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  • 251. Arwidson, Claes
    et al.
    Bernspång, Lars
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Verification of numerical forming simulation in high strength steels2004Inngår i: Proceedings of the Conference Innovations in metal forming, 2004Konferansepaper (Fagfellevurdert)
  • 252.
    Arén, Björn
    Luleå tekniska universitet.
    Hot working of powders: procedure and products1982Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This dissertation can be divided into two main parts. The first deals with the hot working conditions of mild steels in powder forging, the second with the deformation behaviour of powder forged products; both monolithic and compound parts are studied. Hot forging of low alloy steel powders makes it possible to produce complicated parts with properties comparable with homogeneous forged parts, whilst reducing the machining operations necessary to a minimum. This powder forging technique is a high production rate method, and can only be applied to mass produced components. It also gives the possibility of complete reconstruction of the shape of the part. Forging of porous preforms however require certain precautions to be made to prevent working faults and to optimize the densification process. These things are therefore studied with some interest. In order to compete with other high production rate methods such as hot forging, the powder forged parts must be economically advantageous. Therefore the most attractive parts to be produced are those in which substantial material is saved, where most machining operations can be excluded and in which material of substantial structure is required. A method to produce compound structural parts of high strength hardened steel and ductile mild steel from alloyed powders is presented. It involves the manufacture of compound preforms by forging and their heat treatment to produce an optimal structure. On examining the deformation behaviour of compound parts produced, a substantial rise in the fracture toughness is seen, which is associated with the interface between hard and ductile material zones.

    Fulltekst (pdf)
    FULLTEXT01
  • 253.
    Arén, Björn
    Luleå tekniska universitet.
    Powder forging of compound steels: P. 11982Rapport (Annet vitenskapelig)
  • 254.
    Arén, Björn
    Luleå tekniska universitet.
    Powder forging of compound steels: P. 21982Rapport (Annet vitenskapelig)
  • 255.
    Arén, Björn
    et al.
    Luleå tekniska universitet.
    Li, Wen Bin
    Luleå tekniska universitet.
    The approach of near net shape by direct hot isostatic pressing of powders1985Rapport (Annet vitenskapelig)
  • 256.
    Arén, Björn
    et al.
    Luleå tekniska universitet.
    Li, Wen-Ben
    Luleå tekniska universitet.
    Investigation of hot iostatic pressing of metal powder to near net shape1988Inngår i: HIP: proceedings of the International Conference on Hot Isostatic Pressing, Luleå, 15-17 June 1987 / [ed] Tore Garvare, Luleå: CENTEK , 1988, s. 117-124Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Shape control during HIP is influenced by many variables such as container properties, powder filling method, initial density, temperature and pressure cycle, etc. Experiments have been performed using ASP30 aimed at the study of how to produce parts near net shape through the optimization of these variables. Special care was given to the powder filling and laser welded sheet metal containers of special geometry was used. The results indicate that the temperature and pressure cycles during the first stages of the HIP-process have a pronounced effect on the final shape. A HIP-cycle which will result in a close shape is also presented. Graphs

  • 257.
    Arén, Björn
    et al.
    Luleå tekniska universitet.
    Navara, Eric
    Luleå tekniska universitet.
    Modelling shape change of parts produced by hot isostatic pressing1988Inngår i: Powder Metallurgy, ISSN 0032-5899, E-ISSN 1743-2901, Vol. 31, nr 2, s. 101-105Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Models for analysing shape changes during the hipping of powder compacts enclosed in preshaped containers are presented. Two limiting mechanisms are considered: homogeneous densification and the motion of a sharply defined densification front through the compact as hipping proceeds. The shape changes and shrinkages occurring in a number of simple one, two, and three dimensional shapes are analysed and illustrated. Although some of the assumptions implicit in the model are at variance with the conditions found in practice, it is concluded that useful insights are provided into the shape changes occurring during hipping

  • 258.
    Arén, Björn
    et al.
    Luleå tekniska universitet.
    Navara, Eric
    Luleå tekniska universitet.
    Modelling shape change of parts produced by hot isostatic pressing of powders1988Inngår i: Powder Metallurgy, ISSN 0032-5899, E-ISSN 1743-2901, Vol. 31, nr 2, s. 101-105Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Models for analysing shape changes during the HIPping of powder compacts enclosed in preshaped containers are presented. Two limiting mechanisms are considered: homogeneous densification and the motion of a sharply defined densification front through the compact as HIPping proceeds. The shape changes and shrinkages occurring in a number of simple one, two, and three dimensional shapes are analysed and illustrated. Although some of the assumptions implicit in the model are at variance with the conditions found in practice, it is concluded that useful insights are provided into the shape changes occurring during HIPping

