Material Influence on Crenellation Effectiveness in Damage Tolerant Design

Crenellation is a novel concept to heighten the fatigue resistance of the airframe structures without increasing weight. In this work the impact of materials on the effectiveness of crenellations was investigated. Two candidate alloys for future fuselage skin: AA2139 and AA2198 were selected for this task. Firstly the microstructure and texture of those materials were investigated. Then flat and crenellated panels made of the two alloys were tested under service-related biaxial loading conditions. A removable δ5 clip gauge was applied at the crack tip to monitor the crack closure behavior during the fatigue tests. After fatigue tests the fracture surfaces were examined to interpret the respective fatigue behavior. It was found that sharp textured AA2198 alloy, which also showed tortuous shear lip morphology, has higher fatigue resistance and larger fatigue life improvement of crenellations compared with the nearly randomly textured AA2139 alloy. The correlation between the sharp texture and tortuous shear lip morphology was discussed. The source of the additional fatigue resistance and the increased crenellation efficiency in AA2198 was also analyzed.