Characterization of composite materials, such as Asphalt Concrete (AC) and other engineering materials is required to provide data for design and construction. This is usually carried out through various performance tests, which are always time consuming for specimen preparation, equipment calibration and test setting up. For materials with time- and temperature-dependent properties, this procedure requires fabrication of a large number of specimens in order to get reasonably comprehensive results. Furthermore, for materials that consist of phases with significant differences in properties, macroscopic homogeneous assumption or microscopic statistic approximation will lead to complex correction schemes. This will add complexity in material characterization. On the other hand, the homogeneity based interpretation of test results makes it difficult to understand the interaction between different components. The objective of this research is to develop a numerical testing method for material characterization based on x-ray tomography and finite element method. The introduction of tomography technology, such as x-ray tomography into engineering field makes it possible to obtain material microstructure without disturbing the phase configuration. Along with the development of image analysis technology, image data can be manipulated to obtain digitalized sample reconstruction and to build finite element geometric model. Based on well developed material models that sufficiently capture the essential behavior of individual material component, we developed a framework of numerical tests for characterization of composite material. The geometric model imports the microstructural data of the sample, the configuration of aggregates, voids and flakes, through x-ray tomography and image processing. The voids distribution as well as density variation was quantified to verify the model microscopic characteristics. FORTRAN programs were developed to automatically achieve data transfer and model generation, e.g. boundary identification and ABAQUS simulation model generation. Material model was studied and selected for different material components. Viscoplastic material models were evaluated and calibrated in ABAQUS. Monotonic loading and repeated loading were considered in the study to validate the model for most characterization needs. The digital model was validated through small sample tests and was implemented and used in various material characterizations. For the wood panel characterization, the anisotropic elastic properties were studied while the viscous and plastic responses were studied for asphalt concrete. Factors affecting the accuracy and the limitations of the application were determined. It is worth noting that further advance and data collection will make the calibration of material model more accurate. Nevertheless, the work can be extended to other regimes, such as high speed impact especially where the actual testing is complicated to setup. / Ph. D.

The fatigue damage is one of the most common distresses observed on the asphalt concrete pavement. The initiation and propagation of the fatigue damage is a complicated phenomenon and very difficult to detect. In order to thoroughly understand the fatigue of asphalt concrete, the behaviors of the key components of asphalt concrete under cyclic loading are investigated respectively. A new experiment method is developed to test the performances of asphalt binder, mastic and mixture under cyclic loading, which provides a tool to unify the fatigue test method for both binding medium and asphalt mixture. Using the new fatigue test method, the effects of loading magnitude, temperature and loading rate to the performance of the asphalt binder under cyclic loading are estimated. Mastic and mixture specimens are prepared by adding fillers and controlled-size aggregates into the asphalt binder. The effects of filler content to the performance of mastic specimen are discussed. The differences between the test results of mastic and mixture are compared and analyzed. Incorporated with the new fatigue test, x-ray tomography system is used in this study to: 1. Analyze the structure change of the mastic specimen before and after the fatigue test. 2. Compare the void content differences between the mastic and mixture specimens. 3. Reconstruct the 3-D internal structures of mastic and mixture specimens to build up the digital specimens. The digital specimens are used in the fatigue simulation of the asphalt binder, mastic and mixture specimens based on the finite element method. The asphalt binder, filler and aggregate are treated as different materials. Damage parameter is introduced to model the degradation of elastic modulus of the asphalt binder caused by fatigue damage. Direct cyclic analysis available in ABAQUS is used to obtain the response of the material after large number of loading cycles. The basalt fibers are dispersed into the asphalt binder and mastic specimens, the effects of the basalt fiber to the performances of the binder and mastic at low temperature are analyzed using both experimental and FEM modeling methods. / Ph. D.

