Andrew J. Gunnion

On the Certification of Bonded Repairs to Primary Composite Aircraft Components

Airworthiness certification is required when bonded repairs are made to primary composite structure in situations where damage has reduced or has the potential to reduce residual strength to below the design ultimate strength. Generally, certification of bonded primary structure poses many difficulties. As most repairs are one-off events meeting these certification requirements is especially challenging since demonstration by testing will generally not be possible or cost-effective. This paper discusses options for addressing the two key issues relating to certification: (a) how to validate initial and enduring bond strength of adhesive bonds, mainly given the inability of conventional non-destructive inspection to provide this assurance and (b) how to develop acceptable generic design allowables for bonded repairs which represent actual failure modes – especially for cyclic loading, since validation by testing of simulated repairs will generally be infeasible. It is concluded that proof testing of bonded repair coupons is a promising approach for validating bond strength and fatigue testing of representative bond joint specimen can provide generic allowables for patch design. For hidden structure or very high value repairs structural health monitoring of repairs based on a strain-transfer approach offers considerable promise.

Optimum Shapes for Minimising Bond Stress in Scarf Repairs

Abstract 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.

Engineering solutions for complex composite material behaviour spanning time and temperature scales

The issue of time and temperature dependencies is considered in the behaviour of advanced fibre-reinforced polymer composite materials. Currently, for the vast majority of analyses of composite structures, time and temperature are considered invariant. In an effort to further improve the design of composite structures, more advanced analyses are now being developed to accurately capture the behaviour under a range of conditions. Various events and load cases in which time and temperature are critical are described in general terms. The specific cases of viscoelastic distortion under mechanical and thermal loading, the behaviour of adhesive joints and the structural response of composites to fire are discussed in detail. The key material response, characterisation methods and analysis approaches developed are described. It is observed that key challenges in the development of improved predictive models are measurement of time- and temperature-dependent material properties and the implementation of efficient multidisciplinary analysis methods.