On the toughening of bulk metallic glasses (BMGs), successful results in the phase-separated Zr63.8Ni16.2Cu15Al5 BMG have achieved compressive ductility over 15% through the computational-thermodynamic approach. In this study, the phase-separated Zr63.8Ni16.2Cu15Al5 BMG was compressed to nominal strains of 3%, 7%, and 10% at low strain rates (~10-4 s-1) and the results demonstrated that the BMG exhibited apparent uniform deformation initially, followed by visible local shear bands development. Afterwards, a single shear along the principal shear plane was soon developed and mainly dominated the whole deformation process. The principal shear contributed more than 2/3 of the overall plastic strain until failure. It was also found that the local shear strain varied along the principal shear plane and decreased monotonically from the shear band initiation site. Subsequently, in-situ compression experiments were conducted to monitor the change of sample shape during deformation in order to properly correlate with the stress-strain curve. The observed images showed that there was a one-to-one correspondence between the intermittent sample sliding and flow serration in the plastic region of stress-strain curve. Further investigations on flow serration were conducted on the Pd40Ni40P20 BMG through the compression experiments equipped with high-sensitivity strain gauges directly attached to two opposite sides of the test sample. There was an accompanied displacement burst when a shear band starts to propagate during deformation and this displacement burst would be accurately captured by the high-sensitivity strain gauges. Based on the displacement-time profile for one serration, shear-band propagating speed can be estimated and found to be insensitive to the applied strain rates (or the applied crosshead speeds). The disappearance of flow serration at high strain rates should be a result that the signal of displacement burst was overwhelmed by the applied strain rate. Using the shear strain rate data, the measured viscosity within a propagating shear band was found to be relatively low, which is in similar to the viscosity values reported in the supercooled liquid region during homogeneous deformation. In comparison with shear band propagation in the brittle Mg58Cu31Y6Nd5 and Au49Ag5.5Pd2.3Cu26.9Si16.3, moderately ductile Cu50Zr43Al7 and Pd40Ni40P20, and highly ductile phased-separated Zr63.8Ni16.2Cu15Al5 systems, the ductility of BMGs appears to be closely related to the dynamics during shear band propagation. The more ductile in nature the metallic glass is, the slower the shear band propagating speed would become. We also made attempts to investigate the shear band propagation in the porous Mo particles reinforced Mg58Cu28.5Gd11Ag2.5 bulk metallic glass composites (BMGCs) with up to 10% compressive failure strain. It was found that flow serration was absent in the stress-strain curve. Using high-sensitivity strain gauges, no distinct displacement burst was detected in the displacement-time profile. The diappearance of flow serration for the current porous Mo particles reinforced Mg58Cu28.5Gd11Ag2.5 BMGC is apparently associated with the lack of long-range shear band propagagtion. By employing the approach of separating the homogeneous amorphous matrix into many individual compartments, only short-range shear band propgagation is possible in the current Mg-based BMGC. An effective free spacing considering the spacing between two porous Mo particles and porous Mo particle size was applied to interpret the development of shear band propagation and is a useful indicator for the design of BMGC with high ductility.

<p> A yield criterion depending on stress, strain and their histories is revised so as to achieve better correlation with experimental data. It is shown that this simple criterion exhibits a reasonable Bauschinger effect. Theoretical expressions for revised yield stresses for two different types of tests are derived for this function. </p> <p> The purpose of the experimental work in this thesis was to determine the degree of correlation between the proposed function and experimental data. Two types of tests were carried out. The first test was essentially for evaluating the constants appearing in the yield criterion. These values of constants were then used to predict the gross tensile stress-strain curves for specimens cut from sheets which had undergone plastic bending in one direction and contained residual stresses prior to tensile loading. An approximate method to calculate these residual stresses is also outlined. </p> <p> Conclusions are deduced by comparing the experimental and theoretical results for these tests and suggestions are made for future research. </p> / Thesis / Master of Engineering (ME)

