Nonwoven materials are engineered fabrics, produced by bonding constituent fibres together by mechanical, thermal or chemical means. Such a technology has a great potential to produce material for specific purposes. It is therefore crucial to develop right products with requested properties. This requires a good understanding of the macro and micro behaviours of nonwoven products. In last 40 years, many efforts have been made by researchers to understand the performance of nonwoven materials. One of the main research challenges on the way to this understanding is to link the properties of fibres and the fabric's random fibrous microstructure to the mechanisms of overall material's deformation. The purpose of this research is to study experimentally and numerically the deformation mechanisms of a low-density thermally bonded nonwoven fabric (fibre: Polypropylene; density: 20 gsm). The study started with tensile experiments for the nonwoven material. Specimens with varying dimensions and shapes were tested to investigate the size-dependent deformation mechanisms of the material. Based on obtained results, representative dimensions for the material are determined and used in other experimental and numerical studies. Then standard tensile tests were performed coupled with image analysis. Analysis of the obtained results, allowed the tensile behaviour of the nonwoven material to be determined, the initial study of the effects of material's nonuniform microstructure was also implemented. Based on the experimental results obtained from tensile tests, continuous finite-element models were developed to simulate the material properties of the nonwoven material for its two principle directions: machine direction (MD) and cross direction (CD). Due to the continuous nature of the models, they were only used to establish the mechanical behaviour of the material by treating it as a two-component composite. The effects of bond points, which are a stiffer component within the material, were analysed. Due to the limitations of the continuous FE models, experimental studies were performed focused on the material s microstructure. The latter was detected using an x-ray Micro CT system and an ARAMIS optical strain analysis system. According to the obtained images, the nonwoven fabric is a three-component material. The effects of material's microstructure on stress/strain distributions in the deformed material were studied using advanced image analysis techniques. Based on the experimental results, a new stress calculation method was suggested to substitute the traditional approach, which is not suitable for the analysis of the low density nonwoven material. Then, the fibres orientation distribution and material properties of single fibres were measured due to their significant effects on overall mechanical properties. Finally, discontinuous finite-element models were developed accounting for on the material's three-component structure. The models emphasised the effects of the nonuniform and discontinuous microstructure of the material. Mechanical properties of fibres, the density of fibrous network, the fibres orientation distribution and the arrangement of bond points were used as input parameters for the models, representing features of the material's microstructure. With the use of the developed discontinuous models, the effects of material's microstructure on deformation mechanisms of the low-density nonwoven material were analysed.

Laminated carbon fibre-reinforced polymer (CFRP) composites are already well established in structural applications where high specific strength and stiffness are required. Damage in these laminates is usually localised and may involve numerous mechanisms, such as matrix cracking, laminate delamination, fibre debonding or fibre breakage. Microstructures in CFRPs are non-uniform and irregular, resulting in an element of randomness in the localised damage. This may in turn affect the global properties and failure parameters of components made of CFRPs. This raises the question of whether the inherent stochasticity of localised damage is of significance for application of such materials. This PhD project is aimed at developing numerical models to analyze the effect of material randomness on delamination damage in CFRP materials by the implementation of the cohesive-zone model (CZM) within the framework of the finite-element (FE) method. Both the unidirectional and cross-ply laminates subjected to quasi-static loading conditions were studied. The initiation and propagation in delamination of unidirectional CFRP laminates were analyzed. The CZM was used to simulate the progress of that failure mechanism in a pre-cracked double-cantilever beam (DCB) specimen loaded under mode-I employing initially, a two-dimensional FE model. Model validation was then carried out comparing the numerical results with experimental data. The inherent microstructural stochasticity of CFRP laminates was accounted for in the simulations, and various statistical realizations for a half-scatter of 50% of fracture energy were performed, based on the approximation of that parameter with the Weibull s two-parameter probability density function. More detailed analyses were undertaken employing three-dimensional DCB models, and a number of statistical realizations based on variation of fracture energy were presented. In contrast to the results of two-dimensional analyses, simulations with 3D models demonstrated a lower load-bearing capacity for most of the random models as compared to the deterministic model with uniform material properties. The damaged area and the crack lengths in laminates were analyzed, and the results showed higher values of those parameters for random realizations compared to the uniform case for the same levels of applied displacement. The effect of material randomness on delamination in CFRP cross-ply laminates was also investigated. Initially, two-dimensional finite-element analyses were carried out to study the effect of microstructural randomness in a cross-ply laminate under bending with the direct introduction of matrix cracks with varying spacings and delamination zones. A considerable variation in the stiffness for cases with different crack spacings suggested that the assumption of averaged distributions of defects can lead to unreliable predictions of structural response. Three-dimensional uniform, deterministic cross-ply laminate models subjected to a tensile load were analyzed to study the delamination initiation and propagation from the tips of a pre-existing matrix crack. The material s stochasticity was then introduced, and a number of random statistical realizations were analyzed. It was observed that by neglecting the inherent material randomness of CFRP laminates, the initiation conditions for delamination as well as the character of its propagation cannot be properly detected and studied. For instance, the delamination crack length value for all the simulated random statistical realizations predicted its higher magnitudes compared to the uniform (deterministic) case for the same value of applied strain. Furthermore, the location of delamination initiation was shown to be different for different random statistical realizations. Another aspect, emphasizing the importance of microstructural randomness, was the scatter in the magnitudes of global strain at the instance of initiation and subsequent propagation of delamination. In summary, the material randomness in CFRPs can induce randomness in localised damage and it can affect the global properties of laminates and critical failure parameters. These effects can be investigated computationally through the use of stochastic cohesive-zone elements.

