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  • 1.
    Gustafsson, David
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Parareda, S.
    Eurecat, Centre Tecnologic de Catalunya, Spain.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Olsson, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Effect Of Cutting Clearance And Sandblasting On Fatigue Of Thick CP800 Steel Sheets2022Ingår i: Svenska Mekanikdagar 2022 / [ed] Pär Jonsén; Lars-Göran Westerberg; Simon Larsson; Erik Olsson, Luleå tekniska universitet, 2022Konferensbidrag (Refereegranskat)
  • 2.
    Gustafsson, David
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Parareda, Sergi
    Eurecat, Plaça de la ciència 2, 08242 Manresa; CIEFMA, EEBE, Universitat Politècnica de Catalunya-BarcelonaTech, 08019 Barcelona, Spain.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Olsson, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Effect of cutting clearance and sandblasting on fatigue of thick CP800 steel sheets for heavy-duty vehicles2022Ingår i: Hot Sheet Metal Forming of High-Performance Steel: proceedings / [ed] Mats Oldenburg; Jens Hardell; Daniel Casellas, Wissenschaftliche Scripten , 2022, s. 315-322Konferensbidrag (Refereegranskat)
    Abstract [en]

    Effect from manufacturing processes on fatigue properties of high-strength thick steel sheets have gained increased attention the recent years, due to new demands on the heavy-duty vehicle (HDV) industry to reduce the carbon footprint. The aim of this study is to add knowledge of the effect of shear cutting clearance on the fatigue behaviour of complex phase CP800 thick steel sheets. In addition, sandblasting and its effect on the fatigue properties are studied. Service loads are fluctuating loads acting on chassis component making fatigue an important failure mode. The fatigue strength usually follows the yield strength of the material and hence weight could in theory be saved by using steels of higher strength. However, in the presence of edge defects this relation does not necessarily hold, this leads to large safety factors of the design and under-utilization of the high-strength material. Thus, an increased knowledge about the effect from manufacturing processes on fatigue properties is important for the quest to achieve weight reduction. This is particularly true for thick sheets which, to the best of our knowledge, are less investigated than their thinner counterparts, but of high importance for the HDV development.

     

    In this paper, empirical results from fatigue testing of complex phase steel CP800, subjected to punching and trimming, are presented. Results for different cutting clearances are compared as well as the effect of sandblasting. A fast fatigue testing method called Rapid fatigue test based on stiffness evolution is utilized. The results show the improvement obtained by using sandblasting as well as illustrating the effect of different cutting clearances. These results can be used as a guidance for design and production of HDV components, where cutting clearance is set. Furthermore, the results can be used as an input for discussions whether the extra costs associated with sandblasting is motivated or not for components made from high strength, thick steel sheets.

  • 3.
    Hammarberg, Samuel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Modeling of Ultra High Strength Steel Sandwiches with Lightweight Cores2019Ingår i: Hot sheet metal forming of high-performance steel: Proceedings / [ed] Mats Oldenburg, Jens Hardell, Daniel Casellas, Wissenschaftliche Scripten , 2019, s. 313-320Konferensbidrag (Refereegranskat)
    Abstract [en]

    Legislation, du to greenhouse gas emissions, is forcing the automotive industry to reduce emissions and energy consumption. High-performance lightweight materials and structures are essential for meeting these demands. In this work, two types of lightweight sandwich materials are investigated and developed; one intended for crash applications (Type I) and another for stiffness applications (Type II). In order to predict the final properties of the sandwich materials, numerical modeling strategies are established. To achieve reasonable computational time, homogenization is adopted to overcome the complex core geometries of the sandwich materials. Type I, based on press-hardened boron steel, consists of a perforated core between two face plates. Evaluation of energy absorption during crash is conducted by utilizing numerical deformation models of a hat-profile geometry. The intention is to compare the energy absorption of the hat-profile based on the Type I sandwich to a hat-profile based on solid steel with equivalent weight. Type II, based on press-hardened boron steel, consists of a bidirectionally corrugated core between two face plates. The geometry of the bidirectional core requires a large amount of finite elements for precise discretization, causing impractical simulation times. This is adressed by suggesting an equivalent material formulation, to reduce the computational time. The results from Type I indicate an increased specific energy absorption capacity of 20 % as compared to solid steel. From the equivalent material procedure of Type II, it is found that the computational cost is reduced by 95 % with a maintained accuracy for structural stiffness. Validation is carried out by subjecting the sandwich to three-point bending. Good agreement is found between numerical and experimental data. Thus, this work shows that sandwich materials are an interesting and promising approach for reducing weight of vehicle components while maintaining performance, in terms of stiffness and crashworthiness.   

