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I've got an elastomer, NinjaFlex, that I have been tensile testing.

I 3D printed the tensile test specimens. I made some specimens with 100% fill and no voids and some specimens with an 80% infill (with intentional voids). When I put a side crack in both specimens, the 80% infill specimens failed later than the 100% fill specimens.

I was hoping anyone could point me in the right direction of some theory which may explain why this is happening? My initial thoughts are the crack growth dissipates into the voids for the 80% whereas the 100% has no stopping mechanism and continues straight through the specimen.

Best regards!

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    $\begingroup$ I imagine it would be some combination of crack deflection and tip blunting, but it would depend heavily on the infill geometry. $\endgroup$ – wwarriner Apr 13 '17 at 1:32
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    $\begingroup$ Welcome to Engineering.SE! This is exciting research, but perhaps some pictures and some diagrams would be useful for clarification. As indicated in other answers, some of this is still new theory for theorists, and as such the most we can do is speculate. $\endgroup$ – Mark Apr 13 '17 at 16:13
  • $\begingroup$ Goes the crack go with or against the grain pattern for how the material is bonded? What is the orientation of cavities compared to the direction of crack propagation? It might be that the crack propogates towards a cavity and then stops. The cavity may act similar to a drilled hole at the "tip" of the crack, which reduces the stress concentration factor, and therefore stops the spread. $\endgroup$ – JMac Apr 13 '17 at 16:18
  • $\begingroup$ Do any of the answers posted solve the question you've asked or is further answers/explaination required? $\endgroup$ – Diesel Sep 24 '17 at 17:08
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One of the reasons for the difference that you're seeing is that both actually have voids.

When 3D printing your extruder prints lines which don't fully bond in the XY plane, they're close but not perfectly solid. Remember, voids are typically measured in mincrometers or nanometers so you won't easily be able to see them.

With your 80% fill sample each hole you print has a single complete path around it preventing the crack formation in it, and then the lines to will in the rest of the layer span between these printed circles (or hexes in the case with 3D printed parts).

Depending on the printer you're using you may be able to change the infill pattern, with the same 80% fill. I would expect that different types of fill will have different loading results. Hex would likely be stronger than diamond, diamond stronger than Cat fill.. etc.

I'd love to see some results if you run more tests! This is an area I've been meaning to do some more formal research in for a while.

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Printing samples will lead to fiber effects. In effect, you are dealing with an anisotropic material. After a quick experiment with paper, you can see what is happening. However, when you intentionally put voids, the fibers now twist and turn in a different pattern, resulting in potentially increased strength.

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