Study on the Stress Softening Effect of Polyurethane Under Biaxial Tension

Multi‐stage cyclic tensile tests under uniaxial and biaxial loading were conducted on a polyurethane (PU) material. The results indicate that PU exhibits isotropic mechanical behavior and a pronounced Mullins effect under both uniaxial and biaxial loading conditions. Within the framework of pseudo‐elastic theory, the second‐ and third‐order Ogden models, as well as the Mooney–Rivlin pseudo‐elastic model, were established to describe the stress–stretch response of PU exhibiting the Mullins effect. The goodness of fit was used to evaluate the performance of these models. Results show that during the primary loading phase, the second‐ and third‐order Ogden models outperform the Mooney–Rivlin model, whereas in the unloading phase, all three models achieve coefficients of determination ( R 2 ) above 0.95, indicating good agreement with the experimental data. Overall, the third‐order Ogden pseudo‐elastic model provides the most accurate description of the Mullins effect in PU. Furthermore, for the third‐order Ogden model, the composite parameters m / r and m · r follow quadratic relationships with the maximum stretch ratio.