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  • 1.
    Asgharzadeh, Mohammadali
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    A model for precipitation strengthening accounting for variations of particlesize and spacing based on strain gradient plasticity in 3D2018Manuskript (preprint) (Övrigt vetenskapligt)
  • 2.
    Asgharzadeh, Mohammadali
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    A shearing/looping transition model for precipitation strengthening2018Manuskript (preprint) (Övrigt vetenskapligt)
  • 3.
    Asgharzadeh, Mohammadali
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Strengthening effects of particle-matrix interaction analyzed by anaxi-symmetric model based on strain gradient plasticity2018Manuskript (preprint) (Övrigt vetenskapligt)
  • 4.
    Asgharzadeh, Mohammadali
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Dahlberg, Carl F. O.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    A 3D model for the analysis of plastic flow properties of randomly-distributed particles2018Manuskript (preprint) (Övrigt vetenskapligt)
  • 5.
    Asgharzadeh, Mohammadali
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Dahlberg, Carl F. O.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    A 3D model for the analysis of plastic flow properties ofrandomly-distributed particlesManuskript (preprint) (Övrigt vetenskapligt)
  • 6. Barsoum, I.
    et al.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Pingle, S.
    The effect of stress state on ductility in the moderate stress triaxiality regime of medium and high strength steels2012Ingår i: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 65, nr 1, s. 203-212Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Experiments on double notched tube specimens subjected to tension and torsion were conducted by Barsoum and Faleskog (2007) [8,9]. In this study a complementary experimental investigation was conducted on tensile round circumferentially notched bar specimens. The results from the current study were compared with the results from the double circumferentially notched tube specimens with stress triaxiality larger than 0.7 in order to asses the influence of the Lode parameter on ductility in the moderate stress triaxiality regime. The effective plastic strain, the stress triaxiality T and the Lode parameter L were determined at the center of the notch up to the point of onset of failure by means of finite element. The influence of the Lode parameter on the failure strain was significant for the high strength and low hardening material, whereas for the medium strength and high hardening material the influence of the Lode parameter was less distinguished. The experimental results were then analyzed with the micromechanical model proposed by Barsoum and Faleskog (2011) [15], which is based on the assumption that ductile failure is a consequence of that plastic deformation localizes into a band. The band consists of a square array of equally sized cells, with a spherical void located in the center of each cell, which allows for studying a single 3D unit cell with fully periodic boundary conditions. The unit cell is subjected to a proportional loading such that it resembles the stress state, in terms of T and L, from the experiments. The micromechanical model captures the experimental trend and the influence of L on ductility very well. It is found that the Lode parameter sensitivity increases by the combination of increase in the yield strength and decrease in strain hardening. The fractographical analysis reveals that this Lode parameter sensitivity is associated with the failure characteristics of the material.

  • 7. Barsoum, I.
    et al.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Pingle, S.
    The influence of the lode parameter on ductile failure strain in steel2011Ingår i: ICM11, 2011, s. 69-75Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this study an experimental investigation was conducted on tensile round circumferentially notched bar specimens. The results were compared to the experimental result on double notched tube specimens subjected to tension and torsion conducted in [5]. The comparison was done for moderate stress triaxiality levels larger than 0.7 with the objective to assess the influence of the Lode parameter on the ductile failure strain. The effective plastic strain, the stress triaxiality T and the Lode parameter L were determined at the center of the notch up to the point of failure by means of finite element based on J2-plasticity. The influence of the Lode parameter on the failure strain was remarkable for the high strength and low hardening material, whereas for the medium strength and high hardening material the influence of the Lode parameter was less prominent. The experimental results were then analyzed with the micromechanical model proposed in [6-7] which is based on the assumption that ductile failure is a consequence of that plastic deformation localizing into a band of imperfections. It is found that the micromechanical model captures the experimental trend and thus the influence of L on the ductility very well. It is found that the Lode parameter sensitivity increases with increase in the yield strength. The fractographical analysis reveals that Lode parameter sensitivity is associated with the failure characteristics of the material.

  • 8.
    Barsoum, Imad
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Influence of the Lode parameter on ductility in the moderate stress triaxiality regime2008Rapport (Övrigt vetenskapligt)
  • 9.
    Barsoum, Imad
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Micromechanical analysis of rupture mechanisms in mixed mode ductile fracture2006Ingår i: Fracture of Nano and Engineering Materials and Structures - Proceedings of the 16th European Conference of Fracture, Kluwer Academic Publishers, 2006, s. 1169-1170Konferensbidrag (Refereegranskat)
  • 10.
    Barsoum, Imad
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Micromechanical analysis on the influence of the Lode parameter on void growth and coalescence2008Rapport (Övrigt vetenskapligt)
  • 11. Barsoum, Imad
    et al.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Micromechanical analysis on the influence of the Lode parameter on void growth and coalescence2011Ingår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 48, nr 6, s. 925-938Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A micromechanical model consisting of a band with a square array of equally sized cells, with a spherical void located in each cell, is developed. The band is allowed a certain inclination and the periodic arrangement of the cells allow the study of a single unit cell for which fully periodic boundary conditions are applied. The model is based on the theoretical framework of plastic localization and is in essence the micromechanical model by Barsoum and Faleskog (Barsoum, I., Faleskog, J., 2007. Rupture mechanisms in combined tension and shear-micromechanics. International Journal of Solids and Structures 44(17), 5481-5498) with the extension accounting for the band orientation. The effect of band inclination is significant on the strain to localization and cannot be disregarded. The macroscopic stress state is characterized by the stress triaxiality and the Lode parameter. The model is used to investigate the influence of the stress state on void growth and coalescence. It is found that the Lode parameter exerts a strong influence on the void shape evolution and void growth rate as well as the localized deformation behavior. At high stress triaxiality level the influence of the Lode parameter is not as marked and the overall ductility is set by the stress triaxiality. For a dominating shear stress state localization into a band cannot be regarded as a void coalescence criterion predicting material failure. A coalescence criterion operative at dominating shear stress state is needed.

