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  • 351.
    Tofique, Muhammad Waqas
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Very high cycle fatigue of duplex stainless steels and stress intensity calculations2014Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Very high cycle fatigue (VHCF) is generally considered as the domain of fatigue lifetime beyond 10 million (107) load cycles. Few examples of structural components which are subjected to 107-109 load cycles during their service life are engine parts, turbine disks, railway axles and load-carrying parts of automobiles. Therefore, the safe and reliable operation of these components depends on the knowledge of their fatigue strength and the prevalent damage/failure mechanisms. Moreover, the fatigue life of materials in the VHCF regime is controlled by the fatigue crack initiation and early growth stage of short cracks.

    This study was focussed on the evaluation of fatigue properties of duplex stainless steels in the VHCF regime using the ultrasonic fatigue testing equipment. The ultrasonic fatigue tests were conducted on the cold rolled duplex stainless strip steel and hot rolled duplex stainless steel grades. Two different geometries of ultrasonic fatigue test specimens were tested. Considerable attention was devoted to the evaluation of fatigue crack initiation and growth mechanisms using the high resolution scanning electron microscopy. The fatigue crack initiation was found to be surface initiated phenomena in all the tested grades, albeit different in each case.

    The second part of this thesis work was the development of a distributed dislocation dipole technique for the analysis of multiple straight, kinked and branched cracks in an elastic half plane. Cracks with dimensions much smaller than the overall size of the domain were considered. The main goal of the development of this technique was the evaluation of stress intensity factor at each crack tip. The comparison of results from the stress intensity factor evaluation by the developed procedure and the well-established Finite Element Method software ABAQUS showed difference of less than 1% for Jacobi polynomial expansion of sixth order in the dipole density representation.

  • 352.
    Tofique, Muhammad Waqas
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Bergström, Jens
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Burman, Christer
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Very high cycle fatigue crack initiation mechanisms in different engineering alloys2016Ingår i: 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21) / [ed] F. Iacoviello, L. Susmel, D. Firrao, G. Ferro,, Elsevier, 2016, s. 1181-1190Konferensbidrag (Refereegranskat)
    Abstract [en]

    The fatigue crack initiation mechanisms prevalent in high strength martensitic steel grades, hot rolled plate duplex stainless steels, cold rolled strip duplex stainless steel and a super alloy grade were compared. The fatigue testing of all the grades was conducted in the VHCF regime using an ultrasonic fatigue testing equipment operating at 20 kHz. Scanning electron microscope (SEM) observations of the fracture surfaces revealed the presence of a microstructure controlled initial growth of short fatigue cracks in all the tested grades. Fracture surfaces of the failed specimens of a high strength martensitic steel grade revealed the typical fine granular area (FGA) within the fish-eye area around the internal inclusions. Fatigue crack initiation in the cold rolled strip duplex stainless steel grade occurred at surface defects left over by the cold rolling process of this grade. However, the presence of FGA around the surface crack initiating defect was observed similar to the internal crack initiations in the high strength martensitic steels. By mapping the FGA size development during VHCF loading, as obtained from fracture surfaces, FGA growth results were obtained. A similar study on hot rolled plate duplex stainless steel grades, 2304 SRG and LDX 2101, revealed the presence of an initial crystallographic growth region (CGR) in which crack growth direction is changed by microstructural barriers such as phase and grain boundaries. The early plastic fatigue damage accumulation occurred predominantly in one phase or at the austenite-ferrite phase boundaries. On the other hand, an initial transcrystalline fatigue crack growth was observed in the Ni-based super alloy grade Inconel 718.

  • 353.
    Tofique, Muhammad Waqas
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Bergström, Jens
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för maskin- och materialteknik. Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Materialvetenskap.
    Burman, Christer
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Hallbäck, Nils
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap.
    Gåård, Anders
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Materialvetenskap. Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för maskin- och materialteknik.
    Fatigue strength, crack initiation, and localized plastic fatigue damage in VHCF of duplex stainless steels2016Ingår i: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 87, nr 7, s. 899-910Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The fatigue strength of two-duplex stainless steel grades, 2304 SRG and LDX 2101, with austenitic–ferritic microstructure is tested using ultrasonic fatigue testing equipment operating at 20 kHz. The testing is conducted in tension-compression mode with the load ratio R=-1. The fatigue strength is evaluated at 107, 108, and 109 load cycles and the estimates of fatigue strength are higher for the LDX 2101 grade. The fatigue crack initiation mechanisms are analyzed using a scanning electron microscope. The fatigue cracks, in all cases, appear to initiate due to accumulation of plastic fatigue damage at the surface. In the 2304 SRG grade, accumulation of fatigue damage occurs at the external surface of fatigued specimens in the form of extrusions at the grain/phase boundaries and in the form of individual slip lines in the austenite phase. Meanwhile, in the LDX 2101 grade accumulation of plastic fatigue damage in the form of extrusions and intrusions occurs mainly within the ferrite grain. When the crack is microstructurally short, the crack growth appears to be crystallographic in nature and the crack appears to change its direction propagating from one grain into another.

  • 354.
    Tofique, Muhammad Waqas
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Bergström, Jens
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Hallbäck, Nils
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Burman, Christer
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Fatigue initiation and strength of duplex stainless steel strip specimens in the very high cycle fatigue regime2014Ingår i: Very high cycle fatigue 6 (VHCF6), 2014Konferensbidrag (Refereegranskat)
    Abstract [en]

    Fatigue studies of cold-rolled duplex stainless strip steel were performed in the very high cycle fatigue life region. The duplex austenitic-ferritic microstructure gives this grade a combination of high mechanical strength and high corrosion resistance. Fatigue properties of thin steel strips are particular due to cold rolling introducing a very fine microstructure. Crack initiation and fatigue strength are controlled by steel microstructure and alloying. The initiation and growth of the very short initial fatigue crack in very high cycle fatigue are unclear and subject to different descriptions. Fatigue test data of thin strip specimens at very high fatigue lives are scarce due to testing difficulties. For practical reasons testing must be performed at ultrasound test frequencies which involves fixturing problems. A test setup including the load chain ultrasonic horn, fixture and specimen was designed for resonance with a horse-shoe design of a screw fixture. The design of the horse-shoe fixture and the specimens along with FEM calculation of eigenfrequency are presented. Fatigue testing was performed at 20 kHz in R=-1 conditions up to fatigue life of 107 to 5*109 cycles. Fatigue strength was tested and crack initiation was studied on the fracture surface using FEG-SEM at the initiation site.

  • 355.
    Tofique, Muhammad Waqas
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Bergström, Jens
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013). Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Materialvetenskap.
    Svensson, Krister
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Very High Cycle Fatigue of cold rolled stainless steels, crack initiation and formation of Fine Granular Area2017Ingår i: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 100, nr 1, s. 238-250Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    Fatigue tests of cold rolled strip materials, a duplex stainless steel and a martensitic stainless steel, were performed using an ultrasonic fatigue test equipment operating at 20 kHz under a completely reversed tension compression load ratio R =-1. Fatigue test data (SN data) was generated in the VHCF regime and fracture surfaces of the failed specimens were investigated using a Scanning Electron Microscope (SEM). In the duple stainless steel grade, fatigue failures were found to be initiated at surface defects on the side surfaces or corners, created due to cold rolling, of the strip specimens. Features of a Fine Granular Area (FGA) were observed around the crack initiating surface defects on the fracture surfaces. In the martensitic stainless steel grade, fatigue crack initiation occurred due to aluminium-silicon oxide inclusions or surface defects created due to cold rolling of the material. In situ Focussed Ion Beam (FIB) technique was used to extract cross-sections from the FGA around the crack initiating defect on the fracture surface. Transmission Electron Microscope (TEM) investigations of the extracted cross-sections revealed FGAs in immediate vicinity of the crack initiating surface defects. The observed fine grained layers seemed to be composed of nano-sized grains and, thus, could be distinguished from the bulk material. The FGA around the surface crack initiating defects seems to have formed due to localized plastic deformation by stress concentration at the defects.

