Experimental study evaluating the effect of stepped patch external ply material on the impact resistance of CFRP aircraft repaired plates

This study investigates the influence of external ply material composition in a stepped patch on the impact resistance of carbon fiber–reinforced polymer (CFRP) plates subjected to repeated low-velocity impacts. The objective is to enhance the durability of repaired composite structures used in aerospace, marine, and automotive applications. Three repaired CFRP configurations were manufactured using a manual vacuum-assisted layup technique, each incorporating a different external ply material: carbon, Kevlar, or a hybrid combination. Intact and repaired plates were subjected to four consecutive 18 J impacts using a drop-weight testing system. After each impact, damage was assessed using ultrasonic C-scan to quantify the delaminated area. The results indicate that the damage tolerance of the repaired laminates strongly depends on the material used in the external ply. The most severe damage was observed near the edge of the outermost Kevlar layer in the patch. Although this configuration exhibited higher energy absorption during impact, it experienced significantly greater damage and the lowest impact resistance, along with the largest permanent deformation. These findings highlight the critical role of external ply material selection in improving the structural durability of composite repairs.