Subject: Re: extruded polystyrene core questions
From: Mike Crawford
Date: 5/3/2018, 5:55 PM
To: harryproa@yahoogroups.com.au

<<I have not seen a reference supporting delamination being related to tensile strength. Would you mind sharing? Typically delamination is related to over stressing the part afaik.>>

DELAMINATION, TENSILE STRENGTH, A

  After a section of a laminated panel is compressed via impact or heavy weight, the skin needs to pop back up while remaining connected to the core material.

  This requires three things: 

    - A core that will allow itself to be compressed non-destructively, so it can go back to its original shape.

    - A core with enough tensile strength to stay connected to the skin as it springs back, and also to not separate internally, inside the core, as this happens.

  That's why alumium honeycomb is a poor choice for a surface that could experience impact (walking, jumping, dropping a battery, hitting a log, grounding, whatever).  If the skin in the area does spring back into shape after an impact, the core separates from the skin and remains compressed.  

  HexWeb ACG core is almost three times stronger in compression than many polypropylene cores, but it's not recommended for any area that could experience impact because it deforms permanently when crushed.

DELAMINATION, TENSILE STRENGTH, B

  Sometimes sections of the core are in tension as hulls experience combined wracking and wave forces while sailing.


<<Because many different home built aircraft have been flying for going on 50 years... Delamination is unheard of. Unless builder error. Or violent accident.>>

  True.  But aircraft rarely experience impact or point loading other than on structures designed for it.  A boat's hull, on the other hand, has masts falling on it. logs hitting it, grounding loads, kids jumping up and down, wrenches dropping and 35 kg batteries falling on it, and so forth.

  The standard treatment of an offshore boat would, in fact, often qualify as a violent accident for an airplane.

  If that composite airplane's wing hit a log, the manufacturer would probably insist on an x-ray, and/or rebuilding the entire section.

  They'd look at me as if I were crazy if I were to say "Ahh, I'll just patch it with a sticky rubber sheet, or some underwater epoxy, and later use some penetrating epoxy and some extra cloth on it once I get to an airport with service hangar".


<<Many of these have been known to be stored, outdoors, in northern costal states, for their entire lives, with the freezing rain, snow, sleet, and hail>>

  Yes.  And so do many houses with shingle roofs.

  Yet both would fair differently if subjected to substantial forces while submerged, and then left submerged for days, weeks, or months. 

  If you look at the IP waterproof-ness ratings, IPX6, resistant to heavy water jets, is a lower rating than IPX7 (often specified as immersion in 1 meter for 30 minutes), which is a lower rating than IPX8 (often specified to as 2 meters for 30 minutes, but actually refers to a specific pressure and duration).

  The big water ingress question involves standing water, or being submerged under water, for long periods of time.  And if it's a boat, then being stressed by forces, impacts, or abrasion that breaks the integrity of your gel coat, paint, and/or epoxy barrier coat.


<<XPS has given years of service to some. It serves in some pretty tough environments.>>

  Definitely.  And many would say it's a great material for airplanes. 

  But airplanes aren't boats.  Boats regularly experience stresses, impacts, immersion, and abrasion that you wold not see on aircraft.

        - Mike




'.' eruttan@yahoo.com [harryproa] wrote on 5/3/2018 12:51 AM:
 


On May 2, 2018 11:41:04 PM UTC, "Rick Willoughby rickwill@bigpond.net.au [harryproa]" <harryproa@yahoogroups.com.au> wrote:
>Delamination is right on the point. XPS has a tensile strength of 0.4MPa.

Do you mean that was the value of the grade you speced in your prototype hull? You could have used a much better spec XPS. Why did you chose .4MPa?

>H80 PVC foam has a tensile strength of 2.5MPa. That is a massive difference as it relates to delamination.

I have not seen a reference supporting delamination being related to tensile strength. Would you mind sharing? Typically delamination is related to over stressing the part afaik.

>I made a long slender prototype hull from a block of XPS. The skin was 200gsm carbon fibre. Over a few uses water found its way through pinholes and some surface scratches and that initiated progressive delamination through various processes like handling and thermal expansion of the water including a few periods of ice in cold weather.

While i appreciate you sharing your experience. It seems like you are suggesting this is always true. Is that what you mean?

Do you think if you had taken care to not have pinholes, it would have happened the same way?

Because many different home built aircraft have been flying for going on 50 years. Many of these have been known to be stored, outdoors, in northern costal states, for their entire lives, with the freezing rain, snow, sleet, and hail one would expect a tied down aircraft to experience. Along with the sweat inducing tarmac temperature in the same day.

Delamination is unheard of. Unless builder error. Or violent accident.

>Once water can get in, it pumps the bond by forming a bubble under the skin.. The temperature of the water in the bubble could range from 10C to 40C in a matter of hours. The expansion works away at the edges of the bubble and extends it.
>
>A thin skin with pinholes on H80 PVC foam does not deteriorate the same way.

Your theory is that water works on XPS epoxy interface different than PVC epoxy interface? Why would that be? Usually these aircraft will sometimes notice a bubble if there is a water infiltration problem. Which is easily fixed. The shearing of a whole skin off a core does not happen.

Another explanation is the composite put too much stress on the core and it failed. And the XPS skin interface is where that failure shows up. This seems to agree with Robs observations too. Or perhaps the surface was contaminated. (with silicon?)

>The bond must be strong enough to prevent the thermal fretting of the bond.

Do you have a reference for this? Cause my experiences suggest that this characterization is not supported by the thousands of flying machines I am aware of. These see way more thermal cycling than a boat ever would. Its cold at 20k ft and 200mph. And hot parked on the tarmac. How many times a day they see that just depends on how many 100$ hamburgers one feels like in a day.

>XPS is a lower cost means of quickly building a disposable hull. PVC H80 is good for a durable hull that will give years of service.

XPS has given years of service to some. It serves in some pretty tough environments.

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Posted by: "." <eruttan@yahoo.com>
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