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  • 1.
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Branching of a dissolution driven stress corrosion crack2009In: Proceedings of the International Conference on Crack Paths 2009, European Structural Integrity Society , 2009Conference paper (Other academic)
    Abstract [en]

    Stress corrosion cracking occurs due to the synergistic interaction between mechanical load and corrosion reactions. Some types of stress corrosion crack branch heavily. Here, branching during dissolution driven crack growth is studied using an adaptive FE method. A strain-assisted evolution law is used for the inherently blunted crack. No criterion for crack growth is needed as for a sharp crack, neither for the growth direction. Several simulations are performed with different degrees of load biaxiality. It is found that large biaxiality promotes branching, but no conditions for when branching takes place is found. Instead, branching seems to occur rather randomly due to the perturbation sensitivity of a dissolution driven crack. Also crack growth rates for branched cracks are investigated, and it is found that both constant growth rates can be reached, as well as decreasing rates and eventual arrest. The cracks follow a mode I crack path, however local changes may occur due to the perturbation sensitivity.

  • 2.
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Dissolution Driven Fracture - Simulation of Crack Growth2006In: Proceedings of 19th Nordic Seminar on Computational Mechanics, Structural Mechanics, LTH, Sweden. , 2006, p. 130-134Conference paper (Refereed)
    Abstract [en]

    The growth of a crack subjected to corrosion fatigue is studied using adaptive finite elements. The crack growth is the result of a repeated cycle of dissolution of the material, formation of a protective oxide film and break-down of the oxide film due to straining at the surface. The dissolution rate is assumed to be proportional to this stretching. The growth of a semi-infinite crack lying in an infinite strip subjected to different degrees of mixed-mode loading is studied.

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  • 3.
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Spänningskorrosion - modellering av utveckling från ytojämnheter via gropar till sprickor2007In: Abstracts Svenska mekanikdagarna, Luleå, Sweden, 2007Conference paper (Other academic)
    Abstract [sv]

    Den kombinerade effekten av mekanisk belastning och en korrosiv miljö kan leda till spänningskorrosion. Vid anodisk spänningskorrosion löses materialet upp och bildar korrosionsprodukter. I vissa fall bildas en ytbeläggning, t ex en oxidfilm, som verkar skyddande och därmed bromsar korrosionsprocessen. Om en dylik film inte skapas, eller om den kontinuerligt bryts ned, kan korrosionsprocessen fortgå. I en plan yta kan gropar bildas, och någon eller några av dessa kan i sin tur utvecklas till en kritisk spricka. I denna studie undersöks utvecklingen av korrosionssprickor med hjälp av en modell där korrosionshastigheten är proportionell mot den elastiska töjningsenergin och kurvaturen i varje punkt på ytan. Det är känt sedan tidigare att kurvaturen påverkar benägenheten för atomer att lossna från eller binda vid en yta, vilket leder till att en obelastad yta planar ut vid t ex etsning. Om ytan däremot är belastad kommer en ytojämnhet med ett specifikt frekvensinnehåll att utvecklas. Vågor med längder kortare än en viss kritisk våglängd kommer att avklinga. På detta sätt leder korrosionen till att gropar bildas i ytan och vissa av dessa fördjupas allt mer tills de blir sprickliknande. Dessa spänningskorrosionssprickor kommer dock att ha en finit radie i spetsen som är given av det lokala spänningstillståndet. Simuleringar är gjorda med en adaptiv metod. För varje tidsinkrement beräknas töjningarna i ytan i en finita element-analys, därefter bestäms ytevolutionen och sedan genereras ett nytt elementnät för den uppdaterade geometrin. Resultat från simuleringarna visar utvecklingen från en plan yta, med en initiell pertubation, via gropbildning till sprickor.

  • 4.
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    The discrete nature of the growth and arrest of microstructurally short fatigue cracks modelled by dislocation technique2005In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 27, no 1, p. 21-32Article in journal (Refereed)
    Abstract [en]

    Growth of a microstructurally short edge crack in the neighbourhood of a grain boundary has been studied using a dislocation technique. Influence of the distance between the crack tip and a high-angle grain boundary was studied for 5000 load cycles. Growth rate was found to change stepwise during crack advance, both accelerating and decelerating, and finally retarding before crack arrest. Crack propagation is ruled by the competition between the shielding effect from the dislocations in the plastic zone and the stress field increase due to crack extension, and clearly shows a discrete behaviour at growth below the fatigue threshold load.

  • 5.
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    The influence of biaxial loading on branching of a dissolution driven stress corrosion crack2010In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 77, no 11, p. 1989-1997Article in journal (Refereed)
    Abstract [en]

    Stress corrosion cracking occurs due to the synergistic interaction between mechanical load and corrosion reactions. In this study, branching during anodic dissolution driven crack growth is studied using an adaptive FE procedure. The crack has an inherent blunt tip due to the dissolution, and the growth is treated as a moving boundary problem with a strain-assisted evolution law. Simulations are performed with different degrees of load biaxiality. It is found that increasing biaxiality promotes branching. No conditions for when branching takes place are found. Crack growth rates for branches are investigated, and it is found that, after an initial acceleration, constant growth rates can be reached, as well as decreasing speed and eventual arrest. The influence of T-stresses and perturbations sensitivity are discussed.

  • 6.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Kao-Walter, Sharon
    Ståhle, Per
    Malmö högskola, School of Technology (TS).
    Fracture Mechanisms of a Thin Elastic Plastic Laminate2006In: Fracture of Nano and Engineering Materials and Structures: Proceedings of the 16th European Conference of Fracture, Alexandroupolis, Greece, July 3–7, 2006, 2006, p. 927-928Conference paper (Other academic)
  • 7.
    Bjerkén, Christina
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Massih, Ali
    Malmö högskola, Faculty of Technology and Society (TS).
    Kinetics of orientational phase ordering near line defects in crystals2015In: International Conference on Solid-Solid Phase Transformation in Inorganic Materials (PTM 2015), TMS PTM-2015: A combination print-CD volume , 2015, p. 36-36Conference paper (Other academic)
    Abstract [en]

    General properties of directed ordering near line defects in elastic crystals undergoing phase transition are studied using the two-component time-dependent Ginzburg-Landau equation. Upon quenching the system below its transition point, the temporal evolution of the order parameter in the vicinity of the defect is evaluated. The development of vortices is explored and their interaction with the structural defect is examined. Finally, phase transitions in improper ferroelectrics in the context of the model are discussed.

