The use of magnesium and its alloys, as the lightest structural materials, to decrease the weight, improve the fuel efficiency and reduce the greenhouse gas emissions has significantly increased in the automotive and aerospace industries in recent years. However, magnesium alloys are commonly used as die casting products. The current application of wrought magnesium alloy products is limited because of their poor ductility at room temperature due to the formation of a strong texture and restricted active deformation modes in wrought magnesium products. Moreover, to support the application of magnesium alloys in automobile and airplane components, their dynamic mechanical response must be determined to evaluate their behavior during impact events such as car crash and bird strike in airplanes. Therefore, in this research study, the dynamic mechanical behavior of magnesium alloys at high strain rates was investigated. The effects of initial texture, composition, strain rate and grain size on the deformation mechanism were also determined. Split Hopkinson Pressure Bar was used to investigate the dynamic mechanical behavior of the magnesium alloys. Texture analysis on the alloy prior and after shock loading was done using X-ray diffraction. Scanning electron microscopy was used to study the microstructural evolution in the alloys before and after shock loading. Chemical analysis and phase identification were done by energy dispersive spectroscopy and X-ray diffraction analysis, respectively. Additionally, twinning type and distribution was determined by means of orientation imaging microscopy whereas dislocation types and distribution was determined using transmission electron microscopy. A visco-plastic self-consistent simulation was used to corroborate the experimental textures and possible deformation mechanisms. The dynamic mechanical behavior of cast AZ and AE magnesium alloys with different chemistries was investigated at strain rates ranging between 800 to 1400 s-1 to determine the effects of composition on the response of the alloys to shock loading. It was found that an increase in the aluminum content of the AZ alloys increased the volume fraction of β-Mg17Al12 and Al4Mn phases, strength and strain hardening but, on the other hand, decreased the ductility and twinning fraction, particularly extension twinning fraction, for all the investigated strain rates. In addition, increasing the strain rate resulted in considerable increase in strength of the alloys. Texture measurements showed that shock loading of the AE alloys resulted in development of a stronger (00.2) basal texture in samples with higher content of yttrium at the investigated strain rates. Increasing the yttrium content of the cast AE alloys decreased twinning fraction but increased dislocation density and volume fraction of the Al2Y second phase. As a result, the tensile strength and ductility of the alloys increased which is an interesting result for high-strain rate applications of AE alloys in comparison to AZ alloys. The dynamic mechanical behavior of rolled AZ31B and WE43 magnesium alloys were also studied at strain rates ranging between 600 to 1400 s-1. A strong (00.2) basal texture was observed in all shock loaded AZ31B samples. It was also observed that increasing the strain rate led to an increase in strength and ductility, but to a decrease in twinning fraction. A high degree of mechanical anisotropy was found for all investigated strain rates so that the lowest strength was registered for the samples cut along the direction parallel to the rolling direction. Furthermore, it was found that at high strain rates, fine-grained AZ31B alloy exhibits better ductility and strength compared to coarse-grained alloy. However, the hardening rate of coarse-grained alloy was higher. In the case of rolled WE43 alloy, it was found that the strength and ductility increased and twinning fraction decreased with increase in strain rate. Furthermore, another effect of increase in strain rate was the higher activation of pyramidal <c+a> slip systems. In addition, degree of stress and strain anisotropy is low particularly at higher strain rates, which is mainly related to the weak initial texture of the samples due to the presence of rare earth elements. Furthermore, strength and ductility were found to decrease with increasing grain size, while twinning fraction, activity of double and contraction twins and strain hardening rate increase with increasing grain size. In both AZ31B and WE43 alloy, the presence of <c+a> dislocations was confirmed at high strain rates using ‘g.b’ analysis confirming activation of pyramidal <c+a> slip systems during dynamic shock loading.

No description available.

