In the design and development stages of mechanical structures, it is important to predict the dynamic characteristics of modified structures. Since time and cost are critical in design and development stage, structural modification methods predicting the dynamic responses of modified structures from those of the original structure and modification properties are very important, especially for large systems. In this thesis structural modification methods are investigated and an effective structural modification method for modifications with additional degrees of freedom is adapted to structures with distributed modifications and the performance of the method is investigated. A software program is developed in order to apply the structural modification method with additional degrees of freedom. In the software, the dynamic response of the modified structure is predicted by using the modal analysis results of ANSYS for the original structure and dynamic stiffness matrix of the modifying structure. In order to validate the approach used and the program developed, the dynamic analysis results obtained for modified structures by ANSYS are compared with those obtained by using the software. In order to investigate the performance of the structural modification method in real applications, the method is applied to a scaled aircraft model, and the results are compared with experimental results. In order to demonstrate the importance of using the structural modification method with additional degrees of freedom for distributed modification, lumped and distributed models are used for a distributed modification and results are compared. It is concluded in this study that using structural modification methods with additional degrees of freedom for a distributed modification increases the accuracy of the results, and it is observed that the method adapted is efficient for local modifications.

Osteoporosis is a metabolic bone disease that needs to be properly diagnosed. The current diagnosing procedure of osteoporosis is based on the mineral density of bones measured by common methods such as dual energy X-ray absorptiometry (DXA). However, due to the deficiencies and limitations of these common methods, investigations on the utilization of other non-invasive diagnosing methods have been executed. For instance, using vibration measurements seems to be a promising technique in diagnosing metabolic bone diseases such as osteoporosis and also in monitoring fracture healing. Throughout this study, bone structural modal parameters obtained from vibrations experiments with decreasing mineral density are examined and therefore, it is aimed to find a new approach to detect osteoporosis or progressing osteoporosis by investigating a relation between structural dynamic properties and mineral density of bone. The main advantage of this study is that loss factor, which is an inherit property of bone, is investigated since in the previous studies mainly the changes in natural frequency of bones with the state of osteoporosis is examined. In this thesis, both in vitro and in vivo experiments are carried out on human tibia specimens. The measured frequency response functions (FRFs) are analyzed using modal identification techniques to extract the modal parameters of the human tibia. The results obtained from in vitro experiments show that loss factor may be a powerful tool in diagnosing osteoporosis, however due to the difficulties encountered in the case of in vivo experiments makes the use of this parameter as a diagnosing tool difficult. It is also seen from in vivo experiments that there is a weak correlation between the natural frequencies of tibia and BMD measurements of patients. Therefore, in order to investigate the parameters affecting the natural frequencies of tibia, finite element (FE) model of human tibial bone is constructed. Using this FE model tibia, the effect of boundary conditions of experiments and geometry of the bone on natural frequencies of bone is examined. These analyses show that the effect of both boundary conditions and geometry of tibia is very high. Therefore, it is concluded that if the necessary conditions are satisfied, the using natural frequency information of tibia seems to be a possible and practical method that can be used to detect progressing osteoporosis. Also, using the FE model of tibia, the changes of natural frequencies of tibia with the variation in elastic modulus are investigated.

Hydrodynamic and thermal analyses performed during the conceptual design of an unmanned underwater vehicle are presented in this study. The hull shape is determined by considering alternative shapes and the dimensions are determined from the internal arrangement of components. Preliminary thermal analyses of the watertight section are performed with a commercial software called FLUENT to check the risk of over-heating due to the heat dissipation of devices. Performance of the proposed hull design is analyzed by FLUENT. Before simulations of the vehicle, validation studies are performed. Models 4159, 4158 and 4154 of Series 58 are chosen as the experimental reference. Their total resistance coefficients are compared with the results of the validations analyses. Mesh densities, turbulence models, near wall modeling approaches and inlet turbulence intensities are varied to understand their effects on the accuracy of predictions. A suitable turbulence modeling approach is chosen to analyze forward and vertical motions of the vehicle to check whether speed requirements are fulfilled. Hull configurations with and without appendages are used to observe their effects on total drag. It is observed that the proposed design satisfies speed requirements of the vehicle and no overheating is expected in the watertight section.

Shaped charges are explosive devices with a high penetration capability and are used for both civilian and military purposes. In civilian applications shaped charge devices are used in demolition works, oil drilling and mining. In the military applications, shaped charges are used against different kinds of armors, primarily as anti-tank devices. This thesis work involves the modeling and simulation of shaped charge devices, with the focus being on anti-tank warhead design. Both numerical simulation and analytical calculation methods are used to predict shaped charge performance / in the aspects of jet formation, breakup and penetration. The results are compared within themselves and with the data available in the literature. AUTODYN software is used for the numerical simulations. Different solver and modeling alternatives of AUTODYN are evaluated for jet formation and penetration problems. AUTODYN&rsquo / s Euler solver is used to understand how the jet formation is affected by the mesh size and shape and the presence of air as the surrounding medium. Jetting option in the AUTODYN-Euler simulations are used to simulate jet formation as an alternative to simulations performed using AUTODYN&rsquo / s Euler solver. In the jetting option liner elements are modeled as Lagrangian shell elements, rather than Eulerian elements. Analytical codes are written to study the jet formation, breakup and penetration processes. Many alternative formulas that can be used in the analytical calculations are listed and discussed. Parameters of these formulas are varied to investigate their effects on the results. Necessary constants for the analytical formulas are obtained using the results of AUTODYN simulations.

In this thesis, automatic routing technologies for unmanned air vehicles are investigated. A route planner that minimizes the fuel consumption and maximizes the survivability is developed. While planning the route, using more than one objective entails the auto-routing problem to multi-objective optimization considerations. In this work, these considerations are handled with search algorithms. In order to assess the route options, a fuel consumption model and a survivability model are utilized for the route planner. As the assessment models are established, required computational time is taken into account without deteriorating the fidelity.

