One of the biggest industries to use non destructive testing is the aerospace industry. Parts are tested before the plane is assembled, but also during maintenance, several NDT-techniques are used to ensure all parts are safe.The biggest problem with planes is the fatigue in all components. Since a plane functions at different pressures, the forces on the components differ all the time, causing fatigue cracks even when the forces enacted upon the plane remain way below the critical load.
There is no easy way to prevent those cracks from forming, but the cracks propagate on the outside of the plane, so the most important technique used to detect those cracks early enough, is just simple visual inspection. Of course not all areas can be check with the naked eye and cracks sometimes are so small they aren’t detectable with visual inspection, so when a plane is grounded for a thorough inspection, ultrasonic inspection and eddy currents are used to detect even the smallest cracks.
Ultrasonic inspection uses the propagation of ultrasonic waves through the material to detect possible defects, on which the waves reflect differently than on the other material. Eddy currents is a similar procedure, but uses electromagnetic induction to detect the flaws. This obviously can only be used on conductive materials.
A second problem with airplanes is corrosion. The air outside is often moist compared to the air inside the cabin. When the doors are opened, this air gets inside the airplane. When the plane takes off again and reaches the desired altitude, the air outside of the plane is so cold, it causes the outside of the plane to freeze , making the water in the air inside the cabin condensate on the inside of the aircraft skin. the water collects in the lower areas of the plane and can accelerate corrosion. To detect those problems, x-ray imaging is needed to detect the deep crevices corrosion can cause and also allows the maintenance crew to find small areas of water that have penetrated the aircraft skin.
These methods have made air travel safer over the past few decades, but using them becomes increasingly difficult, because the aerospace industry is evolving away from metals in favor of composites, which pose a whole new set of challenges in terms of detecting flaws.