Dissertations / Theses on the topic 'Nanoindentation testing'

Inverse analysis of nanoindentation data has attracted increasing interest in industry due to its ability to estimate the bulk tensile properties of materials and potentially offers an alternative technique to conventional characterisation methods. Inverse analysis of nanoindentation data is particularly valuable in applications where conventional techniques are not suitable due to either the scale of characterisation (very small regions) or because the testing is expensive and time consuming. Despite using best practices to minimise sources of error in the experimental data, given the scale of the indentations, the heterogeneity of material microstructure can create significant variability in the data, ultimately affecting the reliability of the inverse analysis solution. This thesis proposes and discusses pragmatic approaches to mitigate the effects of material heterogeneity on the accuracy of the inverse problem solution as well as of nanoindentation data in general. The work has involved finite element analysis modelling, nanoindentation and tensile testing. One mitigation approach consisted in the implementation and verification of a new ‘multi-objective’ function inverse analysis methodology where the bias of selecting only one experimental nanoindentation curve as representative of the homogenised response of the material is overcome. The new approach uses all the experimental curves generated from a grid of nanoindentations and employs a weighted averaging procedure. This methodology was applied to S355 steel samples through recording nanoindentation and tensile test data. Despite the variation present in the experimental nanoindentation load-depth curves, this being in the order of 13%, the ‘multi-objective’ function approach was found to estimate the tensile parameters with an error margin as low as 3-6% compared to an error margin of 9-20% for the conventional method. A framework of activities was also undertaken to monitor the variation of the measured nanoindentation properties (e.g. hardness) as function of the indentation depth, in relation to the average grain size of the material. Commercial purity aluminium 1050 samples (with varying average grain sizes) and S355 steel were employed as test materials. These results in addition to those from other materials were used to construct a look-up plot of the hardness COV values as function of the normalised nanoindentation depths (normalised with respect to the average grain diameter). The plot is based on upper and lower bound curves and intends to provide guidance on the selection of the nanoindentation testing parameters to minimise the variability of the indentation response.

Current body armor technologies need further improvements in their design to help reduce combat injuries of military and law enforcement personnel. Kevlar-based body armor systems have good ballistic resistance up to a certain ballistic threat level due to limitations such as decreased mobility and increased weight [1,2]. Kevlar fibers have been modified in this work using a nano-scale boron carbide coating and a marked increase in the puncture resistance has been experimentally observed. It is hypothesized that this improvement is due to the enhancement of the mechanical properties of the individual Kevlar fibers due to the nano-scale coatings. This study presents a comprehensive experimental investigation of individual Kevlar fibers based on nanoindentation to quantify the cause of the enhanced puncture resistance. The experimental setup was validated using copper wires with a diameter size in the same order of magnitude as Kevlar fibers. Results from nanoindentation did not show significant changes in the modulus or hardness of the Kevlar fibers. Scanning Electron Microscopy revealed that the coated fibers had a marked change in their surface morphology. The main finding of this work is that the boron carbide coating did not affect the properties of the individual fibers due to poor adhesion and non-uniformity. This implies that the observed enhancement in puncture resistance originates from the interaction between fibers due to the increase in roughness. The results are important in identifying further ways to enhance Kevlar puncture resistance by modifying the surface properties of fibers.

Nanoindentation has brought in many features of research over the past decade. This novel technique is capable of producing insights into the small ranges of deformation. This special point has brought a lot of focus in understanding the deformation behavior under the indenter. Nickel, iron, tungsten and copper-niobium alloy system were considered for a surface deformation study. All the samples exhibited a spectrum of residual deformation. The change in behavior with indentation and the materials responses to deformation at low and high loads is addressed in this study. A study on indenter geometry, which has a huge influence on the contact area and subsequently the hardness and modulus value, has been attempted. Deformation mechanisms that govern the plastic flow in materials at low loads of indentation and their sensitivity to the rate of strain imparted has been studied. A transition to elastic, plastic kind of a tendency to an elasto-plastic tendency was seen with an increase in the strain rate. All samples exhibited the same kind of behavior and a special focus is drawn in comparing the FCC nickel with BCC tungsten and iron where the persistence of the elastic, plastic response was addressed. However there is no absolute reason for the inconsistencies in the mechanical properties observed in preliminary testing, more insights can be provided with advanced microscopy techniques where the study can be focused more to understand the deformation behavior under the indenter. These experiments demonstrate that there is a wealth of information in the initial stages of indentation and has led to much more insights into the incipient stages of plasticity.

