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Dissertations / Theses on the topic 'Brittle-ductile transition'

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Published: 4 June 2021
Last updated: 7 September 2023

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1

Samuels, J. "The brittle to ductile transition in silicon." Thesis, University of Oxford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.382682.

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2

Serbena, F. C. "The brittle-ductile transition of NiAl single crystals." Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.294341.

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3

Ellis, M. "The ductile to brittle transition in BCC metals." Thesis, University of Oxford, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.306220.

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4

Bezenšek, Boštjan. "Elastic-plastic crack problems in the ductile-brittle transition." Thesis, University of Glasgow, 2003. http://theses.gla.ac.uk/6946/.

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Margins in defect assessment procedures such as BS 7910 and R6/4 have been examined for cleavage and ductile tearing from complex and re-characterised defects. A range of crack profiles with re-entrant sectors developed from two co-planar surface breaking defects by fatigue has been examined experimentally and numerically. Both studies show enhanced crack driving forces in the re-entrant sector combined with a loss of crack tip constraint. Cleavage failures from complex and re-characterised defects demonstrated that the re-characterisation procedure is not conservative when cleavage occurs at small fractions of the limit load. Failures close to the limit load benefit from constraint loss which counteract the amplified crack driving forces in re-entrant sectors and cause re-characterised defects to be more detrimental than the original complex defects. Benefit may be taken from statistical size effects, which are strongly dependent on the crack geometry. Experimental fatigue and ductile tearing studies show similar development of complex cracks towards the re-characterised shape and re-characterisation procedures, such as those given in BS 7910 and R6/4, are conservative for fatigue and ductile tearing. A procedure has been developed to quantify enhanced temperature margins due to constraint loss by comparing the self similar stress fields at a critical local fracture stress (the Ritchie-Knott-Rice approach) and through the Weibull stress. Agreement with the experimental data has been demonstrated and the temperature dependence of the material parameters has been discussed. Following Li (1997) and Karstensen (1996), a toughness mapping technique was discussed that allows mode I toughness to be translated into mixed-mode I+II toughness for stress controlled fracture. In support of the arguments, toughness of Mode I and mixed-mode I+II configurations was measured on a mild steel. The experimental data clearly show increased cleavage toughness for unconstrained mode I and mixed-mode fields and the correlation with the predictions from the numerical models was demonstrated.
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5

Joseph, T. D. "Materials for Fusion Reactors - The Brittle-Ductile Transition in Vanadium." Thesis, University of Oxford, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.491624.

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This thesis describes experiments investigating the brittle-ductile transition in single and polycrystalline vanadium using four-point bend specimens. Pre-cracks were created in the beams by machining a notch into the tensile surface using EDM, which produced a network of sharp cracks in the base ofthe notch. Single crystal material was vacuum annealed at 1000°C and 400°C. The material annealed at 1000°C was had a low concentrations of dislocations and interstitial hydrogen. It produced a sharp BOT typical of dislocation free material, with an activation energy of 0.26 eV ± 0.07 eV. Material annealed at 400°C had a low concentration of interstitial hydrogen and a normal dislocation density and produced a soft transition, typical of metallic materials with active dislocations, with an activation energy of 0.22 eV ± 0.02 eV. Unannealed single crystal material had a high concentration of interstitial hydrogen and a normal dislocation density and produced a soft transition with an activation energy of 0.14 eV ± 0.04 eV. Its behaviour was thought to be dominated by the effects of interstitial hydrogen. Annealed polycrystalline vanadium was found to be ductile at temperatures of 77 K and above. Unannealed polycrystalline vanadium was ductile at 77 K and at high temperatures but exhibited a region of brittle failure between approximately 100 K and 300 K. A number of unusual phenomena were observed in this material and were attributed to competing effects of interstitial hydrogen. A number of attempts to measure the velocity of dislocations in vanadium were unsuccessful but direct imaging of the plastic zones of a number of samples using EBSD was used to investigate dislocation activity at different points in the transition curves.
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6

Venkatachalam, Sivaramakrishnan. "Predictive Modeling for Ductile Machining of Brittle Materials." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/19774.

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Brittle materials such as silicon, germanium, glass and ceramics are widely used in semiconductor, optical, micro-electronics and various other fields. Traditionally, grinding, polishing and lapping have been employed to achieve high tolerance in surface texture of silicon wafers in semiconductor applications, lenses for optical instruments etc. The conventional machining processes such as single point turning and milling are not conducive to brittle materials as they produce discontinuous chips owing to brittle failure at the shear plane before any tangible plastic flow occurs. In order to improve surface finish on machined brittle materials, ductile regime machining is being extensively studied lately. The process of machining brittle materials where the material is removed by plastic flow, thus leaving a crack free surface is known as ductile-regime machining. Ductile machining of brittle materials can produce surfaces of very high quality comparable with processes such as polishing, lapping etc. The objective of this project is to develop a comprehensive predictive model for ductile machining of brittle materials. The model would predict the critical undeformed chip thickness required to achieve ductile-regime machining. The input to the model includes tool geometry, workpiece material properties and machining process parameters. The fact that the scale of ductile regime machining is very small leads to a number of factors assuming significance which would otherwise be neglected. The effects of tool edge radius, grain size, grain boundaries, crystal orientation etc. are studied so as to make better predictions of forces and hence the critical undeformed chip thickness. The model is validated using a series of experiments with varying materials and cutting conditions. This research would aid in predicting forces and undeformed chip thickness values for micro-machining brittle materials given their material properties and process conditions. The output could be used to machine brittle materials without fracture and hence preserve their surface texture quality. The need for resorting to experimental trial and error is greatly reduced as the critical parameter, namely undeformed chip thickness, is predicted using this approach. This can in turn pave way for brittle materials to be utilized in a variety of applications.
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7

Narström, Torbjörn. "Microstructural aspects of the ductile-to-brittle transition in pressure vessel steels." Doctoral thesis, KTH, Materials Science and Engineering, 2000. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3007.

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8

Hütter, Geralf. "Multi-scale simulation of crack propagation in the ductile-brittle transition region." Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2013. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-121281.

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In the present thesis the crack propagation in the ductile-brittle transition region is studied on two scales with deterministic models. In the macroscopic model the ductile failure is described by a non-local Gurson-model whereas the discrete void microstructure is resolved around the crack tip in the microscopic model. The failure by cleavage is not evaluated by means of a post-processing criterion but is modeled equivalently using a cohesive zone model on both scales. Thus, cleavage is not a priori identified with unstable crack propagation but the transition between stable and unstable mode of propagation is a result of the simulation. The problem of handling completely failed material within the framework of non-local damage models is pointed out. A method to overcome this problem is proposed and successfully applied. The case of contained plastic yielding at the crack tip is addressed with a modified-boundary layer model. The macroscopic simulations reproduce many features which are known from experiments like the formation of stretch zones, cleavage after initial ductile tearing, pop-ins with crack arrest, among others. The microscopic simulations substantiate the understanding of the macroscopically observed behavior. Systematic parameter studies are performed. Starting with considerations on the limit cases like pure ductile failure or the lower-ductile brittle transition region allows to separate the effects of the different constitutive parameters. Based on these results, a methodology is proposed to extract the macroscopic material parameters from experiments. This scheme is successfully applied to experimental data from literature. The results show that the behavior of a low-constraint specimen can be reliably predicted with the parameters extracted from a high-constraint specimen
In der vorliegenden Arbeit wird die Rissausbreitung im spröd-duktilen Übergangsbereich auf zwei Skalen mittels deterministischer Modelle untersucht. Das duktile Versagen wird im makroskopischen Modell durch ein nichtlokales Gurson-Modell beschrieben, während im mikroskopischen Modell die Porenmikrostruktur im Bereich um die Rissspitze diskret aufgelöst wird. Das mögliche Versagen durch Spaltbruch wird nicht, wie üblich, nachträglich durch ein spannungsbasiertes Kriterium bewertet. Stattdessen wird der Spaltbruch auf beiden Skalen durch ein Kohäsivzonenmodell abgebildet. Somit wird die Spaltbruchinitiierung nicht a priori mit instabiler Rissausbreitung gleichgesetzt. Vielmehr ist die Stabilität der Rissausbreitung ein Ergebnis der Simulationen. Außerdem wird das Problem der der Handhabung vollständig ausgefallenen Materials im Rahmen nichtlokaler Schädigungsmodelle herausgestellt. Es wird eine Methode vorgestellt, dieses Problem zu behandeln und erfolgreich angewendet. In den Simulationen wird der Fall vollständig eingebetteten, plastischen Fließens untersucht. Die Simulationen mit dem makroskopischen Modell geben viele Effekte wieder, die aus Experimenten bekannt sind. Dazu zählen die Ausbildung von Stretchzonen, die Spaltbruchinitiierung nach anfänglichem, duktilem Reißen oder lokale Instabilitäten mit Rissarrest. Die mikroskopischen Simulationen tragen zum Verständnis des makroskopisch beobachteten Verhaltens bei. In der vorliegenden Arbeit werden systematische Parameterstudien durchgeführt. Zunächst werden Grenzfälle wie das rein duktile Versagens oder der Spaltbruch in Abwesenheit der Mikroporen untersucht, um die Einflüsse der einzelnen Materialparameter abzugrenzen. Ausgehend von diesen Ergebnissen wird eine Prozedur vorgeschlagen, die Materialparameter des makroskopischen Modells Schritt für Schritt aus Experimenten zu bestimmen. Diese Prozedur wird erfolgreich auf experimentelle Daten aus der Literatur angewendet. Die Ergebnisse zeigen, dass es das entwickelte Modell erlaubt, das Verhalten einer Bruchmechanikprobe mit geringer Dehnungsbehinderung an der Rissspitze mit denjenigen Materialparametern vorherzusagen, die an Proben mit einer hohen Dehnungsbehinderung ermittelt wurden
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9

Cullers, Cheryl Lynne. "Deformation mechanisms of NiA1 cyclicly deformed near the brittle-to-ductile transition temperature." Thesis, Georgia Institute of Technology, 1993. http://hdl.handle.net/1853/20050.

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10

Cheng, Shiwang. "Tensile Deformation of Polymer Glasses: Crazing, the Brittle-Ductile Transition and Elastic Yielding." University of Akron / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=akron1382525654.

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11

Gally, Brian James 1970. "Experimental investigation of the brittle to ductile transition in fracture of single crystal silicon." Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/85252.

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12

Xu, Wenting. "Molecular Dynamics Simulations of the Size-dependent Brittle-to-ductile Transition of Silicon Nanowires." University of Cincinnati / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1613751523441622.

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13

VanDeVelde, Sharon Ann. "Distribution and Transport of Water in Natural Quartz Arenites Near Brittle-Ductile Transition Conditions." Bowling Green State University / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1237586551.

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14

Ueda, Tadamasa. "Seismogenic deformation structures in the brittle-ductile transition regime: a case study of ultramafic pseudotachylytes and related deformed rocks in the Balmuccia peridotite body, Italy." 京都大学 (Kyoto University), 2016. http://hdl.handle.net/2433/204571.

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15

Novovic, Milorad. "Effects of microstructure and prestraining on ductile to brittle transition in carbon-manganese weld metals." Thesis, University of Birmingham, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.246758.

