A Von Mises stress analysis in the simplest terms is a method to simulate a load on a structure and return visual “hot spots” (structural stress). In the design world, this is basically a commodity tool from an ever increasing number of vendors. All of the FEA/FEM vendors that typically integrate with CAD systems or utilize CAD files for design fidelity, perform meshing of the CAD object for analysis.

So what about the manufactured object?

In the design phase, usually involving several iterations between design modification and FEA testing, the final design is achieved with the designed structural strength and safety factor. Typically, the manufacturing process is theoretically presumed when the design model is created during this process. Downstream, the object is manufactured and may be either digitally scanned for traditional FEA, or subjected to physical (often destructive) testing.

In the case of digital scanning, which includes both optical and X-ray (3D CT), typical output is a mesh that can be returned to a traditional FEA application. As is often the case, the mesh from scan data is simplified as the mesh captured by digital scanning is often far too large for traditional FEA software. This poses the risk of deviant results of what will be analyzed, that is… it is not exactly the same object that was scanned. The second and typically less obvious loss of information is; what happened “inside” the shell of the object scanned during the manufacturing process?

3D CT

This scanning technology not only captures the outside, but the inside as well, including the defects that may have been introduced during the manufacturing phase. Utilizing this process creates a volume object instead of a point cloud or mesh. Yes, a point cloud or mesh can be extracted from a volume or “voxel” based object, but these become shells that no longer possess internal defect information.

Analyzing the volume without meshing

Volume Graphics VGSTUDIO MAX 3.0 introduced an add-on module in service pack 3 that can perform a Von Mises stress analysis using voxels, no meshes. This method retains the full fidelity of the manufactured object, as it was CT scanned. The analysis can be performed on the the as-built object with or without the internal defects included in the analysis. Without internal defects can provide comparative data to the as-designed FEA, which is similar in many ways to the optical scanning approach. With internal defects at full scanning fidelity is a new animal altogether, and has several implications – including safety factor of the design, failure analysis and lean manufacturing.

Example

The following video, which may take a moment to load, presents a simplified demonstration of voxel based Von Mises analysis on a volume object with internal defects. Only the visuals from the 3D view are presented here, sans tabular defect data. The object is steel and the loading is a mere 90 lbforce (400 Newtons), therefore the stress never gets too “hot” – but the main idea here is – internal defects affect structural fidelity, even very small ones.

For the simple demonstration… 

So are those 3 significant decimal places really what they appear to be from a scan?

Lately I’ve been thinking about what precision manufacturing and measurement means to several industrial communities, and it gets pretty murky at times. For the record, I spent 13 years on a previous career path as a job shop machinist. That body of work defined “precision” for me, as I regularly maintained cylindricity & taper & size tolerances of 0.0005 inch or about 13 μm. Whichever unit du jour one may prefer, that translates roughly to 1/4th the diameter of a human hair. Those measurements were performed real-time during the machining process with certified analog instruments, and then on to quality control for final inspection, over 35 years ago.

Fast forward to today, I’m going to talk a little about digital inspection techniques/systems  that prevail in today’s manufacturing environments and additive manufacturing.

Ed Stanley is a post doctoral researcher at the University of Florida and the California Academy of Science. To the pleasure of many, Ed is far from being a stuffy academic. His work in herpetology (reptiles & amphibians) is quite extensive with openly shared collections on both his personal website at http://www.edwardstanley.weebly.com and at http://morphosource.org/

How in the world do I know this guy?

AM or Additive Manufacturing or 3D Printing or Rapid Prototyping seemingly changes names every few years to refresh itself a bit, then there are the myriad of methods for AM (I’ll stick with that acronym today). There is jetted, SLA, cLip, FDM, SLS, SLM, DMLS, DMP, EBeam, oh my… what a list (and that’s just a sampling). Well, without spending too much time on the acronyms, my time here is better spent on what seems to be the golden haired child of the bunch… metal printing.

Factoid: Metal printing is not new, it’s been around for many years, but the technology has advanced a bit more rapidly as of late due to the need to make the hype look more like truth – yes, metal printing has been known to be hyped beyond it’s capabilities at times, but it’s all meant in the best way.

Without further adieu; the meat of my matter here is pretty simple: what is in a metal print?

With the advent of 3D scanning comes some very logical downstream applications – 3D printing, reverse engineering, metrology/inspection and applications to the arts. The cost of 3D scanning hardware is quite literally… all over the road-map, with simple low cost IR scanners under 500USD to CT scanning hardware over 1M USD. While high end CT scanning provides more than just the “skin” of the part, data collected is often output to show only the skin. This skin or surface I am referring to is commonly exchanged between software programs as an STL or polygon mesh file.

The 3D data captured by whatever method, has no intelligence – it is the equivalent of a 3D picture.

Arts News -BERLIN • Two German artists walked into the Neues Museum in central Berlin last October and used a mobile device to secretly scan the 48cm-tall bust of Queen Nefertiti, a limestone-and-stucco sculpture more than 3,000 years old that is one of Germany’s most visited attractions. They used the data to create copies of the bust and delivered them to Egypt.. Read more at straitstimes.com.

Source: ‘Swiping’ ancient artefact with 3D scanner, Arts News & Top Stories – The Straits Times

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