  • 4.
    Hammarberg, Samuel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Larsson, Simon
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Ultra high strength steel sandwich for lightweight applications2020Ingår i: SN Applied Sciences, ISSN 2523-3963, E-ISSN 2523-3971, Vol. 2, nr 6, artikel-id 1040Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Methods for reducing weight of structural elements are important for a sustainable society. Over the recent years ultra high strength steel (UHSS) has been a successful material for designing light and strong components. Sandwich panels are interesting structural components to further explore areas where the benefits of UHSS can be utilized. The specific properties of sandwich panels make them suitable for stiffness applications and various cores have been studied extensively. In the present work, bidirectionally corrugated UHSS cores are studied experimentally and numerically. A UHSS core is manufactured by cold rolling and bonded to the skins by welding. Stiffness is evaluated experimentally in three-point bending. The tests are virtually reproduced using the finite element method. Precise discretization of the core requires large amounts of computational power, prolonging lead times for sandwich component development, which in the present work is addressed by homogenization, using an equivalent material formulation. Input data for the equivalent models is obtained by characterizing representative volume elements of the periodic cores under periodic boundary conditions. The homogenized panel reduces the number of finite elements and thus the computational time while maintaining accuracy. Numerical results are validated and agree well with experimental testing. Important findings from experimental and simulation results show that the suggested panels provide superior specific bending stiffness as compared to solid panels. This work shows that lightweight UHSS sandwiches with excellent stiffness properties can be manufactured and modeled efficiently. The concept of manufacturing a UHSS sandwich panel expands the usability of UHSS to new areas.

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  • 5.
    Hammarberg, Samuel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Larsson, Simon
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Moshfegh, Ramin
    Lamera AB, Odhners gata 17, 42130 Västra Frölunda, Sweden.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Calibration of orthotropic plasticity- and damage models for micro-sandwich materials2022Ingår i: SN Applied Sciences, ISSN 2523-3963, E-ISSN 2523-3971, Vol. 4, nr 6, artikel-id 182Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Sandwich structures are commonly used to increase bending-stiffness without significantly increasing weight. In particular, micro-sandwich materials have been developed with the automotive industry in mind, being thin and formable. In the present work, it is investigated if micro-sandwich materials may be modeled using commercially available material models, accounting for both elasto-plasticity and fracture. A methodology for calibration of both the constitutive- and the damage model of micro-sandwich materials is presented. To validate the models, an experimental T-peel test is performed on the micro-sandwich material and compared with the numerical models. The models are found to be in agreement with the experimental data, being able to recreate the force response as well as the fracture of the micro-sandwich core.

  • 6.
    Hammarberg, Samuel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Larsson, Simon
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Moshfegh, Ramin
    Lamera AB, A Odhners Gata 17, 421 30 Västra Frölunda, Sweden.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Novel Methodology for Experimental Characterization of Micro-Sandwich Materials2021Ingår i: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 14, nr 16, artikel-id 4396Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Lightweight components are in demand from the automotive industry, due to legislation regulating greenhouse gas emissions, e.g., CO2. Traditionally, lightweighting has been done by replacing mild steels with ultra-high strength steel. The development of micro-sandwich materials has received increasing attention due to their formability and potential for replacing steel sheets in automotive bodies. A fundamental requirement for micro-sandwich materials to gain significant market share within the automotive industry is the possibility to simulate manufacturing of components, e.g., cold forming. Thus, reliable methods for characterizing the mechanical properties of the micro-sandwich materials, and in particular their cores, are necessary. In the present work, a novel method for obtaining the out-of-plane properties of micro-sandwich cores is presented. In particular, the out-of-plane properties, i.e., transverse tension/compression and out-of-plane shear are characterized. Test tools are designed and developed for subjecting micro-sandwich specimens to the desired loading conditions and digital image correlation is used to qualitatively analyze displacement fields and fracture of the core. A variation of the response from the material tests is observed, analyzed using statistical methods, i.e., the Weibull distribution. It is found that the suggested method produces reliable and repeatable results, providing a better understanding of micro-sandwich materials. The results produced in the present work may be used as input data for constitutive models, but also for validation of numerical models.