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The effects of microstructure and prestraining on toughness of a multipass carbon-manganese (C-Mn) weld metal have been investigated. The toughness has been assessed in terms of both Charpy impact energy and Crack Tip Opening Displacement values. An extensive Charpy impact test experimental programme has been carried out on the C-Mn pressure vessel steel weld metal resulting in comprehensive databases for the weld metal in as-received and prestrained conditions. Charpy specimens were machined from multipass submerged arc weld metals with various notch tip locations and orientations relative to the weld metal. The impact energy was evaluated over a range of temperatures from -196 to 150°C. Mechanical prestraining levels of 2,9, and 12% have been applied to impact specimens to provide the trends in impact toughness that approximate some effects of irradiation embrittlement in service. Statistical interpretation of the results, in conjunction with the measurements from the fracture surfaces of Charpy specimens and fractography, has been carried out to provide a quantitative assessment of transition temperature shifts from the full and sub-populations of impact energy data pertaining to a particular notch root location relative to the weld microstructure. Impact energy values of a multipass submerged-arc C-Mn weld metal in both as-received and prestrained conditions are found to be subject to substantial scatter in the transition and upper shelf regions. Lower shelf impact energy values are found to be insensitive to microstructure and prestraining. The microstructure is found to have a significant influence on the ductile to brittle transition curve. Impact toughness for specimens with notch root locations in the reheated microstructure is greater than the impact toughness of specimens with notches located in the as-deposited microstructure. This trend is observed in both transition and upper shelf regions and is valid for both as-received and prestrained material conditions. The OT40Js hift between the transition curves corresponding to the as-received specimens with notch root locations on the centreline of the last deposited bead and in reheated microstructure has been estimated to be approximately 50°C. Prestraining (cold deformation) is found to provide the same trends in Charpy impact energy transition curve as irradiation embrittlement. Prestraining thus reduces the as-received upper shelf energy level and shifts the as-received transition curve towards higher temperatures. The magnitude of these effects depends on the amount of prestraining. The as-received and 2% prestrained datasets are found to be very similar, as are the datasets obtained after 9 and 12% prestraining.The transition curve for specimens with the notch root located in the reheated microstructure in the as-received material condition and transition curve for the specimens with notch root located on the centreline of the last deposited bead in the 9% prestrained material condition form the boundaries of the region of all possible locations for the transition curves along the temperature axis. The width of this "window" at the 40 J energy level, i. e. the OT4o4 temperature shift between the "best" (upper bound) dataset in as-received condition and the "worst" (lower bound) impact energy dataset in 9% prestrained material is estimated to be 102°C. The microstructure is also found to have a marked influence on the CTOD values of the as-received material with reheated microstructure showing better CTOD toughness across the whole temperature range examined. Prestraining at the 9% level shifts the onset of plastic collapse to lower values of the opening displacement resulting in the reduction in upper shelf CTOD values of more than 50% relative to the as-received upper shelf CTOD values. The transition region is also shifted along the temperature axis.
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16

Pallaspuro, S. (Sakari). "On the factors affecting the ductile-brittle transition in as-quenched fully and partially martensitic low-carbon steels." Doctoral thesis, Oulun yliopisto, 2018. http://urn.fi/urn:isbn:9789526218977.

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Abstract From the largest discontinuities to the smallest of the elements, various factors can threaten structural integrity. Susceptibility to these factors elevates with higher yield strengths. As-quenched low-carbon steels with a martensitic or martensitic-bainitic microstructure are modern ultra-high-strength structural steels. They can possess sufficient toughness, formability, and weldability, and are typically used in weight-critical and high-performance structures. Common problems with as-quenched steels with a yield strength of 900 MPa or more are that they do not obey the conventional correlation between the fracture toughness reference temperature T₀ and the impact toughness transition temperature T₂₈ᴊ used in many standards and structural integrity assessment procedures, and a lack of design rules in general. This thesis studies the relationship between the T₀ and T₂₈ᴊ to provide additional knowledge for future standardisation, the microstructural features governing the toughness at these temperatures on both global and local scale, and whether hydrogen embrittlement is present at subzero temperatures. It uses steels produced via laboratory rolling and quenching as well as from pilot-scale and full-scale industrial production, studying them with standardised toughness tests, microstructural characterisation, fractography, and cohesive zone modelling. As-quenched steels have a distinct correlation between T₀ and T₂₈ᴊ. An improved general T₀ – T₂₈ᴊ correlation applies to a wide range of steels. T₂₈ᴊ correlates closely with a dynamic reference toughness, which can be used together with the fraction of detrimental {100} cleavage planes near the main fracture plain to effectively estimate the transition temperatures. On a local scale, centreline segregation decreases the effective coarse grain size, which more than compensates for the harmful effects associated with the higher hardness and inclusion content of the centreline, resulting in increased fracture toughness. Hydrogen embrittlement causes a decrease in fracture toughness and local deformability, thereby increasing T₀ while leaving T₂₈ᴊ unaffected. Overall, the results show that high toughness demands good control of effective coarse grain size and hydrogen content
Tiivistelmä Tekijät suurimmista epäjatkuvuuskohdista aina pienimpään alkuaineeseen voivat uhata rakenteellista eheyttä, minkä lisäksi alttius näille kasvaa materiaalin myötölujuuden kasvaessa. Modernit karkaistun tilan ultralujat matalahiiliset rakenneteräkset voivat silti omata riittävän sitkeyden, muovattavuuden ja hitsattavuuden. Tyypillisiä käyttökohteita näille ovat painon suhteen kriittiset ja korkean suorituskyvyn rakenteet. Yleinen ongelma myötölujuudeltaan noin ja yli 900 MPa karkaistun tilan teräksillä on se, että ne eivät noudata perinteistä murtumissitkeyden referenssilämpötilan T₀ ja iskusitkeyden transitiolämpötilan T₂₈ᴊ välistä korrelaatiota, jota käytetään useissa standardeissa ja suunnitteluohjeissa, jotka eivät myöskään vielä salli näin lujien terästen käyttöä. Tämä väitöstyö tutkii transitiolämpötilojen T₀ ja T₂₈ᴊ välistä suhdetta edistääkseen näiden terästen sisällyttämistä standardeihin, haurasmurtuma-sitkeyteen vaikuttavia mikrorakenteellisia tekijöitä sekä yleisellä että paikallisella tasolla, ja vetyhaurautta matalissa lämpötiloissa. Koeteräkset ovat laboratoriovalmisteisia, tuotantokokeita ja tuotantolaatuja. Niitä tutkitaan standardisoiduilla sitkeyskokeilla, mikrorakenteen karakterisoinnilla, fraktografialla ja koheesiovyöhykettä hyödyntävällä mallinnuksella. Tulokset osoittavat karkaistun tilan terästen omaavan erityisen korrelaation T₀ ja T₂₈ᴊ välillä. Muokattu, ultralujat teräkset huomioiva yleinen T₀ – T₂₈ᴊ -korrelaatio soveltuu laajalti eri terästyypeille. T₂₈ᴊ korreloi läheisesti dynaamisen referenssisitkeyden kanssa, jonka avulla yhdessä haitallisten {100} lohkomurtumatasojen osuuden kanssa voidaan estimoida joukko transitiolämpötiloja. Paikallisella tasolla keskilinjasuotauma pienentää efektiivistä karkeiden rakeiden kokoa, mikä suotauman suurista sulkeumista ja kovuudesta huolimatta parantaa murtumissitkeyttä. Vetyhauraus taas huonontaa sitkeyttä ja paikallista muodonmuutoskykyä myös matalissa lämpötiloissa nostaen T₀ lämpötiloja. Kokonaisuutena erinomainen transitiolämpötilasitkeys vaatii efektiivisen karkearaekoon ja vetypitoisuuden minimointia
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17

Brauer, Nancy A. "Fluid inclusions as a monitor of progressive grain-scale deformation during cooling of the Papoose Flat pluton, eastern California." Thesis, Virginia Tech, 1997. http://hdl.handle.net/10919/36556.

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Analyses of fluid inclusions and microstructures within the Papoose Flat pluton were used to investigate the chemistry and temperatures of fluids circulating with the pluton during cooling. Based on previous microstructural analyses, the interior of this late Cretaceous granitic to granodioritic pluton has been divided into three domains: i) a central core characterized by magmatic microstructures, ii) a middle domain of high temperature (>500°C) solid-state deformation, and iii) an outermost domain characterized by relatively low temperature (<5000°C) solid-state deformation. According to previously published anisotropy of magnetic susceptibility analyses and pluton cooling models, plastic flow occurred in both the outer part of the pluton and within its aureole rocks while the core of the pluton was still molten. Solid-state deformation is proposed to have stopped when the pluton interior cooled through its solidus less than 100,000 years after magma emplacement.

Microstructural analysis of samples from all three domains confirmed the transition from magmatic flow in the core of the pluton to solid-state deformation at the pluton margin. However, weakly developed solid-state microstructures overprint the dominant magmatic microstructures in samples from the core domain. The existence of solid-state microstructures in all three domains indicates that deformation continued during and after crystallization of the interior of the pluton.

Two phase, low salinity (< 26 wt% NaCl equivalent), liquid-rich aqueous fluid inclusions predominate within both quartz and feldspar grains in all samples. Throughout the pluton, the majority of fluid inclusions are hosted by deformed grains. Feldspar-hosted primary inclusions are associated with sericitic alteration. Inclusions were also observed in feldspar as secondary or pseudosecondary inclusions along fractures. Inclusions in quartz are frequently found near lobate grain boundaries or near triple junctions; linear pseudosecondary inclusion assemblages are commonly truncated against lobate boundaries between adjacent quartz grains, indicating that discrete microcracking events occurred during plastic deformation.

Homogenization temperatures overlap for all three microstructural domains. Coexisting andalusite and cordierite in the contact aureole, and the intersection of the Mus + Qtz dehydration reaction with the granite solidus, indicate trapping pressures between 3.8 and 4.2 kb. Ninety-eight percent of the calculated fluid inclusion trapping temperatures at 3.8 - 4.2 kb are below the granite solidus of 650°C. Seventy-six percent of the trapping temperature data fall within the more restricted range of 350-500°C; i.e. at temperatures which are lower than the commonly cited brittle-ductile transition temperatures for feldspar at natural strain rates, but above those for quartz. No correlation could be established between trapping temperatures and either host mineral or microstructural domain within the pluton.

The similar, relatively low trapping temperatures indicate that the majority of inclusions preserved in all three domains were trapped during the late low strain magnitude stages of solid-state deformation. The most common fluid inclusion trapping temperatures (400-500°C) in all three microstructural domains are similar to the deformation temperatures indicated by microstructures and crystal fabrics in the outer part of the pluton; these trapping temperatures are obviously lower than temperatures associated with contemporaneous solid state and magmatic flow in the pluton interior. The similar trapping temperatures within the pluton core and margin must indicate that the inclusion-trapping event migrated from the margin to the core of the pluton as it cooled, because fluid inclusions would rapidly equilibrate to a density appropriate for the PT conditions of their host minerals.
Master of Science

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18

Arsène, Sylvie. "Effet de la microstructure et de la température sur la transition ductile-fragile des zircaloy hydrures." Châtenay-Malabry, Ecole centrale de Paris, 1997. http://www.theses.fr/1997ECAP0550.

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Le zircaloy est largement utilisé dans les centrales nucléaires, notamment pour les tubes de gainage. Ces tubes sont susceptibles de subir une pression interne lors de leur séjour en réacteur. Notre but est de déterminer l'influence de la microstructure, de la température, de l'hydruration et finalement de l'irradiation sur la transition ductile-fragile et les propriétés mécaniques des tubes sollicites en traction dans la direction circonférentielle. Pour les besoins de l'étude, nous avons propose une amélioration de l'essai de traction sur anneau classique. Les modélisations de cet essai avec la méthode des éléments finis ont permis de vérifier que les contraintes et les déformations sont bien homogènes dans la partie entaillée de l'anneau et de proposer une méthode pour dépouiller les résultats expérimentaux et de la valider sur les alliages connus. Des essais sur anneau effectues sur deux alliages de zirconium (détendu et recristallisé), pour différentes concentrations en hydrogène, à 20°C et à 300°C ont permis de déterminer l'influence de la température et l'hydruration sur le comportement et la rupture de ces alliages. L’observation des surfaces et des profils de rupture a mis en évidence les paramètres physiques et mécaniques gouvernant la transition ductile-fragile dans les différents cas. La distance interhydrures est le paramètre primordial dans le cas du matériau détendu alors que la longueur d'hydrures radiaux contigus est le facteur déterminant dans le cas du matériau recristallisé. La ductilité importante du zircaloy détendu de notre étude et les résultats des observations au MEB nous ont incité à étudier plus en détail les hydrures, leur rupture et leur comportement. Les observations, faites lors d'essais a 20°C dans l'enceinte du MEB, ont montre que les hydrures se déforment de 20 a 80% avant de rompre dans les alliages hydrures a 1000 ppm. Nous avons pu évaluer la loi de comportement des hydrures puis la contrainte a rupture des hydrures par méthode inverse grâce à une modélisation micro-macro. Nous proposons une hypothèse expliquant la plasticité importante observée.
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19

Voskoboinikov, Roman E. "Analytical Description of Brittle-to-Ductile Transition in bcc Metals. Nucleation of dislocation loop at the crack tip." Forschungszentrum Dresden, 2010. http://nbn-resolving.de/urn:nbn:de:bsz:d120-qucosa-29522.

