Phased Array Ultrasonic Testing
Restrictions in use of industrial radiography has increased in recent times due to health hazards, hence the need for alternative weld inspection methods and techniques such as Time of Flight Diffraction (TOFD) and
Phased Array Ultrasonic Testing (PAUT) have become indispensable. NDE has become more relevant in recent times due to its ability of sizing the flaws accurately. Along with sizing of flaw, characterization of flaw is also very important hence the need for combination of TOFD and phased array techniques have been evolved.
When PA and TOFD techniques are performed together judiciously, the quality of testing increases when compared with radiography, i.e. the integrity of weld joints inspected with TOFD or PA is higher than those inspected using radiography. At times the inspection cost for PA + TOFD combine seem to be higher than conventional radiography testing but since these advanced ultrasonic methods do not involve any radiation hazards and other jobs can be carried out in the vicinity of such testing, leading to a direct saving associated besides time saved in production/fabrication. Also the speed of PA + TOFD inspection in one scan shortens overall inspection time and eliminates the time-loss associated with RT.
Theory of Time of Flight Diffraction (TOFD)
TOFD employs two longitudinal wave (L-wave) angle beam transducers arranged symmetrically opposite to each other and facing each other, straddling the weld or base material under test.
One probe acts like a transmitter of ultrasonic energy while the other probe receives the ultrasonic energy. The transducer, pulsar, and amplifier characteristics are selected to generate as broad distribution of energy as possible over the material under test providing full weld coverage.
Advantages of diffraction phenomenon:
The procedure for testing is based on API 941 using different approaches like:
- Attenuation measurement
- Velocity measurement
- Spectral analysis
- Analyzing back scattered signals
- Testing weld joints and HAZ using high frequency shear wave ultrasound
- Advanced ultrasonic testing like Phased array and TOFD
Phased Array Ultrasonic Technology Theory(PAUT Inspection)
Ultrasonic waves are mechanical vibrations induced in an elastic medium (the test piece) by the piezocrystal probe excited by an electrical voltage. Typical frequencies of ultrasonic waves are in the range of 0.1 MHz to 50 MHz. Most of the industrial applications require frequencies between 0.5 MHz to 15 MHz.
Most conventional ultrasonic inspections use monocrystal probes with divergent beams. The ultrasonic field propagates along an acoustic axis with a single refracted angle. The divergence of this beam is the only "additional" angle, which might contribute to detection and sizing of miss-oriented small cracks.
Assume the monoblock is cut in many identical elements, each with a width much smaller than its length. Each small crystal may be considered a line source of cylindrical waves. The wavefronts of the new acoustic block will interfere, generating an overall wavefront.
The small wavefronts can be time-delayed and synchronized for phase and amplitude, in such a way as to create an ultrasonic focused beam with steering capability.
The main feature of phased array ultrasonic technology and phased array ultrasonic testing is the computer controlled excitation (amplitude and delay) of individual elements in a multielement probe. The excitation of piezo-composite elements can generate an ultrasonic focused beam with the possibility of modifying the beam parameters such as angle, focal distance and focal spot size through software. The sweeping beam is focused and can detect in specular mode the miss-oriented cracks. These cracks may be located randomly away from the beam axis. A single crystal probe, with limited movement and beam angle, has a high probability of missing miss-oriented cracks, or cracks located away from the beam axis (see Figure 1).
To generate a beam in phase and with a constructive interference, the various active probe elements are pulsed at slightly different times. As shown in Figure 2, the echo from the desired focal point hits the various transducer elements with a computable time shift. The echo signals received at each transducer element are time-shifted before being summed together. The resulting sum is an A-scan that emphasizes the response from the desired focal point and attenuates various other echoes from other points in the material.
- During transmission, the acquisition instrument sends a trigger signal to the phased array instrument. The latter converts the signal into a high voltage pulse with a pre-programmed width and time delay defined in the focal laws. Each element receives one pulse only. This creates a beam with a specific angle and focused at a specific depth. The beam hits the defect and bounces back.
- The signals are received, then time-shifted according to the receiving focal law. They are then reunited together to form a single ultrasonic pulse that is sent to the acquisition instrument.
- The delay value on each element depends on the aperture of the phased array probe active element, type of wave, refracted angle and focal depth. There are three major computer-controlled beam scanning patterns (see also chapter 3 and 4):
- Electronic Scanning: the same focal law and delay is multiplexed across a group of active elements (See Figure 4); scanning is performed at a constant angle and along the phased array probe length (aperture). This is equivalent to a conventional ultrasonic transducer performing a raster scan for corrosion mapping or shear wave inspection. If an angled wedge is used, the focal laws compensate for different time delays inside the wedge.
- Dynamic Depth Focusing or DDF (along the beam axis): scanning is performed with different focal depths. In practice, a single transmitted focused pulse is used, and refocusing is performed on reception for all programmed depths (see Figure 5).
- Sectorial Scanning (also called azimuthal or angular scanning): the beam is moved through a sweep range for a specific focal depth, using the same elements; other sweep ranges with different focal depths may be added. The angular sectors may have different values.
Advantages with TCR Advanced and Olympus MX2 machine
TCR Advanced team of NDT consists of ASNT level III (UT, MT, PT, ET, LT and VT) and NDT level II personnel qualified as per guide lines of SNT TC-1A.
The OmniScan MX2 family of ultrasonic flaw detectors with touch screen interface offers increased testing efficiencies and powerful new on-board and PC-based software features, ensuring superior manual and advanced AUT (Auto ultrasonic testing) application performance with fast setups, test cycles, and reporting.
This second generation OmniScan MX2 increases testing efficiency, ensuring superior, advanced AUT application performance with faster setups, test cycles, and reporting, in addition to universal compatibility with more than 10 phased array and ultrasound modules. Designed for NDT (non-destructive testing) experts, this high-end, scalable platform delivers true next-generation NDT performance.
The OmniScan MX2 offers a high acquisition rate and new powerful software features for efficient manual and automated inspection performance-all in a portable, modular instrument.
Courtesy: Olympus Corporation