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  • 8.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Massih, Ali
    Malmö högskola, School of Technology (TS).
    Nucleation of second-phase near elastic defects in crystalline solids2011In: Proceedings of MS&T'11, TMS , 2011Conference paper (Other academic)
    Abstract [en]

    The problem of heterogeneous nucleation of second-phase in alloys in the vicinity of elastic defects is considered. The defect can be a dislocation line or a crack tip residing in a crystalline solid. We use a compressible Ising-like model, represented by the Ginzburg-Landau equation, to describe the spatiotemporal evolution of the order parameter in the environs of the defect. The model accounts for the elasticity of the solid and the interaction of order parameter field with the elastic field of the defect. A finite volume numerical method is used to solve the governing partial differential equation for the order parameter. We examine the nature of the phase transition and discuss the phase diagram topology near and away from the defect. We discuss our calculations in light of observations of formation of hydrides in zirconium and titanium alloys.

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  • 9.
    Bjerkén, Christina
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Massih, Ali
    Malmö högskola, Faculty of Technology and Society (TS).
    Oriented ordering near line defects in crystals2017In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 97, no 27, p. 2437-2467Article in journal (Refereed)
    Abstract [en]

    General properties of directed ordering near line defects, in particular an edge dislocation, in elastic crystals undergoing phase transition are studied using the two-component time-dependent Ginzburg-Landau equation in two dimensions or 2D-XY model. The associated Landau potential comprises a sixth-order term, cubic anisotropy terms and the field of the dislocation. In thermodynamic equilibrium, the phase diagram for the model is delineated. Upon quenching the system below its transition point, the temporal evolution of the order parameter components in the vicinity of the defect is numerically evaluated. The development of vortices, emanated from the model, is explored and their interaction with the dislocation is examined. The dislocation produced a vortex free circular region whose diameter grew almost linearly with time. The time-dependence of vortex density for various settings of the Landau potential coefficients are evaluated. The vortex density ( in 2D) decreased inversely with time, albeit faster in the absence of dislocation. By computing the two-point correlation function, we established that the dynamic scaling law is satisfied for the considered model if the distance is scaled by L = t(1/2) or by its half-width L-1/2 for a dislocation free crystal. Finally, phase transitions in improper ferroelectrics in the context of the model are discussed.

  • 10.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Massih, Ali
    Malmö högskola, School of Technology (TS).
    Phase ordering kinetics of second-phase formation near an edge dislocation2014In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 94, no 6, p. 569-593Article in journal (Refereed)
    Abstract [en]

    The time-dependent Ginzburg-Landau (TDGL) equation for a single component non-conservative structural order parameter is used to study the spatio-temporal evolution of a second phase in the vicinity of an edge dislocation in an elastic crystalline solid. A symmetric Landau potential of sixth-order is employed. Dislocation field and elasticity modify the second-order and fourth-order coefficients of the Landau polynomial, respectively, where the former makes the coefficient singular at the origin. The TDGL equation is solved numerically using a finite volume method, where a wide range of parameter sets is explored. Computations are made for temperatures both above and below the transition temperature of a defect-free crystal Tc0. In both cases, the effects of the elastic properties of the solid and the strength of interaction between the order parameter and the displacement field are examined. If the system is quenched below Tc0, a steady state is first reached on the compressive side of the dislocation. On the tensile side, the growth is held back. The effect of thermal noise term in the TDGL equation is studied. We find that if the dislocation is introduced above Tc0, thermal noise supports the nucleation of the second phase, and a steady state will be attained earlier than if the thermal noise was absent. For a dislocation-free solid, we have compared our numerical computations for a mean-field (spatially averaged) order parameter versus time with the late time growth of the ensemble-averaged order parameter, calculated analytically, and find that both results follow upper asymptotes of sigmoid curves.

  • 11.
    Bjerkén, Christina
    et al.
    Division of Materials Engineering, Department of Mechanical Engineering, Lund University.
    Melin, S
    Division of Solid Mechanics, Department of Mechanical Engineering, Lund University.
    A study of the influence of grain boundaries on short crack growth during varying load using a dislocation technique2004In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 71, no 15, p. 2215-2227Article in journal (Refereed)
    Abstract [en]

    The propagation of short cracks in the neighbourhood of grain boundaries have been investigated using a technique were the crack is modelled by distributed dislocation dipoles and the plastic deformation is represented by discrete dislocations. Discrete dislocations are emitted from the crack tip as the crack grows. Dislocations can also nucleate at the grain boundaries. The influence on crack growth characteristics of the distance between the initial crack tip and the grain boundary has been studied. It was found that crack growth rate is strongly correlated to the dislocation pile-ups at the grain boundaries. (C) 2004 Elsevier Ltd. All rights reserved.

  • 12.
    Bjerkén, Christina
    et al.
    Department of Mechanical Engineering, Lund University.
    Melin, S
    Department of Mechanical Engineering, Lund University.
    A tool to model short crack fatigue growth using a discrete dislocation formulation2003In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 25, no 6, p. 559-566Article in journal (Refereed)
    Abstract [en]

    A method is presented that combines the modelling of cracks by distributed dislocation dipoles with developing plasticity represented by discrete dislocations moving along slip bands. Crack growth is due to the emission of dislocations from the crack tip along preferred slip planes. Eventual annihilation of dislocations occurs by reunion with the corresponding displacement steps of the crack surface. Crack surface overlap is not allowed. The equilibrium state for each load increment is solved iteratively, allowing various crack geometries. The method is applied to the problem of a short edge crack growing in mode I due to fatigue loading. It is shown that the development of a local plastic zone and the propagation of the crack can be monitored in detail. (C) 2003 Elsevier Science Ltd. All rights reserved.

  • 13.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Melin, Solveig
    Dislocation distribution in the plastic zone and growth of ashort fatigue crack2005In: Zeszyty naukowe / Mechanika;1, Politechnika opolska , 2005, p. 29-36Conference paper (Refereed)
  • 14.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Melin, Solveig
    Growth of a short fatigue crack - A long term simulation using a dislocation technique2009In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 46, no 5, p. 1196-1204Article in journal (Refereed)
    Abstract [en]

    In this study, the growth of a short edge crack during more than 14 000 cycles of fatigue loading is investigated in detail. An edge crack, in a semi-infinite body with no pre-existing obstacles present, is modelled in a boundary element approach by a distribution of dislocation dipoles. The fatigue cycles are fully reversed (R = -1), and the load range is well below the threshold for long fatigue cracks. The developing local plasticity consists of discrete edge dislocations that are emitted from the crack tip. The movements of discrete dislocations are restricted to slip along preferred slip planes. The present model is restricted to a 2D plane strain problem with a through-thickness crack, assuming no 3D irregularities. A remote load is applied perpendicular to the crack extension line, and the material parameters are those of a BCC crystal structure. The competition between influence of the global loading on and local shielding of the crack tip governs the crack growth. The growth rate increases in discrete steps with short periods of retardation, from approximately the size of Burgers vector, b, up to 25 b per cycle as the length of the crack is tripled. The plastic zone changes from having an elongated, slender form to include a low angle grain boundary, which, eventually, divides into two parts. The crack growth is found to change from constant acceleration to constant growth rate as the event of the low-angle grain boundary split is approached. ; The results are compared to long crack characteristics, for which linear elastic fracture mechanics and Paris law can be used to predict fatigue crack growth. The exponent in Paris law varies between 1 and 0 in the present study, i.e. smaller than typical values for ductile BCC materials. The ratio between static and cyclic plastic zone sizes is found to increase during crack growth, and the angle of the general plastic zone direction increases, showing a tendency towards long crack values. The characteristics of the simulated crack growth, found in the present study, are typical for below-threshold growth, with slow acceleration, constant growth rate. and, eventually, either arrest or transition to long crack growth behaviour, as reported in the literature.