This study deals with crystal orientation effect along with the effects of microstructure on the pile-ups which affect the nanoindentation measurements. Two metal classes, face centered cubic (FCC) and body centered cubic (BCC, are dealt with in the present study. The objective of this study was to find out the degree of inaccuracy induced in nanoindentation measurements by the inherent pile-ups and sink-ins. Also, it was the intention to find out how the formation of pile-ups is dependant upon the crystal structure and orientation of the plane of indentation. Nanoindentation, Nanovision, scanning electron microscopy, electron dispersive spectroscopy and electron backscattered diffraction techniques were used to determine the sample composition and crystal orientation. Surface topographical features like indentation pile-ups and sink-ins were measured and the effect of crystal orientation on them was studied. The results show that pile-up formation is not a random phenomenon, but is quite characteristic of the material. It depends on the type of stress imposed by a specific indenter, the depth of penetration, the microstructure and orientation of the plane of indentation. Pile-ups are formed along specific directions on a plane and this formation as well as the pile-up height and the contact radii with the indenter is dependant on the aforesaid parameters. These pile-ups affect the mechanical properties like elastic modulus and hardness measurements which are pivotal variables for specific applications in micro and nano scale devices.

Mechanical characterization of four polymer nanocomposite systems and two pure polymer reference systems was performed. Alumina (Al2O3) and magnetite (Fe3O4) nanoparticles were embedded in poly(methyl methacrylate) (PMMA) and polystyrene (PS) matrices. Mechanical testing techniques utilized include tensile testing, dynamic mechanical analysis (DMA), and nanoindentation. Consistent results from the three techniques proved that these nanocomposite systems exhibit worse mechanical properties than their respective pure polymer systems. The interphase, an interfacial area between the nanoparticle filler and the polymer matrix, was investigated using two approaches to explain the mechanical testing results. The first approach utilized data from thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM) to predict the structure and density of the interphase for the four nanocomposite systems. The second approach analyzed the bonding between the polymer and the nanoparticle surfaces using Fourier Transform Infrared Spectroscopy (FT-IR) to calculate the density of the interphase for the two PMMA-based nanocomposite systems. Results from the two approaches were compared to previous studies. The results indicate that Al2O3 nanoparticles are more reactive with the polymer matrix than are Fe3O4 nanoparticles, but neither have strong interaction with the polymer matrix. The poor interaction leads to low density interphase which results in the poor mechanical properties.

