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Dahlberg, Carl F. O.ORCID iD iconorcid.org/0000-0002-9509-2811
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Publications (10 of 17) Show all publications
Gudmundson, P. & Dahlberg, C. F. O. (2019). Dislocation based strain gradient plasticity model for prediction of length scale dependent initial yield strength. In: 6th International Conference on Material Modelling (ICCM6): . Paper presented at 6th International Conference on Material Modelling (ICCM6).
Open this publication in new window or tab >>Dislocation based strain gradient plasticity model for prediction of length scale dependent initial yield strength
2019 (English)In: 6th International Conference on Material Modelling (ICCM6), 2019Conference paper, Oral presentation with published abstract (Refereed)
Abstract [en]

Many experimental studies have shown a plastic strengthening effect for structural length scales approaching microstructural dimensions. Both increases in initial yield strength and strain hardening have been observed. Over the last 30 years different strain gradient plasticity (SGP) theories have been developed in order to capture these length scale dependences. However, up to now no generally accepted theory has emerged. In the present presentation, focus is directed into a physically based SGP model for initiation of plastic deformation.

The plastic behavior is governed by a dissipative part that primarily controls the hardening at moderate plastic strains and an energetic part that is of importance for the initiation of plastic flow. It is shown that a model based on the self-energies of dislocations can be translated into an internal free energy in terms of plastic strain gradients. Similarly, the dissipative part of the model is based on the Taylor model, which also gives a direct connection to dislocation theory.

In this way, a physical connection is made between the SGP framework and dislocation mechanics. It is shown that the same length scale emerges for both the energetic and the dissipative part of the model. Apart from a non-dimensional factor of the order of unity, the length scale can be defined as the Burgers vector divided by the strain for initiation of plastic deformation.

When the structural length scale approaches this microstructural length scale, strengthening effects result. The three-dimensional SGP model is specialized to the simple load cases of tensile tension with a passivation layer that prohibits plastic deformation on the surfaces as well as pure bending with free and fixed boundary conditions for plastic strain. Simulations of initial yield stress for varying thicknesses are compared to experimental observations reported in the literature. It is shown that the model in a good way can capture the length scale dependences. Also upper bound solutions are presented for a spherical void in an infinite volume as well as torsion of a cylindrical rod. The model is as well applied to derive a prediction for the Hall-Petch effect.

Keywords
strain gradient plasticity
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-255868 (URN)
Conference
6th International Conference on Material Modelling (ICCM6)
Note

QC 20190902

Available from: 2019-08-14 Created: 2019-08-14 Last updated: 2019-09-02Bibliographically approved
Dahlberg, C. F. O. & Boåsen, M. (2019). Evolution of the length scale in strain gradient plasticity. International journal of plasticity, 112, 220-241
Open this publication in new window or tab >>Evolution of the length scale in strain gradient plasticity
2019 (English)In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 112, p. 220-241Article in journal (Refereed) Published
Abstract [en]

An equivalence is assumed between a microstructural length scale related to dislocation density and the constitutive length scale parameter in phenomenological strain gradient plasticity. An evolution law is formed on an incremental basis for the constitutive length scale parameter. Specific evolution equations are established through interpretations of the relation between changes in dislocation densities and increments in plastic strain and strain gradient. The length scale evolution has been implemented in a 2D-plane strain finite element method (FEM) code, which has been used to study a beam in pure bending. The main effect of the length scale evolution on the response of the beam is a decreased strain hardening, which in cases of small beam thicknesses even leads to a strain softening behavior. An intense plastic strain gradient may develop close to the neutral axis and can be interpreted as a pile-up of dislocations. The effects of the length scale evolution on the mechanical fields are compared with respect to the choice of length evolution equation.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2019
Keywords
Length scale evolution, Strain gradient plasticity, Size effects, Dislocation mean free path, Dislocation microstructure
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-241186 (URN)10.1016/j.ijplas.2018.08.016 (DOI)000454468400013 ()2-s2.0-85053428373 (Scopus ID)
Note