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Nucleation of dislocation loop at the crack tip in a material subjected to uniaxial loading is investigated. Analytical expression for the total energy of rectangular dislocation loop at the crack tip is found. Depencence of the nucleation energy barrier on dislocation loop shape and stress intensity factor at the crack tip is determined. It is established that the energetic barrier for nucleation of dislocation loop strongly depends on the stress intensity factor. Nucleation of dislocation loop is very sensitive to stress field modifiers (forest dislocations, precipitates, clusters of point defects, etc.) in the crack tip vicinity.
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20

Hennecke, Thomas [Verfasser], and Thomas [Akademischer Betreuer] Sonar. "Mathematical modeling of the ductile-brittle-transition and an application to ferritic steel / Thomas Hennecke ; Betreuer: Thomas Sonar." Braunschweig : Technische Universität Braunschweig, 2013. http://d-nb.info/1175822906/34.

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21

Voskoboinikov, Roman E. "Analytical Description of Brittle-to-Ductile Transition in bcc Metals. Nucleation of dislocation loop at the crack tip." Forschungszentrum Rossendorf, 2002. https://hzdr.qucosa.de/id/qucosa%3A21779.

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Nucleation of dislocation loop at the crack tip in a material subjected to uniaxial loading is investigated. Analytical expression for the total energy of rectangular dislocation loop at the crack tip is found. Depencence of the nucleation energy barrier on dislocation loop shape and stress intensity factor at the crack tip is determined. It is established that the energetic barrier for nucleation of dislocation loop strongly depends on the stress intensity factor. Nucleation of dislocation loop is very sensitive to stress field modifiers (forest dislocations, precipitates, clusters of point defects, etc.) in the crack tip vicinity.
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22

Wang, Hongjian. "Evaluation of Orthogonal Cutting as a Method for Determining Mechanical Properties of Polymers." Thesis, The University of Sydney, 2017. http://hdl.handle.net/2123/16136.

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The purpose of this thesis is to study and evaluate orthogonal cutting as a novel testing method for simultaneously determining the yield stress and fracture toughness of polymeric materials. The work is of particular importance to the fracture toughness characterisation of the tough and ductile thermoplastic polymers as the toughness of such materials is difficult to measure via conventional fracture mechanics testing techniques. In addition, the new cutting theory is applied to the study of machining characteristics of the thermosetting epoxies and epoxy composites. The main focus is to understand the mechanism of brittle- to- ductile transition in the cutting-induced material separation process. An energy based analysis of the orthogonal cutting process was adopted for the determination of the yield stress and fracture toughness, i.e. σY and Gc of a polymeric workpiece material. It is found that varying tool rake angle from 0° to 30° does not exert significant influence on the determination of Gc. However, Gc shows a decreased tendency with decreasing depth of cut as the crack-tip plastic zone ahead of the tool cutting edge is constrained by the limited depths of cut. It is also observed that simultaneous bending and shearing take place on the shear plane during the orthogonal cutting chip formation process. The inclusion of the bending energy in the cutting analysis can correct the previously derived high values of yield stress. In addition, the brittle- to- ductile chip formation behaviour in machining of the brittle epoxies is clarified. The transition is ascribed to ductile-regime cutting as the depth of cut is reduced to be comparable with the size of plane strain crack-tip plastic zone. Furthermore, not only the depth of cut, but also the ratio of Gc/σY of a material controls the characteristics of the cutting-induced material separation process.
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23

Tankoua, Yinga Franck. "Transition ductile-fragile des aciers pour gazoducs : Étude quantitative des ruptures fragiles hors plan et corrélation à l’anisotropie de microtexture." Thesis, Paris, ENMP, 2015. http://www.theses.fr/2015ENMP0014/document.

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La bonne ténacité des aciers pour gazoducs aux basses températures est nécessaire pour éviter la propagation de fissures de manière catastrophique. Cette étude vise à améliorer la compréhension physique et l'évaluation quantitative du comportement à rupture des aciers pour gazoducs laminés à chaud, en nous intéressant plus particulièrement aux températures au pied de la transition ductile-fragile .La résilience de ces aciers est généralement validée à l'aide d'essais dits drop weight tear tests (DWTT), après lesquels le faciès de rupture doit contenir moins de 15% de zone fragile. Pour les aciers mis en forme par laminage thermomécanique (TMCP), des ruptures fragiles hors plan, comme le délaminage (qui se propage dans le plan de laminage de la tôle), et la rupture fragile en biseau le long des plans dits thêta (inclinés de 40° autour de la direction de laminage par rapport au plan de laminage) apparaissent dans la transition ductile-fragile. Ces modes de rupture, observés lors des essais de résilience (DWTT, Charpy) et de ténacité (CT), affectent la résistance à la rupture de ces aciers.L'anisotropie de l'écoulement plastique, puis celle de la sensibilité à la rupture par clivage ont été caractérisées en fonction de la température, à l'aide d'essais de traction sur des éprouvettes lisses et entaillées conçues pour cette étude. L'analyse mécanique de ces essais à l'aide de calculs par éléments finis a permis de déterminer des contraintes critiques de clivage dans les directions perpendiculaires au plan de laminage et aux plans thêta. Les valeurs obtenues dans ces directions sont de 25% inférieures à celles correspondant aux directions de laminage et travers long.L'anisotropie de la contrainte critique de clivage a été quantitativement corrélée à l'anisotropie de microtexture du matériau. Des entités appelées « facettes de clivage potentielles » ont été définies et mesurées dans cette étude, comme des régions contenant un plan {100} défavorablement orienté et dans lesquelles les fissures de clivage se propagent sans être arrêtées. Par exemple, un plan contenant 20% de facettes de clivage potentielles aurait une contrainte critique de clivage de 20% moins élevée qu'un plan présentant seulement 10% de facettes de clivage potentielles.La taille et la forme de ces facettes de clivage potentielles évoluent avec la déformation plastique. Par conséquent, la contrainte critique de clivage est affectée par l'historique de déformation. Dans le cas du délaminage, les facettes de clivage potentielles s'allongent au cours d'un chargement dans la direction travers long, conduisant à une augmentation de leur taille effective et par conséquent à une diminution (qui peut atteindre 30%) de la contrainte critique de clivage dans le plan de la tôle. Cette diminution facilite in fine l'apparition du délaminage. De plus, la présence de micro-fissures ductiles facilite la rupture par délaminage en modifiant l'état de contrainte local. Un critère a ainsi été proposé pour prédire numériquement l'amorçage du délaminage dans des éprouvettes de traction et/ou de résilience Charpy.L'application de cette approche à des échantillons traités thermiquement et à des échantillons pré-déformés a montré que la sensibilité au délaminage pouvait être contrôlée en modifiant la texture locale initiale du matériau
High toughness of pipeline steels at low temperature is required to avoid catastrophic propagation of brittle crack. The aim of the study is to improve physical understanding and quantitative assessment of the toughness behavior of hot-rolled pipeline steels, focusing on the lower part of the ductile to brittle transition temperature range.The impact toughness of these steels is commonly validated using drop weight tear tests (DWTT), on the basis of fracture surfaces that must exhibit less than 15% of brittle fracture appearance. In thermomechanical control processed steels, brittle out-of-plane cracks such as delamination (which propagates along the rolling plane), and brittle tilted fracture (BTF) along theta-planes (tilted around RD by 40° with respect to rolling plane), have been characterized in the ductile to brittle transition temperature range, for both industrial (DWTT) and laboratory Charpy impact tests. In both cases, as well as in fracture toughness tests, such brittle out-of-plane cracking has been shown to impair the impact toughness.The anisotropy in plastic flow and sensitivity to cleavage fracture has been characterized as a function of temperature, by using tensile tests on specifically designed smooth and notched specimens. From finite element mechanical analysis of these tests, critical cleavage stresses normal to the rolling plane and the theta-plane are considerably lower (around 25%) than for planes normal to the rolling and transverse directions.The anisotropy in critical cleavage stress has been quantitatively correlated to microtexture anisotropy. So-called “potential cleavage facets” have been defined and measured in this study, as regions with unfavorably oriented {100} planes, which are taken as unit crack paths for cleavage propagation. A sample containing 20% of potential cleavage facets had a critical cleavage stress 20% lower than a sample with only 10% of potential cleavage facets.The size and shape of these potential cleavage facets evolve during plastic deformation. Therefore, the critical cleavage stress was found to be affected by plastic strain history. In the case of delamination, potential cleavage facets along the rolling plane were elongated during loading, their area was increased and the corresponding critical cleavage stress decreased by around 30% with respect to the undeformed case. This made delamination cracking easier. Moreover, the presence of a ductile crack at the initiation site of delamination locally modified the stress state and also facilitated delamination occurrence. A criterion has been developed to numerically predict the onset of delamination in tensile and Charpy specimens.Application of this approach to heat-treated and to prestrained specimens eventually showed that it was possible to modify the sensitivity to delamination by strongly modifying the initial microtexture anisotropy
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24

Day, Erik. "Characterization of Structures and Deformation in the Brittle-Ductile Transition, Western Termination of the Chugach Metamorphic Complex, Southern Alaska." ScholarWorks@UNO, 2007. http://scholarworks.uno.edu/td/529.

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Field mapping along the western termination of the Chugach Metamorphic Complex (CMC) revealed a new D2 structure, the Bremner foliation fan. This structure has a high strain, vertically foliated core, with moderate-shallow dipping cleavages to the north and south which rollover into the core. The fan appears to have propagated from below by a shear-zone at depth (O'Driscoll, 2006). Description of the Bremner foliation fan revealed that the Bremner shear zone of Pavlis and Sisson (2003) is a D3 structure, and that the Stuart Creek fault is a younger brittle fault with 90km of dextral offset. Electron Back Scatter Diffraction (EBSD) and microstructural observations revealed deformation mechanisms were primarily compositionally controlled, whereas P-T-t conditions played a secondary role. Other factors, deemed to be of lesser influence included increased pressure solution caused by high fluid mobility, a lack of pore space, high levels of free aluminum, and a load-bearing framework of phyllosilicates.
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25

Palazzin, Giulia. "Transition-fragile ductile en zone de subduction : le rôle du quartz." Thesis, Orléans, 2016. http://www.theses.fr/2016ORLE2001/document.

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La transition d’un comportement séismique/instable à un comportement aséismique/stable est observée dans la partie en aval des zones sismogéniques (12-15 km de profondeur). Cette transition est supposée être contrôlée par l’activation de la plasticité de basse température du quartz à ~350°C. À cause de la grande profondeur à laquelle cette transition a lieu, le seul moyen pour étudier les processus physiques qui agissent en ces contestés, est l’étude des anciens prismes d’accrétion exhumés actuellement dans des chaines de montagnes. Le mélange tectonique de Hyuga et l’unité de Morotsuka appartiennent au prisme fossile de Shimanto et sont des unités metasédimentaires déformées à des températures peu inférieures ou égales à la limite fragile/ductile (~250 et ~340°C respectivement). Les résultats des observations de microstructures en microscopie optique et en microscopie électronique à balayage (diffraction des électrons rétrodiffusés) confirment que 1) la pression dissolution et une intense microfracturation sont les mécanismes de déformation principaux du quartz dans le mélange de Hyuga et localement l’activation de la plasticité du quartz est aussi observée; 2) dans l’unité de Morotsuka la recristallisation dynamique du quartz est pleinement active. Ces considérations indiquent que la température n’est pas le seul paramètre qui control l’activation de la plasticité du quartz, et laisse supposer la participation de l’effet adoucissant de l’eau. Avec le but de mieux comprendre le rôle de l’eau sur la rhéologie quartz, des expériences en Presse Griggs ont été menées, le matériel du départ étant de porphyroclasts de quartz (immergés dans une matrice sec) à la fois très riches en eau (provenant du mélange tectonique de Hyuga) et secs (quartz du Brésil). Ces expériences montrent l’effet très adoucissant de l’eau, qui à parité de conditions de déformation, favorise la migration de joint des grains dans le quartz de Hyuga tandis que le quartz du Brésil reste indéformé à exceptions de ses bordures extérieures. L’eau « en excès » est expulsée dans la matrice pour le quartz de Hyuga et stockée dans des bandes de cisaillement C’; l’eau incorporée par le quartz de Brésil n’est pas suffisantes pour favoriser la recristallisation dynamique
The trasition from instable seismic to stable aseismic behaviour is observed in at the lower limit of the seismogenic zones in subduction zones (12-15 km). This transition is supposed to be controlled by the onset of quartz low grade plasticity at about 350°C. Due to inaccessibility of these geodynamic contests, the only way to study the physical processes acting at these depth are exhumed accretionary prisms exposed in mountain chains. The Hyuga tectonic mélange and the Foliated Morotsuka are metasedimentary units constituting the Shimanto accretionary prism (Japan). They were deformed at temperatures of ~250°C and ~340°C respectively, so slightly lower or equal to the temperature transition. Results by optical microscopy and EBSD reveal that 1) quartz deformation mechanisms active in Hyuga Tectonic Mélange are pressure solution and microfracturation accompanied by local quartz low grade plasticity; 2) dynamic recrystallization is totally active in quartz of the Foliated Morotsuka. These considerations allow to consider the role of water in triggering quartz plasticity especially in such water-rich contest as subduction zones. With the aim to better understand the role played by water on quartz rheology, we deformed high hydrated (from Hyuga unit) and dry (classic Brazil) quartz porphyroclasts within a quartz matrix, with the Griggs apparatus. These experiments show the weakening water effect on quartz strength. At the same deformation conditions, the high hydrated Hyuga quartz show recrystallization by grain boundary migration while the dry Brazil porphyroclasts are mostly undeformed, at exception of the outer recrystallized rims. The exceeding water expulsed from Hyuga quartz is stored in C’ shear bands in the matrix; water absorbed by dry Brazil porphyroclasts is not enough to promote dynamic recrystallization
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26