The excessive presence of liquid water in a gas diffusion layer (GDL) hinders the access of reactant gases to the active sites of the catalyst layer leading to decreased performance of a polymer electrolyte membrane fuel cell (PEMFC). Therefore, GDLs are usually treated with a hydrophobic agent to render their fibres more hydrophobic in order to facilitate gas transport and water removal. Numerous studies have been conducted to investigate water transport in PEMFCs in recent years; however, the behaviour of liquid water in a GDL at a pore-level is poorly understood. Macroscopic models fail to incorporate the influence of the structural morphology of GDLs on liquid water transport behaviour. Experimental methods are not conducive towards a good understanding at a microscopic level because of the diminutive size of the GDLs porous structure. Alternatively, the Lattice Boltzmann (LB) method has gathered interest as it is found to be particularly useful in fluid flow simulations in porous media due to its capability to incorporate the complex boundaries of actual GDL structures. To date, most studies on fluid transport in GDLs integrated artificial structures generated by stochastic simulation techniques to the LB models. The stochastic-based model, however, does not represent closely the microscopic features of the actual GDL as manufactured. In addition, comparison of liquid water transport behaviour in different GDL structures using the LB method is rare since only a single GDL material has been utilised in most of those studies. This thesis aims to develop our understanding of liquid water transport behaviour in GDLs with morphologically different structures under varying wettability conditions based on the LB method and the X-ray computed tomography (XCT) technique. GDLs with paper and felt structures were reconstructed into 3D digital volumetric models via the XCT process. The digital models were then incorporated into a LB solver to model water saturation distribution through the GDL domains. The GDL wettability was also altered so that the effect on liquid water behaviour in the GDL could be examined. This project is divided into three main sections. In the sensitivity analysis, the effect of image resolution on gas permeability through the X-ray reconstructed GDL was carried out using a single-phase LB model. It was found that the resolution variation could significantly affect the resulting gas permeability in both principal and off-principal directions, as well as computational time. An optimum resolution, however, exists at 2.72 μm/pixel, which consumed 400 times less computational time with less than 8% difference in the resulting permeability compared to the base resolution. This study also served as a guideline for selecting a resolution for generating the XCT images of the GDLs which were utilised in the following studies. In the structure analysis, the structures of the paper and felt GDLs were generated using the XCT and the key properties of each GDL, including thickness, porosity, permeability and tortuosity, were characterised. The thickness and the through-plane porosity distributions of each GDL were examined based on the tomography images. The resulting local through-plane porosity distributions were then used to calculate through-plane permeability and tortuosity distributions using an analytical model available in the literature. This study revealed the heterogeneity of the GDLs and how the heterogeneous nature of the GDL structures affects others properties of the GDLs. In this study, the absolute through-plane permeability and tortuosity of the X-ray-reconstructed GDL samples were also characterised using the single-phase LB model. The results from the two models were then compared and validated against data in the literature. In the water transport analysis, the two-phase LB model was employed to examine the effects of GDL structures on the behaviour of liquid water in the GDLs, including invasion patterns, saturation distribution and breakthrough behaviour under varying GDL wettability conditions. It was found that wettability was responsible for invasion patterns and water saturation levels whilst the GDL structure was mostly responsible for breakthrough occurrence and saturation distribution. It was observed that water travelled with stable displacement saturating all pores in hydrophilic GDLs, while it travelled with capillary fingering causing decreased saturation in hydrophobic GDLs, about 50% in the highly hydrophobic cases. The GDL structure was found to play a key role in breakthrough behaviour in the hydrophilic GDL as it was seen that the through-plane fibres in the felt structure and the through-plane binders in the paper structure encouraged water removal from the GDL in the thickness direction. Conversely, the GDL structure was found to have negligible influence on breakthrough in the hydrophobic GDL. Each GDL structure, however, contributed to a distinct difference in water distribution in the GDL with hydrophobic wettability. The work presented in this thesis contributes to the understanding of liquid water transport behaviour in the GDLs under the combined effects of the GDL structures and wettability conditions, which is essential for the development of effective PEMFC water management and the design of future GDL materials.

In computed tomography (CT) cross-sectional images of the attenuation distribution within a slice are created by scanning radiographic projections of an object with a rotating X-ray source detector compound and subsequent reconstruction of the images from these projection data on a computer. CT can be made very fast by employing a scanned electron beam instead of a mechanically moving X-ray source. Now this principle was extended towards high-energy electron beam tomography with an electrostatic accelerator. Therefore a dedicated experimental campaign was planned and carried out at the Budker Insitute of Nuclear Physics (BINP), Novosibirsk. There we investigated the capabilities of BINP’s accelerators as an electron beam generating and scanning unit of a potential high-energy electron beam tomography device. The setup based on a 1 MeV ELV-6 (BINP) electron accelerator and a single detector. Besides tomographic measurements with different phantoms, further experiments were carried out concerning the focal spot size and repeat accuracy of the electron beam as well as the detector’s response time and signal to noise ratio.

