Patch repair of perforation damage in composite laminates and its influence on the echo characteristics of grazing incidence electromagnetic waves

In fifth-generation aircraft, carbon fiber reinforced polymer composites are extensively utilized. The pursuit of ultimate stealth performance necessitates that repairs to composite skin damage consider not only the restoration of mechanical strength but also the recovery of stealth capabilities. This paper focuses on the damage assessment of mechanical and stealth performance for perforation damage in composite skin panels, and explores effective single-patch repair methods. The results indicate that perforation damage significantly affects the tensile strength of composite laminates. For a quasi-isotropic composite panel with a width of 50 mm, a 10 mm diameter perforation can reduce the strength by more than 40%. Perforation also has a substantial impact on the radar cross section (RCS) under grazing incidence; a 10 mm diameter perforation can increase the average RCS of a 2 mm thick skin by 4.85 dBsm for incidence angles between 0° and 15°. The repair effectiveness of patch repair for composite perforation damage is related to the patch shape. Rectangular patches fail to simultaneously meet the requirements for strength and stealth recovery. Although diamond-shaped patches can effectively reduce RCS, they significantly compromise the restoration of tensile strength. The tapered-end patch design satisfies both mechanical strength and stealth performance restoration requirements. For a quasi-isotropic composite panel with a 10 mm diameter perforation and a thickness of 2 mm, using a patch with a 1∶60 taper ratio for patch repair achieves a strength recovery rate of 81.54%, while substantially restoring stealth performance.