Hütter, Geralf [Verfasser], Meinhard [Akademischer Betreuer] Kuna, Meinhard [Gutachter] Kuna, and Viggo [Gutachter] Tvergaard. "Multi-scale simulation of crack propagation in the ductile-brittle transition region / Geralf Hütter ; Gutachter: Meinhard Kuna, Viggo Tvergaard ; Betreuer: Meinhard Kuna." Freiberg : Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2013. http://d-nb.info/1220698784/34.

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27

Bernaudin, Maxime. "Rhéologie des failles lithosphériques : vers une compréhension géologique et mécanique de la zone de transition sismique-asismique." Thesis, Montpellier, 2017. http://www.theses.fr/2017MONTT145/document.

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Ces vingt dernières années, le développement de réseaux haute résolution sismologiques et géodésiques denses a permis la découverte de nouveaux signaux géophysiques parmi lesquels on trouve les trémors non-volcaniques (Non-volcanic tremor, NVT, Obara 2002) et les glissements lents épisodiques (Slow Slip Event, SSE, Dragert et al., 2001). La combinaison de NVT et de SSE est communément observée le long des frontières de plaques, entre la zone sismogénique bloquée à faible profondeur et la zone en fluage ductile à plus grande profondeur (Dragert et al., 2004). Cette association définie des glissements et trémors épisodiques (Episodic Tremor and Slip, ETS), systématiquement associés à des surpressions de fluides et à des conditions proches de la rupture. Dans cette thèse, nous proposons de combiner une étude microstructurale de roches exhumées avec une approche par modélisation numérique afin de reproduire et de mieux comprendre la mécanique des glissements et trémors épisodiques.Nous nous sommes concentrés sur des roches continentales provenant de la Zone de Cisaillement Est du Tende (Corse, France), correspondant à une zone de cisaillement Alpine kilométrique ayant enregistré une déformation dans la zone de subduction (10 kb / 400-450°C, Gueydan et al., 2003). Ces conditions pression-température sont cohérentes avec la localisation des ETS dans les zones de subduction. Les analyses microstructurales et EBSD de ces roches mettent en évidence des localisations de la déformation le long de zones de cisaillement centimétriques contrôlées par une rhéologie dépendante de la taille des grains. La microfracturation de la phase dure (ici du feldspath) et le colmatage de ces microfractures correspondent, respectivement, à de processus de réduction et d’augmentation de la taille des grains.La plupart des récentes modélisations des ETS sont basées sur une loi frictionnelle dite rate-and-state, associant les SSE et les NVT à un cisaillement sur un plan. Contrairement à ces modèles, nous souhaitons modéliser l’ensemble de la roche (et non pas uniquement un plan) avec une rhéologie ductile dépendante de la taille des grains directement guidée par nos observations microstructurales (avec microfracturation et colmatage), Nous faisons l’hypothèse que les SSE peuvent résulter d’une localisation ductile de la déformation et non d’un glissement sur des fractures. Durant la localisation de la déformation, le pompage des fluides peut déclencher une fracturation de la roche par surpression de fluide, ce qui pourrait être la signature des NVT. Le modèle numérique 1D présenté ici nous permettra de valider ces hypothèses. En suivant la loi de Darcy, notre approche nous permet également de prédire les variations de la pression de pore en fonction des variations de la porosité/perméabilité et du pompage des fluides.Les résultats numériques montrent que l’évolution dynamique des microstructures, dépendante des fluides, définie des cycles de localisation ductile de la déformation liés aux augmentations de la pression de fluide. Notre modèle démontre que la disponibilité des fluides et l’efficacité du pompage des fluides contrôlent l’occurrence des ETS. Nous prédisons également les conditions pression-température nécessaires au déclenchement des ETS : 400-500°C et 30-50 km de profondeur en subduction, et ~500°C et 15-30 km de profondeur le long des zones de décrochement. Ces conditions PT sont cohérentes avec les exemples naturels.Aussi simple soit-elle, notre modèle mécanique s’appuyant sur des observations de terrain décrit correctement la relation entre surpressions de fluides, rhéologie dépendant de la taille des grains et le déclenchement des ETS. Des travaux restent à entreprendre comme par exemple la comparaison directe de nos résultats avec des données géophysiques (GPS) ou bien l’introduction d’un nouvelle assemble minéralogique, comme par exemple des roches mafiques pour prendre en compte des minéralogies océaniques
These last twenty years, the development of dense and highly sensitive seismologic and geodetic networks permits the discovery of new geophysical signals named non-volcanic tremor (Obara 2002) and slow slip events (Dragert et al., 2001). The combination of non-volcanic tremor and transient slow slip is commonly observed at plate interface, between locked/seismogenic zone at low depths and stable/ductile creep zone at larger depths (Dragert et al., 2004). This association defines episodic tremor and slip, systematically highlighted by over-pressurized fluids and near failure shear stress conditions. In this thesis we propose to combine a microstructural analysis of exhumed rocks with a modeling approach in order to accurately reproduce and understand the physics of episodic tremor and slip.We focus on continental rocks from the East Tenda Shear Zone (Corsica, France), a kilometer-wide localized Alpine shear zone that record HP/LT deformation (10kb / 400-450°C, Gueydan et al., 2003). Such pressure-temperature conditions are consistent with the location of episodic tremor and slip in subduction zone. Microstructural and EBSD analyses on these rocks describe a pattern of strain localization in centimeter-scale shear zones guiding by a grain size-sensitive creep. Microfracturing of the strong phase (feldspar here) and the sealing of these microfractures act, respectively, as grain size decrease and grain size increase processes.Most of recent modeling approaches of episodic tremor and slip are based on the rate-and-state variable friction law, describing slow slip event and non-volcanic tremor as slow shear slip on a plane. In contrast with such models, we wish to model the entire rock volume, with a ductile grain size-sensitive rheology guided by our microstructural observations (e.g. microfracturing and sealing as grain size variation processes). We hypothesize that slow slip events may result from ductile strain localization and not transient slip on fractures. Fluid pumping during strain localization may trigger whole rock fracturing at near lithostatic conditions that can be the signature of non-volcanic tremor. The 1D numerical model presented here will allow us to validate these assumptions.We also can predict pore fluid pressure variation as a function of changes in porosity/permeability and strain rate-dependent fluid pumping following the Darcy’s flow law. The fluid-enhanced dynamic evolution of microstructure defines cycles of ductile strain localization related to the increase in pore fluid pressure. We show that slow slip events can be ductile processes related to transient strain localization, while non-volcanic tremor can correspond to fracturing of the whole rock at peak of pore fluid pressure. Our model shows that the availability of fluids and the efficiency of fluid pumping control the occurrence of episodic tremor and slip. We also well predict the temperature and depth ranges of episodic tremor and slip: 400-500°C and 30-50 km in subduction zones and ~500°C and 15-30 km in strike slip settings, consistent with natural examples.As simplistic as it is, our field-guided mechanical model well describe, at first order, the relation between high pore fluid pressure, grain size-sensitive rheology and episodic tremor and slip. Some efforts remain to be done like a real fit of geophysical data (GPS) or the introduction of the new mineralogical assemblage, such as mafic rocks to reproduce oceanic environment
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28

Wenman, Mark Roger. "Micromechanisms of fracture in the ductile-to-brittle transition region of a MnMoNi steel weld metal and a burn resistant beta titanium alloy." Thesis, University of Birmingham, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.404130.

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29

Abed, El Nabi Firas. "Effets de taille sur la transition fragile-ductile dans les nanopiliers de silicium : étude par simulation numérique." Thesis, Poitiers, 2016. http://www.theses.fr/2016POIT2253/document.

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Pour des intérêts technologiques, la compréhension des mécanismes de déformation des nano-structures est essentielle afin d'éviter que la relaxation des contraintes ne génère des défauts aux conséquences parfois catastrophiques. De plus, dans les nano-objets semi-conducteurs, les expériences montrent une transition fragile-ductile qui dépend de la taille des systèmes : ils sont ductiles pour des dimensions inférieures à quelques centaines de nanomètres, fragiles au-delà. Nous avons abordé ce problème via des calculs de dynamique moléculaire pour simuler des tests de déformation de nano-fils, et nous avons choisi le silicium comme prototype de matériau semi-conducteur. Nous avons dans un premier temps analysé des grandeurs mesurables comme les coefficients d'élasticité et la limite d'élasticité en fonction de différents paramètres, et montré notamment que la limite d'élasticité diminue quand la hauteur du nano-fil augmente. L'analyse à l'échelle atomique des systèmes déformés nous a permis de décomposer le comportement global des nano-fils en mécanismes élémentaires ; nous avons ainsi montré que la nucléation d'une première dislocation est à l'origine de l'ensemble des comportements, ductiles et fragiles. Après cette nucléation initiale, le comportement global du nano-fil est déterminé par la compétition entre la nucléation d'autres dislocations et l'ouverture de cavités. Finalement, nous avons essayé d'estimer quantitativement les degrés de ductilité et de fragilité des nano-fils en analysant l'énergie relaxée pendant le régime plastique par ces deux mécanismes élémentaires, et de rationaliser ainsi le rôle de la taille du système sur la transition fragile-ductile
For technological interest, the understanding of the deformation mechanisms at the nano-scale is essential in order to prevent stress relaxation mechanisms that could lead to defects formation and/or to catastrophic failure. Furthermore, recent experimental findings showed in semiconductor nano-objects, a size dependent brittle to ductile transition: they are ductile below a few hundreds of nanometers, brittle above that scale. To investigate this behavior, we have used molecular dynamics as a tool to simulate deformation tests of nanowires and we have used silicon as a prototypical semiconductor material. First we analyzed a number of measurable quantities such as the elasticity coefficients and the elasticity limit with respect to various parameters and we found that the elasticity limit decreases when the length of the nanowire increases. An analysis of the atomic structure of the deformed systems allowed us to decompose the overall mechanical behavior of the nanowires into elementary mechanisms; we thus showed that the nucleation of a first dislocation was systematically at the origin of ductility and brittleness. After the initial dislocation nucleation, the competition between further dislocation nucleation events and cavities opening, determine the overall mechanical behavior of the nanowire. Finally, we tried to estimate quantitatively the degree of ductility and brittleness of the nanowires by analyzing the amount of energy released by those two elementary mechanisms during the plastic regime and we rationalized the role of the size of the deformed systems on the brittle to ductile transition
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30

LIU, JIANNING. "Understand the mechanical behaviors of polymer glasses under extension and compression." University of Akron / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=akron1542042896236543.

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31

Merabet, Amina. "Etude par microscopie électronique du silicium aux petites échelles : comportement mécanique et structure atomique des défauts." Thesis, Aix-Marseille, 2018. http://www.theses.fr/2018AIXM0554/document.