  • 356.
    Tofique, Muhammad Waqas
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Bergström, Jens
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Svensson, Krister
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Johansson, Sten
    Department of Materials Engineering, Linköping University, Sweden.
    Peng, Ru Lin
    Department of Materials Engineering, Linköping University, Sweden.
    EBSD and TEM analysis of plastic fatigue damage accumulation responsible for fatigue crack initiation and propagation in VHCF of duplex stainless steelsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Fatigue test data (stress-life) of two duplex stainless steel grades, LDX 2101 and 2304 SRG, in the Very High Cycle Fatigue regime (VHCF) is presented. The fatigue testing is conducted using an ultrasonic fatigue testing equipment operating at 20 kHz under fully reversed tension-compression condition (R=-1). Scanning Electron Microscope (SEM) studies of the fracture surfaces and external surfaces of failed specimens is conducted. Electron Back Scattered Diffraction (EBSD) studies of the axially cut surface of the failed specimens is done to analyse grains near the external surface and crack initiation site. Analysis of accumulation of plastic fatigue damage and growth of cracks in the Crystallographic Growth Region (CGR) is carried out. Transmission Electron Microscope (TEM) analysis of thin foils cut from failed specimens of LDX 2101 is carried out to examine the effect of fatigue loading on dislocation structure. SEM studies of the CGR show features like grain boundaries and fatigue striations. The barrier effect of grain and phase boundaries on short fatigue crack propagation is observed. ECCI images and EBSD analysis show that Persistent Slip Bands (PSBs) are observed in ferrite grains in LDX 2101 grade. On the other hand, no PSBs are observed in any of the grains in 2304 SRG. The TEM observations in thin foils cut from the failed specimen of LDX 2101 show stacking faults in austenite grains. Stacking faults were observed to stop at the grain and phase boundaries.

  • 357.
    Tofique, Muhammad Waqas
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Hallbäck, Nils
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Materialvetenskap. Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Development of the distributed dislocation dipole technique for the analysis of closure of complex fractures involving kinks and branches2018Ingår i: European journal of mechanics. A, Solids, ISSN 0997-7538, E-ISSN 1873-7285, Vol. 69, s. 168-178Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper presents the development of the distributed dislocation dipole technique (DDDT) for the analysis of straight, kinked and branched cracks where parts of the cracks may close during loading. The method has been developed for plane problems. Crack cases in which closure occurs are analyzed by reformulating the Buecicner's principle, taking into account the contact stresses at the contacting portions of the crack surfaces. Stress intensity factors corresponding to opening and the in-plane sliding mode of deformation at the crack tips are computed. Several test cases involving straight, kinked and/or branched cracks where parts of the cracks undergoes crack surface closure when subjected to the outer loading are analyzed. The results obtained from the DDDT are compared to those obtained from a Finite Element Method (FEM) analysis of the same crack cases. This comparison shows that the computation of stress intensity factors for the cases involving crack surface closure are less accurate than those for fully open crack cases. However, for the cases under consideration, the stress intensity factors were still computed with a maximum difference of approximately 2 per cent compared to the FEM calculations if Jacobi polynomial expansions of at least the twelfth order were used to represent the crack surface opening and sliding displacements. In most cases under consideration, sixth order Jacobi polynomial expansions were sufficient to obtain results within that margin of deviation.

  • 358.
    Tofique, Muhammad Waqas
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Hallbäck, Nils
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Development of the distributed dislocation dipole technique for the analysis of closure of complex fractures involving kinks and branchesManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    This paper presents the development of the distributed dislocation dipole technique (DDDT) for the analysis of crack surface closure of crack cases involving kinks and branches. Crack cases in which closure occurs are analyzed by reformulating the Bueckner's principle taking the contact stresses at the contacting portions of the crack surfaces into account. Stress intensity factors corresponding to opening and sliding mode of deformation at the crack tips are computed. Three test cases involving kinked and/or branched cracks with at least one of the crack segments undergoing crack surface closure when subjected to remote tensile loading are analyzed. The results obtained from the DDDT are compared to those obtained from the Finite Element Method (FEM) analysis of the same crack cases. This comparison shows that the computation of stress intensity factors for the crack cases involving crack surface closure are less acurate compared to fully open crack cases. However, the stress intensity factors are still computed to an accuracy of within 2 percent if the Jacobi polynomial expansions of at least the sixth order are used to represent the crack surface opening and sliding displacements. Higher order Jacobi polynomials lead to increased accuracy.

  • 359.
    Tofique, Wagas
    et al.
    Karlstad Univ, Dept Phys & Engn Sci, S-65188 Karistad, Sweden..
    Bergström, Jens
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Materialvetenskap. Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013). Karlstad Univ, Dept Phys & Engn Sci, S-65188 Karistad, Sweden..
    Svensson, Krister
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Materialvetenskap. Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013). Karlstad Univ, Dept Phys & Engn Sci, S-65188 Karistad, Sweden..
    Johansson, S.
    Linkoping Univ, Dept Mat Engn, Linkoping, Sweden..
    Peng, R. L.
    Linkoping Univ, Dept Mat Engn, Linkoping, Sweden..
    ECCl/EBSD and TEM analysis of plastic fatigue damage accumulation responsible for fatigue crack initiation and propagation in VHCF of duplex stainless steels2017Ingår i: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 100, s. 251-262Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Fatigue test data of duplex stainless steel grades, LDX 2101 and 2304 SRG, in the Very High Cycle Fatigue (VHCF) regime is presented. Fatigue testing was conducted using ultrasonic fatigue test equipment operating at 20 kHz under fully reversed tension-compression load condition. Scanning Electron Microscope (SEM) analysis of the fracture surfaces and external surfaces of failed specimens was conducted. Electron Channelling Contrast Imaging (ECCI) and Electron Back Scattered Diffraction (EBSD) studies of the axially cut surface of the failed specimens was done to analyse the accumulation of plastic fatigue damage and fatigue crack growth in the grains adjacent to the external surface and crack initiation site. Transmission Electron Microscope (TEM) analysis of thin foils cut from failed specimens of LDX 2101 was carried out to examine the effect of fatigue loading on dislocation structure. SEM studies of the Crystallographic Growth Region (CGR) showed features like grain boundaries and fatigue striations on the fracture surfaces. SEM analysis of the external surfaces of fatigue loaded specimens showed inhomogeneous accumulation of plastic fatigue damage. ECCl/EBSD analysis showed Persistent Slip Bands (PSBs) in ferrite grains in LDX 2101 grade but no PSBs were observed in any grains of 2304 SRG specimens. The barrier effect of grain and phase boundaries on short fatigue crack propagation was observed. TEM analysis of thin foils cut from the failed specimens of LDX 2101 showed stacking faults in austenite grains and they were seen to stop at the grain and phase boundaries. (C) 2017 Elsevier Ltd. All rights reserved.

  • 360.
    Torstensson, Alexander
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Testutrustning för att bedöma knäckvinklar på pappersprodukter: Utveckling av metod och konstruktion av en knäckvinkeltestare för bestruktna pappersprodukter2019Självständigt arbete på grundnivå (högskoleexamen), 15 poäng / 22,5 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Projektet är en del av examen för studenter inom ​högskoleingenjörsprogrammet för innovationsteknik och design​ på Karlstads Universitet och fortgår under vårterminen 2019.

    Projektet har utförts på uppdrag av BillerudKorsnäs och målet har varit att ta fram en utrustning som kan användas för att bedöma knäckvinklar i bestrykningar på pappersprodukter.

    BillerudKorsnäs är ett multinationellt företag med hela världen som marknad och har produktionsanläggningar i Sverige, Finland och Storbritannien. BillerudKorsnäs tillverkar pappersprodukter samt bedriver forskning och utveckling inom pappersindustrin.

    Projektet har delats upp i fyra faser: uppstartsfas, konceptfas, konstruktionsfas och utvärderingsfas. Tillverkning av prototyp var en del av konstruktionsfasen och eftersom BillerudKorsnäs valde att ta över tillverkning så landade projektet i att bli ett konstruktionskoncept.

