Effects of Hydrostatic Pressure on the Mechanical Behavior of Polymers: Polyurethane, Polyoxymethylene, and Branched Polyethylene

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Abstract
The mechanical behavior of polyurethane, polyoxymethylene and branched low density polyethylene is evaluated at various levels of hydrostatic pressure from atmospheric up to 6.9 Kb. Polyurethane undergoes a “ductile-brittle-ductile” transition accompanied by decreased strain hardening and also exhibits intrinsic yield behavior in compressive tests at higher pressures. A study was made of polyoxymethylene with emphasis below 1.4 Kb and in the vicinity of 5.5 Kb. The effect on the mechanical behavior in the 5.5 Kb region is attributed to a possible pressure-induced shift of a β-transition. Additional data not reported by Sardar et al. are discussed. Tests on branched low density polyethylene show that it behaves under pressure in a way similar to that of medium density polyethylene. With increased pressure, failure of the specimens tested, occurs by necking to a fine point. The elastic modulus versus pressure data indicates a possible pressure-induced shift of a β-transition up to room temperature. Above this pressure (2.4 Kb) the rate of increase is slowed because the material is largely in the glass state. A modified form of the Hu-Pae yield criterion which predicts the linear dependence of the yield stress on the hydrostatic pressure is introduced.