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De récents travaux consacrés à l’étude des propriétés des matériaux aux petites échelles ont souligné des différences exceptionnelles dans le comportement mécanique des nano-objets par rapport aux matériaux massifs. Dans le cas du silicium, une transition fragile-ductile à température ambiante a été observée lorsque la taille des échantillons est réduite. Cependant, les défauts et les mécanismes à l’origine de ce changement de comportement n’ont pas été clairement identifiés. Ce travail repose sur l’étude post mortem de nanopiliers déformés, en utilisant différentes techniques de microscopie électronique. Les nanopiliers étudiés ont été préparés par gravure plasma et déformés en compression à température ambiante. Les résultats obtenus durant cette thèse, confirment la différence de comportement des nano-objets par rapport au matériau massif. Par ailleurs, une grande variété de défauts produits lors de la compression a été observée. L’orientation cristallographique de l’axe de sollicitation semble avoir un impact important sur les mécanismes à l’origine du comportement ductile observé. La comparaison entre images HRTEM expérimentales et simulées témoigne de la propagation simultanée de dislocations partielles et parfaites dans les plans {111}. De plus, des événements plastiques ont également été observés dans des plans {115}. Divers mécanismes de déformation possibles impliqués lors de la compression des piliers sont décrits à partir des observations microscopiques. Un modèle tenant compte de l’influence sur la mobilité des dislocations des interactions entre systèmes de glissement est proposé afin d’expliquer la transition fragile-ductile observé aux petites échelles
Several recent works devoted to the study of the properties of materials at small scales have revealed exceptional differences in the mechanical behavior of nano-objects as compared to bulk material. In the case of silicon, a brittle-ductile transition at room temperature has been observed when the sample size decreases. However, the extended defects and mechanisms behind this behavioral change have not been clearly identified. This work is based on the post mortem study of deformed nanopillars, using different electron microscopy techniques. The studied nano-pillars of 100 nm in diameter were prepared by plasma etching (RIE) and deformed in compression at room temperature. The results obtained during this thesis confirm the difference in the behavior of nano-objects compared to bulk material. Moreover, a large variety of defects produced during plastic deformation has been observed. The crystallographic orientation of the deformation axis seems to have a significant impact on the mechanisms behind the observed ductile behavior. The comparison between experimental and simulated HRTEM images notably evidences the simultaneous propagation of partial and perfect dislocations in {111} planes. In addition, unexpected plastic events have also been observed in {115} planes. Various possible deformation mechanisms involved during the nano-compression of the pillars are described, based on the microscopic observations. Finally, a model considering the influence of interactions between various activated systems on the mobility of dislocations is proposed to explain the brittle-ductile transition observed at small scales in silicon
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32

de, Carvalho Antónia. "Controls on the diversity of the fault slip styles at the brittle-ductile transition: examples from the Cape Fold Belt, Nuy Valley, South Africa." Master's thesis, Faculty of Science, 2019. http://hdl.handle.net/11427/31360.

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Crustal deformation models have a first-order rheological division, with pressure-dependent brittle deformation predominating at shallow depths, and temperature-dependent viscous deformation occurring in the deeper levels of the crust. The brittle-ductile transition zone separates these two regimes, it occurs at approximately 350°C for quartz and it is characterised by mixed-mode brittle and viscous deformation. Complex fault zones exhumed to the surface may preserve evidence that can explain the mechanics and the complex slip behaviour of faults. Fault rocks response to applied shear stress is affected by environmental conditions during deformation (such as temperature and pressure), composition of fault zone, fluid presence and strain rate. Thus, the interplay of these factors determines the slip style of a specific fault and may lead to multiple slip styles that overprint each other. The Nuy Valley area in Worcester, Western Cape, South Africa, exposes a section through the deeper parts of the Cape Fold Belt, where the Malmesbury Group schists experienced thrust faulting in response to crustal shortening. Individual thrust faults are manifested in different ways, with quartz-cemented breccias, limestone mylonites, abundant quartz veining and cataclasites attesting to faulting occurring by a diversity of slip style, which allows investigating how the interplay of the controlling factors lead to the observed diversity of fault rock. Through mineral equilibria modelling, the pressure-temperature conditions under which faulting occurred was determined to lie between 5 - 8 KPa and 250 - 420C, with fluid content lines indicating low amounts of dehydration during peak metamorphism. The exhumed fault being analysed in this study was active at 10 - 15 km deep at 25C.km-1 geothermal gradient. The temperature over this transition is relatively constant and short ranged throughout geological evolution of Worcester and the cyclic superposition of ductile and brittle deformation and change in slip styles along fault zones as found in Nuy Valley cannot be justified by ambient temperature and pressure oscillations. Lithotype and competency of wallrocks play an essential role in deformation partitioning by being crucial determinants of rheological properties, and accounts for the coexistence of brittle and ductile fabrics but not for cyclic overprint of slip styles. Fluid presence is evidenced by an intense network of quartz veins and hydraulic breccias and contributes to the weakening and strengthening of wallrock during deformation. Slip style diversity in the study area is considered to the result of the interplay of compositional variabilities, fluid flow and strain rate variations associated with the seismic cycle.
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Tioguem, Teagho Frank. "Lien entre microstructure et transition ductile-fragile des aciers trempés-revenus à haute résistance." Thesis, Paris Sciences et Lettres (ComUE), 2019. http://www.theses.fr/2019PSLEM040.

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Cette thèse s’inscrit dans un projet dans lequel Ascometal souhaite améliorer le développement de nuances d’aciers à haute résistance destinées à la fabrication des pièces de forage offshore. Le projet vise notamment la prédiction, au moyen de la microstructure, de la courbe de transition ductile - fragile en résilience. Les objectifs de la thèse sont doubles à savoir : améliorer la compréhension du lien entre microstructure et propriétés à rupture, puis mettre au point un modèle prédictif de la résilience basée sur la microstructure. Nous avons adopté une démarche scientifique en quatre étapes: 1) la construction d’une base de données constituée de six microstructures. Les microstructures ont été caractérisées quantitativement à différentes échelles. 2) La caractérisation du comportement élastoplastique en variant la température d’essai, la vitesse de sollicitation, la direction de prélèvement des échantillons et le revenu. 3) La caractérisation du comportement à rupture au moyen des essais Charpy instrumentés dans une gamme de température allant de -196°C à +60°C. 4) L’établissement des liens quantitatifs entre chacune des microstructures et leurs propriétés à rupture. Dans certains cas, le recours aux outils numériques a été adopté pour accéder aux contraintes locales et les relier aux entités microstructurales. Les résultats ont permis d’aborder les discussions autour des effets : de la vitesse de sollicitation, du revenu, de la température d’essai et la direction de sollicitation sur le comportement élastoplastique des aciers de microstructure martensite et bainite supérieure. Pour les essais à rupture, l’analyse fractographique a permis d’accéder aux mécanismes de rupture dans les domaines de rupture ductile, de transition et de rupture fragile. Les observations de l’endommagement en post-mortem sur coupe polie ont permis de discuter l’effet du revenu sur l’endommagement ductile des aciers trempés-revenus à haute résistance. Au niveau du plateau ductile, un lien quantitatif est proposé entre la distance intercarbures et l’énergie totale à rupture, il permet d’expliquer l’augmentation de l’énergie à rupture. Au niveau du plateau fragile, l’approche locale de la rupture sur des éprouvettes axisymétriques entaillées est mise à contribution pour déterminer les contraintes critiques de clivage à -196°C. Un lien quantitatif a été établi entre la contrainte critique de clivage à -196°C et la taille des carbures. L’effet de la bainite supérieure sur la contrainte critique de clivage est également abordé. Enfin, un modèle global de la résilience basé sur la microstructure est proposé. Il s’appuie sur la distance intercarbures et la température de transition en cristallinité pour prédire la courbe de transition ductile-fragile des aciers à haute résistance de l’étude. Il semble enfin indispensable d’étendre l’approche proposée sur d’autres nuances de même grade afin de disposer d’une base de données plus large, ce qui devrait rendre le modèle global robuste
This thesis is a part of Ascometal project, which aims at guiding the development of new high strength steels used foroffshore drilling parts in oil and gas domain. The aim of the project is to predict the ductile-to-brittle transition curve in Charpyimpact tests by considering microstructural parameters. The main two objectives of this study are: improving the knowledge of the link between the microstructure and fracture properties and building a microstructurally-based predictive tool for the impacttoughness. The chosen scientific approach includes four steps described as follows: 1) building of a database of six microstructures. These microstructures were quantitatively characterized at different scales. 2) The elastic-plastic behavior was studied by varying the test temperature, the strain rate, the sampling direction and the tempering temperature. 3) The fracture behavior was studied by using an instrumented Charpy impact test in a test temperature range of -196°C to +60°C. 4) Establishment of the links between microstructure and fracture properties. Mechanical analysis by the finite element method was used in some cases in order to determine the local stress. In this work we discuss the effect of strain rate, the test temperature, the sampling direction and the tempering temperature on the elastic-plastic behavior of steels containing martensite and upper bainite. Fractography was used to characterize the fracture mechanism in the ductile region, in the transition and in the brittle domains. The effect of tempering of ductile fracture damage is also discussed. The local approach to fracture is adopted for the purpose of determining the critical cleavage stress at -196°C. A quantitative link is established between the critical fracture stress and the carbide size. The effect of the upper bainite on the cleavage fracture stress is also discussed. Finally, a microstructurally-based global model for impact toughness is proposed in this thesis. The model takes the intercarbide spacing and the fracture appearance transition temperature into account to describe the instrumented Charpy impact curve. Complementary studies have to be made in order to deeply understand the dependence of the fracture appearance temperature on the austenite grain size, the packet size and the upper bainite fraction; they will make the global model for impact toughness more robust
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34

Tran, Thi Phuong Huyen. "Étude expérimentale de la localisation de la déformation par corrélation d’images sur un analogue de roche soumis à différentes conditions de chargement triaxiaux." Thesis, Université Côte d'Azur (ComUE), 2016. http://www.theses.fr/2016AZUR4083/document.

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Ce travail présente une caractérisation expérimentale de la localisation de la déformation et la rupture sur un matériau synthétique analogue de la roche dans différentes conditions de chargement axisymétrique. L’évolution de la localisation de la déformation a été caractérisée par la technique de corrélation d’images bidimensionnelle. Deux gammes d’essais en compression et en extension ont été réalisées à différentes pressions de confinement Pc. Profitant de la technique DIC et de la caractérisation détaillée des propriétés constitutives de GRAM1 par Mas et Chemenda (2014; 2015), nous montrons que l'initiation de la localisation des bandes de déformation est précédée par la déformation élastique uniforme puis la déformation élasto-plastique. La localisation de la déformation est initiée dans le régime de dilatance (la dilatance β est positive) et d’adoucissement (le module d’écrouissage h est négatif). Au cours de l'évolution du chargement, la déformation à l'intérieur de la bande de localisation de la déformation devient compactive (β <0) et est accompagnée par un durcissement du matériau (h> 0). Ceci provoque dans un premier temps l’élargissement de la bande puis l’apparition de nouvelles bandes qui se forment successivement. La formation de ces nouvelles bandes provoque un ralentissement de la déformation dans les bandes préexistantes, ce qui densifie progressivement le réseau. Dans les essais en extension axisymétrique, les fractures se forment perpendiculaires à la contrainte principale mineure σ3 sous un état de contrainte extensif à faible Pc puis compressif quand Pc augmente. Nos résultats présentent une transition continue de la fracture extensive à la fracture cisaillante avec une augmentation de la contrainte compressive. Ceci est en accord avec les résultats obtenus pour les essais en extension réalisées sur des roches naturelles (Ramsey et Chester, 2004 ; Bobich 2005)
This work presents an experimental characterization of the strain localization and the rupture on a granular rock analogue material under different conditions of axisymmetric loading. The evolution of the strain localization was characterized by the two-dimensional image correlation technique. Two series of triaxial compression and extension tests were carried out at different confining pressure Pc. Taking advantage of the DIC technique and detailed characterization of GRAM1’s constitutive properties by Mas and Chemenda (2014, 2015), we show that the initiation of deformation localization bands is preceded by the uniform elastic and then elasto-plastic deformation. The strain localization is initiated in the dilatant regime (positive dilatancy factor β) and strain-softening (negative hardening modulus h). During the band evolution, the deformation within it becomes compactive (β<0) and is accompanied by the material hardening (h>0), which causes the band to widen and new bands to form successively. The formation of new bands causes a slowing down the deformation within the prior bands, which makes the progressively densified band network to continuously evolve. In axisymmetric extension tests, the fractures are formed perpendicular to the minor principal stress σ3 in an extensive stress state at low Pc then compressive when Pc increases. Our results show a continuous transition from the extensive fracture to shear fracture with an increase of compressive stress. This is suitable of the results obtained for extension tests performed on natural rocks (Ramsey and Chester, 2004; Bobich, 2005)
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35

Hémery, Samuel. "Influence du sodium liquide sur le comportement mécanique de l'acier T91." Phd thesis, Ecole Centrale Paris, 2013. http://tel.archives-ouvertes.fr/tel-00978485.