  • 15.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Melin, Solveig
    Influence of low-angle grain boundaries on short fatigue crack growth studied by a discrete dislocation method2008In: Proceedings 17th European Conference on Fracture, Brno 2008, VUTIUM Brno, Czech Republic , 2008Conference paper (Other academic)
    Abstract [en]

    The growth of short fatigue cracks in or below the threshold regime are highly influenced by the local microstructure. In the present study, the influence of low-angle boundaries on the growth and arrest of microstructurally short edge cracks in crystalline material is investigated using a discrete dislocation method. The crack is modelled by boundary elements consisting of dislocation dipoles. An external load that is fully reversed during the fatigue cycle is applied to obtain a crack growth rate of a few Burgers vector per cycle. The crack growth and the developing plasticity are given by the emission, and eventual annihilation, of dislocations at the crack tip. The low-angle grain boundary is modelled as an array of aligned dislocations. It is found that the developing local plasticity, is crucial for the growth rates. Different stress ranges are used and both positive and negative low-angle grain boundaries are considered. It is found that depending on sign of the of the grain boundary, different growth behaviour are found. Generally, a positive low-angle grain boundary gives a low, rather stress range independent growth rate. A negative grain boundary results in increasing crack growth when the stress range is increased. The distance between the crack tip and the low-angle boundary is found not to significantly influence the crack growth characteristics, for the geometries and load ranges considered in this study.

  • 16.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Ortiz, Michael
    Evolution of anodic stress corrosion cracking in a coated material2010In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 165, no 2, p. 211-221Article in journal (Refereed)
    Abstract [en]

    In the present paper, we investigate the influence of corrosion driving forces and interfacial toughness for a coated material subjected to mechanical loading. If the protective coating is cracked, the substrate material may become exposed to a corrosive media. For a stress corrosion sensitive substrate material, this may lead to detrimental crack growth. A crack is assumed to grow by anodic dissolution, inherently leading to a blunt crack tip. The evolution of the crack surface is modelled as a moving boundary problem using an adaptive finite element method. The rate of dissolution along the crack surface in the substrate is assumed to be proportional to the chemical potential, which is function of the local surface energy density and elastic strain energy density. The surface energy tends to flatten the surface, whereas the strain energy due to stress concentration promotes material dissolution. The influence of the interface energy density parameter for the solid-fluid combination, interface corrosion resistance and stiffness ratios between coating and substrate is investigated. Three characteristic crack shapes are obtained; deepening and narrowing single cracks, branched cracks and sharp interface cracks. The crack shapes obtained by our simulations are similar to real sub-coating cracks reported in the literature.

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  • 17.
    Bjerkén, Christina
    et al.
    Department of Mechanics and Materials, Lund University.
    Persson, Christer
    Department of Mechanics and Materials, Lund University.
    A numerical method for calculating stress intensity factors for interface cracks in bimaterials2001In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 68, no 2, p. 235-246Article in journal (Refereed)
    Abstract [en]

    This paper presents a method For obtaining the complex stress intensity factor (or alternatively the corresponding energy release rate and mode mixity) for an interface crack in a bimaterial using a minimum number of computations. A crack closure integral method for homogeneous materials developed by Rybicki and Kanninen has been modified to include mismatch in material properties. This was achieved directly from the nodal forces at the crack tip and the displacements near the tip as obtained from a finite element analysis using only four-node constant strain elements. Numerical calculations for tensile and mixed mode loading showed good agreement with results from corresponding analytical solutions. The main advantages of this method are that it is straightforward and easy to use and that the number of calculations needed to obtain the stress intensity Factors can be held to a minimum. (C) 2000 Elsevier Science Ltd. All rights reserved.

  • 18.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Rimoli, Julian
    Ortiz, Michael
    Stress corrosion crack growth beneath a stiff coating: influence of chemical potential and interface toughness2009In: Proceedings 12th International Conference on Fracture, Ottawa 2009, ICF12 , 2009Conference paper (Other academic)
    Abstract [en]

    In the present paper we investigate the influence of corrosion driving forces and interfacial toughness for a coated material subjected to mechanical loading. If the protecting coating is cracked, the substrate material may become exposed to a corrosive media. For a stress corrosion sensitive substrate material, this may lead to detrimental crack growth. A crack is assumed to grow by anodic dissolution, inherently leading to a blunted crack tip. The rate of dissolution along the crack surface is assumed to be proportional to the chemical potential, which is function of the local surface energy density and the elastic strain energy density. The surface energy tends to flatten the surface, whereas the strain energy due to stress concentration promotes material dissolution. The evolution of the crack surface is modelled as a moving boundary problem using an adaptive finite element method. The crack shapes obtained by our simulations are remarkably similar to real stress corrosion cracks reported in the literature.

  • 19.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Ståhle, Per
    Malmö högskola, School of Technology (TS).
    Branching of growing corrosion fatigue cracks2005Conference paper (Other academic)
    Abstract [en]

    Strain-driven corrosion of branching cracks, initiated from a virtually plane surface, has been studied using a moving boundary technique. The material is assumed linear elastic and is subjected to fatigue loading under plain strain conditions. The surface of the material is covered by a protective film. During loading this film can be damaged if it is strained above a threshold value, thus revealing an unprotected surface. Corrosion advances by material dissolution, eventually evolving into cracks. The rate of surface evolution is a function of the degree of protective film damage. During unloading the protective film is assumed to develop and heal the surface. A low frequency cyclic loading is applied to ensure that total healing is assumed. The moving boundary technique, simulating corrosion, results in arc-shaped crack tips, rather than singular crack tip points, thus no crack growth criterion is needed in the analysis. For each load step, the strain distribution is found using the finite element method, followed by required the movement the boundary and then remeshing. The crack growth has been investigated for at least 2000 cycles. A more or less pronounced branching of the cracks is found to develop. The crack branches can be classified in three groups; main cracks that grows with maximum rate and branches further, branch cracks that initially retards and then find a steady state growth rate that is a fraction of maximum speed, and finally, arresting cracks that after a period of retardation stop growing. The crack patterns are realistic, showing a sort of self-similarity with tree-like structure, cf. the picture below that shows a typical finite element result. The width of a crack branch together with the shielding from the applied stresses, caused by the other branches and main cracks, seem to govern the evolution of the crack branch. A steady-state growth rate is achieved during parts of the evolution as the crack width and the strain field surrounding the crack tip is in balance, i.e. the crack widens while the crack grows longer. As the bluntness of the tip reaches an upper limit, branching results.

