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

Abstract 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 and 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 (to reduce resin bleed at panel edges) (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 a 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 a reliable quantitative assessment of adhesive bondline geometry during cure processes, offering potential for real-time quality control and manufacturing process optimization.