1,721,204 research outputs found
Applications of computational biomechanics in reconstructive cranio-maxillofacial and plastic surgery
No full textIn recent years, the advent of microsurgical transplantation made new surgical techniques with autologous transplants possible. However, these procedures rely mainly on the individual skills and visual judgment of the operator and there is a distinct lack of objective planning tools. Computer aided applications, like the ones proposed in this work, bear the potential to improve the surgical outcome decisively and may provide valuable guidelines for the surgeons at the stage of operation planning. Part I gives a general introduction to the field of research that is addressed in this work. Part II is showing the clinical tasks and the current state of research in the relevant fields. In this part chapter 4 describes the modeling of the human anatomy under special consideration of the constitution of biological tissues. Though this is a medical topic it is important to mention that it is described from the perspective as an engineer. In chapter 5 the theoretical constitutive modeling is described in detail with the corresponding mathematical formulations. Furthermore, comprehensive reviews of literature on material parameters of the relevant biological tissues are included. In part III the methods are shown which have been utilized in this work and the specially developed and implemented algorithms are described. In chapter 6 the utilized imaging technologies and experimental devices are shown and their working principles are explained. The algorithms that were developed in the scope of this work are explained in detail in chapter 7. The parts IV and V comprehend the concrete applications of the developed methods and processes to clinical tasks in the two different disciplines of medicine: In chapter 8 a method of in vivo acquisition of the constitutive parameters of the female breast is introduced. The work uses a combination of MR images acquired in prone position and 3-D surface scans in upright standing position. Based on a collective of healthy volunteers the applicability of the method could be shown. Different material formulations that have been published in literature were utilized and the suitability for the test person collective was evaluated. Furthermore, an optimization of material parameters has been undertaken to find an optimal set of constitutive parameters individually for each volunteer. Chapter 9 shows a computer aided method to predict the flap volume at the abdominal region for breast reconstructions with tissue transplants. This study was conducted based on CT-Angio data of patients who underwent breast reconstruction surgery. Eventually, chapter 10 shows two applications of computational approaches in the planning stage of reconstructive surgeries after tumor removal. In this scope two innovative surgical procedures that have been introduced by other researchers were transferred to process chains that use combinations of modern imaging techniques and numerical simulations to find the optimal shape of a transplant flap from the abdominal region to reconstruct the missing breast. The first chapter of the cranio-maxillofacial part, chapter 11, introduces a method of statistical evaluation of the complex shape of the mandible bone. The outcome of an anatomical study on 65 mandibles from body donors is presented and a method is described that allows to produce a shape variable mesh of consistent topology which is essential for the generation of standardized finite element simulation models that may be individualized to represent the individual anatomical shapes of particular patients. The impact of geometric variations on the load distribution on an average model of the mandible was investigated in that study. Chapter 12 describes the biomechanical experiments that have been performed both with artificial bone models and with ex vivo mandible specimens that have been reconstructed with bone transplants from the fibula, scapula or iliac crest, respectively. The specimens were loaded in an experimental device that allowed the modeling of biting loads and muscle forces that mimic the physiological strain of the mandible bone. From these studies several valuable clinical conclusions could be drawn for mandibular reconstructions with autologous bone grafts. In chapter 13 a clinical application of a developed computer aided algorithms is shown. The reconstruction of an orbital floor in a patient after traumatic fracture with a calvarial bone graft from the patient’s skull was planned with the novel approach. Out of multiple different positions of the harvesting region, the best accordance towards the desired shape of the orbital floor that needs to be reconstructed is calculated. The donor region that was proposed by the computer was used as a guideline in an actually performed surgery
A micromechanical constitutive model and instability analysis of dielectric elastomer
No full textThis work develops a micromechanical constitutive model for electroelasticity effects in the dielectric elastomer. The coupled electroelasticity properties and behaviours are modeled based on polymer chain segment response with respect to an applied external voltage or electric field. The micromechanical model is then implemented in the three energy systems of membrane structure, i.e., membranal, stretch gradient, and bending energies, in order to obtain their corresponding instability boundaries for wrinkling, thinning or snap-through, and bending modes, respectively. From comparison with the experimental results, the model shows a good capability to predict these of instability boundaries. This is especially true for the thinning instability where it can be used to predict failure of dielectric elastomer membrane due to electrical breakdown
Experimentelle Untersuchungen und Beschreibung des deformationsinduzierten anisotropen Werkstoffverhaltens von verstärkten Elastomeren
