Review on composite bonding repairs: Scientific challenges on machining, surface preparation, and structural health monitoring

Adhesively bonded repairs of thermoset fiber-reinforced polymer composites offer structural and weight advantages over mechanical fastening; however, they are generally not fully certified for primary load-bearing structures. This review evaluates the primary factors contributing to this lack of certification by focusing on surface and subsurface damage introduced during conventional machining, inconsistent or inadequate surface preparation. In addition, the limited capability of non-destructive testing techniques to detect all bond-line defects, quantify bond strength, and assess bond quality is also discussed. This review also examines the damaged material removal and surface preparation processes used in composite bonded repairs, comparing conventional machining with non-conventional approaches. Emphasis is given on using the abrasive water jet machining (AWJM) process for damage removal and surface preparation within a single repair process. The process of controlling induced damage, mitigation of thermal degradation, and improving surface morphology before bonding is also discussed. Additionally, bonded repairs remain sensitive to defects originating from patch fabrication, curing, environmental exposure, and in-service loading; improvements in machining and thus surface preparation alone cannot guarantee repair quality. Therefore, the role of structural health monitoring (SHM) as a necessary complementary process for certification is also discussed. The review also provides a comprehensive assessment of fiber-optic, piezoelectric, and piezoresistive sensing techniques applied to composite bonded repairs, evaluating their ability to detect damage initiation, propagation, and localization throughout the service life. It further identifies key technical barriers and outlines future research pathways toward the reliable qualification and certification of bonded composite repairs in primary aerospace structural applications.