Friction is one of the important parameters in metal forming processes since it affects metal flow in the die, forming load, strain distribution, tool and die life, surface quality of the product etc. The range of coefficient of friction in different metal forming applications is not well known and the factors affecting variation are ambiguous. Commercially available FEA packages input the coefficient of friction as constant among the whole process which is not a realistic approach. In this study, utility of user-subroutines is integrated into MSC SuperForm v.2004 and MSC Marc v.2003 FEA packages, to apply a variable coefficient of friction depending on the contact interface conditions. Instead of using comparatively simple friction models such as Coulomb, Shear (constant) models, friction models proposed by Wanheim-Bay and Levanov were used to simulate some cold forging operations. The FEA results are compared with the experimental results available in literature for cylinder upsetting. Results show that, large variation on the coefficient of friction is possible depending on the friction model used, the part geometry and the ratio of contact normal pressure to equivalent yield stress. For the ratio of contact normal pressure to equivalent yield stress values above 4, coefficient of friction values are approximately same for both friction models.

As metals have high formability at elevated temperatures, hot forming is preferred and widely used in manufacturing of complicated geometries. The term hot forming is usually used if the whole workpiece is processed at elevated temperatures. However, for certain products high formability is required only locally. Forming by local heating is proposed to provide ease of manufacturing of local forms on the workpiece. Also, tools can be simplified by this method. In this study, local laser heating procedures are applied to obtain local forms on cylindrical bulk metal products in a single step. Locally heated workpieces are formed between two flat dies. Both solid and hollow products have been investigated experimentally and by finite element modeling. The experimental studies and finite element analyses are done simultaneously in order to obtain optimum local deformation characteristics. Three different materials together with different initial geometries and various local laser-heating procedures are applied to search for the process window. The limits of applicability are determined and examples of application are supplied.

The aim of this study is to design a computer interface, which links the user to commercial Finite Element Analysis (FEA) program, MSC.Marc-Mentat to make automatic FE analysis of an excavator boom by using DELPHI as platform. Parametrization of boom geometry is done to add some flexibility to interface called OPTIBOOM. Parametric FE analysis of a boom shortens the design stages and helps to find the optimum design in terms of stresses and mass.

Roll-forging is a metal forming process and mainly used for preform forging of long parts prior to press or hammer forging in the industry. Variable cross sections through the length of billet can be obtained by roll-forging to acquire an adequate distribution of material to the next forging stages. In the design of process and dies used in roll-forging, there are some empirical techniques in literature. However these techniques only provide approximate reduction ratio and elongation during the process and the geometry of the workpiece at the end of each stage cannot be determined exactly by using these techniques. In this study, multistage roll-forging process has been analyzed by using a finite element analysis software to examine material flow, temperature, stress and strain distribution at each roll-forging step. The geometries at the intermediate stages have been determined. Computer results have been compared to experimental results and good agreement has been observed.

Background Surface treatments and irregularities in the surfaces may affect the fracture of ceramics. The effects of various treatments on the surface texture of different types of ceramic cores/substructures was therefore qualitatively, quantitatively and numerically evaluated. Since fractures in ceramics are not fully understood, the fracture behavior in dental ceramic core/substructures was also studied using both established laboratory methods and newly developed numerical methods. Methods The surfaces of dental ceramic cores/substructures were studied qualitatively by means of a fluorescence penetrant method and scanning electron microscopy, quantitatively evaluated using a profilometer and also numerical simulation. In order to study fracture in zirconia-based fixed partial denture (FPD) frameworks, fractographic analysis in combination with fracture tests and newly developed two-dimensional (2D) and three-dimensional (3D) numerical modeling methods were used. In the numerical modeling methods, the heterogeneity within the materials was described by means of the Weibull distribution law. The Mohr–Coulomb failure criterion with tensile strength cut-off was used to judge whether the material was in an elastic or failed state. Results Manual grinding/polishing could smooth the surfaces on some of the types of dental ceramic cores/substructures studied. Using the fluorescence penetrant method, no cracks/flaws apart from milling grooves could be seen on the surfaces of machined zirconia-based frameworks. Numerical simulations demonstrated that surface grooves affect the fracture of the ceramic bars and the deeper the groove, the sooner the bar fractured. In the laboratory tests the fracture mechanism in the FPD frameworks was identified as tensile failure and irregularities on the ceramic surfaces could act as fracture initiation sites. The numerical modeling codes allowed a better understanding of the fracture mechanism than the laboratory tests; the stress distribution and the fracture process could be reproduced using the mathematical methods of mechanics. Furthermore, a strong correlation was found between the numerical and the laboratory results. Conclusion Based on the findings in the current thesis, smooth surfaces in areas of concentrated tensile stress would be preferable regarding the survival of ceramic restorations, however, the surfaces of only some of the ceramic cores/substructures could be significantly affected by manual polishing. The newly developed 3D method clearly showed the stress distribution and the fracture process in ceramic FPD frameworks, step by step, and seems to be an appropriate tool for use in the prediction of the fracture process in ceramic FPD frameworks.