  • 12.
    Barsoum, Imad
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Rupture mechanisms in combined tension and shear - Experiments2007Ingår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 44, nr 6, s. 1768-1786Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An experimental investigation of the rupture mechanisms in a mid-strength and a high-strength steel were conducted employing a novel test configuration. The specimen used was a double notched tube specimen loaded in combined tension and torsion at a fixed ratio. The effective plastic strain, the stress triaxiality and the Lode parameter were determined in the centre of the notch at failure. Scanning electron microscopy of the fractured surfaces revealed two distinctively different ductile rupture mechanisms depending on the stress state. At high stress triaxiality the fractured surfaces were covered with large and deep dimples, suggesting that growth and internal necking of voids being the governing rupture mechanism. At low triaxiality it was found that the fractured surfaces were covered with elongated small shear dimples, suggesting internal void shearing being the governing rupture mechanism. In the fractured surfaces of the high-strength steel, regions with quasi-cleavage were also observed. The transition from the internal necking mechanism to the internal shearing mechanism was accompanied by a significant drop in ductility.

  • 13.
    Barsoum, Imad
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Rupture mechanisms in combined tension and shear - Micromechanics2007Ingår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 44, nr 17, s. 5481-5498Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A micromechanics model based on the theoretical framework of plastic localization into a band introduced by Rice is developed. The model consists of a planar band with a square array of equally sized cells, with a spherical void located in the centre of each cell. The periodic arrangement of the cells allows the study of a single unit cell for which fully periodic boundary conditions are applied. The micromechanics model is applied to analyze failure by ductile rupture in experiments on double notched tube specimens subjected to combined tension and torsion carried out by the present authors. The stress state is characterized in terms of the stress triaxiality and the Lode parameter. Two rupture mechanisms can be identified, void coalescence by internal necking at high triaxiality and void coalescence by internal shearing at low triaxiality. For the internal necking mechanism, failure is assumed to occur when the deformation localizes into a planar band and is closely associated with extensive void growth until impingement of voids. For the internal shearing mechanism, a simple criterion based on the attainment of a critical value of shear deformation is utilized. The two failure criteria capture the transition between the two rupture mechanisms successfully and are in good agreement with the experimental result.

  • 14. Bolinder, T.
    et al.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Evaluation of the influence of residual stresses on ductile fracture2015Ingår i: Journal of Pressure Vessel Technology-Transactions of the ASME, ISSN 0094-9930, E-ISSN 1528-8978, Vol. 137, nr 6, artikel-id 061408Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this work, the significance of residual stresses on ductile fracture is investigated by a set of experiments that are analyzed by finite element simulations. The treatment of residual stresses as expressed in fracture assessment procedures such as R6 is believed to be very conservative for ductile materials, when fracture occurs at high primary loads. Earlier numerical studies have reinforced this belief. This is supported in the current study. Tests on notched 3PB specimens with and without residual stresses were conducted on two ferritic steels. The residual stresses were introduced by applying a compressive preload on notched specimens. The tests were designed to achieve crack initiation at load levels around the plastic limit load. The crack growth in the tests was measured by a compliance method and by color marking of the crack surface. The crack tip driving force J was evaluated numerically for specimens with and without residual stresses. The experimental results show that the residual stresses clearly contribute to J at low primary loads. However, this contribution diminishes as the primary loads increase. The experimental results were also compared with results evaluated using the R6 procedure. These comparisons revealed overly high conservatism in R6 for cases with residual stresses compared to the ones for cases without residual stresses where less conservatism was evident.

  • 15. Bolinder, T.
    et al.
    Sattari-Far, I.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Evaluation of the influence of residual stresses on ductile fracture2011Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this work the significance of residual stresses for ductile fracture was investigated. The treatment of residual stresses as expressed in fracture assessment procedures such as the R6 method is believed to be very conservative for ductile materials, when fracture occurs at high primary loads. Earlier numerical studies have reinforced this belief. Tests on notched 3PB specimens with and without residual stresses were conducted on two ferritic steels. The residual stresses were introduced by applying a compressive pre-load on notched specimens. The tests were designed to achieve crack initiation at load levels around the limit load. The crack growth in the tests was measured by a compliance method and by colour marking of the crack surface. The crack-tip driving force J was evaluated numerically for specimens with and without residual stresses. The experimental results show that the residual stresses clearly contribute to J at low primary loads. However, this contribution diminishes as the primary loads increase. The experimental results were also compared with results evaluated using the R6 procedure. These comparisons revealed an overly high conservativeness in R6 for cases with residual stresses compared to the conservativeness for cases without residual stresses.