  • 259.
    Arén, Björn
    et al.
    Luleå tekniska universitet.
    Navara, Eric
    University of Zimbabwe.
    Modelling the shape change of parts produced by hot iostatic pressing of powders1988Inngår i: HIP: proceedings of the International Conference on Hot Isostatic Pressing, Luleå, 15-17 June 1987 / [ed] Tore Garvare, Luleå: CENTEK , 1988, s. 107-115Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Densification in a HIP cycle is non-uniform because of the temperature gradient in the powder body. This gradient gives rise to a densification front, which moves from the surface inward. The densification front will, in reality, be more or less diffuse, but may be considered as extremely sharp for the purpose of modelling the shape and dimensional changes of idealized powder bodies. The geometrical considerations presented show that extreme shape change can occur in a practical HIP cycle. As an example, a square cross-section powder body acquires a star-like shape, in which the angle at the edges depends solely on the densification ratio. The results may serve as a base for an improved powder capsule design, taking into account the effects of non-uniform densification

  • 260.
    Arén, Björn
    et al.
    Luleå tekniska universitet.
    Navara, Eric
    Luleå tekniska universitet.
    Shape Development of Powder Bodies Modelled as a Result of the Densification Process in Hot Isostatic Pressing1988Inngår i: Proceedings of the 1988 International Powder Metallurgy Conference: International powder metallurgy conference : Papers / [ed] P. Ulf Gummeson; Donald A. Gustafson, Princeton, NJ: Metal powder industries federation , 1988, s. 23-36Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The densification of a powder body in a HIP process is non-uniform because of an existing temperature gradient in the powder body. This gradient gives rise to a densification front, which moves from the surface inwards. This front will, in reality, be more or less diffuse, but may be considered as extremely sharp for the purpose of modelling the shape and dimensional changes of idealized powder bodies. The geometrical considerations presented show that extreme shape change can occur in a practical HIP cycle. As an example, a cubic powder body will get a star-like shape, in which the edge angles will depend solely on the densification ratio. Considered are the effects caused by uniform and non-uniform heating in the densification of various powder bodies defined by flat and/or curved body surfaces, and the results may serve as a base for an improved understanding of the effects of non-uniform densification and its importance for the powder body design.

  • 261.
    Arén, Björn
    et al.
    Luleå tekniska universitet.
    Navara, Erik
    Luleå tekniska universitet.
    Modelling the shape change of parts produced by hot isostatic pressing of powders1987Rapport (Annet vitenskapelig)
  • 262.
    Arén, Björn
    et al.
    Luleå tekniska universitet.
    Nilsson, Annika
    Luleå tekniska universitet.
    Die tool dimensioning by finite element method1987Inngår i: Powder Metallurgy, ISSN 0032-5899, E-ISSN 1743-2901, Vol. 30, nr 2, s. 87-96Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Stress and strain are studied using the finite element method in the dimensioning of prestrained die tools for powder compaction. The die geometry has been varied and the die has also been subjected to different types of load. The study shows that the mean pressure transverse to the pressing direction is a dimensioning factor, but that the distribution itself is not very important. For a linear pressure distribution, the stress and strain within the die will differ little from results produced under the assumption of an exponential pressure distribution. Attention is also paid to the possibilities of treating the three-dimensional situation in a real die by different methods of two-dimensional FEM analysis

  • 263.
    Arén, Björn
    et al.
    Luleå tekniska universitet.
    Åkerberg, L G
    Luleå tekniska universitet.
    Jernlund, T
    Luleå tekniska universitet.
    Skärbarhet hos sintergods1983Rapport (Annet vitenskapelig)
  • 264.
    Ashby, M. F.
    et al.
    Cambridge University.
    Easterling, Kenneth E.
    Luleå tekniska universitet.
    A first report on diagrams for grain growth in welds1982Inngår i: Acta Metallurgica, ISSN 0001-6160, Vol. 30, nr 11, s. 1969-1978Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Data for carbide dissolution, grain growth and martensite formation in the heat-affected zone of a weld can be assembled into a diagram showing the extent of each, for different weld cycles, and at different points in the zone. The diagrams are based on elementary kinetic models for grain growth and carbide dissolution, integrated over the weld cycle; and on experimental data. The sets of poorly-known kinetic constants which appear in such a treatment are determined by fitting the equations to data from real or simulated welds, at certain fixed points. Diagrams are presented for six steels. As well as summarising much data they allow the effect of change in weld procedure, or of preheat, to be predicted.

