Investigations on the influence of design parameters on the strength of CFRP repairs

Traditional repair methods involving riveted joints necessitate the creation of numerous holes in still-intact material, thereby compromising the structural integrity of the area. Load transmission between the adherents occurs within this weakened zone. This disadvantage can be avoided through adhesive-based repairs. Presently, aviation authorities only certify the use of bonded repairs for very small areas of damage. For the experimental investigation of bonded repairs, a standardized specimen at the coupon level, the EN6066 specimen, is available. However, this specimen represents a considerable simplification compared to actual repairs. Therefore, effects that have a significant influence may no longer occur, or only to a small extent, when they are analysed on more realistic specimens. In this study, "Cupped" and "Straight" repair configurations, differing in the arrangement of repair plies, were experimentally tested at both coupon and element levels under quasi-static loading. Significant variations were observed, strength differences between Cupped and Straight at the coupon level vary by about 27%, whereas the configurations exhibited only a 2% strength difference when tested at the element level. These disparities primarily are caused by the distinct detailing of the represented repair geometry, illustrating the challenges associated with designing specimens for investigating repair methods.