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  • 1.
    Abiri, Olufunminiyi
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Non-local models in manufacturing simulations2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Ductile fracture presents challenges with respect to material modelling andnumerical simulations of localization. The strain and damage localization maybe unwanted as it indicates a failure in the process or, as in the case ofmachining and cutting, a wanted phenomenon to be controlled. The latterrequires a higher accuracy regarding the modelling of the underlying coupledplastic and fracturing/damage behaviour of the material, metal in the currentcontext as well as the stability and robustness of the simulation procedure.This aim of this work is to develop, evaluate and implement formulations thatcan efficiently and reliably handle localization problems in machiningsimulations. The focus is on non-local models. The non-local models extendthe standard continuum mechanics theory by using non-local continuumtheory in order to achieve mesh independent results when simulating fractureor shear localization.The non-local damage model is implemented and various formulations areevaluated in a Matlab™ based finite element code. The chosen algorithm wasthen implemented in commercial software. The implementations remedy themesh sensitivity problem and gives convergent solution for metal cuttingsimulations with reasonable cost. The length scale associated with the nonlocalmodels are in the current context considered as a numericalregularization parameter. The model has been applied in machiningsimulations and compared with measurements from industry.Keywords: Finite element simulation; Non-local damage; Plasticity; Machining

  • 2.
    Abiri, Olufunminiyi
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Simplifications of non-local damage models2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Ductile fracture presents challenges with respect to material modelling and numerical simulations of localization. The strain and damage localization may be unwanted as it indicates a failure in the process or, as in the case of machining and cutting, a wanted phenomenon to be controlled. The latter requires a higher accuracy regarding the modelling of the underlying coupled plastic and fracturing/damage behaviour of the material, metal in the current context as well as the robustness of the simulation procedure. The focus of this thesis is on efficient and reliable finite element solution of the localization problem through the non-local damage model. The non-local damage model extends the standard continuum mechanics theory by using non-local continuum theory in order to achieve mesh independent results when simulating fracture or shear localization. In this work, the non-local damage model and its various simplifications are evaluated in an in-house finite element code developed using Matlab™. The accuracy, robustness, efficiency and costs of the models are investigated and also compared to a general multi-length scale finite element formulation. A numerical study versus published data is used to demonstrate the validity of the model. The explicit non-local damage variant will be implemented in a commercial finite element code for use in machining simulation

  • 3.
    Abiri, Olufunminiyi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Non-local damage models in manufacturing simulations2013Conference paper (Refereed)
    Abstract [en]

    Localisation of deformation is a problem in several manufacturing processes. Machining is an exception where it is a wanted feature. However, it is always a problem in finite element modelling of these processes due to mesh sensitivity of the computed results. The remedy is to incorporate a length scale into the numerical formulations in order to achieve convergent solutions. Different simplifications in the implementation of a non-local damage model are evaluated with respect to temporal and spatial discretisation to show the effect of different approximations on accuracy and convergence.

  • 4.
    Abiri, Olufunminiyi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Non-local damage models in manufacturing simulations2015In: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 49, p. 548-560Article in journal (Refereed)
    Abstract [en]

    Localisation of deformation is a problem in several manufacturing processes. Machining is an exception where it is a wanted feature. However, it is always a problem in finite element modelling of these processes due to mesh sensitivity of the computed results. The remedy is to incorporate a length scale into the numerical formulations in order to achieve convergent solutions. Different simplifications in the implementation of a non-local damage model are evaluated with respect to temporal and spatial discretisation to show the effect of different approximations on accuracy and convergence.

  • 5.
    Abiri, Olufunminiyi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Qin, Hao
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Comparison of Multiresolution Continuum Theory and Nonlocal Dame model for use in Simulation of Manufacutring Processes2016In: International Journal for Multiscale Computational Engineering, ISSN 1543-1649, Vol. 14, no 1, p. 81-94Article in journal (Refereed)
    Abstract [en]

    Modelling and simulation of manufacturing processes may require the capability to account for localization behavior, often associated with damage/fracture. It may be unwanted localization indicating a failure in the process or, as in the case of machining and cutting, a wanted phenomenon to be controlled. The latter requires a higher accuracy regarding the modelling of the underlying physics, as well as the robustness of the simulation procedure. Two different approaches for achieving mesh-independent solutions are compared in this paper. They are the multiresolution continuum theory (MRCT) and nonlocal damage model. The MRCT theory is a general multilength-scale finite element formulation, while the nonlocal damage model is a specialized method using a weighted averaging of softening internal variables over a spatial neighborhood of the material point. Both approaches result in a converged finite element solution of the localization problem upon mesh refinement. This study compares the accuracy and robustness of their numerical schemes in implicit finite element codes for the plane strain shear deformation test case. Final remarks concerning ease of implementation of the methods in commercial finite element packages are also given.

  • 6.
    Abiri, Olufunminiyi
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. University of Johannesburg, South Africa.
    Svoboda, Ales
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Wedberg, Dan
    Controlling Thermal Softening Using Non-Local Temperature Field in Modelling2016In: Journal of Machining and Forming Technologies, ISSN 1947-4369, Vol. 8, no 1-2, p. 13-28Article in journal (Refereed)
    Abstract [en]

    One of the aims of this work is to show that thermal softening due to the reduced flow strength of a material with increasing temperature may cause chip serrations to form during machining. The other purpose, the main focus of the paper, is to demonstrate that a non-local temperature field can be used to control these serrations. The non-local temperature is a weighted average of the temperature field in the region surrounding an integration point. Its size is determined by a length scale. This length scale may be based on the physics of the process but is taken here as a regularization parameter.

  • 7.
    Abiri, Olufunminiyi
    et al.
    Institute of Intelligent Systems, University of Johannesburg.
    Wedberg, Dan
    AB Sandvik Coromant.
    Svoboda, Ales
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Non-Local Modelling of Strain Softening in Machining Simulations2017In: IOP Conference Series: Materials Science and Engineering, ISSN 1757-8981, E-ISSN 1757-899X, Vol. 225, article id 012053Article in journal (Refereed)
    Abstract [en]

    Non-local damage model for strain softening in a machining simulation is presented in this paper. The coupled damage-plasticity model consists of a physically based dislocation density model and a damage model driven by plastic straining in combination with the stress state. The predicted chip serration is highly consistent with the measurement results. 