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  • 7.
    Hammarberg, Samuel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Lindkvist, Göran
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Evaluation of Perforated Sandwich Cores for Crash ApplicationsManuskript (preprint) (Övrigt vetenskapligt)
  • 8.
    Hammarberg, Samuel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Lindkvist, Göran
    Jonsén, Pär
    Homogenization, Modeling and Evaluation of Stiffness for Bidirectionally Corrugated Cores in Sandwich PanelsManuskript (preprint) (Övrigt vetenskapligt)
  • 9.
    Hammarberg, Samuel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Larsson, Simon
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Numerical evaluation of lightweight ultra high strength steel sandwich for energy absorption2020Ingår i: SN Applied Sciences, ISSN 2523-3963, E-ISSN 2523-3971, Vol. 2, nr 11, artikel-id 1876Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Legislation regarding greenhouse gas emissions forces automotive manufacturers to bring forth new and innovative materials and structures for weight reduction of the body-in-white. The present work evaluates a lightweight ultra high strength steel sandwich concept, with perforated cores, for energy absorption applications. Hat-profile geometries, subjected to crushing, are studied numerically to evaluate specific energy absorption for the sandwich concept and solid hat-profiles of equivalent weight. Precise discretization of the perforated core requires large computational power. In the present work, this is addressed by homogenization, replacing the perforated core with a homogeneous material with equivalent mechanical properties. Input data for the equivalent material is obtained by analyzing a representative volume element, subjected to in-plane loading and out-of-plane bending/twisting using periodic boundary conditions. The homogenized sandwich reduces the number of finite elements and thereby computational time with approximately 95%, while maintaining accuracy with respect to force–displacement response and energy absorption. It is found that specific energy absorption is increased with 8–17%, when comparing solid and sandwich hat profiles of equivalent weight, and that a weight saving of at least 6% is possible for equivalent performance.

  • 10.
    Jonsson, Simon
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Impact crash tests of high-strength steels using 3D high-speed digital image correlation and finite element analysis2022Ingår i: 8th International Conference Hot Sheet Metal Forming of High-Performance Steel CHS2: May 30th - June 2nd, 2022, Barcelona, Spain / [ed] Mats Oldenburg, Jens Hardell, Daniel Casellas, Auerbach: Verlag Wissenschaftliche Scripten , 2022, s. 119-126Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    The automotive industry is currently adapting to a new reality, where anthropogenic emissions need to decrease significantly. To meet present and future demands of vehicle design, press harden-ing techniques to produce complex geometries with high strength and ductility as well as good precision are of great interest. New generations of hot forming steels enable both further weight reductions by using thinner sheets as well as better crash performance due to its ability to improve the structural integrity of the body-in-white. To promote the use of these new steel grades, it is important to study their performance using well-instrumented lab scale test of full-scale compo-nents. Since these tests are often time consuming and expensive, calibrating constitutive models with tensile specimens and using finite element analysis is a more cost-effective alternative. How-ever, these calibrated models should be validated against full-scale experiments to verify their effectiveness in predicting the material behaviour in complex crash environments. In this paper, a high-speed 3D digital image correlation experiment is performed on a crash box under axial com-pression. The material is a hot forming steel grade with a specified tensile strength of 1000 MPa. The axial crash tests are modelled based on a visco-plastic model calibrated by high-speed tensile tests. The computed results in terms of force response and obtained deformations agree well with the corresponding measurements.

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  • 11.
    Jonsson, Simon
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Impact crash tests using high-speed 3D digital image correlation2022Ingår i: Svenska Mekanikdagar 2022 / [ed] Pär Jonsén; Lars-Göran Westerberg; Simon Larsson; Erik Olsson, Luleå tekniska universitet, 2022Konferensbidrag (Refereegranskat)
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  • 12.
    Jonsson, Simon
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Evaluation of Crashworthiness Using High-Speed Imaging, 3D Digital Image Correlation, and Finite Element Analysis2023Ingår i: Metals, E-ISSN 2075-4701, Vol. 13, nr 11, artikel-id 1834Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    To promote the use of newhigh-strengthmaterials in the automotive industry, the evaluation of crashworthiness is essential, both in terms of finite element (FE) analysis aswell as validation experiments. Thiswork proposes an approach to address the crash performance through high-speed imaging combined with 3D digital image correlation (3D-DIC). By tracking the deformation of the component continuously, cracks can be identified and coupled to the load and intrusion history of the experiment. The so-called crash index (CI) and its decreasing rate (CIDR) can then be estimated using only one single (or a few) component, instead of a set of components with different levels of intrusion and crushing. Crash boxes were axially and dynamically compressed to evaluate the crashworthiness of TRIP-aided bainite ferrite steel and press-hardenable steel. Acalibrated rate-dependent constitutivemodel, and a phenomenological damage model were used to simulate the crash box testing. The absorbed energy, the plastic deformation, and the CIDR were evaluated and compared to the experimentally counterparts. When applying the proposed method to evaluate the CIDR, a good agreement was found when using CI:s reported by other authors using large sets of crash boxes. The FE analyses showed a fairly good agreement with some underestimation in terms of energy absorptions. The crack formation was overestimated resulting in too high a predicted CIDR. It is concluded that the proposed method to evaluate the crashworthiness is promising. To improve the modelling accuracy, better prediction of the crack formation is needed and the introduction of the intrinsic material property, fracture toughness, is suggested for future investigations and model improvements.

  • 13. Kajberg, Jörgen
    Displacement field measurement using digital speckle photography for characterisation of materials subjected to large deformations and high strain rates2003Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    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.

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  • 14. Kajberg, Jörgen
    High strain-rate experiments using high-speed photography2002Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    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.

