Sharif University of Technology Scientia Iranica 1026-3098 Articles in Press 2025 02 05 Loading-rate dependent Poisson's ratio of polyurethane auxetic foams at different loading strains; a user-defined material model and experimental verification 23813 10.24200/sci.2025.64578.9043 EN Ali Behbahani Department of Mechanical Engineering, Sharif University of Technology, Azadi Ave, Tehran, Iran Amir Nourani Department of Mechanical Engineering, Sharif University of Technology, Azadi Ave, Tehran, Iran Journal Article 2024 05 24 This study investigated the combined effects of loading rate and applied strain on the behavior of auxetic materials and developed a user-defined material model to capture these effects during loading. First, test specimens were made of conventional polyurethane foams according to ASTM D-3574; then, with a thermo-mechanical process, the conventional foam was converted to auxetic polyurethane foam. Tensile experiments were conducted by applying different strains (i.e., 10, 40, and 80%) and loading rates (i.e., 0.01, 1, and 5 s-1). Displacements were tracked during the tests by digital image correlation. Statistical analysis using analysis of variance (ANOVA) showed that both strain and loading rate significantly affect Poisson’s ratio (p˂0.0001). It was also found that transient strain auxeticity (TSA) depends on the applied loading rate. The user-defined material was developed from the experiments and used in a finite element model (FEM) to capture Poisson's ratio variations because of changing loading circumstances. The model's predictive ability (i.e., the model's validity) was examined by performing different experiments (i.e., different from those used for developing the model), showing a maximum difference of 10% compared to measured Poisson’s ratio-strain curves from experiments.

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