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Hello all, I just wanted to post up my findings and see if anyone else has done similar or more advanced tests using the 3D PET core. I've just been messing around with various materials and cores to gain a better understanding of their strengths and weaknesses, and I was quite surprised by the performance of the PET core. Given my break tests were very crude, I wasn't trying to compile data but rather see how each material fares in different conditions. So basically what I did, was I laminated some 300x300mm panels using 2 layers of 650gsm quadaxial fibreglass on each side (so 4 layers total) of the core materials I was using; 3mm Soric, 3/5/10/15mm 3D PET Core, and 10mm Divinycell. All these panels were infused with Renlam K3600 epoxy resin, so the Divinycell was drilled at 25mm intervals and then scored between the holes to facilitate the resin flow. Then what I was wanting to find out was how each panel compared in terms of stiffness, impact strength, compression strength, and bond strength (bond between the laminate and the core). Again I will just point out that my test methods were crude, this was just a bit of a fun experiment for my own benefit, so please don't judge me too harshly on that haha. First I compared each panels stiffness by attempting to flex them by hand. As you can imagine, they were stiff. To try to get an actual measurement of flex, I did a crude 3-point bend test by placing the ends of each panel on the edge of a brick, and then placed a controlled weight (another brick) in the middle of each panel one at a time and measured their deflection. Fairly predictable results; stiffness increased with thickness of the laminate. Between the 3mm Soric vs 3mm PET, and the 10mm Divinycell vs 10mm PET, there was very little difference and I can't really say that any difference was NOT due to a variation in the test method. The next test is the one that piqued my interest: I attempted to break each panel with various forms of load. My first test method: my blackbelt sister. I held each panel and got my sister to side-kick each one. Most of the cores did nothing but hurt my sisters foot, but the 10mm and 15mm 3D PET cores didn't do so well. The 10mm core cracked noticeably on the edge. The 15mm panel broke completely. Not in the way that I would have expected, the fibreglass held together but the core itself actually broke away from the fibreglass. Initially I thought the bond had failed, but when I cut it apart to have a better look, I could see that in actual fact it was the core that had sheared. The fibreglass was still bonded to the core, but the core had torn away from itself, somewhat like you would expect of a failed bond between a laminate and a foam like polyurethane. I thought that was interesting, so I infused another 10 and 15 mm panel, and placed each one between bricks like I did for the bend test. This time I tried jumping on them. The 10mm core didn't break this way, but the 15mm core sheared again in the exact same way. None of the other panels failed in this way, and I am wondering if that might be because they are thinner cores and shearing thin cores would be very difficult to do. The interesting thing is that I could not break the 10mm Divinycell, I even tried a sledge hammer in the end but I just couldn't get it to shear. I just wanted to get some discussion going and hear from people more knowledgeable than myself about why the PET core appears to have such poor shear strength compared to the other cores?
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A very interesting test, albeit the side kick break test was not the most scientific! I am hoping your sisters foot didnt get any sharp shards of broken fibreglass in them!!
The failure method does not sound too extreme to me. You are talking about massive point loading - generally a bad situation and not ideal for any composite let alone a foam cored panel.
In terms of the performance of the 3D core, it may well be the structural shape of it with limited strength between the cells that hasnt helped. Yes you get some strength from the resin "honeycomb" matrix that fills the voids between the foam cells, but it is not as dense as with Soric and certainly the more uniform structure of the solid foam core is likely to be superior.
If youve got a few photos of where the core sheered, it might help readers to see visually exactly whats going on.
The sheer strength tests used to get the figures you see in technical data for most cores are done in a different way, usually a more uniform sheer loading over a set sized test piece so in that sense you are comparing a point loading force to a large area loading force and as such i would expect the results to be totally different.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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Yes, to be honest I was expecting to damage the panels with that one. My sisters foot is fine though  the interesting thing for me was why it's was the PET cores that broke. I'd be interested to know if Soric also came in 10 and 15mm thicknesses, would it suffer from the same damage? I suspect not given the density. But I was surprised that the thicker panels broke, I had my money on the 7/5mm panels giving up first. Of course I wasn't wasn't expecting the cores to to shear. Also I was hugely impressed by the divinycell. That stuff just would not break, not a chance.
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What would be interesting is a plot of core density vs breakage force. I would expect the denser / more rigid cores to be stronger. Any that stand out as having a greater force per unit density would be interesting to look at. Unfortunately this may require a little more care than a kicking but a good kicking may be more representative of real world loadings. Its a toughie!
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Hello, thank you for sharing your tests with the different cores. I also tested the 3D PET core in 3mm and wasn't very happy with its infusion characteristics. I did just a layer of 3k cf on both sides. I have now tested the soric xf 3mm and really like how it worked. Im sure it is heavier due to the more resin intake but it seemed a lot more solid. I wanted like a .50" wall thickness overall on my project and thought the 3D PET would do this. But now I believe I will be going with a 6mm Soric XF and a 2mm Soric TF layers for the core.
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What were you unhappy with?
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