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• 1.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Modeling of Ultra High Strength Steel Sandwiches with Lightweight Cores2019Ingår i: CHS² 2019 - 7th International Conference on Hot Sheet Metal Forming of High Performance Steel / [ed] Mats Oldenburg, Jens Hardell, Daniel Casellas, 2019, s. 313-320Konferensbidrag (Refereegranskat)
• 2. Kajberg, Jörgen
Displacement field measurement using digital speckle photography for characterisation of materials subjected to large deformations and high strain rates2003Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)

In many technical processes, material is deformed under conditions involving large deformations (strains) and/or high strain rates. Examples of such processes are collisions, impact, penetration, metal forming, powder compaction and crack propagation. For description of these kinds of situations a variety of constitutive models, based on both physical foundations and empirical considerations, is available. Common for all models is that they contain material parameters, which have to be estimated by utilising experimental methods. For material characterisation under quasi-static conditions standardised tension tests of uniaxially loaded specimens are commonly used. With these tests stress-strain relations are obtained up to moderate strain values, whereupon the onset of strain localisation, so-called necking, restricts their validities. Correction methods have been developed to compensate for the onset of necking (e.g. Bridgman's correction method for round bars). The Taylor impact test and the split Hopkinson bar arrangement are frequently used methods for the investigation of incompressible (volume conserving) materials in the high strain rate regime. Typically, the specimens are short and stubby cylinders, which ideally facilitate a homogeneous state of loading necessary for a simple interpretation of the experimental results. In this thesis a methodology is suggested for characterisation of materials subjected to large deformations and high strain rates, where neither homogeneously loaded specimens nor incompressible behaviour are necessary. Experimental methods similar to standardised tension tests and split Hopkinson bar arrangement are complemented with an optical method, digital speckle photography (DSP), for in-plane point-wise displacement and strain measurements. By using a common digital camera in the former tests and a high-speed camera with a CCD-unit (Charged Coupled Device) in the latter tests digitised images are obtained for the method of DSP. An inverse method is used to estimate the material parameters in constitutive models. Three-dimensional numerical simulations of the specimens are performed by the finite element method (FEM). By adjusting the parameters to give a best fit between experimental and numerical results (displacements and strains) in least-square sense optimal values are obtained. In the quasi-static tension tests true strain values up to 0.8 were obtained for a hot-rolled steel. The mild steel specimens in the high strain rate tests were subjected to strain rates of magnitudes 10^2-10^3 1/s.

• 3. Kajberg, Jörgen
High strain-rate experiments using high-speed photography2002Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)

In many technical processes, material is deformed under conditions that contain high strain rates. Examples of such processes are collisions, impact, penetration, metal forming and crack propagation. Constitutive models including viscoplasticity have been proposed for these kinds of situations. Common for all models is that they contain material parameters, which are not well known. Experimental techniques like Taylor impact test and split Hopkinson pressure bar have been designed for the investigation of materials subjected to high strain rates. In these methods high strain rates are achieved by subjecting short specimens to rapid loading. For a simple interpretation of experimental results, a homogeneous state of stress and strain is desirable. Short specimens imply high strain rate but if the length and width are similar a nonhomogeneous state of stress and strain will result and the reliability in the evaluated quantities decreases. With the testing techniques mentioned above, it is difficult to use specimens, which are short but slender. In this thesis an experimental method to study material behaviour at high strain rates is developed. In contrast to the classical techniques, this method does not require a homogeneous state of stress and strain. A very small specimen (sub mm) and several larger specimens (up to 5 mm) have been used in the experiments. They are subjected to rapid tensional loading in devices similar to the Hopkinson bar arrangement. For the larger specimens a complete split Hopkinson bar is used, while one of the bars, namely the incident bar, has been omitted when the shorter specimen is tested. The deformation of the specimens is captured with a high-speed camera of image converter type. For the small specimen of sub mm size, the extension of its entire length is evaluated. The estimated strain rate reached well over 10^4 1/s. The larger specimens are evaluated using digital speckle photography (DSP) to give in-plane strain fields. Strains in the domain, 0.01-0.25, are evaluated and strain rates up to 3000 1/s are achieved.