Butt fusion welding process is an extensively used method of joining for high density polyethylene (HDPE) pipe. With the increasing number of HDPE resin and pipe manufacturers and the diversity of industries utilising HDPE pipes, a wide range of different standards have evolved to specify the butt fusion welding parameters with inspection and testing methods, to maintain quality and structural integrity of welds. There is a lack of understanding and cohesion in these standards for the selection of welding parameters; effectiveness, accuracy, and selection of the test methods and; correlation of the mechanical properties to the micro and macro joint structure. The common standards (WIS 4-32-08, DVS 2207-1, ASTM F2620, and ISO 21307) for butt fusion welding were used to derive the six welding procedures. A total of 48 welds were produced using 180 mm outer diameter SDR 11 HDPE pipe manufactured from BorSafe™ HE3490-LS black bimodal PE100 resin. Three short term coupon mechanical tests were conducted. The waisted tensile test was able to differentiate the quality of welds using the energy to break parameter. The tensile impact test due to specimen geometry caused the failure to occur in the parent material. The guided side bend specimen geometry proved to be too ductile to be able to cause failures. A statistical t-test was used to analyse the results of the short term mechanical tests. The circumferential positon of the test specimen had no impact on their performance. Finite element analysis (FEA) study was conducted for the long term whole pipe tensile creep rupture (WPTCR) test to find the minimum length of pipe required for testing based on pipe geometry parameters of outer diameter and SDR. Macrographs of the weld beads supplemented with heat treatment were used to derive several weld bead parameters. The FEA modelling of the weld bead parameters identified the length to be a key parameter and provided insight into the relationship between the geometry of the weld beads and the stresses in the weld region. The realistic bead geometry digitised using the macrographs contributed a 30% increase in pipe wall stress due to the stress concentration effect of the notches formed between the weld beads and the pipe wall. The circumferential position of the weld bead had no impact on the pipe wall stresses in a similar manner to the results of the different mechanical tests. IV Nanoindentation (NI) and differential scanning calorimetry (DSC) techniques were used to study the weld microstructure and variation of mechanical properties across the weld at the resolutions of 100 and 50 microns, respectively. NI revealed signature 'twin-peaks and a valley' distribution of hardness and elastic modulus across the weld. The degrees of crystallinity obtained from DSC followed the NI pattern as crystallinity positively correlates with the material properties. Both techniques confirm annealing of the heat affected zone (HAZ) material towards the MZ from the parent material. The transmission light microscopy (TLM) was used to provide dimensions of the melt zone (MZ) which displays an hour glass figure widening to the size of the weld bead root length towards the pipe surfaces. Thermal FEA modelling was validated using both NI and TLM data to predict the HAZ size. The HAZ-parent boundary temperature was calculated to be 105 ⁰C. The 1st contribution of the study is to prove the existence of a positive correlation between the heat input calculated from FEA and the energy to break values obtained from the waisted tensile test. The 2nd contribution providing the minimum length of pipe for WPTCR based on the pipe dimensions. The 3rd contribution is the recommendation for the waisted tensile test with the test using the geometry designed to minimise deformation of the loading pin holes. The 4th contribution related the weld bead parameters to pipe wall stresses and the effect of notches as stress concentrators. The 5th contribution is a new method of visualising a welding procedure that can be used to not only compare the welding procedures but also predict the size of the MZ and the HAZ. The 6th contribution of the study is the proposal of new weld bead geometry that consist of the MZ bounded by the HAZ, for butt fusion welded joints of HDPE pipes.

One of the many promising applications of metal/ceramic joining is in biomedical implantable devices. This work is focused on vacuum brazing of C.P titanium to 96% alumina ceramic using pure gold as the filler metal. A novel method of brazing is developed where resistance heating of C.P titanium is done inside a thermal evaporator using a Ta heating electrode. The design of electrode is optimized using Ansys resistive heating simulations. The materials chosen in this study are biocompatible and have prior history in implantable devices approved by FDA. This research is part of Boston Retinal implant project to make a biocompatible implantable device (www.bostonretina.org). Pure gold braze has been used in the construction of single terminal feedthrough in low density hermetic packages utilizing a single platinum pin brazed to an alumina or sapphire ceramic donut ( brazed to a titanium case or ferrule for many years in implantable pacemakers. Pure gold (99.99%) brazing of 96% alumina ceramic with CP titanium has been performed and evaluated in this dissertation. Brazing has been done by using electrical resistance heating. The 96% alumina ceramic disk was manufactured by high temperature cofired ceramic (HTCC) processing while the Ti ferrule and gold performs were purchased from outside. Hermetic joints having leak rate of the order of 1.6 X 10-8 atm-cc/ sec on a helium leak detector were measured. Alumina ceramics made by HTCC processing were centreless grounded utilizing 800 grit diamond wheel to provide a smooth surface for sputtering of a thin film of Nb. Since pure alumina demonstrates no adhesion or wetting to gold, an adhesion layer must be used on the alumina surface. Niobium (Nb), Tantalum (Ta) and Tungsten (W) were chosen for evaluation since all are refractory (less dissolution into molten gold), all form stable oxides (necessary for adhesion to alumina) and all are readily thin film deposited as metals. Wetting studies are also performed to determine the wetting angle of pure gold to Ti, Ta, Nb and W substrates. Nano tribological scratch testing of thin film of Nb (which demonstrated the best wetting properties towards gold) on polished 96% alumina ceramic is performed to determine the adhesion strength of thin film to the substrate. The wetting studies also determined the thickness of the intermetallic compounds layers formed between Ti and gold, reaction microstructure and the dissolution of the metal into the molten gold.