During this thesis work a coupled thermo-mechanical finite element model (FEM) was builtto simulate hot rolling in the blooming mill at Sandvik Materials Technology (SMT) inSandviken. The blooming mill is the first in a long line of processes that continuously or ingotcast ingots are subjected to before becoming finished products. The aim of this thesis work was twofold. The first was to create a parameterized finiteelement (FE) model of the blooming mill. The commercial FE software package MSCMarc/Mentat was used to create this model and the programing language Python was used toparameterize it. Second, two different pass schedules (A and B) were studied and comparedusing the model. The two pass series were evaluated with focus on their ability to healcentreline porosity, i.e. to close voids in the centre of the ingot. This evaluation was made by studying the hydrostatic stress (σm), the von Mises stress (σeq)and the plastic strain (εp) in the centre of the ingot. From these parameters the stress triaxiality(Tx) and the hydrostatic integration parameter (Gm) were calculated for each pass in bothseries using two different transportation times (30 and 150 s) from the furnace. The relationbetween Gm and an analytical parameter (Δ) was also studied. This parameter is the ratiobetween the mean height of the ingot and the contact length between the rolls and the ingot,which is useful as a rule of thumb to determine the homogeneity or penetration of strain for aspecific pass. The pass series designed with fewer passes (B), many with greater reduction, was shown toachieve better void closure theoretically. It was also shown that a temperature gradient, whichis the result of a longer holding time between the furnace and the blooming mill leads toimproved void closure.

The research reported in this PhD thesis provides a novel approach to estimate 3D pitting corrosion kinetics of austenitic stainless steel with exposure to chloride-containing aqueous environments. A quasi-in-situ X-ray computed tomography (X-ray CT) approach was developed, with the aim of providing an experimental methodology to estimate 3D pitting corrosion kinetics under different exposure conditions. The first part summarises a set of preliminary investigations to identify the pitting corrosion behaviour of three austenitic stainless steels (type 303 bar, type 304 plate and type 304L wire) with different inclusion contents. All observed pit densities were related to the inclusion contents, providing confidence in moving to the next stage of the project, for conducting in-situ corrosion studies using X-ray CT. The second section describes the construction of an in-situ electrochemical cell for X-ray CT studies, the aim being to provide an experimental methodology to estimate 3D pitting corrosion kinetics. Pit growth kinetics of individual pits were estimated from segmented 3D X-ray CT data. The evolution of pit current densities, associated pit stability products, and diffusivity parameters over time were obtained. The study also showed that the kinetics of multiple pits could be estimated using this novel approach, based on separating the current response of each pit over time. This was obtained by electrochemical polarisation control and measuring the total current evolution. The third section discusses the effect of plastic strain on 3D pitting corrosion kinetics. Several in-situ X-ray CT experiments were conducted, with a focus on obtaining 3D pit growth, passivation, and re-activation kinetics, to elucidate the effect of applied strain on pit stability and growth. This section explains a possible mechanism for the re-activation of pre-existing corrosion pits, showing that pits grew more rapidly during reactivation than those grown before plastic strain was applied. A marked difference in pit morphology with fractured lacy metal covers was observed with the application of strain. The implications of this observation are discussed in light of stress corrosion crack nucleation mechanisms.

In this thesis, the relation between shear load and displacement for the lead-free solder (Sn3.0Ag0.5Cu) and the tin-lead solder (63Sn37Pb) are investigated. Except that, a new shear strength of the solder balls is suggested with considering the plastic strain energy of the solder balls. Three diameters of the Sn/Ag/Cu and Sn/Pb solder balls are studied. The variation of the plastic strain energies for the balls undergone different number of thermal cycles is compared. The effect of high temperature aging on the shear strength is also discussed. The difference between the failure fractures of the Sn/Ag/Cu and Sn/Pb solder ball are executed by using SEM. The experimental results show that the failure mechanism for the Sn/Ag/Cu is quite different from the Sn/Pb solder ball. Generally, the lead-free Sn/Ag/Cu solder is much ductile than the Sn/Pb solder ball in the shear test. Also the better fatigue performances are observed for the Sn/Ag/Cu solder balls.