  • 15. Kajberg, Jörgen
    et al.
    Lindkvist, Göran
    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)
    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

  • 16. Kajberg, Jörgen
    et al.
    Sjödahl, Mikael
    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)
  • 17.
    Kajberg, Jörgen
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Sundin, Karl-Gustaf
    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)
    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.

  • 18.
    Kajberg, Jörgen
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Sundin, Karl-Gustaf
    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)
    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.

  • 19.
    Kajberg, Jörgen
    et al.
    Luleå tekniska universitet.
    Sundin, Karl-Gustaf
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Melin, L.Gunnar
    Luleå tekniska universitet.
    Ståhle, Per
    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)
    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.

  • 20.
    Kajberg, Jörgen
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Wikman, Bengt
    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)
    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.

  • 21.
    Lindroos, Matti
    et al.
    Integrated Computational Materials Engineering, VTT Technical Research Centre of Finland Ltd., Finland.
    Andersson, Tom
    Integrated Computational Materials Engineering, VTT Technical Research Centre of Finland Ltd., Finland.
    Laukkanen, Anssi
    Integrated Computational Materials Engineering, VTT Technical Research Centre of Finland Ltd., Finland.
    Suarez, Laura
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Terva, Juuso
    Metso Outotec Finland Oy, Finland.
    Kallio, Marke
    Metso Outotec Finland Oy, Finland.
    Micromechanical and multi-scale modeling of manganese containing slag comminution in the design of energy efficient secondary raw material beneficiation processes2021Ingår i: Minerals Engineering, ISSN 0892-6875, E-ISSN 1872-9444, Vol. 170, artikel-id 106995Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Efficient separation of valuable metals from various slags is of great interest for the industry to effectively utilize valuable raw materials. Present work focuses on modeling the deformation and damage behavior of manganese containing slag materials at the microstructural level, which dictates the macroscopic material behavior and allows one to investigate possibilities to perform metal separation after comminution of the slags. The model includes finite element micromechanical description of the material behavior and slag microstructure. Computational micromodels are constructed based on direct input characterization data and statistically representative synthetic models. The damage model treats brittleness and ductility of the material together with phase specific material behavior, all relevant to comminution of the slag. Finally, a simplified jaw crusher simulation accounts for freeing materials, assisting the evaluation of empirical random breakage, all together with a microstructural particle study which is analyzed against micromechanical modeling. Crystal plasticity level simulations of surface deformation and hardening in jaw crusher are presented to couple macroscale crushing events with microscale deformation of wear parts. The work overall presents a workflow and proposes a methodology how digitalization and multi-scale material modeling can contribute to the development of efficient comminution means for hard to process secondary raw materials.

  • 22.
    Lundholm, Erik
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Åkerström, Paul
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Influence of the initial material microstructure on the tensile properties after austenitisation and quenching of boron steelsManuskript (preprint) (Övrigt vetenskapligt)
  • 23.
    Marth, Stefan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Djebien, Slim
    Nagoya Institute of Technology, Japan.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Häggblad, Hans-Åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Stepwise modelling method for post necking characterisation of anisotropic sheet metal2021Ingår i: Modelling and Simulation in Materials Science and Engineering, ISSN 0965-0393, E-ISSN 1361-651X, Vol. 29, nr 8, artikel-id 085001Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Modelling and simulation are important tools during design and development processes. For accurate predictions of, e.g. manufacturing processes or final product performance, reliable material data is needed. Usually, the applied material models are calibrated by utilising direct methods such as conventional uniaxial tensile/compression tests but also inverse methods are occasionally applied. Recently, an effective inverse method, the stepwise modelling method (SMM), was presented. By using SMM, the flow stress from initial yielding, beyond necking to final fracture, can be determined. However, the method is developed for sheet materials having isotropic von Mises hardening. In this paper the SMM is extended for post necking characterisation of anisotropic sheet metals using the Barlat yield 2000 criterion. The novel method was applied to analyse the post necking plasticity of the widely used aluminium alloy AA6016 in T4 condition and the aluminium alloy AA5754 in H111 condition. The latter alloy has reported to show serrated yielding, also known as the Portevin–Le Chatelier effect. The obtained flow stress curves agree well with the curves form conventional uniaxial tensile tests up to the point of necking and show credible post necking predictions to final fracture. Furthermore, SMM showed that it could handle the effect of serrated yielding for AA5754-H111. Hence, the novel approach can be used to characterise the post necking hardening of a variety of anisotropic sheet metals and thereby contributes to efficient and reliable material model calibration.

