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Ultrasonic time-of-flight-diffraction method

Ultrasonic time-of-flight-diffraction method(TOFD method) is a reflection method using separate emitting and receiving transducers and based on the reception and analysis of the propagation time of waves diffracted on discontinuity.

Time-of-flight-diffraction method(TOFD) (Fig. 2.3, d) is based on the reception of waves diffused at the ends of the defect, and both longitudinal and transverse waves can be emitted and received. The figure shows the case when transverse waves are emitted, and longitudinal waves are received.  However, the option when the longitudinal waves are emitted and received is practical used, since they are the first to come to the receiver and using this feature they are easy to be distinguished from transverse waves. The main information characteristic is the time of the signal arrival.

The TOFD technique was developed in the 1970s to accurately measure the size of defects recorded by other methods. However, thanks to the development of technology, the TOFD technique is now the most accurate and reliable tool for detecting defects, which can be used to quickly collect data.

Technology

When using the TOFD method, two transducers in the pulse generator – receiver mode are used to transmit ultrasonic waves to the inspected object. The transducers generate both longitudinal and transverse waves, which allow you to control the whole object.

The figure shows an example of ultrasonic wave propagation. Two different longitudinal waves are introduced into the material.

The first wave moves between the transducers directly under the material surface and is called the lateral wave. The second wave falls at an angle on the back wall and is reflected to the receiver. These waves form transverse waves that propagate throughout the material. In case of a defect, the diffraction wave is reflected from its edge or ends and captured by the receiving transducer.

Dataanalysis

Ultrasonic signals are recorded in rectified mode, which allows you to present both positive and negative cycles of single-wave detection. The screen displays a set of horizontal lines; each line corresponds to a certain wave cycle.

Due to the difference in speed and distance, the lateral wave is displayed above the others as the first section of horizontal lines. The next section of lines corresponds to the longitudinal wave reflected from the back wall. Finally, again, due to the difference in velocity, the transverse waves are represented by the last section in the chart, which is located at the bottom of the screen.

An example of the TOFD data chart is shown in figure 2.

Defects’ detection

TOFD sensitivity High compared to other methods greatly facilitates the detection of defects and determination of their dimensions (length, depth). Figure 3 presents data obtained during ultrasonic testing of a real v-shaped single weld with a wall thickness of 12.7mm. As can be seen from figure 3, the TOFD image clearly shows the defects. Figure 4 also shows a real TOFD image of a single v-shaped weld with a wall thickness of 6.35mm, where an external longitudinal crack at the weld boundary, a crack at the root of the weld, and slag inclusionscan be distinguished.

The frequencies and angles of the ultrasonic beams directly depend on the thickness of the material being inspected. In the oil refining and petrochemical industry UT is carried out for narrow objects with thickness mainly from 6.35 to 25.4mm (0.25 — 1″), although it is possible to inspect materials with a thickness of 101.6mm (4 inches).

The TOFD technique is used mainly for inspection of welded seams of pipelines and tanks. It is a useful tool in detecting defects when fabricating equipment, as well as defects resulting from operation. With the help of the TOFD technique, the following defects can be detected and determined: weakening of the welded joint during processing, cracks in the weld seams of coke drums, surface corrosion. The TOFD inspection method includes single parallel passes, which accelerates data collection. With the TOFD method, it is possible to scan about 150 linear meters of welds in one working day.

AdvantagesofTOFDmethod

There is no need to interrupt the process to perform the inspection.

  • High speed of inspection.
  • Accurate determination of defect depth and length.
  • High sensitivity: detect defects at depths greater than 0.10 inch (2.54mm).
  • Ability to inspect products made of ferrous and non-ferrous metals.
  • Detailed survey reports in digital form.
  • Availability of results for follow-up surveys in case of failure or for monitoring.

OmniScan MX2 (Olympus)

The OmniScan MX2 series of ultrasonic flaw detectors with touch screen offers new powerful PC-based software with an expanded set of functions in manual mode and automatic ultrasonic inspection mode. The ability of quick configuration and creation of reports increases the efficiency of the workflow.

OmniScan MX, which is the result of more than 10 years of leadership in the production of non-destructive testing platforms, is the most successful portable modular PA system developed by Olympus today. Thousands of such systems are used around the world.

Olympus brings to your attention a new PA module with TOFD support, a new UT module, as well as new software applications (NDT SetupBuilder and an updated version of OmniPC) that extend the capabilities of OmniScan MX2.