Piecewise Parameter Optimization of the Neo-Hookean Model for Hyperelastic Silicone Rubber with Large Deformation

Soft actuators are increasingly being used in robotics and biomedical applications. They use hyperelastic materials, such as silicone rubber, to generate large reversible deformations. However, it is not easy to model the mechanical behavior of silicone rubber under large deformations. It is difficult to accurately predict its nonlinear hyperelastic behavior and thus to accurately design and control these actuators. We have created an optimized Neo-Hookean constitutive model of Ecoflex 00-30 silicone rubber. This method is based on the combination of theory and experiments, whose goal is to enhance the usefulness of the model for performance analysis of soft actuators. Dumbbell-shaped specimens were tested in uniaxial tension on a ZQ-990LB testing machine in a controlled environment at 25.4 °C and 57.4% relative humidity (RH). Stretch ratios varied between 1 and 8.6 and tensile speeds up to 500 mm/min were used. Stress-strain curves and fracture behavior were captured by the experiments. The Neo-Hookean model was then fitted and optimized using a global least-squares optimization approach. Two changes were made: piecewise segmentation of the data, and variable weight factors for uniaxial and equibiaxial tensile data. This accuracy was better for each of the stretch ratios. The optimized material parameters yielded curves that were in close agreement to the experimental data-significantly better than fitting using conventional single regime, particularly in each of the segmented ranges. The model breaks the range of deformations into segments, and in each segment it reflects the response of the silicone rubber to the various loadings. The results provide a good theoretical foundation for modeling the mechanics, analyzing the kinematics and developing intelligent control strategies for pneumatic soft actuators. This should help propel their engineering applications in dynamic environments.