  • 24.
    Marth, Stefan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Golling, Stefan
    Gestamp R&D, Box 828, 97 125, Luleå, Sweden.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Häggblad, Hans-Åke
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Prediction of ductile fracture for Aluminium Alloy AA6082 in T6 condition in various directions and stress triaxialities based on full field measurementsManuskript (preprint) (Övrigt vetenskapligt)
  • 25.
    Oldenburg, Mats
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Sundin, Karl-Gustaf
    Wikman, Bengt
    Kajberg, Jörgen
    Åkerström, Paul
    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)
  • 26.
    Sandin, Olle
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Rodriguez, Juan Manuel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära. School of Applied Sciences and Engineering, EAFIT University, Carrera 49 No. 7 South-50, Medellín, Colombia.
    Larour, Patrick
    voestalpine Stahl GmbH, voestalpine-Straße 3, 4020, Linz, Austria.
    Parareda, Sergi
    Unit of Metallic and Ceramic Materials, Eurecat, Centre Tecnològic de Catalunya, Plaça de la Ciència, 2, 08243, Manresa, Spain.
    Frómeta, David
    Unit of Metallic and Ceramic Materials, Eurecat, Centre Tecnològic de Catalunya, Plaça de la Ciència, 2, 08243, Manresa, Spain.
    Hammarberg, Samuel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Casellas, Daniel
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära. Unit of Metallic and Ceramic Materials, Eurecat, Centre Tecnològic de Catalunya, Plaça de la Ciència, 2, 08243 Manresa, Spain.
    A particle finite element method approach to model shear cutting of high-strength steel sheets2024Ingår i: Computational Particle Mechanics, ISSN 2196-4378Artikel i tidskrift (Refereegranskat)
  • 27.
    Sjöberg, Ted
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Calibration and Validation of Three Fracture Criteria for Alloy 718 Subjected to High Strain Rates and Elevated Temperatures2018Ingår i: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 71, s. 34-50Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The aerospace industry has an important role to play in lowering greenhouse gas emissions and thereby reducing its ecological footprint. Aerospace manufacturers do therefore push components and materials closer to their limits in order to design for minimal weight. An example of component is the metallic turbine case of the aircraft engine for protection of the surrounding structure in situations such as blade-off events. This implies design, where knowledge regarding mechanical material response from onset of yield to fracture, at both high strain rates and elevated temperatures, are needed. Alloy 718 is a nickel-based superalloy commonly used in the containment structures of the hot parts of the aircraft engines. Three established criteria, Modified-Mohr-Coulomb, Maximum Shear Stress and Magnitude of Stress Vector, have been applied to characterise the fracture behaviour of Alloy 718 supplied to conditions present at a blade-off event. The calibrations of the criteria were based on high-speed tensile tests of thin sheet specimens with different geometries for varying different stress states. The fracture strains were determined using high-speed photography combined with digital image correlation. The temperature and strain rate were varied from 20 to 650 °C and 1 to 1000 s−1 respectively. The calibrated criteria were validated through finite element analyses and reverse impact testing at room temperature up to 650 °C. Discs were fired against a instrumented rod with different shapes of their tips for obtaining varying stress states. It was found that the Modified-Mohr-Coulomb locus showed the best agreement with measured fracture strains in the calibration experiments. This criterion did also predict the fractures of discs in the reverse impact tests fairly well. The possibility to accurately predict fracture facilitates the use of modern numerical software for containment design as a complement to time-consuming and expensive full-scale tests.

  • 28.
    Sjöberg, Ted
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Calibration and Validation of Three Fracture Criteria for Alloy 718 Subjected to High Strain Rates and Elevated TemperaturesManuskript (preprint) (Övrigt vetenskapligt)
  • 29.
    Sjöberg, Ted
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    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)
    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.

  • 30.
    Sjöberg, Ted
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Marth, Stefan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Häggblad, Hans-åke
    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)
    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

  • 31.
    Sjöberg, Ted
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Sundin, Karl-Gustaf
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Oldenburg, Mats
    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)
    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

  • 32.
    Suarez, Laura
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Mechanical characterization of slag by optical techniques: A case of study2022Ingår i: Svenska Mekanikdagar 2022 / [ed] Pär Jonsén; Lars-Göran Westerberg; Simon Larsson; Erik Olsson, Luleå: Luleå tekniska universitet, 2022Konferensbidrag (Refereegranskat)
    Ladda ner fulltext (pdf)
    fulltext
  • 33.
    Suarez, Laura
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära. LKAB Minerals AB, Box 952, 97128, Luleå, Sweden.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Valorization of Air-Cooled EAF Manganese Slag in Comminution Processes: an Investigation into the Breakage Characterization2023Ingår i: Mining, Metallurgy & Exploration, ISSN 2524-3462, Vol. 40, nr 6, s. 2449-2462Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In recent years, slag, a residue from pyrometallurgical processes, has become more attractive in circular economy frameworks to increase the efficient use of resources throughout the life cycle of steel products and help in the reduction of carbon emissions. Its applicability is strongly dependent on the particle size, and therefore, the optimization of breaking processes should be approached by increasing the knowledge of the dynamics of slag to promote fracture. Increasing the knowledge on the mechanical response of manganese slag opens up the potential for the development of cost-effective numerical models, e.g., constitutive models based on inverse engineering calibration frameworks or digital twins. In this study, rate-dependent tests of manganese slag have been performed using a split Hopkinson pressure bar device for testing its dynamic mechanical response. In order to obtain information about the crack initiation and fracture process, 2D ultra-high speed imaging was implemented with a sampling frequency of 663,200 fps for diametrically loaded specimens. Full-field deformation measurements using digital image correlation (DIC) techniques showed a staggered fracture process where failure points on mechanical response curves vary due to the internal events happening in the material. Localized frictional occurrences and inertial effects acting inside the pre-cracked matrix have a strong effect on the global mechanical response, and therefore, a great variability of strengths was obtained.