  • 16.
    Bonnaud, Etienne L.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Explicit, Fully Implicit and Forward Gradient Numerical Integration of a Hyperelasto-Viscoplastic Constitutive Model for Amorphous Polymers Undergoing Finite Deformation2010Rapport (Övrigt vetenskapligt)
    Abstract [en]

    Following the growing use of amorphous polymers in an expanding range of applications, interest for polymer mechanical modeling has greatly increased. Together with reliable constitutive models, stable, accurate and rapid integration algorithms valid for large deformations need to be developed. Here, in a framework of hyperelasto-viscoplasticity and multiplicative split formulation, three integration algorithms (explicit, fully implicit and forward gradient) are applied to the polymer model developed by Anand and Gurtin (2003) and respective stability is investigated. The explicit and fully implicit algorithms were furthermore implemented in a commercial Finite Element code and simulations of a simple tensile test are shown to capture the actual deformation behavior of polymers.

  • 17.
    Boåsen, Magnus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Stec, Mateusz
    Efsing, Pål
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    A generalized probabilistic model for cleavage fracture with a length scale - Influence of stress state and application to surface cracked experiments2019Rapport (Övrigt vetenskapligt)
    Abstract [en]

    A probabilistic model for the cumulative probability of failure by cleavage fracture with a material related length scale is further developed in this study. A new generalized effective stress measure is proposed, based on a normal stress decomposition of the stress tensor, capable of describing a state of normal stress in the range from the mean stress to the maximum principal stress. The effective stress measure associated with a material point is evaluated from the stress tensor averaged over the material related length scale. The model is shown to be well capable to predict both the fracture toughness at loss of both in-plane and out-of-plane constraint by model application to two different datasets from the open literature. The model is also shown to be well capable of predicting the probability of failure of discriminating experiments on specimens containing semi-elliptic surface cracks. A comparison where the master curve methodology is used to predict the probability of failure of the experiments is also included.

  • 18.
    Boåsen, Magnus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Stee, Mateusz
    Swedish Def Res Agcy FOI, SE-16490 Stockholm, Sweden..
    Efsing, Pål
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    A generalized probabilistic model for cleavage fracture with a length scale - Influence of stress state and application to surface cracked experiments2019Ingår i: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 214, s. 590-608Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A probabilistic model for the cumulative probability of failure by cleavage fracture with a material related length scale is further developed in this study. A new generalized effective stress measure is proposed, based on a normal stress decomposition of the stress tensor, capable of describing a state of normal stress in the range from the mean stress to the maximum principal stress. The effective stress measure associated with a material point is evaluated from the stress tensor averaged over the material related length scale. The model is shown to be well capable to predict both the fracture toughness at loss of both in-plane and out-of-plane constraint by model application to two different datasets from the open literature. The model is also shown to be well capable of predicting the probability of failure of discriminating experiments on specimens containing semi-elliptic surface cracks. A comparison where the master curve methodology is used to predict the probability of failure of the experiments is also included.

  • 19.
    Bremberg, Daniel
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    A numerical procedure for interaction integrals developed for curved cracks of general shape in 3-D2015Ingår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 62, s. 144-157Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This study presents a numerical procedure for the evaluation of interaction energy integrals used to extract mixed-mode stress intensity factors. The interaction energy integral is expressed as a domain integral, and the proposed numerical procedure delivers accurate results for three-dimensional cracks with curved crack fronts and curved crack surfaces for a rather general set of integration domains. It is clearly shown that, when the curvature of the crack surface becomes sufficiently large, special care must be taken in the evaluation of both the volume and the area integrals involved. To improve the accuracy in the evaluation of the former, a composite rule for the Gaussian quadrature scheme is employed. Four benchmark geometries with available analytical solutions are considered. Firstly, mesh design parameters for planar cracks with straight and curved crack fronts are established. Secondly, non-planar cracks with straight and curved crack fronts are employed to examine the accuracy of the numerical procedure.

  • 20.
    Bremberg, Daniel
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Faleskog, Jonas
    Mesh design for direct and indirect evaluation of mixed mode stress intensity factors in three dimensionsRapport (Övrigt vetenskapligt)
  • 21. Daehli, L. E. B.
    et al.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Borvik, T.
    Hopperstad, O. S.
    Unit cell simulations and porous plasticity modelling for recrystallization textures in aluminium alloys2016Ingår i: 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), Elsevier, 2016, s. 2535-2542Konferensbidrag (Refereegranskat)
    Abstract [en]

    The well-known Gurson model has been heuristically extended to incorporate effects of matrix anisotropy on the macroscopic yielding of porous ductile solids. Typical components of recrystallization textures for aluminium alloys were used to calibrate the Barlat Y1d2004-18p yield criterion using a full-constraint Taylor homogenization method. The resulting yield surfaces were further employed in unit cell simulations using the finite element method. Unit cell calculations are invoked to investigate the evolution of the approximated microstructure under pre-defined loading conditions and to calibrate the proposed porous plasticity model. Numerical results obtained from the unit cell analyses demonstrate that anisotropic plastic yielding has great impact on the mechanical response of the approximated microstructure. Despite the simplifying assumptions that underlie the proposed constitutive model, it seems to capture the overall macroscopic response of the unit cell. However, to further enhance the numerical predictions, the model should be supplemented with a void evolution expression that accounts for directional dependency, and a void coalescence criterion in order to capture the last stages of deformation.

