A review of Shakedown and Ratchetting concepts and their extensions is presented in an attempt to recount all the aspects of the problems considered in this research programme. The concept of Stress Concentration Factor was the first to be further investigated, by analysing two representative types of structures operating under severe stress concentration, namely; two-bar structures and cylindrical vessels with variable thickness subjected to cyclic mechanical loads. The material behaviour considered are: elastic-perfectly plastic and isotropic hardening. Such an analytical investigation allowed the assessment of the influence of the Stress Concentration Factor below and above the limit of reversed plasticity. The primary aim of this research was to develop simplified techniques capable of solving thermal loading problems in the presence of steady mechanical loads. A simplified technique was then developed to analyse a tube subjected to a complex thermal loading simulating the fluctuation of level of sodium in Liquid Metal Fast Breeder Reactors (LMFBR). The technique was also able to include a second important aspect of shakedown problems which is cases of multiple mechanical loads. The construction of bi-dimensional Bree type diagrams, from tri-dimensional ones obtained for such cases, allowed an easy assessment of the modes of deformation of the structure. The effects of the temperature on the yield stress were explored. A third aspect of thermal cyclic problems investigated was the experimental verification of the reliability of the extended Upper Bound Theorem proposed in Chapter 2. This was achieved by experimental tests on portal frames at 400°C. Contours representing states of constant of deformation were obtained from the experimental measurements. A fourth aspect of the problem was the development of theoretical technique to estimate the transient plastic deformation in excess of the shakedown limit which allowed the construction of theoretical contours directly comparable with the experimental ones. The fifth and major contribution of this thesis was the development of a general technique for the analysis of axi-symmetric shells based in a displacement formulation for the Finite Element Method. Limit analysis and shakedown problems were reduced to minimization problems by developing a technique to obtain consistent relationship between the displacement field and the plastic strain field. Such a technique, based upon a Galerkin type of approach, consist of minimizing the difference between the two representations of the strain within the element; in terms of nodal displacement and in terms of plastic multipliers. The problem was then solved by Linear Programming. Finally, the conclusions and proposal for future work are presented.

This thesis reports research to investigate the mechanical, thermal and acoustic properties of biodegradable foams in block forms based on wheat starch and developed at Brunel University's School of Engineering & Design, in order to exploit the potential environmental benefits of this renewable and biodegradable class of materials. Two emergent novel technologies have been developed based on a combination of the extrusion foaming of starch in conjunction with the natural adhesive characteristics of moistened starch to produce block foams. Regular Packing & Stacking (RPS), and Compression Bonded Loosefill (CBL), are foam fabrication technologies which have both demonstrated the potential to produce bulk foams based on wheat starch with unique structures and properties - a new class of foam materials in the form of macro-composites reinforced by a network of high-density bonding interfaces. This thesis, as part of a Department of Trade & Industry/Technology Strategy Board funded project, reports an investigation into the following areas to address the scientific and technical issues involved in the further development of the materials and their applications. - The basic properties of the raw materials used in the manufacture of CBL and RPS foams are outlined and the fabrication and preparation of these starch-based foams are described. The limitations of these production techniques are discussed with preliminary work and suggestions made for their enhancement. - Research into the mechanical properties of the CBL and RPS foams includes compression, tensile, creep and dynamic impact tests, whilst the mechanical behaviour of the foams subject to high temperature and high humidity conditions is also reported. - Research into the thermal properties of CBL and high density RPS foams includes testing of the material's thermal conductivity. This aspect of the research also involved a case study detailing the use of RPS in a commercial thermal insulation application. - Research into the acoustic properties of CBL and RPS foams includes tests for sound absorption coefficient and sound transmission loss. - Data obtained from these tests are benchmarked against data pertaining to the mechanical, thermal and acoustic properties of conventional polymer foams in order to provide a basis on which to identify the potential cushioning, thermal insulation and acoustic insulation applications of the starch-based materials. The research has demonstrated the following: - Potential cushioning applications include those limited