QC 20190121

Available from: 2019-01-21 Created: 2019-01-21 Last updated: 2019-03-11Bibliographically approved
Dahlberg, C. F. O. & Ortiz, M. (2019). Fractional strain-gradient plasticity. European journal of mechanics. A, Solids, 75, 348-354
Open this publication in new window or tab >>Fractional strain-gradient plasticity
2019 (English)In: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 75, p. 348-354Article in journal (Refereed) Published
Abstract [en]

We develop a strain-gradient plasticity theory based on fractional derivatives of plastic strain and assess its ability to reproduce the scaling laws and size effects uncovered by the recent experiments of Mu et al. (2014, 2016, 2017) on copper thin layers undergoing plastically constrained simple shear. We show that the size-scaling discrepancy between conventional strain-gradient plasticity and the experimental data is resolved if the inhomogeneity of the plastic strain distribution is quantified by means of fractional derivatives of plastic strain. In particular, the theory predicts that the size scaling exponent is equal to the fractional order of the plastic-strain derivatives, which establishes a direct connection between the size scaling of the yield stress and fractionality.

Place, publisher, year, edition, pages
Elsevier, 2019
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-246428 (URN)10.1016/j.euromechsol.2019.02.006 (DOI)000471361100027 ()2-s2.0-85062243267 (Scopus ID)
Note

QC 20190326

Available from: 2019-03-26 Created: 2019-03-26 Last updated: 2019-07-29Bibliographically approved
Gudmundson, P. & Dahlberg, C. F. O. (2019). Isotropic strain gradient plasticity model based on self-energies of dislocations and the Taylor model for plastic dissipation. International journal of plasticity, 121, 1-20
Open this publication in new window or tab >>Isotropic strain gradient plasticity model based on self-energies of dislocations and the Taylor model for plastic dissipation
2019 (English)In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 121, p. 1-20Article in journal (Refereed) Published
Abstract [en]

A dislocation mechanics based isotropic strain gradient plasticity model is developed. The model is derived from self-energies of dislocations and the Taylor model for plastic dissipation. It is shown that the same microstructural length scale emerges for both the energetic and the dissipative parts of the model. Apart from a non-dimensional factor of the order of unity, the length scale is defined as the Burgers vector divided by the strain for initiation of plastic deformation. When the structural length scale approaches this microstructural length scale, strengthening effects result. The present model predicts an increased initial yield stress that is controlled by the energetic contribution. For larger plastic strains, the hardening is governed by the dissipative part of the model. The theory is specialized to the simple load cases of tension with a passivation layer that prohibits plastic deformation on the surfaces as well as pure bending with free and fixed boundary conditions for plastic strain. Simulations of initial yield stress for varying thicknesses are compared to experimental observations reported in the literature. It is shown that the model in a good way can capture the length scale dependencies. Also upper bound solutions are presented for a spherical void in an infinite volume as well as torsion of a cylindrical rod. The model is as well applied to derive a prediction for the Hall-Petch effect.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2019
Keywords
Strain gradient plasticity, Size effects, Initial yield stress, Dislocation mechanics
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-261940 (URN)10.1016/j.ijplas.2019.05.004 (DOI)000487566600001 ()
Note

QC 20191015

Available from: 2019-10-15 Created: 2019-10-15 Last updated: 2019-10-15Bibliographically approved
Asgharzadeh, M., Faleskog, J. & Dahlberg, C. F. O. (2018). A 3D model for the analysis of plastic flow properties of randomly-distributed particles.
Open this publication in new window or tab >>A 3D model for the analysis of plastic flow properties of randomly-distributed particles
2018 (English)Manuscript (preprint) (Other academic)
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-239905 (URN)
Note

