Quantitative Assessment of Adhesively-bonded Joints Using Ultrasonic Guided Waves

Maintenance is a costly but vital part of an aircraft lifespan. Regulations demand scheduled and rigorous inspection to ensure safety and airworthiness in the long term. Studies estimate that up to a third of the total operational cost of an aircraft is spent in maintenance checks. Consequently, design choices are made to take into account these requirements, which mean that conservative but easy to maintain designs are often preferred. Adhesive bonding is a typical example of a design choice that is overlooked due to the lack of efficient and reliable non-destructive maintenance and inspection. The mechanical advantages of adhesive bonding make it a promising substitute to mechanical joining methods but a consistent lack of confidence in the long term behaviour of these joints is limiting its broad application. Available NDI methodologies are limited both in reliability and versatility, and weak bonds can still go undetected. A guided waves based method has been developed and used to inspect a set of different adhesive interfaces in especially manufactured specimens. The method estimates an equivalent bondline length which can be directly correlated to the residual strength of the inspected bond. The tests showed good correlation even in cases where conventional ultrasonic methods (C-scan) were unable to detect any defects. The proposed method promises to further enhance the detection of weak bonds in bonded joints providing a rapid and accurate evaluation and direct correlation with mechanical strength of inspected bonds.