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Identification of soft tissue mechanical material model and corresponding parameters from in vivo experimental data by using inverse finite element method

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2008
Üsü, Kerem
The purpose of this thesis is to search for the best material model for soft biological tissues in general. Different sections of human body exhibit different responses like stress relaxation, creep, hysteresis and preconditioning to external loading conditions. These body sections can be assumed as viscoelastic, poroelastic or pseudoelastic. After making the choice of the material model from one of these for the current study, the finite element model and the material code to be used with this model have been created. The material code has also been tried on a simple finite element model before implementing to the real model to prove the fact that it is working properly. Then, the constants in the code which simulates the in vivo experimental data that was obtained by indenting the elliptic indenter tip into the forearm, medial part as close as possible, have been derived by inverse finite element method. Consequently, the characteristic behaviors of the soft tissue could be simulated. Despite the big size of the finite element model and very long submission times (up to one day for preconditioning simulation), relaxation and creep behaviors could be simulated with the maximum normalized sum of square errors of 0.74 % and 0.43 %, respectively. The number of square errors for the hysteresis and preconditioning behaviors appeared as 2.56 % and 3.89 % which are also acceptable values. These values prove that these material models are well suited for the simulation of the behavior of soft biological tissues. By using different experimental data obtained from other sections of human body, simulation of the behavior of different soft tissues can be achieved by using these material models.