  • 8.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Analysis of multiple solutions in bifurcation diagrams to avoid unexpected dynamics2009Conference paper (Refereed)
    Abstract [en]

    In mechanical applications it is essential that unexpected dynamics are avoided. The industry wants to build reliable machines that are not sensitive to initial conditions. Therefore, a simple method has been developed to extract all sets of stable bifurcation diagrams. The method gives a designer a good overview of possible dynamics and thereby the possibility to select a safe operating region. The method is described and demonstrated with a rub-impact rotor. The practical usage of this method is to help the designer to determine if parameter ranges exist where coexistent solutions will appear. Thereby one can design the system to work in parameter ranges where only one acceptable solution exists.

  • 9.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dynamics of some vibro-impacting systems with amplitude constraints1995Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis concerns the dynamics of some vibro-impacting systems with fixed or moving amplitude constraints. It is based on and includes five papers, marked A to E. Simple models of three different vibro-impacting systems with applications in the fields of impact hammers, granular flow and disk brakes in vehicles are analysed. A 2-DOF (two-degree-of-freedom) model of a threshold-limited impact hammer is studied (Paper A). The stability of a class of periodic motions is analysed. For some parameter values these periodic motions are found to be qualitatively similar to the ones observed for a corresponding 1-DOF system. At other parameter combinations, however, new kinds of periodic or chaotic motions can be observed. For low damping, phenomena resembling antiresonance for linear systems can also be observed. Granular shear flows show a transitional behaviour in the rapid flow regime as the shear speed or the concentration of the grains is varied. The motion can, for example, change from smooth and orderly to erratic and turbulent. Some aspects of this transitional behaviour in granular shear flow are studied numerically, analytically and experimentally (Papers B, C and D). Simple vibro-impacting models are suggested to get some analytical insight into the dynamics of shear layers. Results from a 1-DOF model show that for high forcing frequencies, which correspond to high shear speeds, periodic as well as chaotic motions can exist, whereas, for low forcing frequencies the vibrations are completely damped out to a stationary state (Paper B). Stability of this stationary state is studied analytically (Paper C), and experimentally (Paper D), where the motions of granular particles in a transparent shear cell are followed by using video techniques. For low shear speeds a single shear layer adjacent to the bottom boundary of the shear cell is observed. As the shear speed is increased, a transition to a random like state involving many layers is found to occur. In order to understand the phenomenon of squeal in disk brakes, a 3-DOF model is suggested to simulate the dynamics of a brake pad. The region of contact between the brake pad and the disk is described by using a coefficient of friction and distributed stiffness. The brake pad is allowed to have adjustable support locations and possibilities of impacts with its surroundings. The equilibrium state of the pad is determined by using a static analysis. The assumption is that the instability of this stationary state is a possible explanation of squeal, therefore, the stability is analysed in detail. Examples of different kinds of pad motions are presented. A rich variety of motions are found to exist including periodic, seemingly chaotic, stationary behaviour in slip, vibrations with full contact with the disk, stick-slip and impacts.

  • 10.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Multiple solutions in a rub-impact Jeffcott rotor2004In: Extended abstracts, proceedings: 10th international symposium on transport phenomena and dynamics of rotating machinery / [ed] Dieter Bohn, Aachen: Inst. of steam and gas turbines, RWTH , 2004Conference paper (Refereed)
  • 11.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Multiple solutions in an amplitude limited Jeffcott rotor including rubbing and stick-slip effect2005In: Proceedings ASME 2005 International Design Engineering Technical Conferences & Computers and Information in Engineering Conference: DETC 2005, American Society of Mechanical Engineers , 2005, p. 1091-1096Conference paper (Refereed)
    Abstract [en]

    The non-linear behaviour of rub-impact rotors have been studied in several papers. In such systems rich dynamics have been found together with the coexistence of solutions within some specific parameter ranges. In this paper an attempt is made to find all stable solutions for an amplitude limited Jeffcott rotor including rubbing and stick-slip effect. The recently suggested "multi bifurcation diagram method" is used to find and extract stable sets of bifurcation diagrams. A system is chosen where the linear stationary amplitude only exceeds the clearance in a narrow region near the natural frequency. Therefore large regions in frequency are expected to have only the linear stationary response. The results show that it is only for very low frequencies that one single solution exists. Even though periodic motions are dominant, there exist large ranges in frequency with quasi-periodic or chaotic motions. For the studied cases, three coexisting stable solutions are most common. In one case as many as four stable solutions was found to coexist. For rotors with large clearances (no impacts necessary) it is still possible to find several coexisting motions. For all cases the stick motion is the most severe one with large amplitudes and high backward whirl frequencies. In real situations the consequence of this stick motion is machine failure. These high amplitude motions were found to be stable over large frequency ranges. From the stability analysis it was found that this rolling motion can be avoided by low spin speed, low contact stiffness, low coefficient of friction, small ratio of disc radius/clearance or high damping ratio. In a design situation the parameters are seldom known with high accuracy. Therefore, it is of interest to know all solutions for parameter intervals. The multi-bifurcation diagram can be used in such situations to design a robust machine or at least be prepared for unwanted dynamics.

  • 12.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Review of Mechanical Vibrations by S.S. Rao2007In: Proceedings of the Institution of mechanical engineers. Part C, journal of mechanical engineering science, ISSN 0954-4062, E-ISSN 2041-2983, Vol. 221, no 1, p. 135-Article in journal (Other academic)
  • 13.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Rotor dynamical analysis of failure due to rotor-stator contact in an overhung rotor2006In: Proceedings - 7th International Conference on Rotor Dynamics: September 25 - 28, 2006, Vienna, Austria / [ed] H. Springer; H. Ecker, Vienna: Institute of Mechanics and Mechatronics, Vienna University of Technology , 2006, article id 264Conference paper (Refereed)
    Abstract [en]

    Two new 1MW fans in a remote gas cleaning plant have failed within 12 days of operation. Due to high costs a rapid repair and modification was performed without any deeper investigation or measurements. Since the driving frequency was far below the critical speed, a theory was that contact between rotating and stationary part was the reason for failure. In this paper the rotor dynamics is studied by an amplitude limited 4DOF overhung rotor. Simulations indicate that in addition to the ordinary unbalance response there can exist a high amplitude rolling contact mode between the disc and the stationary parts. By analysis of basin of attraction it is concluded that contact is a possible cause for the failure if the damping ratio is low, coefficient of friction is high and the rotor is eccentrically positioned relative to the stator. Measurements on the system are however necessary before a satisfying explanation can be given.