• 4. Kajberg, Jörgen
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Characterisation of materials subjected to large strains by inverse modelling based on in-plane displacement fields2004Ingår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 41, nr 13, s. 3439-3459Artikel i tidskrift (Refereegranskat)

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

• 5. Kajberg, Jörgen
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Optical method to study material behaviour at high strain rates2000Ingår i: IUTAM Symposium on Field Analysis for Determination of Material Parameters - Experimental and Numerical Aspects: proceedings of the IUTAM Symposium held in Abisko National Park, Kiruna, Sweden, July 31 - August 4, 2000 / [ed] P. Ståhle, Dordrecht: Encyclopedia of Global Archaeology/Springer Verlag, 2000, s. 37-49Konferensbidrag (Refereegranskat)
• 6.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
High-Temperature Split-Hopkinson Pressure Bar with a Momentum Trap for Obtaining Flow Stress Behaviour and Dynamic Recrystallisation2014Ingår i: Strain, ISSN 0039-2103, E-ISSN 1475-1305, Vol. 50, nr 6, s. 547-554Artikel i tidskrift (Refereegranskat)

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.

• 7.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Material characterisation using high-temperature Split Hopkinson pressure bar2013Ingår i: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 213, nr 4, s. 522-531Artikel i tidskrift (Refereegranskat)

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.

• 8. Kajberg, Jörgen
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik. Luleå tekniska universitet. Luleå tekniska universitet.
High strain-rate tensile testing and viscoplastic parameter identification using microscopic high-speed photography2004Ingår i: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 20, nr 4-5, s. 561-575Artikel i tidskrift (Refereegranskat)

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.

• 9.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Viscoplastic parameter estimation by high strain-rate experiments and inverse modelling: speckle measurements and high-speed photography2007Ingår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 44, nr 1, s. 145-164Artikel i tidskrift (Refereegranskat)

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.

• 10.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Material characterisation using advanced experiments and inverse methods2006Ingår i: 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, 2006Konferensbidrag (Övrigt vetenskapligt)
• 11.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Fracture behaviour of Alloy 718 at high strain rates, elevated temperatures, and various stress triaxialities2017Ingår i: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 178, s. 231-242Artikel i tidskrift (Refereegranskat)

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.

• 12.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Experimental characterisation of the evolution of triaxiality stress state for sheet metal materials2017Ingår i: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 66, s. 279-286Artikel i tidskrift (Refereegranskat)

• 13.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Experimental Characterization of Triaxiality Stress State Evolution for Sheet Metal MaterialsManuskript (preprint) (Övrigt vetenskapligt)
• 14.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Reverse ballistic experiment resembling the conditions in turbine blade off event for containment structures2016Ingår i: Thin-walled structures, ISSN 0263-8231, E-ISSN 1879-3223, Vol. 107, s. 671-677Artikel i tidskrift (Refereegranskat)

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

• 15.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Kajberg, JörgenLuleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.Jonsén, PärLuleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Towards the Discrete Element Modeling of Rock Drilling2019Proceedings (redaktörskap) (Refereegranskat)

Rock drilling is involved in a lot of areas today, e.g.in mining-, oil- and geothermal industries. Depending onthe conditions, different drilling methods are used. Twoprevalent methods for drilling in rock are the regular rotarydrilling method, where the rock is crushed by a weight-onbitand rotation, and the percussive rotary drilling method,where the rock is initially crushed by the impact energy andexperiences further splitting from the rotary motion of thebit.In the GeoFit project, the main goal is to develop costeffective enhanced geothermal systems on energy efficientbuilding retrofitting. As a part of this project, LTU willlead the simulation work regarding the application of noveldrilling techniques. This involves the development anddemonstration of modeling strategies, based on particlebased methods and FEM, of the drilling and excavation process.The work is focused on virtual predictions of the drillbit in terms of wear rate, pressure distribution and vibrationsin the adjacent soil and rock. From this prediction,the drilling parameters, tool performance and selection ofmaterials will be improved in order to reduce tool damageand predict maintenance.Using the Discrete Element Method of the rock and arigid FEM formulation of the drill bit, LTU has conductedsimulations of the aforementioned drilling methods. Thewear has been evaluated using two different wear laws; Archard’sand Finnie’s law.The Discrete Element Method, as initially formulated byCundall (1971), is a discontinuous method where the materialof interest is modeled using discrete rigid spheres, allof which follow the Newtonian laws of motion. This inherentdiscontinuous behavior is what makes this method suitablefor modeling problems of large deformations, which ishighly relevant in the rock drilling process.In the current applications, the LS-Dyna software hasbeen used. Here, each particle contact is realized by usinga penalty-based contact. Friction is also included at theperimeters of the particles based on the Coulombs frictionlaw. In order to model the cohesion between particles forthe rock model, the parallel bond model [1] has been used.Here, the cohesion is represented by contact forces in thenormal- and tangential direction, given by

