Introduction
TWI worked alongside Brunel University London and GMI Aero SAS on the ‘Sealed Without A Kiss (SWAK): Non-Destructive Testing of Bonded Assemblies’ project to investigate different non-destructive testing (NDT) technologies and models for determining the quality of bonds in aerospace composites, with a focus on kiss bonding.
The aerospace industry has increasingly looked towards adhesive joints as an alternative to conventional techniques such as riveting, bolting or welding. The increased use of adhesively bonded parts for aircraft structures and components reduces weight and fuel consumption, cutting aircraft emissions as a result. These adhesive joints have fewer fatigue and stress concentration issues than the more conventional joining techniques, but they can also be susceptible to manufacturing defects and environmental degradation.
The ‘SWAK’ project worked to find a reliable, cost-efficient inspection solution for composites and adhesively bonded hybrid assemblies, specifically for kissing bonds and other hard to find structural defects.
Kissing Bond Testing
Kissing bond defects, also called zero-volume dis-bonds between adhesive and adherend, appear to show solid-to-solid contact but there will be no tensile strength or volume at the interface between the adhesive and the adherend. Because the bond appears to have been made, these defects are difficult to locate using NDT techniques. Of course, these defective bonds can be dangerous as they compromise the joint strength. Kissing bond defects can occur during joint manufacture because of poorly prepared adherents.
The associated safety concerns will only become of greater concern as new models of aircraft a built with 50% composite materials. Structural failure has been shown to be the second most common cause of aircraft incidents (16.5%), which raises genuine concerns as adhesive bonds are used on more aeroplanes.
The project team tested a range of NDT techniques to locate discontinuities in kissing bonds, including computed tomography, thermography, high frequency C scans, ultrasonic phased array inspections, laser shock testing, and guided wave NDT.
Each of these techniques was evaluated for its effectiveness in locating kissing bond defects in a range of assemblies. The project team also investigated inline process inspection and mechanical testing during the manufacturing process.
The SWAK Project not only worked to deliver safety improvements through zero defect NDT and mechanical testing techniques for bonded structures and geometries in aircraft structures, but also sought to reduce labour costs for composite products due to there being fewer, stronger assemblies with fewer parts.