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Detecting Kissing Bond Defects in Aerospace Composites

Thu, 27 January, 2022

TWI has been working alongside GMI Aero SAS and Brunel University London on a project to study non-destructive testing technologies and models for determining the quality of bonds in aerospace composites, with a focus on kiss bonding.

Adhesive joints are increasingly used to join structural elements in aircraft as they offer less stress concentration and fewer fatigue problems than conventional techniques such as bolting, riveting or welding. However, adhesive joints can be susceptible to environmental degradation and manufacturing defects, which creates a need to effectively inspect the joints.

This need is set to continue increasing as new models of aircraft are made from over 50% composite materials as air transport fleets continue to grow in number. The safety concerns are very real too as structural failure is the second most common cause of aircraft incidents, accounting for some 16.5% of incidents.

The ‘Sealed Without A Kiss (SWAK): Non-Destructive Testing of Bonded Assemblies’ project is developing a manufacturing control procedure to generate kissing bond defects in bonded joints made from composite materials and then detect them using non-destructive testing (NDT) techniques.

About Kissing Bonds

Also known as a zero-volume dis-bond between adhesive and adherend, a kissing bond defect will show intimate solid-to-solid contact but the dis-bond has no tensile strength or volume at the interface between the adhesive and adherend. These factors make these dis-bonds difficult to detect using non-destructive means.

Kissing bond defects can be very dangerous as they compromise the joint strength and can occur during joint manufacture due to a poor preparation of the adherents.

Testing for Kissing Bonds

The project team have tested a range of NDT techniques to look for discontinuities within 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 is being evaluated for effectiveness in detecting kissing bond defects in a range of assemblies.

The project aims to deliver the highly reliable and widespread implementation of adhesively bonded assemblies for the aerospace industry through NDT methods, inline process inspection and mechanical testing during the manufacturing process.

This increased use of adhesively bonded parts for aircraft structures and components, will not only reduce weight and fuel consumption but also cut emissions from aircraft as a result. The SWAK Project will also deliver safety improvements through its zero defect NDT and mechanical testing techniques for bonded structures and geometries in aircraft structures, while there will also be reduced labour costs for composite products due to there being fewer, yet stronger, assemblies with fewer parts.


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

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