Disertační práce se zabývá studiem strukturních a mechanických vlastností anorganických materiálů. Cílem je nalezení jednotlivých fází ve zkoumaném materiálu a hlavně lokalizace (mechanicky) nejslabšího místa, jeho ovlivnění a následně výroba materiálu o lepších mechanických vlastnostech. Z důvodu velkého množství použitých metod je základní teorie vložena vždy na začátku příslušné kapitoly. Taktéž z důvodu značného množství výsledků jsou na konci kapitol uvedeny dílčí závěry. Práce je rozdělena na tři části, kdy první se zabývá seznámením s možnostmi modelování mikro-mechanických vlastností a provedením experimentů umožňujících posouzení rozsahu platnosti některého modelu. V druhé části je provedeno shrnutí současných možností indentačních zkoušek pro měření mechanických vlastností strukturních složek betonu a praktické zvládnutí metodiky vhodné k užití pro výzkum materiálů zkoumaných domovským pracovištěm. V třetí části je navržena metoda identifikace nejslabších článků struktury anorganických pojiv a její ověření na konkrétním materiálu zkoumaném na domovském pracovišti. V této dizertační práci jsou použity tyto metody: kalorimetrie, ultrazvukové testování, jednoosá pevnost v tlaku, nanoindentace, korelativní mikroskopie a rastrovací elektronová mikroskopie s energiově disperzním spektrometrem. Dílčími výsledky jsou kompletní charakterizace cementových materiálů, upřesnění stávajících poznatků a nalezení optimálního postupu pro charakterizaci. Hlavním výsledkem je inovativní přístup vedoucí k pozitivnímu ovlivnění materiálu.

Les nanostructures ont des propriétés mécaniques qui diffèrent de celles des matériaux massifs. La compréhension des propriétés mécaniques aux échelles nanométriques requièrent la mise en place d’essais mécaniques combinés à des observations structurales. Au cours de cette thèse nous avons développé un microscope à force atomique (AFM) dédié permettant de solliciter mécaniquement un nano-objet unique sur une ligne de lumière synchrotron. Les possibilités offertes par cette nouvelle approche expérimentale sont démontrées sur deux exemples de sollicitation mécanique in situ: (i) la nanoindentation in situ de cristaux d’or combinée à la diffraction cohérente des rayons X; (ii) la flexion trois points de nanofils d’or associée à la micro-diffraction de Laue. Ces expériences permettent d'accéder au comportement élastique ainsi qu’au comportement plastique du nanomatériau et permettent de déterminer la limite d'élasticité et le type de défauts induits par le chargement mécanique
Nanostructures were found to exhibit different mechanical properties compared to their bulk counterpart. For obtaining further insight into the mechanical behaviour on the nanoscale, mechanical tests are combined with observation techniques allowing for monitoring the structural evolution. Within this thesis a special atomic force microscope has been developed which is compatible with different X-ray diffraction techniques at synchrotron sources for in situ mechanical testing on single nano-objects. The great potential of the new experimental approach is demonstrated on two kinds of in situ mechanical tests: (i) in situ nano-indentation on Au crystals with coherent X-ray diffraction. (ii) In situ three point bending tests on Au nanowires with μLaue diffraction. These experiments give access to the elastic as well as the plastic behavior of the nanomaterial and allows for determining the elastic limit and the type of defects induced by the mechanical loading

