Search

When the elastic properties of structured materials become direction-dependent, the number of their descriptors increases. For example, in two-dimensions, the anisotropic behavior of materials is described by up to 6 independent elastic stiffness parameters, as opposed to only 2 needed for isotropic materials. Such high number of parameters expands the design space of structured materials and leads to unusual phenomena, such as materials that can shear under uniaxial compression. However, an increased number of properties descriptors and the coupling between shear and normal deformations render the experimental evaluation of material properties more challenging. In this paper, we propose a methodology based on the virtual fields method to identify six separate stiffness tensor parameters of two-dimensional anisotropic structured materials using just one tension test, thus eliminating the need for multiple experiments, as it is typical in traditional methods. The approach requires no stress data and uses full-field displacement data and global force data. We show the accuracy of our method using synthetic data generated from finite element simulations as well as experimental data from additively manufactured specimens., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Team Sid Kumar

Atherosclerosis is a lipid-driven inflammatory disease of the arteries. It causes increased thickness of the intima layer of an artery, due to build-up of, among others, lipid deposits. Eventually, this could cause rupture of the blood vessel’s wall, leading to thrombus forming and possibly to clinical events. Clinically, the degree of stenosis (degree of narrowing of the lumen) is used as a metric for risk assessment. However, narrow arteries do not necessarily rupture, while wide arteries can still rupture. Since rupture is a mechanical event, (mechanical) stress in the arteries could be a better metric to assess rupture. Stress could be calculated through a chosen constitutive model. However, these constitutive models require patient-specific constitutive parameters, which presently are challenging to obtain. To retrieve the patient-specific constitutive parameter(s), the Virtual Fields Method (VFM) could be used. This is a method based on the principle of virtual work. By minimizing the difference between the internal and external virtual work, the constitutive parameter(s) can be calculated. In this study, five atherosclerotic plaques were modeled with a neo-Hookean material model, subjected to intraluminal pressure in a Finite Element (FE) environment. The nodal deformations were used to apply the VFM in MATLAB. This was done at three levels: level 1 assumed that, for each model, a single constitutive parameter dictated the stress behavior of the entire atherosclerotic model. At level 2, the models were divided into four mechanically relevant components: intima, wall, calcified tissue, and lipid component. The ground truth for the constitutive parameter of each component was unique (120 kPa, 250 kPa, 5 kPa, and 423 kPa for intima, wall, lipid, and calcified tissue component respectively). For each of the components, a constitutive parameter was calculated. At the last level, the nodes, Mechanical Engineering | BioMechanical Design

Atherosclerosis is a lipid-driven inflammatory disease of the arteries. It causes increased thickness of the intima layer of an artery, due to build-up of, among others, lipid deposits. Eventually, this could cause rupture of the blood vessel’s wall, leading to thrombus forming and possibly to clinical events. Clinically, the degree of stenosis (degree of narrowing of the lumen) is used as a metric for risk assessment. However, narrow arteries do not necessarily rupture, while wide arteries can still rupture. Since rupture is a mechanical event, (mechanical) stress in the arteries could be a better metric to assess rupture. Stress could be calculated through a chosen constitutive model. However, these constitutive models require patient-specific constitutive parameters, which presently are challenging to obtain. To retrieve the patient-specific constitutive parameter(s), the Virtual Fields Method (VFM) could be used. This is a method based on the principle of virtual work. By minimizing the difference between the internal and external virtual work, the constitutive parameter(s) can be calculated. In this study, five atherosclerotic plaques were modeled with a neo-Hookean material model, subjected to intraluminal pressure in a Finite Element (FE) environment. The nodal deformations were used to apply the VFM in MATLAB. This was done at three levels: level 1 assumed that, for each model, a single constitutive parameter dictated the stress behavior of the entire atherosclerotic model. At level 2, the models were divided into four mechanically relevant components: intima, wall, calcified tissue, and lipid component. The ground truth for the constitutive parameter of each component was unique (120 kPa, 250 kPa, 5 kPa, and 423 kPa for intima, wall, lipid, and calcified tissue component respectively). For each of the components, a constitutive parameter was calculated. At the last level, the nodes, Mechanical Engineering | BioMechanical Design

