Differences between Radiographic and Ultrasonic Testing of Welds

12 October 2018

Radiographic and ultrasonic weld testing, industry-standard inspection solutions for sure, are both held in high regard. They’re industry standards, seeing deeply inside opaque fuse zones with specially designed sensory mechanisms. Two different technologies dominate their operational architectures, but they both capture similar datasets, which are then rendered on a screen of some sort. Focusing on radiographic weld testing first, this technology is based on the propagation of radioactive frequencies.

Weld Inspection Technology: Radiographic Testing

The principles in use here are simple enough to determine. The equipment sends out particles from a section of the electromagnetic spectrum that can non-destructively penetrate metal. Picture an X-Ray tube or radioactive isotope inside an intricate housing. The particles leave the sensory section, penetrate the weld, and they strike concealed discontinuities. Some of the energy contained in the discharge is absorbed while the rest passes cleanly through the fuse area. As a result of the absorption, an image is exposed on radiographic film.

Weld Inspection Technology: Ultrasonic Testing

In this NDT procedure, the ultra-high frequency vibrational energy is directed to a selected weld joint. Upon reaching an internal weld discontinuity, the energy is reflected. The ultrasonic testing (UT) equipment uses pulse reflection contact technology, as supplied by a fast-vibrating crystal. Essentially, the equipment transducer converts an electronic signal into mechanical vibrations, those ultrasonic vibrations propagate through an oil-filled coupling, and the discharged energy sinks deep into the weld. Reflected back to the source, tiny signal changes are translated into an image, which is then displayed as a trace signal on a cathode-ray oscilloscope.

Display and Rendering Differences

Weld inspectors are trained to understand the images captured on radiographic film. They see the discontinuities and weld defects in the image. For ultrasonic equipment, the trained eye sees the peaks and signal furrows, the wrinkles and gutters, and those return echoes all have meaning. They translate to clean and accurate welds, to hidden discontinuities, and other flaws. Watching closely, the lines of data mean nothing to a casual observer, but the amplitude and distance between those different signal spikes can be read like a book by a welding inspector.

Clearly, both equipment rigs can only be operated by a trained professional. Radiation is dangerous stuff, and false positives are obviously costly. Wielding the equipment expertly, a radiographic tech gains access to both sides of the weld site. Incidentally, there’s also double-wall signal imaging, which does away with this requirement. The interpretation of the film image also requires an expert eye, of course. As for the ultrasonic weld tests, only a trained professional can make sense of the spikes and amplitude-tuned data floating across an oscilloscopes screen.


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