Expansion differences from steel to hdpe

[Jkeroack]

First off, I don't even know if this is the right place for this. I found this site while looking for work related information. I k ow this is probably "elementary or below" most of
You but I really need the help. I also don't know if this is the right place for the question.

I am building a truck camper. The frame is 2" steel square tubing, and the walls, which will be fastened by bolts, to the frame. Is planned to be "puck board" or HDPE (high density polyurethane?) I live in northern Canada so our temperature fluctuates from +30 to -40. I am worried about the hdpe contracting during the winter and cracking the walls.
Is this a valid concern? And how could one co
Compensate for this?

 

[JedClampett]

HDPE has coefficients of expansion that are about 10 times steel and concrete. So if you anchor HDPE panels to a steel frame and experience temperature differences, something has to give.
So you have to accomodate the extra expansion. you can slot the holes on one side and not tighten the bolts as much at the slotted hole locations. But if the boards are supported on four sides, this will get messy. Maybe you can oversize the holes on two sides. Or you can build a wave in the HDPE and let it allow for expansion. For HDPE pipes we use a serpentine pattern to allow for expansion, but of course, those are one dimensional issues.

 

[rb1957]

alpha for steel is approx 10 micro inches/inch /degC, so something 4' long (lets say 50") heated (or cooled) 20degC will expand 10*50*20 micro inches = 0.01".
so if the plastic CoE is 10x steel, it'll expand 0.1" ...

you could compensate with "sloppy" (over-sized) holes in the cover (and large penny washers), but then you'd probably have them over-sized in the 1st place for ease of assembly.