Moisture ingress detected in composites with non-destructive testing method

Moisture ingress in aircraft honeycomb sandwich structures is an ongoing issue that has attracted significant attention from aircraft operators and maintenance facilities. The phenomenon  can lead to skin-to-core bonding degradation, thereby affecting structural integrity. The aerospace sector has thus been looking for an easy-to-use and generally applicable method of detecting such ingress.

Researchers at Singaporean research institute A*STAR have proposed an answer: vacuum-assisted active thermography. This method applies infrared (IR) radiation to detect water ingress and incipient defect levels and entails applying vacuum-based cooling to reduce the boiling temperature of water down to room temperature by locally reducing the pressure in the region of interest. In tests applied to composite samples incorporating metal screws, the researchers were easily able to distinguish a dry screw from a wet one. When the system is applied to a dry screw, the infrared signal drops sharply but recovers within a few seconds to the original temperature. If there is moisture along the screw, however, water starts to evaporate. Not only does this result in a larger drop in IR signal: the signal remains low for several tens of seconds. Hence, the signature of a problematic screw can easily be identified..

The penetration rout of moisture into the honeycomb is debatable but it is a common observation that water tends to agglomerate at discontinuities in the structure. A*STAR's research will reportedly enable a better understanding of the penetration mechanism of fluid and moisture into composite materials, and their effect on structural integrity and surface adhesion property. "This is significant as today's aircraft structures are commonly made up of composite materials to reduce the weight of the aircraft in order to be more fuel efficient," says A*STAR. "Understanding the characteristics of composites when exposed to extensive water penetration will enable users to take preventive and corrective measures during maintenance, repair and overhaul (MRO) activities," it adds.