In the past two decades depth-sensing indentation has becomea widely used technique to measure the mechanical properties ofmaterials. This technique is particularly suitable for thecharacterisation of materials at sub-micro or nano scale thoughthere is a tendency to extend its application to the micro ormacro scale. The load-penetration depth curve of depth-sensingindentation is a characteristic of a material and can be usedfor analysing various mechanical properties in addition tohardness. This thesis deals with the mechanicalcharacterisation of bulk materials, thin films and coatings,gradient materials, and composites using depth-sensingindentation. Finite element method has been resorted to as atool to understand the indentation behaviour of materials. The piling-up or sinking-in behaviour of materials plays animportant role in the accurate determination of materialsproperties using depth-sensing indentation. Finite elementsimulations show that the piling-up or sinking-in behaviour isdetermined by the material parameters, namelyE/σyratio and strain hardening exponent orexperimental parameterhe/hmaxratio, and the contact friction. Anempirical model has been proposed to relate the contact area ofindentation to theE/σyratio and thehe/hmaxratio and used to predict thepiling-up orsinking-in of materials. The existence of friction is found toenhance the sinking-in tendency of materials. A generalrelationship between the hardness and the indentationrepresentative stress valid for both soft and hard materialshas been obtained. A possible method to estimate the plasticproperties of bulk materials has been suggested. Measuring the coating-only properties requires theindentation to be done within a critical penetration depthbeyond which substrate effect comes in. The ratio of thecritical penetration depth to the coating thickness determinedby nanoindentation is independent of coating thickness andabout 0.2 for gold / nickel, 0.4 for aluminium / BK7 glass, and0.2 for diamond-like-carbon / M2 steel and alumina / nickel.Finite element simulations show that this ratio is dependent onthe combination of the coating and the substrate and moresensitive to differences in the elastic properties than in theplastic properties of the coating/substrate system. Thedeformation behaviour of coatings, such as, piling-up of thesoft coatings and cracking of the hard coatings, has also beeninvestigated using atomic force microscope. The constraint factors, 2.24 for WC phase and 2.7 for WC-Cocemented carbides, are determined through nanoindentation andfinite element simulations. A modified hardness model of WC-Cocemented carbides has been proposed, which gives a betterestimation than the Lee and Gurland hardness model. Finiteelement method has also been used to investigate theindentation behaviour of WC-Co gradient coatings. Keywords:depth-sensing indentation, nanoindentation,finite element method, atomic force microscope, mechanicalproperties, hardness, deformation, dislocations, cracks,piling-up, sinking-in, indentation size effect, thin coatings,composite, gradient materials, WC-Co, diamond-like-carbon,alumina, gold, aluminium, nickel, BK7 glass, M2 steel.

The stress corrosion cracking (SCC) condition for Alloy 600 was determined for various stress modes: constant-strain standard C-ring, and indentation, used to localize cracks for interrogation with x-ray techniques such as micro Laue diffraction (MLD). The SCC cracks nucleated on both the indentation edge, where finite element analysis showed that the maximum residual tensile stresses lie, and the surface in tension (bulge) on 150-kgf conically indented mill-annealed specimens (0.02 wt% C) in de-aerated solution of 10% caustic at 150 mVRE (pseudo-reference: A600), 315 OC for 48 hr. On the C-rings, the cracks nucleated at the lateral outer surface of apex, where maximum tensile stresses lie, in less than 12 hours, and propagated into the cross section. Also, corrosion tests on as-received A600 30-min ZrO2 surface mechanical attrition treated (SMAT) specimens suggested an intergranular attack type of behavior in 50% caustic at 210 mVRE (pseudo-reference: A600), 280°C for 24 hr.