  • 14.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Two vibroimpacting systems with amplitude constraints1992Licentiate thesis, comprehensive summary (Other academic)
  • 15.
    Aidanpää, Jan-Olov
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Gupta, R.B.
    Periodic and chaotic behaviour of a threshold-limited two-degree-of-freedom system1993In: Journal of Sound and Vibration, ISSN 0022-460X, E-ISSN 1095-8568, Vol. 165, no 2, p. 305-327Article in journal (Refereed)
    Abstract [en]

    A two-degree-of-freedom impact oscillator with proportional damping is considered. The maximum displacement of one of the masses is limited to a threshold value by a rigid wall, which gives rise to a non-linearity in the system. Impacts between the mass and the wall are described by a coefficient of restitution. The behaviour of the system is rich and includes features like period doublings, period havings, jumps, chaos, etc. Periodic motions of the system are studied by analytical methods. The influence of system parameters such as damping, coefficient of restitution, distribution of masses and clearance, etc., is studied for some extreme values of these parameters. The stability of a class of periodic motions is investigated. Parameter ranges which result in stable periodic multiple impacts are identified. Application of the results to the design of impact tools is discussed.

  • 16.
    Aidanpää, Jan-Olov
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Gustavsson, Rolf K.
    Vattenfall, VRD.
    Lundström, Niklas L. P.
    Umeå university.
    Karlsson, Martin
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Calleecharan, Yogeshwarsing
    Nässelqvist, Mattias
    Karlberg, Magnus
    Lundin, Urban
    Uppsala University.
    Developments in rotor dynamical modeling of hydropower units2009In: Proceedings of the IUTAM Symposium on Emerging Trends in Rotor Dynamics: held in New Delhi, India, March 23 - March 26, 2009 / [ed] Kshitij Gupta, Dordrecht: Encyclopedia of Global Archaeology/Springer Verlag, 2009Conference paper (Refereed)
  • 17.
    Aidanpää, Jan-Olov
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindkvist, Göran
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dynamics of a rubbing Jeffcott rotor with three blades2011In: Chaos Theory: Modeling, Simulation and Applications: Selected Papers from the 3rd Chaotic Modeling and Simulation International Conference (CHAOS2010) / [ed] Christos H. Skiadas; Ioannis Dimotikalis; Charilaos Skiadas, Singapore: World Scientific Publishing Co Pte Ltd , 2011, Vol. Part II. Invited and Contributed Papers, p. 97-104Conference paper (Refereed)
  • 18.
    Aidanpää, Jan-Olov
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lundström, Niklas L. P.
    Umeå university.
    Complex dynamic responses due to deviations of generator shape2009In: 10th Conference on Dynamical Systems - Theory and Applications: DSTA-2009, December 7-10, Łódź, Poland, 2009Conference paper (Refereed)
    Abstract [en]

    Earlier measurements in large synchronous generators indicate the existence of complex whirling motion, and also deviations of shape in both the rotor and the stator. These non-symmetric geometries produce an attraction force between the rotor and the stator, called Unbalanced Magnetic Pull (UMP). The target of this paper is to analyse responses due to certain deviations of shape in the rotor and the stator. In particular, the perturbation on the rotor is considered to be of oval character, and the perturbations of the stator are considered triangular and eccentric. It is concluded that harmful conditions can occur due to high amplitudes especially when the whirling of the rotor changes from forward to backward. During maintenance of hydro power generators the shapes of the rotor and stator are frequently measured. The results from this paper can be used to evaluate such measurements and to explain the existence of complex whirling motion.

  • 19.
    Aidanpää, Jan-Olov
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Shen, Hayley
    Clarkson University, Potsdam, NY.
    Onset of transition for cohesive and viscous granular flows1998In: Journal of engineering mechanics, ISSN 0733-9399, E-ISSN 1943-7889, Vol. 124, no 10, p. 1073-1079Article in journal (Refereed)
    Abstract [en]

    A theory for determining the onset of rapid flow or termination of quasi-static flow in a cohesive granular material mixed in a viscous interstitial fluid is presented. This theory is based on a one-dimensional model introduced in an earlier paper. Linear stability analysis is used to find the combination of material properties and flow rate for which a quasi-static flow becomes unstable, indicating the onset of rapid flow. This theory yields a quantitative criterion to select appropriate constitutive laws for viscous and cohesive granular material. This criterion is a linear approximation of the theoretical result. While it provides the critical condition for quasi-static flow to terminate, it does not give the transitional constitutive law before a rapid flow is fully developed, for which a second power law dependence on the strain rate applies. Nonetheless, the analysis and modeling effort presented in this work are necessary first steps toward a general quantification of how granular materials, with identical physical composition, can behave in drastically different ways.

  • 20.
    Aidanpää, Jan-Olov
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Shen, Hayley H.
    Gupta, Ram B.
    Uppsala universitet.
    Experimental and numerical studies of shear layers in granular shear cell1996In: Journal of engineering mechanics, ISSN 0733-9399, E-ISSN 1943-7889, Vol. 122, no 3, p. 187-196Article in journal (Refereed)
    Abstract [en]

    The stability of a shear layer inside a granular material in a gravity field is studied experimentally and numerically. A shear cell is built of transparent acrylic to visualize the motion of the granular material. This shear cell consists of two concentric cylinders containing layers of uniform spheres in the annular space between the cylinders. The shearing motion of the spheres is produced by rotating the bottom boundary of the cell. Friction of the cylinder walls resists the shear motion, thus creating a single shear layer adjacent to the bottom boundary, while the rest of the layers above move with constant speed as a solid body. As the rotation speed of the bottom boundary increases, two layers adjacent to the bottom boundary begin to shear. This shearing zone quickly thickens and dilates as the rotational speed increases. The transition of this shear motion from a single layer to many layers of shearing is studied by video recording. The initiation of this transition is observed to depend on the material properties and the number of layers overlaying the shear layer. A one-dimensional numerical model is constructed to bring insight into this transitional phenomenon.