    Det här projektet har syftat till att utveckla en testutrustning för bestrykningar på pappersprodukter. Testutrustningen ska användas för att bedöma sprickbildning i bestrykningar och utvärdera när en bestrykning har tillräckligt stora sprickor för att betraktas som undermålig.

    Med det framtagna konceptet kan pappersprover med enkelhet och hög noggrannhet testas för att bedöma sprickbenägenheten. Konstruktionen har en inbyggd högupplöst kamera så att pappersprovet kan analyseras i en dator samtidigt som testsekvensen körs. Konstruktionen klarar av att vika pappersprover upp till 180° och är anpassad för att monteras i en intern utrustning som finns att tillgå på BillerudKorsnäs anläggning i Grums. Utrustningen liknar en dragprovmaskin som kan användas för att mäta bland annat krafter och hastigheter samt även avstånd som är en nödvändighet för att beräkna knäckvinklar i den här konstruktionen.

  • 361.
    Uggla, Claes
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Global cosmological dynamics for the scalar field representation of the modified Chaplygin gas2013Ingår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. D88, s. 064040-1-064040-7Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper we investigate the global dynamics for the minimally coupled scalar field representation of the modified Chaplygin gas in the context of flat Friedmann-Lemaître-Robertson Walker cosmology. The tool for doing this is a new set of bounded variables that lead to a regular dynamical system. It is shown that the exact modified Chaplygin gas perfect fluid solution appears as a straight line in the associated phase plane. It is also shown that no other solutions stay close to this solution during their entire temporal evolution, but that there exists an open subset of solutions that stay arbitrarily close during an intermediate time interval, and into the future in the case when the scalar field potential exhibits a global minimum.

  • 362.
    Uggla, Claes
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Recent developments concerning generic spacelike singularities2013Ingår i: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 45, s. 1669-1710Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This is a review of recent progress concerning generic spacelikesingularities in general relativity. For brevity the main focus is on singularities in vacuum spacetimes, although the connection with, and the role of, matter for generic singularity formation is also commented on. The paper describes recent developments in two areas and show how these are connected within the context of the conformally Hubble-normalized state space approach. The first area is oscillatory singularities in spatially homogeneous cosmology and the connection between asymptotic behaviour and heteroclinic chains. The second area concerns oscillatory singularities in inhomogeneous models, especially spike chains and recurring spikes. The review also outlines some underlying reasons for why the structures that are the foundation for generic oscillatory behaviour exists at all, which entails discussing how underlying physical principles and applications of solutiongenerating techniques yield hierarchical structures and connections between them. Finally, it is pointed out that recent progress concerning generic singularities motivates some speculations that suggest that a paradigm shift concerning their physical role, and what mathematical issues to address,might be in order.

  • 363.
    Uggla, Claes
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Wainwright, John
    University of Waterloo, Canada.
    Asymptotic analysis of perturbed dust cosmologies to second order2013Ingår i: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 45, nr 8, s. 1467-1492Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Nonlinear perturbations of Friedmann-Lemaitre cosmologies with dust and a cosmological constant Lambda>0 have recently attracted considerable attention. In this paper our first goal is to compare the evolution of the first and second order perturbations by determining their asymptotic behaviour at late times in ever-expanding models. We show that in the presence of spatial curvature K or a cosmological constant, the density perturbation approaches a finite limit both to first and second order, but the rate of approach depends on the model, being power law in the scale factor if Lambda>0 but logarithmic if Lambda=0 and K<0. Scalar perturbations in general contain a growing and a decaying mode. We find, somewhat surprisingly, that if Lambda>0 the decaying mode does not die a way, i.e.  it contributes on an equal footing as the growing mode to the asymptotic expression for the density perturbation. On the other hand, the future asymptotic regime of the Einstein-de Sitter universe (K=Lambda=0) is completely different, as exemplified by the density perturbation which diverges; moreover, the second order perturbation diverges faster than the first order perturbation, which suggests that the Einstein-de Sitter universe is unstable to perturbations, and that the perturbation series do not converge towards the future. We conclude that the presence of spatial curvature or a cosmological constant stabilizes the perturbations. Our second goal is to derive an explicit expression for the second order density perturbation that can be used to study the effects of including a cosmological constant and spatial curvature.

  • 364.
    Uggla, Claes
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Wainwright, John
    Univ Waterloo, Dept Appl Math, Waterloo, Canada.
    Dynamics of cosmological perturbations at first and second order2018Ingår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 98, nr 10, artikel-id 103534Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper we give five gauge-invariant systems of governing equations for first and second order scalar perturbations of flat Friedmann-Lemaitre universes that are minimal in the sense that they contain no redundant equations or variables. We normalize the variables so that they are dimensionless, which leads to systems of equations that are simple and ready-to-use. We compare the properties and utility of the different systems. For example, they serve as a starting point for finding explicit solutions for two benchmark problems in cosmological perturbation theory at second order: adiabatic perturbations in the superhorizon regime (the long wavelength limit) and perturbations of ACDM universes. However, our framework has much wider applicability and serves as a reference for future work in the field.

  • 365.
    Uggla, Claes
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Wainwright, John
    University of Waterloo, Canada.
    Second order cosmological perturbations: simplified gauge change formulas2019Ingår i: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 36, nr 3, s. 1-19, artikel-id 035004Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper we present a new formulation of the change of gauge formulas in second order cosmological perturbation theory which unifies and simplifies known results. Our approach is based on defining new second order scalar perturbation variables by adding a multiple of the square of the corresponding first order variables to each second order variable. A bonus is that these new perturbation variables are of broader significance in that they also simplify the analysis of second order scalar perturbations in the super-horizon regime in a number of ways, and lead to new conserved quantities.

  • 366.
    Uggla, Claes
    et al.
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap. Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Wainwright, John
    University of Waterloo.
    Second order density perturbations for dust cosmologies2014Ingår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. D90, s. 043511-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present simple expressions for the relativistic first and second order fractional density perturbations for FL cosmologies with dust, in four different gauges: the Poisson, uniform curvature, total matter and synchronous gauges. We include a cosmological constant and arbitrary spatial curvature in the background. A distinctive feature of our approach is our description of the spatial dependence of the perturbations using a canonical set of quadratic differential expressions involving an arbitrary spatial function that arises as a conserved quantity. This enables us to unify, simplify and extend previous seemingly disparate results. We use the primordial matter and metric perturbations that emerge at the end of the inflationary epoch to determine the additional arbitrary spatial function that arises when integrating the second order perturbation equations. This introduces a non-Gaussianity parameter into the expressions for the second order density perturbation. In the special case of zero spatial curvature we show that the time evolution simplifies significantly, and requires the use of only two non-elementary functions, the so-called growth supression factor at the linear level, and one new function at the second order level. We expect that the results will be useful in applications, for example, studying the effects of primordial non-Gaussianity on the large scale structure of the universe.

  • 367.
    Uggla, Claes
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Wainwright, John
    University Waterloo, Canada.
    Second-order cosmological perturbations: New conserved quantities and the general solution at super-horizon scale2019Ingår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 100, nr 2, artikel-id 023544Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The study of long-wavelength scalar perturbations, in particular the existence of conserved quantities when the perturbations are adiabatic, plays an important role in e.g., inflationary cosmology. In this paper we present some new conserved quantities at second order and relate them to the curvature perturbation in the uniform density gauge, zeta, and the comoving curvature perturbation, R. We also, for the first time, derive the general solution of the perturbed Einstein equations at second order, which thereby contains both growing and decaying modes, for adiabatic long-wavelength perturbations for a stress-energy tensor with zero anisotropic stresses and zero heat flux. The derivation uses the total matter gauge, but results are subsequently translated to the uniform curvature and Poisson (longitudinal, zero shear) gauges.

