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Finite Element Analysis (FEA) is a tool commonly used by engineers to solve solid mechanics problems. With FEA, engineers can predict how a part will behave under a given set of loads and constraints dictated by how the part is used. FEA is often used to determine the displacement profile of the part, as well as to know whether the part will fail. Engineers use this information to validate their designs.
A key input to an FEA is data on the materials used to build the part. Depending on the material properties, the stress and displacement profiles will look different, and therefore the failure thresholds. Traditional manufacturing methods have well-defined material datasheets that can be used as input for an FEA, but 3D printing poses three key new challenges linked to the layering process:
(1) Imperfect bonding between layers causes anisotropy of the part’s properties. Therefore, properties along the vertical axis are different from properties along the horizontal axes.
(2) The wide range of printing parameters combinations (infill %, layer height, infill pattern) lead to very different outcomes.
(3) In the case of Fused Deposition Modeling (FDM), the presence of an outer surface (see definition here) makes the properties of the part dependent on its geometry.
Using Optimatter Data to enable FEA for 3D Printing
3D Matter started addressing these challenges by developing OptiMatter, a model that forecasts the properties of printed materials depending on the printing parameters used. However, OptiMatter does not address the geometrical side of the equation. Now, 3D Matter has developed a proprietary methodology to conduct Static Linear Finite Element Analysis on 3D printed parts, with a focus on FDM parts. Thanks to this new tool, we can accurately predict if a printed part will yield or not.
This articles shows the results we obtain on a case study example and provides empirical validation of the accuracy of our model.
For any material characterized in OptiMatter, we can conduct static linear FEA. In this section, we will run the analysis with the most widely used 3D printing material (PLA), on the Ocke Stool, a file designed by Big Rep:
First, let’s define the manufacturing and loading conditions for the two cases we are going to study:
Note: A high load is applied to amplify the differences between Injection Molding and FDM, and to get to a point where the stool is under enough stress that it may break. Under “normal” conditions (an 80kg human for example), the stool shows no issue with either manufacturing process.
We run the FEA for the two manufacturing configurations we have defined, and here are the results for displacement:
As expected, displacement is greater for the 3D printed part than for the Injection molded part, though it remains low (less than 3mm). PLA is a very rigid plastic.
Here are the results from the stress analysis:
However, a cross-section analysis reveals that the infill, which is weaker, is also under a high stress (~7MPa):
While the outer surface is strong enough to sustain the load, the infill is not and therefore the object will yield:
*Sources: Injection Molding: Ingeo PLA 3251D data sheet; 3D Printing: 3D Matter’s OptiMatter model www.optimatter.com
In this analysis, we are deliberately using a load beyond normal conditions to see if FDM printing could be used for manufacturing the Ocke stool. Under a high load (500kg), here are the conclusions for the Ocke stool:
- Injection molded, it will not yield: the maximum stress we observe (~3 MPa) is much lower than the yield stress of [injection molded PLA] (62 MPa)
- FDM 3D printed, the infill will yield in the middle leg:
- The outer surface is under a ~15 MPa stress, still lower than the yield stress of [FDM PLA outer surface] (31 MPa)
- However, the infill is also under a ~7 MPa stress, which is much higher than the yield stress of [FDM PLA at 10% infill] (2 MPa)
- FDM part will yield, and potentially fail (a non-linear FEA would be needed to confirm)
So, under a 500kg load and using the defined printing parameters, our initial analysis shows that Injection Molding is the better manufacturing option. However, further analysis can provide two viable options for FDM:
- Limit the load to a maximum of 200kg
- Modify the printing parameters: at 50% infill for instance, the stool can support 500kg
This is just one example of how you can utilize FEA for 3D printing to develop end-use parts subject to significant loads.
3D Matter’s new FEA tool remains in development, but early empirical confirmation tests are very promising. In this section, we describe two of the tests we carried out to confirm our FEA model.
For this first test, we measure the displacement of an actual tensile test against the predicted displacement from the FEA.
Here are the displacement profiles we get for each process:
And here is a compiled view to compare both processes side by side:
As you can see the displacement is close for each process, especially for the larger displacements. Also, the trend is the same: increasing displacement as we get farther from the constrained area.
To test the stress profile, we studied a key threshold data point: yield stress. We use FEA for 3D printing to simulate the hook’s behavior under 3 different loads around the yield stress given by the stress-strain curve, and compare the results.
First, let’s look at the results for the 220N stress analysis:
The yield stress for the hook’s outer surface is 40 MPa, according to OptiMatter. On the FEA, we are using a neon-green color for areas over 40MPa. As we can see on the graphs, both the FEA and the tensile test indicate no yield.
Now let’s look at the 290N stress analysis:
We can see that for the tensile test, the stress-strain plot is starting to curve, indicating that the yield point is close. Similarly, we can start seeing some neon-green on the inside edges of the hook, which means that the hook is starting to yield.
Finally, let’s take a look at the 360N stress analysis:
This time, the stress-strain plot has clearly started to curve, meaning that it passed the yield point. Same conclusion on the right, where the interior of the hook is now all neon-green, meaning that the hook has yielded.
In conclusion, early tests confirm that 3D Matter’s data and modeling is accurate enough to be very useful to industrials, engineers and designers. 3D Matter combines its unique FEA procedure with data from OptiMatter to offer the most advanced analysis of FDM parts on the market.
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