The unique problem that NASA and other organizations or companies are facing with their high-value additively manufactured parts is that the technology isn’t just redefining the way objects are made, it’s redefining the way in which such objects are tested and evaluated, as well. When AM parts are used in critical applications where they have a direct impact on the safety and success of a project, ensuring their quality and reliability is of paramount importance. But, since these parts are often complex and built as one-piece in one-go, inspecting or testing them using traditional methods without compromising the part is difficult, to say the least.
As LaNetra Tate, principal investigator for NASA’s Space Technology Mission Directorate explains, “[With 3D printing], you can print an entire part. With traditional methods you can build, inspect, build, inspect, but with 3D printing, you’re doing it in one swoop. We need to understand how we are going to verify and qualify and certify these parts.”
With parts made using traditional manufacturing processes, it is much easier to predict the properties and performance of the part with certainty. These processes also have well-established standards and procedures for verification or testing. For AM, these are only just being developed by ASTM’s committee F42 and other international bodies working to standardize AM. For instance, in the aerospace industry, using the same destructive tests that are implemented for traditionally manufactured parts wouldn’t be viable for AM parts because they are often one-offs and extremely expensive to make. Also, since AM parts are created layer by layer, their properties can be more unpredictable than what traditional testing anticipates.
For NASA, this is where their Nondestructive Evaluation (NDE) Working Group, or NNWG, comes in. Dr. Jess Waller, material scientist at NASA with Jacobs Technology Inc. highlights their role, “The unique nature of AM parts leads to correspondingly unique product qualification challenges, and this is where NDE fits in. You have parts with complex geometry that are difficult to inspect through traditional means. NDE is uniquely suited to inspect AM parts with all the unique inspection requirements.”
NNWG will develop NDE methods for verifying and inspecting AM parts initially for those that are used in launch applications and then, later, for those used in space. For AM, these methods can be developed to be used while the part is being built, evaluating it at every layer, or for parts post-manufacturing. Waller points out that, “It will be the first of its kind – the standard for AM parts – and NASA is taking the lead on that. It’s a unique opportunity we have with additive that’s not available at all with traditional methods. You can develop an NDE build record of your part that tells you what the properties of that part are, layer by layer.”
Of course, this would require new and innovative NDE methods. Typical eddy-current testing or penetrant testing can prove problematic, since unfinished AM parts have rough surfaces and require polishing. Even x-ray Computer Topography (CT), although well suited for evaluating deep internal features and properties, has its limitations in that it cannot detect cracks that are perpendicular to the x-ray beam. Yet NDE seems to be the way to go, A it optimizes the testing and standardization of complex AM parts, while being potentially cost-effective with wide applicability due to its non-invasive approach. There are already several organizations (government and commercial) such as SpaceX, THE European Space Agency, Lockheed Martin, Boeing, working on this in conjunction with NASA, , and, soon enough, there should be a whole new set of NDE methods for AM parts, not just for space or aerospace, but for applications across industries.