This thesis was most concerned with the mechanical response to irradiation of two in-house produced oxide dispersion strengthened (ODS) steels and two non-ODS coun- terparts. The steels, manufactured by Dr. M. J. Gorley (University of Oxford), were me- chanically alloyed from gas-atomised Fe-14Cr-3W-0.2Ti, with the addition of 0.25Y₂O₃ powder in the case of the ODS variants. The powders were hot isostatic pressed at consolidation temperatures of 950 °C and 1150 °C. The four steels were designated 14WT 950 (non-ODS), 14YWT 950 (ODS), 14WT 1150 (non-ODS) and 14YWT 1150 (ODS), and were used in the as-produced condition. Initially, the macroscale elastic modulus and yield stress were determined using a four-point flexure test, employing digital image correlation (DIC) as a strain gauge. The microcantilever size eects were then characterised, and it was determined that the yield stress signicantly diverged from macroscale values at microcantilever beam depths of < 4.5 μm. Using knowledge of this, the in-house produced alloys were irradiated with 2 MeV protons at the Surrey Ion Beam Centre (University of Surrey, UK) to a displacement damage of ∼ 0.02 dpa and 0.2 dpa (Bragg peak). This was to produce a deep irradiated layer for the fabrication of large microcantilevers with reduced size effects. The cross-sectional surface of the irradiated layer was then exposed and inclined linear arrays of 250 nm deep indents were placed across the damage prole. 14WT 1150 (non-ODS) revealed a clear proton damage prole in plots of hardness against irradiation depth, 14WT 950 (non-ODS) also showed modest hardening in the region of the Bragg peak. No appreciable hardening was observed in either 14YWT specimens, attributed to the fine dispersion of nanoscale oxides providing a high number density of defect sink sites. However, a large bimodal variation in hardness was measured in both ODS variants. This was investigated using EBSD and EDX, and was determined to be caused by a pronounced heterogeneity of the microstructure. While Hall-Petch strengthening and changes in the local chemistry had some effect on the measured hardness, the most likely cause of the large variation in local hardness was heterogeneity in the nanoscale oxide population. Microcantilevers were fabricated out of the irradiated layer cross-section in 14WT 1150 and 14YWT 1150. Larger microcantilevers, with ∼ 5 μm beam depth, were placed with their beam centre at ∼ 0.026 dpa. Smaller microcantilevers, with ∼ 1.5 μm beam depth, were placed with their beam centre at the Bragg peak, 0.2 dpa. Both the large and the small microcantilevers fabricated in 14WT 1150 (non-ODS) displayed significant irradiation hardening. In the ODS variant, 14YWT 1150, irradiation hardening appeared to be reduced. The work in this thesis successfully showed that it was possible to extract a close approximation of the macroscale yield stress from shallow irradiated layers, providing that the irradiation condition is carefully chosen in response to known size dependent behaviour. This thesis also investigated the size dependent behaviour of microcantilevers using a lengthscale dependent crystal plasticity UMAT, developed by Dunne et al. and implemented within ABAQUS 6.14-2 commercially available nite element software. The simulation of the GND density evolution with increasing plastic strain allowed their contribution to the microcantilever size effect, through mobile dislocation pinning, to be determined. This novel approach to modelling size effects in three dimensional finite element microcantilever models demonstrated that while it was possible to simulate a lengthscale-dependent response in finite element microcantilever models, the constitutive equation for the plastic velocity gradient needs to be more physically based in order the match the experimentally derived results; for example, a lengthscale-dependent term relating to the dislocation source density of the material. Although the apparent reduction of irradiation hardening in ODS in-house produced alloys showed great promise, these alloys also displayed a large amount of scatter in measured hardness and yield stress, attributed to the pronounced heterogeneity in the microstructure. Alloys with such signicant microstructural heterogeneity are not suitable for engineering or commercial use.

Thesis (Ph. D.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2008.
Committee Chair: Rami M Haj-Ali; Committee Member: Arash Yavari; Committee Member: Donald W. White; Committee Member: Erian Armanios; Committee Member: Kenneth M. Will.