  • 20.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Ståhle, Per
    Malmö högskola, School of Technology (TS).
    Simulation of corrosion fatigue crack growth under mixed-mode loading2008In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 75, no 3-4, p. 440-451Article in journal (Refereed)
    Abstract [en]

    The kinking of a corrosion crack due to mixed-mode fatigue loading is studied using an adaptive finite element procedure. The rate of material dissolution is assumed to be proportional to the stretching of the corroding surface. The dissolution of material is governed by a corrosion law, where no criterion is needed for neither crack growth nor growth direction. The problem is treated as a general moving boundary problem. The kink angles are found to be in very good agreement with results for sharp cracks using criteria reported in the literature.

  • 21.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Ståhle, Per
    Malmö högskola, School of Technology (TS).
    The path of a growing crack - A simulation of the fracture process2006Conference paper (Other academic)
    Abstract [en]

    The growth of a crack subjected to corrosion fatigue is studied using adaptive finite elements. The crack is the image of a deep corrosion pit, and the growth is the result of a repeated cycle of dissolution of the material, formation of a protective oxide film and break-down of the oxide film. The break-down of the film is governed by the strain at the surface and the dissolution rate is assumed to be proportional to this stretching. A threshold strain is assumed to exist below which the oxide film remains intact. With this model, no criterion is needed, neither for crack growth, nor for prediction of the growth direction. The reason is that both are immediate results of the evolution of the body shape. The growth of a semi-infinite crack lying in an infinite strip subjected to different degrees of mixed-mode loading is studied and the results are compared to crack path criteria for sharp cracks. Additionally, the path of a corrosion fatigue crack starting at the surface of an elastic layer attached to a stiff substrate is simulated. The result showed some agreement with experimental results found in the literature.

  • 22.
    Bjerkén, Christina
    et al.
    Malmö högskola, School of Technology (TS).
    Ståhle, Per
    Malmö högskola, School of Technology (TS).
    Gunnars, Jens
    The Path of a Growing Crack - Experiment and Simulation2006In: Proceedings of Optical methods, 2006Conference paper (Other academic)
  • 23. Blomqvist, Jacob
    et al.
    Massih, Ali
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    A First Principle Atomistic Pre-study of a Multiscale Model of Hydride Induced Embrittlement in Zirconium2010Conference paper (Other academic)
    Abstract [en]

    Hydrogen induced embrittlement and associated cracking effects in Zirconium alloys impose a serious problem, for example, within the nuclear industry where these materials are used in a range of applications, including fuel claddings. To get a fuller understanding of the hydrogen diffusion, hydride formation and phase transformations taking place at conditions of external stress and at elevated temperature it is crucial to augment experimental measurements with multi-level modelling. It is, for example, possible to simulate hydride phase-transformation trough phase-field calculations[1]. In order to get accurate results from these calculations, however, it is important to have basic physical information, such as interfacial energies and elastic constants. The latter were obtained completely from first principle calculations of density functional theory. In pure zirconium, at low hydrogen concentrations and at low temperatures, hydrogen atoms are dissolved into the matrix and preferentially occupy tetrahedral sites within the thermodynamically stable aZr hexagonal closed packed unit cell. At higher concentrations the various face centred cubic and tetragonal hydride phases start to form. Using the plane-wave code DACAPO, within the generalized gradient approximation, the lattice parameters and the ionic positions of bulk aZr, dZrHx and gZrHx were relaxed with respect to total energies. The formation energies as well as bulk moduli were then obtained by fitting total energies as a function of lattice parameters with a 3rd or 4th order polynomial. By comparing the present results with previous calculations and experiments it was possible to validate the methods for further use. The results, thus, comprise a foundation and serves to verify methods and codes. The next step is to compute more complex properties such as surface free energies, or interfacial energies, of bi-phase Zr-H systems, so that the full multiscale model can be implemented. 1. V. Vaithyanathan et al., Acta Materialia 52 (2004) p. 2973

  • 24.
    Blomqvist, Jakob
    et al.
    Malmö högskola, School of Technology (TS).
    Johansson, Johan
    Alvarez, Anna-Maria
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Structure and Thermodynamical Properties of Zirconium Hydrids from First Principle2011Conference paper (Other academic)
    Abstract [en]

    Zirconium alloys are used as nuclear fuel cladding material due to their mechanical and corrosion resistant properties together with their favorable cross-section for neutron scattering. At running conditions, however, there will be an increase of hydrogen in the vicinity of the cladding surface at the water side of the fuel.The hydrogen will diffuse into the cladding material and at certain conditions, such as lower temperatures and external load, hydrides will precipitate out in the material and cause well known embrittlement, blistering and other unwanted effects. Using phase-field methods it is now possible to model precipitation build-up in metals, for example as a function of hydrogen concentration, temperature and external load, but the technique relies on input of parameters, such as the formation energy of the hydrides and matrix. To that end, we have computed, using the density functional theory (DFT) code GPAW, the latent heat of fusion as well as solved the crystal structure for three zirconium hydride polymorphs: delta-ZrH1.6, gamma-ZrH, and epsilon-ZrH2.

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  • 25.
    Blomqvist, Jakob
    et al.
    Malmö högskola, School of Technology (TS).
    Olofsson, Johan
    Alvarez, Anna-Maria
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Structure and Thermodynamical Properties of Zirconium hydrides from first-principle2012In: Proceedings of the 15th international conference on environmental degradation of materials in nuclear power systems-water reactors, John Wiley & Sons, 2012, p. 671-679Conference paper (Other academic)
    Abstract [en]

    Zirconium alloys are used as nuclear fuel cladding material due to their mechanical and corrosion resistant properties together with their favorable cross-section for neutron scattering. At running conditions, however, there will be an increase of hydrogen in the vicinity of the cladding surface at the water side of the fuel. The hydrogen will diffuse into the cladding material and at certain conditions, such as lower temperatures and external load, hydrides will precipitate out in the material and cause well known embrittlement, blistering and other unwanted effects. Using phase-field methods it is now possible to model precipitation build-up in metals, for example as a function of hydrogen concentration, temperature and external load, but the technique relies on input of parameters, such as the formation energy of the hydrides and matrix. To that end, we have computed, using the density functional theory (DFT) code GPAW, the latent heat of fusion as well as solved the crystal structure for three zirconium hydride polymorphs: \delta-ZrH1.6, \gamma-ZrH, and \epsilon-ZrH2.

