3D Printing Design Tips - Part 1: Stress Concentrations
This will be the first in what I hope to be a series of tips for optimizing your 3D printed part design for mechanical performance. An apology in advance: I have a masters degree in mechanical engineering and 15+ years of industry experience. I'm used to speaking to people in 'engineeringese' with all of the made up technical words that entails. I am going to do my best to express things in layman's terms as much as possible. One of the awesome things about 3D printing is that anyone can do it; you certainly don't need to be a mechanical engineer to run a 3D printer or to design parts to send to a bureau. So, I'm going to try to structure this writeup in a way that anyone can understand it. Inevitably, I'll end up throwing in some jargon without realizing it. Please call me on it and I will revise.
Many times, people do not associate 3D Printed parts with strength, but this does not need to be the case. Paying attention to a few fundamental design / mechanics of materials principles can greatly increase the strength of your printed part. Combining these principles with advancements in available materials will allow you to produce parts that can meet the structural requirements of more and more demanding applications.
In this first installment, we'll talk about stress concentrations. When you put load on a part, the way that the load is distributed through the part is referred to as stress. Materials are rated based on the amount of stress that they can carry. The stress generated in the part is generally proportional to the cross sectional area of the part. I think that as 3D Printer people, cross sectional area should be easy to understand; it is the area of the part when you slice it against the direction of loading. Of course the stress exists in the full 3 dimensions, but thinking of the two dimensional space first is adequate to get the point here. (Making things more complicated: what exactly is a cross sectional area on something that has X perimeters and Y% infill? We'll ignore that for now and pretend that the prints are solid since the concept still basically holds, but this makes things significantly more complex if we really try performing stress calculations.)
Think of a diving board. The amount of stress in the board is higher with a large adult jumping on it than with a small child jumping on it. Let's say that the board is rated to handle a 400lb person. Now drill some holes in the board. You will find that the load capacity is reduced and that it is reduced by more than the small percentage of material you have drilled out. Now instead of drilling out a hole, cut two sharp notches in the side. The capacity will be reduced even more significantly. In both cases, the point of failure will move to the feature you've added.
This principle where the ability of an object to carry load is reduced by adding sharp features is due to stress concentrations. Both the hole and the notches are called stress risers. They change the way that the stress is distributed in a part and increase it dramatically at the location of the stress riser. Any change in geometry is a stress concentration.
Removing Stress Risers
So what? How can you take this knowledge and use it to improve your designs?
Three words for you.
Fillets, Fillets, Fillets.
Fillets are features that you add to the geometry to round out sharp corners.
Red piece on the right has a sharp corner. This is a stress riser. By smoothing out that sharp corner with a fillet as in the part on the left, strength will be more than doubled.
In many traditional manufacturing methods, fillets become necessary simply because features may be oriented in such a way that it is not practical to get a sharp tool in there. Even when you can, sharp mill bits typically don't do better than about a 0.005" radius, so you naturally get fillets unless you pay $$ for more exotic manufacturing processes.
With 3D Printing, no such limitation exists. You can make perfect sharp transitions that are terribly stress risers quite easily.
Here's another example. I was at an event and another attendee showed me this part. He said, "the design geometry suits our needs perfectly, but it keeps breaking." To which I replied, 'oh, is it breaking right here?' and pointed to the sharp points at the ends of the semicircle. At which point, as if on queue, the part in his hand actually broke at those points as he handed it to me. The ends of the semicircles are pretty significant stress risers.
The corners of the D shaped cutouts present nasty stress risers. It doesn't help that the material around it is relatively thin. Expect sharp features like this to be a potential failure site and avoid if possible.
Now that we've learned about stress concentrations, what would you do to fix this part? Fillets? That could work. You could move the flat surface back and add fillets. You could also adopt a simpler solution and simply make the semi-circle into a full circle. This may come across as a surprise to some. Removing material will actually make the part stronger! Yes, it will still be weakest at that point, but the amount of load it can carry will be significantly increased by removing that stress concentration.
I hope that this article will help you improve your 3D Printed designs. Our next article will elaborate a little bit on how properties of the filament you print with affect stress concentrations (spoiler: stress concentrations are much more significant in PLA than they are in Ninja Flex).