Looking Below the Surface with Ultrasonic Phased Array
Time may be on the side of Mick Jagger, as the song goes, but if you are inspecting the integrity of nuclear power plant components during a scheduled outage, time is likely against you. Time is a luxury that most inspectors do not possess in their daily jobs. They are required to quickly and efficiently perform nondestructive testing (NDT) on various components within the nuclear power plant facility. In addition to the obvious financial impacts ($1M+ per day) of prolonging planned outages, time constraints also are put in place for the safety and health of the inspectors.
For instance, while working in highly radioactive or contaminated environments of the plant, inspectors have a limited window of time to volumetrically inspect components for flaws, such as cracks, defects or other anomalies that might lead to leakage or failure during operation.
In the interest of simplicity, flaws are limited to cracks for our purposes. Ultrasonic phased array volumetric line scanning is emerging as a powerful tool that allows for quick inspections of components that had been very time-consuming using earlier technologies.
The ApproachUltrasonic phased array volumetric line scanning relies on principles of constructive interference from several sources of ultrasonic energy. A phased array probe consists of several individual piezoelectric elements that can be energized at precise times to produce the desired sound field in the component. The individual elements are housed in a probe casing, which may be mounted on a wedge that is tailored for various component shapes. Using advanced electronics and control software, a unified ultrasonic beam at a chosen angle, direction and focus can be formed.
More importantly, in phased array ultrasonic testing (PA-UT) several of these beams can be formed at various angles and focal depths almost simultaneously by controlling the time delay sequence for the firing of each of the elements. For example, one can focus at a constant beam length of 50 millimeters (1.97 inches) over a range of 30 to 70 degrees at 1-degree increments azimuthally, thereby effectively inspecting a significant volumetric region of the component almost instantaneously. Examining an equivalent volume of material using conventional ultrasonic testing would take significantly longer; hours instead of minutes.
Phased array line scanning is configured so that the beam is directed in an orthogonal direction to the scan motion. For example, the scan path could be in the circumferential direction around the pipe, while the region of insonification is directed to an area of interest, such as the weld of a dissimilar metal joint. Scanning with an encoded scanner yields spatial information for the location of any detected indications. A typical line scan can be completed within a few seconds to a few minutes depending on the area being scanned. Ideally, two or more line scans would be acquired at slightly different axial positions in order to provide complementary data on the region of interest.
Line Scan DataThe raw ultrasonic data received from the transducer are stored digitally as an array of amplitude (intensity) vs. time values that can be plotted and viewed in what is known as an A-Scan view. The A-Scan represents information at one angle and at one circumferential location contained within the scan.
As a single A-Scan can be difficult to interpret and so advanced, software is used to compile all the data based on position, time and intensity. The compiled data is then displayed visually in a color-coded format based on intensity. The most common views for displaying compiled line scan data are the sectorial view, which displays depth and angle information at a specific circumferential location, and the end view, which displays depth and scan (circumferential) information at a single angle.
Detection of FlawsThe detection of flaws is the fundamental objective of an inspection of a component in a nuclear power plant.
What is detection? What attributes indicate a flaw? Detection can be simply defined as the presence of a signal that is above the background noise level. Within PA data there often will be a baseline level of intensity that is caused by the reflection of ultrasonic energy from the component material itself. Signals above this noise level must be taken into consideration as possible flaws. One must keep in mind there can be multiple signal regions that stand out, but are not necessarily flaw-related. Frequently, there are inner diameter (ID) geometrical reflections from the counter-bore in a pipe. These signals, however, are fairly uniform, strong and constant near the ID region within the scan. This is unlike typical flaw characteristics that tend to be localized and may exhibit some through-wall extent.
Volumetric merging is a common analysis procedure performed on PA line scan data that allows the inspector to rapidly review the entire section of data for signals and flaws. This merging of the data results in images similar to those previously described, but in a composite sense. The sectorial and end views display the images made from all data collected in the scan. Gates can be utilized to isolate certain regions of interest. Image resolution may be compromised or blurred with volumetric merging, but in general, the technique works well for rapid flaw detection.
Characterization of FlawsAfter identifying a signal as a potential flaw-a circumferentially oriented crack for this particular case-it is important to characterize the flaw. Of particular interest are the length and through wall depth measurements. Customarily, the length of an indication can be obtained from the merged line scan data by placing the measurement and reference cursors in the end view at the -6dB points signifying that at these locations the signal intensity is half of the maximum intensity. This measurement can only be obtained if encoded data were collected.
The through-wall extent measurement can more readily be obtained from the non-volumetric merged data. This type of data allows the analyst to scroll through the various angles and circumferential locations to find the best view of the base and tip of the crack. Generally, the crack tip signal will be much lower in amplitude than the base. Typically, the inspector is looking for a signal above background noise levels in the region above the crack base.
PA line scanning is a fast volumetric NDE method for inspecting regions of interest in various components. The technique provides a means for flaw detection and characterization and also is useful for identifying regions to acquire additional and more time- consuming raster data for enhanced flaw characterization.