  • 368.
    Uggla, Claes
    et al.
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap. Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Wainwright, John
    University of Waterloo.
    Simple expressions for second order density perturbations in standard cosmology2014Ingår i: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 31, nr 10, s. 105008-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper we present four simple expressions for the relativistic first and second order fractional density perturbations for ΛCDM cosmologies in different gauges: the Poisson, uniform curvature, total matter and synchronous gauges. A distinctive feature of our approach is the use of a canonical set of quadratic differential expressions involving an arbitrary spatial function, the so-called comoving curvature perturbation, to describe the spatial dependence, which enables us to unify, simplify and extend previous seemingly disparate results. The simple structure of the expressions makes the evolution of the density perturbations completely transparent and clearly displays the effect of the cosmological constant on the dynamics, namely that it stabilizes the perturbations. We expect that the results will be useful in applications, for example, studying the effects of primordial non-Gaussianity on the large scale structure of the universe.

  • 369.
    Uggla, Claes
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013). Karlstad Univ, Dept Phys, S-65188 Karlstad, Sweden..
    Wainwright, John
    Univ Waterloo, Dept Appl Math, Waterloo, ON N2L 3G1, Canada..
    The general solution at large scale for second order perturbations in a scalar field dominated universe2019Ingår i: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, nr 6, artikel-id 021Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper we consider second order perturbations of a flat Friedmann-Lemaitre universe whose stress-energy content is a single minimally coupled scalar field with an arbitrary potential. We derive the general solution of the perturbed Einstein equations in explicit form for this class of models when the perturbations are in the super-horizon regime. As a by-product we obtain a new conserved quantity for long wavelength perturbations of a single scalar field at second order.

  • 370.
    van Stam, Jan
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörs- och kemivetenskaper (from 2013).
    Ericsson, Leif
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Deribew, Dargie
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Moons, Ellen
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Morphology in Dip-Coated Blend Films for Photovoltaics Studied by UV/VIS Absorption and Fluorescence Spectroscopy2018Ingår i: Organic Electronics and Photonics: Fundamentals and Devices / [ed] S. Reineke, K. Vandewal, SPIE-INT SOC OPTICAL ENGINEERING , 2018, artikel-id UNSP 106870AKonferensbidrag (Refereegranskat)
    Abstract [en]

    Blend thin films, prepared by dip-coating, of polyfluorene (F8 or PFO), acting as an electron donor, and [6,6]-phenyl-C-61-butyric acid methyl ester (PC60BM), acting as the electron acceptor, have been characterized by UV/VIS absorption spectroscopy, static and dynamic fluorescence, and atomic force microscopy. Four different solvents were used for the film preparation; the monohalogenated fluorobenzene and chlorobenzene and their dihalogenated counterparts o-difluorobenzene and o-dichlorobenzene, respectively. Fluid mechanics calculations were used to determine the withdrawal speed for each solvent, in order to prepare wet films of comparable thicknesses. The resulting dry films were also of similar thicknesses. It was found that the choice of solvent influences the ability for F8 to form its beta-phase.

  • 371.
    van Stam, Jan
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörs- och kemivetenskaper.
    Hansson, Rickard
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Lindqvist, Camilla
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Ericsson, Leif
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik. Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Materialvetenskap.
    Moons, Ellen
    Karlstads universitet, Fakulteten för teknik- och naturvetenskap, Avdelningen för fysik och elektroteknik.
    Fluorescence spectroscopy studies on polymer blend solutions and films for photovoltaics2015Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects, ISSN 0927-7757, E-ISSN 1873-4359, Vol. 483, s. 292-296Artikel i tidskrift (Refereegranskat)
  • 372.
    van Stam, Jan
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörs- och kemivetenskaper.
    Lindqvist, Camilla
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Hansson, Rickard
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Ericsson, Leif
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Moons, Ellen
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Fluorescence and UV/VIS absorption spectroscopy studies on polymer blend films for photovoltaics2015Ingår i: Proceedings of SPIE: Physical Chemistry of Interfaces and Nanomaterials XIV / [ed] Hayes, SC; Bittner, ER, SPIE - International Society for Optical Engineering, 2015, Vol. 9549, s. 95490L1-95490L9Konferensbidrag (Refereegranskat)
  • 373.
    van Stam, Jan
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörs- och kemivetenskaper (from 2013).
    Van fraeyenhoven, Paulien
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörs- och kemivetenskaper (from 2013). UC Leuven Limburg, Leuven, Belgium.
    Andersén, Mikael
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörs- och kemivetenskaper (from 2013).
    Moons, Ellen
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Comparing Morphology in Dip-Coated and Spin-Coated Polyfluorene:Fullerene Films2016Ingår i: Proceedings of SPIE: Organic Photovoltaics XVII / [ed] Zakya H. Kafafi, Paul A. Lane, Ifor D.W. Samuel, SPIE - International Society for Optical Engineering, 2016, Vol. 9942, s. 99420D-1-99420D-10, artikel-id UNSP 99420DKonferensbidrag (Refereegranskat)
  • 374.
    Vanhecke, Ruud
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik. University College Leuven Limburg.
    Identification of green solvents for organic solar cells using P3HT and PC60BM2015Självständigt arbete på grundnivå (kandidatexamen), 10 poäng / 15 hpStudentuppsats (Examensarbete)
    Abstract [en]

    The importance of renewable energy sources is becoming clearer and clearer as unsustainable energy sources are running out and global warming is getting worse. Energy derived from sunlight is already commonly used, but more energy can be produced from sunlight when solar cells become cheaper. Organic solar cells use organic compounds as semiconductors which can be prepared from solutions, resulting in lower production costs.

    However, these semiconductors;Poly(3-hexylthiophene) and [6,6]-Phenyl-C61-butric acid methyl ester, are commonly dissolved inhalogenated and aromatic solvents. These solvents have toxic properties, which is why alternative solvents should be identified. Potential solvents were predicted by using the Hansen solubilityparameters and thin films were prepared by spin-coating the solutions.

    The thin films were evaluated with absorption spectroscopy, the fluorescence spectroscopy and atomic force microscopy. Since the alternative solvents had lower solubilites than the commonly used solvents, i.e., chlorobenzene, ortho-dichlorobenzene and chloroform, the absorption of the films with new solvents were lower as well. Using tetrahydrofuran resulted in the highest absorption of the used solvents, while xylene hada better film morphology.

    Increasing the absorption was attempted by spin-coating multiple thin films on top each other or by using a lower rotational speed. Spin-coating multiple films had nouniform effect on the absorption, while lowering the rotational speed increased the absorption, but not enough to equal the original absorption. Contrasting the results of the absorption spectra, the morphology improved when multiple layers were used while the film with the lower rotational speed’s morphology got worse.

    In conclusion, tetrahydrofuran and xylene are the best alternative solvents and using multiple layers as well as decreasing the rotational speed show improvements oneither the morphology or the absorption.

  • 375.
    Vestlund, Jimmy
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Streamlining tool design and manufacturing process for blancing and function test equipment to a propeller hub assembly2014Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [en]

    Rolls-Royce AB in Kristinehamn, Sweden, part of Rolls-Royce Marine, is a leading developer and supplier of water jet and propeller based propulsion equipment. Its low volume production series and wide product variety offered to its customers along with an increasingly competitive market has entailed an increased demand on both delivery time and cost reductions. The on-site manufacturing engineering department is responsible for developing all tools and fixtures, and programming required to maintain the on-site production, assembling and quality testing. As part of the departments streamlining efforts this study aimed on evaluating streamlining possibilities related to the existing tool design used for static balancing and function testing controllable pitch propeller assemblies before packaging and shipping, along with the related tool development and manufacturing processes has been conducted.The process evaluation started from the point when a hub assembly design was finalized until when a manufactured tool was delivered for use in production. Work focused on locating inefficient activities and product properties, with respect to tool cost and lead time, followed by setting up an amendment proposal, implementing it and producing an alternate tool design of which the effects on tool cost and manufacturing lead time would be evaluated. Post evaluating the current state of the process and product a set based front loaded product development methodology known as Modular Function Deployment was chosen to be the applied method. This application resulted in a modular tool design that avoided the determined most inefficient manufacturing operation combination of welding and annealing. Modularity increased manufacturing flexibility, enabling more concurrent manufacturing, to reduce the lead time. The tool design also applied integral properties by identifying the common components and features between tool sizes. This led to reducing manufacturing and material costs. Possible lead time reduction for manufacturing was determined to be 35-45%, 3-4weeks, in comparison with the original tool design due to increased parallel manufacturing and avoiding inefficient manufacturing methods. The estimated cost reduction for combined development and manufacturing was determined to be 105K SEK the initial year followed by 175K SEK the second year assuming the current tool manufacturing rate. The combined effects of reduced cost and lead time would be beneficial to Rolls-Royce AB by contributing to an increase in delivery reliability and competitive prices on the market

