Non Destructive Testing Overview
Nondestructive Testing plays a critical role in assuring that aircraft structural components and systems perform their function reliably. NDT Qualified personnel define and implement appropriate tests to locate and characterize material conditions and flaws that could adversely affect safety. NDT testing methods allow parts and materials to be inspected without damaging them or preventing their future usefulness. Appropriate testing methods can locate a defect as well as measure its size, shape, and orientation.
The NDT methods that are used to perform an inspection depend upon the type of material, the geometry of the material and the type of measurement or defect characteristics expected. NDT inspection methods that are used most often are visual (VT), penetrant (PT), magnetic particle (MT), electromagnetic (ET), and ultrasonic (UT).
Visual and Optical Testing (VT)
Visual inspection involves using an inspector's eyes to look for defects. The inspector may also use special tools such as magnifying glasses, mirrors, or borescopes to gain access and more closely inspect the subject area. Visual examiners follow procedures that range from simple to very complex.
Test objects are cleaned of all paint, grease or dirt and then coated with a fluorescent dye solution. The penetrant will be drawn into any surface crack through capillary action. Excess dye is then removed from the surface, and a developer is applied which acts as blotter, drawing trapped penetrant out of imperfections open to the surface. Ultraviolet light is then used to make the “bleedout” fluoresce brightly, allowing surface breaking imperfections to be readily seen. After inspection the part is thoroughly cleaned.
Magnetic Particle Testing (MT)
Magnetic Particle Testing is accomplished by inducing a magnetic field in a ferromagnetic material and then dusting the surface with iron particles (either dry or suspended in liquid). Surface and near-surface imperfections distort the magnetic field and concentrate iron particles near imperfections providing a visual indication of a flaw.
Electromagnetic Testing (ET) or Eddy Current Testing
Electrical currents are generated in a conductive material by an induced alternating magnetic field. The electrical currents are called eddy currents because they flow in circles at and just below the surface of the material. Interruptions in the flow of eddy currents, caused by imperfections, dimensional changes, or changes in the material's conductive and permeability properties, can be detected with the specialized measuring equipment.
Ultrasonic Testing uses transmission of high-frequency sound waves into a material to detect imperfections or to locate changes in material properties. The most commonly used ultrasonic testing technique is pulse echo, wherein sound is introduced into a test object and reflections (echoes) are returned to a receiver from internal imperfections or from the part's geometrical surfaces.