    Ladda ner fulltext (pdf)
    fulltext
  • 34.
    Suarez, Laura
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Forsberg, Fredrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Mechanical Characterization of Highly Heterogeneous Brittle Materials by Optical Techniques2022Ingår i: Minerals Engineering, ISSN 0892-6875, E-ISSN 1872-9444, Vol. 185, artikel-id 107704Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Fragmentation processes like crushing and grinding are complex and extensively energy-consuming activities in the mining and mineral processing industry. Numerical analysis of different materials and loading conditions will gain more knowledge and support in the improvement of the efficiency of the fragmentation process. Although, a challenge is the lack of experimental data both for calibration and validation hampering the development of constitutive models. As a case of study, a mechanical characterization of pre-processed MnSiFe-slags was performed. Diametral and axial compressive tests under quasi-static conditions were used to load the mineral material and obtain a strain  field (ε) during increasing and cyclic loading until failure accounting for progressive damage. The evolution of the strain captured by digital image correlation (DIC) techniques exposed a mechanical behavior of composite-like material where random failure of the components caused high variability of the elastic parameters. These were found to be load dependent and they are strongly related to the ability of the material to internal rearrangement during loading. Irreversible damage affects the structure of the material and is perceived as non-linearities in the load-strain curves. It was found a degradation of the material under repetitive loading decreasing of the elastic modulus perceived as a weakening of the matrix and dominant behavior of the inclusions on the mechanical response of the material.

  • 35.
    Suarez, Laura
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Dynamic characterization of manganese slag core material using Split Hopkinson Pressure BarManuskript (preprint) (Övrigt vetenskapligt)
  • 36.
    Wessling, Albin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Towards the Discrete Element Modeling of Rock Drilling2019Konferensbidrag (Refereegranskat)
    Ladda ner fulltext (pdf)
    fulltext
  • 37.
    Wessling, Albin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Dynamic Compressive and Tensile Characterisation of Igneous Rocks Using Split-Hopkinson Pressure Bar and Digital Image Correlation2022Ingår i: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 15, nr 22, artikel-id 8264Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The dynamic fracture process of rock materials is of importance for several industrial applications, such as drilling for geothermal installation. Numerical simulation can aid in increasing the understanding about rock fracture; however, it requires precise knowledge about the dynamical mechanical properties alongside information about the initiation and propagation of cracks in the material. This work covers the detailed dynamic mechanical characterisation of two rock materials—Kuru grey granite and Kuru black diorite—using a Split-Hopkinson Pressure Bar complemented with high-speed imaging. The rock materials were characterised using the Brazilian disc and uniaxial compression tests. From the high-speed images, the instant of fracture initiation was estimated for both tests, and a Digital Image Correlation analysis was conducted for the Brazilian disc test. The nearly constant tensile strain in the centre was obtained by selecting a rectangular sensing region, sufficiently large to avoid complicated local strain distributions appearing between grains and at voids. With a significantly high camera frame rate of 671,000 fps, the indirect tensile strain and strain rates on the surface of the disc could be evaluated. Furthermore, the overloading effect in the Brazilian disc test is evaluated using a novel methodology consisting of high-speed images and Digital Image Correlation analysis. From this, the overloading effects were found to be 30 and 23%. The high-speed images of the compression tests indicated fracture initiation at 93 to 95% of the peak dynamic strength for granite and diorite, respectively. However, fracture initiation most likely occurred before this in a non-observed part of the sample. It is concluded that the indirect tensile strain obtained by selecting a proper size of the sensing region combined with the high temporal resolution result in a reliable estimate of crack formation and subsequent propagation.