  • 376. Vilardell, A. M.
    et al.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Fredriksson, Gunnel
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Cabanettes, F.
    University of Lyon.
    Sova, A.
    University of Lyon.
    Valentin, D.
    University of Lyon.
    Bertrand, P.
    University of Lyon.
    Influence of surface topography on fatigue behavior of Ti6Al4V alloy by laser powder bed fusion2018Ingår i: Procedia CIRP, Elsevier, 2018, s. 49-52Konferensbidrag (Refereegranskat)
    Abstract [en]

    This article deals with the understanding of the influence of surface topography on fatigue behavior of Ti6Al4V alloy specimens produced by laser powder bed fusion (LPBF). The same laser parameters and scan strategy were used for all specimens, giving a sample density higher than 99.5 %. Two different surface topographies were obtained by using the top and side surfaces of the specimens. The surface topography and morphology were investigated by optical surface profilometry and focus variation microscopy. Four-point bending fatigue test was performed on specimens with top and side surfaces as the highest stressed surface respectively. Machined specimens were used as reference. The features of the fracture surface, such as crack initiation and propagation, were analyzed by focus variation and scanning electron microscopy (SEM). Both, fatigue results and fracture surface investigations, were correlated and discussed in relation to surface topography and microstructure, as well as manufacturing parameters. 

  • 377.
    Vilardell, Anna M.
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Fredriksson, Gunnel
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Yadroitsev, I.
    Cent Univ Technol Free State, Dept Mech & Mechatron Engn, South Africa.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Fracture mechanisms in the as-built and stress-relieved laser powder bed fusion Ti6Al4V ELI alloy2019Ingår i: Optics and Laser Technology, ISSN 0030-3992, E-ISSN 1879-2545, Vol. 109, s. 608-615Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The influence of a stress-relief treatment on impact and fatigue properties of Ti6Al4V ELI samples manufactured by laser powder bed fusion was analyzed. The heat treatment resulted in removal of residual stresses, coarsening of needles and formation of precipitations between needles. In both, impact and fatigue tests, crack development was correlated to microstructural features. Fracture analysis was carried out by means of optical and electron microscopy to reveal the influence of microstructure on crack development. Ductile fracture was the dominating fracture mode at impact testing. Pore formation and coalescence were the main crack formation mechanisms. The microstructural changes led to a decrease in impact toughness after heat treatment. Presumably, this was a result of the precipitations between needles. Fatigue results showed multiple crack nucleation at the surface in both, as-built and stress-relieved material. The crack propagation rate was slightly higher and the crack was less deflected in the stress-relieved material due to the stress relief and coarsening of the microstructure.

  • 378.
    W. Lindvall, Fredrik
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Adhesive wear testing and modelling of tool steels sliding against sheet metals2014Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Sheet metal forming is a manufacturing method used because of its versatility. Sheets are plastically deformed by a tool to create a product. A tool is expected to last for several 100,000 forming operations and efforts are made to optimize the tools. A common type of wear referred to as galling is the adhesion of sheet material to the tools. This problem has become more prevalent as new high strength sheet materials have been developed at the same time as lubricants have become heavily regulated. This has forced the development of new tool steels with improved resistance to galling. There are many parameters influencing the response to galling. In this work the influence of surface preparation, contact geometry, material selection and lubrication has been investigated. The surface of the forming tools has a large influence on the tools effective life. To refurbish a forming tool is expensive and often requires special shops and hand polishing. The influence on galling of different surface preparations suited for sheet metal forming was investigated using a strip-reduction equipment. The contact conditions of a tool sliding against metal sheets were investigated using FE models. The contact conditions were calculated for a U-bending test and for a sliding-on-flat-surface wear tester. The results were compared to those found in literature. One model incorporated the surface roughness of a sheet as measured by optical profilometry. The strength of the interface between the tool and the sheet material determine if material can be transferred onto the tool. The interface between the tool and adhered sheet material was closely studied using transmission electron microscopy of thin lamellas produced by focused ion beam milling. This showed sheet material adhering to the tool without the formation of an interlayer. Finally, several different combinations of tool steels and sheet materials were tested with regards to their ability to withstand galling.

  • 379.
    Walan, Susanne
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för miljö- och livsvetenskaper (from 2013).
    Jeannie, Flognman
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Kilbrink, Nina
    Karlstads universitet, Fakulteten för humaniora och samhällsvetenskap (from 2013), Institutionen för pedagogiska studier (from 2013).
    Building with focus on stability and construction: Using a story as inspiration when teaching technology and design in preschool2019Ingår i: Education 3-13, ISSN 0300-4279, E-ISSN 1475-7575Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this study children's learning of the concept, stability, during some building activities were investigated. It was also examined how a story can create meaning, having the children build for some animals in the story. Two preschool teachers and 10 children participated. Data consisted of video-recordings from activities with the children and was analysed through thematic coding. The findings showed that the children enjoyed to build and showed an understanding of how to build stable constructions, however without using the word stable. There were connections to the story and new stories were also made by the children during building activities.

  • 380.
    Wallberg, Elin
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik, Science, Mathematics and Engineering Education Research (SMEER).
    Utveckling av empatistation för att få förståelse för hur det är att leva av lite luft2015Självständigt arbete på grundnivå (högskoleexamen), 15 poäng / 22,5 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Projektet är ett examensarbete för högskoleingenjörsprogrammet i innovationsteknik och design vid Karlstads universitet. Bakom uppdraget ligger Experio Lab som använder sig av empatistationer som en del i deras arbetsform. Empatistationerna innehåller simuleringsverktyg för att simulera olika funktionsnedsättningar. För att ge användaren ytterligare förståelse förekommer även information och/eller aktiviteter vid stationen.

    Första delen i projektet fokuserar på vad simuleringsverktyg är samt vad det finns för olika simuleringsverktyg ämnade att ge empati hos användaren. Projektet kom efter hand att handla om att utveckla ett av Experio Labs befintliga empatistationer.

    Under projektet Utveckling av empatistation för att få förståelse av hur det är att leva av lite luft, har det analyserats hur empatistationen för simulering av kroniskt obstruktiv lungsjukdom, KOL-simulatorn, kan utvecklas. Detta för att ge användaren en djupare förståelse för hur det kan vara att leva med andningssvårigheter. Det finns många olika orsaker till andningssvårigheter men när det kommer till andningssvårigheter orsakade av sjukdom är det främst sjukdomar i luftrör och lungor som är orsaken. Detta projekt fokuserar på obstruktiva andningssvårigheter vilket innebär ett ökat flödesmotstånd i luftrören där astma och KOL är de vanligaste.

    Genom litteraturstudier, intervjuer med personer insatta i ämnet och personer som lever med andningssvårigheter kom projektet efter hand att bestå av tre delar som tillsammans skapar helhetskonceptet. Huvuddelen är ett simuleringsverktyg för andningssvårigheter. För att ge användaren kunskap och förståelse för hur det kan vara att leva med andningssvårigheter skapades även ett informationsblad samt scenarier.

    Projektets resultat har medfört en mer fysiologisk och verklighetstrogen grund för empatistationen. Att ta del av processen vid andning med motstånd, få information om innebörden och påvisa hur personer med andningssvårigheter kan uppleva sin vardag ökar kunskapen hos användaren.

