Creating kiss-bonds for non-destructive inspection

A novel method for creating kiss-bonds within a composite structure is presented. Kiss-bonds are a defect type whereby there is a loss of structural continuity within the material, yet the material remains in intimate contact across the defect. This results in a local loss of strength, which can then develop into a serious structural issue under in-service loads. With lightweight structures being at a premium for performance and emissions considerations, ensuring that a composite structure is both optimised and safe is a priority. This requires a thorough knowledge of the structural implications of kiss-bonds, as well as an ability to detect and characterise kiss-bonds in structures. Satisfying these requirements has been hindered by the difficulty in manufacturing – in a controlled, repeatable manner – test panels that contain representative kiss-bonds, for mechanical characterisation and Non-Destructive Inspection (NDI). In addition, there is currently no method to reliably detect kiss-bonds within repair bond-lines, directly hindering the design of bonded repairs. Kiss-bonds are therefore impeding the optimisation of composite structures and repairs. The method presented in this work addresses these issues by creating controllable and repeatable kiss-bonds for scientific interrogation. Areas of two adjacent plies are pre-cured before being incorporated within a laminate and autoclave cured. Following consolidation, no bonding occurs between the pre-cured areas. The robustness of the technique – in terms of repeatedly creating kiss-bonds – was evaluated via ultrasonics and laser shearography.