A base de solo-cimento é um material de alta qualidade e durabilidade empregada na pavimentação rodoviária. Atualmente no Brasil, sua dosagem é realizada a partir de ensaios mecânicos, de forma a definir um valor mínimo de cimento que apresente resistência satisfatória. O foco da pesquisa consiste no estudo de prováveis diferenças na forma da adição do cimento: (i) mistura realizada com o solo seco, procedimento comumente adotado para dosagem, ou ainda (ii) mistura realizada com solo úmido, condição que representa o processo de produção em pista, ou usina. Ensaios mecânicos de resistência e rigidez foram realizados para as duas condições de umidade de mistura e para quatro tempos de cura (3, 7, 14 e 28 dias). As amostras moldadas a partir da umidade higroscópica apresentaram maiores valores de resistência à compressão simples e módulo de resiliência, porém menores de valores de resistência à tração por compressão diametral. O grau de saturação obtido na compactação Proctor das amostras de solo mostram que o solo compactado na umidade de campo atinge menores índices de vazios, resultando em massa específica aparente seca superiores ao solo compactado a partir da umidade higroscópica. Através do acompanhamento de um trecho experimental, foram realizadas medidas de bacias de deflexão com equipamento FWD, com o objetivo de retroanalisar os módulos resilientes das camadas constituintes da estrutura do pavimento. Os valores de módulo de resiliência retroanálisados apresentaram variações quando comparados com as determinações de rigidez determinadas em laboratório. / The soil-cement base-layer is a high quality and durable material employed in construction road. In Brazil, the soil-cement mix design is based on mechanical tests, in order to define a minimum cement content to assure satisfactory strength. The aim of this research is to study possible differences in the way cement is added to the mixture: (i) soil in the dry moisture condition (hygroscopic), common procedure adopted in laboratory for the mixture design, or (ii) soil in the wet moisture condition, common procedure during the road construction, or in the mix plant. Mechanical tests for strength and stiffness measurement were performed using both initial moisture conditions, and for four curing times (3, 7, 14, and 28 days). The sample compacted in the hygroscopic moisture condition showed higher strength in ultimate compressive strength (UCS) and resilient modulus (RM) tests. The degree of saturation calculated for the soil samples from the compaction test (Proctor) showed that the soil at wet moisture condition decreased the air voids. It resulted in higher dry density when compared with the soil at the dry moisture condition. One experimental test site, with soil-cement in the base layer was constructed and monitored. Its structural evaluation with Falling Weight Deflectometer (FWD) was used for backcalculation of the resilient modulus of the pavement layers. The backcalculated resilient modulus results were smaller than the values obtained from laboratory sample testing.

A base de solo-cimento é um material de alta qualidade e durabilidade empregada na pavimentação rodoviária. Atualmente no Brasil, sua dosagem é realizada a partir de ensaios mecânicos, de forma a definir um valor mínimo de cimento que apresente resistência satisfatória. O foco da pesquisa consiste no estudo de prováveis diferenças na forma da adição do cimento: (i) mistura realizada com o solo seco, procedimento comumente adotado para dosagem, ou ainda (ii) mistura realizada com solo úmido, condição que representa o processo de produção em pista, ou usina. Ensaios mecânicos de resistência e rigidez foram realizados para as duas condições de umidade de mistura e para quatro tempos de cura (3, 7, 14 e 28 dias). As amostras moldadas a partir da umidade higroscópica apresentaram maiores valores de resistência à compressão simples e módulo de resiliência, porém menores de valores de resistência à tração por compressão diametral. O grau de saturação obtido na compactação Proctor das amostras de solo mostram que o solo compactado na umidade de campo atinge menores índices de vazios, resultando em massa específica aparente seca superiores ao solo compactado a partir da umidade higroscópica. Através do acompanhamento de um trecho experimental, foram realizadas medidas de bacias de deflexão com equipamento FWD, com o objetivo de retroanalisar os módulos resilientes das camadas constituintes da estrutura do pavimento. Os valores de módulo de resiliência retroanálisados apresentaram variações quando comparados com as determinações de rigidez determinadas em laboratório. / The soil-cement base-layer is a high quality and durable material employed in construction road. In Brazil, the soil-cement mix design is based on mechanical tests, in order to define a minimum cement content to assure satisfactory strength. The aim of this research is to study possible differences in the way cement is added to the mixture: (i) soil in the dry moisture condition (hygroscopic), common procedure adopted in laboratory for the mixture design, or (ii) soil in the wet moisture condition, common procedure during the road construction, or in the mix plant. Mechanical tests for strength and stiffness measurement were performed using both initial moisture conditions, and for four curing times (3, 7, 14, and 28 days). The sample compacted in the hygroscopic moisture condition showed higher strength in ultimate compressive strength (UCS) and resilient modulus (RM) tests. The degree of saturation calculated for the soil samples from the compaction test (Proctor) showed that the soil at wet moisture condition decreased the air voids. It resulted in higher dry density when compared with the soil at the dry moisture condition. One experimental test site, with soil-cement in the base layer was constructed and monitored. Its structural evaluation with Falling Weight Deflectometer (FWD) was used for backcalculation of the resilient modulus of the pavement layers. The backcalculated resilient modulus results were smaller than the values obtained from laboratory sample testing.