Micro drilling has been applied in the interconnection and precision manufacturing industries extensively. As a promising machining technique, Ultrasonically Assisted Drilling (UAD) has become increasingly popular in both academia and industry in recent years. In this thesis, modelling techniques and experiments for Ultrasonically Assisted Micro Drilling (UAMD) are investigated. Representative work on modelling of micro drills and UAD has been documented and categorised. Existing gaps in the literature are identified and the aims of this research are formulated. Using the Finite Element (FE) technique, a hybrid model is developed to realise modelling for the whole drill bit without compromising the computation efficiency, even when the drill has a complicated geometry (small diameter flute, multiple step shanks, etc). A specific drill model (Φ0.3 mm diameter, 2 step shanks) is chosen for a case study in order to evaluate the model. The hybrid tool shows sufficiently accurate results and impressive computation efficiency in the evaluation. For vibration modelling, force modelling and experimental work, a standard Φ1 mm drill with 1 step shank is used across the chapters. First of all, FE analysis is conducted on the whole drill and normal modes are solved with boundary condition as fixed simply supported. A 2 Degree-of-Freedom (DOF) model is then built considering rotation and the ultrasonic excitation to solve the transverse vibration with boundary conditions consistent with the FE model. The asymmetric geometric characteristics of the drill bit are taken account of through using the first two fundamental modes in the FE model. Potential parametric resonances are discussed in the numerical simulation. Other vibration characteristics are also discussed with varying parameters such as ultrasonic frequency, ultrasonic amplitude and rotational speed. In order to extend the vibration model, a nonlinear thrust force model has been developed for incorporation into the 2 DOF model. The force model considers ultrasonic parameters, feed rate, material properties and the nonlinearity of the UAMD process. Force reduction during the UAMD process is explained qualitatively with the model and a full range of feed rates have been simulated to study their effect on the force reduction. The limitations of this model have also been explained. A high speed UAMD system was designed to examine the effects of key parameters. Experiments with different ultrasonic frequencies, amplitudes and rotational speeds were conducted and the influences of these parameters on thrust force were investigated. With the thrust force data from these experiments, a correlation study to the simulation results based on the force model is carried out. The study identifies the limitations on the current one dimensional force model and leads to recommendations for the further development of the force model. Further work is identified for both modelling and experiments, and the present models can be expanded to suit the research and development of UAMD techniques.

Bone cutting is widely used in orthopaedic, dental and neuro surgeries and is a technically demanding surgical procedure. Novel surgical methods are continually introduced in orthopaedic, neuro and dental surgeries and are aimed at minimising the invasiveness of the operation and allowing more precise cuts. One such method that utilises cutting with superimposed ultrasonic vibration is known as ultrasonically- assisted cutting (UAC). The main concern in bone cutting is the mechanical and thermal damage to the bone tissue induced by high-speed power tools. Recent technological improvements are concerned with the efforts to decrease the force required by the surgeon when cutting the bone as well as increases in surgery speed. A programme of experiments was conducted to characterise properties of a bone and get a basic understanding of the mechanics of bone cutting. The experiments included: (a) nanonindentation and tension tests to obtain the properties for the finite element (FE) bone cutting model, (b) high-speed filming to observe the chip formation process, which influences thermomechanics of the cutting process in conventional drilling (CD) and ultrasonically-assisted drilling (UAD) and, (c) plane cutting and drilling experiments to measure the levels of force and temperature rise in the bone tissue. Novel two-dimensional finite element (FE) models of cortical bone cutting were developed for conventional and ultrasonically-assisted modes with the MSC.MARC general FE code that provided thorough numerical analysis of thermomechanics of the cutting process. Mechanical properties such as the elastic modulus and strain-rate sensitivity of the bone material were determined experimentally and incorporated into the FE models. The influence of cutting parameters on the levels of stress, penetration force and temperature in the bone material was studied using conventional cutting (CC) and ultrasonically-assisted cutting (UAC). The temperature rise in the bone material near the cutting edge was calculated and the effect of cutting parameters on the level of thermal necrosis was analysed. The necrosis depth in bone was calculated as a distance from the cut surface to the point where the thermal threshold level was attained. Comparative studies were performed for the developed FE models of CC and UAC of bone and the results validated by conducting experiments and using data from scientific publications. The main outcome of the thesis is an in-depth understanding of the bone cutting process, and of its possible application in orthopaedics. Recommendations on further research developments are also suggested.

