| 1. |
- Floros, Dimosthenis, 1982, et al.
(författare)
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A numerical investigation of elastoplastic deformation of cracks in tubular specimens subjected to combined torsional and axial loading
- 2016
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Ingår i: International Journal of Fatigue. - : Elsevier BV. - 0142-1123. ; 91, s. 171-182
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Tidskriftsartikel (refereegranskat)abstract
- Numerical simulations to investigate crack deformation in a pre-cracked cylindrical specimen subjected to mixed mode axial/torsional loading are carried out. Results are quantified in terms of crack face displacements and contrasted to experimental trends in the literature.The results aid in the understanding and interpretation of experimental results, in design of future experiments and in the development of models to predict mixed mode crack propagation. The analyses identify elastoplastic phenomena as major mechanisms to explain experimental results of mixed mode crack growth. These phenomena are interlinked in the general case, which significantly complicates the development of a general engineering model.
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| 2. |
- Floros, Dimosthenis, 1982, et al.
(författare)
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Computation of material forces based on a gradient-enhanced mixed variational formulation
- 2016
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Ingår i: Proceedings of 29th Nordic Seminar on Computational Mechanics – NSCM29.
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Konferensbidrag (refereegranskat)abstract
- Configurational (or material) forces are computed based on a previously derived thermodynamically consistent definition of material forces, in conjunction with a gradient-enhanced constitutive theory. The primary problem solved for is based on a mixed variational formulation. Under the proposed mixed formulation, no nodal smoothing of internal variables for the computation of material forces is required at the post-processing. Results are shown in terms of energetic quantities which stem from the computed material forces. The mesh sensitivity of the latter quantities is examined and compared to respective results based on local constitutive theory in a "standard"displacements-based variational formulation.
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| 3. |
- Floros, Dimosthenis, 1982, et al.
(författare)
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Evaluation of crack growth direction criteria on mixed-mode fatigue crack growth experiments
- 2019
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Ingår i: International Journal of Fatigue. - : Elsevier BV. - 0142-1123. ; 129
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Tidskriftsartikel (refereegranskat)abstract
- The predictive capabilities of different fatigue crack growth direction criteria in finite element simulations are investigated. Crack growth direction criteria based on stress intensity factors, energetic measures and kinematic (displacement) measures are evaluated for mixed-mode fatigue crack growth experiments in the literature. More specifically, the development of the experimentally found crack path is simulated numerically, and the directions from the different criteria are compared to the known crack path along the development of the fracture. The simulated experiments featured proportional and non-proportional loading. Of the evaluated criteria, those based on energetic and displacement measures are able to accurately capture the tensile-mode fatigue crack growth direction for all examined experiments. Furthermore, the displacement-based criterion is able to capture the direction of shear-mode crack growth as well as the transition from shear- to tensile-mode growth and the subsequent tensile-mode growth. Modeling the cyclic elastic plastic material response does not improve the accuracy of the predicted directions for the considered cases.
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| 4. |
- Floros, Dimosthenis, 1982, et al.
(författare)
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Evaluation of mixed-mode crack growth criteria under rolling contact conditions
- 2018
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Ingår i: Proceedings of the 11th International Conference on Contact Mechanics and Wear of Rail/wheel Systems, CM 2018. - 9789461869630 ; , s. 253-260
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Konferensbidrag (refereegranskat)abstract
- Fatigue crack growth direction criteria are evaluated towards a twin-disc experiment from the literature. The experiment is modelled numerically at different crack lengths. Based on numerically evaluated stresses, strains and displacements, a range of crack growth direction criteria from the literature are evaluated and compared to the experimentally found crack growth direction. In the considered experiment, the growth is essentially shear driven. Of the evaluated criteria employed to a linear elastic analysis, the Maximum Shear Stress (MSS) and Vector Crack-Tip Displacement (VCTD) criteria are found to be able to capture the experimentally found crack growth direction throughout the fatigue life.
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| 5. |
- Floros, Dimosthenis, 1982, et al.
