In the world of welding, there is often a need to inspect the internal integrity of a weld. Obviously, it would be a somewhat counter-productive to cut into the metal in order to verify the weld is solid all the way through.
This is where Nondestructive Testing (NDT) methods come in to play, of which, Radiographic and Ultrasonic are the two most common.
Radiographic Welding Inspection
The Radiographic inspection process works well for joints that are accessible from both sides. Overall, this method is relatively slow, expensive, and limited in application. The process is roughly the same as if you were to go to the hospital and get a chest x-ray. Radioactive isotope gamma rays are produced by an x-ray tube and focused on the test metal. The radiation penetrates the weld and an image is imprinted on photographic x-ray film. Porosity, cracks and inclusion will appear as dark areas outlines on the x-ray film.
The most positive aspect of the radiographic inspection method is the ease with which a technician can read and interpret the x-ray images, as the inclusions and various imperfections are generally very obvious on the x-ray film. The radiographic process, however, is quite dangerous, as it involved gamma radiation, and should only be conducted by a qualified technician.
Ultrasonic Welding Inspection
The ultrasonic inspection method can be used to inspect thicker metal that may only be accessible from one side. The ultrasonic unit projects a very high frequency sound beam into the test metal. The sound beam is created by a transducer excited with high-voltage electricity that causes a crystal to vibrate. A conductive gel, known as a couplant, must be applied to the test metal in order for the ultrasonic beam to penetrate the surface.
The sound waves travels unencumbered through the test metal, until they hit an inclusion or imperfection and bounce back to the ultrasound machine, and are displayed on a cathode-ray oscilloscope (in the ultrasound machine). The amplitude of the returning wave and the length of the delay determine the location of the inclusion.
An ultrasonic inspection enables a technician to determine the exact location of specific inclusion of discontinuity. Properly interpreting the returning waves, and reading the ultrasonic scope to decipher the location of a particular inclusion is a complex process, requiring the services of a highly competent ultrasonic technician, with specialized training and concentrated knowledge of the ultrasonic inspection procedure.