Arterial mechanics : noninvasive identification of constitutive parameters and residual stress

Sammanfattning: This thesis concerns the mechanical modelling of arteries, and particularly a method to identify parameters describing the mechanical properties using only clinically obtainable in vivo measurements. The artery is modelled as a fibre reinforced, incompressible, thick-walled cylinder subjected to large deformations and a residual stress field. The residual stress field is parameterized using two methods: the classical opening angle method and the virtual configuration method. In the former method, the parameterization is obtained from the geometry of the cylindrical sector that an artery springs open into after a radial cut through the wall, while the latter method is based on a more general approach where the artery is relieved of stress by a local tangent map.The model parameters are identified in a minimization problem. This is a well known technique for parameter identification; however, a simultaneous identification of the material and the residual stretch parameters has not been done for soft tissues before. Two particular diffculties are encountered in the minimization: first, the non convexity of the objective function, and second, the amount of information available in the measured pressure radius response is limited and the solution must be checked for over parameterization. The thesis studies these aspects and tries to relieve some of the problems by introducing physical or physiologically motivated constraints on the minimization.The results presented in the four papers show that the method is feasible. It is also shown that the opening angle method can fail to give a true parameterization of the residual stress and that the virtual configuration method is preferable.