Structural multi-mechanism model with anisotropic damage for cerebral arterial tissues and its finite element modeling

A structural multi-mechanism constitutive equation is proposed to describe the anisotropic and damage behavior of cerebral arterial tissue. The arterial tissue is modeled as a non-linear, incompressible and inelastic material. In this model, new deformation criteria are proposed for the recruitment of collagen fibers and degradation of internal elastic lamina (IEL), two important features of early stage aneurysm formation. This structural anisotropic model is formulated by modifying a previous multi-mechanism model to include the fibrous nature of collagen fibers and incorporates morphological information such as fiber orientation and dispersion. An anisotropic damage model is included to characterize tissue weakening and softening before failure of the IEL, ground matrix or collagen fibers. Two possible damage mechanisms are formulated in this model: mechanical damage dependent on material strains and enzymatic damage induced by hemodynamic stresses.;The elastin/ground matrix and collagen fibers are treated as separate components of arteries. The elastin and ground matrix, which are represented by an isotropic response, bear loads at low strain level, and degrade gradually due to damage or disrupt due to eventual failure. The collagen fibers are recruited into load-bearing and subfailure damage at higher strain levels. Two approaches are considered for modeling their anisotropic behavior. In the first, they are characterized by the anisotropic behavior of N fibers. In the second, the collagen fibers are arranged in two helically oriented families with dispersion in their orientation. The fiber distribution is modeled by an orientation density function or distribution parameter. The fiber orientation and dispersion can be prescribed from arterial histology studies, or identified from stress-strain response as structural parameters.;Pressure inflation test data for cerebral arteries are used to evaluate the constitutive model. It is found to fit the mechanical response of uniaxial test well. There is a need for additional experimental data to further evaluate and develop this model. The constitutive model is implemented in commercial finite element analysis package for numerical computation. The numerical implementation is validated by analytical solutions. The numerical model is used for the study of arterial microstructural behavior in complex biomechanical procedure of angioplasty surgery.