Smart Patch repair solution for cure monitoring of bonded repairs in composite aircraft structures

Abstract

Composite repair solutions are applied to recover the strength and functionality of composite structures in service without having to replace the whole structural part which is impractical for large sections. However, such bonded repairs are still not implemented in primary aircraft structures given the inability to provide repeatable results, ensure uniform bondline and the absence of nondestructive testing methods for detecting poor bond. In this paper, a dielectric-based technique is proposed to monitor the curing of composite bonded repairs based on Impedance Spectroscopy using smart patches embedded in the laminate by relying on drop-on-demand inkjet printed circuits which provided indicators of the polymer's curing stage whilst introducing minimal thickness in the bondline. The inkjet printed circuit is bonded to the composite plates using a newly developed procedure for high-level applications. The experimental campaign followed a building block approach to iteratively upscale the information from single printed IDTs in polymeric films to large circuits bonded to repaired composite parts with multiple tracks and IDTs. Numerous parameters were explored with the respective influence on the cure monitoring procedure of the thermoset adhesive layer, like the IDT design, the presence of imperfections at the bondline and instrumentation noise. Finally, the embedded smart patch was successfully employed for the detection of multiple impact damages and respective severity on the repair patch.