  • 21.
    Aidanpää, Jan-Olov
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Shen, Hayley H.
    Department of Civil and Environmental Engineering, Clarkson university.
    Gupta, Ram B.
    Division of Solid Mechanics, School of Engineering, Uppsala University.
    Stability and bifurcations of a stationary state for an impact oscillator1994In: Chaos, ISSN 1054-1500, E-ISSN 1089-7682, Vol. 4, no 4, p. 621-630Article in journal (Refereed)
    Abstract [en]

    The motion of a vibroimpacting one-degree-of-freedom model is analyzed. This model is motivated by the behavior of a shearing granular material, in which a transitional phenomenon is observed as the concentration of the grains decreases. This transition changes the motion of a granular assembly from an orderly shearing between two blocks sandwiching a single layer of grains to a chaotic shear flow of the whole granular mass. The model consists of a mass-spring-dashpot assembly that bounces between two rigid walls. The walls are prescribed to move harmonically in opposite phases. For low wall frequencies or small amplitudes, the motion of the mass is damped out, and it approaches a stationary state with zero velocity and displacement. In this paper, the stability of such a state and the transition into chaos are analyzed. It is shown that the state is always changed into a saddle point after a bifurcation. For some parameter combinations, horseshoe-like structures can be observed in the Poincare sections. Analyzing the stable and unstable manifolds of the saddle point, transversal homoclinic points are found to exist for some of these parameter combinations

  • 22.
    Aidanpää, Jan-Olov
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Shen, H.H.
    Clarkson University, Potsdam, NY.
    Gupta, R.
    Luleå tekniska universitet.
    Babic, M.
    University of Notre Dame, Notre Dame, IN.
    A model for the transitional behavior of simple shear flows of disks1992In: Studies in Applied Mechanics, ISSN 0922-5382, Vol. 31, p. 301-310Article in journal (Refereed)
    Abstract [en]

    A simple one-dimensional model is suggested to simulate some aspects of the dynamics of granular shear flows. The model consists of a viscoelastic packet bouncing between two harmonically oscillating walls. The amplitude and frequency of the wall motion are related to the concentration and shear rate respectively. The dynamics of the system is studied and it is shown that periodic as well as chaotic motions are possible for different parameters and initial conditions. Results are presented in the form of time histories, power spectral densities, phase diagrams, Poincaré maps and bifurcation diagrams. Some of the characteristics present in the two-dimensional granular shear flows are present in this simple model. It is promising that such analysis can provide better understanding of the transitional behavior of the constitutive relation between the rapid shear and the quasi-static flows.

  • 23.
    Aidanpää, Jan-Olov
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Shen, H.H.
    Gupta, R.B.
    Babic, M.
    One-dimensional model for the transition from periodic to chaotic motions in granular shear flows1993In: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 16, no 1-2, p. 153-161Article in journal (Refereed)
    Abstract [en]

    A simple one-dimensional mechanical model to simulate some aspects of the dynamics of granular flow is suggested. The model consists of a visco-elastic packet bouncing between two oscillating walls. The motion of the walls is prescribed to be harmonic. The amplitude and frequency of the wall motion are related to the concentration and shear rate, respectively. The dynamics of the system is studied for various parameter combinations and it is shown that periodic as well as chaotic motions are possible for different parameters and initial conditions. Results are presented in the form of time histories, power spectral densities, phase diagrams, Poincare plots and bifurcation diagrams. This simple one-dimensional model presents many features that are analogous to those observed in the two-dimensional simple shear flow of disks.

  • 24.
    Alam, Md. Minhaj
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Barsoum, Z
    Royal Institute of Technology, Department of Aeronautical and Vehicle Engineering, Stockholm.
    Jonsén, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Häggblad, Hans-Åke
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Influence of defects on fatigue crack propagation in laser hybrid welded eccentric fillet joint2011In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 78, no 10, p. 2246-2258Article in journal (Refereed)
    Abstract [en]

    Fatigue cracking of laser hybrid welded eccentric fillet joints has been studied for stainless steel. Two-dimensional linear elastic fracture mechanics analysis was carried out for this joint geometry for four point bending load. The numerical simulations explain for the experimental observations why the crack propagates from the lower weld toe and why the crack gradually bends towards the root. Lack of fusion turned out to be uncritical for the initiation of cracks due to its compressive stress conditions. The linear elastic fracture mechanics analysis has demonstrated in good qualitative agreement with fatigue test results that lack of fusion slightly (<10%) reduces the fatigue life by accelerating the crack propagation. For the geometrical conditions studied here improved understanding of the crack propagation was obtained and in turn illustrated. The elaborated design curves turned out to be above the standard recommendations

  • 25.
    Alam, Md. Minhaj
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Barsoum, Z.
    Royal Institute of Technology.
    Jonsén, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Kaplan, Alexander
    Häggblad, Hans-Åke
    The influence of surface geometry and topography on the fatigue cracking behaviour of laser hybrid welded eccentric fillet joints2010In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 265, no 6, p. 1936-1945Article in journal (Refereed)
    Abstract [en]

    Laser hybrid welding of an eccentric fillet joint causes a complex geometry for fatigue load by four point bending. The weld surface geometry and topography were measured and studied in order to understand the crack initiation mechanisms. The crack initiation location and the crack propagation path were studied and compared to Finite Element stress analysis, taking into account the surface macro- and micro-geometry. It can be explained why the root and the upper weld toe are uncritical for cracking. The cracks that initiate from the weld bead show higher fatigue strength than the samples failing at the lower weld toe, as can be explained by a critical radius for the toe below which surface ripples instead determine the main stress raiser location for cracking. The location of maximum surface stress is related to a combination of throat depth, toe radius and sharp surface ripples along which the cracks preferably propagate.