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Nous avons étudié la sensibilité du T91 à la fragilisation par le sodium liquide. Une procédure expérimentale a été mise en place afin de procéder à des essais mécaniques en sodium sous atmosphère inerte. Grâce à l'introduction d'une étape préliminaire d'exposition au sodium, la mouillabilité du T91 par le sodium liquide et la structure de l'interface sodium/acier ont pu être étudiés en fonction des différents paramètres d'exposition. Une réduction significative des propriétés mécaniques est observée quand le mouillage de l'acier par le sodium est bon. L'utilisation de différentes teneurs en oxygène et en hydrogène suggère que l'oxygène joue un rôle primordial dans l'amélioration de la mouillabilité du T91. La sensibilité du phénomène de fragilisation à la vitesse de sollicitation et à la température a été caractérisée. A partir de ces résultats, l'existence d'une transition fragile/ductile, fonction de ces deux paramètres a été mis en évidence. L'étude de cette transition suggère qu'une étape de diffusion du sodium dans les joints de grains du T91 est limitante pour la fragilisation. Des analyses en microscopie électronique en transmission et par cartographies d'orientation de fissures arrêtées ont également permis de constater que le mode de rupture est interlatte ou intergranulaire, fournissant ainsi une image cohérente du processus de fragilisation. La même méthodologie a été appliquée à l'acier non allié XC10. Les résultats montrent un comportement en tous points similaire à celui de l'acier T91 et suggèrent donc un mécanisme commun pour les aciers cubiques centrés. De plus, ils confirment que la transition fragile ductile observée semble la conséquence d'une vitesse de propagation de fissure fragile relativement limitée. Cette propagation est activée thermiquement avec une énergie d'activation d'environ 50 kJ/mol. Enfin, il a été montré que l'acier austénitique 304L est également sensible à la fragilisation par le sodium liquide. Certaines surfaces de rupture témoignent clairement d'une rupture intergranulaire, cependant des interrogations persistent à propos du chemin de fissuration.
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36

Hamouche, Zehoua. "Étude de la fragilisation des aciers T91 et 316L par l'eutectique plomb-bismuth liquide." Phd thesis, Université Paris-Est, 2008. http://tel.archives-ouvertes.fr/tel-00461665.

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L'objectif de cette étude est d'aboutir à une meilleure compréhension de la fragilisation par les métaux liquides (FML) à travers l'étude des systèmes T91/Pb-Bi et 316L/Pb-Bi et notamment d'en établir les mécanismes mis en jeu lors du contact entre ces aciers sous tension et le métal liquide. Ce travail s'inscrit dans le cadre du projet MEGAPIE-TEST mis en place pour étudier la faisabilité d'une cible de spallation au plomb-bismuth liquide. L'effet de l'eutectique plomb-bismuth liquide sur le T91 et le 316L a été étudié en fonction de la température et de la vitesse de déformation, en utilisant des éprouvettes CCT adaptées à l'étude de propagation de fissures. La présence de Pb-Bi modifie le mécanisme de rupture du T91 au détriment de la germination, croissance et coalescence des cavités. La rupture procède alors par décohésion des bandes de cisaillement. L'effet fragilisant du Pb-Bi est très marqué aux très faibles vitesses de déformation. Une transition fragile-ductile se produit aux grandes vitesses de déformation (~10-5 m.s-1 à 160°C). Les propriétés mécaniques du 316L ne sont pas autant affectées par la présence de Pb-Bi, toutefois une transition réelle est observée sur les faciès de rupture, où là également il y a compétition entre l'effet fragilisant du métal liquide et la rupture ductile. Le mécanisme suggéré dans ce travail est fondé sur la localisation de la déformation en pointe de la fissure combinée au phénomène de réduction d'énergie de surface induite par adsorption de métal liquide (effet Rebinder) et ne fait intervenir aucun processus diffusionnel en particulier aux joints de grains.
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37

Dragan, Rajnović. "Uticaj mikrostrukture na prelaznu temperaturu ADI materijala." Phd thesis, Univerzitet u Novom Sadu, Fakultet tehničkih nauka u Novom Sadu, 2015. http://www.cris.uns.ac.rs/record.jsf?recordId=94932&source=NDLTD&language=en.

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U disertaciji je izvršena karakterizacija mikrostrukture i mehaničkih osobinanelegiranog austemperovanogi nodularnog liva (ADI materijala), kao i uticajamikrostrukture na prelaznu temperaturu u intervalu od -196 do +100°C.Utvrđeno je da mehaničke osobine ADI-ja zavise od morfologije ausferitnemikrostrukture i količine zadržanog austenita, tj. parametara austemperovanja.Na osnovu mehaničkih osobina utvrđen je i opseg procesiranja u skladu sastandardima ASTM, ISO i EN. Zaključeno je da prelazna temperatura ADImaterijala zavisi od količine i stabilnosti zadržanog austenita. U višemtemperaturnom opsegu (iznad cca. -25°C) dominantna je količina zadržanogaustenita, dok na nižim temperaturama, stabilnost. Visoka obogaćenostugljenikom, stabilnog zadržanog austenita sprečava stvaranje martenzita naniskim temperaturama, a time i pojavu krtosti kod ADI-ja.
The object of this thesis was to characterize microstructure and mechanical propertiesof the unalloyed ADI material (Austempered Ductile Iron). In addition, the influence ofmicrostructure on the ductile to brittle transition temperature (DBTT) by Charpy impacttest in temperature interval from 196 to +100°C has been studied. The all propertiesobtained depend on the morphology of microstructure and the amount of retainedaustenite, i.e. on the austempering parameters. According to the mechanical propertiesand standard requirements (ASTM, ISO and EN) the processing window has beenproposed, also. It was found that DBTT is influenced by amount and stability of retainedaustenite. In upper temperature range (above cca. 25°C) the most influence factor onDBTT is amount of retained austenite, while at the lower temperatures the stability ismore prominent. Stability of high carbon retained austenite at lower temperaturesprevents transformation to martensite and thus the embrittlement of ADI.
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38

Jacquet, Lucie. "Rupture par clivage des aciers inoxydables ferritiques 18Cr-2Mo : influence de la précipitation et de la taille des grains." Thesis, Université Grenoble Alpes, 2021. http://www.theses.fr/2021GRALI019.

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Parmi les nombreuses nuances d’aciers inoxydables, les nuances ferritiques sont utilisées dans de nombreux secteurs grâce à leur résistance à la corrosion et à des propriétés spécifiques comme leur conductibilité thermique ou leurs propriétés ferromagnétiques. Leur teneur résiduelle en nickel en font des aciers économiques en comparaison des aciers austénitiques, plus massivement produits. Toutefois, l’utilisation de cette famille d’aciers en dessous de la température ambiante est limitée par sa fragilité mécanique, la moindre mobilité des dislocations dans le réseau cubique centré rendant l’écoulement plastique plus difficile aux basses températures. La précipitation et la taille des grains sont deux des paramètres microstructuraux qui influent sur la température de transition ductile/fragile des aciers ferritiques. L’objectif de cette thèse est d’étudier et de comprendre l’effet de ces paramètres sur cette température de transition et sur la rupture par clivage des aciers à matrice 100% ferritique contenant 18% de chrome et 2% de molybdène.Pour ce faire, trois coulées ont été élaborées avec la même teneur en carbone et en azote à laquelle s’ajoute, pour deux d’entre elles, du titane ou du niobium. La gamme de traitements thermomécaniques a été choisie pour fabriquer six microstructures distinctes pour lesquelles varient la taille des grains, la nature et la localisation de la précipitation et la teneur en carbone et en azote en solution solide. Ces microstructures sont caractérisées par microscopies optique et électroniques et par diffusion centrale des neutrons aux petits angles. La mise en place d’un essai de traction avec des éprouvettes axisymétriques entaillées permet de déterminer la contrainte de rupture par clivage de chaque microstructure à 20°C et à -40°C, puis de discuter des mécanismes qui régissent la rupture fragile.Les microstructures élaborées sont finalement classées en deux catégories. Pour les microstructures Ti, Nb et NbG, pour lesquelles l’ajout de titane ou de niobium permet d’éviter la précipitation de carbures et nitrures de chrome, la contrainte de rupture par clivage est dépendante de la taille et de la morphologie des particules intragranulaires. Les Ti(N,C), de 4 à 5 µm au maximum, dont la forme s’apparente à celle d’un cube, rendent la microstructure Ti plus fragile que la microstructure Nb caractérisée par des Nb(C,N), certes plus nombreux mais de plus petites dimensions. La taille des grains est un facteur de deuxième ordre sur la contrainte de rupture par clivage de ces microstructures comme cela est mis en évidence avec l’étude de la rupture fragile de la microstructure NbG, pour laquelle la taille des grains a été multipliée par 10 par rapport à celle de la microstructure Nb.Dans les microstructures CrP, Cr et Cr0, la présence ou non de carbone et d’azote en solution solide sursaturée et les précipitations intra et intergranulaires sont les conséquences des traitements thermiques choisis. Les précipités dans les joints de grains, même s’ils sont de petites dimensions (quelques dizaines à quelques centaines de nm), modifient significativement l’amorçage de la rupture par clivage. La température de transition ductile/fragile est, au premier ordre, dépendante de la limite d’élasticité des microstructures. La contrainte de rupture par clivage n’est plus dépendante de la taille des particules intragranulaires comme c’est le cas pour les microstructures Ti, Nb et NbG mais de la précipitation aux joints de grains et de la dépendance de la contrainte de friction de réseau à la solution solide (modèle de Smith).Au vu de ces résultats, des pistes pour limiter la fragilité des nuances ferritiques sont proposées
Among stainless steel grades, ferritic stainless steels are used in many applications thanks to their corrosion resistance and attractive properties such as their thermal conductivity and ferromagnetic properties. Their residual nickel content make them economical steels compared to austenitic steels, which are more heavily produced. However, the use of this family of steels is limited by its relatively low toughness at room temperature and below, coming from the lattice friction of the body centered cubic structure that hinders plastic flow. Precipitation and grain size are two microstructural parameters that influence the ductile to brittle transition temperature of ferritic steels. The objective of this thesis is to study and understand the effect of these parameters on the ductile to brittle transition temperature and on the cleavage fracture of ferritic stainless steels containing 18% chromium and 2% molybdenum.To do so, three castings were produced with the same carbon and nitrogen content. Titanium or niobium were added to two of them. Thermomechanical treatments were chosen to produce six different microstructures for which the grain size, the nature and the location of the precipitation and the carbon and nitrogen content in solid solution vary. These microstructures are characterized by optical and electron microscopies and small-angle neutron scattering. The implementation of a tensile test with notched round specimens allows the determination of the cleavage fracture stress of each microstructure at 20 ° C and -40 ° C and discuss the brittle fracture mechanisms.The microstructures developed are finally divided into two groups. For Ti, Nb and NbG microstructures, for which the addition of titanium or niobium prevents the precipitation of chromium carbides and nitrides, the cleavage critical stress is dependent on the size and morphology of the intragranular particles. The 4 to 5 µm long cubic Ti (N, C) make the Ti microstructure more brittle than the Nb microstructure characterized by more numerous but smaller Nb (C, N). The grain size is a second order parameter on the cleavage fracture stress of these microstructures as shown by the study of the brittle fracture of the NbG microstructure, for which grain size is ten times larger.In CrP, Cr and Cr0 microstructures, the presence, or absence, of carbon and nitrogen in solid solution and intra and intergranular precipitations are the consequences of the chosen heat treatments. The presence of precipitates in the grain boundaries, even if they are small (few dozens to few hundred nm), significantly modifies the initiation of cleavage fracture and the value of the ductile to brittle transition temperature. The ductile to brittle transition temperature depends in the first order on the yield stress of microstructures. The cleavage fracture stress is no longer dependent on the size of the intragranular particles as in Ti, Nb and NbG microstructures but on the precipitation of chromium carbides and nitrides at the grain boundaries and on the dependence of the lattice friction stress on the solid solution (Smith's model).In the light of these results, alternatives to limit the embrittlement of ferritic grades are suggested
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39

Skrzypczak, Mathieu. "Compréhension des mécanismes physico-chimiques intervenant lors du broyage à sec du carbonate de calcium en présence d'agents de broyage." Ecully, Ecole centrale de Lyon, 2009. http://www.theses.fr/2009ECDL0021.