Low back pain is an ailment that affects a significant portion of the community. However, due to the complexity of the spine, which is a series of interconnected joints, and the loading conditions applied to these joints the causes for back pain are not well understood. Investigations of damage or failure of the spinal structures from a mechanical viewpoint may be viewed as a way of providing valuable information for the causes of back pain. Low back pain is commonly associated with injury to, or degeneration of, the intervertebral discs and involves the presence of tears or lesions in the anular disc material. The aim of the study presented in this thesis was to investigate the biomechanical effect of anular lesions on disc function using a finite element model of the L4/5 lumbar intervertebral disc. The intervertebral disc consists of three main components - the anulus fibrosus, the nucleus pulposus and the cartilaginous endplates. The anulus fibrosus is comprised of collagen fibres embedded in a ground substance while the nucleus is a gelatinous material. The components of the intervertebral disc were represented in the model together with the longitudinal ligaments that are attached to the anterior and posterior surface of the disc. All other bony and ligamentous structures were simulated through the loading and boundary conditions. A high level of both geometric and material accuracy was required to produce a physically realistic finite element model. The geometry of the model was derived from images of cadaveric human discs and published data on the in vivo configuration of the L4/5 disc. Material properties for the components were extracted from the existing literature. The anulus ground substance was represented as a Mooney-Rivlin hyperelastic material, the nucleus pulposus was modelled as a hydrostatic fluid in the healthy disc models and the cartilaginous endplates, collagen fibres and longitudinal ligaments were represented as linear elastic materials. A preliminary model was developed to assess the accuracy of the geometry and material properties of the disc components. It was found that the material parameters defined for the anulus ground substance did not accurately describe the nonlinear shear behaviour of the tissue. Accurate representation this nonlinear behaviour was thought to be important in ensuring the deformations observed in the anulus fibrosus of the finite element model were correct. There was no information found in the literature on the mechanical properties of the anulus ground substance. Experimentation was, therefore, carried out on specimens of sheep anulus fibrosus in order to quantify the mechanical response of the ground substance. Two testing protocols were employed. The first series of tests were undertaken to provide information on the strain required to initiate permanent damage in the ground substance. The second series of tests resulted in the acquisition of data on the mechanical response of the tissue to repeated loading. The results of the experimentation carried out to determine the strain necessary to initiate permanent damage suggested that during daily loading some derangement might be caused in the anulus ground substance. The results for the mechanical response of the tissue were used to determine hyperelastic constants which were incorporated in the finite element model. A second order Polynomial and a third order Ogden strain energy equation were used to define the anulus ground substance. Both these strain energy equations incorporated the nonlinear mechanical response of the tissue during shear loading conditions. Using these geometric data and material properties a finite element model of a representative L4/5 intervertebral disc was developed. When the measured material parameters for the anulus ground substance were implemented in the finite element model, large deformations were observed in the anulus fibrosus and excessive nucleus pressures were found. This suggested that the material parameters defining the anulus ground substance were overly compliant and in turn, implied the possibility that the stiffness of the sheep anulus ground substance was lower than the stiffness of the human tissue. Even so, the mechanical properties of the sheep joints had been shown to be similar to those of the human joint and it was concluded that the results of analyses using these parameters would provide valuable qualitative information on the disc mechanics. To represent the degeneration of the anulus fibrosus, the models included simulations of anular lesions - rim, radial and circumferential lesions. Degeneration of the nucleus may be characterised by a significant reduction in the hydrostatic nucleus pressure and a loss of hydration. This was simulated by removal of the hydrostatic nucleus pressure. Analyses were carried out using rotational loading conditions that were comparable to the ranges of motion observed physiologically. The results of these analyses showed that the removal of the hydrostatic nucleus pressure from an otherwise healthy disc resulted in a significant reduction in the stiffness of the disc. This indicated that when the nucleus pulposus is extremely degenerate, it offers no resistance to the deformation of the anulus and the mechanics of the disc are significantly changed. Specifically, the resistance to rotation offered by the intervertebral disc is reduced, which may affect the stability of the joint. When anular lesions were simulated in the finite element model they caused minimal changes in the peak moments resisted by the disc under rotational loading. This suggested that the removal of the nucleus pressure had a greater effect on the mechanics of the disc than the simulation of anular lesions. The results of the finite element model reproduced trends observed in both the healthy and degenerate intervertebral disc in terms of variations in nucleus pressure with loading conditions, axial displacement of the superior surface and bulge of the peripheral anulus. It was hypothesised that the reduced rotational stiffness of the degenerate disc may result in overload of the surrounding innervated osseoligamentous anatomy which may in turn cause back pain. Similarly back pain may result from the abnormal deformation of the innervated peripheral anulus in the vicinity of anular lesions. Furthermore, it was hypothesised that biochemical changes may result in the degeneration of the nucleus, which in turn may cause excessive strains in the anulus ground substance and lead to the initiation of permanent damage in the form of anular lesions. With further refinement of the components of the model and the methods used to define the anular lesions it was considered that this model would provide a powerful analysis tool for the investigation of the mechanics of intervertebral discs with and without significant degeneration.