Particles exist in great abundance in nature, such as in sands and clays, and they also constitute 75% of the materials used in industry (e.g., mineral ores, formulated pharmaceuticals, dyes, detergent powders). When a load is applied to a bulk assembly of soil particles, the response of a geomaterial at the bulk (macro) scale, originates from the changes that take place at the particle scale. If particle breakage occurs, the shape and size of the particles comprising the bulk are changed; this induces changes in the contact network through which applied loads are transmitted. As a result, changes at the micro-scale can significantly affect the mechanical behaviour of a geomaterial at a macro-scale. It is therefore unsurprising that the mechanisms leading to particle breakage are a subject of intense research interest in several fields, including geomechanics. In this thesis, particle breakage of two dry granular materials is studied, both experimentally and numerically. The response of the materials is investigated under different stress paths and in all the tests grain breakage occurs. High resolution x-ray computed micro-tomography (XCT) is used to obtain 3D images of entire specimens during high confinement triaxial compression tests and strain controlled oedometric compression tests. The acquired images are processed and measurements are made of the temporal and spatial evolution of breakage, local variations of porosity, volumetric and shear strain and grading. The evolution and spatial distribution of quantified breakage including the resulting particle size distribution for the whole specimen and for specific areas, are presented and further related to the localised shear and volumetric strains that developed in the specimens. In addition, the discrete element method (DEM) was used to provide further micro-mechanical insight of the underlying mechanisms leading to particle breakage. Classical DEM simulations, using a Hertz-Mindlin contact model and non-breakable spheres, was first deployed to study the initiation and likelihood of particle breakage under oedometric compression. Moreover, a bonded DEM model was used to create clumps that represent each particle and simulate breakage of particles under single particle compression. The DEM model parameters were obtained from results of single particle compression test and the models were validated against the quantitative 3D information of the micro-scale, acquired from the XCT analysis.

L’utilisation des matériaux constitués de fibres synthétique ou naturelle est en pleine expansion et concerne de nombreux secteurs : industrie automobile, aéronautique, électrique, filtration de l’air ou applications médicales. Malgré des procédés de fabrication et des natures de fibres différents, ces matériaux ont pour point commun d’être constitués d’un réseau de fibres liées entre elles par des liaisons. Les papiers et les cartons sont, par exemple, constitués de fibres de cellulose naturelles liées chimiquement. A l’heure actuelle, les mécanismes de fissuration dans de tels milieux sont encore mal compris. Ils dépendent fortement (a) des propriétés des constituants : géométrie et propriétés mécaniques des fibres et des contacts fibre-fibre, (b) des caractéristiques des réseaux fibreux : géométrie et arrangement des fibres, et des caractéristiques du réseau poreux induit : porosité, distribution de taille des pores, répartition spatiale des pores, etc. et (c) des modes de sollicitations mécaniques. Dans ce type de matériaux, les effets d’échelles doivent être pris en compte pour compléter les approches mécaniques traditionnelles. Les récents progrès en mécanique expérimentale et en simulation numérique permettent de mener une telle étude de l’échelle de la fibre à celle du réseau fibreux.Cette thèse a donc pour but de mettre en place des outils d’analyse des microstructures et des mécanismes de fissuration dans les milieux fibreux à faible densité. Pour cela, (i) des essais de micromécaniques seront couplés à des méthodes d’imagerie (ESEM, microtomographie à rayons X, stéréocorrélation) afin de caractériser expérimentalement les milieux et leur endommagement (ii) Cette étude vient compléter les travaux expérimentaux menés dans les deux laboratoires 3SR et LGP2 (ANR ANAFIB http://anafib.hmg.inpg.fr/spip.php?rubrique1) et sera complétée par des simulations numériques des essais réalisés en collaboration avec Per Isaksson de l’Université d’Uppsala (Suède). / Materials made up of synthetic or natural fibres are increasingly developed in various domains: papermaking, composite, automotive and aeronautic industries for structural, packaging, air filtration or medical applications. Despite the variety of manufacturing processes of such materials, all of them can be considered as being formed by a network of fibres interconnected via bonds. For instance, in the case of materials made up of natural cellulosic fibres such as papers or boards, fibres are chemically linked.Crack propagation phenomena in such materials remain poorly understood even though it can be presumed that such mechanisms depend on:- (i) the geometrical and mechanical properties of the constituents of individual fibres and fibre-fibre bonds,- (ii) the architecture of the fibrous network, for example the spatial distributions of fibres, bonds and pores and the size distributions of pores and bonds,- (iii) the applied mechanical loadings.In such materials, scale effects must be investigated in order to improve the classical approaches used to understand crack propagation mechanisms. Recent progresses in both experimental mechanics and numerical simulation approaches allow such a study from the fibre scale up to the fibre network scale to be carried out.The proposed PhD aims first at developing an original experimental approach to analyse microstructure changes and crack propagation phenomena for low density papers. For that purpose x-ray microtomography or ESEM, and stereo-correlation experiments will be carried out to investigate microstructural changes and deformation mechanisms at all relevant scales (see the illustration given in ).

