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Non-linear acoustic NDT, a new era for critical AM applications

Exclusive interview with Theta Technologies CEO Steve Butler and CTO James Watts

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As additive manufacturing technologies become increasingly sophisticated and more and more viable for the production of end-use parts for critical applications, new questions and challenges inevitably arise. Once AM processes have been validated, AM companies and adoptive industries must reckon with how to ensure the process is scalable and industry-ready. Often, when we talk about this we focus on the scalability of post-processing and a need for greater workflow automation. But there is also another piece of the puzzle that must be addressed. Part testing and qualification. Theta Technologies, a UK-based company that specializes in NDT (non-destructive testing), is bringing to market a new testing process—called non-linear acoustics (NLA)—that could help manufacturers confidently implement AM for the production of critical parts.

Founded in 2007 as a spinout from Exeter University, Theta Technologies is currently on the precipice of becoming a full commercial business, following a multi-year transition led by Theta’s CEO Steve Butler. We spoke to Butler and CTO James Watts about the company’s mission and how its unique NDT technology could be a game-changer for industrial AM users.

Theta Technologies non-linear acoustics

Theta’s Non-Linear Acoustics

Non-linear acoustics technology is a non-destructive testing method that can rapidly and cost-efficiently detect defects or inconsistencies in a 3D printed part using audible and ultrasound frequencies. According to the Theta executives, their patented NLA-based technology is capable of detecting cracks, delamination and creep smaller than a millimeter regardless of the complexity of the printed component.

“Theta Technologies delivers accurate pass/fail results in a matter of seconds, dramatically enhancing the efficiency of metal additive manufacturing production processes,” they explain. “Our NDT solutions are so simple to operate that testing can take place without the presence of highly skilled operators; helping to reduce costs in the process.”

But how does the process work? Watts explains it in more detail: “All components have a characteristic acoustic signature, which is influenced by the material and the shapes and sizes of the component’s features. We exploit the fact that the acoustic signature of a flawed component changes as we change the excitation, whereas the signature of a flaw-free sample remains unchanged. We can thus detect the nonlinear response of a flaw in the sample.”

Theta Technologies, a UK company that specializes in NDT, is bringing to market a new process: non-linear acoustics or NLA
Steve Butler, CEO of Theta Technologies

In other words, the process uses sound waves to excite the part with a linear stimulus. If the part has any internal cracks or flaws, it will send back a nonlinear response. This testing technique can be used in different ways to either quickly analyze a structure’s overall integrity or to generate a detailed picture of the part’s structure and any possible inconsistencies. It goes without saying that Theta’s Non-Linear Acoustic technology also differs from Linear Acoustic testing processes. There are three key differences: NLA ignores dimensional differences between components, removing the risk of false negatives for fractional variation in a part’s dimensions; it also isn’t influenced by surface imperfections, enabling 3D printed parts to be accurately tested before polishing; and it doesn’t require a ‘known good’ to identify a fault in the part.

“We have two main NLA test systems, which work in different ways to identify flaws in components,” Watts continues. “Our nonlinear resonance (NLR) systems excite the whole component at once, and report a ‘Damage Index’ that rapidly shows the response of a flaw anywhere within the component.  Our scanning NLA systems use a localized acoustic signal to generate an image of the part, showing regions of high nonlinear response that correspond to flaws. For AM parts that tend to be very complex shapes, our NLR systems are particularly useful because they don’t depend on surface finish, and can tell the difference between a flaw and intricate internal detail in less than a minute.”

Theta Technologies is currently preparing to launch both its NLA-based solutions on the market. Its first product, expected in June 2022, will be the NLR go/no go system. The second, the full NLA scanning system, is expected to be released 9-12 months later. “If customers would like to send us samples we would be happy to demonstrate our capability to help them make a decision to buy a machine later,” Butler specifies. At the same time, the company is working towards fully qualifying its NDT technique in cooperation with international standards panels. The company is also looking for partners to adopt its NDT technology to further explore what types of parts yield the best results.

A good fit for AM

While not specifically engineered for additive manufacturing, Theta’s NLA technology is highly suited for AM and offers certain advantages over other NDT methods, like dye penetrant inspection and X-rays. This is primarily because NLA can quickly identify even the smallest inconsistencies in the most complex structures.