  • 26.
    Blomqvist, Jakob
    et al.
    Malmö högskola, School of Technology (TS).
    Olsson, Pär
    Malmö högskola, School of Technology (TS).
    Steuwer, Axel
    Maimaitiyili, Tuerdi
    Malmö högskola, School of Technology (TS).
    Massih, Ali
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Towards a multiscale understanding of hydride induced embrittlement in zirconium and titanium: theory and experiments2013In: Thermec' 2013 Rio Hotel Las Vegas USA, Abstract book, 2013, article id 140Conference paper (Other academic)
  • 27.
    Ehrlin, Niklas
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Fisk, Martin
    Malmö högskola, Faculty of Technology and Society (TS).
    Cathodic hydrogen charging of Inconel 7182016In: AIMS Materials Science, ISSN 2372-0484, Vol. 3, no 4, p. 1350-1364Article in journal (Refereed)
    Abstract [en]

    The nickel based superalloy IN718 is known to be prone to hydrogen sensitivity, causing degradation of its mechanical properties. Therefore, during mechanical testing of hydrogen charged samples, a well-defined hydrogen distribution is essential to better understand the influence of hydrogen on dislocation movement and plasticity behavior. The possibility of charging cylindrical specimens of IN718 with hydrogen using cathodic charging is investigated. The method is based on an electro-chemical process using a molten salt electrolyte. The resulting hydrogen concentration is measured for various radii, and it is shown that the anisotropic diffusion coefficient resulting from electromigration, inherent in the charging method, must be taken into account as it has a major impact on the charging parameters of IN718. Also, no evidence of degassing during storage is found. Further, changes in surface roughness were examined by SEM, and only limited surface degradation is observed, which is not considered to significantly affect the results.

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  • 28.
    Ehrlin, Niklas
    et al.
    Malmö högskola, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Fisk, Martin
    Malmö högskola, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Flow stress model for hydrogen degraded Inconel 7182017In: ICF 2017 - 14th International Conference on Fracture, Vol 1 / [ed] Emmanuel E. Gdoutos, International Conference on Fracture , 2017, p. 233-234Conference paper (Refereed)
    Abstract [en]

    A mechanism based material model has been developed to simulate hydrogen enhanced localized plasticity for superalloy IN718 in the present work. The model accounts for the increase in mobility of the moving dislocation during plastic deformation, whenever exposed for hydrogen. Two different hydrogen charging methods have been used: cathodic charging with molten salt as electrolyte, and high temperature gas chamber charging. Tensile tests performed at a strain rate of 5×10−5 s-1 show a clear difference between charged samples and uncharged samples. No or small differences could be seen between the two different charging methods. The material model was fitted against the experimental curves using a minimising method alternating fitting parameters. The agreement between the experimental value and the model was good.

  • 29.
    Ehrlin, Niklas
    et al.
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Fisk, Martin
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM). Division of Solid Mechanics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden.
    Bjerkén, Christina
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Flow stress model for hydrogen degraded Inconel 7182018In: Mechanics of materials, ISSN 0167-6636, E-ISSN 1872-7743, Vol. 119, p. 56-64Article in journal (Refereed)
    Abstract [en]

    For life time estimation, it is desirable to capture the lowering of yield strength and premature failure that some alloys exhibits when subjected to hydrogen. For this, a mechanism based material model has been developed to simulate the hydrogen enhanced localized plasticity (HELP) for the superalloy IN718. The model accounts for the increase in mobility of moving dislocations during plastic deformation, whenever hydrogen is present in the material. Tensile tests performed at four different strain rates: 5 x 10(-5), 5 x 10(-4), 5 x 10(-3) and 5 x 10(-2) s(-1) show a difference in yield behaviour between hydrogen pre-charged and uncharged samples. No strain rate dependency of the hydrogen effect could be concluded. Two different hydrogen charging methods have been used: cathodic charging with molten salt as electrolyte, and high temperature gas charging. No differences in the tensile response could be seen between the two different charging methods. The proposed model was fitted against the experimental curves using a minimizing method and model parameters were obtained. Comprising iteratively updated parameters, the model is suited for implementation in finite element software.

  • 30.
    Forslind, B
    et al.
    Karolinska Institutet.
    Wiren, C
    Lund University.
    Malmqvist, KG
    Assessment of Qualitative and Quantitative Data From Pathological Hairs: A Critical-evaluation of Scanning Electron-microscope and Proton-induced X-ray-emission Analyses1987In: Scanning Microscopy, ISSN 0891-7035, Vol. 1, no 1, p. 159-167Article in journal (Refereed)
    Abstract [en]

    Analysis of single hair fibres in genetic disorders is a desirable complement to the clinical diagnosis. The scanning electron microscope (SEM) allows detailed study of the surface morphology of hair fibres which may explain some mechanical characteristics of the pathological hair. Quantitative elemental data may indicate biochemical or metabolic abnormalities. In this preliminary study we assess the feasibility of combining SEM and proton induced X-ray emission (PIXE) analysis on single hair fibres from 12 cases of genetic disease influencing the integument status. We conclude that SEM is a valuable tool in the analysis of hair pathology. The macro-PIXE technique involves some methodological and technical problems which in many cases are likely to be solved by using a proton microbeam. However, this means that routine methods have to be abandoned and careful selection of the material for analysis is an imperative necessity.

  • 31. Havir, Jiri
    et al.
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Ståhle, Per
    Malmö högskola, School of Technology (TS).
    Slack Edges of Paper2006Conference paper (Other (popular science, discussion, etc.))
  • 32.
    Hejman, Ulf
    et al.
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Dissolution driven crack branching in polycarbonate2011In: Fatigue and Fracture of Engineering Materials and Structures, Vol. 34, no 4, p. 227-239Article in journal (Refereed)
    Abstract [en]

    Stress corrosion, in the form of chemically assisted crack growth, in polycarbonate is examined with focus on crack branching characteristics. Cracks with finite width are observed; this is to be expected for dissolution driven cracking. The cracks branched repeatedly and crack widths before and after branching are measured. Both symmetric and asymmetric branching is found. The dissolution rate is assumed to be a linear function of the strain along the crack surface. In the literature, it is proposed that the crack width is proportional to the square of the mode I stress intensity factor. Energy considerations lead to that the sum of branch widths must equal the width of the unbranched crack. The results from the present study correspond fairly well with this assumption. The branching angle is found to be 32º ± 12º, which is in line with results for sharp cracks reported in the literature. The mean growth direction of the branches is found to deviate slightly from the expected straight. No significant correlation between angles and crack widths is found. The scatter in results is mainly addressed to the inherent perturbation sensitivity of stress corrosion cracking. Also numerically simulations of crack branching is performed. These results show promising agreement with the experiments.

