Adhesive stress criterion for the design of composite scarf joints

Kurzfassung

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. References [1] FAA (ed.). AMC 20-29 Composite Aircraft Structure: AMC 20-29; 2010. [2] Wang CH, Gunnion AJ. On the design methodology of scarf repairs to composite laminates. Composites Science and Technology 2008;68(1):35–46. [3] JOHNSON CL. Effect of ply stacking sequence on stress in a scarf joint. AIAA JOURNAL 1989;27(1):79–86. [4] Yoo J, Truong V, Park M, Choi J, Kweon J. Parametric study on static and fatigue strength recovery of scarf-patch-repaired composite laminates. Composite structures 2016;140:417–32. [5] Liu B, Xu F, Yan R, Ji Z, Li W. Parameters sensitivity and optimization for composite scarf repair. Journal of Reinforced Plastics and Composites 2014;33(23):2164–73. [6] Hart-Smith LJ. Adhesive-bonded scarf and stepped-lap joints. Hampton, Virginia, USA; 1973. [7] Harman AB, Wang CH. Improved design methods for scarf repairs to highly strained composite aircraft structure. Composite Structures 2006;75(1-4):132–44. [8] Erdogan F, Ratwani M. Stress Distribution in Bonded Joints*. Journal of Composite Materials 1971;5(3):378–93. [9] Baker AA, Dutton S, Kelly D. Composite materials for aircraft structures. 2nd ed. Reston, VA: American Institute of Aeronautics and Astronautics; 2004.