La vibration et le rayonnement acoustique des structures sont des problèmes d'une importance capitale en ingénierie. Souvent, on s'intéresse à identifier les sources vibratoires responsables de ces phénomènes afin de les contrôler ou les éliminer. Cependant, des contraintes pratiques peuvent rendre impossible la mesure directe de ces sources à l'aide de capteurs traditionnels. Une méthode indirecte est alors employée pour déterminer les forces d'excitation à partir d'une mesure du mouvement vibratoire engendré sur la structure. Ce travail doctoral présente une telle approche permettant de cartographier spatialement le chargement dynamique sur des plaques minces et des membranes en utilisant la méthode des champs virtuels, une méthode dédiée aux problèmes d'identification. Pour éviter de modifier la réponse de la structure, les mesures vibratoires sont effectuées à l'aide d'une technique sans contact. Présentement, l'instrument le plus courant pour ce genre d'application est le vibromètre laser à balayage. Le vibromètre a été utilisé précédemment pour valider l'approche d'identification de chargements avec la méthode des champs virtuels, mais l'action de balayer successivement les points de mesure crée deux inconvénients : (1) le temps d'acquisition est long pour un grand maillage de points et (2) les données fournies par la vibrométrie limitent généralement l'application de la méthode d'identification à des excitations stationnaires en espace et en fréquence. Afin de contourner ces limitations, une technique optique appelée la déflectométrie est employée pour mesurer la réponse vibratoire. Elle permet de faire une acquisition simultanée à tous les points de mesure. Ces mesures sont aussi résolues en temps grâce à l'utilisation d'une caméra haute vitesse. Par conséquent, le temps d'acquisition est réduit à seulement quelques secondes pour des mesures qui prendrait plusieurs heures à balayer par vibrométrie laser, tout en fournissant un volume d'informations supérieur, Vibration and the acoustic radiation of structures are issues of utmost importance in engineering. Often, we are interested in identifying the vibration sources responsible for these phenomena in order to control or eliminate them. However, due to practical constraints, direct measurement of these sources using traditional sensors may be impossible. Therefore, an indirect method is used to determine the excitation forces from a measurement of the induced vibration on the structure. This doctoral work presents such an approach for spatially mapping the dynamic loading on thin plates and membranes using the virtual field method, a dedicated method for identification problems. To avoid modifying the response of the structure, the vibration measurements are performed using a contactless technique. Currently, the most common instrument for this type of application is the scanning laser vibrometer. The vibrometer was previously used to validate the loading identification approach using the virtual field method, but the action of successively scanning the measurement points has two drawbacks: (1) the acquisition time is long for a large mesh of points and (2) the data provided by vibrometry generally limit the application of the identification method to stationary excitations in space and frequency. In order to circumvent these limitations, an optical technique called deflectometry is used to measure the vibration response. It enables simultaneous acquisition at all measurement points. Furthermore, these measurements are time-resolved due to the use of a high-speed camera. As a result, the acquisition time is reduced to only a few seconds for measurements that would take several hours to scan by laser vibrometry, while also providing a higher volume of information. By combining deflectometry and the identification method, this study demonstrates the ability to correctly solve loading distributions at small spatial scales applied to a structure over time. Validati

The majority of cardiovascular clinical events, which are the main causes of mortality and morbidity worldwide, are caused by atherosclerotic plaque rupture. This biomechanical event occurs when the local plaque stresses exceed its strength. The plaque stresses can be assessed by computational models to predict these events. Current approaches to obtaining the plaque material stiffness properties that these models require as input have large computational costs and are therefore far from being implemented for clinical use. This study aims to develop, validate, and apply for the first time, an approach to obtaining the material stiffness properties of atherosclerotic plaque tissue much faster by employing the virtual fields method (VFM). With this method, the virtual work principle is employed with boundary problem specific, kinematically admissible virtual fields to solve energy balance equations for the material stiffness parameters that are of interest. In this study a method is presented for obtaining the virtual fields for the specific application of intraluminally pressurised atherosclerotic plaque tissue. For the purpose of validation, full field displacement maps were computed at 100 mmHg using Finite Element (FE) models based on histological slides of atherosclerotic plaque tissue. To mimic a realistic situation, the resolution and noise levels of a clinical and high frequency ultrasound scanner were used. Although higher resolution deformation maps with smaller noise levels were shown to provide more accurate results, the VFM-based technique demonstrated good performance for both the high frequency and clinical ultrasound scanner settings tested. VFM was also used in a single case study to estimate the c1 material parameter for a Neo-Hookean incompressible material model in the case of an atherosclerotic human coronary artery. The estimated c1-values for this case were: 21.5 kPa for diseased intima, 13.3 kPa for lipid, and 23.6 kPa for

The majority of cardiovascular clinical events, which are the main causes of mortality and morbidity worldwide, are caused by atherosclerotic plaque rupture. This biomechanical event occurs when the local plaque stresses exceed its strength. The plaque stresses can be assessed by computational models to predict these events. Current approaches to obtaining the plaque material stiffness properties that these models require as input have large computational costs and are therefore far from being implemented for clinical use. This study aims to develop, validate, and apply for the first time, an approach to obtaining the material stiffness properties of atherosclerotic plaque tissue much faster by employing the virtual fields method (VFM). With this method, the virtual work principle is employed with boundary problem specific, kinematically admissible virtual fields to solve energy balance equations for the material stiffness parameters that are of interest. In this study a method is presented for obtaining the virtual fields for the specific application of intraluminally pressurised atherosclerotic plaque tissue. For the purpose of validation, full field displacement maps were computed at 100 mmHg using Finite Element (FE) models based on histological slides of atherosclerotic plaque tissue. To mimic a realistic situation, the resolution and noise levels of a clinical and high frequency ultrasound scanner were used. Although higher resolution deformation maps with smaller noise levels were shown to provide more accurate results, the VFM-based technique demonstrated good performance for both the high frequency and clinical ultrasound scanner settings tested. VFM was also used in a single case study to estimate the c1 material parameter for a Neo-Hookean incompressible material model in the case of an atherosclerotic human coronary artery. The estimated c1-values for this case were: 21.5 kPa for diseased intima, 13.3 kPa for lipid, and 23.6 kPa for