Over the past several decades increasing focus has been paid to the impact of structures where energy, during the impact event, needs to be absorbed in a controlled manner. This has led to considerable research being carried out on energy absorbers, devices designed to dissipate energy during an impact event and hence protect the structure under consideration. Energy absorbers have found common usage in applications such as vehicles, aircraft, highway barriers and at the base of lift shafts. A type of energy absorber which has received relatively limited attention in the open literature is the tapered rectangular tube. Such a structure is essentially a tube with a rectangular cross-section in which one or more of the sides are inclined to the tube's longitudinal axis. The aim of this thesis was to analyse the impact and energy absorption response of tapered and non-tapered (straight) rectangular tubes. The energy absorption response was quantified for both axial and oblique loading, representative of the loading conditions typically encountered in impact applications. Since energy absorbers are commonly used as components in energy absorbing systems, the response of such a system was analysed which contained either straight or tapered rectangular tubes as the energy absorbing components. This system could typically be used as the front bumper system of a vehicle. Detailed finite element models, validated using experiments and existing theoretical and numerical models, were used to assess the energy absorption response and deformation modes of straight and tapered tubes under the various loading conditions. The manner in which a thin-walled tube deforms is important since it governs its energy absorption response. The results show that the energy absorption response of straight and tapered rectangular tubes can be controlled using their various geometry parameters. In particular, the wall thickness, taper angle and the number of tapered sides can be effectively used as parameters to control the amount of absorbed energy. Tapered rectangular tubes display less sensitivity to inertia effects compared with straight rectangular tubes under impact loading. This is beneficial when the higher crush loads associated with inertia effects need to be reduced. Furthermore, though the energy absorption capacity of thin-walled rectangular tubes diminishes under oblique impact loading, the capacity is more maintained for tapered rectangular tubes compared with non-tapered rectangular tubes. Overall, the results highlight the advantages of using tapered rectangular tubes for absorbing impact energy under axial and oblique loading conditions. Understanding is gained as to how the geometry parameters of such structures can be used to control the absorbed energy. The thesis uses this knowledge to develop design guidelines for the use of straight and tapered rectangular tubes in energy absorbing systems such as for crashworthiness applications. Furthermore, the results highlight the importance of analysing thin-walled energy absorbers as part of an energy absorbing system, since the response of the absorbers may be different to when they are treated on their own.