  • 33.
    Hejman, Ulf
    et al.
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Environmentally assisted initiation and growth of multiple surface cracks2010In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 47, no 14-15, p. 1838-1846Article in journal (Refereed)
    Abstract [en]

    The initial stages of stress corrosion on an amorphous polymer is investigated. This is done by exposing stressed specimens of polycarbonate to an acetone and water solution. The surface develops two distinct features of degradation that appear on different length scales when subjected to tensile stress. Small pits form on the surface and make it rough. These pits are in the order of micrometers, and are found to be randomly distributed. They occur even without load and seem to slightly increase in number with increasing stress. In the millimeter domain, visible to the bare eye, surface cracks are formed transverse to the direction of loading. The occurrence of cracks is seen to have a positive stress threshold value, exceeding which, a linear increase of number of cracks with stress is found. The manners in which the cracks grow and coalesce on the surface are examined. It is seen that they do not meet crack tip to crack tip. Instead, they avoid each other initially and coalesce crack tip to crack side. The results are discussed in the light of mechanical considerations. A stress analysis for a few configurations of meeting cracks supports the experimental observations. With assumptions of stress corrosion crack growth and coalescence, a simulation of cracks growing from randomly distributed initiation sites is performed. Similar crack patterns as obtained in the experiments are found.

  • 34.
    Hejman, Ulf
    et al.
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Investigation of Branching in Polycarbonate Due to Stress Corrosion2008In: Proceedings 17th European Conference on Fracture, Brno 2008, VUTIUM Brno, Czech Republic , 2008Conference paper (Other academic)
    Abstract [en]

    In this study, experiments have been made to investigate how the width of stress corrosion cracks change when branching occurs. The amount of dissolved material is investigated using and optical microscope. Crack width before and after branching and characteristic crack angels are measured. It was found that the total width of the branches is preserved after branching and a connection to the stress intensity factor at the crack tips is suggested. It is assumed that quasistatic calculations for branched cracks, found in literature, can be applied to these cracks. Experiments have been performed on polycarbonate with acetone as a dissolving agent. Experimental results are compared with numerically simulated results as well as with data found in literature.

  • 35.
    Hejman, Ulf
    et al.
    Malmö högskola, School of Technology (TS).
    Ståhle, Per
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Experiment and simulation of a crack growing by material dissolution2007In: Abstracts 1st Bertram Broberg symposium on mechanics of materials;1, 2007Conference paper (Other academic)
    Abstract [en]

    Crack growth and crack path under a fracture mechanical experiment in polycarbonate is studied. The material, a polycarbonate, is photo elastic witch makes it possible to do an optical stress field analysis. The crack path and crack branching is also predicted by an adaptive finite element analysis. The crack growth is a result of stress induced, chemically activated material dissolution that completely determines the geometry of the specimen, including the shape and length of the crack. In this case, the mechanisms and processes leading to crack growth are assumed to be incorporated in a general material dissolution process. As a consequence of this assumption there is no need for a crack criterion or crack path hypothesis. The lack of crack path criterion imposes an arbitrary length scale where the crack width scales with the square of the applied load but the crack growth rate is independent of the load. Numerical calculations suggest that the crack is capable of branching and that the branches will continue to grow at the same rate as the original crack. The numerical results are verified by the photo elastic experiments and a reasonable agreement is established.

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  • 36.
    Hejman, Ulf
    et al.
    Malmö högskola, School of Technology (TS).
    Ståhle, Per
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Experiment och simulering av materialupplösningsspricka2007In: Abstracts Svenska mekanikdagarna, Luleå, Sweden, Luleå tekniska universitet, 2007Conference paper (Other academic)
    Abstract [sv]

    Sprickväg och sprickgrening studeras under brottmekaniska experiment i polykarbonat. Detta material är spänningsoptiskt och fotoelastiska analyser utförs. Sprickväg och sprickgrening beräknas också med hjälp av en adaptiv finit element-metod. Spricktillväxten är ett resultat av spänningsunderstödd, kemiskt eller biologiskt aktiverad, materialupplösning, som helt bestämmer provkroppens geometri inklusive sprickans form och längd. På så sätt är de processer som leder till sprickpropagering inkluderade i en allmän materialupplösningsprocess. Som en konsekvens av detta behövs varken brottkriterium eller sprickvägshypotes. Den fotoelastiska undersökningen jämförs med de numeriska resultaten. Avsaknaden av brottkriterium gör att sprickpropageringsprocessen saknar längdskala. Därmed följer att sprickans bredd skalar med kvadraten på den pålagda lasten medan spricktillväxthastigheten blir oberoende av pålagd last. Förhållandet gör vidare att sprickan förgrenas och att alla grenar fortsätter att växa utan att förlora fart.

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  • 37.
    Krause, Andreas M.
    et al.
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Olsson, Pär A T
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM). Lund University.
    Music, Denis
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Bjerkén, Christina
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Interstitial diffusion of hydrogen in M7C3 (M=Cr,Mn,Fe)2023In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 218, article id 111940Article in journal (Refereed)
    Abstract [en]

    To increase the understanding of the role of carbide precipitates on the hydrogen embrittlement of martensitic steels, we have performed a density functional theory study on the solution energies and energy barriers for hydrogen diffusion in orthorhombic M7C3 (M = Cr, Mn, Fe). Hydrogen can easily diffuse into the lattice and cause internal stresses or bond weakening, which may promote reduced ductility. Solution energies of hydrogen at different lattice positions have systematically been explored, and the lowest values are -0.28, 0.00, and 0.03 eV/H-atom for Cr7C3, Mn7C3, and Fe7C3, respectively. Energy barriers for the diffusion of hydrogen atoms have been probed with the nudged elastic band method, which shows comparably low barriers for transport via interstitial octahedral sites for all three systems. Analysis of the atomic volume reveals a correlation between low solution energies and energy barriers and atoms with large atomic volumes. Furthermore, it shows that the presence of carbon tends to increase the energy barrier. Our results can explain previous experimental findings of hydrogen located in the bulk of CrC precipitates and provide a solid basis for future design efforts of steels with high strength and commensurable ductility.

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  • 38.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Mean-field strain in zirconium hydride2010Report (Other academic)
    Abstract [en]

    The formation of hydrides in zirconium alloys will cause degradation of the materials. Despite extensive study, the mechanisms and effects of hydrogen embrittlement have remained unclear. In this report, we had presented the calculation results of mean-field strain by different hydride phases in zirconium alloys, like δ-hydride and γ-hydride. The results of δ-hydride are compared with experimental data obtained by X-ray diffraction method by Bai et al.\cite{Baib} and note \cite{Ali} of Massih. The result of δ-hydride is identical with Massih but have some differences with Bai et al. We had also compared our calculated mean-field strain by γ-hydride with δ-hydride. The γ-hydride showed lower value than δ.