to the range of static loads within the capabilities of the materials, taking into account the resilience of CBL and RPS which is likely to be compromised by successive impacts. - Tensile forces tend to exploit weaknesses in the macrostructure of these materials. By implication the behaviour of the materials under shear forces would be expected to be similarly compromised. - CBL and RPS exhibited dimensional shrinkage, density increase and significantly reduced mechanical properties under conditions of high temperature and humidity. This suggests that neither CBL nor RPS foams would be suitable for applications in regions where tropical conditions may be encountered unless used in conjunction with other protective materials which would not acutely increase the environmental burden of the products. - Low-density RPS and CBL foams exhibit lower thermal conductivities and hence higher thermal insulation properties compared to many commercially available polymer foams of similar densities. As such these foams have the potential to be used in applications in which a measure of thermal insulation is required. A case study based on an existing commercial application in which the temperature of chilled products must be maintained over a 24 hour period reinforced these findings. - The performance of CBL and RPS starch foams would not provide sufficient functionality to be employed in applications in which dedicated acoustic performance is required, although their sound absorption capabilities may facilitate overall marketability for applications in which a degree of acoustic performance is required if used in conjunction with other materials which demonstrate good acoustic performance. It is anticipated that this work will make significant contributions toward advances in the development of these novel technologies, specifically in terms of establishing an understanding of the properties of the starch-based materials and in identifying potential applications. The research results should thus provide a fundamental element in the basis for the industrial development of these renewable and biodegradable materials.

No description available.

Indiana University-Purdue University Indianapolis (IUPUI) / This study represents a cost-effective method to advance the physical and mechanical properties of carbon fiber-reinforced polymer (CFRP) prepreg composite materials, where electrospun multiwalled carbon nanotubes (CNTs)/epoxy nanofibers fabricated and deposited in between the layers of traditional CFRP prepreg composite. CNT-aligned epoxy nanofibers were uniformly formed by an optimized electrospinning method. Electrospinning is considered one of the most flexible, low-cost, and globally recognized methods for generating continuous filaments from submicron to tens of nanometer diameter. Nanofilaments were incorporated precisely on the layers of prepreg to accomplish increased adhesion and interfacial bonding, leading to increased strength and enhancements in more mechanical properties. As a result, the modulus of the epoxy and CNT/epoxy nanofibers were revealed to be 3.24 GPa and 4.84 GPa, leading to 49% enhancement. Furthermore, interlaminar shear strength (ILSS) and fatigue performance at high-stress regimes improved by 29% and 27%, respectively. Barely visible impact damage (BVID) energy improved considerably by up to 45%. The thermal and electrical conductivities were also increased considerably because of the highly conductive CNT networks present in between the CFRP layers. The newly introduced approach was able to deposit high content uniform CNTs at the ply interface of prepregs to enhance the CFRP properties, that has not been achieved in the past because of the randomly oriented high viscosity CNTs in epoxy resins.

This study represents a cost-effective method to advance the physical and mechanical properties of carbon fiber-reinforced polymer (CFRP) prepreg composite materials, where electrospun multiwalled carbon nanotubes (CNTs)/epoxy nanofibers fabricated and deposited in between the layers of traditional CFRP prepreg composite. CNT-aligned epoxy nanofibers were uniformly formed by an optimized electrospinning method. Electrospinning is considered one of the most flexible, low-cost, and globally recognized methods for generating continuous filaments from submicron to tens of nanometer diameter. Nanofilaments were incorporated precisely on the layers of prepreg to accomplish increased adhesion and interfacial bonding, leading to increased strength and enhancements in more mechanical properties. As a result, the modulus of the epoxy and CNT/epoxy nanofibers were revealed to be 3.24 GPa and 4.84 GPa, leading to 49% enhancement. Furthermore, interlaminar shear strength (ILSS) and fatigue performance at high-stress regimes improved by 29% and 27%, respectively. Barely visible impact damage (BVID) energy improved considerably by up to 45%. The thermal and electrical conductivities were also increased considerably because of the highly conductive CNT networks present in between the CFRP layers. The newly introduced approach was able to deposit high content uniform CNTs at the ply interface of prepregs to enhance the CFRP properties, that has not been achieved in the past because of the randomly oriented high viscosity CNTs in epoxy resins.

