Active Thermography Inspection of Water Ingress into the Sandwich Structure of an Helicopter Blade

Water ingress into the sandwich structures of aircraft components is a relatively common issue of even modern aircraft. Considering the variety of conditions of operation of SAR helicopter systems, a combination of moisture and water ingress into the critical structures is very likely – with a rotor blade partially manufactured from the sandwich structure being the most exposed. The water ingress can lead to the degradation of mechanical properties, but also the change of mass, and, especially for rotors, balance problems. This paper presents the results of active thermography inspections of controlled water ingress into the cells of the aluminum honeycomb/aluminum face-sheets sandwich structure with external thermal stimulation via halogen lamps. The ability of the active thermography inspection to detect water ingress of selected volumes is assessed. Additional factors impacting the sensitivity of the active thermography inspection, such as the position of the rotor blade, and duration of the heating pulse, are explored as well. Furthermore, the thermal pulse generated by rapid cooling as opposed to the heating of the tested specimen was tested. The resulting thermal contrast was evaluated by a principal component analysis. Active thermography inspection proved to yield excellent results for moisture/water ingress, assuming some defined criteria are met.