abstract: Polymer and polymer matrix composites (PMCs) materials are being used extensively in different civil and mechanical engineering applications. The behavior of the epoxy resin polymers under different types of loading conditions has to be understood before the mechanical behavior of Polymer Matrix Composites (PMCs) can be accurately predicted. In many structural applications, PMC structures are subjected to large flexural loadings, examples include repair of structures against earthquake and engine fan cases. Therefore it is important to characterize and model the flexural mechanical behavior of epoxy resin materials. In this thesis, a comprehensive research effort was undertaken combining experiments and theoretical modeling to investigate the mechanical behavior of epoxy resins subject to different loading conditions. Epoxy resin E 863 was tested at different strain rates. Samples with dog-bone geometry were used in the tension tests. Small sized cubic, prismatic, and cylindrical samples were used in compression tests. Flexural tests were conducted on samples with different sizes and loading conditions. Strains were measured using the digital image correlation (DIC) technique, extensometers, strain gauges, and actuators. Effects of triaxiality state of stress were studied. Cubic, prismatic, and cylindrical compression samples undergo stress drop at yield, but it was found that only cubic samples experience strain hardening before failure. Characteristic points of tensile and compressive stress strain relation and load deflection curve in flexure were measured and their variations with strain rate studied. Two different stress strain models were used to investigate the effect of out-of-plane loading on the uniaxial stress strain response of the epoxy resin material. The first model is a strain softening with plastic flow for tension and compression. The influence of softening localization on material behavior was investigated using the DIC system. It was found that compression plastic flow has negligible influence on flexural behavior in epoxy resins, which are stronger in pre-peak and post-peak softening in compression than in tension. The second model was a piecewise-linear stress strain curve simplified in the post-peak response. Beams and plates with different boundary conditions were tested and analytically studied. The flexural over-strength factor for epoxy resin polymeric materials were also evaluated. / Dissertation/Thesis / Ph.D. Mechanical Engineering 2011