(författare)
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Evaluation of mixed-mode crack growth direction criteria under rolling contact conditions
- 2020
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Ingår i: Wear. - : Elsevier BV. - 0043-1648. ; 448-449
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Tidskriftsartikel (refereegranskat)abstract
- Fatigue crack growth direction criteria are evaluated towards a twin-disc experiment from the literature. The experiment is modelled numerically at different crack lengths accounting for the effect of crack-face friction. Based on numerically evaluated stresses, strains and displacements, a range of crack growth direction criteria from the literature are evaluated and compared to the experimentally found crack growth directions. In the considered experiment, the growth is found to be essentially shear driven. Of the evaluated criteria employed to linear elastic plane strain analyses, the Maximum Shear Stress Range (MSSR) and Vector Crack-Tip Displacement (VCTD) criteria are found to be able to capture the experimentally obtained crack growth direction throughout the fatigue life. Crack-face friction was found to impose a small effect on directions predicted by these criteria.
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| 6. |
- Floros, Dimosthenis, 1982
(författare)
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Finite element procedures for crack path prediction in multi-axial fatigue
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Rolling Contact Fatigue (RCF) cracks in rails are among the most detrimental railway track defects regarding reliability and cost. The cracks typically grow in shear mode up to a certain length at which they might arrest or kink into a more tensile-driven growth. This growth scheme appears as a result of non-proportional loading, large plastic deformations at the rail surface and primary compression with crack-face friction. In contrast, most existing crack growth criteria in the literature feature quantities that are susceptible to the limitations of small-scale yielding, e.g. Stress Intensity Factors (SIFs), tensile-mode growth and unloaded crack-faces. Consequently, the range of validity of the existing criteria may be questioned in the non-linear crack growth setting of RCF. In a study of the role of inelastic deformation on the crack loading, elastic–plastic simulations are carried out in pre-cracked tubular specimens subjected to mixed-mode cyclic loading. The crack loading is quantified via the Crack-Tip Displacements (CTDs) in modes I and II. Shakedown and ratcheting effects in the ranges of the CTDs are compared to trends of crack growth curves from experiments in the literature. It is concluded that the ranges of the CTDs can be used for qualitative crack growth assessment in the examined load cases. In addition, a gradient-enhanced mixed variational formulation is developed for overcoming the numerical difficulties associated with the computation of Configurational Forces (CFs) for inelasticity. The mesh sensitivity of the CFs acting on an embedded discrete singularity is investigated. Results highlight that the proposed formulation provides sufficient regularity for the computation of CFs, which may then be used in the formulation of criteria for RCF crack propagation. Predictions of the multi-axial fatigue crack path are performed based on instantaneous crack growth direction criteria. To this end, a generic model for load cycle evaluation is proposed and implemented on criteria based on CFs and CTDs. The predicted directions are compared towards mixed-mode fatigue crack growth experiments from the literature. Of the evaluated criteria, the ones based on CFs and CTDs accurately predict the tensile-mode growth. Classical SIF-based criteria seem to handle tensile-mode growth under moderate shear-mode loading. Moreover, the criterion based on CTDs captures the shear-mode growth and the tensile-mode growth as well as the transition between them. The latter growth schemes essentially resemble the RCF crack growth. In an investigation of the influence of various railway operational parameters on predicted RCF crack growth directions, the coefficient of friction at the wheel–rail interface was found the most influential as compared to the wheel tonnage and crack-face friction. The latter had no effect on predicted directions, due to crack-tip opening at the instances of maximum shear CTDs.
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| 7. |
- Floros, Dimosthenis, 1982, et al.
