A unified viscoelastic-viscoplastic constitutive framework for optically clear adhesives: Experiments, modeling, and...

Optically clear adhesives (OCAs) are extensively employed as bonding layers in display manufacturing. As compliant interlayers in foldable display stacks, they undergo pronounced time and temperature -dependent deformation during repeated bending, which makes reliable constitutive characterization essential for accurate stress prediction and design optimization. Therefore, accurately characterizing their mechanical behavior is fundamental to bending-stress analysis and design optimization of foldable displays. Existing studies emphasize viscoelastic or hyperelastic behavior yet overlook the coupled viscoelastic-viscoplastic response. We propose a unified constitutive framework combining a generalized Maxwell representation with the Qian-Liu viscoplastic formulation to characterize OCA mechanics. The proposed model is calibrated against stress-relaxation experiments conducted at 30, 60, and 85 °C, from which the corresponding material parameters are identified; the fitting results demonstrate that the model captures the coupled viscoelastic-viscoplastic behavior under these conditions. The framework is further implemented into an ABAQUS UMAT and validated via finite-element simulations of a simplified foldable-display structure, which corroborate the experimental observations. The framework closes a gap in OCA constitutive modeling and delivers a calibrated, numerically validated tool for structural optimization and long-term reliability assessment of flexible OLED devices.