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  • 39.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Steuwer, A.
    Wang, Z.
    Daniels, J.
    Andrieux, J.
    Blomqvist, Jakob
    Malmö högskola, Faculty of Technology and Society (TS).
    Zanellato, O.
    In situ observation of gamma-ZrH formation by X-ray diffraction2017In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 695, p. 3124-3130Article in journal (Refereed)
    Abstract [en]

    We report on the measurement of the formation of gamma-ZrH during in situ gaseous charging. The measurements were undertaken using high-energy synchrotron X-ray diffraction. Experimental observation shows that gamma-ZrH can form at 180 degrees C from a mixture of alpha+delta while dehydrogenating at slow cooling rates. The observation is further supported by ex situ laboratory X-ray diffraction on deuterated Zr powder that has undergone a similar heat-treatment cycle. The crystal structure of gamma-ZrH refinement agrees with the reported P4(2)/n structure found in the literature. (C) 2016 Elsevier B.V. All rights reserved.

  • 40.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Blomqvist, Jakob
    Malmö högskola, Faculty of Technology and Society (TS).
    Steuwer, Axel
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Zanellato, Olivier
    Blackmur, Matthew
    Andrieux, Jerome
    Ribeiro, Fabienne
    In Situ Hydrogen Loading on Zirconium Powder2015In: Journal of Synchrotron Radiation, ISSN 0909-0495, E-ISSN 1600-5775, Vol. 22, no 4, p. 995-1000Article in journal (Refereed)
    Abstract [en]

    For the first time, various hydride phases in a zirconium-hydrogen system have been prepared in a high-energy synchrotron X-ray radiation beamline and their transformation behaviour has been studied in situ. First, the formation and dissolution of hydrides in commercially pure zirconium powder were monitored in real time during hydrogenation and dehydrogenation, then whole pattern crystal structure analysis such as Rietveld and Pawley refinements were performed. All commonly reported low-pressure phases presented in the Zr-H phase diagram are obtained from a single experimental arrangement.

  • 41.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Blomqvist, Jakob
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Steuwer, Axel
    Malmö högskola, School of Technology (TS).
    Blackmur, Matthew
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Safdar, Adnan
    Residual stress and hydrogen effect on Ti-6Al-4V alloys produced by Electron Beam Melting2013Conference paper (Other academic)
    Abstract [en]

    There is an internal, self-balanced stress known as residual stress (RS) that exists in all alloy systems without any external applied forces. Depending on the compressive or tensile nature and magnitude of the RS, it significantly affects the mechanical properties of the materials. Therefore, it is crucial to know the nature and magnitude of RS in material for safe and economical operation. In this work, we used unique, multipurpose, high energy (50-150 KeV) beamline I12-JEEP (Joint Engineering, Environment and Processing) at Diamond Light Source in UK with Energy Dispersive X-Ray Diffraction (EDXRD) setup to map up the RS states in one of the most popular titanium alloys with a code name Ti6Al4V. This type of titanium alloy is widely used in biomedical and aerospace industry because of their excellent combination of a high strength/weight ratio and good corrosion resistance. The Ti6Al4V which we were investigated are produced using electron beam melting (EBM) technique as a function of EBM processing parameters. In addition to relation between RS and processing parameters of EBM, the hydride formation versus processing parameters and as well as the effect of residual stress to the hydride precipitation in EBM built Ti6Al4V were investigated. To find out the effect of EBM processing parameters to the residual stress development, various samples produced with different beam size, scanning speed and different building thickness were investigated. From each type of the sample four specimens were prepared and three of them loaded with hydrogen in different concentrations, i.e. the hydrogen concentration of the various samples are 262, 772, 951 and 1410 ppm. Other than these alloy samples we also measured clean and hydrided original powder samples which are used for make these solid samples in our studies. After data collection, the whole pattern fitting method Rietveld and Pawely were performed with structure analysis software package Topas-Academic and GSAS. .

  • 42.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Blomqvist, Jakob
    Malmö högskola, Faculty of Technology and Society (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Steuwer, Axel
    Zanellato, Oliver
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Hölzel, M
    Exact crystal structure of the zirconium hydride determined by neutron powder diffraction2012Conference paper (Other academic)
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  • 43.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Steuwer, A.
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Blomqvist, Jakob
    Malmö högskola, Faculty of Technology and Society (TS).
    Hoelzel, M.
    Ion, John Christopher
    Malmö högskola, Faculty of Technology and Society (TS).
    Zanellato, O.
    The preparation of Zr-deuteride and phase stability studies of the Zr-D system2017In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 485, p. 243-252Article in journal (Refereed)
    Abstract [en]

    Deuteride phases in the zirconium-deuterium system in the temperature range 25-286 degrees C have been studied in-situ by high resolution neutron diffraction. The study primarily focused on observations of delta ->gamma transformation at 180 degrees C, and the peritectoid reaction alpha + delta <-> gamma at 255 degrees C in commetcial grade Zr powder that was deuterated to a deuterium/Zr ratio of one to one. A detailed description of the zirconium deuteride preparation route by high temperature gas loading is also described. The lattice parameters of alpha-Zr, delta-ZrDx and epsilon-ZrDx were determined by whole pattern crystal structure analysis, using Rietveld and Pawley refinements, and are in good agreement with values reported in the literature. The controversial gamma-hydride phase was observed both in-situ and ex-situ in deuterated Zr powder after a heat treatment at 286 degrees C and slow cooling. (C) 2017 Elsevier B.V. All rights reserved.

  • 44.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Steuwer, A.
    Blomqvist, Jakob
    Malmö högskola, Faculty of Technology and Society (TS).
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Blackmur, M. S.
    Zanellato, O.
    Andrieux, J.
    Ribeiro, F.
    Observation of the δ to ε Zr‐hydride transition by in‐situ synchrotron X‐ray diffraction2016In: Crystal research and technology (1981), ISSN 0232-1300, E-ISSN 1521-4079, Vol. 51, no 11, p. 663-670Article in journal (Refereed)
    Abstract [en]

    We investigate the formation and dissolution of hydrides in commercially pure zirconium powder in-situ using highenergy synchrotron X-ray radiation. Experimental results showed a continuous phase transition between the delta and epsilon zirconium hydride phases with indication of a second order phase transformation.