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Ce travail s’inscrit dans un effort de compréhension des mécanismes physico-chimiques intervenant au cours du broyage à sec du carbonate de calcium (CaC03). Le processus de réduction de taille est généralement facilité par la présence d'agents de broyage, qui permettent de réduire significativement la consommation énergétique du procédé pour atteindre des poudres de granulométries comparables. Nous avons ainsi cherché à déterminer quels étaient les différents effets des additifs sur le broyage et à quantifier leur importance relative sur Ie broyage. Dans une première approche, les forces de surface ont été considérées. Nous avons pu mettre en évidence une tendance à l'augmentation de l’efficacité de broyage lorsque la tension superficielle de l’additif utilisé diminuait. L'énergie de surface de la poudre broyée, caractérisée par la méthode de Washburn, n’a en revanche pas pu être corrélée à un quelconque impact sur le rendement du procédé. Dans un deuxième temps, nous avons considéré le comportement mécanique du CaC03. Ceci a permis de montrer l’existence d’une taille critique de transition ductile-fragile comprise entre 1 et 2 µ. Cette taille de transition correspond à la limite de broyage observée industriellement. Par ailleurs, le CaC03 est sensible aux phénomènes de fatigue, qui conduisent à terme à la rupture des particules malgré des charges appliquées très faibles. Enfin, nous avons mis en évidence le rôle significatif des additifs sur la modification de la dureté de surface. Le durcissement de la surface conduit potentiellement à la fragilisation du matériau et donc tend à favoriser le broyage fin. Les résultats de broyage obtenus lors de tests à petite échelle conduisent à penser que l’efficacité d'un additif résulte de sa capacité à limiter les forces d'agglomération par une faible tension superficielle et par un durcissement de sa surface
This work aims at understanding the physicochemical mechanisms that occur while processing the dry grinding of calcium carbonate (CaC03). The particles size reduction is helped by grinding aid addition that permits to significantly reduce power consumption. We try to define the different effects of additives on grinding and to quantify their relative importance on the final result. In a first attempt, surface forces were considered and it was shown that low surface tension additive was generally related to high grinding efficiency. On the other side, the surface energy of the powder, determined using the Washburn method, does not seem to influence the final process efficiency. In a second time, we considered the CaCO3 mechanical behavior and showed the existence of a critical size of ductile-brittle transition comprised between 1 and 2 µm. This dimension corresponds to the grinding limit observed experimentally in CaCO3 dry grinding. The material seems to be very sensitive to the fatigue process and it appears fracture can occur after a repetition of low applied stress. Finalyy, we emphasized the additive effect on the CaCO3 surface hardness. This hardening may increase the material brittleness and therefore eases grinding. Small scale grinding experiments highlight the fact that the grinding aid efficiency results both from a low liquid surface tension that reduces surface forces between particles and from a material hardening due to additive presence
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40

Otoboni, José Antonio. "Estudo da usinabilidade do ZERODUR® no torneamento de ultraprecisão com ferramenta de diamante de ponta única." Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/18/18146/tde-31102013-171149/.

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Alguns espelhos usados em câmeras de satélite devem apresentar qualidade superficial elevada. Eles são normalmente fabricados em ZERODUR®, um material vitrocerâmico, por meio de processos abrasivos. Observou-se que a qualidade da superfície do material deteriora-se algum tempo após a usinagem, necessitando assim de retrabalho. A causa mais influente desse fenômeno é o crescimento de trincas devido à corrosão sobtensão. Estas trincas são geradas pelas condições impostas nos processos convencionais de lapidação e polimento. Este trabalho apresenta um estudo sobre a usinabilidade do ZERODUR® utilizando torneamento de ultraprecisão com ferramenta de diamante de ponta única como uma alternativa a estes métodos tradicionais. Amostras de ZERODUR® foram submetidas a testes de indentação e riscamento a fim de se estudar as propriedades mecânicas do material, a transição dúctil-frágil e os mecanismos de remoção de material. Com base nesses testes, foi delineado um experimento fatorial do tipo 23 para avaliar a influência das seguintes variáveis de usinagem: profundidade de corte, nos níveis 0,4 e 0,2 \'mü\'m, taxa de avanço, nos níveis 0,3 e 0,1 \'mü\'m/rev e ângulo de saída da ferramenta, nos níveis -5º e -20º. As amostras do experimento foram usinadas em um torno de ultraprecisão com a ferramenta de diamante de ponta única. A qualidade superficial de cada combinação foi avaliada por microscopia eletrônica de varredura e perfilometria óptica interferométrica. Os resultados dos experimentos foram analisados estatisticamente, por meio de análise de variância (ANOVA). Para os intervalos das variáveis testadas verificou-se que o ângulo de inclinação da ferramenta é o que mais afeta a qualidade superficial. Ângulos com inclinações mais negativas (-20º) proporcionaram as melhores qualidades superficiais (em torno de 200 nm). Nos resultados não houve evidência de que o corte do material tenha ocorrido dentro do regime dúctil. Porém, a rugosidade superficial encontrada em algumas combinações de corte sugere que o torneamento de ultraprecisão pode ser uma alternativa ao processo lapidação que antecede o polimento. Este trabalho de caráter exploratório contribui com estimativas de alguns parâmetros ótimos para usinagem do ZERODUR®, fornecendo subsídios para pesquisas futuras sobre este tema.
Some mirrors used in satellite cameras must present a high surface quality. They are usually made of ZERODUR®, a brittle glass ceramic, by means of abrasive processes. It was observed that the surface quality of the material deteriorates some time after the machining, thus requiring rework. The most influential cause of this phenomenon is the crack growth due to stress corrosion. These cracks are generated by the conventional machining processes of lapping and polishing. This work presents a study on the machinability of ZERODUR® using ultraprecision turning with single-point diamond tool as an alternative to these traditional methods. ZERODUR® samples were subjected to indentation and nano-scratching tests in order to study the ductile-brittle transition and material removal mechanisms. After that, a \'2 POT.3\' factorial experiment was designed in order to assess the influence of three machining parameters on the resulting surface roughness, which are: depth of cut (tested at levels 0,4 and 0,2 \'mü\'m), feed rate (tested at levels 0,3 and 0,1 \'mü\'m/rev) and rake angle of the cutting tool (tested at levels -5º e -20º). The samples were turned with single-point diamond tool using different combinations of the parameters at the levels described above. The surface quality of each sample was evaluated using an interferometric optical profiler and a scanning electron microscope (SEM). The results of the experiments were statistically evaluated by means of Analysis Of Variance (ANOVA). For the ranges tested, it was found that the rake angle of the tool was the most influential parameter. The angle of -20º provided the best values for surface quality, which were around 200 nm. There was no evidence that the cut of the material occurred in the ductile regime, however, the obtained surface roughness showed that the ultraprecision turning may be a feasible alternative for the lapping process of optical components. This exploratory research contributes to the existing knowledge by providing estimates for optimal parameters of ZERODUR® machining, furnishing empirical basis for future research in this field.
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41

Stec, Mateusz. "Micromechanical modeling of cleavage fracture in polycrystalline materials." Doctoral thesis, Stockholm : Hållfasthetslära, Kungliga Tekniska högskolan, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-9773.

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42

Morgan, Robert Edward. "Ductile-brittle transitions in pipe grade polyethylene." Thesis, Imperial College London, 1994. http://hdl.handle.net/10044/1/7399.

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43

Ferreira, Dario Magno Batista. "Metodologia para simulação computacional da distribuição de temperaturas para identificar sub-regiões reaquecidas da ZAC e avaliar suas influências nas propriedades mecânicas na soldagem multipasse de aço API 5L X80." Universidade de São Paulo, 2017. http://www.teses.usp.br/teses/disponiveis/3/3133/tde-08012018-110211/.

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Em projetos de oleodutos e gasodutos utilizam-se aços de alta resistência e baixa liga (ARBL), como o aço API 5L X80. Na soldagem multipasse destas tubulações, a zona afetada pelo calor (ZAC) do passe de raiz é submetida a um novo ciclo térmico pelos passes de soldagem subsequentes. Isto resulta em alterações nos valores das propriedades físicas. Nos aços ARBL, a ZAC de grãos grosseiros reaquecida intercriticamente (IC-ZACGG) pode se tornar uma zona frágil localizada, isto é, uma zona com maior dureza. Consequentemente, falhas estruturais podem ocorrer, ocasionando paradas não desejadas no transporte de fluidos. O objetivo deste trabalho é desenvolver uma metodologia baseada no modelo de fontes de calor distribuídas de Mhyr e Gröng, para avaliar o fluxo de calor na soldagem considerando as propriedades físicas dependentes da temperatura. Estender a aplicação desta ferramenta em soldagens multipasses para identificar sub-regiões da ZAC de um passe anterior sendo afetada pela ZAC de passes subsequentes. As isotermas simuladas foram validadas através de medições realizadas em macrografias de juntas soldadas. Os ciclos térmicos simulados foram validados através das temperaturas máximas atingidas e pelos tempos de resfriamento de 800 a 500 ºC (?t8-5) dos ciclos térmicos experimentais. Ao aplicar a metodologia proposta, foi possível delimitar com acurácia as regiões reaquecidas da ZAC e analisar os efeitos dos passes subsequentes em cada uma das sub-regiões da ZAC do passe de raiz. A IC-ZACGG na região do passe de raiz foi localizada, mas não se comportou como zona frágil devido à boa soldabilidade do aço API 5L X80 comprovada pelos ensaios de dureza e de tenacidade ao impacto Charpy-V.
In pipelines projects, the high strength low alloy (HSLA) steels are used, such as the API 5L X80 steel. During the multipass welding of these pipes, the heat affected zone (HAZ) of the root pass is subjected to a new thermal cycle by the subsequent welding passes. This results in changes in the values of the physical properties. In the HSLA steels, the intercritical reheated coarse-grained heat-affected zone (IR-CGHAZ) can become a local brittle zone, that is, a region with greater hardness. Consequently, structural failures could happen, causing undesired shutdowns in fluid transportation. The objective of this work is to develop a methodology based on the distributed heat sources model of Mhyr and Gröng, to evaluate the heat flux in the welding considering the temperature-dependent physical properties. Extend the application of this tool in multipass welds to identify HAZ subregions of a previous pass being affected by the HAZ of subsequent passes. The simulated isotherms were validated through measurements made on macrographs of welded joints. The simulated thermal cycles were validated through the maximum temperatures reached and the cooling times from 800 °C to 500 ºC (?t8-5) of the experimental thermal cycles. By applying the proposed methodology, it was possible to accurately delimit reheated HAZ regions and analyze the effects of subsequent passes in each of the root pass HAZ subregions. The IRCGHAZ in the root pass region was localized, but it did not behave as a brittle zone due to the good weldability of the API 5L X80 steel as proven by the hardness and Charpy-V impact tests.
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44

Sun, Xun. "Ab initio Investigation of Al-doped CrMnFeCoNi High-Entropy Alloys." Licentiate thesis, KTH, Materialvetenskap, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-251330.