  • 22. Daehli, Lars Edvard Bryhni
    et al.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Borvik, Tore
    Hopperstad, Odd Sture
    Unit cell simulations and porous plasticity modelling for strongly anisotropic FCC metals2017Ingår i: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 65, s. 360-383Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The macroscopic behaviour of anisotropic porous solids made from an aggregate of spherical voids embedded in a plastically anisotropic matrix material is investigated by means of unit cell simulations. Plastic yielding of the polycrystalline matrix is governed by the anisotropic yield criterion Yld2004-18p. Generic texture components for face-centred cubic crystals resembling those that typically emerge during rolling and annealing processes are applied in the study. A numerical method for systematic prescription of external stress states is presented and employed in the unit cell calculations. To preclude shear effects in the unit cell model, the material symmetry axes are restricted to coincide with the principal stress directions. This excludes the possibility to properly study the ductile failure mechanism and the current work is thus mainly concerned with the void growth phase. Various stress states ranging from biaxial tension to highly constrained regions in the vicinity of crack tips are employed in the study. The numerical results demonstrate that the matrix anisotropy has a marked effect on the unit cell response, both in terms of void growth and stress-strain curves. Furthermore, the void shape evolves quite differently depending upon the direction of the major principal stress relative to the material axes. A heuristic extension of the Gurson model that incorporates matrix plastic anisotropy is presented and subsequently used to describe the constitutive behaviour of the porous ductile solid. Numerical data from the unit cell analyses are used as target curves in the calibration process of the porous plasticity model. A sequential least-square optimization procedure is invoked to minimize the overall discrepancy between the unit cell calculations and the homogenized response of the plastically anisotropic porous solid for all the imposed stress states. The anisotropic porous plasticity model demonstrates predictive capabilities for the range of stress states covered in this study.

  • 23.
    Dahlberg, Carl F. O.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    An improved strain gradient plasticity formulation with energetic interfaces: theory and a fully implicit finite element formulation2013Ingår i: Computational Mechanics, ISSN 0178-7675, E-ISSN 1432-0924, Vol. 51, nr 5, s. 641-659Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A fully implicit backward-Euler implementation of a higher order strain gradient plasticity theory is presented. A tangent operator consistent with the numerical update procedure is given. The implemented theory is a dissipative bulk formulation with energetic contribution from internal interface to model the behavior of material interfaces at small length scales. The implementation is tested by solving some examples that specifically highlight the numerics and the effect of using the energetic interfaces as higher order boundary conditions. Specifically, it is demonstrated that the energetic interface formulation is able to mimic a wide range of plastic strain conditions at internal boundaries. It is also shown that delayed micro-hard conditions may arise under certain circumstances such that an interface at first offers little constraints on plastic flow, but with increasing plastic deformation will develop and become a barrier to dislocation motion.

  • 24.
    Dahlberg, Carl F. O.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Hållfasthetslära (Avd.).
    Energetic interfaces and boundary sliding in strain gradient plasticity; investigation using an adaptive implicit finite element methodArtikel i tidskrift (Refereegranskat)
  • 25.
    Dahlberg, Carl F. O.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Strain gradient plasticity analysis of the influence of grain size and distribution on the yield strength in polycrystals2014Ingår i: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 44, s. 1-16Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Plane strain models of polycrystalline microstructures are investigated using strain gradient plasticity (SGP) and a grain boundary (GB) deformation mechanism. The microstructures are constructed using a non-linear constrained Voronoi tessellation so that they conform to a log-normal distribution in grain size. The SGP framework is used to model the grain size dependent strengthening and the GB deformation results in a cut-off of this trend below a certain critical grain size. Plastic strain field localization is discussed in relation to the non-local effects introduced by SGP and a material length scale. A modification of the Hall-Petch relation that accounts for, not only the mean grain size, but also the statistical size variation in a population of grains is proposed.

  • 26.
    Dahlberg, Carl F. O.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Niordson, Christian F.
    Legarth, Brian Nyvang
    A deformation mechanism map for polycrystals modeled using strain gradient plasticity and interfaces that slide and separate2013Ingår i: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 43, s. 177-195Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Small scale strain gradient plasticity is coupled with a model of grain boundaries that take into account the energetic state of a plastically strained boundary and the slip and separation between neighboring grains. A microstructure of hexagonal grains is investigated using a plane strain finite element model. The results show that three different microstructural deformation mechanisms can be identified. The standard plasticity case in which the material behaves as expected from coarse grained experiments, the nonlocal plasticity region where size of the microstructure compared to some intrinsic length scale enhances the yield stress and a third mechanism, active only in very fine grained microstructures, where the grains deform mainly in relative sliding and separation.

