Actually, because of creep properties typical in plastics, you don't even need high stress to get significant relaxation over time. It's no a good application for plastics to design for press fits.
Not sure if this is really an ME or EE area but .... I am looking at an applicaton for a reed switch and am trying to select an actuating magnet. The reed switches are spec'd for sensing in terms of mmf in units of ampere-turns. Magnets, typically, are spec'd in terms of flux density in units of...