Processing–Structure–Performance Relationships in Stretch Blow‐Molded Polyethylene Terephthalate Bottles: Optical Quantification of Deformation and Stress‐Crack Resistance

Stress crack failure in the base region of polyethylene terephthalate (PET) bottles remains a critical reliability concern in carbonated beverage applications. Conventional indicators such as wall‐thickness distribution and visual inspection provide only indirect and often unreliable predictors of stress‐crack resistance (SCR), as they do not capture the underlying deformation history developed during stretch blow molding (SBM). In this study, circular‐polariscope‐based digital photoelasticity is employed as a non‐contact, full‐field diagnostic technique to resolve deformation morphology in 3.0 L PET bottle bases produced under a structured design of experiments. Isochromatic fringe patterns enable classification of deformation into three zones: uniform radial stretching near the gate, primary biaxial stretching, and secondary biaxial stretching toward the petaloid valleys. Within the investigated process window, variations in pre‐blow pressure (0.5–0.9 MPa) produced measurable changes in deformation morphology, with radial stretching distance decreasing from approximately 9.0 mm at 0.5 MPa to 7.5–8.1 mm at 0.9 MPa, accompanied by a reduction in secondary biaxial stretching area from approximately 550 mm 2 to below 400 mm 2 . The results demonstrate that circular‐polariscope‐based digital photoelasticity can experimentally capture the influence of processing conditions on deformation distribution and stretching behavior within the PET bottle base.