QC 20181211

Available from: 2018-12-05 Created: 2018-12-05 Last updated: 2019-09-24Bibliographically approved
Gudmundson, P. & Dahlberg, C. F. O. (2018). Physically based strain gradient plasticity model for length scale dependent yield strength. In: 9th International Conference on Multiscale Materials Modeling (MMM2018), Osaka, Japan, October 28- November 2, 2018: . Paper presented at 9th International Conference on Multiscale Materials Modeling (MMM2018), Osaka, Japan, October 28- November 2, 2018.
Open this publication in new window or tab >>Physically based strain gradient plasticity model for length scale dependent yield strength
2018 (English)In: 9th International Conference on Multiscale Materials Modeling (MMM2018), Osaka, Japan, October 28- November 2, 2018, 2018Conference paper, Oral presentation with published abstract (Refereed)
Abstract [en]

Many experimental studies have shown a plastic strengthening effect for structural length scales approaching microstructural dimensions. Both increases in initial yield strength and strain hardening have been observed. Over the last 30 years different strain gradient plasticity (SGP) theories have been developed in order to capture these length scale dependences. However, up to now no generally accepted theory has emerged. In the present paper, focus is directed into a physically based SGP model for initiation of plastic deformation. The plastic behavior is governed by a dissipative part that primarily controls the hardening at moderate plastic strains and an energetic part that is of importance for the initiation of plastic flow. It is shown that a model based on the self-energies of dislocations can be translated into an internal free energy in terms of plastic strain gradients. In this way a physical connection is made between the SGP framework and dislocation mechanics. A microstructural length scale can then be defined as the Burgers vector divided by the strain for initiation of plastic deformation. When structural length scales approach this microstructural length scale, strengthening effects result. It the Taylor model is used for the dissipative part, the same microstructural length scale appears. The so developed three-dimensional SGP model is specialized to the simple load cases of tensile tension with a passivation layer that prohibits plastic deformation on surfaces as well as pure bending with free and fixed boundary conditions for plastic strain. Simulations for varying thicknesses are compared to experimental observations reported in the literature. It is shown that the model in a good way can capture the length scale dependences. Suggestions for improvement of the dislocation theory based model for the internal free energy are discussed.

Keywords
strain gradient plasticity
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-255869 (URN)
Conference
9th International Conference on Multiscale Materials Modeling (MMM2018), Osaka, Japan, October 28- November 2, 2018
Note

QC 20190902

Available from: 2019-08-14 Created: 2019-08-14 Last updated: 2019-09-02Bibliographically approved
Dahlberg, C. F. O., Mitchell-Thomas, R. C. & Quevedo-Teruel, O. (2017). Reducing the Dispersion of Periodic Structures with Twist and Polar Glide Symmetries. Scientific Reports, 7, Article ID 10136.
Open this publication in new window or tab >>Reducing the Dispersion of Periodic Structures with Twist and Polar Glide Symmetries
2017 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 10136Article in journal (Refereed) Published
Abstract [en]

In this article, a number of guiding structures are proposed which take advantage of higher symmetries to vastly reduce the dispersion. These higher symmetries are obtained by executing additional geometrical operations to introduce more than one period into the unit cell of a periodic structure. The specific symmetry operations employed here are a combination of p-fold twist and polar glide. Our dispersion analysis shows that a mode in a structure possessing higher symmetries is less dispersive than in a conventional structure. It is also demonstrated that, similar to the previously studied Cartesian glide-symmetric structures, polar glide-symmetric structures also exhibit a frequency independent response. Promising applications of these structures are leaky-wave antennas which utilize the low frequency dependence.

National Category
Other Physics Topics Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-214497 (URN)10.1038/s41598-017-10566-w (DOI)000408781000032 ()2-s2.0-85028581605 (Scopus ID)
Note

QC 20171002

Available from: 2017-10-02 Created: 2017-10-02 Last updated: 2017-10-02Bibliographically approved
Dahlberg, C. F. O. (2016). Spatial distribution of the net Burgers vector density in a deformed single crystal. International journal of plasticity, 85, 110-129
Open this publication in new window or tab >>Spatial distribution of the net Burgers vector density in a deformed single crystal
2016 (English)In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 85, p. 110-129Article in journal (Refereed) Published
Abstract [en]

