|
|
Exploitation of Experiments for Improvement of Level Constitutive Models of Aortic Aneurysm Tissues
Man, Vojtěch ; Horný,, Lukáš (referee) ; Rosenberg, Josef (referee) ; Burša, Jiří (advisor)
This paper deals with the problem of abdominal aortic aneurysms (AAA), taking into account the possibility of using mechanical tests of aortic tissues for improvement of level of their constitutive models. First part of thesis deals with the introduction into the problem, description of the structure of the wall of the healthy aorta, its main components and the degenerative changes which lead to formation of AAA. This is followed by a brief excursion into constitutive modeling, which focuses closely on the description of the models used to describe the mechanical behavior of soft tissues. The theoretical part is then supplemented by a narrower selection of constitutive models used for modeling aortic wall and intraluminal thrombus, together with published results, which are reviewed and discussed at the end of this section. The main part of this thesis is devoted to tests of mechanical properties of arterial tissues. First, the methodology is presented together with the description of the customizations of the laboratory equipments together with the test rig. In addition, attention is focused on the results of mechanical tests of intraluminal thrombus, where the results of both uniaxial tensile tests and equbiaxial testing are presented. Also the influence of distance ILT from the lumen on the mechanical properties of the thrombus is examined. Another area of interest is the investigation of the effect of elastase on the chnage of mechanical properties of pig aorta. In this case, porcine aortas are experimentally tested only by biaxial testing, and the time of elastase action to alter the mechanical properties is analyzed so that the resulting tissue has a similar stress-strain response as aneurysmal tissue. Finally, the results of experimental measurements, limitations and other possible ways of research are summarized.
|
|
|
Numerical implementation of incremental minimization principle for materials with multiple rate-independent dissipative mechanisms
Frost, Miroslav ; Moskovka, Alexej ; Sedlák, Petr ; Valdman, Jan
The incremental energy minimization approach is a compact variational formulation of the evolutionary boundary value problem for constitutive models of materials with a rate-independent response. Although it can be easily applied to many conventional models, its main advantages arise when applied to models with multiple strongly coupled dissipation mechanisms, where the direct construction of the coupled yield conditions and flow rules may be challenging. However, this usually requires a more complex numerical treatment of the resulting sequence of time-incremental boundary value problems resolved via the finite element method. This contribution presents, compares and discusses two genuine minimization approaches - the staggered solution procedure relying on alternating minimization and the monolithic approach employing global minimization - for an advanced constitutive model of shape memory alloys.
|
|
|
Exploitation of Experiments for Improvement of Level Constitutive Models of Aortic Aneurysm Tissues
Man, Vojtěch ; Horný,, Lukáš (referee) ; Rosenberg, Josef (referee) ; Burša, Jiří (advisor)
This paper deals with the problem of abdominal aortic aneurysms (AAA), taking into account the possibility of using mechanical tests of aortic tissues for improvement of level of their constitutive models. First part of thesis deals with the introduction into the problem, description of the structure of the wall of the healthy aorta, its main components and the degenerative changes which lead to formation of AAA. This is followed by a brief excursion into constitutive modeling, which focuses closely on the description of the models used to describe the mechanical behavior of soft tissues. The theoretical part is then supplemented by a narrower selection of constitutive models used for modeling aortic wall and intraluminal thrombus, together with published results, which are reviewed and discussed at the end of this section. The main part of this thesis is devoted to tests of mechanical properties of arterial tissues. First, the methodology is presented together with the description of the customizations of the laboratory equipments together with the test rig. In addition, attention is focused on the results of mechanical tests of intraluminal thrombus, where the results of both uniaxial tensile tests and equbiaxial testing are presented. Also the influence of distance ILT from the lumen on the mechanical properties of the thrombus is examined. Another area of interest is the investigation of the effect of elastase on the chnage of mechanical properties of pig aorta. In this case, porcine aortas are experimentally tested only by biaxial testing, and the time of elastase action to alter the mechanical properties is analyzed so that the resulting tissue has a similar stress-strain response as aneurysmal tissue. Finally, the results of experimental measurements, limitations and other possible ways of research are summarized.
|
|
|
Konstitutivní modelování tepenné stěny
Horný, L. ; Chlup, Hynek ; Adámek, T. ; Zitny, R. ; Macková, H.
Inflation tests and uni–axial extension tests of arterial tissue were performed. Experimental data were used in nonlinear regression analysis to identify material model. Arterial tissue was assumed to be incompressible hyperelastic material. 5–parameter strain energy density function based on combination of isotropic Neo–Hookean expression and Fung–type orthotropic expression was used. Computational model for material parameters identification was based on thick–wall tube with axial pre–strains. Residual strains were included. Internal structure of arterial wall was not considered. Fitted material models correspond to experimental data very well.
|
|
|
The Common Mechanical Factor of Shape Memory
Kafka, Vratislav ; Vokoun, David
Shape memory is observed in elastic bodies, springs, bi-metals, in many mechanical actuators, such as thermostats and others, in many biological materials, and in special "shape memory alloys". It is shown that all systems or materials have one common factor: at least one elastic or thermoelastic continuous substructure, in the case of "shape memoryalloys" this substructure is on the atomic scale.
|
guest ::
