I'm going to stick the 5 main or basic methods:
(1) Visual
(2) Penetrant
(3) Magnetic Particle
(4) Radiographic
(5) Ultrasonic
Nondestructive Testing methods are used to verify the quality of a product by methods that will not damage the item or reduce it's usefulness. We usually think of NDT in terms of welded products, castings, forgings, basically steel or metal items. the truth is it can used on many other materials as well.
Each method has it's own limitations and advantages, these characteristics of the available methods are key in determining which method is best for a particular item.
The size, shape and location of expected flaws, also known as discontinuities, will also effect the choice of method. If a flaw is connected to the surface and you can access that surface, maybe Liquid Penetrant, or Magnetic Particle testing would be options. If it is a subsurface flaw we would probably want a "Volumetric" test method such as Radiography or Ultrasonic.
No single method is guaranteed to find all flaws. Depending on the flaws and the requirements of the specification regarding allowable discontinuities it may be necessary to use a combination of NDE methods. The choice will likely be based on POD, Probability Of Detection, most codes and standards written have to take this into account.
All NDE methods need to be performed in accordance with a written procedure that lays out all the parameters of the test based on the material to be inspected, including thickness, material specification (ie: SA516-70 vs 316 stainless steel).
The NDE method required is usually selected by the design engineer, possibly in consultation with a NDE Level III.
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