$\left\{\begin{array}{l}F_{n}=K_{n} u_{n} \\ \left|F_{s}\right|=K_{s}\left|u_{s}\right|\end{array}\right.$

where un is the gap (or overlap) between the spheres, us isthe relative displacement and Kn and Ks are the normalandtangential contact stiffnesses. A bond breaks instantaneouslyonce the normal or tangential force exceeds itsmaximum value, c or c respectively.A challenge with the DEM is how to relate the macroscopicparameters, e.g elastic modulus and Poisson’s ratio, to the microscopic parameters, Kn, Ks, c and c. Forthe present applications, parameters from previous researchhas been used [2,3].Future work consists of laboratory experiments, usinge.g. the unconfined stress test, Brazilian test and Split-Hopkinson bar, in order to calibrate the different types ofrocks often encountered during drilling in geothermal applications.Further, the constitutive relationships for thedrill bit will be determined in order to model wear and geometrychanges.

• 16.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Kajberg, JörgenLuleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.Jonsén, PärLuleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
Towards the Discrete Element Modeling of Wear in Rock Drilling Bits2019Proceedings (redaktörskap) (Refereegranskat)

Two commonly used methods for rock drilling are the regular rotary drilling method, where the rockis crushed by a weight-on-bit and rotation, and the percussive rotary drilling method, where the rock isinitially crushed by the impact energy and experiences further splitting from the rotary motion of the bit.In this study, wear of drill bits, pressure distributions and vibrations in the adjacent soil and rock will beinvestigated by togehter using FEM, DEM and CFD.DEM has been used to model rock behaviour where the cohesion between the particles was realized byusing the parallel bond model [1] with microscopic parameters [2,3]. Each particle contact was realizedby using a penalty-based contact model and friction was included at the perimeters of the particles basedon the Coulombs friction law. The drill bit has been modeled using a FEM formulation with steelproperties for the drill body and tungsten carbide properties for the cutters. Further, the transportationof drill cuttings via drilling fluid was modeled by coupling DEM and FEM to an incompressible CFDsolver. The drill bits were driven by applying torque and weight-on-bit, as well as a percussive force forthe percussive drilling method. The fluid dynamics were driven via a pressure difference between the topof the drill bit and the back of the drill. Both air and water has been used as drilling fluids.Two different wear models have been used, the first one being Archard’s abrasive wear law, which relatesthe removed volume to the hardness of the target material, and the second one being Finnie’s wear lawfor particle impact. Here the erosion due to particle impact is obtained as a function of impact angle andyield stress of the target material.The preliminary simulations based on rock characteristics from literature show promising results regardingwear and transportation of drill cuttings. Future mechanical characterisation of different types ofrocks often encountered during drilling in geothermal applications will be conducted to improve the accuracyof the numerical models. The mechanical properties of the rock materials will be characterized bye.g. unconfined stress test, Brazilian test and Split-Hopkinson pressure bar. Further, the constitutive relationshipsand wear coefficients for the drill bits will be determined in order to model wear and geometrychanges.The study is a part of the H2020-project GEOFIT, which is acknowledged for financial support, grantagreement number 792210.

1 - 16 av 16
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