Comparison of Stepped and Straight Design of Bonded Scarf Joints

Abstract

Structural bonding and bonded repairs of composite materials become more and more important in aerospace industry. Depending on the application, two different repair designs are common in the repair of aircraft structures – Straight scarf and stepped lap repairs. From a sole mechanical point of view straight scarfs are more efficient compared to stepped joints. Peel stresses are minimized due to the smoother contour. A recent experimental study conducted by the author’s partners in the German national research project FACTOR (funded by the German Ministry of Commerce) showed a differing behavior. Two scarf ratios (1:20 and 1:40) for a straight as well as a stepped design are investigated giving unexpected results. The presented work summarizes the experimental data and explains the necessity of a numerical study. Several numerical simulations are carried out validating the experimental data. A 2D plain strain model is used to investigate the influence of several parameters. Material data for the parent laminate, the repair laminate and the repair adhesive are derived from literature. The geometrical and material non-linear simulations are described and four joint configurations analysed. The differing failure cascades and the specific characteristics of the investigated stacking sequence are discussed. Recommendations for future experimental designs to study the difference of repair configurations are given