Establishing weld integrity without destroying the component
February 14, 2002
This article outlines the differences in radiographic and ultrasonic weld inspection, the two most common methods if nondestructive testing. It gives an overview of both methods, including how they are used.
Radiographic and ultrasonic weld inspection are the two most common nondestructive testing (NDT) methods used to detect discontinuities within the internal structure of welds. The obvious advantage of both of these testing methods is their ability to help establish the weld's internal integrity without destroying the welded component.
Radiographic testing (RT) usually is suitable for testing welded joints that can be accessed from both sides, with the exception of double-wall signal image techniques used on some pipe. Although this is a slow and expensive NDT method, it is a dependable way to detect porosity, inclusions, cracks, and voids in weld interiors.
RT makes use of X-rays or gamma rays. X-rays are produced by an X-ray tube, and gamma rays are produced by a radioactive isotope. The basic principle of radiographic weld inspection is the same as that of medical radiography. Penetrating radiation is passed through a solid object (in this case, a weld rather than part of the human body) onto photographic film, creating an image of the object's internal structure on the film.
The amount of energy absorbed by the object depends on its thickness and density. Energy not absorbed by the object causes exposure of the radiographic film. These areas will be dark when the film is developed. Areas of the film exposed to less energy remain lighter. Therefore, areas of the object where the thickness has been changed by discontinuities, such as porosity or cracks, will appear as dark outlines on the film. Inclusions of low density, such as slag, will appear as dark areas on the film, while inclusions of high density, such as tungsten, will appear as light areas.
All discontinuities are detected by viewing the weld shape and variations in the density of the processed film. This permanent film record of weld quality is relatively easy to interpret if personnel are properly trained. Only qualified personnel should conduct radiography and radiographic interpretation because false readings can be expensive and can interfere seriously with productivity, and because invisible X-ray and gamma radiation can be hazardous.
Ultrasonic testing (UT) can be used on ferrous and nonferrous materials and often is suited for testing thicker sections accessible from one side only. In general, it can detect finer linear or planar defects than can RT.
UT makes use of mechanical vibrations similar to sound waves but of higher frequency. A beam of ultrasonic energy is directed into the object to be tested. This beam travels through the object with insignificant energy loss, except when it is intercepted and reflected by a discontinuity.
The ultrasonic contact pulse reflection technique is used in UT. This system uses a transducer, which converts electrical energy into mechanical energy. The transducer is excited by a high-frequency voltage that causes a crystal to vibrate mechanically. The crystal probe becomes the source of ultrasonic mechanical vibration. These vibrations are transmitted into the test piece through a coupling fluid, usually a film of oil, called a couplant.
When the ultrasonic waves pulse strikes a discontinuity in the test piece, it is reflected back to its point of origin. Thus, the energy returns to the transducer. The transducer now serves as a receiver for the reflected energy.
The initial signal, or main bang; the returned echoes from the discontinuities; and the echo of the rear surface of the test piece all are displayed by a trace on the screen of a cathode-ray oscilloscope. The detection, location, and evaluation of discontinuities become possible because the velocity of sound through a material is nearly constant, making distance measurement possible, and the relative amplitude of a reflected pulse is more or less proportional to the size of the reflector.
One of the most useful characteristics of UT is its ability to determine the exact position of a discontinuity in a weld. This testing method requires a high level of operator training and competence and depends on establishing and applying suitable testing procedures.
Tony Anderson is a regular contributor to this section and technical services manager with AlcoTec Wire Corporation, 2750 Aero Park Drive, Traverse City, MI