  • 27.
    Faleskog, Jonas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Barsoum, Imad
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Micromechanics of rupture in combined tension and shear2007Ingår i: Mechanical Behavior of Materials X, Pts 1and 2 / [ed] Nam, SW; Chang, YW; Lee, SB; Kim, NJ, STAFA-ZURICH: TRANS TECH PUBLICATIONS LTD , 2007, Vol. 345-346, s. 681-684Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    A micromechanics model based on the theoretical framework of plastic localization into a band introduced by Rice [1] is developed. The model employed consists of a planar band with a square array of equally sized cells, with a spherical void located in the centre of each cell. The micromechanics model is applied to analyze the rupture mechanisms associated with mixed mode ductile fracture. The stress state is characterized by the stress triaxiality T and the Lode parameter mu which adequately describe the stress state ahead of a crack tip under mixed mode loading of an isotropic elasto-plastic material. The main focus is the influence of mu on void growth and coalescence behavior. It is shown that the Lode parameter exerts a strong influence upon this behavior.

  • 28.
    Faleskog, Jonas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Barsoum, Imad
    Tension-torsion fracture experiments-Part I: Experiments and a procedure to evaluate the equivalent plastic strain2013Ingår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 50, nr 25-26, s. 4241-4257Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ductile failure experiments on a double notched tube (DNT) specimen subjected to a combination of ten-sue load and torque that was applied at a fixed ratio is presented. The experimental results extend those in Barsoum and Faleskog (2007a) down to zero stress triaxiality. A new and robust evaluation procedure for such tests is proposed, and a simple relation for the equivalent plastic strain at failure for combined normal and shear deformation, respectively, is developed. Tests were carried out on the medium strength medium hardening steel Weldox 420, and the high strength low hardening steel Weldox 960. The experimental results unanimously show that ductile failure not only depends on stress triaxiality, but is also strongly affected by the type of deviatoric stress state that prevails, which can be quantified by a stress invariant that discriminates between axisymmetric stressing and shear dominated stressing, e.g., the Lode parameter. Additional experiments on round notch bar (RNB) specimens are recapitulated in order to give a comprehensive account on how ductile failure depends on stress triaxiality, ranging from zero to more than 1.6, and the type of stress state for the two materials tested. This provides an extensive experimental data base that will be used to explore an extension of the Gurson model that incorporates damage development in shear presented in Xue et al.

  • 29.
    Faleskog, Jonas
    et al.
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Kroon, Martin
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Öberg, Hans
    KTH, Tidigare Institutioner, Hållfasthetslära.
    A probabilistic model for cleavage fracture with a length scale - parameter estimation and predictions of stationary crack experiments2004Ingår i: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 71, nr 1, s. 57-79Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This study presents a large experimental investigation in the transition temperature region on a modified A508 steel. Tests were carried out on single-edge-notch-bend specimens with three different crack depth over specimen width ratios to capture the strong constraint effect on fracture toughness. Three test temperatures were considered, covering a range of 85 degreesC. All specimens failed by cleavage fracture prior to ductile tearing. A recently proposed probabilistic model for the cumulative failure by cleavage was applied to the comprehensive sets of experimental data. This modified weakest link model incorporates a length scale, which together with a threshold stress reduce the scatter in predicted toughness distributions as well as introduces a fracture toughness threshold value. Model parameters were estimated by a robust procedure, which is crucial in applications of probabilistic models to real structures. The conformity between predicted and experimental toughness distributions, respectively, were notable at all the test temperatures.

  • 30.
    Faleskog, Jonas
    et al.
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Nilsson, Fred L.
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Conditional failure probabilities in weakest link modelling2004Ingår i: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 125, nr 04-mar, s. 349-351Artikel i tidskrift (Refereegranskat)
  • 31.
    Faleskog, Jonas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Stec, Mateusz
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Micromechanics and probabilistic modeling of cleavage microcrack nucleation and growth caused by particle cracking2008Ingår i: Journal of the mechanics and physics of solids, ISSN 0022-5096, E-ISSN 1873-4782Artikel i tidskrift (Övrigt vetenskapligt)
  • 32. Guo, T. F.
    et al.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Shih, C. F.
    Continuum modeling of a porous solid with pressure-sensitive dilatant matrix2008Ingår i: Journal of the mechanics and physics of solids, ISSN 0022-5096, E-ISSN 1873-4782, Vol. 56, nr 6, s. 2188-2212Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The pressure-sensitive plastic response of a material has been Studied in terms of the intrinsic sensitivity of its yield stress to pressure and the presence and growth of cavities. This work focuses on the interplay between these two distinctly different mechanisms and the attendant material behavior. To this end, a constitutive model is proposed taking both mechanisms into account. Using Gurson's homogenization, an tipper bound model is developed for a voided solid with a plastically dilatant matrix material. This model is built around a three-parameter axisymmetric velocity field for a unit sphere containing a spherical void. The void is also subjected to internal pressure; this can be relevant for polymeric adhesives permeated by moisture that vaporizes at elevated temperatures. The plastic response of the matrix material is described by Drucker-Prager's yield criterion and an associated flow rule. The resulting yield surface and porosity evolution law of the homogenized constitutive model are presented in parametric form. Using the solutions to special cases as building blocks, approximate models with explicit forms are proposed. The parametric form and an approximate explicit form are compared against full-field Solutions obtained from finite element analysis. They are also studied for loading under generalized tension conditions. These computational simulations shed light on the interplay between the two mechanisms and its enhanced effect on yield strength and plastic flow. Among other things, the tensile yield strength of the porous solid is greatly reduced by the internal void pressure, particularly when a liquid/vapor phase is the source of the internal pressure.