Thesis (Ph.D)--Civil and Environmental Engineering, Georgia Institute of Technology, 2010.
Committee Co-Chair: Kimberly Kurtis; Committee Co-Chair: Lawrence Kahn; Committee Member: Arun Gokhale; Committee Member: James Lai; Committee Member: T. Russell Gentry. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Nanoindentation is a widely recognized method for characterizing the mechanical properties of thin films and small volumes. This dissertation reports the results of finite element analyses of elastic and elastic-plastic indentation by a rigid cone aimed at improving methods for measuring of contact area, hardness and elastic modulus by nanoindentation methods. Analytical and finite element results are presented which show that corrections to Sneddon's solution are needed to properly describe elastic indentation by a cone. Since most nanoindentation methods are based on Sneddon's solution, these corrections have important consequences for making accurate mechanical property measurements. Elastic-plastic finite element simulations are presented which show that pile-up can significantly affect the accuracy of nanoindentation measurements. It is shown that an experimentally measurable parameter, the ratio of the final depth to the total depth of indentation, is useful in determining the amount of pile-up in the material. An investigation of plastic zones and stresses in indented materials reveals important correlations between them and the nanoindentation behavior of the material. Implications of these results for indentation cracking are also discussed. A long standing problem in nanoindentation is why load-displacement data obtained during unloading fit well to a power relation with a power law exponents in the range 1.25-1.50. Finite element simulations combined with elastic contact analyses are presented which provide a simple explanation for this behavior. General recommendations are made for improving of nanoindentation methods for measuring mechanical properties.

This thesis presents the research results of a correlation and model based on nano and macroindentation hardness measurements. The materials used to develop and test the correlation include bulk tantalum and O1 tool steel. Following the literature review and a detailed description of the experimental techniques, the results of the nanoindentation hardness measurements are presented. After applying the methods and correlation recommended here, the results should give an accurate value of hardness in the Vickers scale for microstructural features that are too small to be precisely and exclusively measured using the traditional macroindentation hardness technique. The phenomena and influential factors in nanoindentation hardness testing are also discussed. These phenomena and theories are consistent with the microstructural behavior predicted in the Nix and Gao model for mechanism-based strain gradients. Implementing the correlation factors and/or correlation curve, accurate results can be found for metals over a broad hardness range. Initially, this research may impact the pipeline division of the petroleum industry by providing a correlation to the Vickers scale for nanoindentation testing of microstructural features. This thesis may also provide a research methodology to develop hardness correlations for materials other than metals. This thesis consists of eight chapters. Following an introduction in Chapter I, the research motivations and objectives are highlighted in Chapter II. Chapter III explains the multi-scale indentation techniques used in this thesis and Chapter IV presents the materials preparation techniques used. Then, the results are presented in Chapter V, followed by the factors affecting nanoindentation hardness in Chapter VI. Finally, Chapters VII and VIII reveal the indentation contact analysis, correlation, and conclusions of this research, respectively.

碩士
逢甲大學
機械工程學所
97
The development of flexible electronics, which use flexible polymer materials to replace the high hardness glass substrate, has been the focus in the lately years. Polymer materials usually have high viscoelasticity which will affect the quality of products. Therefore, studying the viscoelastic properties of flexible films is an important part for flexible industry. In general, the force acting on materials is a change with time. In this study, we apply acoustic shaker to excite a source of harmonic vibration instead of contact exciting method, and we utilize the variation of sound pressure when sound transmits through the materials to define the relation between transmitted sound pressure and strain. Then the experimental results combine the dynamic viscoelastic theory to calculate the dynamic modulus and loss factor of sample, which method is the transmitted sound acquisition. To confirm the reliability of the transmitted sound acquisition, we also apply dynamic nanoindentation testing to compare with the results. Using dynamic nanoindentation testing to measure the samples, which are the same material and different thicknesses, will get the similar result. If the samples are composite films, the experimental results will be rude due to the thickness of plated layer and the surface roughness. However, the transmitted sound acquisition method excites the total sample and makes it vibration, so it can obtain the precise and complete viscoelastic properties of polymer films. The trend of transmitted sound acquisition is similar to dynamic nanoindentation testing, so we can acquire the dynamic viscoelasticity of the flexible films by transmitted sound acquisition. From the results of transmitted sound acquisition, the viscousity of PET films will become larger when the thickness increases. ITO film will lead to the vicoelasticity of PET substrate occur big variation substantially with frequency raises. The coating layer of diffusion can effectually decrease the stiffness and vicoelasticity of its substrate, which the deformation will reduce the influence from time increase.