abstract: The mechanical behavior of Pb-free solder alloys is important, since they must maintain mechanical integrity under thermomechanical fatigue, creep, and mechanical shock conditions. Mechanical shock, in particular, has become an increasing concern in the electronics industry, since electronic packages can be subjected to mechanical shock by mishandling during manufacture or by accidental dropping. In this study, the mechanical shock behavior of Sn and Sn-Ag-Cu alloys was systematically analyzed over the strain rate range 10-3 - 30 s-1 in bulk samples, and over 10-3 - 12 s-1 on the single solder joint level. More importantly, the influences of solder microstructure and intermetallic compounds (IMC) on mechanical shock resistance were quantified. A thorough microstructural characterization of Sn-rich alloys was conducted using synchrotron x-ray computed tomography. The three-dimensional morphology and distribution of contiguous phases and precipitates was analyzed. A multiscale approach was utilized to characterize Sn-rich phases on the microscale with x-ray tomography and focused ion beam tomography to characterize nanoscale precipitates. A high strain rate servohydraulic test system was developed in conjunction with a modified tensile specimen geometry and a high speed camera for quantifying deformation. The effect of microstructure and applied strain rate on the local strain and strain rate distributions were quantified using digital image correlation. Necking behavior was analyzed using a novel mirror fixture, and the triaxial stresses associated with necking were corrected using a self-consistent method to obtain the true stress-true strain constitutive behavior. Fracture mechanisms were quantified as a function of strain rate. Finally, the relationship between solder microstructure and intermetallic compound layer thickness with the mechanical shock resistance of Sn-3.8Ag-0.7Cu solder joints was characterized. It was found that at low strain rates the dynamic solder joint strength was controlled by the solder microstructure, while at high strain rates it was controlled by the IMC layer. The influences of solder microstructure and IMC layer thickness were then isolated using extended reflow or isothermal aging treatments. It was found that at large IMC layer thicknesses the trend described above does not hold true. The fracture mechanisms associated with the dynamic solder joint strength regimes were analyzed. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2012

abstract: An accurate knowledge of the complex microstructure of a heterogeneous material is crucial for quantitative structure-property relations establishment and its performance prediction and optimization. X-ray tomography has provided a non-destructive means for microstructure characterization in both 3D and 4D (i.e., structural evolution over time). Traditional reconstruction algorithms like filtered-back-projection (FBP) method or algebraic reconstruction techniques (ART) require huge number of tomographic projections and segmentation process before conducting microstructural quantification. This can be quite time consuming and computationally intensive. In this thesis, a novel procedure is first presented that allows one to directly extract key structural information in forms of spatial correlation functions from limited x-ray tomography data. The key component of the procedure is the computation of a “probability map”, which provides the probability of an arbitrary point in the material system belonging to specific phase. The correlation functions of interest are then readily computed from the probability map. Using effective medium theory, accurate predictions of physical properties (e.g., elastic moduli) can be obtained. Secondly, a stochastic optimization procedure that enables one to accurately reconstruct material microstructure from a small number of x-ray tomographic projections (e.g., 20 - 40) is presented. Moreover, a stochastic procedure for multi-modal data fusion is proposed, where both X-ray projections and correlation functions computed from limited 2D optical images are fused to accurately reconstruct complex heterogeneous materials in 3D. This multi-modal reconstruction algorithm is proved to be able to integrate the complementary data to perform an excellent optimization procedure, which indicates its high efficiency in using limited structural information. Finally, the accuracy of the stochastic reconstruction procedure using limited X-ray projection data is ascertained by analyzing the microstructural degeneracy and the roughness of energy landscape associated with different number of projections. Ground-state degeneracy of a microstructure is found to decrease with increasing number of projections, which indicates a higher probability that the reconstructed configurations match the actual microstructure. The roughness of energy landscape can also provide information about the complexity and convergence behavior of the reconstruction for given microstructures and projection number. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2017