The objective of this study is to develop and implement practical and accurate methods for prediction of the workpiece dynamics during a complete machining cycle of the workpiece, so that FRFs of the workpiece can be used in chatter stability analysis. For this purpose, a structural modification method is used since it is an efficient tool for updating FRFs due to structural modifications. The removed mass is considered as a structural modification to the finished workpiece in order to determine the FRFs at different stages of the process. The method is implemented in a computer code and demonstrated on representative parts such as turbine blades. The predictions are compared and verified with the data obtained using FEA. The FRFs are used in chatter stability analyses, and the effect of part dynamics on stability is studied.

Rapid prototyping methods embrace a family of manufacturing methods that are developed to speed up the prototyping stage of product design. The sole needed input for production being the solid model of the part, mold/tool-free production characteristics and the geometric part complexity that can be achieved due to layer-by-layer production have extended the applicability/research areas of these methods beyond prototyping. Local pore formation in part that occurs as a result of the discrete manufacturing nature of rapid prototyping methods can be viewed as an opportunity for material development. In this thesis, the manufacturing-internal (porous) structure-mechanical property relations of porous materials are investigated. These porous parts are produced via Selective Laser Sintering (SLS) which is a rapid prototyping method. The elastic modulus, tensile strength, rupture strength and Poisson&rsquo / s ratio of uniform porous specimens with known porosities are determined through standardized mechanical tests for polymeric materials. The mechanical property variation profiles in graded materials are determined using the mechanical properties of uniform parts. The mechanical behavior of uniform and graded materials under applied loads are modeled using finite element method and simulation results are compared to the results of mechanical tests performed on graded materials. In addition, feasibility of producing resin filled composite parts from these uniform and graded porous parts are sought. Porous parts (both uniformly and graded) that are infiltrated with epoxy resin have been characterized mechanically and the results have been compared with the uninfiltrated porous parts.

Capturing what is going on and what may happen related to vehicle handling behaviour in cases of desired or non-desired actions and interventions has a crucial importance. Strategies implemented to improve vehicle stability or algorithms and control modules designed to compensate the non-desired effects on handling behavior may appear to be inadequate as the vehicle goes through uncountable experiences. The importance of understanding and introducing the possible sources of undesired effects which may be encountered throughout driving action cannot be underemphasized. One of the possibilities that may lead the driver face with unexpected results concerning vehicle&rsquo / s handling is suspension damper failure, which has not yet been dealt with adequately in the literature. The fast developing technology and consequently the expanding utilization of chassis electronics and electronic vehicle components make the investigation of damper failure phenomenon essential since reliability decreases by the continuously increasing introduction of electronic means. In this study, possible failure types of dampers including electrical failure are taken into account, their effects on vehicle stability under critical driving conditions are examined. Shortcomings and comments are made on criticality of failed damper and its failure point. This work as a result, constitutes a particular contribution to the literature in that it brings up a concrete knowledge to the stated research area.

Numerical model usage has substantially increased in many industries. It is the aerospace industry that numerical models play possibly the most important role for development of optimum design. However, numerical models need experimental verification. This experimental verification is used not only for validation, but also updating numerical model parameters. Verified and updated models are used to analyze a vast amount of cases that structure is anticipated to face in real life. In this thesis, structural finite element model updating of a utility helicopter fuselage was performed as a case study. Initially, experimental modal analyses were performed using modal shakers. Modal analysis of test results was carried out using LMS Test.lab software. At the same time, finite element analysis of the helicopter fuselage was performed by MSC.Patran &amp / Nastran software. v Initial updating was processed first for the whole helicopter fuselage then, tail of the helicopter was tried to be updated. Furthermore, a new method was proposed for the optimum node removal location for getting better Modal Assurance Criterion (MAC) matrix. This routine was tried on the helicopter case study and it showed better performance than the Coordinate Modal Assurance Criterion (coMAC) that is often used in such analyses.

Selective laser sintering is a rapid prototyping method (RP), which was originally developed, along with other RP methods, to speed up the prototyping stage of product design. The sole needed input for production being the solid model of the part, the mold/tool-free production characteristics and the geometric part complexity that can be achieved due to layer-by-layer production have extended the applicability/research areas of these methods beyond prototyping towards new applications and material development. Local pore formation in a part that occurs as a result of the discrete manufacturing nature of selective laser sintering is normally considered a defect. In the current research, this is viewed as an opportunity for material development: Exploitation of rapid prototyping methods to produce composites/functionally graded materials with controlled porous structures. That the material interior structure (porous structure) and exterior shape are formed during the same course renders selective laser sintering process as an attractive manufacturing alternative for producing complex-geometry composite/porous materials, which may be difficult or impossible to manufacture with other techniques. In this thesis, the use of selective laser sintering (a rapid prototyping method) in producing uniformly porous and graded polymeric graded porous structures is studied. The material used was polyamide powder (PA 2200) and the selective laser sintering machine used was the EOSINT P 380 system. In this research, three process parameters of the SLS system, the hatching distance, the laser power and the laser scanning speed were varied to produce parts that have different porosities. Porous parts with a homogenous porous microstructure (uniformly porous parts) could be produced, as well as graded porous parts. The results of uniformly porous structure production were utilized to build graded porous structures by imparting different porosities along a certain direction within a single part. Both, uniformly porous and graded structures were characterized physically and mechanically. The porous parts (both uniformly porous and graded porous) were infiltrated with epoxy resin to produce epoxy-PA composites and graded materials. The physical and mechanical properties of these parts were compared with those of the uninfiltrated (porous PA) structures