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Item type:Publication, Access status: with files , Defect localization and modulation indices separation in Aluminum structures using vibro-acoustic modulation This study addresses the mathematical description of the Vibro-Acoustic Modulation (VAM) response to separate modulation indices (MIs) and introduces a new modulation model that describes phase shifts in the measured response. Furthermore, a robust, baseline-free method for the localization of real cracks in plate-like aluminum structures is presented, based on a novel short-time VAM-based localization algorithm that tracks the first upper sideband at ω + Ω. Both topics are described theoretically and validated experimentally.Publicationtype: Doctoral Thesis34 28 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Dynamik der Hüftschaftimplantation(2026); In cementless hip arthroplasty, mallet blows are used to advance the rasp during the preparation of the femoral canal, to implant the prosthesis, and to join the implant components. The strength of the mallet blows is crucial for a stable press fit, while the dynamics of the involved instruments and the interaction with the femur-tissue system are complex and individual. Therefore, the aim of this study is to investigate the influence of patient- and surgical approach-specific characteristics on the dynamic behavior of the femur-tissue system as well as the force transmission of surgical instruments. The experimentally determined parameters of the femur-tissue system of the human bodies examined varied depending on the positioning on the operating table, which in turn depended on the surgical approach. However, for both surgical approaches examined, the values resulted in a relatively low natural frequency (lateral: 6.7 Hz, direct anterior: 9.6 Hz). Consequently, metal-on-metal mallet blows or comparable short impulses from automated surgical impaction tools lead to overcritical excitation. As a result, the implant is inserted into the femur before the bone itself even starts moving. The experimental and numerical analyses revealed a significant attenuation of the mallet force by the impactor. The design of the impactor had a significant influence on the force transmission: lighter impactors transmitted higher peak forces than heavier variants. Even minor design modifications resulted in differences in the transmitted force by a factor of two. Due to the overcritical excitation, it is not necessary to make intraoperative adjustments of the surgical technique to the patient's positioning or to the mass and stiffness of the surrounding soft tissue. However, the utilization of different impactors can lead to considerable deviations in force transmission. Without adequate briefing for orthopedic surgeons, changing the implant system, the associated instruments, or the surgical approach can increase the risk of complications such as periprosthetic fractures or aseptic loosening.Publicationtype: Doctoral Thesis49 36 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Systematic design, modeling and simulation of a biped robot(2025); The bipedal walking robot, CENTAUROB, is designed to perform the walking over random terrains. It features two symmetrically assigned 6-DOF parallel kinematic mechanisms as legs. This work represents the development phases from structure synthesis, kinematic and dynamic modelling to walking stability. Different formations of 6-DOF legs were analysed through dissection of basic kinematic chains, and the design adopting double Cardan joints was demonstrated. The passive rotation problem was identified and solved through complete kinematic modelling. The inverse dynamic model was built, and the semi-dynamic walking stability was ensured by analysing foot rotation indicator with the load-split algorithm.Publicationtype: Doctoral Thesis95 58 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , 32 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Hydrodynamic aspects in the design of lift-based wave energy convertersPublicationtype: Doctoral Thesis90 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Experimental investigations of the mechanics of gassy sands - testing methodology, shear tests, and imaging(2024); Most soils in the offshore area contain small amounts of gas; either originating from bacterial metabolistic processes or from thermogenic gas production in deeper layers and subsequent upward migration. For a variety of research questions, for example in the fields of submarine slope stability and carbon capture and storage, the interaction of soil grains, pore water, and gas becomes a relevant aspect. However, the impact of occluded gas bubbles on the soil's stress-strain behaviour has not been thoroughly investigated to date, with the existing studies focusing only on very specific boundary conditions. Particularly the characteristics of the gas phase in the pore space and how they influence the macroscopic stress-strain behaviour remain a matter of theoretical assumptions not validated to date. A holistic