Effects of hydrostatic pressure on shear deformation of polymers

Abstract

The effects of hydrostatic pressure on the torsional shear stress‐strain behavior of polyoxymethylene (POM) and polypropylene (PP) have been investigated. The experimental data show that the shear modulus of polyoxymethylene increases linearly with pressure while the shear modulus of polypropylene increases bilinearly with pressure with the break, the glass transition pressure, occurring at 2.1 kbar. This value coincides with prior results obtained from Young’s modulus–vs–pressure curve for polypropylene. The linear variation of the shear moduli with pressure is compared with the Birch’s equation which was derived from finite strain theory of elasticity. The shear yield stress (2% offset) has been found to increase linearly with pressure for POM but increase nonlinearly with pressure for PP. The Pae yield criterion (J′₂)^1/2 =Jⁿ_k=0α_kJ^k₁ predicts accurately both the linear and the nonlinear variation of the shear stresses with hydrostatic pressure. Other proposed yield criteria, however, predict only linear or nonlinear, but not both. A new phenomenon, which is analogous to the necking in a tensile specimen, has been observed on the torsional shear specimens of polyoxymethylene and polypropylene. This phenomenon has been named girdling. Girdling is a constriction or a reduction of a cross‐sectional area in a narrow band on the shear specimens.