  • 265.
    Ashby, M.F.
    et al.
    Cambridge University.
    Easterling, K.E.
    Luleå tekniska universitet.
    Harrysson, Ralph
    Luleå tekniska universitet.
    Maiti, S.K.
    Indian Institute of Technology.
    The fracture and toughness of woods1985Inngår i: Proceedings of The Royal Society of London. Series A, Mathematical and physical sciences, ISSN 0080-4630, Vol. 398, nr 1815, s. 261-280Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Crack propagation in various woods has been examined by scanning electron microscopy, and the observations related to measurements of fracture toughness. It is found that the toughness is related in a simple way to the density of the wood, which is explained by a straightforward model. The apparent fracture toughness of wood for cracks that lie normal to the grain is larger, by a factor of about 10, than that for cracks which propagate parallel to the grain. This difference can be explained in terms of the fracture mechanics of very anisotropic solids

  • 266.
    Ashby, M.F.
    et al.
    Cambridge University.
    Easterling, Kenneth E.
    Luleå tekniska universitet.
    A first report on welding diagrams1981Rapport (Annet vitenskapelig)
    Abstract [en]

    Data for carbide dissolution and grain growth in the heat affected zone of a weld can be assembled into a diagram showing the extent of each, for different weld cycles, and at different points in the zone. The diagrams are based on elementary kinetic models for grain growth and carbide dissolution, integrated over the weld cycle. The sets of kinetic constants which appear in such a treatment are determined by fitting the equations to data from real or simulated welds, at certain fixed points. Diagrams are presented for six steels. As well as summarizing much data they allow the effect of change in weld procedure, or of preheat, to be predicted.

  • 267.
    Ashby, M.F.
    et al.
    University Engineering, Department, Trumpington St, Cambridge.
    Easterling, Kenneth E.
    Luleå tekniska universitet.
    The transformation hardening of steel surfaces by laser beams-I. Hypo-eutectoid steels1984Inngår i: Acta Metallurgica, ISSN 0001-6160, Vol. 32, nr 11, s. 1935-1948Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Surfaces can be treated with a scanning laser beam to modify their properties. The beam subjects the near-surface to a thermal cycle with an amplitude and shape which depends on the process variables: the beam size, energy, scan rate and on whether the laser is pulse or continuous, and the chemistry and metallurgy of the steel. Approximate solutions to the equations of heat flow are combined with kinetic models to predict the near-surface structure and hardness of plain carbon steels after laser treatment. The results are assembled into diagrams which show immediately the combination of process variables for a desired surface structure and the associated hardness profile

  • 268.
    Ashkin, A.
    et al.
    Luleå tekniska universitet.
    Ashkin, D.
    Luleå tekniska universitet.
    Babushkin, O.
    Luleå tekniska universitet.
    Investigation of α-sialon formation by high temperature X-ray diffraction1994Inngår i: Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795, Vol. 89-91, s. 373-377Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A technique for following sialon formation in situ by high temperature x-ray diffraction (HT-XRD) was developed. The composition chosen for study was an yttrium α-sialon with x = 0.4. Powder compacts containing silicon nitride, aluminum nitride and yttria powders were pre-sintered at 1350 °C and then studied by HT-XRD at temperatures between 1450 and 1580 °C and nitrogen pressures of 0.11 MPa. The furnace was made from graphite coated with porous silicon nitride/silicon carbide. The coating prevented silicon carbide formation in the sample up to 1600 °C. X-ray diffraction results show the formation of a Y10Al2Si3O18N4 phase at 1350 °C, which dissolved to form α-sialon and other phases at higher temperatures. The amounts of α-sialon formed are similar to the amounts reported by other authors. An empirical method was used for the calculation of activation energy for the silicon nitride to α-sialon transformation and resulted in a value of 330 kJ mol

