engineering

Thermal acoustic imaging

Thermal acoustic imaging is a engineering 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 Thermal acoustic imaging rather than just read about it. In short: Thermal acoustic imaging (TAI) is an active thermographic inspection process developed by Pratt and Whitney (P&W) in 2005. TAI is a nondestructive testing (NDT) method to detect internal and external cracking of hollow-core metallic turbofan engine fan blades.

Key takeaways

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

Reference excerpt

Thermal acoustic imaging (TAI) is an active thermographic inspection process developed by Pratt and Whitney (P&W) in 2005. TAI is a nondestructive testing (NDT) method to detect internal and external cracking of hollow-core metallic turbofan engine fan blades. Considered proprietary by P&W, TAI is performed to inspect the PW4000 112-inch (2,800 mm) diameter fan blades in an enclosed air-conditioned room within the engine manufacturer's overhaul and repair facility in East Hartford, Connecticut.

Technical description In the TAI process, sound energy is applied to excite the fan blade. If a discontinuity exists in the metal, the excitation will cause each side of the contacting discontinuity to move, resulting in frictional heating. The frictional heating is detected on the surface of the fan blade by a thermal imaging sensor. To examine a complete fan blade, the convex and concave surfaces of the fan blade airfoil are divided into zones and the computer controlled thermal sensor takes an image of each zone while sound energy is applied. After both sides of the fan blade have been completely scanned, the images are processed by a computer and then displayed on a monitor for evaluation by an inspector. The computer can enhance the image to assist the inspector in evaluating any indications. Some indeterminate indications may require reinspection, which in turn may require repainting of the fan blade and repeating the TAI process. If a fan blade has an indication the inspector is not able to evaluate conclusively, the inspector should forward the images along with the fan blade to a process engineer for further evaluation, including by ultrasonic or x-ray inspection.

History In 2005, when TAI was initiated, P&W, following standard NDT industry practice, categorized the TAI as a new and emerging technology that allowed TAI to be performed without establishing a formal training program and certification requirements. In 2018, P&W continued to categorize TAI as a new and emerging technology, despite the manufacture and subsequent TAI inspection of over 9,000 fan blades. In the final report on the 2018 United Airlines Flight 1175 (UA1175) contained engine failure of its PW4000-112 series engine, where the fractured fan blade was found to have had a rejectable indication at the previous TAI inspection that was not properly identified, the National Transportation Safety Board faulted P&W for this, concluding the probable cause of the UA1175 incident was:

the fracture of a fan blade due to P&W's continued classification of the TAI inspection process as a new and emerging technology that permitted them to continue accomplishing the inspection without having to develop a formal, defined initial and recurrent training program or an inspector certification program. The lack of training resulted in the inspector making an incorrect evaluation of an indication that resulted in a blade with a crack being returned to service where it eventually fractured. After this incident, P&W initiated an overinspection and reviewed the TAI inspection records for all 9,606 previously inspected PW4000 112-inch fan blades. During the overinspection, there were two fan blades that were in service at Korean Air and United Airlines that had TAI indications that could not be resolved. Subsequent x-ray inspection of both revealed peening shot in the cavity in the area where the previous TAI indication had been reported. P&W also reported that between December 2004 and the time of the UA1175 incident in 2018, cracks had been detected in five PW4000 112-inch fan blades. One was identified visually and the other four were detected by TAI. On February 23, 2021, four days after a similar contained engine failure incident that occurred in another PW4000 engine on United Airlines Flight 328 (UA328), the U.S. Federal Aviation Administration (FAA) issued an emergency airworthiness directive that required U.S. operators of airplanes equipped with Pratt & Whitney PW4000-112 engines to inspect these engines before further flight. After reviewing the available data and considering other safety factors, the FAA determined that operators must conduct a TAI inspection of the large titanium fan blades located at the front of each engine. FAA noted that TAI technology can detect cracks on the interior surfaces of the hollow fan blades, or in areas that cannot be seen during a visual inspection. The previous inspection interval for this engine was 6,500 flight cycles.

References This article incorporates public domain material from websites or documents of the National Transportation Safety Board.

Illustrations

Worked examples

Example 1 — a first encounter with Thermal acoustic imaging

Start with the simplest possible case. Write down what Thermal acoustic imaging claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, 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 Thermal acoustic imaging 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 Thermal acoustic imaging 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 Thermal acoustic imaging

In research: Thermal acoustic imaging appears in engineering 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 Thermal acoustic imaging 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: Thermal acoustic imaging is common in secondary-school and first-year university syllabi. It links to neighbouring topics 2005 introductions, 21st-century inventions, Acoustics, so understanding it makes those chapters shorter.
In everyday life: Look for Thermal acoustic imaging 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 Thermal acoustic imaging in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Thermal acoustic imaging means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Thermal acoustic imaging out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Thermal acoustic imaging in simple terms?

Why does Thermal acoustic imaging matter?

Because it connects several engineering 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 Thermal acoustic imaging?

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 Thermal acoustic imaging.