  • 381.
    Wang, Yuming
    et al.
    Nanjing Tech University, China; Linkoping University .
    Jafari, Mohammad Javad
    Linköping University .
    Wang, Nana
    Nanjing Tech University, China.
    Qian, Deping
    Linkoping University.
    Zhang, Fengling
    Linkoping University.
    Ederth, Thomas
    Linkoping University.
    Moons, Ellen
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Wang, Jianpu
    Nanjing Technical University, China.
    Inganas, Olle
    Linköping University.
    Huang, Wei
    Nanjing Technical University, China.
    Gao, Feng
    Linkoping University.
    Light-induced degradation of fullerenes in organic solar cells: a case study on TQ1:PC71BM2018Ingår i: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 6, nr 25, s. 11884-11889Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The stability of organic solar cells (OSCs) is critical for practical applications of this emerging technology. Unfortunately, in spite of intensive investigations, the degradation mechanisms in OSCs have not been clearly understood yet. In this report, we employ a range of spectroscopic and transport measurements, coupled with drift-diffusion modelling, to investigate the light-induced degradation mechanisms of fullerene-based OSCs. We find that trap states formed in the fullerene phase under illumination play a critical role in the degradation of the open-circuit voltage (V-OC) in OSCs. Our results indicate that the degradation is intrinsic to the fullerenes in OSCs and that alternative acceptor materials are desired for the development of stable OSCs.

  • 382.
    Wassborg, Pär
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Tribological evaluation of the contact between upper compression ring and cylinder liner with different surface coatings2016Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [en]

    The constant pursuit in the automotive industry to increase the engines performance, new solutions are always developed and tested to reduce the friction and increase the efficiency in the engine. One component that contributes to friction losses is the piston ring pack where the top compression stands for up to 40 %. This master thesis collaborated with Scania’s material science department Basic engine and covers the friction and wear of four different materials on the cylinder liner surface against the top compression ring.The four tested materials were grey cast iron with different honing quality and three atmospheric plasma sprayed coatings with titanium oxide, chromium oxide and Metco’s mixture F2071 which is a stainless steel mixed with a ceramic. A martensitic steel piston ring with a chromium coated sliding surface was used for all the testing in the Cameron-Plint TE77 test-rig. This is a pin-on-disc test method and the parameters used for testing is set to replicate the environment the ring is exposed to at the top dead centre.The test-rig has been in Scania’s possession for a long time and has not always given a satisfying result. An uneven contact between the ring and liner has been a problem resulting in only worn edges of the liner specimen. The piston ring holder was therefore redesigned to be able to adjust the radius of the ring. This allowed a good conformability between the ring and liner to be obtained.The tested materials were evaluated according to friction and wear. Friction was measured with the test-rig and the wear was calculated with surface profiles that were measured before and after testing. Worn surfaces were studied in a SEM to verify which wear mechanism that was active. The changes of the surfaces was studied with the use of following surface parameters Ra, Rk, Rpk, Rvk and if there was a connection between these parameters and friction and wear coefficient.Independent of honing quality showed the grey cast iron lowest friction coefficient just under 0.13, the F2071 liner showed a friction coefficient just above 0.13. Both oxide layers showed similar friction where the chromium oxide had a friction just below 0.15 and the titanium oxide lay just above 0.15. Lowest wear coefficient had the chromium oxide followed by F2071, titanium oxide and the bad honed grey cast iron. These three liners showed almost the exact same wear coefficient. Worst wear coefficient had the grey cast iron with a good honing quality. A mild abrasive wear mechanism was active during the wear test and vague wear marks was found on the surface. There is no connection between wear coefficient and friction and the change in surface roughness during the test does not affect the friction.

  • 383.
    Weinesson, Linnéa
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013), Science, Mathematics and Engineering Education Research (SMEER).
    Climate Neutral Management: Implement Circular Economy in the Construction Industry2019Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Byggbranschen släpper ut en betydande mängd växthusgaser i atmosfären, använder mycket resurser och genererar mycket avfall. Dessa handlingar är inte hållbara och industrin har därför skapat en färdplan över huruvida de ska bli klimatneutrala till 2045. Några saker som lyfts fram i denna färdplan är vikten av ledarskap och en omställning till cirkulär ekonomi, däremot har befintlig forskning inte berört sambandet mellan dessa i byggprojekt. Denna studie syftar till att förstå och bedöma implementeringen av cirkulär ekonomis påverkan på ledarskapet av byggprojekt. För att uppfylla detta syfte utformades ett teoretiskt ramverk bestående av potentiella tillämpningar av cirkulär ekonomi i byggprojekt. Sambandet mellan dessa tillämpningar och projektledningen undersöktes sedan i en empirisk studie som bestod av en kvalitativ fallstudie med dokumentanalyser, projektobservationer och intervjuer med olika projektledarroller inom ett svenskt byggföretag. Resultaten antyder att implementeringen påverkar ledarna till att fokusera mer på helheten och tänka genom metodvalen. Förskottsplanering är väsentligt samt att diskutera lösningar för att optimera resursanvändningen, ha god och tidig kommunikation med rätt leverantörer och andra byggarbetsplatser, och en noggrann mängdning av godkända material samt fokusera på hanteringen av dessa. Dessutom är det viktigt att göra en ordentlig inskrivning av nyanställda samt att leda dessa mot högt satta miljömål. Slutligen visade resultatet att det är svårt att precisera endast en ledare som påverkas av en specifik tillämpning, eftersom alla är involverade i implementeringen av cirkulär ekonomi.

  • 384.
    Yadroitsev, I.
    et al.
    Cent Univ Technol, Dept Mech & Mechatron Engn, ZA-9300 Bloemfontein, Free State, South Africa.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Yadroitsava, I.
    Cent Univ Technol, Dept Mech & Mechatron Engn, ZA-9300 Bloemfontein, Free State, South Africa.
    Du Plessis, A.
    Univ Stellenbosch, CT Scanner Facil, ZA-7602 Stellenbosch, South Africa.
    Qualification of Ti6Al4V ELI Alloy Produced by Laser Powder Bed Fusion for Biomedical Applications2018Ingår i: JOM: The Member Journal of TMS, ISSN 1047-4838, E-ISSN 1543-1851, Vol. 70, nr 3, s. 372-377Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Rectangular Ti6Al4V extralow interstitials (ELI) samples were manufactured by laser powder bed fusion (LPBF) in vertical and horizontal orientations relative to the build platform and subjected to various heat treatments. Detailed analyses of porosity, microstructure, residual stress, tensile properties, fatigue, and fracture surfaces were performed based on x-ray micro-computed tomography, scanning electron microscopy, and x-ray diffraction methods. The types of fracture and the tensile fracture mechanisms of the LPBF Ti6Al4V ELI alloy were also studied. Detailed analysis of the microstructure and the corresponding mechanical properties were compared against standard specifications for conventional Ti6Al4V alloy for use in surgical implant applications. Conclusions regarding the mechanical properties and heat treatment of LPBF Ti6Al4V ELI for biomedical applications are made.

  • 385.
    Yadroitsev, Igor
    et al.
    South Africa.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Yadroitsava, I.
    South Africa.
    A systematic approach to manufacturing parts with desired properties by selective laser melting2015Ingår i: Materials Science and Technology Conference and Exhibition 2015, MS and T 2015, Association for Iron and Steel Technology, AISTECH , 2015, Vol. 1, s. 121-128Konferensbidrag (Refereegranskat)
    Abstract [en]

    Selective laser melting (SLM) is becoming a powerful additive manufacturing technology for different industries: automotive, medical, chemical, aerospace, etc. The extension of applications requires a wide spectrum of powder materials with specific properties. To produce parts with tailored properties by SLM, optimal process-parameters and scanning strategies have to be used for different powders. Numerical simulation allows the estimation of temperature distribution during laser melting and prediction the final microstructures and properties of SLM object. A hierarchical approach, including systematic analysis of SLM parameters necessary to control the final product quality on every level - track, layer, 3D object is suggested and discussed. A series of single tracks, layers and 3D objects were manufactured from metal powders to validate a proposed algorithm. The efficiency of the approach was illustrated by the manufacturing of fully dense samples from AISI 420 stainless steel.

