Development of thermographic inspection routine exploiting phase transition of water for moisture detection in aircraft structures

Penetrated water in the composite sandwich structures has caused problems in aircraft structures. Flight surfaces have been lost during the flights, because moisture corrodes the honeycomb and further reduces the strength of the adhesive. Water can also cause additional defects during the composite repairs, which have resulted because of the expansion of the moisture (in closed cavity), hence causing skin blow core phenomena during the curing cycle (heating) of the repair. Thermographic investigation is done to find a suitable procedure to find penetrated water from the composite aircraft structures by cooling the whole structure, or separated parts of the aircraft, under freezing conditions. Thermographic inspection based on the phase transition of water exploits the phase transition energy that is needed for the water defrosting (melting). Advantage of this method is that no additional excitation source is needed for the tests. Method based on phase transition can be especially exploited during the long period of arctic weather conditions in Finland and other cold areas. Aircraft can be either inspected right after a flight, or it can be left outside in freezing conditions overnight and inspected when it has been brought in to the maintenance hall to warm conditions.