[pt] A modelagem do acoplamento entre os fenômenos mecânicos e térmicos em sólidos inelásticos é considerada neste trabalho. O acoplamento termomecânico é importante em determinadas situações, como por exemplo, no estudo de problemas envolvendo deformações inelásticas cíclicas em estruturas metálicas. Um procedimento sistemático para obtenção de equações constitutivas termodinamicamente admissíveis é apresentado. Através deste procedimento, baseado na Termodinâmica dos Processos Irreversíveis, foi possível obter uma teoria constitutiva para modelar o comportamento anisotérmico de metais e ligas metálicas. Dois tipos de acoplamentos termomecânicos foram identificados: o acoplamento interno, associado à dissipação interna do processo mecânico, e o térmico, associado à dependência dos parâmetros das equações constitutivas com a temperatura. A teoria foi particularizada para materiais elasto-viscoplásticos. Simulações com barras foram realizadas para estudar fenômenos como o aquecimento de metais provocado por solicitações mecânicas complexas e o comportamento de metais submetidos a grandes gradientes de temperatura. Uma variável de dano foi incorporada ao modelo, permitindo estudar a influência do acoplamento termodinâmico em processos de degradação do material como fadiga de baixo ciclo. / [en] The present work is concerned with the modeling of the coupling between mechanical and termal phenomena. The thermomechanical coupling is important in some problems like those involving inelastic cyclic deformation in metallic structures. A systematic procedure to obtain thermodynamically admissible constitutive equations is presented. Such procedure has a strong thermodynamic basis and is used to obtain a constant theory to model the anisothermal behavior of metals and alloys. Two kinds of thermomechanical couplings can be identified: the internal coupling, related with the internal dissipation in the mechanical process and the thermal coupling, related with the dependence of the material parameters in the constitutive equations on temperature. The theory is particularized to elasto-viscoplastic materials. Uniaxial simulations were performed to study the heating of metals due to complex mechanical loadings and the behavior of metals subjected to high temperature gradients. A damage variable is introduced in the model to study the influence of the thermomechanical coupling in processes involving the degradation of the material like in low-cycle fatigue.

This study investigates the cyclic deformation, fatigue behavior, and failure mechanisms for Ti-6Al-4V ELI (extra low interstitial) with and without mean strain/stress. Mean stress effects on fatigue behavior were studied using four strain ratios. Fatigue data generated was used to assess mean stress fatigue life prediction approaches, including stress-based methods such as Goodman, Gerber, Morrow, Walker and Kwofie; as well as strain-based models, such as Morrow, Smith-Watson-Topper, Walker, Kwofie, Ince-Glinka and a modified version of the Smith-Watson-Topper. The stress-based models did not yield reasonable results and data scatter was observed. The strain-based models offered better results, specifically the Morrow approach which provided more accurate fatigue life predictions. Fractography analysis determined that the influence of material defects on fatigue life had no major differences across all the strain ratios considered. Overall observations indicate that inclusions near the surface had great influence on the fatigue behavior.

The goal of this study is to investigate the microstructure and microstructure-based fatigue (MSF) model of additively-manufactured (AM) metallic materials. Several challenges associated with different metals produced through additive manufacturing (Laser Enhanced Net Shaping – LENS®) have been addressed experimentally and numerically. Significant research efforts are focused on optimizing the process parameters for AM manufacturing; however, achieving a homogenous, defectree AM product immediately after its fabrication without postabrication processing has not been fully established yet. Thus, in order to adopt AM materials for applications, a thorough understanding of the impact of AM process parameters on the mechanical behavior of AM parts based on their resultant microstructure is required. Therefore, experiments in this study elucidate the effects of process parameters – i.e. laser power, traverse speed and powder feed rate – on the microstructural characteristics and mechanical properties of AM specimens. A majority of fatigue data in the literature are on rotation/bending test of wrought specimens; however, few studies examined the fatigue behavior of AM specimens. So, investigating the fatigue resistance and failure mechanism of AM specimens fabricated via LENS® is crucial. Finally, a microstructure-based MultiStage Fatigue (MSF) model for AM specimens is proposed. For calibration of the model, fatigue experiments were exploited to determine structure-property relations for an AM alloy. Additional modifications to the microstructurally-based MSF Model were implemented based on microstructural analysis of the fracture surfaces – e.g. grain misorientation and grain orientation angles were added to the MSF code.

No description available.

The improvement in mechanical strength offered by ultra fine- (UF) and nanocrystalline (NC) sized grains is very attractive for potential applications of structural metals. Accumulative Roll-Bonding (ARB) is one of the promising new techniques for producing bulk UF grained metals. There are numerous reports on the monotonic mechanical behavior of various ARBed metals, however there are few, if any, on the cyclic deformation behavior of such metals. The primary objective of this study is to investigate the cyclic deformation behaviour and the related micro-mechanisms of ARBed metals from a fundamental perspective. To achieve this, the microstructure and the deformation behavior of commercial purity aluminum, OFHC copper, and DLP copper after ARB processing have been systematically characterized. The as-ARBed microstructure is found to be composite natured, with constituents of different grain sizes. The three constituents are: (i)UF grained matrix, (ii)NC primary discontinuities, and (iii)conventional sized pre-existing coarse grains. Due to this composite nature, three different cyclic strain accommodation mechanisms were found in the ARBed OFHC copper: (i)conventional dislocation patterns in the large grains, (ii)reactivation of pre-existing shear bands, and (iii)stress/strain driven grain coarsening at sites of strain localization. The order of activation of the mechanisms can be described with a composite approach based on activation energy. The occurrence of grain coarsening is the major contributor to the cyclic softening response observed in OFHC copper. Conversely, the lesser extent of cyclic softening in the other two metals is likely due to the higher microstructure stability of the initial as-ARBed materials. The microstructure stability is believed to be the primary influencing factor for the extent of grain coarsening and cyclic softening. The applied cyclic plastic strain is a secondary influencing factor, although this is generally overshadowed by the limitation of grain coarsening due to the short cyclic lifespan of these metals. The occurrences of shear banding and grain coarsening reported in the present ARBed metals are similarly reported for UF grained metals from other processes, e.g. ECAPed metals. Thus, its relationship to the cyclic deformation response and governing factors are believed to be applicable for UF grained metals in general.

The improvement in mechanical strength offered by ultra fine- (UF) and nanocrystalline (NC) sized grains is very attractive for potential applications of structural metals. Accumulative Roll-Bonding (ARB) is one of the promising new techniques for producing bulk UF grained metals. There are numerous reports on the monotonic mechanical behavior of various ARBed metals, however there are few, if any, on the cyclic deformation behavior of such metals. The primary objective of this study is to investigate the cyclic deformation behaviour and the related micro-mechanisms of ARBed metals from a fundamental perspective. To achieve this, the microstructure and the deformation behavior of commercial purity aluminum, OFHC copper, and DLP copper after ARB processing have been systematically characterized. The as-ARBed microstructure is found to be composite natured, with constituents of different grain sizes. The three constituents are: (i)UF grained matrix, (ii)NC primary discontinuities, and (iii)conventional sized pre-existing coarse grains. Due to this composite nature, three different cyclic strain accommodation mechanisms were found in the ARBed OFHC copper: (i)conventional dislocation patterns in the large grains, (ii)reactivation of pre-existing shear bands, and (iii)stress/strain driven grain coarsening at sites of strain localization. The order of activation of the mechanisms can be described with a composite approach based on activation energy. The occurrence of grain coarsening is the major contributor to the cyclic softening response observed in OFHC copper. Conversely, the lesser extent of cyclic softening in the other two metals is likely due to the higher microstructure stability of the initial as-ARBed materials. The microstructure stability is believed to be the primary influencing factor for the extent of grain coarsening and cyclic softening. The applied cyclic plastic strain is a secondary influencing factor, although this is generally overshadowed by the limitation of grain coarsening due to the short cyclic lifespan of these metals. The occurrences of shear banding and grain coarsening reported in the present ARBed metals are similarly reported for UF grained metals from other processes, e.g. ECAPed metals. Thus, its relationship to the cyclic deformation response and governing factors are believed to be applicable for UF grained metals in general.

The bulk metallic glasses (BMGs) are potential metallic materials due to their interesting properties, such as the high strength, high elastic strain limit, and high wear/corrosion resistance. Over the past four decades, a variety of studies have been done on the characteristics of the mechanical, thermodynamic properties of such category of metallic materials, but there still remain many questions about basic deformation mechanisms and their microstructures so far. Molecular dynamics (MD) simulation can provide significant insight into material properties under the atomic level and see a detailed picture of the model under available investigation in explaining the connection of macroscopic properties to atomic scale. MD simulation is applied to study the material properties and the deformation mechanisms in various binary metallic glasses and intended to examine the feasibility of MD simulation to compare the experimental results obtained in our laboratory over the past few years. The gradual vitrification evolution of atom mixing and local atomic pairing structure of the binary Zr-Ni, Zr-Ti alloys and pure Zr element during severe deformation at room temperature is traced numerically by molecular dynamic simulation. It is found that the icosahedra clusters will gradually develop with the increasing of disorder environment of alloys in the Zr-Ni, Zr-Ti systems, forming amorphous atomic packing. Other compound-like transition structures were also observed in transient in the Zr-Ni couple during the solid-state amorphization process under severe plastic deformation. The crystalline pure Zr can be vitrified in the simulation provided that the rolling speed is high enough and the rolling temperature is maintained at around 300 K. On the other hand, the effective medium theory (EMT) inter-atomic potential is employed in the molecular dynamics (MD) simulation to challenge the study of the diffusion properties in the Mg-Cu thin films. The transition of local structures of Mg-Cu thin films is traced at annealing temperatures of 300, 413, and 500 K. Furthermore, the simulation results are compared with the experimental results obtained from the transmission electron microscopy and X-ray diffraction. The gradual evolution of the local atomic pairing and cluster structure is discussed in light of the Mg and Cu atomic characteristics. Lately, the progress of the cyclic-fatigue damage in a binary Zr-Cu metallic glass in small size scale is investigated using classical molecular-dynamics (MD) simulations. The three-dimensional Zr-Cu fully amorphous structure is produced by quenching at a cooling rate 5 K/ps (ps = 10-12 s-1) from a high liquid temperature. The Nose-Hoover chain method is used to control the temperature and pressure to maintain a reasonable thermodynamic state during the MD-simulation process, as well as to bring the imposed cyclic stress on the subsequent simulation process. Both the stress- and strain-control cyclic loadings are applied to investigate the structural response and free-volume evolution. The overall structure would consistently maintain the amorphous state during cyclic loading. The plastic deformation in simulated samples proceeds via the network-like development of individual shear transition zones (STZs) by the reversible and irreversible structure-relaxations during cyclic loading, dislike the contribution of shear band in large-scale specimens. Dynamic recovery and reversible/irreversible structure rearrangements occur in the current model, along with annihilation of excessive free volumes. This behavior might be able to retard the damage growth of metallic glass and enhance their fatigue life.

The present study is focussed on assessment of cyclic deformation response and fatigue behaviour of bare and precorroded AZ31 magnesium alloys. Corrosion degradation was carried out in a salt spray fog (5% solution of sodium chloride) for 480 and 1000 hrs. Microstructure, mechanical properties, low- and high-cycle fatigue behaviour of experimental material in as-cast condition has been evaluated. Furthermore, the corrosion behaviour of material has been investigated. The fatigue tests have been performed using precorroded specimens to assess influence of corrosion degradation on cyclic deformation response and on low- and high-cycle fatigue behaviour. Influence of local corrosion degradation on initiation of fatigue cracks has been studied.

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