Novel thermographic inspection method to detect the moisture in early stage of the water ingress and a procedure to remove the moisture from the composite structure

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. The number of penetrated water-induced failures is rising, therefore penetrated water in the composite structures operating in arctic conditions has been a research activity in Finland for several years. VTT and Patria have worked recently in close co-operation to develop a method to detect moisture and remove it from the structure efficiently. Procedure is divided to three phases: 1. X-ray inspection, which can indicate suspected water in one or multiple honeycombs. 2. Drying procedure (several hours) 3. Thermographic inspection to verify remaining water. . X-ray inspection can detect assumed water ingress, if it fills most of one or several the honeycomb cells. The drawback is that there is no certainty, if the indication is from water or excess adhesive/resin from manufacturing process. If doubt of water ingress is observed during X-ray inspection, structure will go through special drying procedure. A gentle procedure to remove the water is applied to the honeycomb composite structure, because it is essential not to cause skin blow core effect during the drying phase. Honeycomb composite structure is heated under a low vacuum to vaporize moisture from the structure. Heating takes place several hours. Pulsed thermography is reported to indicate water ingress in the composite structure. However, water can t be distinguished from other liquids easily or at all by this method. Thermography was decided to be exploited by cooling the inspected structure under freezing conditions before inspection. Thermographic inspection based on the phase transition of water exploits the phase transition energy that is needed for the water ingress to be defrosted (melted). Water ingress indication is observed in specific phase transition temperature and insures the presence of water. An other 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. Composite structures can be left outside in freezing conditions overnight and inspected when they have been brought in to warm conditions.