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Kajberg, Jörgen
Publications (10 of 14) Show all publications
Hammarberg, S., Kajberg, J. & Jonsén, P. (2019). Modeling of Ultra High Strength Steel Sandwiches with Lightweight Cores. In: Mats Oldenburg, Jens Hardell, Daniel Casellas (Ed.), CHS² 2019 - 7th International Conference on Hot Sheet Metal Forming of High Performance Steel: . Paper presented at CHS² 2019 - 7th International Conference on Hot Sheet Metal Forming of High Performance Steel, Luleå, Sweden, June 2nd to 5th 2019 (pp. 313-320).
Open this publication in new window or tab >>Modeling of Ultra High Strength Steel Sandwiches with Lightweight Cores
2019 (English)In: CHS² 2019 - 7th International Conference on Hot Sheet Metal Forming of High Performance Steel / [ed] Mats Oldenburg, Jens Hardell, Daniel Casellas, 2019, p. 313-320Conference paper, Published paper (Refereed)
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-75752 (URN)
Conference
CHS² 2019 - 7th International Conference on Hot Sheet Metal Forming of High Performance Steel, Luleå, Sweden, June 2nd to 5th 2019
Available from: 2019-08-29 Created: 2019-08-29 Last updated: 2019-08-29
Sjöberg, T., Marth, S., Kajberg, J. & Häggblad, H.-å. (2017). Experimental characterisation of the evolution of triaxiality stress state for sheet metal materials. European journal of mechanics. A, Solids, 66, 279-286
Open this publication in new window or tab >>Experimental characterisation of the evolution of triaxiality stress state for sheet metal materials
2017 (English)In: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 66, p. 279-286Article in journal (Refereed) Published
Abstract [en]

Sheet metals are often used as safety structures in automotive applications where the fracture behaviour is a key design parameter. Theoretical and experimental observations have shown that the fracture behaviour of many metals depends on the stress state. Modelling the stress state dependency of fracture in Finite Element (FE) simulations has led to the development of advanced stress state dependent fracture criteria. The calibration of advanced fracture models is currently limited by the characterisation methods, which have not developed much during the last decades. Experimental characterisation methods that can determine the stress state accurately are necessary to ensure reliable calibrations of advanced fracture models. In this article, an experimental method to obtain the stress state and its evolution during deformation is presented. The stress state evolution is determined using measured local displacement field data, which were obtained by digital image correlation, coupled with a stepwise modelling method. This article shows that the stepwise modelling method can capture the stress state evolution for three different specimen geometries subjected to tensile loading. The resulting experimentally determined stress state evolutions are compared with the results of FE simulations, and both results are in good agreement. The accurate stress state evolutions characterised directly from experiments using the proposed method enables calibration of advanced fracture models rapidly and reliably

Place, publisher, year, edition, pages
Elsevier, 2017
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-65104 (URN)10.1016/j.euromechsol.2017.07.013 (DOI)000412254300022 ()2-s2.0-85026754631 (Scopus ID)
Note

Validerad; 2017; Nivå 2; 2017-08-15 (andbra)

Available from: 2017-08-15 Created: 2017-08-15 Last updated: 2018-04-12Bibliographically approved
Sjöberg, T., Kajberg, J. & Oldenburg, M. (2017). Fracture behaviour of Alloy 718 at high strain rates, elevated temperatures, and various stress triaxialities. Engineering Fracture Mechanics, 178, 231-242
Open this publication in new window or tab >>Fracture behaviour of Alloy 718 at high strain rates, elevated temperatures, and various stress triaxialities
2017 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 178, p. 231-242Article in journal (Refereed) Published
Abstract [en]

A methodology for fracture characterisation at strain rates up to 1000 s−1, temperatures up to 650 °C, and various stress triaxialities is presented. High-speed photography combined with digital image correlation is used to evaluate the strain at fracture. The methodology was successfully demonstrated on aged nickel based Alloy 718, commonly used in the containment structure of aircraft engines. Tensile specimens with four different geometries were loaded to get a wide range of positive stress triaxialities. All specimens originated from one single heat and batch to ensure consistent mechanical properties. The results showed evident stress state dependency on the failure strain, where lower failure strains were observed at higher stress triaxialities for all combinations of temperatures and strain rates. A coupled relationship between the temperature and the stress triaxiality controlling the fracture strain was found. However, any clear dependency on strain rate was hard to detect.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Mechanical testing, Failure mechanism, High strain rate, High temperature, Digital image correlation
National Category
Metallurgy and Metallic Materials Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-62599 (URN)10.1016/j.engfracmech.2017.04.036 (DOI)000403127100016 ()2-s2.0-85019076687 (Scopus ID)
Note

