Plastic Memory

Unidirectional carbon fiber pultrusions make very good leaf springs. The (thickness vs length) size you have described (in the interesting mixed units) would be too stiff in carbon fiber, but a thinner cross section would work (e.g. .5 mm). (As you can probably guess, the thicker the section, the more the outer surface must stretch for a given bend radius, so a stiff material, like carbon, that can only stretch 1% or so before failure must be thin.)

To get a feel for the amount of stretch required, just calculate the outer circumference vs the centerline circumference, with the assumption that you are bending the piece into a circle. This will get you into the ball park for obtaining some pieces of close to the right thickness for experimentation. (4" is about 100 mm: so the centerline diameter will be roughly 100 mm, the OD will be roughly 101 mm, and the ID will be roughly 99 mm, if the material is 1 mm thick. This puts the stretch of the outer surface much too close to the yield point of carbon fiber -- but at half that thickness, you'd be ok.)

Corvette leaf springs have been glass fiber reinforced for many years, and glass FRP would be another possibility, again, in a slightly thinner size than you've specified.

Shape memory is probably not the term you really want to use. That term is usually used for a material that returns to a previous shape when heated. So, for example, some clear plastic drinking cups will return to a flat disc (the blank used to form the cup) when heated. There are metal shape memory alloys too.

Some plastics will permanently distort, rather than acting as perfect springs, or take a long time to return to their original shape after deflection. The fiber reinforced products avoid this, because the vast majority of the stress is born by the fiber, rather than by the plastic.

Acrylics (e.g. Plexiglass) and polycarbonates (e.g. Lexan) can make pretty good springs. You could buy a small pieces of Lexan to experiment with. 1mm Lexan would bend to a radius of of 12 mm or so without yielding (going beyond its elastic limit).

The "snappiest" material will be carbon fiber reinforced. The least snappy will be materials like polyethylene or standard Nylon. In between, there are loads of plastics that could meet your needs -- it just depends on how stiff you want the spring to be and how much internal dampening you want (or want to avoid). ABS, PVC are common possibilities.

MoronicBumble,

Thank you for an informative post, very much appreciated.

I hadn't thought of a thin carbon or glass fiber but your explanation makes a lot of sense now.

And thanks for pointing out my mixing of the units, I hadn't noticed that either but seeing it now I realize it's a sign of my (v old) age.

It was very kind of you to go to that trouble to make such a detailed and clear reply, you've been a great help.

OP

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