博士
國立清華大學
動力機械工程學系
98
Ever since the exciting discovery of carbon nanotubes (CNTs), there has been a huge growth in research in material science on finding novel nanostructured materials with advanced material properties. Recently, due to the shrink of feature size in IC technology, nanostructured materials, especially one-dimensional (1-D) nanostructures such as CNTs, nanowires and nanorods, have been considered for use in nanoscale electronic or electromechanical devices as active electronic components or interconnects. Despite of their potential, as claimed, for various engineering applications, the thermal-mechanical properties of nanostructured materials remain not fully determined or clear, not mentioning the effects of the relevant influence factors, such as size, crystal structure and defect. Recent progress in computational methods based on molecular dynamics (MD) methods has allowed the characterizations of the mechanical properties of nanomaterials. The study aims at developing an accurate and effective MD simulation model to explore the thermal-mechanical characteristics of nanostructured materials. The study starts from the evaluation of the fundamental mechanical properties of various single/multi-walled carbon nanotubes (S/MWCNTs), including zig-zag, armchair and hybrid types. The study first focuses on the exploration of the effect of the weak inlayer van der Waals (vdW) atomistic interactions on the mechanical properties of S/MWCNTs. The influence of the axial orientation mismatch between the inner and outer layers of MWCNTs on the associated mechanical properties are also addressed, followed by the investigation of the behaviors of the interlayer shear force/strength of MWCNTs. The effectiveness of the MD simulation is demonstrated through the comparison with the theoretical/experimental data available in literature. Besides, due to the limitation of fabrication technologies nowadays, atomistic defects are often perceived in carbon nanotubes (CNTs) during the manufacturing process. Thus, the second goal of the study is to perform a systematic investigation of the effects of atomistic defects on the nanomechanical properties and fracture behaviors of single-walled CNTs (SWCNTs) using MD simulation. Key parameters and factors under investigation include the number, type (namely the vacancy and Stone-Wales defects), location and distribution of defects. The correlation between local stress distribution and fracture evolution is also discussed. To demonstrate the feasibility of the proposed MD model, the present results are compared with the theoretical/experimental data available in literature. The third goal of the study aims to estimate the elastic properties of three different metal nanowires, namely made of gold (Au), silver (Ag) and cobalt (Co), through MD simulations and nanoindentation testing. The investigation also addresses the effects of the length and cross-sectional area of the nanowires, crystal structure, presumed defect and the variation of grain boundary of the metal crystal on the mechanical properties. Furthermore, tensile test simulation for both the Au (gold) and Ag nanowires is carried out, where the ultimate strength and the necking structure are also evaluated. Verification of the MD simulation model in terms of elastic modulus is made using nanoindentation experiment, and the literature theoretical and experimental data. Finally, the last goal of the study is to establish a multi-material MD simulation model to look into the insight of the effects of self-assembly monolayer (SAM) coating on the interfacial adhesion of an Au-epoxy system and on the bondability of the thermocompression-bonded Au-Au joints. Three different types of functionalized alkanethiol SAMs (SH(CH2)nX, X=CH3, OH, NH2) chemisorbed onto Au substrates, are considered in the investigation. The investigation first explores the elastic properties of these SAMs through uniaxial tensile simulation, followed by exploring the effects of the SAMs on the adhesion behaviors of the Au-epoxy system and the Au-Au system, and those of chain lengths and tail groups of the n-alkanethiolates on the adhesion strength. The study also reports a comparative analysis of the effects of the crystal orientation of Au on the associated interfacial behaviors. The calculated results are partly compared with the published experimental data, and also with each other to identify the optimal SAM candidate in terms of adhesion strength for the Au-epoxy system. The achievements made in this study can not only provide a more thorough and clear understanding of the basic mechanical properties and behaviors of the nanostructured materials and the adhesion behaviors at the Au-Au and Au-epoxy bi-material interfaces, but also give a solid foundation for future research on the nanomechanics and industrial application of the nanostructured materials.