  • 269.
    Ashkin, A.
    et al.
    Luleå tekniska universitet.
    Ashkin, D.
    Luleå tekniska universitet.
    Babushkin, O.
    Luleå tekniska universitet.
    Ekström, T.
    Arrhenius laboratory.
    At-temperature observation of phase development in yttrium α-Sialon1995Inngår i: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 15, nr 11, s. 1101-1109Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A powder mixture of α-Si3N4, AlN and Y2O3 corresponding to an yttrium α-sialon composition, Y0.4Si10.2Al1.8O0.6N15.4, was sintered both in a high temperature X-ray diffraction unit and in a regular sintering furnace. X-ray analysis was performed and it was shown that high temperature X-ray diffraction can be used to monitor the kinetics of the α-sialon phase formation during sintering at temperatures between 1450 and 1580 °C as the reactions take place. A variety of yttrium-rich intermediate and secondary phases are formed during sintering; those formed in the HT-XRD unit were not the same as those formed in the regular sintering furnace, but they do not significantly influence the overall reaction sequence or the amount of α-sialon formed. Quantitative analysis of the acquired data was used to evaluate the kinetics of the α-sialon phase formation; the estimated activation energy of the initial stage α-sialon formation was 330kJ/mol. After this initial stage of relatively rapid α-sialon formation the process became inhibited

  • 270.
    Ashkin, Alena
    Luleå tekniska universitet.
    Formation of silicon nitride based materials by nitridation and sintering1995Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Todays demands on silicon nitride based materials for structural applications focus on microstructures rendering high strengths and reliability at reasonable costs. This work was mainly aimed at production of sialons for these types of applications by nitridation with subsequent sintering. For this purpose both the influence of different factors on sialon formation and the effect of additives in nitriding was studied, as well as different sintering methods. Y-alfa-sialons with x=0.1 to 0.9 in the formula Yx(Sil2-4.5x,Al4.5x)(01.5x,N 16-1.5x) prepared from silicon nitride powders were sintered in different ways. The crystalline phase composition varied from alfa/beta sialon at x=0.2 to alfa sialon at x=0.4 and alfa sialon + polytypes at x=0.8. The highest alfa-sialon content and density was obtained for x=0.4 with an excess of yttria after sintering 1h at 1750°C and post HIPing 1h at 1750°C and 200 MPa. No glass encapsualtion was needed as closed porosity was obtained in the sintering step. Less residual glass was also present after this processing than when just HIPing. Sintering without pressure, however was not enough to densify the material. Sintering experiments by HT-XRD and in a conventional furnace of an x=0.4 alfa-sialon composition without excess yttria, showed that the amount of alfa-sialon formed was relatively insensitive to small changes in composition. Assuming that the formation mechanisms during the early stages of sintering (first 90-120 min) did not change with time and temperature, therefore made it possible to determine the kinetics of alfa-sialon formation. The activation energy was estimated as 330 kJ/mol. The effect of additives on nitridation was studied by adding silica or additives for sialon formation (AlN, Al2O3, Y2O3, CaO) to silicon. Formation of silicon oxynitride in the case of silica additions and sialon in the case of sialon additions was then observed after nitriding. The amounts of sialon formed depended on the liquid phase properties of the different compositions and the nitriding conditions. Fast nitridation resulted in more sialon formation. By nitriding with different schedules it was shown that this formation could be controlled and also that the nitridation could be speeded up when additives were present. Large amounts of additives made the pore size distribution insensitive to nitriding gas composition. The presence of hydrogen in the gas, however, did increase the amount of reaction at low temperatures and thereby influenced the phase composition. Densification of the materials nitrided with silica present was not possible by pressureless sintering and standard glass encapsulated HIPing. Most of the nitrided sialon compositions, on the other hand, sintered well by most sintering methods used at temperatures of 1850°C, and resulted in homogeneous microstructures. Sinter HIPing rendered unusually elongated grains in the (alfa-sialon (x=0.4), while the beta-sialon (z=2) had high grain aspect ratios for all sintering methods. HIPing at 1750°C gave the highest densities in most cases but resulted in inhomogeneities in the alfa-sialon. These looked very similar to inhomogeneities obtained when AlN additive powder with larger grain sizes was used. This work shows that nitridation with subsequent sintering of sialon compositions is a very promising way of manufacturing high performance structural ceramics based on silicon nitride.