A two-dimensional deformation field on an indented single crystal, where the only nonzero lattice rotation occurs in the plane of deformation and only three effective in-plane slip systems are activated, is investigated both experimentally and numerically. ElectronBackscatter Diffraction (EBSD) is utilized to probe the lattice rotation field on the sample. The lattice rotation field is utilized to calculate the two non-zero components of Nye'sdislocation density tensor, which serves as a link between plastic and elastic deformation states. The enhanced accuracy of EBSD enabled measurements of the net Burgers vector density, and a new quantity β, which monitors the activity of slip systems in the deformed zone. The β-field is compared to the slip system activity obtained by analytical solution and also by crystal plasticity simulations. A qualitative comparison of the three methods confirms that the β-field obtained experimentally agrees with the slip system activity obtained analytically and by numerical methods.

Place, publisher, year, edition, pages
Elsevier, 2016
National Category
Applied Mechanics
Research subject
Solid Mechanics; Engineering Mechanics; Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-248469 (URN)10.1016/j.ijplas.2016.07.005 (DOI)000383006900006 ()
Note

QC 20190429

Available from: 2019-04-09 Created: 2019-04-09 Last updated: 2019-10-11Bibliographically approved
Dahlberg, C., Saito, Y., Öztop, M. S. & Kysar, J. W. (2014). Geometrically necessary dislocation density measurements associated with different angles of indentations. International journal of plasticity, 54, 81-95
Open this publication in new window or tab >>Geometrically necessary dislocation density measurements associated with different angles of indentations
2014 (English)In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 54, p. 81-95Article in journal (Refereed) Published
Abstract [en]

Experiments and numerical simulations of various angles of wedge indenters into face-centered cubic single crystal were performed under plane strain conditions. In the experiments, the included angles of indenters are chosen to be 60 degrees, 90 degrees and 120 degrees and they are indented into nickel single crystal into the < 00 (1) over bar > direction with its tip parallel to < 1 1 0 > direction, so that there are three effective in-plane slip systems on (1 1 0) plane. Indenters are applied 200 mu m in depth. The midsection of the specimens is exposed with a wire Electrical Discharge Machining (EDM) and the in-plane lattice rotations of the region around the indented area are calculated from the crystallographic orientation maps obtained from electron backscatter diffraction (EBSD) measurement. No matter which angles of indenters are applied, the rotation fields are very similar. There is a strong lattice rotation discontinuity on the line below the indenter tip. The magnitude of the lattice rotation ranges from -20 degrees to 20 degrees. Lower bounds on the Geometrically Necessary Dislocation (GND) densities are also calculated and plotted. The numerical simulations of the same experimental setup are performed. The simulation results of lattice rotation and slip rates are plotted and compared with the experimental result. There is high correlation between the experimental result and the numerical result.

Keywords
Electron backscatter diffraction, Geometrically necessary dislocation density, Indentation, Single crystal plasticity
National Category
Applied Mechanics Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-136245 (URN)10.1016/j.ijplas.2013.08.008 (DOI)000331664900005 ()2-s2.0-84892811951 (Scopus ID)
Note

QC 20140617

Available from: 2013-12-04 Created: 2013-12-04 Last updated: 2017-12-06Bibliographically approved
Dahlberg, C. F. O. & Faleskog, J. (2014). Strain gradient plasticity analysis of the influence of grain size and distribution on the yield strength in polycrystals. European journal of mechanics. A, Solids, 44, 1-16
Open this publication in new window or tab >>Strain gradient plasticity analysis of the influence of grain size and distribution on the yield strength in polycrystals
2014 (English)In: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 44, p. 1-16Article in journal (Refereed) Published
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.

Keywords
Grain size distribution, Hall-Petch effect, Strain gradient plasticity
National Category
Applied Mechanics Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-136243 (URN)10.1016/j.euromechsol.2013.09.004 (DOI)000331662100001 ()2-s2.0-84887373406 (Scopus ID)
Funder
Swedish Research Council, 621-2005-5759
Note

QC 20140331

Available from: 2013-12-04 Created: 2013-12-04 Last updated: 2017-12-06Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-9509-2811

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