Effect of hydrostatic pressure on the deformation behavior of polyethylene and polycarbonate in tension and in compression

Abstract

The stress‐strain response of crystalline high density polyethylene and of amorphous polycarbonate has been determined in tension and in compression at superimposed pressures up to 1104 MPa(160 ksi). Strain softening occurred in the polycarbonate at low pressures but was inhibited by pressure. Tensile necking occurred in both materials, but was promoted by pressure in polyethylene and inhibited in polycarbonate. The initial modulus, E, and the flow stress, σ, at a given offset strain varied linearly with the mean pressure, P, with essentially the same pressure coefficient, α. Thus, E = (1+αP)E0 and σ = (1+αP)σ0, where E0 and σ0 are values at zero mean pressure. In polyethylene, the coefficient, σ0, was the same in tension and compression, indicating that the strength differential between tension and compression was a simple manifestation of pressure‐dependent yielding. In polycarbonate the coefficient, σ0, was different in tension and in compression, implying an effect due to the third stress invariant or to anisotropy. The results suggest a constitutive model for polymers in which the flow stress is linearly dependent on mean pressure, but in which inelastic volume change is negligible. The results also suggest that the pressure dependence of flow stress in polymers is the same as that of the initial modulus.

Modeling of the Postyield Response of Glassy Polymers: Influence of Thermomechanical History

“…This can be avoided by performing experiments directly under superimposed hydrostatic pressure. [40][41][42] Therefore, we determined µ by numerically predicting the yield data obtained from compression tests at different true strain rates and, finally, from the tensile tests under superimposed hydrostatic pressure as reported by Christiansen et al 41 Parts a and b of Figure 6 show that an excellent description was obtained for the material parameters tabulated in Table 2 with an initial S a value of 26.5 for the compression and 33.4 for the yield experiments, representing the difference in thermal history between the two material sets used.…”

Effect of hydrostatic pressure on flow and deformation in highly reinforced particulate composites

“…Here, relative sliding occurs along non-planar surfaces and hence does not preserve volume; yield is therefore pressure-sensitive in such materials. Granular media [1]-[2], polymers [3]- [5], metallic glasses [6]- [7] or fractured ceramics all have a yield stress that varies with the local hydrostatic stress. For isotropic granular media the simplest yield surface corresponds to the Drucker-Prager flow criterion, which traces, in principal stress space, a cone having again the bissectrix as its axis.…”

Numerical simulation of flat-tip micro-indentation of glassy polymers: Influence of loading speed and thermodynamic state

“…An excellent method to obtain the pressure dependence µ is by performing experiments directly under superimposed hydrostatic pressure [60][61][62]. Therefore, µ was determined by numerically predicting the yield data obtained from compression tests at different true strain rates and, finally, from the tensile tests under superimposed hydrostatic pressure as reported by Christiansen et al [60].…”