How to analyze a 3D printed part: a matter of assessing

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I’m tempted to use the word “analyzing” instead of “assessing”, because that’s really what I’ve been writing about in this series. Analysis is the one thing you’re actually doing at every step: you mesh while analyzing what you’re getting at. Then you see how it looks like and you redo if it’s not good.

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You analyze the 3D printed parts and their boundary conditions and working environments and inject them and so on. You’re always analyzing your model and look at every bit of it and iterate over and over again. It’s all analysis.

Now, I want to speak about what comes after the analysis – when you are finally pretty confident about what you have modeled and decide to click the “write the data file”!

Pay close attention to calculation

Calculation is a process you luckily don’t have to do yourself. Gauss and Euler would have loved numerical solving and skipping hundreds and hundreds of matrices and equations. It’s a wonderful era and world to be alive in!

Still, just because a solver will process your input data, put it in equations and solve them, doesn’t mean it will know what to do all the way through. Too often, it will either be stuck in a loop of infinite calculations or stop solving due to a singularity that popped up.

And sometimes (not too often alas, unfortunately) it will give you a hint on the cause. Your contact is lacking in nodes, you didn’t put data in those nodes, your contacts aren’t rigid, are too rigid…etc.

The good news is there are some classic errors that are relatively easy to solve. For example, when it comes to static analysis, contacts create the most fuss. In dynamic, fretting conditions can ruin your day.

There are too many others and I may not be able to list them all. But I will focus on the ones that got to me for 3D printed parts, and those have direct relationship mostly with the 3D printed features:

Void rate

 

To overcome any kind of defect that arises from 3D printing, this specific number comes from one of three places:

  1. Your customer (they’ve done extensive research in R&D and came up with the average number and positions of void within their parts),
  2. the specifications of a certification (you need to put such and such number if you want your part to be such and such certified),
  3. or your personal tests (which I will guarantee you is not as easy as some might think).

All in all, if you put too much, your part will deform too quickly, if you don’t put enough, your part will be a divine unflawed inner peace of a matter. And in both cases, your solver won’t actually solve anything. And the best you can get out of it might be the indication that there is no longer a part to calculate or that material’s hypothesis might log in wrong.

Material theory

 

If you or your customer don’t assess the material and introduce the accompanying theory to it in the software, the solver will absolutely not do it for you. If the solver doesn’t know what to pursue –laminate, composite, elasto-plastic- hypothesis, etc. – it will just drop the whole thing. That’s how delicate things are when it comes to tackling materials!

Correct thermal input

 

I believe I can fairly state that if you’re working with a computer, you can work with excel. Not “macro-visual-basic-little-games” work but rather “extract-data-organize-it-according-to-value” work. So, when you slip up and write 300°C instead of 30°C, then state very, very bluntly that you injected good values (while your solver agonizes along with your part from the heat), you might consider checking your data in excel again. Organize it according to values or compare it with original source to see if there is a mistake. And this is where you can discover the boo-boo. If not, well, good luck!

More types of errors could be explored and I think it’s worthy enough of an extensive catalog of its own. But it’s not the main point of this post. What we are trying to pinpoint here is the results!

Assessing the results: now and later

You’re done with the calculations and are reading the result files now? Check the basic results (that is deformation and displacements) and they will tell you if you’re REALLY done or if it’s a beautiful dream that you will have to give up on sooner than later. You probably don’t need your customer or a more experienced person to tell you that something might be wrong with your model when you displacements of two inches or more –and this value gets worse as the part gets smaller. Just because the solver processes input data with no warnings doesn’t mean the results will actually be irrevocably accurate.

So, once you are truly sure of the veracity of your input and output data, you can proceed to assessing the results: this part can be as wide as you venturing through them, or as narrow as you presenting to the customer the specific information he/she needs. But to draw the big picture here, screenshots of the part’s complete displacements/stress spectrums is a useful start. Then, you can proceed from there into locating the regions with biggest values and narrowing the analysis on them.

Make sure though those regions aren’t the typical “rough-edges-over-constrained” classic ones. In fact, before reading results, you can discard those regions –if you can know them beforehand of course- or ask your software to not give a reading on them. The “critical regions” will require more screenshots and more components’ check. This will help in so many ways I can’t possibly list them all.

Personal recommendations

Shape improvement

 

Through results, stress impact gives insight on how a peculiar shape can support the loads or stand a contact. Therefore, it can help you give recommendation on its features, mainly, thickness, shape or radius. The best scenario is of course when you tell your customer he/she doesn’t need to have so much material somewhere. But this kind of outcome comes after further analysis is performed. Just because static analysis pleads for less thickness doesn’t mean that dynamic or thermic one won’t prove its usefulness and safety.

Further analysis

 

By having critical regions in my hand, and more specifically the X-Y-Z components and their derivatives, I’m able to assess the stress and its direction through the part. That means I am therefore able to give recommendations what further analysis we can have, especially when it comes to failure and working cycles. Because you can’t test every possible section of your 3D printed part and have to start somewhere, the critical regions are the best spots. And directed stress gives moreover a specific plane through which you can study the spread of a crack or a tear.

Moreover, In most cases, my customer is keen to know how many working hours –or cycles – a part can withstand before showing signs of wear. While primary analysis is not the definite answer, it certainly is the ground start of it.

Reference data for the future

 

When your analysis is approved, when your full report is ready and when you’ve presented your results to your customer, you can officially let out a long exhale and proceed to classifying your data.

Why?

Because next time you will be given an assignment, even if it’s a different part, you will have this basic ground covered. At last, you will be able to be a little more confident when arguing about the importance of a thermal stress analysis and how the customer should be paying you more to get your thermic data correctly correlated. Next time, you will be able to spot a lack of data when there is one.

Funny story: once I thought I didn’t have the right to get material data (read, confidentiality issue) and therefore didn’t ask about void rate. This resulted in days of work that weren’t accounted for, and extensive search and several iterations until I contacted the provider of my customer and he told me I could go ahead and ask the guy myself. Eventually, when I told my manager the story, he picked up the phone, called and TADA! I got my number. It served me right and I’m the first one to say so (well the second…My project manager implied it). But the point is to not run away from questions. It will save you a ton of time.

As we’ve seen over this part series, 3D printing analysis is a challenge, an adventure, and a quest of a peculiar kind. But honestly, once the hustle is over and you start to have a foot in these waters, you enjoy both the unknown territory and the realization that there is so much to discover. It’s a priceless experience you can’t earn everywhere.

At first, I was taken aback by the AM and FEA marriage, but throughout and after the process, I realized it was my chance to experiment and put my critical thinking and my technical luggage into practice. And I happened to perform it among senior engineers who could guide me along with an understanding customer and under good conditions. I wonder about other’s experience in this newly discovered territory but feel free to share your own story!


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Source: grabcad How to analyze a 3D printed part: a matter of assessing
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