Le barrage poids en enrochement dont l’étanchéité est assurée par un masque en béton sur sa face amont est de plus en plus utilisé aujourd’hui. Son appellation courante est « CFRD1 ». La conception de ce type d’ouvrage est effectuée en suivant des « règles » de conception empiriques. Dans la dernière décennie, plusieurs barrages de type CFRD de grande hauteur ont subi la rupture de leur masque en béton lors de la première mise en eau, montrant ainsi la défaillance de l’approche empirique.Il s’avère alors nécessaire de comprendre les mécanismes physiques mis en jeu, notamment dans le comportement des enrochements sous fortes contraintes. Ce travail de thèse commence par une recherche bibliographique montrant que les fortes contraintes ont un effet important dans la rupture des particules. Cette rupture est influencée par les caractéristiques des particules, l’assemblage et les conditions mécaniques imposées. Une fois les facteurs influençant la rupture de particules identifiés, on s’intéresse aux modèles de comportement tenant compte de ce phénomène. La plupart des modèles étudiés englobent les différents facteurs d’influence dans un ou plusieurs paramètres difficilement identifiables par les essais mécaniques courants. Un modèle proposé dans le cadre de la thermodynamique prenant en compte la distribution de taille des particules a été retenu pour une analyse plus approfondie. Cette analyse aboutit au besoin d’identifier deux phénomènes importants : i) la distribution de tailles de particules suite à la rupture, et ii) la relation entre la rupture de particules et la dissipation d’énergie par frottement. Concernant le premier phénomène, un modèle probabiliste est proposé. Ce modèle tient compte de l’effet de la taille des particules dans la probabilité de rupture. La comparaison des simulations aux résultats expérimentaux pour un sable montre une bonne approximation de la variation des courbes granulométriques. Concernant la relation entre la rupture de particules et la dissipation d’énergie par frottement, des essais triaxiaux ont été analysés thermodynamiquement sur un plan « énergie reçue/énergie rendue » par le système (l’échantillon de sol). Sur ce plan, on étudie le comportement dissipatif sans rupture qui se traduit par une relation linéaire. Les matériaux présentant une rupture des particules montrent une relation différente. Donc, ce plan permet de mettre en évidence la différence entre l’énergie dissipée par le matériau sans rupture et celui avec rupture de grains. Cette différence est associée au travail mécanique dû à la rupture des particules. Différentes conclusions et perspectives sont proposées à ce point pour le développement de modèles de comportement. Une conséquence directe de la dépendance de la probabilité de rupture avec la taille des particules est l’existence d’un effet d’échelle. Une théorie d’effet d’échelle récemment proposée pour l’enveloppe de rupture a été validée dans le cadre des relations contraintes-déformations. Ceci permet de modéliser la réponse contrainte-déformation d’un matériau contenant des particules de taille significative à partir d’un matériau de granulométrie réduite.Finalement, quelques recommandations de modification aux pratiques existantes dans la conception de barrages à masque amont en béton ont été proposées suite à des analyses de modèles en éléments finis. / The concrete face rockfill dam (CFRD) is a type of dam widely constructed nowadays. The design “rules“ for this type of dam have remained totally empirical. During the last decade, several high CFRDs have experienced the cracking of the concrete face during first reservoir impounding. This shows the limitations of the current empirical state of practice. It is important to understand the different physical mechanisms leading to these problems, especially those concerning the mechanical behavior of rockfill materials. Looking for answers to this problem, this thesis starts by a bibliographic research which shows that particles breakage is an important issue on the mechanical behavior of rockfills. The particles breakage phenomenon is affected by the characteristics of individual particles, the packing conditions of the assembly and the imposed mechanical solicitations. Once the factors affecting particles breakage have been identified, we study the constitutive models that have introduced the particles breakage phenomena. Most of them reduce the influence of the different factors to a couple of parameters, which are not easily identifiable through current laboratory tests. One constitutive model proposed on a thermodynamic framework has been retained for a more detailed analysis. This analysis leads to two main topics of interest: i) the description of the grain-size distribution due to particles breakage, and ii) the relationship between dissipation of energy and particles breakage. Concerning the first topic, a probabilistic model is proposed to describe the evolution of particles breakage and the variation of the grain-size distribution curve. This model takes into account the dependency of breakage probability with particles size. The comparison between model response and laboratory experiences shows good agreement for the variation of the grain-size distribution curves.Concerning the relationship between dissipation of energy by friction and particles breakage, several drained triaxial tests have been studied thermodynamically on a plot called “input/output power” of the system (soil sample). On this plot, we study the dissipative behavior without particles breakage, which is characterized by a straight line. Materials experiencing particles breakage show a different form. Therefore this plot allows identifying the difference between the dissipated energy of materials with and without particles breakage. This difference is associated to the particles breakage phenomena. Several conclusions and future works are proposed at this point related to the description of the mechanical behavior of rockfills.A direct consequence of the dependency of particles breakage on the particles size is the existence of a size-scale effect. A theory recently proposed for the shear-strength envelope has been validated for the stress-strain relationships. This allows estimating the stress-strain response of a material with very coarse particles from a material with reduced grain-size distribution.Finally, some recommendations to the current practice of CFRDs design are proposed, based on analyses with finite element models.

Presently, a critical requirement in electronic assemblies is the reliability of solder joints. Accurate characterization of the mechanical behavior of solder alloys is challenging due to their micro-scale size, microstructural complexity, and complex rate-dependent mechanical behavior. This research presents two mechanical testers designed to acquire accurate mechanical response of the solder alloys. The testers allow using micro-scale test samples that replicate real solder joints in size and soldering pad metallurgy. <div>The first mechanical tester presented in this research is the micro-precision tester. It is capable of monotonic, creep and fatigue test profiles at testing temperatures between 25 and 75◦C. Using a closed-loop control scheme and an external capacitance sensor to minimize measurement of the load train compliance, the tester is capable of precision on the order of 0.1 µm. For load controlled tests, the tester is capable of precision on the order of 0.5 N. The design and construction processes are presented, including rationale for major design choices. Additionally, the development of custom squat-joint samples for use in this tester is presented. These samples allow for increased data reliability while maintaining realistic dimensions. Both validation and test data are presented to demonstrate the capabilities of the micro-precision tester. </div><div>A second mechanical tester, the nano-precision tester, was developed to address the need for increased accuracy as solder geometries shrink. Again, the design choices and limitations are presented, with emphasis on improvements over the micro-precision tester. The load and displacement control are approximately and order of magnitude better than that of the micro-precision tester. Example tests are presented to demonstrate the accuracy and capabilities of the nano-precision tester. </div><div>Finally, the thesis concludes with recommendations on methods to further improve the two testers. Specifically, for the micro-precision tester, thermal expansion during high-temperature testing is a significant concern. For the nano-precision tester, both validation of the tester the capability of multi-temperature testing are future work.<br></div>

No description available.

