AIRBUS INDUSTRIE has developed a new infrared thermographic-inspection technique for detecting water ingress in composite-sandwich structures. The manufacturer says, that the procedure is proving so successful that it has been, recommended for use by all operators of Airbus aircraft.
Infrared thermography is based on the principle that an object emits electromagnetic radiation in proportion to its temperature. An infrared camera, which converts incident thermal radiation into an electronic signal, which can be displayed on a video screen, allows examination of temperatures across the surface of an object.
In water-ingress inspection, the camera highlights the difference between the specific heat of water and composite materials, and, since the specific heat of water is about five times greater than that of composite materials, any changes in temperature take place much more slowly where there is water contamination.
The Airbus technique involves heating an airframe to a predetermined temperature and inspecting it as it cools. To avoid damage, the test structure is heated to 60°C in not less than 15min, the last 5min held at 60°C. Heating is carried out using an electric blanket linked to a temperature-control unit.
The scanning camera used by Airbus is an AGEMA Infrared Systems Thermovision 210, which provides a thermal resolution of 0.1°C at 30°C. With this resolution, it is possible to detect an area measuring 120mm2, contaminated with 10% of water, and located in the opposite skin to the inspection.
While one engineer operates the camera, another engineer views the output on a hand-held video monitor. As areas of water contamination show up on the monitor, they are marked on the test surface.
Airbus says that it has proved reliable. The first official inspection was carried out, in July 1993 and the company recommends it, as a standard service procedure to all Airbus operators.
Source: Flight International