  • 26.
    Alam, Md. Minhaj
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Barsoum, Zuheir
    Kungliga tekniska högskolan, KTH.
    Jonsén, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Häggblad, Hans-Åke
    Kaplan, Alexander
    Fatigue behaviour study of laser hybrid welded eccentric fillet joints: Part I2009In: 12th NOLAMP proceeding 2009: Nordic Laser Materials Processing Conference ; 24th - 26th August 2009 in Copenhagen / [ed] Erling Dam Mortensen, Kgs. Lyngby: ATV-SEMAPP , 2009Conference paper (Refereed)
    Abstract [en]

    Welded joints are a major component that is often responsible for causing a structure failure or for being the point at which fatigue cracking initiates and propagates. Despite tremendous research efforts, the understanding of fatigue behaviour is still limited, particularly for new techniques like laser hybrid welding. Beside a comprehensive state-of-the-art study, the paper presents a fatigue study of laser hybrid welded eccentric fillet joint of stainless steel of 10 mm thickness, with 5 mm displacement. Motivation is to study the influence of the surface geometry shape on fatigue performance under a four point bending test. 13 samples were produced, measuring the toe radii and testing under constant amplitude loading with stress ratio R=0. Different techniques have been used to measure local weld geometry, like plastic replica, a 3D optical profiler and a 3D-digitizer. The influence of the local weld geometry, like the toe radii, on the stress concentration was studied by FE-analysis. Occasionally lack of fusion was observed, which was taken into account in the FE-analysis. Based on the nominal stress approach, SN-curves were designed for laser hybrid welded eccentric fillet joints. Macro hardness tests were carried out and the crack surfaces were observed in order to detect crack initiation and propagation. Correlations between the toe radii, the corresponding stress maxima and crack initiation locations were studied between the different samples and even along the welds.

  • 27.
    Alam, Md. Minhaj
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Barsoum, Zuheir
    Kungliga tekniska högskolan, KTH.
    Jonsén, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Häggblad, Hans-Åke
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Fatigue behaviour study of laser hybrid welded eccentric fillet joints: Part II: State-of-the-art of fracture mechanics and fatigue analysis of welded joints2009In: 12th NOLAMP proceeding 2009: Nordic Laser Materials Processing Conference ; 24th - 26th August 2009 in Copenhagen / [ed] Erling Dam Mortensen, Kgs. Lyngby: ATV-SEMAPP , 2009Conference paper (Refereed)
    Abstract [en]

    Simplified fatigue and fracture mechanics based assessment methods are widely used by the industry to determine the structural integrity significance of postulated cracks, manufacturing flaws, service-induced cracking or suspected degradation of engineering components under normal and abnormal service loads. In many cases, welded joints are the regions most likely to contain original fabrication defects or cracks initiating and growing during service operation. The welded joints are a major component that is often blamed for causing a structure failure or for being the point at which fatigue or fracture problems initiate and propagate. Various mathematical models/techniques for various classes of welded joints are developed by analytically or by simulation software's that can be used in fatigue and fracture assessments. This literature survey compiled useful information on fracture and fatigue analysis of various welded joints. The present review is divided into two major sections- fracture mechanics and fatigue analysis with widely used models. A survey table is also introduced to get the outlook of research trend on fatigue and fracture over last 3 decades. Although tremendous research effort has been implemented on fatigue and fracture analysis of conventional welding, research on relatively new welding technology (laser welding, hybrid laser welding) is still limited and unsatisfactory. In order to give guarantee or make welding standard for new welding technology, further research is required in the field of fatigue and fracture mechanics including FEM and multi-scale modeling.

  • 28.
    Alam, Md. Minhaj
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Barsoum, Zuheir
    Royal Institute of Technology.
    Jonsén, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Häggblad, Hans-Åke
    Kaplan, Alexander
    Geometrical aspects of the fatigue behaviour of laser hybrid fillet welds2009In: Proceedings of the Fatigue Design Conference, Cetim , 2009Conference paper (Refereed)
  • 29.
    Alam, Md. Minhaj
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Barsoum, Zuheir
    Royal Institute of Technology.
    Jonsén, Pär
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Kaplan, Alexander
    Häggblad, Hans-Åke
    The effects of surface topography and lack of fusion on the fatigue strength of laser hybrid welds2009In: Congress proceedings: ICALEO, 28th International Congress on Applications of Lasers & Electro-Optics : November 2 - 5, 2009 - Orlando, FL : Laser Materials Processing Conference, Laser Microprocessing Conference, Nanomanufacturing Conference, poster presentation, gallery, Orlando, Fla: Laser institute of America , 2009, p. 38-46Conference paper (Refereed)
    Abstract [en]

    The geometrical aspects of laser hybrid welding before, during and after the process differ from autonomous laser welding and from arc welding. When studying the fatigue behaviour of laser hybrid welded fillet joints we identified that the microgeometry, i.e. the surface ripples can be more critical than the macrogeometry of the weld surface and even than lack of fusion (LOF), which frequently was detected. The plastic replica method was applied to measure the toe radii at the weld edges while the topography was identified by interferometric profilometry. From metallurgical analysis of the joint interface the tendency to LOF can be explained. Stress analysis was carried out by FEA for the complex joint geometry and bending load situation, showing maximum stress on the weld toes, even when including LOF. It was shown that the position and value of the maximum stress depends on a non-trivial combination of the weld geometry, including possible LOF, and the surface topography. Thus it can be explained that at compressive stress conditions LOF does not contribute significantly to the fatigue strength of laser hybrid welds while the surface topography does. Recommendations for defining and in turn avoiding critical geometrical aspects during the welding process are discussed.

  • 30. Angantyr, Anders
    et al.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    A pareto-based genetic algorithm search approach to handle damped natural frequency constraints in turbo Generator rotor system design2004In: Journal of engineering for gas turbines and power, ISSN 0742-4795, E-ISSN 1528-8919, Vol. 126, no 3, p. 619-625Article in journal (Refereed)
    Abstract [en]

    The detailed design of a turbo generator rotor system is highly constrained by feasible regions for the damped natural frequencies of the system. A major problem for the designer is to find a solution that fulfills the design criterion for the damped natural frequencies. The bearings and some geometrical variables of the rotor are used as the primary design variables in order to achieve a feasible design. This paper presents an alternative approach to search for feasible designs. The design problem is formulated as an optimization problem and a genetic algorithm (GA) is used to search for feasible designs. Then, the problem is extended to include another objective (i.e., multiobjective optimization) to show the potential of using the optimization formulation and a Pareto-based GA in this rotordynamic application. The results show that the presented approach is promising as an engineering design tool

  • 31. Angantyr, Anders
    et al.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Optimization of a rotor-bearing system with an evolutionary algorithm2004In: 10th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery: ISROMAC-2004 / [ed] Dieter Bohn, Aachen: Inst. of steam and gas turbines, RWTH , 2004Conference paper (Refereed)
  • 32. Angantyr, Anders
    et al.
    Andersson, Johan
    Linköping University.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Constrained optimization based on a multiobjective evolutionary algorithm2003In: The 2003 congress on evolutionary computation: CEC 2003, Piscataway, NJ: IEEE Communications Society, 2003, p. 1560-1567Conference paper (Refereed)
    Abstract [en]

    A criticism of Evolutionary Algorithms (EAs) might be the lack of efficient and robust generic might be the lack of officient and robust generic methods to handle constraints. The most widespread approach for constrained search problems is to use penalty methods. EAs have received increased interest during the last decade due to the ease of handling multiple objectives., A constrained Optimization problem or an unconstrained multiobjective problem may in principle be two different ways to pose the same underlying I problem. In this paper an alternative approach for the constrained optimization problem is presented. The method is a variant of a multiobjective real coded Genetic Algorithm (CA) inspired by the penalty approach. It is evaluated on six different constrained single objective problems found in the literature. The results show that the proposed method performs well in terms of efficiency, and that it is rohust for a majority of the test problems.

