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SWAK Project: Determining Aerospace Composite Bond Quality


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.

Programme of Work

The programme of work for the project was split between the project partners as follows:

  • GMI AERO SAS (GMI) - samples manufacture and inspection
  • TWI Limited (TWI) - NDT techniques, mechanical testing and inspection
  • Brunel University London (BUL) - NDT and inline inspection

The project partners completed a range of deliverables during the project, as follows:

  • 1 – Review of adherence models and NDT Methods
  • 2 – Use cases
  • 3 – Evaluation of NDT Methods
  • 4 – Mechanical Testing
  • 5 – TRL demonstration of NDT methods

These deliverables led to the production of documents demonstrating the range of testing and results as well as instructions on how to prepare, test and analyse samples and results to successfully determine if a kissing bond defect is present.



The project partners evaluated a range of NDT techniques for their ability to detect discontinuities within a kissing bond:

  • Computed Tomography: XCT scans were successfully carried out on samples, finding evidence of contamination on the contaminated samples as compared to the reference samples.
  • Thermography: Experiments showed that contaminants allow for incremental heat conduction through the material join region, which, under extreme stress, can result in premature failure of parts. Further experiments are to be performed to determine the uniformity of heat distribution alongside unknown variables such as contaminant and epoxy distributions.
  • High Frequency C Scans: Initial inspection results showed that it as possible to identify the adherend’s geometric signals, however it was not possible to reliably detect a defect of a known size, depth or location in single lap joints. This was most likely due to the nature of the material, since higher frequency ultrasonics with a finer resolution will not penetrate the material as effectively.
  • Laser Shock Testing: Laser shock tests were shown to provide favourable results for detecting kissing bond defects.
  • Guided Wave NDT: Guided waves were shown to be successful in identifying kissing bond defects.
  • Ultrasonic and Phased Array Inspections: Final ultrasonic inspections are to be completed to determine the effectiveness of these NDT techniques for locating kissing bond defects.

In summary, computed tomography and thermography were both shown to be successful for the detection of contaminated kissing bonds, with additional testing being performed using ultrasonic testing in the coming weeks.

Once the final tests are completed, repair samples will be inspected and NDT methods will be demonstrated.


The SWAK project has received funding from the EU Clean Sky initiative under Grant No 831882