In-situ Inspection of Reflowable-Interface Composite Joints During Cure in an Autoclave

In structural bonds, the interface between adherend and adhesive is nearly two-dimensional making it susceptible to minute quantities of contamination, which can cause weak bonds. Regulatory organizations such as the Federal Aviation Administration (FAA) often require redundant load paths in secondary-bonded primary-structures to alleviate the inability to certify bonded performance. To address this issue, the NASA Convergent Aeronautics Solutions (CAS): Adhesive Free Bonding of Composites (AERoBOND) project is investigating reformulated aerospace epoxy-matrix resins to enable reflow and diffusion of the resin at the joint interface during a secondary bonding and cure process. The reflow and intermixing of the matrix resin during assembly can eliminate the material discontinuity at the interface, thereby removing the dependence of bond performance on adhesion at a nearly two-dimensional boundary. AERoBOND process development has evaluated a multitude of parameters including the materials used, the stoichiometric offset of the epoxy resins, time and temperature of cure, and thickness of each ply. Without in-situ process monitoring, the condition of a test article is unknown until mechanical testing is completed. This paper describes the use of an in-situ ultrasonic inspection system to monitor the joining of two composite parts assembled using the AERoBOND technique. This work quantifies the interface by measuring the amplitude of the wave reflection at the joint throughout the cure cycle. In addition, the results indicate when reflow and cure of the epoxy resin at the joint occurred. By using the recently developed in-situ inspection method with a mobile ultrasonic transducer, localized results can be obtained across a large portion of the joint with high resolution.