Monitoring Ultrasonic Time of Flight of the Bondline Across a Large Area of Adhesively Bonded Joints During Autoclave Cure

The certification of adhesively bonded composite joints remains a significant challenge for aerospace applications, requiring reliable methods to assess bondline quality during manufacturing. This study validates in-situ ultrasonic monitoring for real-time assessment of adhesive bondline thickness during autoclave cure. Three secondary-bonded laminates were fabricated using 26-ply unidirectional IM7G/8552 carbon fiber half-panels secondary bonded with FM209-1U unsupported film adhesive under varying bondline conditions: two adhesive plies without edge dam (Panel 1), four adhesive plies without edge dam (Panel 2), and four adhesive plies with edge dam (Panel 3). A custom ultrasonic scanning system (operational up to 180°C) continuously monitored panels throughout autoclave cure, measuring time-of-flight (TOF) data at 1 mm × 1 mm resolution over 225 mm × 25 mm scan areas. A previously calibrated machine learning-based temperature correction factor compensated for temperature-dependent ultrasonic velocity changes through the composite adherends. Panel 1 exhibited approximately half the TOF of Panels 2 and 3, consistent with its reduced adhesive ply count. Panel 2 showed a characteristic dome-shaped thickness distribution resulting from resin bleed at panel edges, while Panel 3 maintained uniform thickness distribution due to edge dam containment of the adhesive. Optical microscopy confirmed strong agreement between ultrasonic TOF measurements and post-cure bondline thicknesses. The study establishes that in-situ ultrasonic monitoring, combined with appropriate temperature correction algorithms, provides reliable quantitative assessment of adhesive bondline geometry during cure processes, offering potential for real-time quality control and manufacturing process optimization.