Three Point Bend Test of Various 3D Printed Samples

Every time I print something off on my Printrbot, I ask myself, "Do I really need to use this infill and wall thickness?" And then I decide to just go with my standard settings (40% Rectilinear fill with 3 layer wall thickness) and don't give it anymore thought. But all of that has changed. 

I recently asked Bruce Hart, director of the Independent Testing Lab at SVSU, if he would allow me to use the lab's Instron tensile testing machine to test out some parts from my printer. Being the great guy that he is, Bruce agreed. What follows is a description of what the test entailed, and some of the conclusions I drew from it. If you don't want read through everything and just want an answer, the inevitable truth is that a solid part has (by far) the best strength to weight ratio.

I designed a simple sample piece that is a basic rectangular solid (20mmx40mmx7mm). There are 2 variables that are easily alterable, namely the wall thickness, and the in-fill density (respectively A and B in the figure below). Note that as in-fill density increases, the "B" dimension in the below figure decreases (inverse relationship). I try each combination of wall thickness of 1, 3 and 5 layers, mixed with infill densities of 0%, 10%, 20%, 40%, 60%, and 100%.


A cross section view of the test sample where A depicts the infill spacing and B depicts the wall thickness.

 All of the pieces were 3D printed under the same conditions (minus the intentionally changed variables). I only test one sample per configuration, because this is my experiment, and I'm on a budget.

One of the samples being printed
One of the samples being printed

The samples were tested in the three-point bend test configuration, where the two outside corners are held static, and a load is applied to the center. The bolt in the figure below was used to concentrate the force on the middle. It should be noted that the results yielded from this test are only going to signify strengths in a three-point bend-like situation, and they might not hold true for something like a crush test.

The test setup
The test setup

The output of the test rig compares how much load is being applied vs how much the part is deflecting. The curve will go up until it hit's its yield force, or in simpler terms; begins to break. The piece can no longer sustain as much weight, and will continue to deflect.

The stress-strain curve of a 40% Infill 3 layer walled sample
The stress-strain curve of a 40% Infill 3 layer walled sample

 All of the samples were tested and one of the first interesting results I found were that for almost all pieces, if the in-fill was less than 50%, the sample would plastically deform, however if it was more than 50% the part would break down the middle.

Piece plastically deformed after test
Piece plastically deformed after test (in-fill < 50%)

Piece with significant fracture after testing
Piece with significant fracture after testing (in-fill > 50%)

Below is all of the weight to strength ratios compared. As expected the best weight-to-strength ratio are the 100% infill variations, because the plastic was allowed to bond in all directions. I found that my standard print setting (40% in-fill, 3 layer walls) was pretty acceptable, and based off the data, increasing the in-fill won't gain me any strength, so I am keeping my general settings the same. However, perhaps this data will be useful for other models with particular needs, and I hope this data below may help other makers as well.

Compiled results compairing the strength to weight ratio of all tested samples
Compiled results comparing the strength to weight ratio of all tested samples

ID Number Infill % Edge Thickness Infill Pattern Mass (g) Yield Force (lb) Force/Mass
0 0 1 layers Rectilinear 1.21 14 11.58
1 10 1 layers Rectilinear 1.93 79 41
2 20 1 layers Rectilinear 2.55 103 40.34
3 40 1 layers Rectilinear 3.67 221 60.17
4 60 1 layers Rectilinear 4.96 335 67.5
5 100 1 layers Rectilinear 6.89 738 107.11
6 0 3 layers Rectilinear 3.19 156 48.84
7 10 3 layers Rectilinear 3.55 188 52.9
8 20 3 layers Rectilinear 4.04 239 59.13
9 40 3 layers Rectilinear 4.76 325 68.32
10 60 3 layers Rectilinear 4.98 313 62.88
11 100 3 layers Rectilinear 4.69 228 48.63
12 0 5 layers Rectilinear 3.84 206 53.59
13 10 5 layers Rectilinear 4.6 320 69.5
14 20 5 layers Rectilinear 5.23 380 72.71
15 40 5 layers Rectilinear 5.82 513 88.1
16 60 5 layers Rectilinear 5.54 429 77.42

Test Video: