We analyze plane strain thermomechanical deformations of a prenotched rectangular plate impacted on one side by a prismatic body of rectangular cross-section and moving parallel to axis of the notch. Both the plate and the projectile are made of the same material. Strain hardening, strain-rate hardening and thermal softening characteristics of the material are modeled by the Johnson-Cook relation. The effect of different material parameters, notch-tip radius, impact speed and the length of the projectile on the maximum tensile principal stress and the initiation and propagation of shear bands at the notch-tip is analyzed. It is found that for high impact speeds or enhanced thermal softening, two shear bands, one at and the other at to it propagate from the notch tip. Otherwise, only one shear band nearly parallel to the notch-ligament initiates at the notch-tip. The notch-tip distortion for high strength materials is quite different from that for low strength materials. The maximum tensile principal stress occurs at a point on the upper surface of the notch-tip and for every set of values of material parameters and impact speeds studied equals about 2.3 times the yield stress of the material in a quasistatic simple tension or compression test. We assume that the brittle fracture occurs when the maximum tensile principal stress equals twice the yield stress of the material in a quasistatic simple tension test and a shear band initiates when the effective plastic strain at a point equals 0.5. The effect of material and geometric parameters on the time of initiation of each failure mode is computed. It is found that for low impact speeds (< 30 m/s), a material will fail due to the maximum tensile principal stress exceeding its limiting value, and at high impact speeds due to the initiation of a shear band at the notch-tip. Results are also computed for a C-300 steel with material parameters given by Zhou et al. For an impact speed of 50 m/s, the shear band speed and the maximum effective plastic strain-rate before a material point melts are found to be 350 m/s and 5 x /s respectively. / Master of Science

The fingerprint of deformation in materials at large strains and at high strain rates is the formation of adiabatic shear bands. Adiabatic shear bands lead to unexpected failure of materials during service. This study investigated the possibility of eliminating adiabatic shear bands from materials subjected to severe deformation at high strain rates by post impact heat treatment. Five groups of cylindrical AISI 4340 steel samples were impacted using the Direct Impact Hopkinson Pressure Bar (DIHPB) developed at the University of Manitoba. Selected impacted samples with distinct transformed shear bands were soaked at 350⁰C to 850⁰C for periods ranging from 30 minutes to 4 hours to investigate how temperature and time affects the properties and structure of the shear bands. Annealing the shear bands at 350⁰C resulted in an increase in hardness of the shear bands and the surrounding material outside the shear bands regardless of the heat treatment before impact, amount of deformation, and the time of annealing. Significant decrease in hardness of the shear bands occurred after post impact annealing at 650⁰C for 30 minutes and 2 hours. Hardness of the shear bands reduced to the same level as that of the impacted material outside the shear bands. However, the initial path of the shear bands in the impacted steel samples could be traced through a “signature” left after the annealing. Post-impact annealing of the steel samples at 750⁰C and 850⁰C resulted in a homogenous microstructure with no trace of the shear bands. The “signatures” which were used to trace the path of the shear bands in impacted samples annealed at 650⁰C disappeared and the hardness across the samples became uniform. The observations from this study show that adiabatic shear bands in typical steel can be eliminated by annealing heat treatment. The temperature of annealing is the most critical parameter and the annealing should be performed above 650⁰C.

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While inelastic mechanical behaviour of crystalline materials is well-understood in terms of lattice defects, bulk metallic glasses (BMGs) pose significant challenges in this respect due to their disordered structure. They can be produced by rapid cooling from the liquid state (among other technique) and, thus can be frozen as vitreous solids. Due to the absence of a long-range order in atomic structure and a lack of defects such as dislocations, BMGs generally show unique mechanical properties such as high strength and elastic limit, as well as good fracture toughness and corrosion resistance. Typically, inorganic glasses are brittle at room temperature, showing a smooth fracture surface as a results of mode-I brittle fracture. At small scale, it was well documented that inelastic deformation of bulk metallic glasses is localised in thin shear bands. So, in order to understand deformation mechanisms of BMGs comprehensively, it is necessary to investigate formation of shear bands and related deformation process. In this thesis, a history of development of BMGs is presented, followed by a review of fundamental mechanisms of their deformation.

In this study, ultrafine-grained commercial purity AA1050 aluminum was produced by equal channel angular extrusion (ECAE).Annealing at 250¢J was able to give a grain size of 0.59£gm. Specimens were compressed along different ECAE axis under a strain rate of 7.1¡Ñ10-4 s-1at room temperature. Compression tests were also performed under 5¡Ñ10-5 s-1, 7.1¡Ñ10-4 s-1 ,and 10-1 s-1 strain rates at 100¢J,150¢J ,and 175¢J. Surface morphology of specimens was observed by optical and scanning electron microscopes to study the generation of shear bands. Texture within shear bands was analyzed by electron backscattered diffraction (EBSD). The present research found that, different compression direction has little effect on the generation of shear bands. Increasing compression temperature and decreasing strain rates have the effect of decreasing the degree of strain localization of shear bands. Shear band deformation is compatible with the uniform deformation occurred outside shear bands. Texture change within shear bands is rotated about an axis perpendicular to the specimen surface, and strengthens the texture component.

No description available.

This research, is initiated to systematically study the microstructure of AISI 4340 steel prior to impact, after impact and after post-impact annealing to determine the effect of the pre-deformation microstructure on the nucleation and initiation of ASBs, and the mechanism of evolution of ASBs during impact. This study used state-of-the-art microstructural characterization techniques such as the FIB and STEM/HRTEM to reveal that initial microstructural inhomogeneity produces nucleation sites for the initiation of ASBs during impact. It was observed that double misfit interfaces and boundary layers, formed around precipitated carbides (interface between reinforcements and matrix), increased the volume fraction of dislocation sources within the pre-impact specimens. It is demonstrated that the intersection of an activated dislocation source with the direction of maximum shear (regions of stress concentrations) within the specimens during impact, is a necessary condition for the points of intersection to act as possible sites for the nucleation and initiation of ASB depending on the rate of dislocation generation, local strain and strain rate. In addition, the structure that evolves after strain localization starts out with elongation of the grains in the shear direction with the initiation of random and transverse dislocation boundaries along the elongated grains. The elongated grains break along the initiated dislocation boundaries as strain/strain rate increases resulting in the creation of smaller elongated-broken grains and nanograins. Boundary refinement of the broken grains occurring through grain rotation and adiabatic heating results in the evolution of refined grains, subgrains and nanograins. The presence of elongated grains, broken grains, refined grains, subgrains and nanograins within the evolved shear band structures demonstrate that the local deformation is dependent on the imposed local strain and strain rate and that these mechanisms occur concurrently during impact. The results obtained, which are specific to the behavior of BCC ferritic Pearlitic hardenable steels, lead to the conclusion that the evolution of ASBs is a simultaneous layering of microstructures initially driven by dislocations which produce the final structures observed in the shear bands at the end of passage of the stress wave. / February 2015

No description available.

Il est bien établi que les propriétés mécaniques et rhéologiques d'une large classe de matériaux vitreux amorphes met en jeu - contrairement aux dislocations dans les cristaux - des rearrangements structuraux localisés formant par un processus de cascade des bandes de cisaillements. Cette localisation de la déformation est observée dans divers systèmes vitreux ainsi que dans des simulations numériques. Cette réponse mécanique complexe reste mal comprise à une échelle microscopique et il n'est pas clair si l'écoulement plastique peut être associé à une origine structurale locale ou à des processus purement dynamiques.Dans cette thèse nous envisageons ces problématiques à l'aide de simulations atomiques athermales sur un système Lennard-Jones modèle. Nous calculons le tenseur élastique moyenné localement sur une échelle nanométrique. A cette échelle, le verre est assimilable à un matériau composite comprenant un échafaudage rigide et des zones fragiles. L'étude détaillée de la déformation plastique à différents taux de cisaillement met en évidence divers régimes d'écoulement. En dessous d'un taux de cisaillement critique dépendant de la taille du système, la réponse mécanique atteind une limite quasistatique (effets de taille fini, cascades d'événements plastiques, contrainte seuil) alors que pour des taux de cisaillement plus importants les propriétés rhéologiques sont fixées par le taux de cisaillement imposé. Dans ce régime nous mettons en évidence la croissance d'une longueur de coopérativité dynamique et discutons de sa dépendance avec le taux de cisaillements.

The purpose of the work presented in this thesis was to explore and understand the mechanisms governing toughness, ductility and ballistic performance in a class of nanostructured carbide-free bainite-austenite steels, sometimes known as ‘superbainite’. The mechanical properties of these alloys have been extensively reported, but their interpretation is not clear. The thesis begins with an introduction to both the relevant nanostructures and some of the difficulties involved in explaining observed properties, alongside a summary of the role of mixed- microstructures in alloy development. An overview of the debate regarding the mechanism of bainite formation is pre- sented in Chapter 2, in the form of a literature survey encompassing the period of explicit recognition of the bainite microstructure. Of note is the role played by the displacive theory of formation in the development of the alloy structures investigated in this thesis. A characterisation of a commonly available bainitic alloy forms the basis for Chapter 4. Observations confirm the nanoscale nature of the structure, although additional phases are found to be present, namely: cementite and martensite. This is explained as resulting from relatively low alloying additions and chem- ical segregation effects, which are modelled using thermodynamic and kinetic approaches. Chapters 5 and 6 contain a comprehensive study of the response of this alloy to the stress concentration present at the notch root of a Charpy impact sample. The work provides evidence of notch root embrittlement due to stress-induced martensite transformation. Results from synchrotron and laboratory X-ray experiments in particular reveal that machining, as well as applied stress, can initiate the austenite-martensite transformation, and methods to mitigate this effect are suggested. An innovative approach is harnessed in Chapter 7, in order to identify exper- imentally the volume fraction at which three-dimensional connectivity (‘percolation’) of austenite is lost in a superbainitic steel. Hydrogen thermal desorption techniques are applied to this problem, inspired by the tendency of such alloys to undergo tensile failure with limited or zero necking. The striking result sheds light on the importance of austenite morphology in restricting the diffusion of hydrogen into a mixed structure. The final set of experimental work is directed towards understanding the damage mechanisms that occur during projectile penetration of a coarser bainitic armour- plate alloy. The formation of adiabatic shear bands is found to be a dominant factor governing the ballistic failure of the plate. The sheared material undergoes severe high-temperature deformation, but does not change phase upon cooling, leading to the proposal of certain methods that could be implemented to improve ballistic resistance of the steel. The totality of the research presented herein is summarised in Chapter 9, which draws attention to new areas of interest that have arisen from the current work, proposing several future directions of investigation. The broader issue of understanding, common to all studies performed thus far, is that of the causes, effects, and extent, of stress-induced transformation to martensite experienced by the retained austenite that is a key feature of superbainite and similar steels.