Thermography for Honeycomb Panel Inspection

Many of the secondary structures on commercial airplanes are fabricated from composite material with honeycomb core and use is increasing on new airplane models. Under adverse conditions, moisture can enter the honeycomb cells through microcracks on the surface of the part or around fasteners or fittings. Therefore, it is necessary to inspect the aircraft structures that are prone to water ingression as part of a typical maintenance program. If the moisture goes undetected, it can accumulate in the honeycomb cells. If enough water fills the honeycomb cells, the water expands upon freezing at altitude, which can lead to disbonds and, in extreme cases, the subsequent loss of the skin. Radiography and electronic thermography are currently approved NDI techniques in the Boeing NDT manuals. This paper discusses the benefits of using thermography for initial water detection and the subsequent repair inspection and describes a low cost chemical thermography method that employs liquid crystals. Both the electronic thermography and liquid crystal methods have been approved for use as alternatives to radiography