Improved Inverse Identification of Anisotropic Polymer Properties via FEMU and VFM With Sensitivity Analysis

The stretch‐blowing process of PET bottles induces significant anisotropy in mechanical properties due to the differential elongation of macromolecular chains. The Young's modulus can nearly double in the longitudinal direction and increase more than threefold in the hoop direction. However, process variability leads to dispersion in elastic properties, making accurate identification essential. The challenge lies in determining all orthotropic elastic parameters from a single test while minimizing experimental uncertainties. In this study, a biaxial testing machine developed in our laboratory was used in combination with digital image correlation (DIC) to measure displacement fields. The orthotropic properties were identified using two inverse methods: Finite Element Model Updating (FEMU) and the Virtual Fields Method (VFM). A cruciform specimen with an optimized hole geometry was designed to enhance sensitivity to material parameters. Sensitivity analysis revealed that FEMU provides more accurate and less scattered results than VFM, benefiting from its ability to handle complex deformation fields. Finite Element simulations further confirmed the robustness of FEMU, demonstrating its superiority in capturing anisotropic behavior with reduced uncertainties. These findings provide valuable insights for improving the characterization of PET materials.