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Overview of Time of Flight Diffraction (TOFD)
Time of Flight Diffraction (TOFD) is a reliable method of nondestructive ultrasonic testing (UT) used to look for flaws in welds. TOFD uses the time of flight of an ultrasonic pulse to find the location of a reflector. It can also be used for weld overlays and the heat affected zones of other components as well such as piping, pressure vessels, clad material, storage tanks, and structural steel.
Like most UT methods, TOFD works by emitting sound waves into a component and measuring the time from them to return. What makes TOFD different from other UT methods is that, rather than measuring only for the high amplitude sound waves that reflects off of the back of the component, it instead measures the response time of low amplitude waves that are diffracted by the tips of cracks.
To do this, TOFD uses a pair of ultrasonic transducers, one as a transmitter and the other as a receiver. The low frequency waves propagate at an angle and only diffract back to the receiver if they hit a defect. If this happens, the time it takes for both waves to make it to the receiver can be used to create a complete image of the weld and identify the size and location of the damage.
TOFD is one of the fastest methods of nondestructive testing because, unlike other types of UT methods, generally only one scan is required to find any defect information within the weld. It can locate and measure the size of many different types of defects with incredible precision. It also has a high degree of repeatability. Because of this, the growth of any flaws can be tracked over time. Finally, it is able to detect damage which would traditionally only be able to be detected through other techniques, such as pulse echo or radiography.
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Internal Rotating Inspection System (IRIS) Long Range Ultrasonic Testing (LRUT) Phased Array Ultrasonic Testing (PAUT)
Articles about Time of Flight Diffraction (TOFD)
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May/June 2018 Inspectioneering JournalBy Samer E. Ibrahim at wood
HTHA of hydrogen-containing equipment can be prevented with appropriate material selection and fabrication, appropriate welding procedures, regular inspection of equipment using proven, effective technologies and equipment operated by qualified technicians, well monitored and controlled temperatures and hydrogen partial pressure.
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March/April 2016 Inspectioneering JournalBy Mike Brown at Sentinel Integrity Solutions
While there are many types of advanced NDT, this article will focus on the use of acoustics and electromagnetism as the bases for conducting examinations.
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November/December 2015 Inspectioneering JournalBy Mike Brown at Sentinel Integrity Solutions
It is often best to rely on properly trained and experienced inspection service providers to determine the proper method for any inspection project. Thus, inspection companies can suggest utilizing the most effective and efficient inspection techniques that will result in the highest probability of detection (POD), while potentially saving the facility operators’ time, effort, and capital.
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July/August 2005 Inspectioneering JournalBy Julian Speck at TWI Ltd., and Bryan Kenzie at TWI
The ultrasonic Time of Flight Diffraction (TOFD) technique was developed for the UK nuclear industry during the 1970s to provide a method for measuring the height of planar flaws. TOFD is now generally recognized as the most accurate ultrasonic technique for measuring the height of embedded planar flaws (eg. Cracks, lack of fusion, etc.) that lies perpendicular to the surface.
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January/February 2005 Inspectioneering JournalBy John Reynolds at Intertek
HTHA falls into multiple categories of corrosion mechanisms, including environmentally assisted cracking, hydrogen assisted cracking, and high temperature degradation. Sometimes HTHA is confused with low temperature hydrogen cracking mechanisms that result from hydrogen being driven into steels by aqueous corrosion reactions.
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July/August 1998 Inspectioneering Journal
The debate about advantages and drawbacks of the application of the TOFD (time of flight diffraction) approach for ultrasonic weld inspection should not forget the original reasons for its introduction in the 1960s. The major advantage at that time had been the better crack detection in comparison to x-ray techniques, especially in view of an increased use of steels and welding technologies with a the presence of diverse cracking phenomena (e.g. cold cracking, transverse cracks etc.).
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January/February 1998 Inspectioneering JournalBy Ed Ginzel
TODF (time-of-flight-diffraction) is proposed as an option to Pulse-echo methods by some practitioners. It suffers from shortcomings that can limit its effectiveness.
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