  • 386.
    Yadroitsev, Igov
    et al.
    Department of Mechanical and Mechatronic Engineering, Bloemfontein, Central University of Technology, Free State, South Africa.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Yadroitsava, Inna
    Department of Mechanical and Mechatronic Engineering, Bloemfontein, Central University of Technology, Free State, South Africa.
    Du Plessis, Anton
    University of Stellenbosch.
    Qualification of DMLS Ti6Al4V (ELI) alloy for biomedical applications2017Ingår i: Proceedings of the 28th Annual International Solid Freedom Fabrication symposium – an Additive Manufacturing conference 2017 / [ed] David L. Bourell, Richard H. Crawford, Carolyn C. Seepersad, Joseph J. Beaman & Scott Fish, Austin: University of Texas , 2017Konferensbidrag (Refereegranskat)
  • 387.
    Yaghoubi, Saba Tahaei
    et al.
    Aalto University, Finland.
    Mousavi, Mahmoud
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013). Aalto University, Finland.
    Paavola, Juha
    Aalto University, Finland.
    Size effects on centrosymmetric anisotropic shear deformable beam structures2017Ingår i: Zeitschrift für angewandte Mathematik und Mechanik, ISSN 0044-2267, E-ISSN 1521-4001, Vol. 97, nr 5, s. 586-601Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper, the size effect on beam structures with centrosymmetric anisotropy is studied within strain gradient elasticity theory. Applying dimension reduction to the three dimensional anisotropic gradient elasticity, the third-order shear deformable (TSD) beam is analysed. A variational approach is used to determine the equilibrium equations of TSD beam together with consistent (classical and non-classical) boundary conditions. The TSD beam theory which is suitable for deep beam structures can be replaced by (less complicated) Euler-Bernoulli beam model for thin beam structures. The anisotropic Euler-Bernoulli beam model is also formulated within the framework of strain gradient theory. This anisotropic beam theory can be used to study size effects for any types of centrosymmetric anisotropy. To address the more practical cases of composite structures, the formulation is simplified for orthotropic and transversely isotropic materials. Finally, the analytical solutions are provided for bending of simply supported (TSD and Euler-Bernoulli) beams as well as clamped Euler-Bernoulli beams. The effect of the crystal orientation with respect to the beam geometry is investigated in these examples.

  • 388.
    Yaghoubi, SabaTahaei
    et al.
    Aalto university, Finland.
    Mousavi, Mahmoud
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013). Aalto university, Finland.
    Paavola, Juha
    Aalto university, Finland.
    Buckling of centrosymmetric anisotropic beam structures within strain gradient elasticity2017Ingår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 109, s. 84-92Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Buckling of centrosymmetric anisotropic beams is studied within strain gradient theory. First, the three dimensional anisotropic gradient elasticity theory is outlined. Then the dimension of the three dimensional theory is reduced, resulting in Timoshenko beam as well as Euler–Bernoulli beam theories. The governing differential equations together with the consistent (classical and non-classical) boundary conditions are derived for centrosymmetric anisotropic beams through a variational approach. By considering von Kármán nonlinear strains, the geometric nonlinearity is taken into account. The obtained nonlinear formulation can be used to study the postbuckling configuration. The analysis of size effect on anisotropic beam structures is missing in the literature so far, while the present model allows one to characterize the size effect on the buckling of the centrosymmetric anisotropic micro- and nano-scale beam structures such as micropillars. As a specific case, the governing buckling equation is obtained for the more practical case of orthotropic beams. Finally, the buckling loads for orthotropic simply supported Timoshenko and Euler–Bernoulli beams as well as a clamped Euler–Bernoulli beam are obtained analytically and the effect of the internal length scale parameters on the buckling load is depicted.

  • 389.
    Yasir, Irshad
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik, Science, Mathematics and Engineering Education Research (SMEER).
    A study of a stochastic differential equation based model for wireless channels2008Självständigt arbete på avancerad nivå (magisterexamen), 10 poäng / 15 hpStudentuppsats (Examensarbete)
    Abstract [en]

    An autoregressive (AR) first order model is often used for modelling wireless channels. This is done in spite of the fact that a satisfactory physical explanation for this model has been missing. However, in the recent paper [Feng, Field and Haykin. 2007] derive a model in form of a first order stochastic differential equation (SDE) from a stochastic description of the scattered electric field. After discretizing this SDE, a physically motivated first order AR model is obtained.

  • 390.
    Yong-an, Min
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013). Shanghai University, China.
    Bergström, Jens
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Xiao-chun, Wu
    Shanghai University, China.
    Luo-ping, Xu
    Shanghai University, China.
    Oxidation and Thermal Fatigue Behaviors of Two Type Hot Work Steels During Thermal Cycling2013Ingår i: Journal of Iron and Steel Research International, ISSN 1006-706X, E-ISSN 2210-3988, Vol. 20, nr 11, s. 90-97Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Thermal fatigue test has been carried out on widely used hot work steel 4Cr5MoSiV1 and a low alloyed steel 3Cr3MoV in temperature range of 200 to 700 degrees C. Tempering resistance, as well as high temperature hardness/strength of steel specimens, works as a dominating material parameter on thermal fatigue resistance. During the heating period, high hardness can depress the inelastic deformation. This deformation is the origination of tensile stress, which acts as the driving force of heat checking during the cooling period. The cyclic strain-oxidation interaction can speed up the damage on surface defects, which plays an obvious role in initiation of thermal cracks. On 4Cr5MoSiV1 steel specimens, borders between the matrix and inclusions such as titanium compounds, or lager carbides such as primary carbides, are focused by strain and attacked by oxidation, and are main initiating places of cracks. While on 3Cr3MoV steel specimens, larger strain causes plastic deformation concentrating around grain boundaries. Then the following oxidation accelerates this grain boundary damage and creates cracks.

  • 391.
    Zhang, C.
    et al.
    Waseda UniversityTokyo.
    Ohashi, T.
    Tokyo Institute of TechnologyToky.
    Saijo, M.
    Tokyo Institute of TechnologyToky.
    Solis, Jorge
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Takeda, Y.
    Tokyo Institute of TechnologyToky.
    Lindborg, A. -L
    Camanio Care ABNackaSweden.
    Takeda, R.
    Leave a Nest Co. Ltd.Tokyo.
    Tanaka, Y.
    Global Information and Telecommunication InstituteWaseda UniversityTokyoJapan.
    A Monte Carlo based computation offloading algorithm for feeding robot IoT system2018Ingår i: Smart Computing and Communication, Springer Verlag , 2018, s. 163-171Konferensbidrag (Refereegranskat)
    Abstract [en]

    Ageing is becoming an increasingly major problem in European and Japanese societies. We have so far mainly focused on how to improve the eating experience for both frail elderly and caregivers by introducing and developing the eating aid robot, Bestic, made to get the food from plate to the mouth for frail elderly or person with disabilities. We expand the functionalities of Bestic to create food intake reports automatically so as to decrease the undernutrition among frail elderly and workload of caregivers through collecting data via a vision system connected to the Internet of Things (IoT) system. Since the computation capability of Bestic is very limited, computation offloading, in which resource intensive computational tasks are transferred from Bestic to an external cloud server, is proposed to solve Bestic’s resource limitation. In this paper, we proposed a Monte Carlo algorithm based heuristic computation offloading algorithm, to minimize the total overhead of all the Bestic users after we show that the target optimization problem is NP-hard in a theorem. Numeric results showed that the proposed algorithm is effective in terms of system-wide overhead.

  • 392.
    Zhang, Hanmin
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Hirvonen Grytzelius, Joakim
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Johansson, Lars
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Thin Mn germanide films studied with XPS, STM, and XMCD2013Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, nr 4, s. 045311-1-045311-6Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Thin Mn germanide films with nanoscale thicknesses on Ge(111) have been studied by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), core-level spectroscopy (CLS), and x-ray magnetic circular dichroism. The 260 C annealing of 16 monolayers of Mn deposited on Ge(111)c(2×8) resulted in a uniform film with intense threefold split √3×√3 LEED spots and Moiré patterns in the STM images. This ultrathin film shows a clear ferromagnetism with a Curie temperature of ∼250 K. High-resolution Ge 3d CLS spectra were recorded with photon energies between 50 and 90 eV at normal and 60 emission angle. To achieve a consistent fit over the energy and angular range three components were used in the line-shape analysis. The low temperature (260 C) annealed film shows significant differences in terms of electronic structure and magnetism in contrast to the high temperature (330 C or above) annealed ones. Our results indicate that the annealing temperature and the Mn coverage play important roles in the formation of a thin magnetic Mn germanide film.