Over the past decades simulation techniques, and in particular finite element method, have been used successfully to predict the response of systems across a whole range of industries including aerospace, automotive, chemical processes, geotechnical engineering and many others. In these numerical analyses, the behaviour of the actual material is approximated with that of an idealised material that deforms in accordance with some constitutive relationships. Therefore, the choice of an appropriate constitutive model that adequately describes the behaviour of the material plays an important role in the accuracy and reliability of the numerical predictions. During the past decades several constitutive models have been developed for various materials. In recent years, by rapid and effective developments in computational software and hardware, alternative computer aided pattern recognition techniques have been introduced to constitutive modelling of materials. The main idea behind pattern recognition systems such as neural network, fuzzy logic or genetic programming is that they learn adaptively from experience and extract various discriminants, each appropriate for its purpose. In this thesis a novel approach is presented and employed to develop constitutive models for materials in general and soils in particular based on evolutionary polynomial regression (EPR). EPR is a hybrid data mining technique that searches for symbolic structures (representing the behaviour of a system) using genetic algorithm and estimates the constant values by the least squares method. Stress-strain data from experiments are employed to train and develop EPR-based material models. The developed models are compared with some of the existing conventional constitutive material models and its advantages are highlighted. It is also shown that the developed EPR-based material models can be incorporated in finite element (FE) analysis. Different examples are used to verify the developed EPR-based FE model. The results of the EPR-FEM are compared with those of a standard FEM where conventional constitutive models are used to model the material behaviour. These results show that EPR-FEM can be successfully employed to analyse different structural and geotechnical engineering problems.

The third generation of hip resurfacing commenced in the U.K. in the 1990’s with the Birmingham Hip Resurfacing system and is now becoming more commonplace as an attractive alternative for young and active patients due to premature failure in total hip replacement in this patient group. However the Swedish National Hip Arthroplasty Register (2010) suggests that premature failure of resurfacing arthroplasty may be more prevalent than first expected. The aim of this study is to investigate, through Finite Element Analysis, the short, medium and long term performance of Poly Methyl Methacrylate (PMMA) bone cement of the femoral component in hip resurfacing arthroplasty. The study takes a forensic engineering approach, analysing the performance of PMMA bone cement in order to provide understanding, awareness and an insight into lifestyle options. Finite Element Analysis explores and models the effect of resting periods during daily activities, patients’ bone quality and PMMA bone cement Young’s modulus on the PMMA bone cement stresses within the femoral hip resurfacing component. Mechanical tests are used to illustrate the use of the Finite Element Analysis results. Contributing to knowledge, this study verifies the significance of high metal-on-metal friction due to resting periods, developing a dynamic FEA model to quantify the premature fatigue failure of PMMA bone cement, within the femoral component of hip resurfacing arthroplasty. A decrease in bone quality added to the effect of resting periods increase the risk of PMMA fatigue failure and PMMA-metal interface failure due to an increase of PMMA tensile and shear stresses, suggesting that patients with low bone quality should avoid hip resurfacing procedures. The use of low PMMA Young’s modulus could greatly enhance the long term success of hip resurfacing arthroplasty generally and specifically reduce the risk of interface failure and PMMA bone cement failure due to resting periods and patient bone quality. Moreover, this study shows that the consequence of PMMA fatigue failure and PMMA-metal interface failure must be included in the design, patient selection, screening process, post-operative rehabilitation and long term lifestyle attributes. This study suggests that occupational therapists and patients with hip resurfacing arthroplasty should be aware of high metal-on-metal friction situations, which could lead to early failure indicated by this research. The deleterious effect of resting periods indicated by this research could be alleviated by appropriate re-initiation of synovial lubrication by movement prior to full loading. Recommendations for further work include the compilation of a PMMA bone-cement fatigue properties database and further development of the FEA modelling technique for application upon other arthroplasty procedures.

