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..