Through the use of generalized spherical harmonic basis functions a spectral representation is used to model the microstructure of cubic materials. This model is then used to predict the macroscopic elastic and plastic properties of materials with cubic crystal symmetry and various sample symmetry including triclinic and axial—symmetric. Building on the work of citeN{Barnett-FractureMechanics} the influence that anisotropy has on the fatigue response of the material is also modeled. This is accomplished through using the effective elastic stiffness tensor in the computation of crack extension force G. The resulting material model and macroscopic property calculations are the foundation for a software package which provides an interface to the microstructure. The Microstructure Sensitive Design interface (MDSi) enables interaction with the material design process and provides tools needed to incorporate material parameters with traditional design, optimization, and analysis software. Therefore the microstructure model can be optimized concurrently with a geometric model to further increase the overall design space. The software is then be used to explore how changes in the microstructure affect the performance of a turbine disc. The additional design space afforded by inclusion of the material parameters show that the total mass of the disk can be lowered by 9.5%. Additionally when the same geometry and loading conditions are considered and only the texture of the material is modified G is reduced be more than an order of magnitude.

Mg-based metal matrix nanocomposites (MMNCs) with mechanical properties, superior to those of coarse-grained composites, are promising structural materials for applications in the automotive and aerospace industries. The research in this area was primarily focused earlier on either micro-scaled reinforcements or nano-scaled reinforcements with very low volume fractions. MMNCs with high volume fractions have not been explored yet. In this research, we study the processing, microstructures and properties of MMNCs containing ceramic nanoparticles up to 30 vol.%. We first investigated the mechanical alloying of Al2O3 nanoparticles and pure Mg under high-energy ball milling conditions. The phase evolution and their distribution were evaluated as a function of milling time. Then, the thermal stability of the formed nanocomposites was investigated by annealing it at high temperatures. It indicated that an exchange reaction had occurred to a large extent between Mg and Al2O3 resulting in the formation of Al and MgO phases. Additionally, the reaction between Al and unreacted Mg led to the formation of Mg-Al intermetallics. Due to the reaction between Mg and Al2O3 during the milling and annealing process, we attempted to synthesize Mg/SiC nanocomposites. The mixed powders containing 0, 5, 10 and 15 vol.% SiC were produced by high energy ball milling and then the powders were consolidated via spark plasma sintering. The phase constitutions and microstructures of the Mg/SiC nanocomposites were characterized. SiC nanoparticles (average particle size ~14 nm) appear to be homogeneously dispersed within the matrix, iv and the average inter-particle spacings of all the Mg/SiC nanocomposites were smaller than 50 nm. Microscopic methods, even at high magnifications did not reveal any significant porosity in the as-processed MMNCs. Mechanical characterization of the Mg/SiC nanocomposites was conducted using the microindentation test. Besides the microhardness test, different intermediate pause times and loading rates were used to evaluate the stiffness and loading rate sensitivity of our samples. The abnormal microhardness and loading rate sensitivity were showed for the Mg-15 vol.% SiC samples. At the same time, the monotonic increase of stiffness with volume fraction was exhibited in the Mg/SiC nanocomposites. Finally, we investigated the quasi-static and dynamic response of Mg/SiC nanocomposites and microcomposites, and discussed the underlying mechanisms. Strain softening was noticed in the milled Mg sample under quasi-static compression. Similarly, the strengthening effect leveling off was also observed in the Mg-15 vol.% SiC samples under either quasi-static or high-strain rate uniaxial compression conditions. No significant plastic deformation was observed in the Mg/SiC nanocomposites. The estimated strain rate sensitivity of all the Mg/SiC nanocomposites in this work was around 0.03, which is much smaller than 0.3 and 0.6, observed for 100 nm and 45 nm grain size pure Mg individually. In particular, the existing models fail in predicting the inverse volume fraction effect, and other mechanisms are yet to be explored. The presence of SiC nanoparticles may play an important role that leads to this difference