The stress corrosion cracking (SCC) condition for Alloy 600 was determined for various stress modes: constant-strain standard C-ring, and indentation, used to localize cracks for interrogation with x-ray techniques such as micro Laue diffraction (MLD). The SCC cracks nucleated on both the indentation edge, where finite element analysis showed that the maximum residual tensile stresses lie, and the surface in tension (bulge) on 150-kgf conically indented mill-annealed specimens (0.02 wt% C) in de-aerated solution of 10% caustic at 150 mVRE (pseudo-reference: A600), 315 OC for 48 hr. On the C-rings, the cracks nucleated at the lateral outer surface of apex, where maximum tensile stresses lie, in less than 12 hours, and propagated into the cross section. Also, corrosion tests on as-received A600 30-min ZrO2 surface mechanical attrition treated (SMAT) specimens suggested an intergranular attack type of behavior in 50% caustic at 210 mVRE (pseudo-reference: A600), 280°C for 24 hr.

Proton Exchange Membrane (PEM) fuel cell stack requires gaskets and seals in each cell to keep the hydrogen and air/oxygen within their respective regions. The stability of the gaskets is critical to the operating life as well as the electrochemical performance of the fuel cell. Chemical degradation of five elastomeric gasket materials in a simulated and an aggressive accelerated fuel cell solution at PEM operating temperature for up to 63 weeks was investigated in this work. The five materials are Copolymeric Resin (CR), Liquid Silicone Rubber (LSR), Fluorosilicone Rubber (FSR), Ethylene Propylene Diene Monomer Rubber (EPDM), and Fluoroelastomer Copolymer (FKM). In order to assess the durability of the materials, observation of chemical degradation level, dynamic mechanical analysis, and micro-indentation test were adopted in this study. This experimental result showed that the influence of the chemical reaction could affect the material surface condition. Also, the chemical reaction could affect material¡¦s mechanical properties had been changed over the soaking time. By considering the level of chemical degradation and mechanical properties, the experimental results showed that EPDM is recommended as the best choice of sealing material for using in a PEMFC.

The mechanical and electrical properties of thin films have been become important and urgent in recent years, especially, the laminated structure made by films stacked over hundreds of layers. For example, the Multi-Layered Ceramic Capacitors (MLCCs) are such structures fabricated by one layer ceramic film interleaves with one layer electrode film repeatedly a hundred times. Thus, the advantages of MLCCs include small volume, mass product, and high capacity. That makes the MLCCs the necessary part of passive components. The Finite element method is adopted in the study. The model is built by the simulation program of ANSYS. After meshing and setting boundary conditions, the numerical process is performed. The numerical simulation was started first by applying a uniformly distributed pressure on the top of near hundred layers of MLCCs before sintering process with the bottom plate fixed. Then, the displacement and stress fields of MLCCs under five pressures were obtained and discussed. In order to visualize the results, the data of displacement and the stress fields were listed in Tables and plot in Figures. In addition to the MLCCs under vertically and uniformly distributed pressure, the slightly slant distributed pressure and gradient distributed pressure had been simulated. Next, the results of changing Young¡¦s modulus had also been received. It is found that the vertical distributed pressure and slant distributed pressure were not the main factor led to the side deformation. The lateral constraint of gradient distributed pressure would influence the deformation of the MLCCs significantly.