Zirconium alloys (zircaloy) have been widely used in light water reactors due to their good thermomechanical properties, corrosion resistance, and low thermal neutron absorption rate. As one of the most important safety barriers, cladding is not only used to encapsulate nuclear fuel, but also to prevent the nuclear fission products from leaking into the coolant. During the operation of nuclear reactors, hydride will form in zircaloy and significantly degrade the tensile strength, ductility, fracture toughness, and creep behavior of the cladding, and eventually leading to the failure of cladding. Therefore, understanding the material properties of zircaloy and its hydrides is crucial to the safety of power plants. In this study, the mechanical Raman spectroscopy and nano-mechancial testing techniques were used to perform thermomechanical measurements and damage analysis of zircaloy-4. The Raman thermometry method was used to measure localized spatially resolved thermal conductivity and establish the potential linkage of microstructure to thermal and mechanical properties of zircaloy-4. The local thermal conductivity values showed to increase with increase in grain size. Nanoindentation and nano-scale impact techniques were used to obtain the viscoplastic constitutive relation of hydrides at elevated temperatures. Based on the obtained viscoplastic model, fracture strength of hydrides was predicted by using finite element method (FEM) simulations. An extended Gurson-Tvergaard-Needleman (GTN) model was used to study the macro-scale fracture behavior of hydrided zircaloy-4 structures. Good agreement between calculated and experimental results was obtained for various boundary conditions.

abstract: Aluminum alloys are ubiquitously used in almost all structural applications due to their high strength-to-weight ratio. Their superior mechanical performance can be attributed to complex dispersions of nanoscale intermetallic particles that precipitate out from the alloy’s solid solution and offer resistance to deformation. Although they have been extensively investigated in the last century, the traditional approaches employed in the past haven’t rendered an authoritative microstructural understanding in such materials. The effect of the precipitates’ inherent complex morphology and their three-dimensional (3D) spatial distribution on evolution and deformation behavior have often been precluded. In this study, for the first time, synchrotron-based hard X-ray nano-tomography has been implemented in Al-Cu alloys to measure growth kinetics of different nanoscale phases in 3D and reveal mechanistic insights behind some of the observed novel phase transformation reactions occurring at high temperatures. The experimental results were reconciled with coarsening models from the LSW theory to an unprecedented extent, thereby establishing a new paradigm for thermodynamic analysis of precipitate assemblies. By using a unique correlative approach, a non-destructive means of estimating precipitation-strengthening in such alloys has been introduced. Limitations of using existing mechanical strengthening models in such alloys have been discussed and a means to quantify individual contributions from different strengthening mechanisms has been established. The current rapid pace of technological progress necessitates the demand for more resilient and high-performance alloys. To achieve this, a thorough understanding of the relationships between material properties and its structure is indispensable. To establish this correlation and achieve desired properties from structural alloys, microstructural response to mechanical stimuli needs to be understood in three-dimensions (3D). To that effect, in situ tests were conducted at the synchrotron (Advanced Photon Source) using Transmission X-Ray Microscopy as well as in a scanning electron microscope (SEM) to study real-time damage evolution in such alloys. Findings of precipitate size-dependent transition in deformation behavior from these tests have inspired a novel resilient aluminum alloy design.
Dissertation/Thesis
Doctoral Dissertation Materials Science and Engineering 2017

Mechanical surface treatments using an elastic-plastic cold working process can develop residual stresses on the surface of a workpiece. Compressive residual stresses on the surface increase resistance against surface crack propagation, so the overall mechanical performance can be improved by this technique. Compressive residual stresses can be created by different methods such as hammering, rolling, and shot peening. Shot peening is a well-established method to induce compressive residual stresses in the metallic components using cold working, and often ascribed to being beneficial to fatigue life in the aerospace and automobile industries. In this method, the surface is bombarded by high-velocity spherical balls which cause plastic deformation of the substrate, leading to a residual compressive stress after shot peening on the surface of the part. Computational modeling is an appropriate and effective way which can predict the amount of produced residual stresses and plastic deformation to obtain surface roughness after shot peening simulation. Finally, an experimental method to measure the magnitude of the residual stress using a nanoindentation technique was developed. The experimental indentation method was compared to both computational predictions (in aluminum) and with x-ray diffraction measurements of stress (in an alloy steel). The current study validates the relation between the nanoindentation method and numerical simulation for assessing the surface residual stresses resulting from single or multiple shot peening processes.