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High-entropy alloys (HEAs) represent a special group of solid solutions containing five or more principal elements. The new design strategy has attracted extensive attention from the materials science community. The design and development of HEAs with desired properties have become an important subject in materials science and technology. For understanding the basic properties of HEAs, here we investigate the magnetic properties, Curie temperatures, electronic structures, phase stabilities, and elastic properties of paramagnetic (PM) body-centered cubic (bcc) and face-centered cubic (fcc) AlxCrMnFeCoNi (0 ≤ x ≤ 5, in molar fraction) HEAs using the first-principles exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) for dealing with the chemical and magnetic disorder. Whenever possible, we compare the theoretical predictions to the available experimental data in order to verify our methodology. In addition, we make use of the previous theoretical investigations carried out on AlxCrFeCoNi HEAs to reveal and understand the role of Mn in the present HEAs. The theoretical lattice constants are found to increase with increasing x, which is in good agreement with the available experimental data. The magnetic transition temperature for the bcc structure strongly decreases with x, whereas that for the fcc structure shows a weak composition dependence. Within their own stability fields, both structures are predicted to be PM at ambient conditions. Upon Al addition, the crystal structure changes from fcc to bcc with a broad two-phase field region, in line with the observations. Bain path calculations suggest that within the duplex region both phases are dynamically stable. Comparison with available experimental data demonstrates that the employed approach describes accurately the elastic moduli of the present HEAs. The elastic parameters exhibit complex composition dependences, although the predicted lattice constants increase monotonously with Al addition. The elastic anisotropy is unusually high for both phases. The brittle/ductile transitions formulated in terms of Cauchy pressure and Pugh ratio become consistent only when the strong elastic anisotropy is accounted for. The negative Cauchy pressure of CrMnFeCoNi is found to be due to the relatively low bulk modulus and C12 elastic constant, which in turn are consistent with the relatively low cohesive energy. Our findings in combination with the experimental data suggest anomalous metallic character for the present HEAs system. The work and results presented in this thesis give a good background to go further and study the plasticity of AlxCrMnFeCoNi type of HEAs as a function of chemistry and temperature. This is a very challenging task and only a very careful pre-study concerning the phase stability, magnetism and elasticity can provide enough information to turn my plan regarding ab initio description of the thermo-plastic deformation mechanisms in AlxCrMnFeCoNi HEAs into a successful research.
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45

Teixeira, Juliana Cristine de Sousa. "Influência do tipo de entalhe em tubo de aço API grau X60 para obtenção da curva de temperatura de transição dúctil–frágil no ensaio de DWTT." Guaratinguetá, 2018. http://hdl.handle.net/11449/154208.

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Orientador: Marcelo dos Santos Pereira
Resumo: O ensaio de queda de peso DWTT (Drop Weigth Tear Test) é um método amplamente utilizado pela indústria de óleo e gás para determinar a capacidade de um material em impedir a propagação de uma trinca. Esse método foi desenvolvido pelo Battelle Memorial Institute, e é realizado em conformidade com a especificação API RP 5L3 "Práticas Recomendadas para a Condução de Testes de Queda de Peso". Com o desenvolvimento dos aços ARBL, o comportamento dos aços vem mostrando algumas particularidades resultantes do processamento termomecânico, e por esse motivo, podem não apresentar o mesmo comportamento à fratura que aços mais antigos, como delaminações ou inclusões não metálicas. Atualmente são propostos dois tipos de entalhe, sendo o tipo prensado, obtido pela estampagem de uma matriz na amostra, e o tipo Chevron, que deve ser usinado. A correlação entre ambos os entalhes pode ser realizada apenas para a análise da porcentagem da superfície dúctil da fratura. Outros tipos de correlação como energia absorvida para impacto, não são recomendados, uma vez que a concentração de tensão para o entalhe Chevron é muito maior, facilitando o rompimento da amostra, enquanto que o entalhe prensado demanda maior energia, uma vez que possui maior encruamento na região. No presente trabalho foram realizados os levantamentos de curvas de temperatura de transição dúctil e frágil (TTDF) do material base do tubo com dimensões de 762 mm x 38,1 mm de aço carbono com grau API X60, através da análise da porce... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: The Drop Weigth Tear Test (DWTT) is a method widely used by the oil and gas industry to determine the ability of a material to prevent the propagation of a crack. This method was developed by the Battelle Memorial Institute, and is performed in accordance with API RP 5L3 "Drop-Weight Tear Tests on Line Pipe" specification. With the development of ARBL steels, the behavior of steels has shown some particularities resulting from thermomechanical processing, and for this reason, they may not present the same fracture behavior as older steels, such as delamination or nonmetallic inclusions. Currently two types of notch are proposed, being the type pressed, obtained by the stamping of a matrix in the sample, and the type Chevron, that must be machined. The correlation between both notches can be performed only for the analysis of the percentage of the ductile surface of the fracture. Other types of correlation as energy absorbed for impact are not recommended, since the stress concentration for the Chevron notch is higher, facilitating the rupture of the sample, while the notched press demands greater energy, since it has greater hardening in the region. In the present study, the ductile and brittle transition temperature (TTDF) curves of the base material of the pipe with dimensions of 762 mm x 38.1 mm of carbon steel with API grade X60 were carried out, through the analysis of the percentage of ductile fracture resulting whose results were shown to be equivalent for both notch: ... (Complete abstract click electronic access below)
Mestre
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46

Fischdick, Acuna Andres Fabricio. "Hybrid Laser Welding in API X65 and X70 Steels." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1462876479.

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47

Ferreño, Blanco Diego. "Integridad estructural de vasijas nucleares en base a la curva patrón obtenida mediante probetas reconstruidas." Doctoral thesis, Universidad de Cantabria, 2008. http://hdl.handle.net/10803/10603.

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En esta tesis se ha caracterizado el acero, virgen e irradiado, de la vasija de una central nuclear española actualmente en servicio, en la región de temperaturas conocida como Zona de Transición Dúctil-Frágil. Para ello, se han ensayado probetas propias de la Mecánica de Fractura, reconstruidas a partir de mitades de probetas de impacto previamente ensayadas. Para describir la tenacidad a fractura en la Zona de Transición se ha empleado el modelo de la Curva Patrón.Finalmente, se ha analizado el impacto del procedimiento de caracterización y del modelo de la Curva Patrón sobre la Integridad Estructural de la vasija, comparando sus predicciones con las que se obtienen de aplicar los procedimientos convencionales que contempla la normativa vigente, representada por el Código ASME. En esta comparación se ha hecho uso del procedimiento FITNET de Integridad Estructural.
In this thesis, the steel, virgin and irradiated, from the vessel of a Spanish Nuclear Plant currently operating, has been characterized in the range of temperatures known as Ductile to Brittle Transition Region. For this purpose, Fracture Mechanics specimens, reconstituted from halves of impact specimens previously broken, have been tested. To describe fracture toughness in the Transition Region, the Master Curve method has been used.Eventually, the influence of the characterization procedure together with the Master Curve method on the Structural Integrity of the vessel has been analyzed, comparing their predictions with those coming from applying conventional procedures proposed by the current regulations, represented by the ASME Code. In this comparison, the FITNET Structural Integrity procedure has been used.
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48

Haskel, Hudison Loch. "Avaliação da tenacidade à fratura e do efeito da microtextura e mesotextura na curva de transição dúctil frágil do aço API 5L X70." UNIVERSIDADE ESTADUAL DE PONTA GROSSA, 2013. http://tede2.uepg.br/jspui/handle/prefix/1426.

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In the present study was investigated the API 5L X70 steel mechanical behavior through tensile and instrumented impact tests. Results from the instrumented Charpy impact test have allowed quantify the forces, energies and deflections of the specimens. It was used to obtain the fracture mechanics parameters such as the dynamic fracture toughness JId, the stress intensity factor for dynamic loading KId, as also range the ductile brittle transition temperature. Simultaneously was evaluated the mechanisms that induce to the delamination phenomenon as well as the investigation of their influence on the absorbed energy during the impact test. It was also analyzed the microtexture and mesotexture of different samples from the brittle, ductile and mixed regions from the ductile brittle transition temperature curve. Results have shown that the longitudinal direction was slightly superior mechanical properties when compared to the transverse direction. The dynamic fracture toughness JId was higher for the L-T specimen orientation when compared to the T-L orientation and it tends to increase with increasing temperature for both directions. The stress intensity factor values for dynamic loading was close to in both L-T and T-L orientations. The microtexture has revealed that deformation process from the impact test preserves some of its characteristics from the prior processing such as phase transformations and recrystallization components. While the mesotexture feature has shown higher energy values in the ductile brittle transition temperature curve, it is resulted from the formation of deformation substructure. Results obtained by scanning electron microscopy and light microscopy have shown that the main factors that may be contributing to the formation of crack-divider type delaminations are banded microstructure and elongated ferrite grains. While the microtexture from the edges delamination has shown that propagation mechanism is intergranular. Taylor factor mapping has shown that the mechanism responsible for the formation of delamination is accompanied by low plastic deformation.
No presente trabalho foi investigado o comportamento mecânico do aço API 5L X70 através de ensaios de tração e impacto instrumentado. O ensaio de impacto Charpy instrumentado permitiu quantificar as forças, energias e deflexões dos corpos de prova. Com a utilização destes dados foram obtidos parâmetros da mecânica da fratura, tais como, a tenacidade à fratura dinâmica JId, o fator de intensidade de tensão para carregamento dinâmico KId, além da faixa de valores da temperatura de transição dúctil frágil. Concomitantemente foram avaliados os mecanismos que contribuem na formação do fenômeno conhecido como delaminação, bem como a investigação de sua influência sobre a energia absorvida durante o ensaio de impacto. Foram também analisadas a microtextura e mesotextura de diferentes amostras retiradas da região frágil, dúctil e mista, da curva de transição dúctil frágil do material. Os resultados mostraram que o material possui propriedades mecânicas de impacto ligeiramente superiores na direção longitudinal à laminação comparado à direção transversal. Os valores encontrados para a tenacidade à fratura dinâmica JId são superiores para corpos de prova com orientação L-T quando comparados para a orientação T-L, e tendem a aumentar com o aumento da temperatura para ambas orientações. Os valores encontrados para o fator de intensidade de tensão para carregamento dinâmico foram próximos para ambas as orientações L-T e T-L. A microtextura revelou que após a deformação no ensaio de impacto o aço preserva algumas das características do seu histórico de processamento tais como componentes de transformações de fase e recristalização. Enquanto que o estudo da mesotextura mostrou que maiores valores de energia na curva de transição dúctil frágil resultam na formação de uma subestrutura de deformação. Resultados obtidos por microscopia eletrônica de varredura e microscopia ótica mostraram que os principais fatores que podem estar contribuindo para formação de delaminações do tipo trinca divisora são a microestrutura bandeada e grãos alongados de ferrita. Enquanto que a microtextura nas bordas da delaminação mostrou que o seu mecanismo de propagação é intergranular. O mapeamento do fator de Taylor mostrou que o mecanismo responsável pela formação da delaminação é acompanhado por pouca deformação plástica.
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49

Dyja, Vanessa. "Interaction entre fluides de différents réservoirs lors de l'évolution d'un prisme orogénique en contexte de déformation partitionnée : les Cordillères bétiques internes (Espagne) : Implications sur le transfert de métaux dans la croute." Thesis, Université de Lorraine, 2014. http://www.theses.fr/2014LORR0029/document.

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La thèse cherche à caractériser les types de fluides à différents niveaux structuraux et types de déformation. Les résultats de la Sierra Bédar indiquent le rôle d'une saumure enrichie en métaux lors des derniers stades de circulation au sein de la croûte ductile dans un domaine encore isolé des fluides de surface. Cette saumure est plus diluée à l'est dans la Sierra Almagrera et où l'impact du système décrochant va devenir de plus en plus important avec du volcanisme et de nombreux gisements. Dans le domaine fragile, des veines de quartz enregistrent des fluides de surface. Suivent des veines de quartz indiquant la réapparition des fluides métamorphiques sous-jacent. La formation de veines de sidérite-pyrite-galène-barytine montre de plus forte salinité et des rapports Cl/Br indiquant la contribution d'une autre source de saumures secondaires issues cette fois des évaporites messiniennes. Le décrochement a modifié le compartimentage de part et d'autre de la transition fragile-ductile
This thesis aim at characterizing the types of fluids concerned structural levels and style of deformation. Results from Sierra Bedar point out the role played by metal-rich brines during late stages of fluid flows within the ductile crust within a domain was still isolated from surficial fluids. Brine concentration drop in the eastern Sierra Almagrera which was progressively more impacted by the major transcurrent system with volcanism and metal ore deposits. At reaching the brittle domain, quartz veins trapped surficial fluids. Soon after, quartz veins indicating the return of metamorphic fluids. Following siderite-pyrite-galena-barite veins display higher salinities and Cl/Br ratios indicate the contribution of another secondary brine coming from early Messinian evaporites. This major transcurrent tectonics clearly modified the partitioning on both sides of the brittle-ductile transition
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50

Demenet, Jean-Luc. "Etude du silicium à basse et moyenne température sous forte contrainte : comportement des dislocations individuelles et plasticité." Poitiers, 1987. http://www.theses.fr/1987POIT2016.

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