The objectives of this research are to characterize the mechanical and thermal performance of natural fiber nonwoven composites and to predict the composite strength and long-term creep performance. Three natural fibers: kenaf, jute, and sunn hemp as potential candidates were compared in terms of physical, thermal and mechanical properties. In order to see the effects of fiber surface chemical treatment, sunn hemp fiber was treated with sodium hydroxide (NaOH) agent. Kenaf fiber was selected for the following study due to the higher specific modulus and the moderate price of kenaf fiber. After alkaline treatment, the moisture content, glass-transition temperature, and decomposition temperature of sunn hemp fiber increased but not significantly. The mechanical performance of kenaf/polypropylene nonwoven composites (KPNCs) in production of automotive interior parts was investigated. The uniaxial tensile, three-point bending, in-plane shearing, and Izod impact tests were performed to evaluate the composite mechanical properties. The thermal properties were evaluated using TGA, DSC, and DMA. An adhesive-free sandwich structure was found to have excellent impact resistance performance. Based on the evaluation of mechanical and vii thermal properties, manufacturing conditions of 230 C and 120 s for 6 mm thick sample and 230 C and 60 s for 3 mm thick samples were selected. The open-hole and pin filled-hole effects on the tensile properties of KPNCs in production of automotive interior parts were investigated. Three specimen width-to-hole diameter (W/D) ratios of 6, 3 and 2 were evaluated. A preliminary model by extended finite element method (XFEM) was established to simulate the composite crack propagation. Good agreement was found between experimental and simulation results. Mechanical properties of the KPNCs in terms of uniaxial tensile, open-hole tensile (OHT), and pin filled-hole tensile (FHT) were measured experimentally. By calculating the stress concentration factor Kt for brittle materials, the net section stress factor Kn for ductile materials, and the strength reduction factor Kr, it was found that KPNC was relatively ductile and insensitive to the notch. The strain rate effects on the tensile properties of KPNC were studied. The strain rate effects confirmed the time-dependence of KPNCs. Afterward, the creep behavior of KPNC and PP performed by DMA was investigated extensively. The linear viscoelastic limit (LVL) was found to be 1 MPa in this study. The long-term creep behavior of KPNC compared to virgin PP plastic was predicted using the time-temperature superposition (TTS) principle. Three-day creep tests were also conducted to verify the effectiveness of TTS prediction. It was found that the master curve for PP fit better with the three-day creep data than KPNC, due to the multiphase thermo-rheological complexity of KPNC. The creep recovery, stress effects and cyclic creep performance were also evaluated. Two popular creep models: the four-element Burgers model and the Findley power law model were used to simulate the creep behavior in this study. It was found that KPNC had higher creep resistance and better creep recoverability than virgin PP plastics. / text

Despite the many attractive features of carbon fiber reinforced polymers (FRPs) composites, they are prone to failure due to delamination. The ability to tailor the fiber/matrix interface FRPs is crucial to the development of composite materials with enhanced structural performance. In this dissertation, ZnO nanowires (NWs) were grown on the surface of carbon fibers utilizing low temperature hydrothermal synthesis technique prior to the hybrid composite fabrication. The scanning electron microscopy revealed that the ZnO nanowires were grown uniformly on the surface of the carbon fabric. The surface grown ZnO NWs functionally-graded the composite material properties and ensured effective load transfer across the interface. To assess the influence of the ZnO NWs growth, reference samples were also prepared by exposing the carbon fabric to the hydrothermal conditions. The damping properties of the hybrid ZnO NWs-CFRP composite were examined using the dynamic mechanical analysis (DMA) technique. The results showed enhanced energy dissipation within the hybrid composite. Quasi-static tensile testing revealed that the in-plane and out-of-plane strengths and moduli of the hybrid FRP composite were also boosted. The interlaminar shear strength (ILSS) measurements suggested the improvement in the mechanical properties of the composite to the enhanced adhesion between the ZnO nanowires and the other constituents (carbon fiber and epoxy). It was necessary thus, to utilize the molecular dynamics simulations (MD) to investigate the adhesion within the CFRP structure upon growing the ZnO nanowires on the surface of the carbon fibers. Molecular models of the carbon fibers, the epoxy matrix and the ZnO nanowires were built. The resulting molecular structures were minimized and placed within a simulation box with periodic boundary