Comparing Nondestructive Testing Methods for Welding
One of the most common uses of nondestructive testing (NDT) is the examination of weld integrity. This is because welds are extraordinarily common in critical industrial applications across the world. The ability to inspect welds nondestructively is essential, as destroying one for testing necessitates replacing it with a new, untested weld. As such, there has been significant technological development in the broad field of nondestructive testing methods for welding.
Many different NDT methods can be used to examine welds. From the most basic visual and somatic tests, techniques increase in speed and effectiveness using a wide array of technologies. Fluid immersion, radioactive waves, electromagnetism, and acoustic techniques all provide means of examining the interior structures of welds. Some techniques, however, are more capable and efficient than others.
Nondestructive Testing Methods Used for Welding
The aim of each nondestructive testing method used for welding is the same: to detect any internal flaws that could cause weld failure. As the ramifications of weld failure range from minor to extreme, so does the importance of testing. Many weld applications can cause severe harm upon failure, requiring a high level of testing capabilities. At the same time, welds are ubiquitous throughout modern industrial society, demanding efficient testing regimens.
The most common types of nondestructive testing for welds, excluding simple sensory examinations, are liquid penetrant, radiography, magnetic particle, eddy current, and ultrasonic testing.
Liquid Penetrant Testing
While simple in principle, liquid penetrant testing can be difficult to carry out efficiently. This technique isolates a testing area, then attempts to pass fluid through it. If the fluid refuses to pass through the weld, then the weld is secure. However, if cracks or flaws run through the weld, then so will the fluid. This shows technicians not only that a flaw exists, but also where it can be found. While this can be an effective method, partitioning areas for testing, as well as transporting and containing fluids, can make it cumbersome. Further, it does nothing to detect sealed voids within an object, flaws that don’t touch the surface.
Radiographic testing uses X-rays to observe the internal structure of welds (and other objects). This technique can detect sealed internal voids, as well as the cracks and flaws observed by liquid penetrant testing. Without the need for fluid containment, this technique can operate more quickly than liquid penetrant as well. However, the radioactivity inherent in this method poses serious issues. Equipment must be carefully treated. Technicians must take significant safety precautions. Non-essential personnel must be kept away from the testing area while testing is underway. These safety concerns create logistical hurdles diminishing the efficiency of radiographic testing.
Magnetic Particle Testing
Magnetic particle testing examines the interior of ferromagnetic objects, such as carbon steel welds, by studying their effects on magnetic fields. An induced magnetic field travels through the object, and any flaws or irregularities are observed by how they impede the magnetic field. Unfortunately, paint is not ferromagnetic, requiring it to be removed before inspections and replaced after. Other surface irregularities may also throw off readings. These handicaps inhibit the efficiency of magnetic particle testing, rendering it cost-prohibitive for large-scale testing regimens.
Eddy Current Testing
Like magnetic particle testing, eddy current testing relies on electromagnetism to detect flaws in welds. Unlike magnetic particle testing, eddy current techniques typically require no surface preparation—they work effectively, despite coats of paint. This dramatically increases the comparative efficiency of eddy current testing. However, eddy current technology cannot sense flaws deep beneath the surface of larger welds, requiring another method to be used.
For speed and capability, ultrasonic testing is the preferred method of nondestructive testing for welds. High-pitched acoustic waves are introduced into a weld via a transducer. Any internal changes in structure—including cracks, voids, or warps—can alter the transmission of these sound waves through the weld. These alterations can be detected from the same side of the weld that the wave was introduced on, allowing the ultrasonic instrument to interpret their reflectance. Sound waves can also be detected on the far side of the weld and interpreted based on their transmittance. Probes with multiple transducers, using a technique called phased array, can quickly scan a broad swath of a weld’s interior. Powerful software can efficiently analyze images, displaying results for technicians to act on.
The Best NDT Method for Welding
While many methods of nondestructive testing can detect failure-predictive flaws in welds, the most efficient, effective method is phased array ultrasonic testing. Without the safety flaws, labor-intensive set-up, or cumbersome ancillary equipment of earlier methods, ultrasonic allows technicians to perform rapid inspections without sacrificing accuracy.