  • 38.
    Wessling, Albin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Static and Dynamic Properties of Kuru Black Diorite and Grey Granite Using Full-Field Deformation MeasurementsManuskript (preprint) (Övrigt vetenskapligt)
  • 39.
    Wessling, Albin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Towards the Discrete Element Modeling of Wear in Rock Drilling Bits2019Konferensbidrag (Refereegranskat)
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    fulltext
  • 40.
    Wessling, Albin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Larsson, Simon
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Ramírez Sandoval, Giselle
    Unit of Metallic and Ceramic Materials, Eurecat, Centre Tecnológic de Catalunya Placa de la Ciéncia, 2, Manresa 08243, Spain.
    Vilaseca Llosada, Montserrat
    Unit of Metallic and Ceramic Materials, Eurecat, Centre Tecnológic de Catalunya Placa de la Ciéncia, 2, Manresa 08243, Spain.
    Discrete Element Modelling of Rock Drilling2022Ingår i: European Geothermal Congress 2022, 2022, artikel-id 272Konferensbidrag (Refereegranskat)
    Abstract [en]

    Percussive rotary drilling is recognized as the mostefficient method for hard rock drilling. Despite clearadvantages over conventional rotary methods, there arestill some uncertainties associated with percussivedrilling. For geothermal applications, drilling accountsfor a large portion of the total cost. Specifically, thewear of drill bits when drilling in hard rock is apredominant cost factor and drilling parameters areoften based on the experience of the field operator.Within the framework of the H2020 project GEOFIT,numerical simulations of percussive drilling areperformed in order to evaluate the rock drilling processand gain insight about the trade-off between wear andRate of Penetration (ROP). In the simulations, the rockmaterial was represented by the Bonded DiscreteElement Method (BDEM), the drill bit by the FiniteElement Method (FEM), the drilling fluid by theParticle Finite Element Method (PFEM) and theabrasive wear on the surface of the drill bit wasrepresented by Archard’s wear law. The drillingsimulations were conducted for two rock materials; asedimentary rock material corresponding to what wasfound when drilling at the GEOFIT pilot site in AranIslands, Ireland, and a harder reference rock similar togranite. The results show that, at a drill bit impact forceof 10 kN, the ROP in the sedimentary rock was 6.3times faster than for granite. When increasing theimpact force to 40 and 50 kN, however, the ROP for thesedimentary rock is only 1.9 and 1.6 times faster,respectively. Furthermore, the wear rate decreased withincreased impact force when drilling in the granite rock.For the sedimentary rock, however, the loadingresulting in the best trade-off between abrasive wearand ROP was the second highest loading of 40 kN,which suggests that an increase in impact energy mayincrease the rate of penetration but may not beeconomically motivated.

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    Discrete_Element_Modelling_Rock_Drilling
  • 41.
    Wessling, Albin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Warlo, Mathis
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Larsson, Simon
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    A Statistical Bonded Particle Model Study on Laboratory Scale Rock DrillingManuskript (preprint) (Övrigt vetenskapligt)
  • 42.
    Wessling, Albin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Larsson, Simon
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    A Brittle and Heterogeneous Bonded Discrete Element Model of Wide Applicability2022Ingår i: Svenska Mekanikdagar 2022 / [ed] Pär Jonsén; Lars-Göran Westerberg; Simon Larsson; Erik Olsson, Luleå tekniska universitet, 2022Konferensbidrag (Refereegranskat)
    Ladda ner fulltext (pdf)
    fulltext
  • 43.
    Wessling, Albin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Larsson, Simon
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    A Statistical Bonded Discrete Element Model for heterogeneous brittle materials2021Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    Numerical modelling of the fracture of heterogeneous brittle materials is of interest for several industries,such as rock excavation and comminution applications. A numerical model of brittle materials needs tobe able to capture the unpredictable results, e.g. with regards to measured strength and fracture pattern,as observed experimentally. In this study a new approach, based on the Parallel Bond Model (PBM) [1] and theWeibull distribution, for modelling brittle heterogeneous materials in 3D is proposed and appliedto the Brazilian Disc Test (BDT) [2]. The PBM is used to generate irregular grains with varying bondstrengths and stiffnesses. For the grain generation, a parent particle is chosen at random in the rockbody and a randomized ellipsoid is generated around the particle. The mean grain bond stiffnesses andstrengths are associated with the grain and all particles within the ellipsoid surface are bonded togetherwithin +/- 10 % of these mean values. Further, the bond parameters of the cement between a grain andits neighbours is scaled based on the mean grain properties. An example of a generated sample is shownin Figure 1 a). In order to evaluate the model, a series of simulations of the BDT were conducted.The effects of the Weibull heterogeneity index and cement strengths on the predicted tensile strengthand crack pattern were evaluated. Specifically, the initiation, propagation, coalescence and branching ofcracks were examined in detail. Apart from demonstrating challenges with the BDT, the results also showthat the proposed model is able to capture key phenomena related to brittle heterogeneous materials, suchas unpredictable fracture pattern and a large variation in tensile strength, see Figure 1 b-c).