conditions. The MD simulations were performed using the force field COMPASS to account for the empirical energy interactions between the different toms in the simulation box. Proper statistical thermodynamics were employed to relate the dynamics of the molecular model to the macroscale thermodynamic states (pressure, temperature and volume). Per the computed potential energies of the different components of the composite, it was found that the polar surfaces in the ZnO structures facilitates good adhesion properties in the graphite-epoxy composite. Besides the attractive mechanical properties of the ZnO nanowires, their piezoelectric and semiconductor properties were sought to design an energy harvesting device. To ensure sufficient charges collection from the mechanically stressed individual ZnO nanowires, a copper layer was sputtered on top of the ZnO nanowires which introduced also a Schottky effect. The mechanical excitation was provided by exposing the device to different vibration environment. The output voltage and currents were measured at the conditions (in terms of frequency and resistive load). It was demonstrated that the electrical output could be enhanced by stacking up similar devices in series or in parallel. Finally, in an attempt to exploit the reversibility of the electromechanical coupling of the energy harvesting device, the constitutive properties of the hybrid ZnO nanowires-CFRP composite were estimated using the Mori-Tanaka approach. This approach was validated by a finite element model (FEM). The FEM simulations were performed on a representative volume element (RVE) to reduce the computational time. The results demonstrated that the mechanical properties of the hybrid ZnO NWs-CFRP composite were better than those for the baseline CFRP composite with identical carbon fiber volume fraction (but with no ZnO NWs) which confirmed the experimental findings. Furthermore, the electro-elastic properties of the hybrid composite were determined by applying proper boundary conditions to the FE RVE. The work outlined in this dissertation will enable significant advancement in the next generation of hybrid composites with improved structural and energy harvesting multifunctionalties. / Ph. D.

Human dental tissues are fascinating nano-structured hierarchical materials that combine organic and mineral phases in an intricate and ingenious way to obtain remarkable combinations of mechanical strength, thermal endurance, wear resistance and chemical stability. Attempts to imitate and emulate this performance have been made since time immemorial, in order to provide replacement (e.g. in dental prosthodontics) or to develop artificial materials with similar characteristics (e.g. light armour). The key objectives of the present project are to understand the structure-property relationships that underlie the integrity of natural materials, human dental tissues in particular, and the multi-scale architecture of mineralized tissues and its evolution under thermal treatment and mechanical loading. The final objective is to derive ideas for designing and manufacturing novel artificial materials serving biomimetic purposes. The objectives are achieved using the combination of a range of characterization techniques, with particular attention paid to the synchrotron X-ray scattering (Small- and Wide-Angle X-ray Scattering, SAXS and WAXS) and imaging techniques (Micro Computed Tomography), as well as microscopy techniques such as Environmental Scanning Electron Microscopy (ESEM), Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). Mechanical properties were characterized by nanoindentation and photoelasticity; and thermal analysis was carried out via thermogravimetric analysis (TGA). Experimental observations were critically examined and matched by advanced numerical simulation of the tissue under thermal-mechanical loading. SAXS and WAXS provided the initial basis for elucidating the structure-property relationships in human dentine and enamel through in situ experimentation. Four principal types of experiment were used to examine the thermal and mechanical behaviour of the hierarchical structure of human dental tissue and contributed to the Chapters of this thesis: (i) In situ elastic strain evolution under loading within the hydroxyapatite (HAp) in both dentine and enamel. An improved multi-scale Eshelby inclusion model was proposed taking into account the two-level hierarchical structure, and was validated against the experimental strain evaluation data. The achieved agreement indicates that the multi-scale model accurately reflects the structural arrangement of human dental tissue and its response to applied forces. (ii) The morphology of the dentine-enamel junction (DEJ) was examined by a range of techniques, including X-ray imaging and diffraction. The transition of mechanical properties across the DEJ was evaluated by the high resolution mapping and in situ compression measurement, followed by a brief description of the thermal behaviour of DEJ. The results show that DEJ is