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Re: Pad eyes for tethers and jacklines
The very thing that makes it easy to drill through is the same thing that makes is completely unsutable for use as a backing plate. Pretty much any metal would be better, even assuming galvanic problems than HDPE.
I didn't want to get into a technical issue since its so clearly a bad idea, but here goes...
HDPE is an extremely week material, and is terrible at absorbing impacts. It was never intended to be used as a backing plate particularly in a critical application, you littlerly are risking your life on a material that simply isn't up to the task. HDPE is for cutting boards not backing plates.
So just a quick chart to give you an idea of what you are using...
Yup HDPE is roughly 1/10 the strength of Aluminium, which is generally not considered a particularly strong metal. And 1/20the the strength of 316, which is a pretty weak steel.
But since you aren't going to put it under tension, but compression so let's see how it hold up there...
Again 1/10 the strength of the other options.
HDPE also has a habit of deforming under load, so if you tighten the nuts down, you are going to push the material out of the way. This is called plastic deformation, and HDPE is highly susceptible to it. Again because it's yield strength is so low. Over time the backing plate will literly move away from the compressive force of the nuts and thin out over the area you most need it to be.
Your concerns about galvanic corrosion, while warranted, are causing you to use a completely unsutable material. If you want to use a plastic, then use G10 which was designed to be used as backing plates. Or switch to a matching bolt/backing plate metal. If you don't want metal or G10, at least use some plywood. It is a much better material for this than HDPE board.