On Fri, Jul 13, 2018 at 2:32 AM, StoneTool owly@ttc-cmc.net [harryproa] <harryproa@yahoogroups.com.au> wrote:
Rob:
There is no question that the shear strength is not on the level of higher density foams. Your proposed test seems not particularly useful without some sort of quantitative comparison with a control.
Agreed. Add a second piece of foam to the set up and see which shears first.
The wrapped foam plank system not only should provide longitudinal stiffening in lieu of stringers, but should also reduce the issue of shear strength. I've come to the conclusion tentatively that wrapping planks with fabric on the bias or 45 degree biaxial glass is probably viable. Obviously it will not pull tightly around the corners, and that should not be a problem if there is a way to slit the corners such as a rotary cutter. Optimally we want a good bond between the wrap and the next ply. This would create a very functional structural shear web, just like a truss beam such as I have built from steel to hold roofs up, but on a much smaller scale. A bit of "frizz" projecting up at the intersections should be an asset so long as they do not make an actual lump. How to slit the corners may be the issue here. The other obvious solution is to stack plank as I suggested before and lightly glue biax to the edges of the whole stack at once, then cut them apart. This could also involve thin spacers to give the "frizz" which would then bond to the surface plies. The spacers could be nothing more than think sheets of linerboard (shoebox material). The challenge of course being to make a consistent cut exactly where it is needed. Tilting each plank up away from the stack close to 90 deg, would create a V that should make this easy...... Obviously only one side at a time would be glassed. Once the system was worked out, it should go pretty fast........
Lots of possibilities (and effort) to make the planks, and infusion would ensure they are firmly stuck to the foam. A possible problem is bending the glassed planks lengthwise. Needs a fair bit on a cat, not so much on a harry, but if the planks don't sit flat, setting up the infusion becomes harder.
Planks narrow enough to overcome the compression weakness would be overkill for the sheer properties. Another solution would be to alternate higher density foam with the lower density. This is similar to wooden boats where stringers are used and foam placed between them.
On Fri, Jul 13, 2018 at 2:32 AM, Bjorn wrote:
Regarding 700kPa foam, I found some interesting quotes on a Swedish forum for pool builders:
"I have now jumped up and down on my 700 insulation and found that no heels bite on it. Not as much as a little depression."
"The pool workers complained for a long time about how hard the insulation was to cut. Just hope that the pool will be seamless."
Later:
"I wrote about the amazing 700 isolation with reference to being able to jump up and down on it like a madman without seeing any dents. That is true, but there are other disadvantages. After having had the pool for almost three seasons, I can say that we do not have a single bump at the bottom, but on the other hand the joints are very visible. One problem that the installers had was cutting this incredibly dense insulation foam"
This made me start thinking and understand that human weight / contact pressure might be the "core" of the issue. Most peoples experience with XPS are based on the lighter foams. And the light foam will not take the load of a person standing on it on a small area of his foot, like the heels, which I guess is the position where a human creates the highest contact pressure.
I measured the contact area of my heel to somewhere between 12-16 cm². With my weight of 70kg, this would create a pressure of 400-600 kPa. I'm impressed that the 700kPa foam could handle jumping on it like a "madman" like in the quote above! But I guess it also is related to the pain threshold of the body/heel. I found some info saying that the pain threshold could be in the area of 250 kPa, so when jumping lik a "madman", the body/senses will probably still not allow you to land too hard/create too much pressure because it would be too painful.
But XPS at 400kPa and below is probably not useful in a construction which a human can walk/jump on. The skin might distribute the load somewhat, but probably not enough with the light fibreglass skins of 0.3-0.5mm which we are talking about here (since they would match the shear strength of this core).end quote
Interesting, thanks. I suspect the joins showing are a result of the resin shrinking (print through). Compression strength is definitely a problem, and people jumping on it is the usual cause of delaminations. Heavier cloth in traffic areas is one solution but 400 foam would appear to be the limit.. My 80 kgs standing on one heel on a piece of unglassed 400 makes a noticable dent, but it springs out when I get off. Something else to test when the samples are glassed.
On 07/08/2018 09:44 PM, Rob Denney harryproa@gmail.com [harryproa] wrote:
That is precisely what I am asking. And also suggesting some ways the lack of compression and shear properties can be enhanced to make the cheaper cores usable. Getting the engineering numbers is the first part. The second part is testing, both bench and full size. Then we can include it in the plans.
We have not done this previously as there was too much going on getting Intelligent Infusion and the resulting boats happening.. Now that this is coming under control, we can focus on further improvements, of which optimising the core is one.
Re your cooked polystyrene:Interesting results. I would be interested to know if there is any difference in shear properties. ie Glue a piece of squashed/heated to a piece of normal then glue a piece of ply (great for stuff like this) on each side with an inch or so sticking out at opposite ends. Put it in the vice and apply pressure to the bits sticking out so the material is loaded in sheer. See what happens.
On Sun, Jul 8, 2018 at 11:02 AM, eruttan@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:
Rob, would a resonable question be for said engineer, "Is 400kpa XPS a resonable core for a harry?", or the corresponding 700kpa material? Or where they can be used safely?
Which is more like asking (where ) will h60 do? Or h45?
Perhaps more interestingly, can some parts of some of the harrys be made out of these as a cost optomization question? And what can't?
I mean, your asking the questions and paying the bills, so you do you. Untill I pony up the cost of a call to Scaled Composites, I got no reson to expect an answer.
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Posted by: Rob Denney <harryproa@gmail.com>
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