  • 33.
    Kroon, Martin
    et al.
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Faleskog, Jonas
    KTH, Tidigare Institutioner, Hållfasthetslära.
    A probabilistic model for cleavage fracture with a length scale-influence of material parameters and constraint2002Ingår i: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 118, nr 2, s. 99-118Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A probabilistic model for the cumulative probability of failure by cleavage fracture with a material related length scale is developed in this study. The model aims at describing the random nature of fracture in ferritic steels in the brittle-to-ductile transition temperature region. The model derives from use of an exponential function to describe the distribution of microstructural entities eligible to take part in the fracture initiation process, where also a dependence on effective plastic strain is incorporated. A nonlocal stress measure, calculated as the average stress in a spherical volume, drives the contribution to failure probability of an infinitesimal material volume. The radius of the spherical volume enters as the material length in this model. This length has a significant influence on failure probability predictions in geometries exposed to strong stress gradients as found ahead of cracks. The material length is associated with a fracture toughness threshold value. In a fracture application three model parameters need to be estimated based on testing; a parameter directly related to the mean fracture toughness, a parameter that primarily is related to crack-tip constraint effects and the material length parameter. The model is explored in a parametric study showing model features in concord with typical features found in toughness distributions from fracture mechanics testing in the transition region.

  • 34.
    Kroon, Martin
    et al.
    KTH, Tidigare Institutioner                               , Hållfasthetslära.
    Faleskog, Jonas
    KTH, Tidigare Institutioner                               , Hållfasthetslära.
    Influence of carbide/ferrite interface and carbide shape on cleavage fracture initiation in ferritic steels2003Rapport (Övrigt vetenskapligt)
  • 35.
    Kroon, Martin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Biomekanik.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Biomekanik.
    Influence of crack deflection into the carbide/ferrite interface on cleavage fracture initiation in ferritic steels2008Ingår i: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 40, nr 9, s. 695-707Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    n this and a companion study (Kroon, M., Faleskog, J., 2005. Micromechanics of cleavage fracture initiation in ferritic steels by carbide cracking. J. Mech. Phys. Solids 53, 171–196), the initiation of cleavage fracture in ferritic steels is studied. The initiation is modelled explicitly in the form of a microcrack, which nucleates in a brittle carbide and propagates into the surrounding ferrite. The carbide is modelled as an elastic cylinder and the ferrite as an elastic viscoplastic material. The crack growth is modelled using a cohesive surface, in which the tractions are governed by a modified exponential cohesive law. The advancing microcrack, which has nucleated in the carbide, may either continue into the ferrite or deflect into the interface between the carbide and the ferrite. Special attention is given to the influence of the mode mixity factor β, which is defined as the ratio between the shear and tensile strength of the interface between the carbide and the ferrite. Crack growth in the interface occurs in shear mode and is driven by a fibre loading mechanism. For mode mixity values β⩽0.2, the crack deflects into the interface. The results indicate that crack growth in the interface can have a profound influence on the macroscopic fracture toughness of ferritic steels.

  • 36.
    Kroon, Martin
    et al.
    KTH, Tidigare Institutioner                               , Hållfasthetslära.
    Faleskog, Jonas
    KTH, Tidigare Institutioner                               , Hållfasthetslära.
    Influence of plastic rate sensitivity on cleavage fracture initiation in ferritic steels2003Rapport (Övrigt vetenskapligt)
  • 37.
    Kroon, Martin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Biomekanik.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Micrornechanics of cleavage fracture initiation in ferritic steels by carbide cracking2005Ingår i: Journal of the mechanics and physics of solids, ISSN 0022-5096, E-ISSN 1873-4782, Vol. 53, nr 1, s. 171-196Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Cleavage fracture in ferritic steels is often initiated in brittle carbides randomly distributed in the material. The carbides break as a result of a fibre loading mechanism in which the stress levels in the carbides are raised, as the surrounding ferrite undergoes plastic deformation. The conditions in the vicinity of the nucleated micro-crack will then determine whether the crack will penetrate or be arrested by the ferrite. The ferrite is able to arrest nucleated cracks through the presence of mobile dislocations, which blunt and shield the microcrack and thus lowers the stresses at the crack tip. Hence, the macroscopic toughness of the material directly depends on the ability of the ferrite to arrest nucleated micro-cracks and in turn on the plastic rate sensitivity of the ferrite. The initiation of cleavage fracture is here modelled explicitly in the form of a micro-crack, which nucleates in a brittle carbide and propagates into the surrounding ferrite. The carbide is modelled as an elastic cylinder or in a few cases an elastic sphere and the ferrite as an elastic viscoplastic material. The crack growth is modelled using a cohesive surface, where the tractions are governed by a modified exponential cohesive law. It is shown that the critical stress, required to propagate a microcrack from a broken carbide, increases with decreasing plastic rate sensitivity of the ferrite. The results also show that a low stress triaxiality and a high aspect ratio of the carbide promote the initiation of cleavage fracture from a broken carbide.

