Effect of tensile strain rate on the mechanical properties of constrained‐uniaxially and simultaneous‐biaxially drawn poly(ethylene terephthalate) sheets

The dependency of the mechanical properties (Young's modulus, yield strength, breaking strain, and breaking energy) of preoriented poly(ethylene terephthalate) (PET) sheets on the tensile deformation speeds was examined and discussed in relation to changes of density and birefringence. The procedures for preorientation were constrained‐uniaxially (CU) and simultaneous‐biaxially (SB) drawings at 65°C. The performance characteristics of the present tensile testing at room temperature were obtained over a wide range of extension rates (1.7 × 10−4−13.1 m/s = 0.29–2.3 × 104%/s) without changing the mode of deformation and the shape of the test pieces. The CU drawn PET is strain‐rate‐independent and mechanically superior in structure in the preextended direction with draw ratio λ > 2.5. In the SB drawn PET such a structure comes into existence at λ > 3, which has, furthermore, no dependency on draw direction (mechanically isotropic). The draw ratio of the latter case corresponded to the birefringence (−Δn/d) of about 5 × 10−2. These results imply a possibility of producing the strain rate (from low to impact speeds) independent, anisotropy‐free, and mechanically superior molded products of PET if adequate extrusion or blow molding methods which induce multiaxial orientation with −Δn/d > 5 × 10−2 are developed.