HDPE design stresses

Could you tell me what design stresses you use when calculating the structure of an HDPE hull?
Do you take into account the deflection of the panels and stiffeners? What maximum values for these parameters do you consider acceptable?
Thank you very much in advance.

 

Not sure about which HDPE you are using, but I know Dow and DuPont used to have design booklets for all their thermoplastics.
Here is a quick google example of collected ones.
https://apps.dtic.mil/sti/tr/pdf/ADA307551.pdf
https://www.distrupol.com/General_Design_Principles_for_Engineering_Polymers.pdf
And there are other design guides out there...most referencing HDPE now seem to be about pipe.

 

Thanks @jehardiman, this is very valuable information that I will keep in my files.
Right now, I need to calculate non-structural HDPE tanks for a passenger ship. This type of tank is not covered by Classification Societies or other regulations, so I can't find a value for the SC safety coefficients to apply to the mechanical properties of HDPE. The SCs don't depend, I suppose, on the type of HDPE I'm going to use or on the mechanical properties that the manufacturer can provide. Hence my initial question.
Thanks again.

 

If that's the case, then the tank is really not ship structure, but cargo. I'd start here or what ever cargo load in the class rules you are using. Is it inside a cofferdam? Like a blackwater tank in a yacht?
https://apps.dtic.mil/sti/tr/pdf/ADA262879.pdf

 

I don't think I'm explaining myself well. I'll clarify a few things to try to get the answer I need.
No, the tank isn't a cargo tank (at least not what I understand as cargo), but a gray water tank.
It's not structural and therefore, as you said, it's not part of the ship's structure. Now, it must be attached to the ship's structure in some way, that's obvious. But that doesn't concern me at this point.
Since it's nonstructural, I insist, the classification societies don't take it into account when it comes to its scantlings at all.
No, it's not located inside a cofferdam; it can't be inside a cofferdam by the very definition of a cofferdam.
The liquid it contains is, in fact, decisive in calculating its scantlings.
It's also important to know the total height of the tank and the height of its overflow.
So, let's forget all of the above and focus on the question. I need to calculate the tank's scantlings, and I'm not sure what value to use for the design stress. I don't need a specific value, just what percentage of the material's yield point or ultimate strength I should apply to obtain the HDPE's design stress.
Thank you very much.

 

Not familiar with higher math :

I think don't worry

 

I see, that clears everything up. Thanks.

 

Isn't a ship's hull just a tank to keep the water out, not in ? So can't scaling the scantlings of a hull of HDPE, of the estimated thickness you need for a tank, work ? Even go one size thicker plate and welds for safety. Grey water burst - could be worse. The internal anti slosh baffles may add strength if designed so. Fishing boats of HDPE, with a wet well, may give an idea. I'm just guessing.
Industrial process tank manufacturers may give further information too; shippers transporting tanks would have minimum requirements of strength and rigidity of cargo, maybe back to Lloyds, etc. specs. Hopefully that's of some use.

 

Thanks for your response, @seasquirt.
I know how to calculate the tank. That is, I know the loads applied to the tank panels and reinforcements, and from that, I can deduce the stresses each element is supporting. What I'm not clear on, although I've made my assumption, is the maximum stress each element can withstand. That should be a value related to the ultimate strength or yield point of the material, a percentage of it, but I don't know what that percentage is.
If it helps, please forget that this is a tank inside a ship. It's just a tank.

 

Not really, tanks operate in tension.
You can use flexible tanks to store liquids but try using one as a boat and it gets squashed.

 

Here are a list of standard having requirements for HDPE boats

• TURK polyethylene crafts
• DNVGL-CP-0072
• AMSA NSCV PART C3
• IRCLASS Hull Structure of Thermoplastic boats
• New Zealand Maritime Rules Part 40A
• DNVGL-CP-0342
• KR Guidelines for Ships of Polyethylene