Adhesive stress criterion for the design of composite scarf joints

Abstract

Nowadays bonded scarf repairs are the state of the art technology to repair composite aircraft structures. Due to current aerospace regulations [1] the size of bonded repairs is limited. Today low scarfing angles generate large mechanical reserves at the cost of big repair sizes. The influence of design parameters on adhesive scarf stresses has been thoroughly investigated by several authors [2–5] in the past. Most of the time finite element models are used to derive material or geometric parameter dependencies on the scarf stresses and strength. There are analytical solutions to the constituent equations of the scarf stresses as found by Hart Smith [6–8] but these are iterative solutions. An easy to use formula for the adhesive stresses is given by Baker et al. [9] deriving a peak stress criterion for a given laminate. A comprehensive study has been performed developing a design method improving the straight forward peak stress criterion to get a better approximation of the adhesive scarf stresses. The proposed formula takes effects resulting from the stacking sequence, adhesive thickness as well as adherent offsets into account. Differences of laminate stacking sequences and their effects on the adhesive stress distribution are discussed. The improved analytical prediction is compared to finite element and experimental results. The proposed method is fast and easy to use, providing an improved precision for composite engineers within the predesign phase of bonded repairs.

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