Thanks for the datasheet on the Divinycell. It had much more info than the one I've been using.
And I wish there was better data on these PS insulation foam boards. The data from Finnfoam is even less detailed than the datasheet I linked to.
Regarding impact strength, I found some properties from a Dielectric corporation. It shows slightly higher fracture toughness for PVC.
But since the data is for electrical purposes, it might not be the same grade as for insulation or structural foam.
This picture also shows that PVC is tougher, but with less of a difference than above (even some overlap):
https://en.wikipedia.org/wiki/Fracture_toughness#/media/File:Fracture_toughness_vs._Strength.jpg (near center)
There also seems to be a "high impact polystyrene" grade. But that is not used for foam according to this pdf:
The high impact PS has almost an order of magnitude higher impact strength, and is tougher than some grades of PVC
Not sure what makes some grades tougher, and if it's possible to extrude foam from the tougher grades. But if that is the case, it could explain our different experiences. Maybe the Finnfoam is made of a less brittle grade?
One thing to note is that the density of PVC is higher than PS. The density ratio is 1.3-1.4. This means that Finnfoam 700 at 48 kg/m3 has almost the same amount of solid plastic as H80. Further, according to some sources, the tensile strength of solid PS is higher than PVC. This should mean that also the compressive strength of the solid plastic is higher. And that could perhaps explain why Finnfoam 700 crushes H60 in a vice, even though the density is lower?
When I get the time, I will try to compare the MX7-7 to Finnfoam 700 regarding shear strength. I'm thinking it could be done by gluing two identical pieces between two pieces of wood (of about the same dimensions but longer), and then apply the force through short ropes attached to the end of these pieces (ropes to remove any bending moments). Maybe the force can be a bucket hanging in the rope, which I slowly fill with water until it drops to the floor. Then I can measure the bucket weight or volume of water to get a rough value of the shear breaking force.
So the above would be the low-tech setup. Then I'm sure I can hack together a high tech setup with a linear motor, a load cell and some electronics, which I have laying around. That would make me able to make a controlled stretch of the foam until failure, and plot the graph of shear force v.s elongation. I would have to fabricate the fixture used in the standard from metal and joints instead of wood and rope, to make sure there is no measurable stretch in the fixture. But that is doable:
On Tue, Nov 20, 2018 at 1:31 AM Rick Willoughby rickwill@bigpond.net.au [harryproa] <harryproa@yahoogroups.com.au> wrote:
I was looking for data on fracture toughness but could only find data for shear on both materials.
This is the link I used for the 30kg/Cu.m Styrofoam properties:It gives shear strength 250kPa and shear modulus 8MPa. From that data I calculated the breaking strain of 3.1%; assuming a brittle failure.This link has the data I used for H80:The table on page 4 gives the shear strain as 12%. The 8% I stated was for the H60 rather than H80 (meaning I transposed columns when I looked down them).If I calculate the breaking strain from the shear values for H80 using stated values 0.9/23 then that gives 3.9% if it was brittle failure, which it isn't. On this basis the H80 is only 25% better than 30kg/Cu.m Styrofoam. However it is the actual breaking strain that matters because the skins are taking the load. As long as the core stays intact the structure retains its strength.The link you provided for the Styrofoam HD 300F-X has a density of 45kg/Cu.m. It gives a shear strength of 0.5MPa and shear modulus of 14MPa so 3.6% breaking strain for brittle failure. Its shear breaking strain may or may not be comparable with Divinycell H80 depending on its failure mode. If you can find actual breaking strain in shear then that would be insightful.My comparison in handling and working 30kg/Cu.m Styrofoam compared with Divinycell H80 (actually these days Gurit PVC80) is that the PVC foam is considerable tougher. I regard the blue Styrofoam as brittle but I have not actually measured the fracture toughness.RickOn 19 Nov 2018, at 4:01 pm, Björn bjornmail@gmail.com [harryproa] <harryproa@yahoogroups.com.au> wrote:Rick, where did you get the number for shear braking strain of only 3% for the XPS?The max compressive strength of these insulation boards is measured at 10% deformation, and I don't expect a large difference regarding the shear strength.My hands on experience with Divinycell and Finnfoam is that the Divinycell feels more brittle. For example, while bending a small sample of Divinycell, it just breaks at a certain point. The Finnfoam deforms and forms a curve instead. It also seems to rebound better from the same compressive or bending force while playing with both.Mx7-7XPS 700The breaking strains are about the same, but with a larger modulus of elasticity for the XPS.
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