Under Stress in a Pandemic: Tensile Testing of 3D-Printed Parts

Back in 2015 I often taught a basic "Introduction to 3D Printing" class at the Jocelyn H. Lee Innovation Lab. One of the questions that would come up was "how strong are 3D-printed parts?" I could give an answer in relative terms and talk about the variables involved, but I wanted to be able to give a concrete answer to a concrete example, so I started some primitive tensile tests on 3D-printed samples using a hanging scale as a tensometer. I simply put the sample under load and tried to capture the maximum reading of the scale prior to the sample breaking. I have a photo of the rig, but the small amount of data I collected is lost on a dead hard-drive. I do remember that some of the samples held up to over 500 lbs.

In the five years since, a LOT of testing has gone into the mechanical properties of 3D-printed parts by academics and professionals who do this for a living. But being cooped up in the house due to a global pandemic, and now in possession of my own 3D printer, I thought it might be fun to try some more tensile testing on my own.

I still have most of the test rig, but the hanging scale was corroded. I salvaged the load cell, put it in a custom enclosure and hooked it up to an OpenScale board from Sparkfun for data logging. I'm not measuring temperature, nor do I have a gauge factor, so each time I use the rig, I calibrate it against another hanging scale. The test coupons were all printed from Inland PLA+ (black) using an arbitrary sample geometry with a minimum cross-section of 6mm square. One published source gave the maximum tensile strength of PLA as 37 MPa, so that would indicate a maximum load rating of around 300-lbs, which seemed about right for this round of tests.

Clearly the "slicing" settings and orientation of printing will have a huge impact of the part's strength. In fact, due to the anisotropic nature of 3D-printed parts, this is something that must be accounted for when printing a specific geometry. All of the samples were printed "flat", and while at first I thought I would simply vary the infill percentage to see the effect on tensile strength, it's clear that the walls will be carrying the majority of the load in tension, so that's the parameter I primarily investigated.

For small numbers of walls (4 or less), the parts generally failed at the neck as expected. But once the wall thickness (number of walls) increased, the failure mode changed to primarily 'de-lamination', where parts of the wall peeled off from fillet of the sample, effectively reducing the minimum cross-section prior to failure. A quick finite-element analysis of a sample modeling a part with only walls (no infill) verified the stress on the corners where the de-lamination was occurring, so the test coupon was re-designed with extra fillets and tested again.

The "extra fillet" may have added a little strength, though it's hard to tell with small sample sizes and wide variance in failure strength with these multiple failure modes. In a final attempt, I tried to optimize all of the 'slicing' settings to generate the strongest possible part in tension: a wall thickness that completely enclosed the minimum thickness of the sample, 100% concentric infill and top and bottom concentric patterns. (The concentric patterns were used in hope of having the lines of extrusion follow the expected lines of stress.) This 'concentric' design did achieve the highest load prior to failure for one of the samples (335-lbs), but more rigorous testing with larger sample sizes would be required to say for certain whether this design is significantly stronger in tension than the others.

In any event, with sufficient wall thickness, the samples were able to meet the 300-lbs tension target, though having a range of failure mechanisms puts a great deal of variance in the data. There are, obviously, several other parameters to investigate:

• Change the layer height and line width (nozzle size)

• Vary printing temperature to improve bonding

• Investigate alternative infills and settings to help with delamination

But for now, this is enough to satisfy my 'material science' craving, and I'll go find some other project to work on.

p.s. If you want to learn more about this project, I put together a video slideshow with further details. I also learned that using PowerPoint to make videos does NOT make for a quality product (and make sure the dogs are sleeping before recording). But I'm always open to feedback and questions, so let me know if you have any!

https://www.youtube.com/watch?v=_bY99EBZOnM