An experimental and anaiytical investigation of the large strain compressive and tensile response of glassy polymers

Abstract

In this investigation, the plastic flow of polycarbonate (PC) was examined by obtaining true stress‐strain data over a range of strain rates at room temperature through homogeneous, uniaxial, constant strain rate compression testing to strains as high as 125 percent. Uniaxial compressive loading conditions give rise to a planar molecular orientation process which results in the observed strain hardening in compression. Uniaxial tensile tests on PC were also conducted. The necked region of the tensile specimen is being cold drawn resulting in a uniaxial state of orientation. Therefore, the observed macroscopic strain hardening in uniaxial tension distinctly differs from that obtained In uniaxial compression, giving different stress‐strain curves. The major differences experimentally obtained between the large strain response in compression and tension indicate a need for an orientation‐based model of the strain hardening process. The experimental program also acts to uncouple the effects of strain softening and strain rate providing more accurate data for future modeling of the true strain softening process. A constitutive law which directly relates the strain hardening response to the state of molecular network stretch in the polymer is used to model and analyze the experiments. The model is found to simulate the observed rate dependent yield and post yield strain softening and hardening of the compressive data over the entire range of strain rates very well. The model is then utilized in a finite element analysis of the tensile tests on PC. Numerical results compared favorably with the experimental data including: load vs, contraction curves, natural draw ratio, and the axial stress‐strain response of the cold drawing region.