Validerad;2017;Nivå 2;2017-06-07 (rokbeg)

Available from: 2017-03-22 Created: 2017-03-22 Last updated: 2018-07-10Bibliographically approved
Sjöberg, T., Sundin, K.-G., Kajberg, J. & Oldenburg, M. (2016). Reverse ballistic experiment resembling the conditions in turbine blade off event for containment structures (ed.). Thin-walled structures, 107, 671-677
Open this publication in new window or tab >>Reverse ballistic experiment resembling the conditions in turbine blade off event for containment structures
2016 (English)In: Thin-walled structures, ISSN 0263-8231, E-ISSN 1879-3223, Vol. 107, p. 671-677Article in journal (Refereed) Published
Abstract [en]

An experimental technique has been developed which allows loading of heated sheet material under impact conditions with simultaneous measurement of the impact force history. The combined characteristics of impact loading at elevated temperature makes the experiment ideal for validation of models used to simulate the containment structure surrounding aircraft engines. In this paper experimental results for Alloy 718 are presented, a nickel based super alloy commonly used in hot parts of the containment structure. The experimental results are then compared to simulations in order to validate previously calibrated material parameters. The basic principle of the validation experiment is based on reverse ballistics, in which a thin circular specimen with free boundaries impacts the end of an instrumented rod. Using induction heating the specimen is heated to temperatures up to 650 °C and a gun driven by compressed air accelerates the specimens to desired velocity. In the reported work velocities are kept low enough to avoid cracking and thus the study is limited to plastic conditions, even though the technique is applicable also for fracture studies. The free boundaries of the experiment makes numerical modelling and simulation straightforward, making it valuable as a validation tool. All numerical simulations are performed using the commercial finite element code LS-Dyna and plastic behaviour of the material was modelled with the Johnson-Cook material model

National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-14178 (URN)10.1016/j.tws.2016.07.002 (DOI)000383813200054 ()2-s2.0-84982803960 (Scopus ID)d87e4938-9599-407f-9604-78c6dbbcdf41 (Local ID)d87e4938-9599-407f-9604-78c6dbbcdf41 (Archive number)d87e4938-9599-407f-9604-78c6dbbcdf41 (OAI)
Note

Validerad; 2016; Nivå 2; 20160825 (andbra)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
Kajberg, J. & Sundin, K.-G. (2014). High-Temperature Split-Hopkinson Pressure Bar with a Momentum Trap for Obtaining Flow Stress Behaviour and Dynamic Recrystallisation (ed.). Paper presented at . Strain, 50(6), 547-554
Open this publication in new window or tab >>High-Temperature Split-Hopkinson Pressure Bar with a Momentum Trap for Obtaining Flow Stress Behaviour and Dynamic Recrystallisation
2014 (English)In: Strain, ISSN 0039-2103, E-ISSN 1475-1305, Vol. 50, no 6, p. 547-554Article in journal (Refereed) Published
Abstract [en]

In hot forming processes at elevated temperatures like wire rolling, microstructural changes such as repeated dynamic recrystallisation and grain growth occur. An experimental method to obtain the flow stress behaviour and to capture the recrystallised microstructure for materials subjected to large deformations, high temperatures between 900 and 1200 °C and high strain rates around 5000 s− 1 is presented. The method is based on the split-Hopkinson pressure bar arrangement complemented with an inductive heat source. Furthermore, a momentum trap is added to ensure that the specimen is loaded only once. By quenching the specimen directly after the single loading, the dynamically recrystallised microstructure is preserved. The quenching is performed within 0.1 s of loading by dropping the specimen into a water bath. By applying the momentum trap technique, the compressive loading of the specimen could be interrupted at a strain level slightly above the strain level corresponding to the peak stress, which is a good estimation for the onset of dynamic recrystallisation.