abstract: Increasing density of microelectronic packages, results in an increase in thermal and mechanical stresses within the various layers of the package. To accommodate the high-performance demands, the materials used in the electronic package would also require improvement. Specifically, the damage that often occurs in solders that function as die-attachment and thermal interfaces need to be addressed. This work evaluates and characterizes thermo-mechanical damage in two material systems – Electroplated Tin and Sintered Nano-Silver solder. Tin plated electrical contacts are prone to formation of single crystalline tin whiskers which can cause short circuiting. A mechanistic model of their formation, evolution and microstructural influence is still not fully understood. In this work, growth of mechanically induced tin whiskers/hillocks is studied using in situ Nano-indentation and Electron Backscatter Diffraction (EBSD). Electroplated tin was indented and monitored in vacuum to study growth of hillocks without the influence of atmosphere. Thermal aging was done to study the effect of intermetallic compounds. Grain orientation of the hillocks and the plastically deformed region surrounding the indent was studied using Focused Ion Beam (FIB) lift-out technique. In addition, micropillars were milled on the surface of electroplated Sn using FIB to evaluate the yield strength and its relation to Sn grain size. High operating temperature power electronics use wide band-gap semiconductor devices (Silicon Carbide/Gallium Nitride). The operating temperature of these devices can exceed 250oC, preventing use of traditional Sn-solders as Thermal Interface materials (TIM). At high temperature, the thermomechanical stresses can severely degrade the reliability and life of the device. In this light, new non-destructive approach is needed to understand the damage mechanism when subjected to reliability tests such as thermal cycling. In this work, sintered nano-Silver was identified as a promising high temperature TIM. Sintered nano-Silver samples were fabricated and their shear strength was evaluated. Thermal cycling tests were conducted and damage evolution was characterized using a lab scale 3D X-ray system to periodically assess changes in the microstructure such as cracks, voids, and porosity in the TIM layer. The evolution of microstructure and the effect of cycling temperature during thermal cycling are discussed. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018

In recent years, the use of textile composites has grown rapidly primarily due to the high strength-to-weight ratio which they offer. The applications of fibre reinforced composites include a range of industries including aerospace, automotive, marine, civil construction, wind energy and sports. The textile reinforcements used for composites include woven, knitted, braided and stitch-bonded preforms. Among these, woven fabrics are the most widely used reinforcements which comprise interlaced warp and weft yarns oriented at 0o and 90o, respectively. This research concerns woven fabrics wherein the interlacing sets of yarns are oriented at bias. The main focus is the development of manufacturing concepts for bias woven preforms. Following a thorough study on existing bias technologies, five bias weaving concepts have been proposed for making bias woven preforms. With regard to the first of these concepts, a Desktop Bias Weaving (DBW) machine has been developed. A range of elementary and compound bias woven preforms have been successfully produced using the DBW machine. The preforms have been consolidated using the vacuum resin infusion process to make textile composites. The mechanical properties of the composite materials have been assessed, and their structure has been analysed to observe tow geometry using advanced imaging techniques such as X-ray tomography. The next step has been the development of advanced Bi-axial Bias (BiB) weaving concepts for producing quasi-isotropic bias woven preforms. Here both sets of interlacing yarns are oriented at bias. Such fabric formation requires a double rapier weft insertion mechanism. With regard to this, four concepts have been proposed and two BiB weaving machines have been developed accordingly. BiB woven preforms based on fundamental plain, twill and satin weaves have been fabricated successfully and impregnated with epoxy resin to make laminates. The weave geometry in the composite samples has been analysed using Scanning Electron Microscopy.