a narrow band of material with graded structure and mechanical properties, rather than a discrete interface. (iii) Further investigation regarding the thermo-mechanical structure-property relationships in human dental tissues was carried out by nanoindentation mapping of the nano-mechanical properties in ex situ thermally treated dental tissues. (iv) In order to understand the details of the thermal behaviour, in situ heat treatment was carried out on both human dental tissues and synthetic HAp crystallites. For the first time the in situ ultrastructural alteration of natural and synthetic HAp crystallites was captured in these experiments. The results presented in this thesis contribute to the fundamental understanding of the structure-property integrity mechanisms of natural materials, human dental tissues in particular. These results were reported in several first author publications in peer-reviewed journals, conference proceedings, and a book chapter.

Polyethylene Terephthalate (PET) is one of the most important polymers in use today for packaging due to its outstanding properties. The usage of PET has grown at the highest rate compared with other plastic packaging over the last 20 years, and it is anticipated that the increase in global demand will be around 6% in the 2010-2015 period. The rheological behaviour, thermal properties, tensile modulus, permeability properties and degradation phenomena of PET/clay nanocomposites have been investigated in this project. An overall, important finding is that incorporation of nanoclays in PET gives rise to improvements in several key process and product parameters together - processability/ reduced process energy, thermal properties, barrier properties and stiffness. The PET pellets have been compounded with carefully selected nanoclays (Somasif MAE, Somasif MTE and Cloisite 25A) via twin screw extrusion to produce PET/clay nanocomposites at various weight fractions of nanoclay (1, 3, 5, 20 wt.%). The nanoclays vary in the aspect ratio of the platelets, surfactant and/or gallery spacing so different effect are to be expected. The materials were carefully prepared prior to processing in terms of sufficient drying and re-crystallisation of the amorphous pellets as well as the use of dual motor feeders for feeding the materials to the extruder. The rheological properties of PET melts have been found to be enhanced by decreasing the viscosity of the PET i.e. increasing the 'flowability' of the PET melt during the injection or/and extrusion processes. The apparent shear viscosity of PETNCs is show to be significantly lower than un-filled PET at high shear rates. The viscosity exhibits shear thinning behaviour which can be explained by two mechanisms which can occur simultaneously. The first mechanism proposed is that some polymer has entangled and few oriented molecular chain at rest and when applying high shear rates, the level of entanglements is reduced and the molecular chains tend to orient with the flow direction. The other mechanism is that the nanoparticles align with the flow direction at high shear rates. At low shear rate, the magnitudes of the shear viscosity are dependent on the nanoclay concentrations and processing shear rate. Increasing nanoclay concentration leads to increases in shear viscosity. The viscosity was observed to deviate from Newtonian behaviour and exhibited shear thinning at a 3 wt.% concentration. It is possible that the formation of aggregates of clay is responsible for an increase in shear viscosity. Reducing the shear viscosity has positive benefits for downstream manufacturers by reducing power consumption. It was observed that all ii three nanoclays used in this project act as nucleation agents for crystallisation by increasing the crystallisation temperature from the melt and decreasing the crystallisation temperature from the solid and increasing the crystallisation rate, while retaining the melt temperature and glass transition temperatures without significant change. This enhancement in the thermal properties leads to a decrease in the required cycle time for manufacturing processes thus potentially reducing operational costs and increasing production output. It was observed that the nanoclay significantly enhanced the barrier properties of the PET film by up to 50% this potentially allows new PET packaging applications for longer shelf lives or high gas pressures. PET final products require high stiffness whether for carbonated soft drinks or rough handling during distribution. The PET/Somasif nanocomposites exhibit an increase in the tensile modulus of PET nanocomposite films by up to 125% which can be attributed to many reasons including the good dispersion of these clays within the PET matrix as shown by TEM images as well as the good compatibility between the PET chains and the Somasif clays. The tensile test results for the PET/clay nanocomposites micro-moulded samples shows that the injection speed is crucial factor affecting the mechanical properties of polymer injection moulded products.

