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  • 1.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Linares Arregui, Irene
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Hazar, Selcuk
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Numerical analysis of plasticity effects on fatigue growth of a short crack in a bainitic high strength bearing steel2016In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 92, p. 36-51Article in journal (Refereed)
    Abstract [en]

    Plasticity effects on fatigue growth were simulated for a physically short crack. The material description comprised the Drucker-Prager yield surface, non-associated flow rule and non-linear combined hardening. The simulated development of the growth limiting parameter agreed with the experimental crack behaviour with early rapid propagation followed by a transition to slow R-controlled growth. The crack was open to the tip without any crack face closure throughout all load cycles. Instead compressive residual stresses developed at the unloaded tip which supplied an explanation to the slow rate of the propagated short crack in this bainitic high strength bearing steel. The material's strength differential effect was the key difference explaining why compressive residual stresses instead of crack face closure was responsible for the short crack effect in this material.

  • 2.
    Hazar, Selcuk
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Lai, J.
    Mechanical modeling of coupled plasticity and phase transformation effects in a martensitic high strength bearing steel2018In: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 117, p. 41-57Article in journal (Refereed)
    Abstract [en]

    The stress and strain induced solid to solid phase transformation of retained austenite in a martensitic high strength bearing steel has been studied. Monotonic tension experiments that were carried out at different temperatures using this high strength steel showed that not only the strain induced but also the stress induced phase change plays a crucial role in the phase transformation of retained austenite to martensite. In the material model, plastic deformation was defined using the Drucker Prager yield surface through a nonassociated flow rule accompanied by nonlinear kinematic and isotropic hardening. The hardening was coupled with stress and strain induced phase transformations. A nonlinear elastic effect based on elastic dilation was included in the constitutive model by extending the bulk modulus with a second order term. For the finite element analysis, the material model was written as a user defined material subroutine (UMAT). The numerical simulations were done using ABAQUS and compared to monotonic tension, compression and cyclic experiments. The results showed that the strength differential effect and the volumetric change under loading are closely related to the transformation of retained austenite to martensite. At low temperatures the effect of stress induced phase transformation on yield strength was noticeable. It was concluded that at certain temperatures both strain and stress induced phase transformations significantly affect mechanical behavior of the high strength steel.

  • 3.
    Hazar, Selcuk
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Lai, Junbiao
    SKF Engn & Res Ctr, POB 2350, NL-3430 DT Nieuwegein, Netherlands..
    Martensite transformation in the fatigue fracture surface of a high strength bearing steel2019In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 220, article id UNSP 106650Article in journal (Refereed)
    Abstract [en]

    Phase transformation of retained austenite (RA) to martensite was studied considering both stress and strain induced transformations during fatigue crack propagation. X-ray diffraction measurements, performed on the fatigue crack surface obtained through push-pull experiments, showed that almost all RA in the fatigue surface transformed to martensite. A material model that can simulate the phase change including nonlinear isotropic and kinematic hardening behaviors and nonlinear elasticity was used to simulate the fatigue crack growth. Numerical results showed that only a very small amount of RA remained in the crack surface, which agreed with the X-ray diffraction measurements. The effect of phase transformation on crack closure was studied and it was observed in the FE simulations that the crack faces close at an earlier stage when phase change has been taken into account than when it is absent.

  • 4.
    Tojaga, Vedad
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Hazar, Selcuk
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). KTH Royal Inst Technol, Dept Solid Mech, SE-10044 Stockholm, Sweden..
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Compressive failure of fiber composites containing stress concentrations: Homogenization with fiber-matrix interfacial decohesion based on a total Lagrangian formulation2019In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 182, article id 107758Article in journal (Refereed)
    Abstract [en]

    Compression failure by fiber kinking limits the structural applications of fiber composites. Fiber kinking is especially prevalent in laminates with holes and cutouts. The latter behavior is characterized by strain localization in the matrix material and fiber rotations. To study fiber kinking on the level of the individual constituents, a homogenization of fiber composites is presented. It is based on a total Lagrangian formulation, making it independent of fiber rotations. It accounts for the microstructure of the composite, including fiber matrix interfacial decohesion, and enables all types of material behavior of the constituents. The response of each constituent of the composite is modeled separately and the global response is obtained by an assembly of all contributions. The model is implemented as a user-defined material model (UMAT) in ABAQUS and used for multiscale modeling of notched unidirectional plies subjected to compression. The model performs well in agreement with a finite element model of an explicit discretization of the microstructure and literature results. The simulations predict the formation of a kink band in near 0-degree plies and show that the open-hole compression strength is sensitive to fiber-matrix interfacial decohesion. The present work suggests a convenient and computationally efficient tool for simulating the elastic-plastic behavior of fiber composites on the fiber matrix level and predicting the compressive strength of laminates.

  • 5.
    Tojaga, Vedad
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Hazar, Selcuk
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Compressive failure of fiber composites containing stress concentrations: Homogenization with fiber-matrix interfacial decohesion based on a total Lagrangian formulation2019In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 182, article id 107758Article in journal (Refereed)
    Abstract [en]

    Compression failure by fiber kinking limits the structural applications of fiber composites. Fiber kinking is especially prevalent in laminates with holes and cutouts. The latter behavior is characterized by strain localization in the matrix material and fiber rotations. To study fiber kinking on the level of the individual constituents, a homogenization of fiber composites is presented. It is based on a total Lagrangian formulation, making it independent of fiber rotations. It accounts for the microstructure of the composite, including fiber-matrix interfacial decohesion, and enables all types of material behavior of the constituents. The response of each constituent of the composite is modeled separately and the global response is obtained by an assembly of all contributions. The model is implemented as a user-defined material model (UMAT) in ABAQUS and used for multiscale modeling of notched unidirectional plies subjected to compression. The model performs well in agreement with a finite element model of an explicit discretization of the microstructure and literature results. The simulations predict the formation of a kink band in near 0-degree plies and show that the open-hole compression strength is sensitive to fiber-matrix interfacial decohesion. The present work suggests a convenient and computationally efficient tool for simulating the elastic-plastic behavior of fiber composites on the fiber-matrix level and predicting the compressive strength of laminates.

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