  • 38.
    Kroon, Martin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Numerical implementation of a J(2)- and J(3)-dependent plasticity model based on a spectral decomposition of the stress deviator2013Ingår i: Computational Mechanics, ISSN 0178-7675, E-ISSN 1432-0924, Vol. 52, nr 5, s. 1059-1070Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A new plasticity model with a yield criterion that depends on the second and third invariants of the stress deviator is proposed. The model is intended to bridge the gap between von Mises' and Tresca's yield criteria. An associative flow rule is employed. The proposed model contains one new non-dimensional key material parameter, that quantifies the relative difference in yield strength between uniaxial tension and pure shear. The yield surface is smooth and convex. Material strain hardening can be ascertained by a standard uniaxial tensile test, whereas the new material parameter can be determined by a test in pure shear. A fully implicit backward Euler method is developed and presented for the integration of stresses with a tangent operator consistent with the stress updating scheme. The stress updating method utilizes a spectral decomposition of the deviatoric stress tensor, which leads to a stable and robust updating scheme for a yield surface that exhibits strong and rapidly changing curvature in the synoptic plane. The proposed constitutive theory is implemented in a finite element program, and the influence of the new material parameter is demonstrated in two numerical examples.

  • 39.
    Kroon, Martin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.), Biomekanik.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Öberg, Hans
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    A probabilistic model for cleavage fracture with a length scale - Parameter estimation and predictions of growing crack experiments2008Ingår i: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 75, nr 8, s. 2398-2417Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A probabilistic model for the cumulative probability of failure by cleavage fracture was applied to experimental results where cleavage fracture was preceded by ductile crack growth. The model, introduced by Kroon and Faleskog [Kroon M, Faleskog J. A probabilistic model for cleavage fracture with a length scale - influence of material parameters and constraint. Int J Fract 2002; 118:99-118], includes a non-local stress with an associated material related length scale, and it also includes a strain measure to account for the number of nucleated cleavage initiation sites. The experiments were performed on single edge cracked bend test specimens with three different crack lengths at the temperature 85 degrees C, which is in the upper transition region for the steel in question. The ductile rupture process is modelled using tile cell model for nonlinear fracture mechanics. The original cleavage fracture model had to be modified in order to account for the substantial number of cleavage initiators being consumed by the ductile process. With this modification, the model was able to accurately capture the experimental failure probability distribution.

  • 40.
    Larsson, Joachim
    et al.
    KTH.
    Faleskog, Jonas
    KTH, Tidigare Institutioner, Hållfasthetslära.
    Massih, A. R.
    Analysis of densification and swelling of solids using pressure dependent plasticity criteria2004Ingår i: CMES - Computer Modeling in Engineering & Sciences, ISSN 1526-1492, E-ISSN 1526-1506, Vol. 5, nr 1, s. 73-80Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider certain constitutive laws for analyzing the elastic-plastic behavior of granular material, which is subjected to compressive hydrostatic stresses and concomitantly undergoing swelling. The plastic yield functions for this kind of materials are pressure and porosity dependent. The constitutive laws are formulated in a finite element (FE) framework for applications to structures involving granular/porous materials. We have employed an implicit and unconditionally stable algorithm for numerical integration of the constitutive relations. The numerical method has been programmed in a FE computer code. The code is then used to study a plane strain problem for a range of model parameters and yield criteria, where the stress response of material under compressive loads is evaluated. Finally the method is applied to the case of B4C powder encased in a stainless steel tube, which is subjected to compressive loads under swelling condition.

  • 41.
    Stec, Mateusz
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    A micromechanics based probabilistic model for cleavage fracture: Application to SE(B) experiments2008Ingår i: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315Artikel i tidskrift (Övrigt vetenskapligt)
  • 42.
    Stec, Mateusz
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Influence of grain size on arrest of a dynamically propagating cleavage crack in ferritic steels-micromechanics2009Ingår i: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 158, nr 1, s. 51-71Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Cleavage fracture in ferritic steels is controlled by several critical steps. First a microcrack must nucleate, grow and overcome barriers, such as grain boundaries. The latter is examined here by use of a periodic, axisymmetric model representing two grains. A microcrack nucleated at the center in one grain is driven by a constant remotely applied stress towards the second grain. The cleavage planes of the grain in which the microcrack is nucleated coincide with the principal loading direction. In the adjacent grain, due to misalignment in possible cleavage planes, the propagation direction changes and separation occurs in mixed mode, involving both normal and shear separations. The temperature dependence of the mechanical properties of the material is accounted for by use of a temperature dependent elasto viscoplastic material model. The largest grain size that can arrest a rapidly propagating microcrack is defined as the critical grain size. The effects of stress state and temperature on the critical grain size are examined. The influence of mismatch in lattice orientation between two adjacent grains in terms of a tilt angle is both qualitatively and quantitatively described. It is shown that the critical grain size is influenced by plastic geometry change and prestraining, which depend on the applied stress state. The results also show that a microcrack can be arrested in an adjacent grain under specific conditions.