assessment of the implications of marine gas occurrence is therefore not possible with the existing knowledge and a requirement for further laboratory investigations can be derived. The overarching objective of this thesis is to forward the general understanding of gassy soil mechanics to allow for a holistic understanding of geological systems such as the continental slopes. A micro-to-macro approach was chosen for the experimental investigations. This approach allows for the analysis of microstructural controls for the macroscopic stress-strain behaviour of gassy soils and therefore offers the potential for a more fundamental understanding of the soil mechanical processes. To this end, a sample preparation methodology is developed which is subsequently employed in macroscopic CU triaxial tests as well as in microscopic microCT experiments. Finally, all experimental results are combined in a joint interpretation. The conclusions drawn from the investigations conducted within the scope of this thesis can be summarised as follows: The advancement of the axis-translation method for gassy soils, as introduced in this thesis, is suitable for a reliable preparation of gassy soil samples under different experimental boundary conditions. The application of the method on two poorly graded model sands was successful in the triaxial as well as microCT experiments. Within the test series two different gas morphologies can be identified in the two investigated gradations: gas clusters that grow by capillary invasion of a stationary grain skeleton in medium sand and macropores within a saturated soil matrix that grow by fracturing in fine sand. Therefore, the grain size is the governing factor for the pore habit of the gas phase. The basic soil mechanical assumptions regarding the distribution of the gas phase within the pore space differ from the observations in this thesis; e. g. in contrast to the literature assumptions capillary forces play a great role in gassy sands. Consequently, the mechanical implications are likewise diverging. The shearing behaviour is impacted differently in the two gradations. The friction angle is slightly elevated in the gassy medium sand compared to its saturated equivalent. In the gassy fine sand, a substantial capillary cohesion is induced by the gas phase. Both model sands fail at significantly lower stress levels than the saturated baseline tests. Thus, a general negative impact of the gas can be deduced.Publicationtype: Doctoral Thesis115 103 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Patientenorientierte Implantatwahl in der RevisionsknieendoprothetikPublicationtype: Doctoral Thesis168 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Zur Geräuschentwicklung bei reibungserregten Schwingungen in KarosseriedichtsystemenPublicationtype: Doctoral Thesis10 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , A time domain boundary element method for fluid-structure interaction analysis on a discontinuous surfaceIn this thesis, the development of the two-dimensional time domain solver cBEM for modeling linear hydrodynamic problems is described. The approach was based on Potential Flow Theory and applied Boundary Integral Equations with that the governing Boundary Value Problem was solved and the interactions of body and surface gravity waves were modeled. The embedding of the High-Order Spectral procedure into the Boundary Element Method in the symmetric Galerkin formulation represents an innovative coupling method for the treatment of hydrodynamic problems. The explicit account of a surface discontinuity represented by the body in the free surface boundary domain, the strategy of incorporating the High-Order Spectral approach in the Boundary Element Method, and the development of suited desingularization techniques for kernel functions up to hypersingular order had been considered within this thesis. As higher-order basis functions had been used for the approximation of solution function space and geometry, the results of cBEM in terms of the boundary quantities, the potential and its normal derivative, were of good accuracy. cBEM can be accounted as the foundation of a highly efficient three-dimensional nonlinear Boundary Element Method solver with possible future application e.g. in the research fields of analyzing nonlinear body motion due to nonlinear wave excitation in numerical wave tanks and for the optimization of ship hull geometries in the early design phase. For offshore operations, the deterministic wave and motion prediction would help to increase safety and cBEM was designed to fit into a holistic approach containing wave inversion, nonlinear wave propagation, and motion prediction. The efficient evaluation of the mixed Boundary Value Problem with highly efficient methods would allow predictions over a period of time that meets industry expectations. The work is structured as follows. In the introduction, the scope of the work is highlighted and limitations and innovations are outlined. In the literature review presented thereafter, the Boundary Element