  • 393.
    Zhang, Hanmin
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013). CMM, Characterization and modeling of materials.
    Holleboom, Thijs
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Johansson, Lars
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Band splitting of quantum wells of thin Ag films on Sn/Si(111)√3×√32017Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, nr 4, s. 041402-1-041402-6Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    High-resolution valence band spectra of ultrathin Ag films on Sn/Si(111)√3×√3 show intrinsic splitting of the quantum-well states (QWSs). Especially at low coverages, the QWSs of such a system display delicate coupling characters with the bulk bands from the substrate. The observed QWS splitting agrees well with the result of the theoretical calculation. We found that the splitting originates from an interface with a finite thickness. In addition, the interface also causes a large sp band splitting due to the Umklapp scattering in the Γ−M direction of the Ag(111) surface Brillouin zone.

  • 394.
    Zhang, Hanmin
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Johansson, Lars
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Electronic structure of PTCDA on Sn/Si(111)-2√3×2√32014Ingår i: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 439, s. 71-78Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The electronic structures of PTCDA on the Sn/Si(111)-2√3×2√3 surface have been thoroughly studied by high-resolution photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS). Upon deposition of PTCDA, there is an unusual charge transfer from the Sn/Si(111)-2√3×2√3 surface to the molecules. This is clearly shown by a new component in the Sn 4d core-level spectra that shifts towards higher binding energy. In contrast to the literature, the charge provided by Sn is donated to the carbonyl C instead of the O atoms. This is revealed by a new component in the C 1s core-level spectra that shifts towards lower binding energy. The charge transfer causes a splitting of the HOMO level in the valence band spectra. As indicated in the NEXAFS spectra, it also induces a splitting of the LUMO level of the molecules. For thick films the NEXAFS results suggest a layer by layer growth mode.

  • 395.
    Zhang, Xiaojing
    et al.
    ABB Corporate Research.
    Lindberg, Therese
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Svensson, Krister
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik.
    Vyatkin, Valeriy
    Department of Computer Science, Electrical and Space Engineering at LuleåUniversity of Technology, Sweden and the Department of Electrical Engineering and Automation, Aalto University, Finland.
    Mousavi, Arash
    Department of Computer Science, Electrical and Space Engineering at LuleåUniversity of Technology, Sweden.
    Power Consumption Modeling of Data Center IT Room with Distributed Air Flow2016Ingår i: International Journal of Modeling and Optimization, ISSN 2010-3697, Vol. 6, nr 1, s. 33-38Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Modern data centers are characterized by large sizes, high energy consumption and complexity involving IT, power supply, ventilation and cooling. Data center energy efficiency is a major concern for data center design and operation. To improve data center energy efficiency through efficient cooling and ventilation, advanced process control and optimization, process models to describe the process power consumption are required. In this work, data center power consumption models are investigated. A concept of distributed air flow control is presented. The objective is to develop a comprehensive data center power consumption model to describe IT room, computer room air handling (CRAH), data center ventilation and cooling characteristics as well as distributed air flow control. Data center operation scenarios with uneven IT load are simulated. Results show that the distributed air flow control can save the cooling energy significantly.

  • 396. Zhang, Xiaojing
    et al.
    Lindberg, Therese
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Xiong, N.
    Vyatkin, Valeriy
    Mousavi, Atash
    Cooling Energy Consumption Investigation of Data Center IT Room with Vertical Placed Server2017Ingår i: Energy Procedia, ISSN 1876-6102, E-ISSN 1876-6102, Vol. 105, s. 2047-2052Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    As energy consumption by cooling data center IT equipment can be over 40 % of total energy consumption, efficient cooling for large data centers is essential for reducing operation costs. Modern data centers are complex systems involving IT facilities, power system, cooling and ventilation systems. In our previous work, literature study was made to investigate available data center energy consumption models; and energy consumption models for data center IT room with distributed air flow control were developed. In this paper, the models are further extended and developed to cover the combined distributed air flow control and vertical placed servers in raised floor ventilation system. Simulation of the three types of ventilation systems with Even load, Idle server and Uneven load scenarios showed that significant cooling energy consumed by a traditional ventilation system can be saved by applying the proposed new concept and method. (C) 2017 The Authors. Published by Elsevier Ltd.

  • 397.
    Züfle, Simon
    et al.
    Fluxim AG, Switzerland; Institute of Computational Physics, ZHAW, Switzerland.
    Hansson, Rickard
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Katz, Eugene A.
    The Ben-Gurion University of the Negev, Israel.
    Moons, Ellen
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Initial photo-degradation of PCDTBT:PC 70 BM solar cells studied under various illumination conditions: Role of the hole transport layer2019Ingår i: Solar Energy, ISSN 0038-092X, E-ISSN 1471-1257, Vol. 183, nr 1, s. 234-239Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    ncapsulated organic solar cells often show a burn-in behaviour under illumination. This burn-in manifests itself as a rapid performance loss followed by a much slower progression of the degradation. Here we investigate the burn-in for PCDTBT:PC 70 BM solar cells under a wide range of illumination intensities. We find that increasing the sunlight concentration from 1 Sun to up to 100 Suns does not change the degradation behaviour, i.e. the dependence of all principal photovoltaic parameters on the dose of solar exposure (in Sun hours). This suggests that the degradation mechanisms under solar concentration (≤100 Suns) are the same as those observed under 1 Sun. This result makes it possible to use concentrated sunlight for accelerated stability assessment of these devices. We also find that devices with PEDOT:PSS as hole transport material show a rapid drop in open-circuit voltage of around 100 mV during the first Sun hour of light exposure. By replacing PEDOT:PSS with MoO 3 this initial process can be prevented and only the much slower part of the photo-degradation takes place.

  • 398.
    Åsberg, Mikael
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Fredriksson, Gunnel
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Hatami, Sepehr
    Sverea IVF, Gotenburg.
    Fredriksson, Wendy
    Bodycote Hot Isostatic Pressing AB.
    Influence of HIP on microstructure, pore elimination and tensile properties of SLM manufactured H13 tool steel2017Konferensbidrag (Övrigt vetenskapligt)
  • 399.
    Åsberg, Mikael
    et al.
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Fredriksson, Gunnel
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Hatami, Sepehr
    Swerea IVF AB.
    Fredriksson, Wendy
    Bodycote Hot Isostatic Pressing.
    Krakhmalev, Pavel
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Influence of post treatment on microstructure, porosity and mechanical properties of additive manufactured H13 tool steel2019Ingår i: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 742, s. 584-589Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Additive manufacturing (AM) is an attractive manufacturing technology in tooling applications. It provides unique opportunities to manufacture tools with complex shapes, containing inner channels for conformal cooling. In this investigation, H13, a widely used tool steel, was manufactured using a laser powder bed fusion method. Microstructure, tensile mechanical properties, hardness, and porosity of the AM H13 after stress relieve (SR), standard hardening and tempering (SR + HT), and hot isostatic pressing (SR + HIP + HT) were investigated. It was found that the microstructure of directly solidified colonies of prior austenite, which is typical for AM, disappeared after austenitizing at the hardening heat treatment. In specimens SR + HT and SR + HIP + HT, a microstructure similar to the conventional but finer was observed. Electron microscopy showed that SR and SR + HT specimens contained lack of fusion, and spherical gas porosity, which resulted in remarkable scatter in the observed elongation to break values. Application of HIP resulted in the highest strength values, higher than those observed for conventional H13 heat treated in the same way. The conclusion is that HIP promotes reduction of porosity and lack of fusion defects and can be efficiently used to improve the mechanical properties of AM H13 tool steel.

  • 400.
    Åström, Maria
    Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
    Vad kännetecknar ämnestexter? (inriktning fysik)2016Övrigt (Övrigt vetenskapligt)
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