3D PET Core break tests

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?