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Session: Session 3: Fatigue life enhancement methods and repair solutions I
Session starts: Monday 26 June, 15:30
Presentation starts: 15:50
Room: Theatre room: plenary

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Matthew Kokaly (PartWorks)
Jude Restis (PartWorks)

Abstract:
Partworks is conducting a test and development program on a new repair for aerospace structures, including hybrid composite and metallic designs. The repair incorporates adding beneficial engineering residual stresses and the use of new technologies to advance beyond the status quo. The overall goal is to drastically reduce the cost and time to make in-service repairs while also providing a longer life and higher reliability compared to what is used today. The use of hybrid, mixed material aerostructures utilizing carbon skins over aluminum substructures in aircraft today has driven higher levels of corrosion than all metal designs of the past. This is largely attributable to the significant galvanic potential difference between metal and carbon fiber composites. Existing repair methods for metal/carbon fiber composite skin bolt hole/fastener sites necessitate extensive and complete removal of corroded material, non-standard hole sizes and individual analysis and validation of each repair instance. Limited to no testing is done to demonstrate effectiveness of each unique repair configuration. The proposed new repair method combines the use of a known engineering residual stress process with new technology and understanding including the use of thin wall bushings and/or rivetless nut plates, a new adhesive/sealant, a refined expansion process to better control the residual stresses developed during installation while maximizing post install compression, deliver standard hole sizes post treatment, incorporate technology to verify the level of residual strain and stresses and an ergonomic design of lightweight tooling. The repair method will capture installation parameters real-time in order to provide go/no-go feedback to the operator. The resulting repair is intended to restore or further enhance fatigue life at the bolt hole while minimizing the amount of material removal. This presentation will describe the progress to date, including unique fatigue test results with the worst-case scenario of severe corrosion left in place during repairs. Early testing shows that the repair is capable of significantly extending the fatigue life, even for severely corroded material, over the standard open hole. We will also share the results from the application of a Digital Image Correlation (DIC) system to validate and quantify the residual strains from the repair and X-Ray Diffraction (XRD) measurements of the residual stress fields. Correlation of the measured DIC residual strains and XRD residual stresses will be shown along with a comparison with the results of a nonlinear finite element analysis (FEA) of the repair process. Future plans will be shared including the greater contribution of FEA to the development process.