<p>In the past two decades depth-sensing indentation has becomea widely used technique to measure the mechanical properties ofmaterials. This technique is particularly suitable for thecharacterisation of materials at sub-micro or nano scale thoughthere is a tendency to extend its application to the micro ormacro scale. The load-penetration depth curve of depth-sensingindentation is a characteristic of a material and can be usedfor analysing various mechanical properties in addition tohardness. This thesis deals with the mechanicalcharacterisation of bulk materials, thin films and coatings,gradient materials, and composites using depth-sensingindentation. Finite element method has been resorted to as atool to understand the indentation behaviour of materials.</p><p>The piling-up or sinking-in behaviour of materials plays animportant role in the accurate determination of materialsproperties using depth-sensing indentation. Finite elementsimulations show that the piling-up or sinking-in behaviour isdetermined by the material parameters, namely<i>E/σ</i><i>y</i>ratio and strain hardening exponent orexperimental parameter<i>h</i><i>e</i><i>/h</i><i>max</i>ratio, and the contact friction. Anempirical model has been proposed to relate the contact area ofindentation to the<i>E/σ</i><i>y</i>ratio and the<i>h</i><i>e</i><i>/h</i><i>max</i>ratio and used to predict thepiling-up orsinking-in of materials. The existence of friction is found toenhance the sinking-in tendency of materials. A generalrelationship between the hardness and the indentationrepresentative stress valid for both soft and hard materialshas been obtained. A possible method to estimate the plasticproperties of bulk materials has been suggested.</p><p>Measuring the coating-only properties requires theindentation to be done within a critical penetration depthbeyond which substrate effect comes in. The ratio of thecritical penetration depth to the coating thickness determinedby nanoindentation is independent of coating thickness andabout 0.2 for gold / nickel, 0.4 for aluminium / BK7 glass, and0.2 for diamond-like-carbon / M2 steel and alumina / nickel.Finite element simulations show that this ratio is dependent onthe combination of the coating and the substrate and moresensitive to differences in the elastic properties than in theplastic properties of the coating/substrate system. Thedeformation behaviour of coatings, such as, piling-up of thesoft coatings and cracking of the hard coatings, has also beeninvestigated using atomic force microscope.</p><p>The constraint factors, 2.24 for WC phase and 2.7 for WC-Cocemented carbides, are determined through nanoindentation andfinite element simulations. A modified hardness model of WC-Cocemented carbides has been proposed, which gives a betterestimation than the Lee and Gurland hardness model. Finiteelement method has also been used to investigate theindentation behaviour of WC-Co gradient coatings.</p><p><b>Keywords:</b>depth-sensing indentation, nanoindentation,finite element method, atomic force microscope, mechanicalproperties, hardness, deformation, dislocations, cracks,piling-up, sinking-in, indentation size effect, thin coatings,composite, gradient materials, WC-Co, diamond-like-carbon,alumina, gold, aluminium, nickel, BK7 glass, M2 steel.</p>

Swelling of a polymer gel is a kinetic process coupling mass transport and mechanical deformation. A comparison between a nonlinear theory for polymer gels and the classical theory of linear poroelasticity is presented. It is shown that the two theories are consistent within the linear regime under the condition of a small perturbation from an isotropically swollen state of the gel. The relationships between the material properties in the linear theory and those in the nonlinear theory are established by a linearization procedure. Both linear and nonlinear solutions are presented for swelling kinetics of substrate-constrained and freestanding hydrogel layers. A new procedure is suggested to fit the experimental data with the nonlinear theory. A nonlinear, transient finite element formulation is presented for initial boundary value problems associated with swelling and deformation of hydrogels, based on nonlinear continuum theories for hydrogels with compressible and incompressible constituents. The incompressible instantaneous response of the aggregate imposes a constraint to the finite element discretization in order to satisfy the LBB condition for numerical stability of the mixed method. Three problems of practical interests are considered: constrained swelling, flat-punch indentation, and fracture of hydrogels. Constrained swelling may lead to instantaneous surface instability. Indentation relaxation of hydrogels is simulated beyond the linear regime under plane strain conditions, and is compared with two elastic limits for the instantaneous and equilibrium states. The effects of Poisson’s ratio and loading rate are discussed. On the study of hydrogel fracture, a method for calculating the transient energy release rate for crack growth in hydrogels, based on a modified path-independent J-integral, is presented. The transient energy release rate takes into account the energy dissipation due to diffusion. Numerical simulations are performed for a stationary center crack loaded in mode I, with both immersed and non-immersed chemical boundary conditions. Both sharp crack and blunted notch crack models are analyzed over a wide range of applied remote tensile strains. Comparisons to linear elastic fracture mechanics are presented. A critical condition is proposed for crack growth in hydrogels based on the transient energy release rate. The applicability of this growth condition for simulating concomitant crack propagation and solvent diffusion in hydrogels is discussed. / text

