Optimum Shapes for Minimising Bond Stress in Scarf Repairs

Bonded scarf repairs are used in composite structures when high strength recovery is needed or when there is a requirement for a flush surface to satisfy aerodynamic or stealth requirements. However, scarf repairs are complex to design and require the removal of significant parent structure, particularly for thick skins. In this investigation, analytical and numerical approaches have been developed to investigate whether an optimum repair shape for a known biaxial load can be determined. The results clearly demonstrate that the strength of a repaired panel can be improved by optimising both the initial damage cut-out shape and the scarf angle distribution

Loaded carbon composite scarf joints subject to impact

Bonded composite scarf repairs are often used when a flush surface is required for aerodynamic or stealth reasons. Such repairs on the external surface of an aircraft are subject to the same impact risk as that of the parent structure. Consequently, it is essential to assess their durability in the case of impact. A previous preliminary experimental study found an instance of catastrophic failure of a composite scarf joint subject to impact whilst prestrained to 3000 μ. It was postulated that this phenomenon is a result of failure in the joint due to the combination of the prestrain and global structural oscillations resulting from the impact event. In this investigation, a previously applied finite element model is extended to more accurately replicate such catastrophic failure. The effect of lay-up sequence on adhesive failure is studied

Experimental and Finite Elements Analysis of Bondline Flaw Criticality in Composite Scarf Joints

Conventional design methods for bonded repairs to composite structure assume that the repair is in pristine condition. However, for a repair to be certified on primary structure, the repair must be designed to address damage tolerance and fatigue strength requirements, in addition to static strength. Therefore, the criticality of bondline flaws in composite scarf joints is analysed within this paper using finite element analysis (FEA). Experimental data is presented for scarf joints with various initial flaw sizes. As a result of both the experimental and FEA studies, it can be concluded that the strength of a scarf joint is considerably and adversely affected by the presence of a flaw