abstract: Dissimilar metal joints such as aluminum-steel joints are extensively used in automobile, naval and aerospace applications and these are subjected to corrosive environmental and mechanical loading resulting in eventual failure of the structural joints. In the case of aluminum alloys under aggressive environment, the damage accumulation is predominantly due to corrosion and is accelerated in presence of other metals. During recent years several approaches have been employed to develop models to assess the metal removal rate in the case of galvanic corrosion. Some of these models are based on empirical methods such as regression analysis while others are based on quantification of the ongoing electrochemical processes. Here, a numerical model for solving the Nernst- Planck equation, which captures the electrochemical process, is implemented to predict the galvanic current distribution and, hence, the corrosion rate of a galvanic couple. An experimentally validated numerical model for an AE44 (Magnesium alloy) and mild steel galvanic couple, available in the literature, is extended to simulate the mechano- electrochemical process in order to study the effect of mechanical loading on the galvanic current density distribution and corrosion rate in AE44-mild steel galvanic couple through a multiphysics field coupling technique in COMSOL Multiphysics®. The model is capable of tracking moving boundariesy of the corroding constituent of the couple by employing Arbitrary Langrangian Eulerian (ALE) method.Results show that, when an anode is under a purely elastic deformation, there is no apparent effect of mechanical loading on the electrochemical galvanic process. However, when the applied tensile load is sufficient to cause a plastic deformation, the local galvanic corrosion activity at the vicinity of the interface is increased remarkably. The effect of other factors, such as electrode area ratios, electrical conductivity of the electrolyte and depth of the electrolyte, are studied. It is observed that the conductivity of the electrolyte significantly influences the surface profile of the anode, especially near the junction. Although variations in electrolyte depth for a given galvanic couple noticeably affect the overall corrosion, the change in the localized corrosion rate at the interface is minimal. Finally, we use the model to predict the current density distribution, rate of corrosion and depth profile of aluminum alloy 7075-stainless steel 316 galvanic joints, which are extensively used in maritime structures. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2015

The presented thesis was created in cooperation with Siemens, s.r.o. and deals with the analysis of the bending of aluminium sheets. These products are used as conductors of electrical energy. The problem happened when the bending angle was 90 ° when there was a violation outside of the bend. The aim of the thesis is to verify whether the bend is correctly designed using analytical and empirical relations and the finite element method. Because of the requirement for running numerical simulation on one of the freely available software the Salome Meca program environment was used. The results of numerical simulation did not confirm that the violation should occur. In deeper analysis appeared doubts whether it is possible to correctly evaluated result only by data from tensile test. In the final part, there are recommended some experiments which would be necessary for the correct calibration of the ductil fracture criteria.

Finite element simulations are carried out to study transient stresses and strains in the pre-cracked (pre-notched) 4340 steel plates impacted by a projectile in the direction of the notch ligament. The computations employed Johnson - Cook model which takes into account strain hardening, strain-rate hardening and thermal softening. An approximate solution of the governing equations is sought by using an explicit finite element code DYNA2D. We analyzed the evolution of the shear and hoop stresses considered to be responsible for two modes of failure: opening crack inclined at 70°, and shear crack inclined at -5° to the notch ligament. At small impact speeds and large notch tip radii failure in the 70° direction is due to the high tensile hoop stress. At high impact speeds and small notch tip radii a failure develops predominantly in the (-5°) -- (-15°) direction, within a zone of the maximum shear stress and compressive hoop stress. / Master of Science

The shear strength, elastic moduli and plastic strain characteristics of scaled-down ballast materials are investigated by use of the parallel gradation technique. Uniformly graded ballast materials chosen for the investigation are limestone, basalt and steel-slag. Steel-slag is a byproduct material of Eregli Iron and Steel Works, which is suitable to meet the durability test requirements as well as the electrical resistivity and the waste contaminants regulatory level. Conventional triaxial testing at a strain rate of 0.4 mm/min is used to obtain these characteristics for the scaled-down materials with a diameter of 100 mm specimen under a confining stress of 35 kPa, 70 kPa and 105 kPa / whereas that of only 35 kPa is used to characterize the accumulated plastic strain. The angle of internal friction, f, and the apparent cohesion, c, may be conservatively taken to be 42o and 35 kPa for all materials. The elastic moduli values for all materials may be predicted within an adequate estimate for the engineering purposes by using the power law parameters, K and n, determined for L-9.5 (D50 = 12.7 mm), the coarsest gradation tested for limestone. K with a reference pressure, pr = 1 kPa and n values for L-9.5, respectively, are 4365 and 0.636 for initial / 8511 and 0.419 for secant / 25704 and 0.430 for unloading-reloading elastic moduli. The unloading-reloading moduli increased, as the number of cycles increased. An increase in unloading-reloading modulus at N = 20 obtained was roughly 15% for scaled-down limestone / 10% for the basalt / and 5% for the steel-slag. The plastic strain after first cycle, &amp / #949 / 1, and the plastic strain coefficient, C can be represented as a function of mean particle size for each material type. For the limestone, basalt and steel-slag prototype size, D50 = 45 mm, &amp / #949 / 1 values of 0.59, 0.43 and 0.75 and C values of 0.54, 1.42 and 0.74 are predicted, respectively.