  • 33.
    Aoki, S.
    et al.
    Tokyo Institute of Technology, Department of Mechanical and Environmental Informatics.
    Amaya, K.
    Tokyo Institute of Technology, Department of Mechanical and Environmental Informatics.
    Noguchi, T.
    Tokyo Institute of Technology, Department of Mechanical and Environmental Informatics.
    Ståhle, P.
    Luleå tekniska universitet.
    Sundin, Karl-Gustaf
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Identification of elastic/visco-plastic material constants under impact loading2000In: 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, 2000Conference paper (Refereed)
    Abstract [en]

    A method for identifying the material constants of an elastic/visco-plastic meterial under tensile impact loading is proposed. In this method, the Kalman filter and the finite element method are used together with a few test data. To obtain data for high strain rates, notched specimens are employed, because arbitrarily shaped specimens are available in this method. To increase the accuracy of identification, a test under a relatively low strain rate is needed in addition to a test under a high strain rate. The optimum strain rate is determined in such a way that the largest eigen value of the covariance matrix of the estimated parameters becomes minimum. A numerical simulation in performed to demonstrate the validity and usefulness of this method

  • 34.
    Azizoğlu, Yağız
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. Luleå University of Technology.
    Modeling of Cold Pilgering of Tubes2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Cold pilgering is a challenging tube forming process in terms of modeling due to the complexity in kinematic of tools, friction condition and material behavior. The process development has mostly been based on simple formulas and costly full-scale tryouts. The aim in this study is to develop validated Finite element models of cold pilgering to increase the understanding of influence of the process parameters on the produced tubes.

    In the course of this thesis, three-dimensional mechanical and thermo-mechanical Finite element models of cold pilgering were developed. The commercial code MSC.Marc was used in the simulations. General 3D models are needed to be able to capture asymmetric deformation in cold pilgering. It was found that tool deflections together with elastic deformation of roll dies have considerable influence on the rolling force. Furthermore, the strain rate and temperature effects on the response of the material and thereby on the rolling force were evaluated.

  • 35.
    Azizoğlu, Yağız
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Gärdsback, Mattias
    Sandvik Materials Technology, R&D, Sandviken.
    Sjöberg, Bengt
    Sandvik Materials Technology, R&D, Sandviken.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Finite Element Analysis of cold pilgering using elastic roll dies2017In: Procedia Engineering, ISSN 1877-7058, E-ISSN 1877-7058, Vol. 207, p. 2370-2375Article in journal (Refereed)
    Abstract [en]

    A finite element model of cold pilgering with elastic roll dies have been developed and used to investigate the influence of roll die deformation on the material flow, contact region, roll separating force and tube dimensions. Full scale experiments were performed to validate the contact surface and tube dimensions. The results show that the influence of roll die flattening is not significant on the contact length. However, elastic deformation of roll die has strong influence on both the wall thickness reduction and roll separating force. Thus it is recommended to consider elasticity of roll dies when forces and tube dimensions are estimated.

  • 36.
    Azizoğlu, Yağız
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Gärdsback, Mattias
    Sandvik Mat Technol, R&D, SE-81181 Sandviken.
    Sjöberg, Bengt
    Sandvik Mat Technol, R&D, SE-81181 Sandviken.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Finite element modeling of tube deformation during cold pilgering2016In: NUMIFORM 2016: The 12th International Conference on Numerical Methods in Industrial Forming Processes / [ed] Saanouni, K; Chenot, JL; Duval, JL, 2016, Vol. 80, p. 1-8, article id 15004Conference paper (Refereed)
    Abstract [en]

    A three-dimensional finite element model of cold pilgering of stainless steel tubes is developed in this paper. The objective is to use the model to increase the understanding of forces and deformations in the process. The focus is on the influence of vertical displacements of the roll stand and axial displacements of the mandrel and tube. Therefore, the rigid tools and the tube are supported with elastic springs. Additionally, the influences of friction coefficients in the tube/mandrel and tube/roll interfaces are examined. A sensitivity study is performed to investigate the influences of these parameters on the strain path and the roll separation force. The results show the importance of accounting for the displacements of the tube and rigid tools on the roll separation force and the accumulative plastic strain.

  • 37.
    Azizoğlu, Yağız
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Gärdsback, Mattias
    Dalarna University.
    Sjöberg, Bengt
    Dalarna University.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Finite Element Modelling of Cold Pilgering of Tubes2015In: Computational Plasticity XIII: Fundamentals and Applications - Proceedings of the 13th International Conference on Computational Plasticity - Fundamentals and Applications,held in Barcelona, Spain, 1-3 September 2015 / [ed] E. Oñate; D.R.J. Owen; D. Peric; M. Chiumenti, Barcelona: International Center for Numerical Methods in Engineering (CIMNE), 2015, p. 716-726Conference paper (Refereed)
    Abstract [en]

    Cold pilgering is a cold forming process used during manufacturing of seamless tubes. The tube with a mandrel inside is fed forward and rotated in stepwise increments, while the roll stand moves back and forth. The total plastic deformation of the tube is such that the cross-sectional area of the tube decreases and the length of the tube increases during the process. However, this is performed in many small incremental steps, where the direction of deformation in a material point changes at each stroke. Most published models of cold pilgering use simplified material models. In reality, the flow stress is dependent on temperature, strain rate, strain history and microstructure. In this work, temperature and strain rate distributions are computed, using a 3D thermo-mechanical FE model, and the influence of temperature and strain rate on the rolling force is investigated. The Johnson-Cook model is employed to describe the flow stress using isotropic hardening. The results show that strain rate and temperature have a significant influence on the roll separation force