(författare)
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On configurational forces for gradient-enhanced inelasticity
- 2018
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Ingår i: Computational Mechanics. - : Springer Science and Business Media LLC. - 1432-0924 .- 0178-7675. ; 61:4, s. 409-432
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we discuss how configurational forces can be computed in an efficient and robust manner when a constitutive continuum model of gradient-enhanced viscoplasticity is adopted, whereby a suitably tailored mixed variational formulation in terms of displacements and micro-stresses is used. It is demonstrated that such a formulation produces sufficient regularity to overcome numerical diffi- culties that are notorious for a local constitutive model. In particular, no nodal smoothing of the internal variable fields is required. Moreover, the pathological mesh sensitivity that has been reported in the literature for a standard local model is no longer present. Numerical results in terms of config- urational forces are shown for (1) a smooth interface and (2) a discrete edge crack. The corresponding configurational forces are computed for different values of the intrinsic length parameter. It is concluded that the convergence of the com- puted configurational forces with mesh refinement depends strongly on this parameter value. Moreover, the convergence behavior for the limit situation of rate-independent plasticity is unaffected by the relaxation time parameter
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| 8. |
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| 9. |
- Floros, Dimosthenis, 1982
(författare)
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The effect of inelastic deformation on crack loading
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Rolling contact fatigue (RCF) cracks in rails are among the most detrimental railway track defects, in relation to reliability and cost. The cracks grow in a mixed mode II & III, which in combination with the rotating stress field in the neighborhood of the crack-tip, large plastic deformations on the rail surface, crack face friction due to the compressive stresses from the wheel load and anisotropic crack growth resistance, add to the complexity related to the study of RCF cracks. In contrast, most existing criteria for RCF crack propagation in the literature feature quantities that are susceptible to limitations, such as small scale yielding (see e.g. stress intensity factors), pure mode I growth, unloaded crack faces. Consequently, the range of validity (and precision) of existing criteria may be questioned.The current work focuses on one of the complicating factors: the role of inelastic deformation on the crack loading. At the first part, a qualitative assessment is performed of the mechanisms that accompany elastoplastic deformations of multi-axially loaded cracks. For this purpose, numerical simulations are carried out in pre-cracked tubular specimens subjected to torsional and axial loading in various load configurations. Elastoplastic deformations are quantified here via the relative deformation of initially aligned crack faces, here denoted as crack face displacement. The range of the crack face displacement over each load cycle accounts for the severity of the crack situation (in a manner analogous to the range of the stress intensity factor). Results are identified as shakedown and ratcheting effects and compared to experimental trends in literature.The work continues with the study of configurational (or material) forces for gradient-enhanced inelasticity. The severity of the crack loading is here measured via energy release rates (a generalization of the J-integral for inelasticity with material dissipation), which stem from the computed material forces. The mesh sensitivity of the energy release rates is investigated for the cases of a smooth interface and an embedded discrete singularity. Results highlight that the proposed gradient-regularized scheme provides sufficient regularity for the computation of material forces. Obtaining a rather mesh insensitive material force field for inelasticity, is considered a necessary step towards the development of a criterion for RCF crack propagation based on material forces.
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| 10. |
- Salahi Nezhad, Mohammad, 1993, et al.
(författare)
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Numerical predictions of crack growth direction in a railhead under contact, bending and thermal loads
- 2022
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Ingår i: Engineering Fracture Mechanics. - : Elsevier BV. - 0013-7944. ; 261
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Tidskriftsartikel (refereegranskat)abstract
- The effect of different operational loading scenarios on predicted crack growth direction for a propagating inclined railhead crack is assessed by 2D finite element simulations. Studied load scenarios include a moving Hertzian contact load, a temperature drop, rail bending due to a passing wheelset, and combinations thereof. The direction of the unbiased crack propagation is predicted using an accumulative vector crack tip displacement criterion. The numerical model is validated for the individual load scenarios. Restraints due to crack face locking are imposed by a threshold parameter, whose influence is also assessed. For combinations of thermal and contact loads, the predicted crack path is found to diverge gradually from transverse growth, corresponding to pure thermal loading, to shallow growth, corresponding to a pure contact load. For combined bending and contact loading, there is a discrete jump in the predicted crack direction as the contact load increased while the bending load is kept constant. These results are well aligned with empirical experience.
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