Bioimpedance spectroscopy has shown potential as a method for characterising biological tissue with the use of a tetrapolar electrode configuration. Brown et al. (2000) demonstrated that the configuration is capable of distinguishing between normal squamous epithelium and Cervical Intra-epithelial Neoplasia (CIN). However little has been done to identify the volumes of tissue that contribute to the measured impedance. Brown et al. employed a probe with a single tetrapolar electrode set thus analysing single points of tissue. The probe was required to be moved in order to "sample" other areas of tissue. This method provides no spatial information of the lesion boundaries. The overall objective of this research was to design and construct an impedance mapping system (IMS) for objective virtual biopsy of lesions by bioimpedance spectroscopy (BIS). Initially freshly excised cervical tissue was to be tested however as the study progressed this proved problematic and bovine blood was chosen as a suitable substitute. Specific aims were to; - .Investigate the spatial sensitivity distribution of the tetrapolar electrode configuration via finite element analysis (FEA). - Design a novel front end multiplexing system and multi-electrode array for mapping the impedance of the tissue of interest. - .Experimentally confirm the efficacy of the approach to identify regions of different impedances and their boundaries using bioimpedance mapping. The present study used finite element analysis (FEA) to investigate the spatial variation in sensitivity of the tetrapolar electrode configuration and identify which volumes of tissue were included in the measured impedance. An impedance mapping device was also designed and constructed utilising the tetrapolar electrode configuration in an expanded array of 25 electrodes. This array allowed the surface of an area of tissue to be mapped and lesion boundaries identified in an objective manner. FEA was also used to model lesions in healthy tissue and the sensitivity fields associated with the tetrapolar configuration. The FEA indicated that anomalous results would be obtained when a lesion was located between a drive and measurement electrode pair. In this case the lesion resulted in an increase in impedance with respect to the impedance of healthy tissue, whereas a lesion should result in a decrease in measured impedance relative to that of healthy tissue. The anomaly was found to produce false negative results for small lesions up to 0.4 mm and even a lesion with radius of approximately 0.75 mm could be undetected as the measured impedance spectrum for such a lesion is similar to that of healthy tissue. Modelling also provided insight into the sensitivity fields for an electrode array and its efficacy in accurately measuring the surface impedance of tissue and lesions of interest. The impedance mapping system (IMS) developed used an array of 25 (5x5) electrodes. The array allows 64 individual tetrapolar measurements to be obtained at 16 locations, providing an impedance map of 49 mm2 on the surface of a tissue sample. Multiple measurements at each location reduce the chance of anomalous results since these can be identified and excluded. Software was developed to display the measured impedance maps and regions of different impedance were easily identified Testing of the IMS using bovine blood showed separation of the measured impedance for a range of haematocrit between 0 - 80%. Introduced volumes of red blood cells (RBC) or clots (to mimic lesions) to the plasma (haematocrit 0%) were also clearly identified using the IMS. It was seen that measurements made at the boundary of 2 different haematocrits (ie 2 volumes of different impedance) resulted in an anomalous result as indicated by the FEA modelling. However it was demonstrated that these anomalies can be used to objectively identify the introduced RBC (lesion) boundaries. A more efficient electrode stepping sequence was also developed taking advantage of the reciprocal nature of the tetrapolar electrode configuration. This development allows for the electrode array to be doubled in size using the same components, and to sample twice the surface area in the same time taken using the initially developed system. In summary, an impedance mapping system has been modelled, designed and developed for tissue characterisation by bioimpedance measurements. The technique has been shown experimentally to be able to detect regions of differ- ent impedance and is in agreement with the finite element analysis performed. Further development of the IMS will allow progressive monitoring of suspect lesions in-vivo and better identification of their spatial distribution for biopsy.

