Constitutive Modelling of Tendons as Fibre-Reinforced Soft Tissues with a Single Fibre Family: Stress-Relaxation Tests for Parameter Identification

Background: Nowadays, flexor hand tendon repair represents a clinical need, and new suture patterns or devices are commonly tested on animal surrogates. Considering the literature, the most frequently adopted animal models for testing are the equivalent tendons taken from porcine specimens. The constitutive modelling of these tendons could open the way to the numerical testing of new repair techniques and the development of digital twins, reducing the use of animal models. Methods: Uniaxial tensile stress-relaxation tests at different strain levels during the loading and unloading phases on porcine tendons were performed. Constitutive formulations based on the assumptions of incompressible and nearly incompressible materials were evaluated. Results: The experimental data were evaluated considering the relaxation tests at different strain levels during both the loading and unloading phases. The experimental tests were used for the material parameter calibration of both models. Conclusions: The stress-relaxation tests conducted at different strain levels during the loading phase showed good agreement with previous findings reported in the literature. Both constitutive model formulations provided a reliable approximation for numerical simulations.