Orientadores: Antonio Jose Ramirez Londono, Sérgio Tonini Button / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-23T08:59:11Z (GMT). No. of bitstreams: 1 Fonseca_EduardoBertonida_M.pdf: 6113689 bytes, checksum: 7822b778729271cdd221fc4ab3b1a0cc (MD5) Previous issue date: 2013 / Resumo: Simulação física da soldagem por atrito com pino não consumível (SAPNC) foi realizada com o objetivo de se determinar as condições termomecânicas impostas ao material durante o processo, as quais são essenciais para o melhor entendimento da soldagem e para o desenvolvimento da simulação numérica da SAPNC. Por isso, ensaios de torção a quente foram realizados no aço inoxidável duplex (AID) UNS S32205 utilizando um simulador termomecânico Gleeble 3800®. Foi desenvolvido um sistema de resfriamento com nitrogênio líquido para se reproduzir, nos ensaios de torção, a história térmica medida durante a SAPNC do AID. Assim, foram reproduzidas as microestruturas da zona termomecanicamente afetada (ZTMA), tanto do lado de avanço quanto do lado de retrocesso, e da zona misturada (ZM) central. As microestruturas foram comparadas em termos de morfologia, de fração volumétrica, de tamanho de grão médio e de fração recristalizada de cada fase, por meio de microscopia óptica e difração de elétrons retroespalhados (EBSD). Além disso, foram analisados os mecanismos de amaciamento de cada fase durante a deformação a quente. Simulações numéricas do ensaio de torção foram realizadas para determinar a deformação verdadeira e a taxa de deformação em cada região simulada. A ZTMA foi simulada fisicamente a 1030 °C, 500 a 750 rpm e 0,50 a 0,75 revolução, o que corresponde a deformações verdadeiras entre 0,50 e 0,65 e taxa de deformação de 11 s-1. Embora tenha sido observada boa semelhança morfológica, o tamanho de grão médio das regiões simuladas se mostrou inferior àquele da ZTMA real. Observou-se recristalização dinâmica contínua da ferrita e recristalização dinâmica parcial da austenita, resultando em estrutura majoritariamente deformada. Já a ZM foi simulada fisicamente a 1130 °C, 500 a 750 rpm e 1,2 revolução, o que corresponde à deformação verdadeira de 1,30 e taxa de deformação de 15 s-1. O refinamento de grão da ZM foi reproduzido pelos ensaios de torção, porém com pequenas diferenças morfológicas devido à falta de mistura mecânica na simulação física. Foi apontada a ocorrência de recuperação e recristalização dinâmicas na austenita e recristalização dinâmica contínua na ferrita / Abstract: Physical simulation of friction stir welding (FSW) was employed to determine the thermomechanical conditions imposed to the material during the process, which are essential for the better understanding of the process and for the development of FSW numerical simulation. Therefore, hot torsion tests were carried out on UNS S32205 duplex stainless steel (DSS) using the thermomechanical simulator Gleeble 3800®. A liquid nitrogen quench system was developed in order to reproduce during the torsion tests the thermal history previously measured during FSW of the DSS. As a result, the microstructures from the thermomechanically affected zone (TMAZ), both advancing and retreating sides, and from the center of the stir zone (SZ) were reproduced. Microstructures were compared in terms of morphology, volume fraction, average grain size, and recrystallized fraction of each phase, by means of optical microscopy and electron backscattered diffraction (EBSD). Moreover, the softening mechanisms of each phase during hot deformation were investigated. Numerical simulation of the torsion tests were carried out in order to determine the true strain and the strain rate associated with each simulated microstructure. The TMAZ was physically simulated at 1030 °C, 500 to 750 rpm, and 0.50 to 0.75 revolution, which corresponds to true strains between 0.50 and 0.65, and strain rate of 11 s-1. Although good morphological resemblance was observed, the average grain size of the simulated regions remained smaller than the real TMAZ. The softening mechanisms observed were continuous dynamic recrystallization of the ferrite and partial dynamic recrystallization of the austenite, which resulted in a highly deformed structure. The SZ was simulated at 1130 °C, 500 to 750 rpm, and 1.2 revolution, which corresponds to true strain of 1.30 and deformation rate of 15 s-1. The grain refinement reported in the SZ was reproduced by the torsion tests, but morphological differences were observed due to the absence of stirring in the physical simulation. Dynamic recovery and dynamic recrystallization of the austenite were observed, as well as the continuous dynamic recrystallization of the ferrite / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica