those adhesive-reliant methodologies, including those involving scarf repair patches, little emphasis has been placed on the mechanical performance of structures from which those repair patches have detached as a consequence of bond-line failure. Meeting the need for such research, a hybrid, numerical-experimental investigation was conducted to evaluate the mechanical performance of scarfed, flat, rectangular, carbon-fiber-reinforced polymer (CFRP) panels (i.e., structures representative of such cases) incorporated as topside components in cantilevered wingbox structures subjected to pure bending. During the experimental phase of the study, conducted at the FAA William J. Hughes Technical Center, the Airframe Beam Structural Test (ABST) fixture, a state-of-the-art test fixture capable of subjecting cantilevered wingbox structures to a wide variety of loading conditions, was used to evaluate the mechanical performance of four 18-ply, nearly-isotropic, CFRP panels installed as topside components in cantilevered wingbox structures subjected to sequences of quasi-static loading, constant-amplitude fatigue loading (in two of four cases), and residual strength loading to failure. Strain gages and three-dimensional (3D) digital image correlation (DIC) systems were leveraged to monitor for deformations, while a bevy of non-destructive inspection (NDI) techniques, including flash thermography, pulse-echo ultrasound, phased-array ultrasound, and acoustic emission (AE), were leveraged to monitor for damage initiation and growth. Ultimately, the results obtained thereby were significant, indicating that wingbox panels configured as such are able to survive conditions in excess of limit-load requirements, regardless of their fatigue load history, but limited, as they were representative of a small number of scenarios; thus, a commercial finite element (FE) program, ABAQUS, was leveraged to develop a numerical model of the structure with which a more thorough and cost-effective study could be conducted. Validated through a comparison of the experimental and numerical strain distributions recorded and computed, respectively, for the half- and full-depth scarf scenarios, the FE model will be used to further investigate the nature of the stresses that develop in such cases and the effects imposed thereon by the characteristics of scarf cutouts, like shape, size, taper ratio, and depth, under a variety of mechanical loading scenarios.

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