The effect of hydrostatic pressure on the shear yield behaviour of polymers

Abstract

The torsional stress-strain behaviour of isotropic poly(methylmethacrylate) (P M M A), poly(ethylene terephthalate) (P E T) and polyethylene has been studied under hydrostatic pressures up to 7 kbar. In P M M A the following important features were observed. First, there is a monotonic increase in the initial slope of the stress-strain curve with increasing pressure. Secondly, there is a substantial increase in the yield stress and the strain to yield as pressure is raised. Thirdly, there is a transition in the mode of failure at elevated pressure, the specimens fracturing in the high pressure region before a drop in stress occurs. Finally, in the high pressure region the fracture stress increases with increasing pressure but the strain at fracture decreases.

The observed yield behaviour can be represented formally in a number of ways, and the results will therefore be discussed accordingly, in an attempt to give a general yield criterion for P M M A. The fracture behaviour has been analysed in terms of the Griffith ideas for fracture of glassy materials, and this will also be discussed.

The results for poly(ethylene terephthalate) (Arnite) differ significantly from those for P M M A. Specimens of Arnite as received from the manufacturers were ductile in torsion at atmospheric pressure, and the torsional yield stress rose monotonically with increasing hydrostatic pressure. Annealing the specimens produced embrittlement at atmospheric pressure, but on testing under conditions where there is no tensile component of stress (i.e. at very low hydrostatic pressures) ductile behaviour was observed.

The contrast between P M M A and Arnite suggests that in the former case there aresurface flaws which are penetrated by the hydraulic fluid at high pressures, whereas in the latter caseinternal flaws are produced by annealing.

Polyethylene remained ductile over the complete pressure range, with a pressure dependence of the tensile yield stress which was similar to that shown by polyethylene terephthalate.