In recent years, tissue engineering has shown great potential in treatment of injured tissues which aims to create artificial structures for cells to regenerate new tissues for replacing the damaged and diseased ones. The selection of scaffold materials is one of the critical factors affecting tissue healing process. Among a wide range of scaffold materials, chitosan (CS) has been demonstrated as an ideal material due to its biocompatibility, nontoxicity, biodegradability, antibacterial activity and favorable strength and stiffness. However, its insufficient mechanical properties limits its feasibility and scope for clinical application, especially for bone scaffolds. The main purpose of the study is to explore the potential of incorporation of nanofillers into CS to enhance the mechanical properties for tissue engineering. In this work, nanodiamond (ND) is applied and studied due to its high surface to volume ratio, rich surface chemistry, high mechanical strength, and excellent biocompatibility. ND/CS nanocomposites with different diamond concentration from 1wt. % to 5wt. % were synthetized through a solution casting method. The microstructure and mechanical properties of the composites were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Differential scanning calorimetry (DSC), and nanoindentation. Compared with pristine CS, the addition of ND resulted in a dramatic improvement of mechanical properties, including a 239%, 276%, 321%, 333%, and 343% increase in Young’s modulus and 68%, 96%, 114%, 118%, and 127% increase in hardness when ND amount is 1wt. %, 2wt. %, 3wt. %, 4wt. %, and 5wt. %, respectively. The strong interaction between ND surface groups and chitosan matrix is of great importance in changing polymer structure and improving mechanical properties. The cell viability and cytotoxicity of the nanocomposite were also studied using MTT (3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide) assay. The results show that the addition of ND has no negative effect on cell viability and the nanocomposites have no cytotoxicity.

TiNi shape memory alloys (SMAs) have been extensively used in various applications. The great interest in TiNi alloys is due to its unique shape memory and superelasticity effects, along with its superior wear and dent resistance. Shape memory and superelastic effects are due to a reversible martensitic transformation that can be induced either thermally or mechanically. In this study, indentation tests at different temperatures, loads and strain rates have been performed on superelastic TiNi alloy. Deformation characteristics of superelastic TiNi under indentation have been compared to AISI 304 steel as a conventional material. Also, in-situ optical microscopy tests with interrupted heating have been employed in order to gain an insight into the coupled deformation and reversible martensitic transformation behaviour of TiNi SMAs under tensile loads. An understanding of the impacts of strain rate and temperature on the deformation behaviour of TiNi SMAs under localized compressive loads has been proposed.

Bioactive glasses react with the human physiological solution in control of their biofunctionality. The stress state in bioactive glasses determines the chemomechanical reaction and their biofunctionality. Using the microindentation technique, the effect of the indentation deformation on the surface damage and material dissolution of 45S5 bioglass was investigated. The indentation-induced residual stresses were calculated. Complete anelastic recoveries of the indentation depths and the impression marks were observed for the first time, which was likely driven by the stored strain energy over the anelastic deformation zone. The indentation-induced local surface damages were revealed before and after the immersion tests in phosphate buffer solution (PBS). The growth of the cracks in the PBS solution displayed the stress-corrosion behavior with the crack-growth speed being a linear function of the indentation load. 45S5-bioglass was crystallized at temperature of 650 ºC. Microindentation technique also was used to study the localized mechanical behavior of the crystallized 45S5-bioglass. The crystallization had little effect on the indentation hardness, and the indentation hardness of the crystallized 45S5-bioglass is the same as that of the corresponding material in vitreous state. The fracture toughness is about 3 times less than that of annealed 45S5-bioglass in vitreous state, suggesting the preference of using bioactive glasses of vitreous state in the implant applications. Also, the effect of crystallization on the material dissolution was examined in phosphate buffer solution. We also studied the growth and mechanical behaviors of the Ca-P precipitate layers formed on 45S5 bioglass in simulated body fluid. The thickness of the Ca-P precipitate layers was proportional to the square root of the immersion time, and the ratio of Ca/P in the Ca-P precipitate layers increased with the immersion time and approached 1.67, corresponding to the stoichiometric hydroxyapaptite (HA).Using the indentation technique, the indentation behavior of the Ca-P precipitate layers was investigated. The indentation hardness of the HA layers formed in SBF was found to be 0.40 GPa, and the contact modulus was 12.0 GPa. The contact modulus of 12.0 GPa is close to that of cortical bone. In this thesis, the primary mechanical properties of the non-crystalline and crystalline bioglass 45S5 were revealed. The relationship between the dissolution rate and localized residual stresses are discussed. With such knowledge, the evaluation of implants with respect to manufacturing processes, control, and service conditions now has another variable to consider and evaluate against performance.