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Time-of-flight diffraction ultrasonics

Time-of-flight diffraction ultrasonics is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Time-of-flight diffraction ultrasonics rather than just read about it. In short: Time-of-flight diffraction (TOFD) method of ultrasonic testing is a sensitive and accurate method for the nondestructive testing of welds for defects. TOFD originated from tip diffraction techniques which were first published by Silk and Liddington in 1975 which paved the way for TOFD.

Time-of-flight diffraction ultrasonics — main illustration
Time-of-flight diffraction ultrasonics — illustration

Key takeaways

  • Time-of-flight diffraction ultrasonics belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Time-of-flight diffraction ultrasonics to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Time-of-flight diffraction ultrasonics from memory before moving on to harder problems.

Reference excerpt

Time-of-flight diffraction (TOFD) method of ultrasonic testing is a sensitive and accurate method for the nondestructive testing of welds for defects. TOFD originated from tip diffraction techniques which were first published by Silk and Liddington in 1975 which paved the way for TOFD. Later works on this technique are given in a number of sources which include Harumi et al. (1989), Avioli et al. (1991), and Bray and Stanley (1997). Bray and Stanley (1997) summarized TOFD as tip-diffraction techniques which utilized the principle that the tips of a crack when struck by a wave will diffract the signals back to the other location on the surface. The depth of these tips can be determined from the diffracted energy. TOFD was invented in the UK in the 1970s initially as a research tool. The use of TOFD enabled crack sizes to be measured more accurately, so that expensive components could be kept in operation as long as possible with minimal risk of failure.

Principle of operation Measuring the amplitude of reflected signal is a relatively unreliable method of sizing defects because the amplitude strongly depends on the orientation of the crack. Instead of amplitude, TOFD uses the time of flight of an ultrasonic pulse to determine the position and size of a reflector. In a TOFD system, a pair of ultrasonic probes sits on opposite sides of a weld. One of the probes, the transmitter, emits an ultrasonic pulse that is picked up by the probe on the other side, the receiver. In undamaged pipes, the signals picked up by the receiver probe are from two waves: one that travels along the surface and one that reflects off the far wall. When a crack is present, there is a diffraction of the ultrasonic wave from the tip(s) of the crack. Using the measured time of flight of the pulse, the depth of a crack tips can be calculated automatically by simple trigonometry.

Applications Phased Array + TOFD combination is commonly used to inspect pipeline welds.

Reliability One of the limitations of TOFD is the "dead zone' created by the lateral wave signal just below the inspection surface (or OD surface in case of the pipe). The dead zone is approximately 5 mm and there is no flaw detection in this zone. Calibration blocks with side drilled holes as shown in Reference and ISO 10863 used to validate the "dead zone" and sizing accuracy.

Features A computerised and automated system for weld inspection. Probes are mounted on a buggy that travels along a weld, recording data as it moves. Compared to conventional methods of ultrasonic testing, TOFD is sensitive to cracks and measures their dimensions accurately. TOFD has a dead zone where it is not sensitive to defects, hence TOFD ought always to be supplemented by a conventional pulse-echo examination or phased array. The pulse echo probes are commonly mounted on the same buggy as the TOFD probes. Requires ultrasound technicians with advanced training.

References

Further reading

Standards International Organization for Standardization (ISO) ISO/DIS 10863:11, Welding – Use of time-of-flight diffraction technique (TOFD) for examination of welds European Committee for Standardization (CEN) EN 583-6, Non-destructive testing – Ultrasonic examination – Part 6: Time-of-flight diffraction technique as a method for detection and sizing of discontinuities EN 15617, Non-destructive testing of welds – Time-of-flight diffraction technique (TOFD) – Acceptance levels

Other sources Engineering Applications of Ultrasonic Time-of-Flight Diffraction, 2nd ed., J. P. Charlesworth and J. A. G. Temple, Research Studies Press, 2002.

External links TOFD: – An Alternate Non-Destructive Testing Procedure to Replace Traditional Methods NDT.net Database Search returns over 500 hits for the search term 'TOFD' TOFD Principles on NDT.net

Illustrations

Time-of-flight diffraction ultrasonics: A TOFD setup with transmit and receive probes. In this case the receive probe sees four indications: one from the lateral wave that has travelled along the upper surface, one from the wave that has reflected off the far surface, and two from the defect in the test object.
A TOFD setup with transmit and receive probes. In this case the receive probe sees four indications: one from the lateral wave that has travelled along the upper surface, one from the wave that has reflected off the far surface, and two from the defect in the test object.
Time-of-flight diffraction ultrasonics: Typical TOFD data, created by aligning the data traces from the above figure vertically and colour-coding them for amplitude. The defect or discontinuity creates a characteristic parabolic indication, due to the apparent change in depth as the probes travel.
Typical TOFD data, created by aligning the data traces from the above figure vertically and colour-coding them for amplitude. The defect or discontinuity creates a characteristic parabolic indication, due to the apparent change in depth as the probes travel.
Time-of-flight diffraction ultrasonics: Manually-guided TOFD probes
Manually-guided TOFD probes

Worked examples

Example 1 — a first encounter with Time-of-flight diffraction ultrasonics

Start with the simplest possible case. Write down what Time-of-flight diffraction ultrasonics claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Time-of-flight diffraction ultrasonics before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Time-of-flight diffraction ultrasonics ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Time-of-flight diffraction ultrasonics

In research
Time-of-flight diffraction ultrasonics appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Time-of-flight diffraction ultrasonics in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Time-of-flight diffraction ultrasonics is common in secondary-school and first-year university syllabi. It links to neighbouring topics Nondestructive testing, so understanding it makes those chapters shorter.
In everyday life
Look for Time-of-flight diffraction ultrasonics outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Time-of-flight diffraction ultrasonics in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Time-of-flight diffraction ultrasonics means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Time-of-flight diffraction ultrasonics out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Time-of-flight diffraction ultrasonics in simple terms?

Time-of-flight diffraction (TOFD) method of ultrasonic testing is a sensitive and accurate method for the nondestructive testing of welds for defects. TOFD originated from tip diffraction techniques which were first published by Silk and Liddington in 1975 which paved the way for TOFD.

Why does Time-of-flight diffraction ultrasonics matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Time-of-flight diffraction ultrasonics?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Time-of-flight diffraction ultrasonics.

Tags

  • Nondestructive testing

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