Methods used in Marine Hydrodynamics are categorized into three main streams, and related research contributions are summarized. Based on this, the research gap is outlined and a global hypothesis defined. The research hypotheses structure and summarize the main concerns according to the global hypothesis and give the frame for the development of cBEM. In chapter two, the theory of Boundary Integral Equations and numerical tools accounted for in the Boundary Element Method approach are summarized. Furthermore, the Linear Wave Theory and the High-Order Spectral method as well as the basic problems of wave-body interaction are described. On the mathematical foundation and the introduction of the computational methods represented here, the steps for the development of the cBEM solver are presented in chapter three. After an overview of the steps, the Boundary Integral Equations for the different solvers are described. The direct formulation of cBEM is given and the methods accounted for in pre-, post-, and processing are introduced. It follows the verification of the approaches including the free surface solver fsBEM, the coupled solver cBEM, and the transient cBEM solver for the continuous and discontinuous surfaces. The validation section, chapter four, shows the application of cBEM for hydrodynamic problems. The submerged and free surface piercing geometries were considered and linear problems with a forced oscillating body, the free surface elevation over a submerged cylinder, and the diffraction of waves due to a body below were compared with literature references. The final discussion in the review of the research hypotheses follows and the conclusions are drawn. By identifying the status of the work described herein, future steps are pointed out. The extension of cBEM to a three-dimensional solver, equipped with efficient solving strategies and nonlinear extensions is outlined and future applications are given. Their potential is highlighted and the required developments are depicted.Publicationtype: Doctoral Thesis79 124 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Physical explanation for vibro-acoustic modulation in a structure due to local and global nonlinearities(2023); This dissertation suggests a physical explanation revealing why any elastic nonlinearity may cause vibro-acoustic modulation (VAM) in a dynamic system that is excited by two waves simultaneously (Xω and XΩ) in case the corresponding frequencies differ significantly (ω >> Ω): The low frequency (LF) vibration XΩ activates the nonlinearity and varies quasi-statically the system’s natural frequency ω0 corresponding to the dominant mode shape. The amplitude and phase of the ultrasonic system response depend on the natural frequency ω0 and are therefore modulated with low frequency Ω. This analytical explanation is built upon the assumption that the system reaches a steady-state vibration (constant amplitude and phase over time) due to ultrasonic excitation Xω before the LF vibration XΩ can change the dy- namic properties of the system significantly. The fulfillment of this assumption is validated experimentally and numerically for an aluminum plate-like structure and excitation frequencies Ω = 10Hz · 2π and 180kHz · 2π < ω < 230kHz · 2π: It is shown that the minimum and maximum amplitudes in the modulated ultrasonic system response match the respective steady-state amplitudes under static loading with a negligible error that is <1%. The beauty of the explanation is that it does not only work for the local nonlinear-elastic behavior of a defect, but also for any non-damage-induced (local or global) nonlinearity in the system which may cause modulation as well. The explanation is employed to conclude and demonstrate the following five analytical findings: 1) Any elastic nonlinearity may cause amplitude modulation and phase modulation at the same time. 2) The nonlinearity can cause envelope functions in the modulated system response with a variety of different characteristic shapes. 3) The maximum amplitude in the modulated system response in time domain can either coincide with the minimum or the maximum stress state corresponding to the LF vibration XΩ. 4) Different nonlinearities in the same system can cause contrary modulations that neutralize each other. 5) The steady-state evaluation of the ultrasonic system response allows—theoretically—VAM applications without exciting the LF vibration XΩ. Finally, three nonlinearities in the same system are investigated separately: Two non-damage-induced nonlinearities (the variation of geometric stiffness and the nonlinear-elastic material behavior of aluminum) and one damage-induced nonlinearity (the contact of fatigue crack surfaces) are considered. Numerical simulations are carried out to quantify and compare the modulation they cause individually in the system response. The results comply with the analytical findings concluded from the physical explanation and emphasize that the conventional VAM evaluation for damage monitoring is error prone.Publicationtype: Doctoral Thesis103 199