  • 43.
    Stec, Mateusz
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Micromechanical modeling of grain boundary resistance to cleavage crack propagation in ferritic steels2009Ingår i: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 160, nr 2, s. 151-167Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In ferritic steels a propagating cleavage microcrack changes its propagation direction as it advances from grain to grain. This is due to differences in the orientation of the cleavage planes of two neighboring grains. In order to reach a cleavage plane in a new grain, a microcrack must first penetrate the grain boundary. Grain boundaries therefore act as natural barriers in cleavage fracture. The influence of a grain boundary and the associated misorientation in cleavage planes on crack arrest is here examined using a 3D finite element model with axisymmetric periodicity, representing two grains whose cleavage planes are tilted and twisted relative to each other. The temperature dependent mechanical properties of ferrite are modeled using a temperature dependent viscoplastic response. The development of the crack front as the microcrack penetrates through a grain boundary is here presented. The influence of the twist misorientation on the critical grain size, defined as the largest grain size that can arrest a rapidly propagating microcrack, is examined in a temperature range corresponding to the ductile to brittle transition (DBT) region. It is shown that when both tilt and twist misorientation are present, the influence of tilt and twist, respectively, on crack growth resistance can be decoupled.

  • 44.
    Stec, Mateusz
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Micromechanical modeling of grain boundary resistance to cleavage fracture propagation2006Ingår i: Fracture of Nano and Engineering Materials and Structures - Proceedings of the 16th European Conference of Fracture, Kluwer Academic Publishers, 2006, s. 105-106Konferensbidrag (Refereegranskat)
  • 45.
    Stec, Mateusz
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Micromechanical modeling of grain boundary resistance to cleavage fracture propagation2007Ingår i: Mechanical Behavior of Materials X, Pts 1and 2 / [ed] Nam, SW; Chang, YW; Lee, SB; Kim, NJ, 2007, Vol. 345-346, s. 825-828Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    A micromechanical model representing two adjacent grains is developed. Rapid crack propagation from one grain into another driven by a constant global stress state is simulated. The normal of the crack face in the grain where the micro-crack initiates coincides with the principle loading direction. In the adjacent grain, the propagation direction changes and separation occurs in a mixed way, involving both normal and shear separation. The largest grain size that can arrest a rapidly propagating micro-crack is defined as the critical grain size. The effects of the global stress state and temperature on the critical grain size is examined. The influence of the mismatch in lattice orientation between two neighboring grains is qualitatively described. The influence of temperature is modeled by a temperature dependent viscoplastic response.

  • 46.
    Stec, Mateusz
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Öberg, Hans
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Validation of two probabilistic cleavage fracture models for half elliptic surface crack2008Ingår i: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315Artikel i tidskrift (Övrigt vetenskapligt)
  • 47. Xue, Zhenyu
    et al.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Hutchinson, John W.
    Tension-torsion fracture experiments - Part II: Simulations with the extended Gurson model and a ductile fracture criterion based on plastic strain2013Ingår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 50, nr 25-26, s. 4258-4269Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An extension of the Gurson model that incorporates damage development in shear is used to simulate the tension-torsion test fracture data presented in Faleskog and Barsoum (2013) (Part I) for two steels, Weldox 420 and 960. Two parameters characterize damage in the constitutive model: the effective void volume fraction and a shear damage coefficient. For each of the steels, the initial effective void volume fraction is calibrated against data for fracture of notched round tensile bars and the shear damage coefficient is calibrated against fracture in shear. The calibrated constitutive model reproduces the full range of data in the tension-torsion tests thereby providing a convincing demonstration of the effectiveness of the extended Gurson model. The model reinforces the experiments by highlighting that for ductile alloys the effective plastic strain at fracture cannot be based solely on stress triaxiality. For nominally isotropic alloys, a ductile fracture criterion is proposed for engineering purposes that depends on stress triaxiality and a second stress invariant that discriminates between axisymmetric stressing and shear dominated stressing.

  • 48.
    Zhou, Tao
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Faleskog, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Hållfasthetslära (Inst.).
    Babu, Prasath
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Odqvist, Joakim
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallografi.
    Yu, Hao
    Univ Sci & Technol Beijing, Sch Mat Sci & Engn, Beijing 100083, Peoples R China..
    Hedström, Peter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Exploring the relationship between the microstructure and strength of fresh and tempered martensite in a maraging stainless steel Fe-15Cr-5Ni2019Ingår i: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 745, s. 420-428, artikel-id DELL AJ, 1985, METALLURGICAL TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, V16, P2131Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Hierarchical microstructure engineering is an efficient design path for ultra-high strength steels. An excellent example of this is maraging stainless steel, which achieves its high-performance by combining the hierarchic martensitic microstructure and nano-sized precipitates. Relating this complex microstructure with mechanical properties, e.g. strength, is not trivial. In the present work, we therefore explore the relationship between the hierarchic microstructure, evolving with the tempering of a Cu-containing maraging stainless steel 15-5 PH, and its strength. Comprehensive microstructure characterization, including the quantification of dislocation density, effective grain size, precipitates and retained austenite fraction is performed after quenching and tempering at 500 degrees C. The microstructure data is subsequently used as input for assessing the evolution of individual strength contributions and thus the increase in strength of tempered martensite contributed by Cu precipitation strengthening is evaluated. It is found that the Cu precipitation and dislocation annihilation are two major factors controlling the evolution of the yield strength of the tempered martensite. The Cu precipitation strengthening is also modelled using our previous Langer-Schwartz-Kampmann-Wagner model based predictions of the Cu precipitation, and modelled precipitation strengthening is compared with the evaluated Cu precipitation strengthening from the experiments. The work exemplifies the promising approach of combining physically based precipitation modelling and precipitation-strengthening modelling for alloy design and optimization. However, more work is needed to develop a generic predictive framework.

1 - 48 av 48
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