  • 38.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dislocation density based constitutive model for Ti-6Al-4V at low strain rates2007In: Ti-2007 : science and technology: proceedings of the 11th World Conference on Titanium (JIMIC 5), held at Kyoto International Conference Center, Kyoto, Japan, 3 - 7 June 2007 / [ed] M. Niinomi, Kyoto: Japan Institute of Metals , 2007, p. 311-314Conference paper (Refereed)
  • 39.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dislocation density based constitutive model for Ti-6Al-4V: including recovery and recrystallisation2007In: Computational plasticity: Fundamentals and Applications / [ed] Eugenio Onate; Roger Owen; Benjamin Suarez, International Center for Numerical Methods in Engineering (CIMNE), 2007, p. 631-634Conference paper (Refereed)
  • 40.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dislocation density model for plastic behaviour of Ti-6-42006In: WCCM VII: 7th World Congress on Computational Mechanics ; Los Angeles, California, USA ; July 16 - 22, 2006, MAdison, Wis: Omnipress , 2006Conference paper (Other academic)
  • 41.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. Swerea MEFOS AB.
    Mechanism-based flow stress model for Ti-6Al-4V: applicable for simulation of additive manufacturing and machining2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Ti-6Al-4V has remarkable properties such as high specific mechanical properties (viz. stiffness, strength, toughness, fatigue resistance), corrosion resistance, biocompatibility etc. These properties make it attractive for applications in aerospace, chemical industry, energy production, surgical implants, etc. Many of these applications have to satisfy high requirements on mechanical properties, which are directly affected by the microstructure. Therefore, it is essential to understand as well as to model the microstructure evolution during manufacturing as well as in-service. Furthermore, this alloy has a narrow temperature and strain rate window of workability.

    This work was initiated as part of a project aimed at performing finite element simulations of a manufacturing process chain involving hot forming, welding, machining, additive manufacturing and heat treatment of Ti-6Al-4V components within the aerospace industry. Manufacturing process chain simulations can compute the cumulative effect of the various processes by following the material state through the whole chain and give a realistic prediction of the final component. Capacity to describe material behavior in a wide range of temperatures and strain rates is crucial for this task.

    A material model based on the dominant deformation mechanisms of the alloy is assumed to have a more extensive range of validity compared to an empirical relationship. Explicit dislocation dynamics based models are not practically feasible for manufacturing process simulation, and therefore the concept of dislocation density, (length of dislocations per unit volume) developed by (Kocks1966; Bergström, 1970) is followed here. This mean field approach provides a representation of the average behavior of a large number of dislocations, grains, etc. Conrad (1981) studied the influence of various factors like solutes, interstitials, strain, strain rate, temperature, etc., on the strength and ductility of titanium systems and proposed a binary additive relationship for its yield strength. The first component relates to long-range interactions and second short-range relates to lattice resistance for dislocation motion. For high strain rate deformation, this short-range term is extended to include the effects of a viscous drag given by phonon and electron drag (Lesuer et al. 2001).  Immobilisation of dislocation by pile-ups gives hardening and remobilization/annihilation by dislocation glide and climb gives restoration. Globularization is also considered to restore the material. The material model is calibrated using isothermal compression tests at a wide range of temperatures and strain rates. Compression tests performed using Gleeble thermo-mechanical simulator is used at low-strain rates and split-Hopkins pressure bar is used at high strain rates for calibration.

    During additive manufacturing depending on the temperature, heating/cooling rates, Ti-6Al-4V undergoes allotropic phase transformation. This transformation results in a variety of textures that can give different mechanical properties.  Based on the texture (Semiatin et al., 1999b; Seetharaman and Semiatin, 2002; Thomas et al., 2005) identified few microstructural features that are relevant to the mechanical properties. The three separate alpha phase fractions; Widmanstatten,  grain boundary, Martensite, and the beta-phase fraction are included in the current model. However, since the strengthening contributions of these individual alpha phases are not known, a linear rule of mixtures for the total alpha-beta composition is developed. This model is calibrated using continuous cooling tests performed by Malinov et al. 2001 with differential scanning calorimeter at varying cooling rates.  

    This mechanism-based model is formulated in such a way that it can be implemented in any standard finite element software. In the current work, this is implemented as subroutines within MSC Marc and used for simulation of hot-forming and additive manufacturing. 

  • 42.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Physically based model for plasticity and creep of Ti-6Al-4V2008Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Albeit Ti-6Al-4V has numerous salient properties, its usage for certain applications is limited due to the challenges faced during manufacturing. Understanding the dominant deformation mechanisms and numerically modelling the process is the key to overcome this hurdle. This work investigates plastic deformation of the alloy at strain rates from 0.001/s to 1/s and temperatures between 20 and 1100 Celsius. Pertinent deformation mechanisms of the material when subjected to thermo-mechanical processing is discussed. A physically based constitutive model based on the evolution of immobile dislocation density and excess vacancy concentration is developed. Parameters of the model are obtained by calibration using isothermal compression tests. Model is compared with relaxation test data to demonstrate its validity.

  • 43.
    Babu, Bijish
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Charles, Corinne
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Physically Based Constitutive Model of Ti-6Al-4V for Arbitrary Phase CompositionIn: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154Article in journal (Refereed)
    Abstract [en]

    The principal challenge in producing aerospace components using Ti-6Al-4V alloy is to employ the optimum process window of deformation rate and temperature to achieve desired material properties. Qualitatively understanding the microstructure-property relationship is not enough to accomplish this goal. Developing advanced material models to be used in manufacturing process simulation is the key to compute and optimize the process iteratively. The focus in this work is on physically based flow stress models coupled with microstructure evolution models. Such a model can be used to simulate processes involving complex and cyclic thermo-mechanical loading.

