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Ultrasonics is the newest of the nondestructive testing technologies. Whereas industrial radiography evolved from medical radiography in the 1930s, magnetic particle detection was used to inspect gun barrels in 1917 and eddy current testing was used in the 1930s, albeit for ferrous sorting, it was not until World War II that industrial ultrasonic inspection became an accepted inspection tool.
Early industrial ultrasonics was very much restricted to the application of compression probes, which were used to detect reducing wall thickness and hence corrosion, to check laminations and to identify simple internal flaws. Data display also was very simple, being in the form of an A-scan on a CRT. The introduction of shear or angle probes greatly expanded the functionality of ultrasonic inspection, as defects could now be positioned rather that merely being located and imagery also evolved to include B-scans and C-scans. Twin crystal transducers also were developed, especially for applications involving coarse grain materials. By having a separate transmitter and receiver in the same element, it was found that it was possible to achieve a quasi-focusing effect where the beams cross, while at the same time enhancing the signal to noise ratio.
Advances in electronics from the 1980s onwards have given rise to great strides in the technology. Computerization has meant that data is now processed, displayed, analyzed and stored in the same instrument and that this data can then be transmitted to remote locations for further analysis or archiving. Cathode ray tubes have been replaced by LED and LCD screens, which can be adjusted in brightness, contrast and even color to aid viewing. Multiplexing electronics has allowed automated ultrasonic testing (AUT) to be used in many fields, notably in the inspection of pipes and tubes.
AUT systems typically employ an array of individual ultrasonic probes positioned on the upstream and downstream sides of a weld, with each probe focused on specific areas of the weld volume. A mechanical drive system provides controlled motion of the complete probe array, which is multiplexed to fire in a given sequence, to provide a comprehensive volumetric ultrasonic picture of the weld or pipe wall. Probes too have changed dramatically and today they can be programmed with given settings so that the operator need only connect to an instrument and the probe will set variables such as frequency and probe drive, while dialogue probes provide instant identification to increase inspection reliability and assist in probe operation calculations.
Improvements in probes, display and probe manipulation technology has also been accompanied by developments in the techniques for detecting, sizing and classifying flaws. The 20dB drop method favored by early practitioners of the NDT “art” has now been effectively replaced by techniques such as time of flight diffraction and the introduction of sophisticated algorithms, which can be used to provide flaw classification information from ultrasonic data. One of the most recent technologies to advance the versatility and reliability of ultrasonic inspection is phased array.
What Is Ultrasonic Phased Array?Phased array technology is a relatively new technique in the industrial sector for inspecting ultrasonically, although it has been used extensively in medical diagnostics. Its origin can be traced to radar where it was developed to overcome the antenna inertia and flexibility problems associated with mechanical scanning systems, although much of the significant work has been carried out in the medical sector.
Phased array systems rely on the computer-controlled excitation of each element in a multi-element probe in terms of the element’s amplitude and the delay between the energizing of consecutive elements. In this way, the small wavefronts created can be time-delayed and synchronized for phase and amplitude such that a focused, steerable beam is produced. This allows a phased array probe to inspect with variable inspection angles and focusing depths almost simultaneously. As a result, a single phased array probe can perform those inspection tasks normally requiring large numbers of conventional probes or multiple scanning passes. This means that inspections are faster, inspection equipment is more flexible as set-up changeover can be achieved very quickly and there is no need to carry different sets of probes for different inspection tasks. In addition, the real time, sector scan imaging of phased array provides an integrated, cross-sectional, easy-to-understand visualization of any area or component under inspection. This imaging is already well accepted in the medical field, particularly in obstetrics.
Improved, comprehensive imaging is one of the major advantages of phased array equipment over conventional ultrasonic techniques. Apart from a standard A-scan, today’s phased array instruments can also produce a real-time sector scan, which is essentially a collection of colorized B-scans. This allows NDT technicians to make better-informed judgments on the condition of components and structures being tested and also allows them to explain the reasons for their assessments more clearly to non-NDT personnel.
Ultrasonic phased array technology is now used in every industrial sector from aerospace and petrochemicals to nuclear power generation and the automotive industry. It is embodied into fixed equipment, used in steel mills and the manufacturing sector and handheld, phased array flaw detectors are now available. Indeed, it is conservatively estimated that in two to five years, 60% of all flaw detectors will be phased array.
One reason for this is the increasing affordability of the technology. The extreme sophistication and complexity of the multi-element phased array probes meant that not very long ago they cost around $4,000 each. However, by adopting the manufacturing technology developed and proven for probes used in the medical field, manufacturers of industrial phased array probes have managed to cut transducer prices by around 75%. In addition, the operating electronics and controlling software are now much more affordable.
The Future Of Phased ArrayPhased array will continue to make inroads into all aspects of inspection. Its progress will be driven by a number of factors. Functionally, phased array systems can minimize inspection times, and make data interpretation easier, benefits that are vital as our inspections skills base continues to decline. They will also find application in the examination of complex geometries and in the inspection of materials that are difficult to inspect with conventional techniques.
Commercially, phased array systems will continue to enjoy price deflation, as volume manufacturing further reduces probe prices and more and more information can be built into sensors.
Technically, phased array systems will become increasingly sophisticated. For example, in the aerospace sector, 3-D graphical information in the sector’s preferred Catia protocol is already being downloaded into instruments so that the workscope of specific inspections is precisely controlled and recorded.
In addition, application and archiving software is currently available and is being developed which combines data from various inspection modalities to provide even more comprehensive interpretation and tracing of inspection results.
However, it is important to bear in mind that phased array inspections will always need to be carried out by trained personnel. Although the imagery is vastly improved compared with the old A-scan displays on CRTs, the raw data is still collected exactly as it always has been. The ability to understand this raw data and to understand the software associated with the analysis and reporting of the data is something that will always have to be taught. This means that it will be something that will have to be learned by NDT technicians, today and tomorrow.