Understanding the mechanical behavior of facial soft tissue has become an important aspect in problems concerning computational simulation of surgical intervention, the evaluation of optimal surgical and suturing procedures, and the design of prosthetic devices. For this purpose, physics- and phenomenologically based continuum constitutive soft tissue models were proposed. Alongside the advancements in modeling, the experimental characterization of time dependent material behavior of facial tissues allows to capture tissue-specific dominant mechanical mechanisms in constitutive models. Validation of predicted material behavior is based on the availability of experimental data on pre-conditioning, stress relaxation at constant strain, and hysteresis loops in cyclic loading and unloading. Rubin and Bodner [1] introduced a three-dimensional nonlinear model for dissipative soft tissue response, which considers changing material properties through history dependent state-variables that are governed by evolution equations. Specifically, the tissue is considered as a composition of an elastic component and a dissipative component. The purely elastic part of this model formulation was used in an anatomically based finite element model of the face, introduced by Barbarino et al. [2], which allowed investigating the mechanical response of superficial soft tissue under different loading conditions.

The work presented here, aims at applying the full set of elastic-viscoplastic constitutive equations by Rubin and Bodner [1] to the face model. The model consists of a set of material parameters that characterizes the dilatation, the elastic distortion of the elastic and dissipative components, the inelastic time-dependent and time-independent responses, and the hardening due to fluid through the tissue. To this end, an extensive experimental campaign based on the suction method is performed for the identification of the model parameters [3, 4]. The inverse problem for the material parameter identification was solved using a new mixed finite element formulation that was derived for the specific form of the elastic-viscoplastic model. This implementation requires the integration of the evolution equations governing the tensorial measures of the elastic distortional and the dissipative part. For this purpose, a strongly objective integrator was developed.

Dr. Mahmood Jabareen studied Civil Engineering at the Technion – Israel Institute of Technology from 1993 – 1997 and obtained his doctoral degree from the Faculty of Civil and Enviromental Engineering in 2005. After spending one and half year at the Faculty of Mechanical Engineering at the Technion – IIT for a post-doc, Dr. Jabareen went to the Faculty of Mechanical Engineering at the Swiss Federal Institute of Technology (ETH) for a second post-doc position from 2007-2009. In 2009, Dr. Jabareen returned to the Technion – IIT as a Faculty member at the Faculty of Civil and Environmental Engineering. His research interests cover aspects in Computational mechanics with contributions ranging from developing finite element formulation over develping constitutive equations for finite plasticity to multi-physics problems including modeling electro-active ploymers and soft biological tissues.