  • 45.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Steuwer, Axel
    Blomqvist, Jacob
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Exact Crystal Structure of the γ-ZrD/δ-ZrD system2010Report (Other academic)
    Abstract [en]

    Zirconium alloys have a strong affinity for hydrogen which leads to hydrogen pick-up during a corrosion reaction when exposed to water. The hydrogen is readily in solution at higher temperature but precipitates as Zirconium hydrides at ambient temperatures. At least three phases are known to exist at ambient temperature depending on hydrogen concentration and quenching rate. However, some controversy exist regarding the exact nature and stability of the γ-ZrH phase, which is closely related to the δ-ZrH phase through ordering of the hydrogen on tetrahedral sites in the matrix. In this report, we give short introduction to our experimental work at FRM II in September 2010. In this work, we use high-resolution neutron powder diffraction on deuterated samples to re-determine and verify the reported structures which essentially date back to the 1960s [1].

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  • 46.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Steuwer, Axel
    Blomqvist, Jakob
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Blackmur, Matthew
    Zanellato, Oliver
    Andrieux, Jerome
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS). Malmö högskola, Faculty of Culture and Society (KS), Medea.
    Fabienne, Ribeiro
    In-situ hydrogen charging of zirconium powder to study isothermal precipitation of hydrides and determination of Zr-hydride crystal structure2013Conference paper (Refereed)
    Abstract [en]

    Zirconium alloys are widely used in the nuclear industry because of their high strength, good corrosion resistance and low neutron absorption cross-section. However, zirconium has strong affinity for hydrogen which leads to hydrogen concentration build-up over time. It is well known that the formation of hydrides will degrade the material and leads to, for example, delayed hydride cracking during high burn up. Even though zirconium hydrides have been studied for several decades, there still remain some controversies regarding the formation mechanisms, exact crystal structure, and stability of various hydride phases. This study uses high resolution synchrotron radiation as a probing tool to observe the precipitation and dissolution of hydrides in highly pure zirconium powder during in-situ hydrogen charging. The experiment enabled the direct observation of the hydride formation and phase transformations. It, also, provided high quality data for crystal structure determination.

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  • 47.
    Maimaitiyili, Tuerdi
    et al.
    Malmö högskola, Faculty of Technology and Society (TS).
    Woracek, R.
    ESS AB, Lund, Sweden.
    Bjerkén, Christina
    Malmö högskola, Faculty of Technology and Society (TS).
    Strobl, M.
    ESS AB, Lund, Sweden.
    Schäfer, N.
    Helmholtz-Zentrum Berlin, Germany.
    Fracture mechanical studies of additive manufactured Ti6Al4V by synchrotron X-ray diffraction2017In: ICF 2017 - 14th International Conference on Fracture, Vol 1 / [ed] Emmanuel E. Gdoutos, International Conference on Fracture , 2017, Vol. 1, p. 253-254Conference paper (Refereed)
    Abstract [en]

    Better understanding of the formation and distribution of hydrides, elastic and plastic strains in deformed polycrystalline, multiphase materials such as digitally manufactured Ti-6Al-4V is important for structural engineering. Polycrystalline Ti-6Al4V alloy samples produced by additive manufacturing techniques called electron beam melting (EBM) have been studied at third generation, high energy synchrotron X-Ray diffraction beam line with energy dispersive X-Ray diffraction setup. The elastic strain in the as built and hydrogenated material determined by extracting unit cell parameters of the existing phases in the system by using whole pattern analysis method Rietveld and Pawley. © 2017 ICF 2017 - 14th International Conference on Fracture. All rights reserved.

  • 48.
    Maimaitiyili, Tuerdi
    et al.
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Woracek, Robin
    Neikter, Magnus
    Boin, Mirko
    Wimpory, Robert C
    Pederson, Robert
    Strobl, Markus
    Drakopoulos, Michael
    Schäfer, Norbert
    Bjerkén, Christina
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Residual Lattice Strain and Phase Distribution in Ti-6Al-4V Produced by Electron Beam Melting2019In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 4, article id 667Article in journal (Refereed)
    Abstract [en]

    Residual stress/strain and microstructure used in additively manufactured material are strongly dependent on process parameter combination. With the aim to better understand and correlate process parameters used in electron beam melting (EBM) of Ti-6Al-4V with resulting phase distributions and residual stress/strains, extensive experimental work has been performed. A large number of polycrystalline Ti-6Al-4V specimens were produced with different optimized EBM process parameter combinations. These specimens were post-sequentially studied by using high-energy X-ray and neutron diffraction. In addition, visible light microscopy, scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) studies were performed and linked to the other findings. Results show that the influence of scan speed and offset focus on resulting residual strain in a fully dense sample was not significant. In contrast to some previous literature, a uniform α- and β-Ti phase distribution was found in all investigated specimens. Furthermore, no strong strain variations along the build direction with respect to the deposition were found. The magnitude of strain in α and β phase show some variations both in the build plane and along the build direction, which seemed to correlate with the size of the primary β grains. However, no relation was found between measured residual strains in α and β phase. Large primary β grains and texture appear to have a strong effect on X-ray based stress results with relatively small beam size, therefore it is suggested to use a large beam for representative bulk measurements and also to consider the prior β grain size in experimental planning, as well as for mathematical modelling.

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  • 49.
    Masic, Alma
    et al.
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Overgaard, Niels
    Malmö högskola, School of Technology (TS).
    Heyden, Anders
    Malmö högskola, School of Technology (TS).
    A Combined Model for Biofilm Development and Biocorrosion2008Conference paper (Other academic)
    Abstract [en]

    We propose a combined model investigating the interplay between a growing biofilm on a metallic surface and environmental suported stress corrosion. The biofilm model incorporates both substrate diffusion, bacteria metabolism and biofilm development, making it possible to estimate the pH at the metallic surface. The biocorrosion model is based on strain driven dissolution. The final model iterates between biofilm development that changes the pH at the surface and biocorrosion that changes the geometry of the substratum.

  • 50.
    Masic, Alma
    et al.
    Malmö högskola, School of Technology (TS).
    Bjerkén, Christina
    Malmö högskola, School of Technology (TS).
    Overgaard, Niels
    Malmö högskola, School of Technology (TS).
    Heyden, Anders
    Malmö högskola, School of Technology (TS).
    Investigation of the Interaction between Biocorrosion and Biofilm Development based on the pH Profile2008Conference paper (Other academic)
    Abstract [en]

    We propose a combined model for investigating the interplay between a growing biofilm on a metallic surface and stress corrosion. The biofilm model incorporates both substrate dissolution, bacteria metabolism and biofilm development, making it possible to estimate the pH at the metallic surface. The biofilm model is base on the well-known Wanner-Gujer model, cf. [1] for biofilm development and applied to a metabolism involving pH, cf. [7,8,9]. Including pH in the metabolism makes it possible to estimate the pH at the substratum as a function of the biofilm thickness, which then can be provided as input to the biocorrosion model.

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    FULLTEXT01
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