A Complete Solution for Weld Inspections: Phased Arrays and Diffraction Sizing

Abstract

Welds have been inspected by radiography for years, but this technology has major drawbacks: low detection rates for critical planar defects (i.e. cracks and lack of fusion), subjective interpretation, no vertical sizing capability, significant safety hazards, licensing issues, plant closures, and is generally slow. For decades, the main alternative was manual ultrasonics, which is also slow, subjective and normally has little hardcopy record. New technology and techniques are now available for improved weld inspections, specifically phased array technology and diffraction techniques. Phased arrays are essentially the industrial version of medical ultrasound, but require a very different approach. In contrast to medical activities, industrial applications typically require a correctly angled beam, with large quantities of data, on variable component geometry, and the ability to save and image defects. All this capability is now packaged in a portable unit, which can be used for rapid and reliable weld inspections. Specifically, the OmniScan MX can now perform a single linear scan of the weld, while the instrumentation performs multiple scans at different angles simultaneously. In addition, phased arrays can perform unique scans, like S-scans (sectorial scans), record and display all data in “top, side, end” views or similar, and perform multi-mode scans. Phased arrays permit electronic rastering in many different modes, which saves considerable inspection time; for example, some estimates show that phased arrays are five or more times quicker than manual scanning. Besides being portable and requiring just a single operator, portable phased arrays are now economically competitive with other weld inspection techniques — and generally provide a much better inspection. The “new” techniques consist of forward and backward diffraction for sizing defects. In reality, both these techniques have been around for years, but new technology has made them more practical. Forward diffraction (TOFD or Time-Of-Flight Diffraction) is now well established, and has been demonstrated for many applications. For most weld inspections, TOFD just requires a single linear pass, with two transducers (or arrays) on either side of the weld. TOFD provides good sizing and defect detection in the midwall, though it has dead zones at the two surfaces. Generally, sizing with TOFD is significantly better than with amplitude techniques, but is limited to defects ∼3 mm and up. TOFD also has the great advantage that it is highly independent of defect orientation, unlike pulse echo techniques. Even better, phased arrays can perform both pulse echo and TOFD simultaneously during a single linear scan, so giving essentially a “complete” weld inspection. Another diffraction sizing technique which has received relatively little attention until recently is “tip back diffraction”. This approach uses the low amplitude signals reflected back from crack tips to size defects. Back diffraction has the great advantages that it is intuitive, and can size defects as small as ∼1 mm, which is generally better than TOFD. However, tip back diffraction has the major disadvantage that it is sometimes difficult to correctly distinguish the crack tips from other signals, and signal-to-noise ratio is usually poor. These two limitations are largely overcome by phased arrays; first, the imaging allows correct determination of the crack tip, and second, new piezo-composite arrays with focusing and filtering give significantly improved signal amplitudes. S-scan imaging can be performed with off-the-shelf phased array equipment. Some of the more advanced phased array techniques include: 2D and 1.5D arrays for improved beam shaping and focusing; curved arrays, also for better sizing; special TRL-PA (Transmit-Receive Longitudinal Wave-Phased Array) probes for austenitic steels; additional beams for special inspections. Examples of these inspections on welds will be given. Overall, a combination of phased arrays with TOFD or back diffraction allows the operator to perform cost-effective inspections with high reliability, repeatability, good defect sizing and full data storage.