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  • 44.
    Wessling, Albin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Larsson, Simon
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    A statistical DEM approach for modelling heterogeneous brittle materials2022Ingår i: Computational Particle Mechanics, ISSN 2196-4378, Vol. 9, nr 4, s. 615-631Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    By utilizing numerical models and simulation, insights about the fracture process of brittle heterogeneous materials can be gained without the need for expensive, difficult, or even impossible, experiments. Brittle and heterogeneous materials like rocks usually exhibit a large spread of experimental data and there is a need for a stochastic model that can mimic this behaviour. In this work, a new numerical approach, based on the Bonded Discrete Element Method, for modelling of heterogeneous brittle materials is proposed and evaluated. The material properties are introduced into the model via two main inputs. Firstly, the grains are constructed as ellipsoidal subsets of spherical discrete elements. The sizes and shapes of these ellipsoidal subsets are randomized, which introduces a grain shape heterogeneity Secondly, the micromechanical parameters of the constituent particles of the grains are given by the Weibull distribution. The model was applied to the Brazilian Disc Test, where the crack initiation, propagation, coalescence and branching could be investigated for different sets of grain cement strengths and sample heterogeneities. The crack initiation and propagation was found to be highly dependent on the level of heterogeneity and cement strength. Specifically, the amount of cracks initiating from the loading contact was found to be more prevalent for cases with higher cement strength and lower heterogeneity, while a more severe zigzag shaped crack pattern was found for the cases with lower cement strength and higher heterogeneity. Generally, the proposed model was found to be able to capture typical phenomena associated with brittle heterogeneous materials, e.g. the unpredictability of the strength in tension and crack properties.

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    fulltext
  • 45.
    Wessling, Albin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Larsson, Simon
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Jonsén, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Modelling Rock Fracture using the Stochastic Bonded Discrete Element Method2022Ingår i: Book of Abstracts: WCCM-APCOM 2022: 15th World Congress on Computational Mechanics & 8th Asian Pacific Congress on Computational Mechanics, Yokohama, Japan Virtual, Barcelona: International Center for Numerical Methods in Engineering (CIMNE), 2022, s. 390-390Konferensbidrag (Refereegranskat)
    Abstract [en]

    Numerical modelling of the fracture of heterogeneous brittle materials is of interest for several industries, such as rock excavation and comminution applications. A numerical model of brittle materials needs to be able to capture the unpredictable results, e.g. with regards to measured strength and fracture pattern, as observed experimentally. In a previous work [1], the Bonded Discrete Element Method [2] was combined with statistical methods in order to generate heterogeneous rock bodies. Grains of random sizes and shapes, consisting of multiple bonded discrete elements, were generated in the body and the micromechanical parameters of these grains were governed by the Weibull distribution [3]. In this work, this modelling approach was used to evaluate the fracture behaviour of experiments commonly found within the field of rock mechanics - the unconfined and confined axial compression test, Brazilian disc test and the three point bend test. For each test, a large set of numerical samples were generated and simulated. The fracture behaviour, e.g. initiation, propagation and coalescence of cracks, were investigated for different levels of heterogeneity and grain cement strengths. The results show that a variety of different fracture modes can be obtained with this modelling approach. Further, the results suggests that the statistical methods employed in this work improves the versatility of the Bonded Discrete Element Method for rock modelling.

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    Abstract
  • 46.
    Wessling, Albin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Larsson, Simon
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    Kajberg, Jörgen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Hållfasthetslära.
    A statistical bonded particle model study on the effects of rock heterogeneity and cement strength on dynamic rock fracture2023Ingår i: Computational Particle Mechanics, ISSN 2196-4378Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Numerical modelling and simulation can be used to gain insight about rock excavation processes such as rock drilling. Since rock materials are heterogeneous by nature due to varying mechanical and geometrical properties of constituent minerals, laboratory observations exhibit a certain degree of unpredictability, e.g. with regard to measured strength and crack propagation. In this work, a recently published heterogeneous bonded particle model is further developed and used to investigate dynamic rock fracture in a Brazilian disc test. The rock heterogeneities are introduced in two steps—a geometrical heterogeneity due to statistically distributed grain sizes and shapes, and a mechanical heterogeneity by distributing mechanical properties using three Weibull distributions. The first distribution is used for assigning average bond properties of the grains, the second one for the intragranular bond properties and the third one for the bond properties of the intergranular cementing. The model is calibrated for Kuru black diorite using previously published experimental data from high-deformation rate tests of Brazilian discs in a split-Hopkinson pressure bar device, where high-speed imaging was used to detect initiations of cracks and their growth. A parametric study is conducted on the Weibull heterogeneity index of the average bond properties and the grain cement strength and evaluated in terms of crack initiation and propagation, indirect tensile stress, strain and strain rate. The results show that this modelling approach is able to reproduce key phenomena of the dynamic rock fracture, such as stochastic crack initiation and propagation, as well as the magnitude and variations of measured quantities. Furthermore, the cement strength is found to be a key parameter for crack propagation path and time, overloading magnitudes and indirect tensile strain rate.

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