A new type of cantilever beam gyroscope is introduced, modeled, and analyzed. The main structure includes a cantilever beam and a rigid body attached to the free end of the beam. The model accounts for the eccentricity, that is the offset of the center of mass of the rigid body relative to the beam's free end. The first and second moments of mass and the rotary inertia appear in the equations of motion and boundary conditions. The common mechanism of electrostatic actuation of microgyroscopes is used with the difference of computing the force at the center of mass resulting in the electrostatic force and moment in the boundary conditions. By using the extended Hamilton's principle, the method of assumed modes, and Lagrange's differential equations, the equations of motion, boundary conditions, and the discretized model are developed. The generalized model simplifies to other beam gyroscope models by setting the required parameters to zero. Considering the DC and AC components of the actuating and sensing methods, the response is resolved into the static and dynamic components. The static configuration is studied for an increasing DC voltage. For the uncoupled system of equations, the explicit equation relating the DC load and the static configuration is computed and solved for the static configuration of the beam-rigid body in each direction. Including the rotation rate, the stationary analysis is performed, the stationary pull-in voltage is identified, and it is shown that the angular rotation rate does not affect the static configuration. The modal frequencies of the beam-rigid body gyroscope are studied and the instability region due to the rotation rate is computed. It is shown that the gyroscope can operate in the frequency modulation mode and the amplitude modulation mode. To operate the beam-rigid body gyroscope in the frequency modulation mode, the closed-form relation of the observed modal frequency split and the input rotation rate is computed. The calibration curves are generated for a variety of DC loads. It is shown that the scale factor improves by matching the zero rotation rate natural frequencies. The method of multiple scales is used to study the reduced-order nonlinear dynamics of the oscillations around the static equilibrium. The modulation equations, the ``slow'' system, are derived and solved for the steady-state solutions. The computational shooting method is employed to evaluate the results of the perturbation method. The frequency response and force response plots are generated. For combinations of parameters resulting in a single-valued response, the two methods are in excellent agreement. The synchronization of the response occurs in the sense direction for initially mismatched natural frequencies. The global stability of the system is studied by drawing phase-plane diagrams and long-time integration of response trajectories. The separatrices are computed, the jump phenomena is numerically shown, and the dynamic pull-in of the response is demonstrated. The fold bifurcation points are identified and it is shown that the response jumps to the higher/lower branch beyond the bifurcation points in forward/backward sweep of the amplitude and the excitation frequency of AC voltage. The mechanical-thermal (thermomechanical) noise effect on the sense response is characterized by using a linear approximation of the system and the nonlinear "slow" system obtained by using the method of multiple scales. To perform linear analysis, the negligible effect of Coriolis force on the drive amplitude is discarded. The second-order drive resonator is solved for the drive amplitude and phase. Finding the sense response due to the thermal noise force and the Coriolis force and equating them computes the mechanical-thermal noise equivalent rotation rate in terms of system parameters and mode shapes. The noise force is included in the third-order equation of the perturbation and equation to account for that in the reduced-order nonlinear response. The numerical results of linear and reduced-order nonlinear thermal noise analyses agree. It is shown that higher quality factor, higher AC voltage, and operating at lower DC points result in better resolution of the microsensor.