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It is one of the most exciting developments in manufacturing: a set of technologies that can produce complex objects in a range of materials, from precious metals to glass, or even concrete.
Additive manufacturing (AM) was once simply referred to as 3D printing, but given the range of materials, scales, applications and industry sectors it is being utilized in, the term has proved unable to fully describe it.
The ability to design a sophisticated part for aviation or space exploration (or even a human organ) and manufacture it inside a machine still seems like something from science fiction, but it is happening worldwide on a daily basis. Once designed, tested and modeled on a computer, an object can be produced onsite and on-demand as required using AM.
Why is Non-Destructive Testing (NDT) Important?
AM-produced parts are being used by NASA in mission-critical situations and in the aviation and power industries where safety and reliability are of prime importance.
The parts manufactured using this technology can be more complex than those they replace, with a single AM part replacing several others. While some quality testing of these parts can be undertaken using existing methods, AM parts also present unique difficulties - not least because NASA is planning in-space robotic manufacturing and assembly using AM.
Although NASA has very specific issues regarding the quality testing of AM parts, many of the problems are generalizable across many industries.
NDT methods are particularly important in an area where AM is replacing traditional methods of metal manufacture (casting, forging and milling), and to some degree ceramic manufacture.
What Tests are Available?
Typical NDT methods involve surface inspections carried out using a 10x visual enhancement or using a fluorescent liquid penetrant.
Dimensional checks can be undertaken using gauges, coordinate measurement machines, and white/red/blue light-scanners. An internal inspection might use radiography, or, if necessary, electromagnetic, ultrasonic, or even Computed Tomography (CT).
The purpose of the testing is the same whatever the method of manufacture. However, the differences between the product of the two methods (AM or non-AM) are significantly different as AM parts are produced in thin layers, rather than as a full-body object.
The textured surface of AM-produced parts renders most surface inspections impossible as superficial artifacts will be mistakenly identified as cracks. Of the tests currently available to validate parts, those most useful for AM manufacturers are radiographic internal inspections.
Radiography, especially Digital Radiography (DR), and CT currently provide the most reliable inspections. They are both used to detect gross defects and to confirm internal geometry. However, CT is more useful for geometry validation, and digital radiography is better at gross defect detection.
Neither method is sensitive enough to confirm if defects are present (or not) at the single-layer level, and each thin layer of an AM part has the possibility of defects. While both DR and CT can find larger defects, the current resolution of these inspections presents real challenges, which have resulted in many manufacturers validating a build with a destructively tested sample.
However, the same defects (if any) are unlikely to occur in each part produced. Therefore, manufacturers must decide whether the convenience and other advantages of AM outweigh the lack of credible quality testing from the ND tests currently available.
How can AM be Reliably Tested?
A comprehensive evaluation tool is needed to resolve the lack of credible testing. The tool can be applied to the specific quality issues that affect AM.
One method which shows promise is Process Compensated Resonant Testing (PCRT). PCRT was developed for the automotive and aerospace industries. It detects structurally defective parts and provides relatively quick feedback about a part's structural integrity. It also does so in a manner that allows for acceptable variation between parts.
PCRT sorts parts by measuring resonant frequencies, which are determined by a part’s stiffness, geometry, and mass. The presence of a structural defect changes the stiffness of a part and consequently changes a part’s resonant frequencies. For a given geometry and mass, the frequency change is proportional to the change in stiffness, and to the severity of the defect. Therefore a part’s resonant frequencies can be a predictor of its structural integrity.
PCRT is a whole-body, structural inspection using statistical processing and pattern recognition tools to segregate non-conforming parts. It is both an internal and external inspection.
With a credible inspection tool such as PCRT, which can validate the integrity of AM-produced parts, manufacturers can be confident of consistent verifiable quality allowing for further expansion of their use.
References and Further Reading