The high stiffness to weight ratio and fatigue resistance make carbon fibre composites suitable for both military and large civil aircraft. The limited ability of current numerical methods to capture the complex growth of damage in laminated composites leads to a conservative design approach applied in today??s composite aircraft structures. The aim of the presented research was to develop an improved methodology for the failure prediction of laminated composites containing delaminations located between arbitrary layers in the laminate, and to extend the investigations to composite structures subjected to barely visible impact damage (BVID). The advantages of fracture mechanics-based methodologies to predict interlaminar failure in composite structures were identified, from which the crack tip element (CTE) approach and the virtual crack closure technique (VCCT) were selected for assessment. Extensive validation of these fracture mechanics methods is presented on a number of composite structures ranging from coupons to large stiffened panels. It was shown that the VCCT was relatively insensitive to the crack front mesh size, whilst predictions using the CTE methodology were significantly influenced by the element size. Based on the obtained results modelling guidelines for the VCCT and CTE were established. Significant contribution of this research to the field of the analysis of composite structures was the development of a novel test method for the evaluation of embedded single and multi-level delaminations. The test procedure of the single delamination specimen was proposed as an analogous test to conventional compression experiments. The transverse test overcame the inherent problems of in-plane compression testing and produced less scatter of experimental measurements. Quantitative analysis of numerical results employing the validated finite element modelling approaches showed that the failure load and location were in agreement with experiments. Furthermore, new modelling techniques for composite structures containing BVID proposed in this research produced good correlation with test data from the compression after impact (CAI) test. The study of BVID provided a significant contribution toward the knowledge of the applicability of implicit FE solvers to predict failure of CAI specimens as well as the criticality of centrally impacted specimens.

Shape control of a structure with distributed piezoelectric actuators can be achieved through optimally selecting the loci, shapes and sizes of the piezoelectric actuators and choosing the electric fields applied to the actuators. Shape control can be categorised as either static or dynamic shape control. Whether it is a transient or gradual change, static or dynamic shape control, both aim to determine the loci, sizes, and shapes of piezoelectric actuators, and the applied voltages such that a desired structural shape is achieved effectively. This thesis is primarily concerned with establishing a finite element formulation for the general smart laminated composite plate structure, which is capable to analyse static and dynamic deformation using non-rectangular elements. The mechanical deformation of the smart composite plate is modelled using a third order plate theory, while the electric field is simulated based on a layer-wise theory. The finite element formulation for static and dynamics analysis is verified by comparing with available numerical results. Selected experiments have also been conducted to measure structural deformation and the experimental results are used to correlate with those of the finite element formulation for static analysis. In addition, the Linear Least Square (LLS) method is employed to study the effect of different piezoelectric actuator patch pattern on the results of error function, which is the least square error between the calculated and desired structural shapes in static structural shape control. The second issue of this thesis deals with piezoelectric actuator design optimisation (PADO) for quasi-static shape control by finding the applied voltage and the configuration of piezoelectric actuator patch to minimise error function, whereas the piezoelectric actuator configuration is defined based on the optimisation technique of altering nodal coordinates (size/shape optimisation) or eliminating inefficient elements in a structural mesh (topology optimisation). Several shape control algorithms are developed to improve the structural shape control by reducing the error function. Further development of the GA-based voltage and piezoelectric actuator design optimisation method includes the constraint handling, where the error function can be optimised subjected to energy consumption or other way around. The numerical examples are presented in order to verify that the proposed algorithms are applicable to quasi-static shape control based on voltage and piezoelectric actuator design optimisation (PADO) in terms of minimising the error function. The third issue is to use the present finite element formulation for a modal shape control and for controlling resonant vibration of smart composite plate structures. The controlled resonant vibration formulation is developed. Modal analysis and LLS methods are also employed to optimise the applied voltage to piezoelectric actuators for achieving the modal shapes. The Newmark direct time integration method is used to study harmonic excitation of smart structures. Numerical results are presented to induce harmonic vibration of structure with controlled magnitude via adjusting the damping and to verify the controlled resonant vibration formulation.