Ultrasonic Testing Displays
In Ultrasonic Testing, the use of display makes the user to understand, locate and identify the flaws. There are a number of display types being used by the machine manufacturer based on the user need. This article details the main types of the UT displays.
The A-scan, as shown in Figure below, is the frequently used UT display type by the UT personnel. A-Scan displays the response along the path of the sound beam for a given position of the probe. It also displays the amplitude of the signal coming from the discontinuity as a function of time on the screen. The ‘x’ axis (right) represents the time of flight and indicates the depth of a discontinuity or back wall (thickness). The ‘y’ axis indicates the amplitude of reflected signals (echoes) and can be used to estimate the size of a discontinuity compared to a known reference reflector.
B- Scan Display
The B-scan display as shown in above figure, right hand side, displays a cross sectional view of the object under test by scanning the probe along one axis. The horizontal axis (left) relates to the position of the probe as it moves along the surface of the object and provides information as to the lateral location of the discontinuity. Echo amplitude is usually indicated by the color or gray scale intensity of echo indications.
The C-scan display, shown in below figure, shows a plan view of the test object. The image is produced by mechanically or electronically scanning in an x-y plane. The ‘x’ and ‘y’ axis form a coordinate system that indicates probe/discontinuity position. Color or gray scale intensity can be used to represent depth of discontinuity or echo amplitude.
The D-scan display (see Figure below) shows a through-thickness view showing a cross-section of the test object perpendicular to the scanning surface and perpendicular to the projection of the beam axis on the scanning surface. The D-scan display is exactly like a B-scan display except that the view is oriented perpendicular to B-scan view in the plane of the plate. The D-scan allows quick discrimination of indications along a weld by presenting their position in depth from the scanning surface. An example of the relationship between all four common ultrasonic displays is shown in below Figure.
Phased Array S-Scan Display
The S-Scan or sector display as shown in below figure. displays two dimensional imaging of ultrasonic reflectors by plotting information from a multitude of angles simultaneously. The image is a cross sectional view of the area where the ultrasound passes through. Location and size information can be measured for any reflectors that are in the Sectorial scan.
Phased Array ultrasonic accomplishes this by using a transducer that contains multiple elements, 8 to 128 commonly, that are excited at intervals to create constructive interference in the wave front of ultrasonic energy. This constructive interference is controlled by the amount of time delay in element excitation and can steer the sound through a range of angles. This array of beam angles is then plotted to create the sector scan. The ultrasonic energy provides responses in a pulse-echo fashion as with conventional straight beam and angle beam techniques.
Time of Flight Diffraction (TOFD) B-scan and D-scan displays
The B-scan and D-scan displays are a different format than the B & D scan displays acquired in any other ultrasonic technique utilizing information provided in a pulse echo fashion. TOFD B & D scan images provide defect sizing information for through wall extent by using diffracted signals rather than pulse echo signals. The TOFD B & D scan displays are created by stacking A-scan displays at a preset interval or collection step and viewing the data in a grayscale image where 100 % amplitude of the sine wave in either the positive or negative direction are plotted as all black or all white with gray images of signals less than 100 % amplitude.
TOFD passes sound energy through a weld area by utilizing a transmitting transducer on one side of the weld and a receiving transducer on the other (see Figure below). Any changes in the material, such as discontinuities, are vibrated by the induced ultrasonic energy. This vibration of discontinuities produces diffracted signals from the discontinuity that are then received by the receiving transducer.
The set of TOFD probes can be manipulated along a weld or across a weld to create scans. Standard TOFD weld inspection is accomplished by moving TOFD probes along the weld, with one transducer on each side of the weld, where the ultrasonic energy is perpendicular to the weld. This is a TOFD D-scan or non-parallel scan. The TOFD probes can also be manipulated across an area parallel to the sound path to evaluate an indication from a position 90 degrees from the perpendicular imaging. This is a TOFD B-scan or parallel scan. This can only be accomplished if the weld cap has been removed for the purpose of weld inspection and is most often used to provide more accurate defect location information once defects have been located with the TOFD D-scan.