Theta Technologies, a UK company that specializes in NDT, is bringing to market a new process: non-linear acoustics or NLA “In our experience, there are limited options for inspecting a component that has been produced to take full advantage of the freedoms that AM offers,” Watts says. “While conventional ultrasound or dye penetrant techniques can theoretically be used for simpler designs, geometric and surface finish issues make this very challenging in practice. For more complex designs, only X-ray CT, visual inspection or functional testing (eg pressure testing) have any real application, but even then they may not be sensitive to smaller flaws that could be critical. X-ray CT in particular is very time consuming and requires management of the radiation safety risk.”

Theta’s technology is also compatible with a wide range of AM materials, including metals, ceramics, composites and even some polymers. “Our technology performs well on metal parts produced by a range of AM techniques: we have applied it to laser powder bed fusion, wire arc and binder-based samples and we continue to explore new applications. We have successfully used our systems on other materials, including composites, ceramics and some polymers, although we see better success with harder materials because propagation losses are smaller.”

James Watts, CTO of Theta Technologies

Watts adds that the company is positioning its NLA technology as particularly well-suited for testing thin-walled parts made from metal. “We are targeting thin-wall metal AM components as a priority, including AM heat exchangers. We have shown that we can identify flaws in these with a simple one-minute test. For heat exchangers in critical aerospace or automotive applications the only current tests (X-ray CT or pressure testing) can identify large flaws but may miss the smaller flaws that go on and cause failure later in life. We see that providing a capable NDT solution for these components will permit adoption of AM where this would otherwise be too high risk.”

The company is also looking to introduce its NLA products to industries that utilize AM for making high integrity components, where it is vital that the risk of failure is minimized. “We are really interested in talking with aerospace and defence companies, power generation businesses particularly nuclear and motorsport organizations,” Butler says.

The market is ripe

It is no coincidence that Theta Technologies is readying its testing solutions for commercial launch with a particular eye on the AM industry. As the industry matures and the number of critical applications for AM grow, so too does the need for efficient, scalable NDT. 

“Until relatively recently, very few AM parts were used for the most critical applications—more commonly they were cosmetic or superficial parts,” Butler explains. “Now, as AM’s potential is understood, attention must turn to proving that functional parts continue to perform as expected through their life.

“A great deal of effort has been expended in qualifying AM processes so their ability to produce good quality materials is understood. In real parts though, the finished part may respond quite differently to the thermal stresses produced in the manufacturing process than a simple cylindrical coupon. For this reason, it doesn’t make sense to simply qualify the machine or process: some sort of NDT is essential to validate the component before allowing its use. Our nonlinear acoustic techniques are a good option for this.”

In some cases, Theta’s NLA technology could be used in combination with other NDT processes, like X-ray CT scanning. This would enable users to quickly and affordably assess which parts should continue through to the next round of testing, which is more time and cost-consuming.

Theta Technologies, a UK company that specializes in NDT, is bringing to market a new process: non-linear acoustics or NLA

Watts explains: “At present we are working with several key AM producers to prove our technique against a range of different component geometries, sizes and materials. We anticipate that in some cases our technique will be first used to filter which components are sent for X-ray CT, so that the time and cost of this process is only incurred on components that are likely to pass.”

As Butler and Watts see it, NDT testing solutions like their soon-to-launch NLR and NLA scanning products will play an important role in the continued adoption of additive manufacturing for critical parts and have the potential to create far more applications opportunities. “Bold claims are made by AM process vendors that AM can replace a wide range of complex manufactured parts and assemblies with smaller, lighter, highly integrated and more cost-effective components,” Watts says. “This is only possible if the AM components can be proven.

“We see that adoption of AM into critical manufacturing processes is hampered by the difficulty of validating the components before use. In some cases we believe that components are even designed to look much like conventionally manufactured components simply to allow them to be inspected more easily. We see that the AM industry, coupled with fast and effective NDT, can answer these fears and enable AM to be used much more widely in applications where it would not previously have been considered.”

Butler sums it up well by saying: “It is clear that the AM market is large and growing fast. For it to reach its full potential it needs to demonstrate consistent product quality using NDT, as is currently the case for conventionally machined and composite components.”

This article was published in collaboration with Theta Technologies.

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Tess Boissonneault

Tess Boissonneault is a Montreal-based content writer and editor with five years of experience covering the additive manufacturing world. She has a particular interest in amplifying the voices of women working within the industry and is an avid follower of the ever-evolving AM sector. Tess holds a master's degree in Media Studies from the University of Amsterdam.

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