Assessing Nonlinear Resonance NDT’s potential to aid additive manufacturing adoption in space applications.

Additive Manufacturing (AM) is transforming the aerospace and space industries, offering the ability to produce parts with increased complexity and reduced mass. This capability is crucial for space applications, where every gram saved translates to substantial cost savings and enhanced mission capabilities. However, the very versatility that makes AM so appealing also introduces significant challenges in part inspection. Ensuring the reliability and structural integrity of these parts is paramount, particularly for the harsh and unforgiving conditions of space.

In this feature, we delve into the groundbreaking potential of Nonlinear Resonance (NLR) Non-Destructive Testing (NDT) technology for AM space applications. We sit down with Professor James Watts, CEO of Theta Technologies, to explore how this innovative NLR technology is overcoming the inspection challenges posed by AM parts, ensuring that they meet the stringent demands of space missions.

The Interview

James, we’re here to talk about space and how Theta’s technology can be deployed to benefit that area. Before we get to that, let’s talk about the opportunity within the space that additive manufacturing has.

Well, we think that AM is really exciting for space because it gives the opportunity to build parts that are much, much lower in mass. Maybe mass reductions of 75%. That means that these parts can be multi-functional, and we can make a real difference to the volume of payloads, the mass of payload, and the size of space flight hardware.

We know that the versatility of additive manufacturing has caused some issues when it comes to part inspection. What’s the importance, certainly within a space context in making sure these parts don’t fail?

So, the main thing with implementing a technology like AM for aerospace and space flight is proving that the parts that you’re going to make, and you’re going to deploy on those missions are going to survive. For that, aerospace and space typically use non-destructive testing, which is great. But the downside with AM is that because you can achieve very, very complex shapes, surface finishes are complex, which is driving much more integration and much bigger parts, there really aren’t that many alternative techniques that are going to work for AM.

Really, X-ray CT is the only technique that is accepted for a non-destructive test for AM parts, but for larger parts, and parts made of denser alloys like Inconel, you just can’t get the resolution for detecting the sorts of flaws that you might be worried about in an AM part. That’s where our technique comes in, because it can detect flaws in parts that are much larger, and it really doesn’t care about the complexity of the parts that you present to it.

Other companies have started to move away from X-ray CT and started to put in-process imaging in place for their inspection process, but there are major pitfalls with that as well aren’t there?

In-process imaging is an effective way of assessing each layer as you build them, but it really doesn’t tell you what happens after that layer has been built. There might be thermal stresses in the parts that really start to accumulate only later in the build. These can easily cause cracking underneath the layer that’s being photographed.

Post-processing can also induce flaws. The support removal process is often quite brutal, and it’s very simple to induce a flaw during support removal, or de-powdering. It’s therefore quite important to have a technique to detect cracks or other flaws that are induced later in the build process.

” It really doesn’t care about the complexity of the parts you present to it.”

Nonlinear resonance looks set to fill that gap. Before we talk about how it’s going to assist the adoption of AM in the space industry, how does this technology work?

So, our technique, nonlinear resonance, works by exciting the part with an acoustic signal. Every mechanical part has a unique acoustic signature, and that’s because of the different features of the part. All have slightly different resonance frequencies, and they interact in different ways. By exciting the part in the right way, we can measure the spectral response of that part. That spectral response is unique for each part.

Even for parts that are normally identical, that are produced by AM, different amounts of support removal might mean that they’re physically slightly different. Usually they are designed so that that doesn’t matter. It doesn’t affect the operation of the part, but it does mean that the unique acoustic signature is different. So, we can’t just rely on the acoustic signature of a part and do spectroscopic comparisons.

Nonlinear Resonance takes that one step further and it looks to see how the unique signature changes as we change the excitation. For a good part, a part which doesn’t have any flaws in it, you wouldn’t expect to see any changes in the acoustic signature of a part. But when you excite a part that has a crack in it, at low amplitude of excitation, you get one acoustic signature, but when you turn the amplitude up, the signature changes. Those changes are the unique signature of a flaw, and they can only come from a flaw within the part.

So, our nonlinear resonance non-destructive test technique looks for those signatures of a flaw within the part. It doesn’t care what the shape of the part is, and it doesn’t care how different the parts are. Although we can measure that, it just looks for the unique signature of a flaw within the part.

Where can this technology fill that gap that is perhaps left by techniques like X-ray and in-process imaging? Where is this most beneficial application in space manufacturing?

So, we see this being used in AM Parts, perhaps larger parts structural elements of a satellite engine components, or thrusters where those parts might be quite complex. They might have manifolds, pipes, fixings, heat-exchanger elements. Parts that are very, very complex, but as a solid, they should have a stable but unique acoustic signature. If we see a crack in one of those parts, we’d see that acoustic signature change as we increase the excitation. That would give us the indication that that part might be a flawed part and we can stop it before there’s lots and lots of post-processing. That later saves costs, time, and money, but we can also detect a part that might fail in service.

Finally, with effective part inspection, how big can AM be for Space applications?

AM is already seeing a lot of deployment within space and space flight. That is fantastic because it means that we’re starting to see the benefits of the technology being deployed. But it’s limited by the lack of good NDT and what I see is that as we’re able to prove that our NDT technique is capable of doing much better-quality validation of the parts, then there might be the opportunity to make parts that are even lower mass, even more optimised, and able to make a real difference to space flight hardware.