National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-3589 (URN)10.1111/str.12115 (DOI)000344872600007 ()2-s2.0-84909631691 (Scopus ID)16a91c2b-95f7-47f9-9119-50d3dfcd37d4 (Local ID)16a91c2b-95f7-47f9-9119-50d3dfcd37d4 (Archive number)16a91c2b-95f7-47f9-9119-50d3dfcd37d4 (OAI)
Note
Validerad; 2014; 20141024 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
Kajberg, J. & Sundin, K.-G. (2013). Material characterisation using high-temperature Split Hopkinson pressure bar (ed.). Paper presented at . Journal of Materials Processing Technology, 213(4), 522-531
Open this publication in new window or tab >>Material characterisation using high-temperature Split Hopkinson pressure bar
2013 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 213, no 4, p. 522-531Article in journal (Refereed) Published
Abstract [en]

In order to characterise the mechanical response of materials in manufacturing processes, such as wire and bar rolling involving very high strain rates, temperatures and level of straining, an experimental device is presented. The device is suitable for testing at strain rates up to approximately 4000 s−1, temperatures up to 1200 °C (≈1500 K) and strains around 0.5. It is based on the classical split Hopkinson pressure bar and is complemented with an inductive heating source for achieving requested temperatures. By keeping the specimen separated from the Hopkinson bars just until an instant before impact (50 ms) considerable cooling and temperature gradients in the specimen are avoided. Three steel grades, two stainless steels and a high-speed steel, were tested. Four different material models whose parameters were fitted to the obtained experimental data were used for mechanical characterisation: two empirically based and two physically based. Overall, one of the physically based models showed the best agreement between experimental results and the predicted flow stresses.

National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-13139 (URN)10.1016/j.jmatprotec.2012.11.008 (DOI)000315311300002 ()2-s2.0-84870987269 (Scopus ID)c50d951c-7287-4631-a2f1-353712c7913a (Local ID)c50d951c-7287-4631-a2f1-353712c7913a (Archive number)c50d951c-7287-4631-a2f1-353712c7913a (OAI)
Note
Validerad; 2013; 20121120 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
Kajberg, J. & Wikman, B. (2007). Viscoplastic parameter estimation by high strain-rate experiments and inverse modelling: speckle measurements and high-speed photography (ed.). International Journal of Solids and Structures, 44(1), 145-164
Open this publication in new window or tab >>Viscoplastic parameter estimation by high strain-rate experiments and inverse modelling: speckle measurements and high-speed photography
2007 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 44, no 1, p. 145-164Article in journal (Refereed) Published
Abstract [en]

A methodology based on inverse modelling for estimating viscoplastic material parameters at high strain-rate conditions is presented. The methodology is demonstrated for a mild steel exposed for compression loading in a split Hopkinson pressure bar arrangement. By using dog-bone shaped specimens nonhomogeneous states of deformation are obtained throughout the entire deformation process. The resulting nonhomogeneous deformation of the specimens is evaluated using digital speckle photography (DSP) to give in-plane point-wise displacement and strain fields. The photographs are captured with a high-speed camera of image converter type, which acquire time resolved images during the impact loading. The experiments are simulated using finite element analysis (FEA), where the material model suggested by Johnson-Cook for high-strain rate conditions are utilised. Experimental and FE-calculated field information are compared in order to estimate the viscoplastic parameter in the Johnson-Cook material model. The estimation is performed by minimising least-square functions that contain the differences in displacements and strains, respectively. The quality of the estimated parameters is studied from statistical point of view.

National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-11673 (URN)10.1016/j.ijsolstr.2006.04.018 (DOI)000242925800009 ()2-s2.0-33750732302 (Scopus ID)ab106070-6828-11dc-a0c3-000ea68e967b (Local ID)ab106070-6828-11dc-a0c3-000ea68e967b (Archive number)ab106070-6828-11dc-a0c3-000ea68e967b (OAI)
Note

Validerad; 2007; 20070921 (pirkko)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
Oldenburg, M., Sundin, K.-G., Wikman, B., Kajberg, J. & Åkerström, P. (2006). Material characterisation using advanced experiments and inverse methods (ed.). In: (Ed.), Zhenhan Yao; Mingwu Yuan; Wanxie Zhong (Ed.), Computational mechanics: Abstracts : abstracts of the papers presented at the regular sessions of the sixth world congress on computational mechanics in conjunction with the second Asian-Pacific congress on computational mechanics, September 5-10, 2004, Beijing, China. Paper presented at World Congress on Computational Mechanics : 05/09/2004 - 10/09/2004. Bejing: Tsinghua University Press
Open this publication in new window or tab >>Material characterisation using advanced experiments and inverse methods
Show others...
2006 (English)In: Computational mechanics: Abstracts : abstracts of the papers presented at the regular sessions of the sixth world congress on computational mechanics in conjunction with the second Asian-Pacific congress on computational mechanics, September 5-10, 2004, Beijing, China / [ed] Zhenhan Yao; Mingwu Yuan; Wanxie Zhong, Bejing: Tsinghua University Press, 2006Conference paper, Meeting abstract (Other academic)
Place, publisher, year, edition, pages
Bejing: Tsinghua University Press, 2006
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-28329 (URN)2198fc20-fa0b-11db-b2dd-000ea68e967b (Local ID)7-302-09343-1 (ISBN)2198fc20-fa0b-11db-b2dd-000ea68e967b (Archive number)2198fc20-fa0b-11db-b2dd-000ea68e967b (OAI)
Conference
World Congress on Computational Mechanics : 05/09/2004 - 10/09/2004
Note
Godkänd; 2006; 20070504 (cira)Available from: 2016-09-30 Created: 2016-09-30 Last updated: 2017-11-25Bibliographically approved
Kajberg, J. & Lindkvist, G. (2004). Characterisation of materials subjected to large strains by inverse modelling based on in-plane displacement fields (ed.). Paper presented at . International Journal of Solids and Structures, 41(13), 3439-3459
Open this publication in new window or tab >>Characterisation of materials subjected to large strains by inverse modelling based on in-plane displacement fields
2004 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 41, no 13, p. 3439-3459Article in journal (Refereed) Published
Abstract [en]