No full textThe first uniaxial extension of reinforced rubber materials causes stress softening (the Mullins effect) not only in this loading direction but in any other direction of subsequent extension. However, the Mullins effect is less pronounced after a change of loading direction than in the direction of the initial loading. Due to the deformation history mechanical behaviour of the the rubber vulcanisate is distinctly anisotropic. To trace the anisotropic Mullins effect, firstly the standard test method for characterization of the isotropic mechanical behaviour must be extended. The appropriate type of specimen enables to perform multiple load steps with alternating load directions. After repeated stretching in the same direction, a subsequent first uniaxial loading in any other direction is characterized by a stiffer stress–strain behavior compared with the stabilized curve of the previous primary load. Due to the high relevance of the primary permanent set for this experimental method and their strong influence on the secondary load step, the direction dependent consideration is studied. On behalf of a more comprehensive evaluation of the anisotropic Mullins effect the role of active filer like carbon black and silica was investigated. In particular, not only the influence of basic properties of carbon blacks, such as specific surface area or structure, but also the influence of the amount incorporated was determined experimentally. The comparison of carbon black and silica filled vulcanisates provided more insights in the anisotropic material behaviour of reinforced elastomers. For the anisotropic (primary) Mullins effect a similar behaviour can be detected regarding the well-known one-dimensional Mullins effect. In summary, the vulcanisates soften high levelly in the primary load (one-dimensional Mullins effect) and in this way, they show a high level deformation induced anisotropy of the Mullins effect. For the visualization of the direction-dependent stress softening the comparison of the Mullins effect and the applied primary strain energy density is proposed to identify the resulting anisotropic material properties of the elastomer
Mikromechanische Materialmodellierung von unverstärkten Kunststoffen für finite Deformationen
No full textThis doctoral thesis deals with the evolution of an effective constitutive model for polymers under finite deformation. Scope of this model is to capture different behaviours under multiple loading directions and strain rates. Especially volumetric change during plastic deformation based on void growth during tensile loading should be implemented. Besides of deriving a new constitutive model, an implementation in commercial FE solver LS-DYNA is done. Basic static loading directions are extended to dynamic tension loading to identify all necessary parameters. For validation purpose these constitutive equations use multiaxial static experiments. In order to identify mechanical material properties for semi-cristalline polypropylene (PP), static loadings like uniaxial tensile tests, uniaxial pressure and simple shear are carried out at room temperature. Optical measurements are done for thickness direction besides of longitudinal and lateral direction to quantify the amount of volumetric strain during plastic deformation. Dynamic loadings of 100 and 300 1/s identify strain rate dependent yielding of tensile deformation. Based on constitutive equations of Haward and Thackray, as well as Eyring and Argon, to describe an one-dimensional yielding of polymers, a three-dimensional network model for polymers is derived by Boyce et al. and Leonov. Their fundamental work on a split into an intermolecular resistance to segment rotation and entropic resistance to molecular alignment is adapted to model softening behaviour and pressure sensitivity. Further extension is based on a coupling of plastic hardening behaviour and void growth during plastic deformation represented by change of first and second invariant of left Cauchy-Green deformation tensor. This combination is introduced as volumetric strain softening. A non-linear dependency of initial shear yield strength on strain rate is implemented to take rate effects of yield process into account. To finalize this thesis a parameter identification is done after implementing into commercial FE solver LS-DYNA, followed by a validation based on multiaxial static load cases
Multi-scale constitutive modeling of textile fabrics
No full textThe present work is dealing with the problem of how to constitutively model textile fabrics and how mechanical behaviors can be predicted by offering additional data. Modeling textiles with prominent anisotropic properties is a complicated process including abundant experimental investigations. The existing phenomenological models by fitting to measured curved of tested materials only can model the behaviors of the considered material with good results, but cannot predict new material behaviors when mechanical properties at the microscale are altered. Therefore, it is demanding to have a constitutive method based on additional information that still can predict various material properties well for both tested and untested materials. The contribution aims to establish a mesoscopic constitutive model based on a deep learning framework trained from virtual experimental data which is directly fitted against an experimental force response and the corresponding two-dimensional displacement field obtained from arbitrary loading. The proposed model is capable of predicting the in-plane material response of materials under arbitrary multi-axial loading conditions. Later, we propose an efficient multiscale modeling approach based on the application of an artificial neural network for the computation of a representative volume element (RVE) at the lower scales. After the network is trained with a wide range of stress-strain data provided from mesoscopic simulations, it has the ability to provide a fast and accurate response to a strain value for a stress value. The trained network results in an ANN-provided (implicit) constitutive law, which can then be fed into the macroscopic finite element framework for the computation of the nonlinear materials. In the manufacturing process of fabrics, the change in curvature of yarns induces residual stress, and it affects dry fabric behaviors. We present an approach to introduce residual stress. A transformation mapping between the two meshes and residual stress can be introduced by nodal displacement from the stress-free RVE to the woven fabric RVE. Then, we seek an equilibrium state of the woven fabric RVE with contact between yarns in simulation. The obtained configuration (in equilibrium) is with residual stress. The processes are validated by different test data with artificial data sets and show good results
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