  • 44.
    Babu, Bijish
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dislocation density based model for plastic deformation and globularization of Ti-6Al-4V2013In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 50, p. 94-108Article in journal (Refereed)
    Abstract [en]

    Although Ti-6Al-4V has numerous salient properties, its usage for certain applications is limited due to the challenges faced during manufacturing. Understanding the dominant deformation mechanisms and numerically modeling the process is the key to overcoming this hurdle. This paper investigates plastic deformation of the alloy at strain rates from 0.001s−1 to 1s−1 and temperatures between 20° C and 1100° C. Pertinent deformation mechanisms of the material when subjected to thermo-mechanical processing are discussed. A physically founded constitutive model based on the evolution of immobile dislocation density and excess vacancy concentration is developed. Parameters of the model are obtained by calibration using isothermal compression tests. This model is capable of describing plastic flow of the alloy in a wide range of temperature and strain rates by including the dominant deformation mechanisms like dislocation pile-up, dislocation glide, thermally activated dislocation climb, globularization, etc. The phenomena of flow softening and stress relaxation, crucial for the simulation of hot forming and heat treatment of Ti-6Al-4V, can also be accurately reproduced using this model.

  • 45.
    Babu, Bijish
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lundbäck, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dislocation density based constitutive model for Ti-6Al-4V used in simulation of metal deposition2007In: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, p. 84-Conference paper (Other academic)
  • 46. Babu, Bijish
    et al.
    Lundbäck, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Physically based constitutive model for Ti-6Al-4V used in the simulation of manufacturing chain2009In: Computational Plasticity X: fundamentals and applications ; proceedings of the X International Conference on Computational Plasticity - fundamentals and applications held in Barcelona, Spain, 02 - 04 September 2009 / [ed] E. Onate; D.R.J. Owen; B. Suarez, International Center for Numerical Methods in Engineering (CIMNE), 2009Conference paper (Refereed)
    Abstract [en]

    Simulations of manufacturing process chain involving forming, welding and heat treatment are complex because of the varying length and time scales and the range of temperatures which trigger the different associated deformation mechanisms. This paper demonstrates the use of a physically based constitutive model in simulation of a manufacturing chain.

  • 47.
    Babu, Bijish
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. Swerea MEFOS.
    Lundbäck, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Simulation of additive manufacturing of Ti-6Al-4V using a coupled physics-based flow stress and microstructure modelManuscript (preprint) (Other academic)
    Abstract [en]

    Simulating the additive manufacturing process of Ti-6Al-4V is very complex owing to the microstructural changes and allotropic transformation occurring during its thermo-mechanical processing. The alpha-phase with a hexagonal close pack structure is present in three different forms; Widmanstatten, grain boundary, and Martensite. A metallurgical model that computes the formation and dissolution of each of these phases is used in this work. Furthermore, a physically based flow-stress model coupled with the metallurgical model is applied in the simulation of direct energy deposition additive manufacturing case.

  • 48.
    Babu, Bijish
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Svoboda, Ales
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Ghassemali, Ehsan
    School of Engineering, Jönköping University..
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dislocation density based plasticity model extended to high strain rate deformation of Ti-6Al-4VManuscript (preprint) (Other academic)
    Abstract [en]

    One of the main challenges in the simulation of machining is accurately describing the material behavior during severe plastic deformation at strain rates ranging six orders of magnitude and temperature between room temperature to nearly melting temperature. High strain rate measurements are performed using Split-Hopkinson Pressure Bar (SHPB) technique at a range of temperatures. The temperature change during deformation is included by computing the plastic work converted to heat energy. A physics-based material model published earlier (Babu and Lindgren, 2013) is extended in this paper to include the high strain rate mechanisms of phonon and electron drag. Characterization of the microstructure is performed using Electron Backscatter Diffraction (EBSD), and a novel method is proposed in this work to quantify the extent of globularization which is compared with model predictions.

  • 49.
    Baer, Máren Ingá
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Crack propagation calculations in non-elastic metals2018Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    It is important to know the lifespan of a component, thus services can be planed. At Siemens Industrial Turbomachinery AB (SIT AB) develop and produce gas and steam turbines, the turbines are heavily strained by thermal and centrifugal forces during their life span which lead to cracking. Today there is no good crack propagation method for materials with large amount of plastic deformation and creep. At SIT a new method, ΔKBDCPP, has been developed that takes these phenomenon in consideration. It is assumed that the whole model has plastic material properties except circular elements at the crack front which have elastic properties. The new method is reviewed by doing simulations on a a specimen model in Franc3D together with the FE program Abaqus. There have been conducted thermo-mechanical fatigue (TMF) tests and 2D simulations on specimen to measure the crack propagation. The TMF tests where immitaded in new simulations with the method ΔKBDCPP. Forces, creep times, cycles and material data where emulated in the model and then simulated. The number of cycles per crack length for the isothermal simulations became higher then for the TMF tests. This is a expected result because the conditions are optimal during simulations, which they may not be during experimental test. The result of ΔKBDCPP are closer to the TMF test results then the 2D simulated ΔKeffective, another crack propagation calculation method. Additionally, out-of-phase (OP) simulations where carried out, in OP the maximum temperature and maximum stress do not coincide. When the temperature is at max the component is held in the minimum stress. For OP simulations the results became irregular, for one of the simulations the number of cycles per crack length became lower for the simulated specimen than for the TMF test, and contrariwise for the other OP simulation. However, it was concluded that the later was correct because the majority of the simulations, and all the isothermal simulations, gave this result. it also was the result for the isothermal simulations. One OP simulation was compared to a 2D simulation and here the method of ΔKBDCPP was also more accurate to the TMF tests. The method of ΔKBDCPP seem to function well however, the solving time for models with creep is long. A model with a small crack and creep is time consuming to simulate, which is a disadvantages.

  • 50.
    Barsoum, Z.
    et al.
    Kungliga tekniska högskolan, KTH.
    Lundbäck, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Simplified FE welding simulation of fillet welds: 3D effects on the formation residual stresses2009In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 16, no 7, p. 2281-2289Article in journal (Refereed)
    Abstract [en]

    In this study two- and three-dimensional finite element welding simulations have been carried out. The welded component studied is a T-type fillet weld which is frequently used in the heavy vehicle machine industry with plate thicknesses of eight and 20 mm, respectively. The software's used for the welding simulations is MSC.Marc and ANSYS. The objective is to study the formation of the residual stresses due to 3D effect of the welding process. Moreover, welding simulations using solid models and contact models in the un-fused weld roots were carried out in order to investigate the possible effect with respect to the residual stresses. Residual stress measurements were carried out using X-ray diffraction technique on the manufactured T-welded structure. The 2D residual stress predictions shows good agreement with measurements, hence the 2D welding simulation procedure is suitable for residual stress predictions for incorporation in further fatigue crack growth analysis from weld defects emanating from the weld toe and the un-fused root.

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