A method for characterisation of materials subjected to large strains beyond the levels when plastic instability occurs in standard tension tests is presented. Thin sheets of two types of hot-rolled steel are subjected to tension loading until fracture occurs. The deformation process is captured with a digital camera and by digital speckle photography (DSP) in-plane pointwise displacement fields are obtained. By numerical differentiation and assuming plastic incompressibility the equivalent plastic strain is determined. The characterisation performed in this paper consists of estimating material parameters in two constitutive models. These models are a piecewise linear plasticity model and a parabolic hardening model. By using inverse modelling including finite element analyses (FEA) of the tension tests the material parameters are adjusted to achieve a minimum in a so-called objective function. The objective function is basically a least-square functional based on the difference between the experimental and FE-calculated displacement and strain fields. Due to the large deformations an adaptive meshing technique is used in order to avoid highly distorted elements. The DSP- technique provided measurements, where the uncertainty of the equivalent plastic strain varied between 0.0015 and 0.0056. The maximum obtained strain was approximately 0.8. The true stress-strain curves based on the estimated parameters are validated in the low strain region by comparison with curves from standard tension tests

National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-6917 (URN)10.1016/j.ijsolstr.2004.02.021 (DOI)000221580300005 ()2-s2.0-2342581618 (Scopus ID)54024210-bb55-11db-b560-000ea68e967b (Local ID)54024210-bb55-11db-b560-000ea68e967b (Archive number)54024210-bb55-11db-b560-000ea68e967b (OAI)
Note
Validerad; 2004; 20070110 (inah)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
Kajberg, J., Sundin, K.-G., Melin, L. & Ståhle, P. (2004). High strain-rate tensile testing and viscoplastic parameter identification using microscopic high-speed photography (ed.). Paper presented at . International journal of plasticity, 20(4-5), 561-575
Open this publication in new window or tab >>High strain-rate tensile testing and viscoplastic parameter identification using microscopic high-speed photography
2004 (English)In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 20, no 4-5, p. 561-575Article in journal (Refereed) Published
Abstract [en]

A combined experimental/numerical method for determination of constitutive parameters in high strain-rate material models is presented. Impact loading, using moderate projectile velocities in combination with small specimens (sub mm) facilitate tensional strain rates in the order of 104-105 s-1. Loading force is measured from one-dimensional wave propagation in a rod using strain gauges and deformation is monitored with a high-speed camera equipped with a microscope lens. A sequence of digital photographs is taken during the impact loading and the plastic deformation history of the specimen is quantified from the photographic record. Estimation of material parameters is performed through so called inverse modelling in which results from repeated FE-simulations are compared with experimental results and a best choice of constitutive parameters is extracted through an iterative optimisation procedure using the simplex method. Results are presented from a preliminary tension test of a mild steel (A533B) at a strain rate well over 104 s-1. The sensitivity of the evaluated material parameters to errors in measured quantities is studied. The method, especially the optical technique for measurement of deformation will be further developed.

National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-5280 (URN)10.1016/S0749-6419(03)00041-X (DOI)000188958200001 ()2-s2.0-0346969843 (Scopus ID)356fe320-8af4-11db-8975-000ea68e967b (Local ID)356fe320-8af4-11db-8975-000ea68e967b (Archive number)356fe320-8af4-11db-8975-000ea68e967b (OAI)
Note
Validerad; 2004; 20061213 (ysko)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
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