Osteoarthritis (OA) is a debilitating joint disease that leads to significant pain, loss of mobility and quality of life. Knee malalignment results in increased joint pressure, which is a primary cause for OA progression. High Tibial Osteotomy (HTO) is a surgical procedure to correct malalignment and redistribute load in the knee joint, reduce peak pressure and delay OA progression. However, clinical outcomes have been unpredictable. Therefore, the aim of this study was to determine the relationship between malalignment and knee contact mechanics. A 3D computational model was created from magnetic resonance images of a cadaveric knee joint. A ligament tuning process was conducted to determine material properties. Finite element analyses were conducted, simulating end of weight acceptance during walking. Different wedge geometries were virtually removed to simulate malalignments from 14° valgus to 16° varus. Contact mechanics were sensitive to soft tissue material properties. In-vitro experiments were compared with computational modelling of the same specimen. Percent full-scale errors for contact force and pressure were less than 8%, demonstrating a unique subject-specific model validation. The native alignment of the cadaveric knee (1° varus) had medial and lateral compartment peak pressures of 4.28 MPa and 2.42 MPa, respectively. The medial:lateral force ratio was 70%:30%. Minimum contact stress did not occur at a Mechanical Axis Deviation (MAD) of zero millimetres nor at the Fujisawa Point, which are common targets for HTO correction. Results showed very strong correlations (r >0.94) between MAD and joint contact loading. This study is the first to demonstrate the relationship between stress (normal, shear, contact pressure) and MAD in a subject-specific model. This is a prerequisite for the development of a tool that could help surgeons make informed decisions on the degree of realignment required to minimise peak joint loading, thereby delaying OA progression.

Dual Energy X-ray Absorptiometry (DXA)-based finite element (FE) modelling has emerged as a potential tool for better assessment of osteoporotic hip fracture risk. Automation of this complex and computationally-intense procedure is the prime requirement for its clinical applicability. The aim of this study was to develop a fully automatic DXA-based finite element tool and assess its discrimination ability and short-term repeatability. The proximal femur was automatically segmented from clinical hip DXA scan and the subject-specific FE model was constructed for simulating sideways fall. Hip fracture risk indices (HFRIs) were calculated using two ways (along a femur cross-section and over a region of interest, ROI). Hip fracture discriminability increased when moved from femur cross-section based to ROI based HFRI calculation. A significant increase in hip fracture discriminability from baseline femoral neck and total hip bone mineral density (BMD) was achieved with ROI based HFRIs. Promising short-term repeatability was observed for HFRIs (coefficient of variation, CV, 3~3.5%). After removing representative poor cases, CVs were less than 3%. These preliminary results establish the potential of the proposed automatic tool for hip fracture risk assessment and justify large-scale clinical evaluation of its ability to predict incident hip fractures. / February 2017

Injury to the medial malleolus, the distal end of the tibia and one of the bones comprising the ankle joint, can occur in various loading scenarios. Open reduction/internal fixation (ORIF) to reattach the malleolar fragment to the proximal tibia can be achieved via various devices, however small fragments are particularly challenging to treat. In this study, computational finite element analysis (FEA) was utilized to investigate the fixation of transverse medial malleolar fractures by two cancellous screws or by a new fixation device, the Medial Malleolar Sled™. Cadaveric testing assessed the performance of the two constructs in both tension and torsion. Following experimentation, the cadaveric study was modeled in SolidWorks and analyzed via FEA to validate the model against the experimental results. Overall, stress analysis was indicative of areas of relatively higher stress concentrations that correlated with failure locations in the experiment. Such results speak to the predictive nature of the tension and torsion models created in the study, and to the general utility of computational modeling for the study of biomechanical systems.

Environmental challenges, such as global warming, growing demand on energy, and diminishing oil sources have accelerated research on alternative energy conversion methods. Thermoelectric power generation is a promising method to convert wasted heat energy into useful electrical energy form. A temperature gradient imposed on a thermoelectric device produces a Seebeck potential. However, this temperature gradient causes thermal stresses due to differential thermal expansions and mismatching of the bonded components of the device. Thermal stresses are critical for thermoelectric devices since they can generate failures, including dislocations, cracks, fatigue fractures, and even breakdown of the entire device. Decreases in power-generation performance and operation lifetime are major consequences of these failures. In order to minimize thermal stresses in the legs without affecting power-generation capabilities, this study concentrates on structural solutions. Thermoelectric devices with non-segmented and segmented legs were modeled. Specifically, the possible effect of various leg geometries, configurations, and dimensions were evaluated using finite-element and statistical methods. Significant changes in the magnitudes and distributions of thermal stresses occurred. Specifically, the maximum equivalent stresses in the rectangular-prism and cylindrical legs were 49.9 MPa and 43.3 MPa, respectively for the temperature gradient of 100ºC. By using cylindrical legs with modified dimensions, decreases in the maximum stresses in legs reached 21.2% without affecting power-generation performance. Moreover, the effect of leg dimensions and coaxial-leg configurations on power generation was significant; in contrast, various leg geometries and rotated-leg configurations had very limited affect. In particular, it was possible to increase power output from 20 mW to 65 mW by simply modifying leg widths and heights within the defined range. It should be noted, however, this modification also increased stress levels. It is concluded that leg geometries, configurations, and dimensions can be redesigned for improved durability and overall performance of thermoelectric devices.