Phased Array Ultrasonic Testing

Phased Array Ultrasonic Testing

Restrictions on the 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 to assess the flaws accurately. Along with sizing of flaw, characterization of flaw is also very important. Hence, the combination of TOFD and phased array techniques have become a need.

When PAUT and TOFD techniques are judiciously performed together, the quality of testing increases when compared with radiography, i.e. the integrity of weld joints inspected with TOFD or PAUT is higher than those inspected using radiography. At times the combined inspection cost for PAUT + TOFD may seem 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 beside time saved in production/fabrication. The speeds of PAUT + TOFD inspection in one scan shortens the overall inspection time and eliminates the time-loss associated with RT.

Read More About the Principle of PAUT & TOFD

Time-of-flight diffraction (ToFD)
and Phased Array

Paut Inspection

Time-of-flight diffraction (ToFD) technique is an ultrasonic NDT technique that relies on the diffraction of ultrasonic energies from 'corners' and 'ends' of internal structures (primarily defects) of the component being tested. TOFD employs two longitudinal waves (L-wave) angle beam transducers arranged symmetrically opposite each other and facing each other, straddling the weld or base material under test. One probe acts as a transmitter of ultrasonic energy while the other probe receives the ultrasonic energy. The transducer, pulsar, and amplifier characteristics are selected to generate a broad distribution of energy as possible over the material under test providing full weld coverage.

The NDT experts at TCR using TOFD, provide amplitude-independent accurate flaw sizing on a wide coverage area. ToFD being an advanced and automated weld examination technique is also useful in Fitness For Purpose (FFP) inspections as well.

ToFD is a fast and effective method of scanning a wide weld area in a very limited period of time. While TOFD is a very powerful and efficient technique, it suffers from limited coverage resulting from two dead inspection zones:

  • The first dead zone: Near the surface, as a result of the lateral wave
  • The second dead zone: At the back-wall, resulting from the width of the back-wall reflection.

By combining the use of TOFD and conventional pulse-echo methods, dead zones in proximity to the front and back surfaces can be improved substantially. TCR Advanced offers services for creating ToFD scan plans and procedures in India. This technique has many advantages including:

Time-of-flight diffraction (ToFD) and Phased Array
  • Wide coverage area using a pair of transducers
  • Accurate flaw sizing; amplitude-independent, Sizing technique using time-of-flight information
  • One-line volume inspection, provides highly efficient scanning
  • Setup is independent of weld configuration
  • Highly sensitive to all kinds of defects with no sensitivity to defect orientation
  • Amplitude-insensitive, acoustical coupling less critical

ToFD is a quick and accurate
tool for flaw sizing

Phased Array technology (using a TCG or DAC) and flaw location indicators with experienced analysts is highly recommended. The TCR team has done a number of noteworthy ToFD projects:

  • 100% weld inspection of Storage Tanks in Kuwait as per API 650 appendix U
  • Random inspection of pipelines in Rabigh, Saudi Arabia as per ASME Code Case 181
  • Pressure Vessel inspection in India as per ASME Code Case 2235-9

ASME Boiler and Pressure Vessel Standard Section VIII Code Case 2235-9 states that it is acceptable to use ToFD for Ultrasonic examination in accordance with ASME Section V. Article 4. ASME Code Case 2235-9 mentions replacing RT with UT has resulted in incorporating ToFD into pressure vessel work for both detection and sizing of flaws. This allows ToFD to be used on all Section VIII pressure vessels. TOFD is perfectly acceptable to use as per Code Case 181 and Code Case 179 of ASME B 31.3 for piping products.

API 579 in its current draft form states that the recommended practice for Fitness-for-Service (crack depth, length, angle, and distance with other surfaces) where breaking or embedded cracks are determined using UT examination techniques, either ToFD or angle beam.

Draft-API 580 states that the Risk Based Inspection Recommended Practice (Base Resource Document recommends automated ultrasonic shear wave testing as a highly effective inspection technique for crack detection and sizing). The capability of the Automated UT technique/type is evaluated using the probability of detection (POD curves from round-robins in the past where ToFD showed the best performance).

British Standards Institute's welding standards policy committee has created BS 7706 as a guide for calibration and setting-up of the Ultrasonic

Time of Flight Diffraction

(ToFD) technique for defect detection, location, and sizing of flaws. Another well-documented guide is the PrEN 583-6.