The role of pre-conditioning frequency in the experimental characterisation of hyper-elastic materials as models for soft tissue applications

Rubber-like materials as many soft tissues can be described as incompressible and hyper-elastic materials. Their comparable elastic behaviour, up to a certain extent, has been exploited to develop and test experimental methodologies to be then applied to soft biological tissues such as aortic wall. Hence, theoretical and experimental simulation of aortic tissue, and more generally blood vessel tissue, has been often conducted using rubbers. Despite all the efforts in characterising such materials, a clear and comprehensive testing procedure is still missing. In particular, the influence of pre-conditioning in the mechanical response of hyper-elastic materials has been often neglected. In this paper, the importance of pre-conditioning is demonstrated by: 1. exploring the effect of stretching frequency applied before the uniaxial tensile test; 2. recognizing the role of specimen geometry and strain amplitude; 3. verifying the impact of experimental data acquisition on finite element predictions. It was found that stress-strain relationship shows a statistical difference between some frequencies of pre-conditioning and its absence. Only certain pre-conditioning frequencies were able to generate repeatable experimental data for strip or dumb-bell shapes. This feature corresponds to a consistent reduction in the scatter of critical pressures obtained by numerical simulations.