    Fulltekst (pdf)
    fulltext
  • 271.
    Ashkin, Alena
    et al.
    Luleå tekniska universitet.
    Ashkin, Daniel
    Luleå tekniska universitet.
    Reaction bonding and post sintering of alpha-sialons1995Inngår i: Ceramics : charting the future: Symposium : 8th World ceramics congress and Forum on new materials : Papers / [ed] P. Vincenzini, TECHNA Srl , 1995, s. 1859-1866Konferansepaper (Fagfellevurdert)
  • 272.
    Ashkin, Daniel
    Luleå tekniska universitet.
    Nitridation Behaviour of Silicon with Clay and Oxide Additions: Rate and Phase Development1997Inngår i: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 17, nr 13, s. 1613-1624Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Samples pressed from fine powders of Si, Si + SiO2, Si + kaolinite and Si + SiO2 + Al2O3 were nitrided by both the nitrogen demand method and a constant rate of heating using a graphite resistance furnace at temperatures of 1100 to 1450 °C. The nitridation rate for different compositions and input parameters were determined by continuously monitoring the time, temperature and pressure within the furnace. By comparing the different compositions, the effect of impurities, silica and particle size on nitriding rate, phase composition and percent theoretical weight gain were determined. Compositions containing high purity additives were found to nitride at higher temperatures and to be more susceptible to weight loss during nitridation. The prime cause for low weight gains was determined to be volatilization of silicon out of the samples. Si2N2O/O′ Sialon was formed in all compositions containing the as-used additives and acted to retain the oxygen within the sample.

  • 273.
    Ashkin, Daniel
    et al.
    Luleå tekniska universitet.
    Haber, R.A.
    Wachtman, J.B.
    Slow crack groth as a function of relative density for vitreous silica1995Inngår i: Ceramics : charting the future: Symposium : 8th World ceramics congress and Forum on new materials : Papers / [ed] P. Vincenzini, TECHNA Srl , 1995Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Crack velocities as a function of KI were studied for gel-derived vitreous silica sintered from relative densities of 0.23 to full density. The gels were formed by mixtures of colloidal silica, potassium soluble silicate and formamide. Slow crack growth was studied by the constant moment double cantilever beam test. Data were analysed in terms of the log v-K plots and the equation v = (v0/A1)exp(-E*+bA2KI)/RT. Experiments showed that porosity had a major effect on the slope of these plots but only a minor effect on the log intercept. The change in log intercept was considered in terms of the pre-exponential A1, the change in slope by the term A2. A2 varied from a value of 1 at full density to a value of 6.71 at 0.23 relative density

  • 274.
    Askari, SJ
    et al.
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Akhtar, Farid
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Chen, GC
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    He, Q
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Wang, FY
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Meng, XM
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Lu, FX
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Synthesis and characterization of nano-crystalline CVD diamond film on pure titanium using Ar/CH 4/H 2 gas mixture2007Inngår i: Materials letters (General ed.), ISSN 0167-577X, E-ISSN 1873-4979, Vol. 61, nr 11-12, s. 2139-2142Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Titanium and Ti alloys have poor tribological properties and deposition of a well adherent diamond coating is a promising way to solve this problem. But diamond film deposition on pure titanium and Ti alloys is always difficult due to the high diffusion coefficient of carbon in Ti, the large mismatch in their thermal expansion coefficients, the complex nature of the interlayer formed during diamond deposition, and the difficulty of achieving very high nucleation density. A nano-crystalline diamond (NCD) film can resolve Ti and Ti alloys weak tribological performance due to its smooth surface. A well-adhered NCD film was successfully deposited on pure Ti substrate by using a microwave plasma assisted chemical vapor deposition (MWPCVD) system in the environment of Ar, CH4 and H2 gases at a moderate temperature. Detailed experimental results on the preparation, characterization and successful deposition of the NCD film on pure Ti are discussed

  • 275.
    Askari, SJ
    et al.
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Akhtar, Farid
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Islam, S.H.
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Qi, H
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Tang, WZ
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Lu, FX
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Two-step growth of high-quality nano-diamond films using CH 4/H 2 gas mixture2007Inngår i: Vacuum, ISSN 0042-207X, E-ISSN 1879-2715, Vol. 81, nr 5, s. 713-717Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Diamond films with fine grain size and good quality were successfully deposited on pure titanium substrate using a novel two-step growth technique in microwave plasma-assisted chemical vapor deposition (MWPCVD) system. The films were grown with varying the methane (CH4) concentration at the stage of bias-enhanced nucleation (BEN) and nano-diamond film deposition. It was found that nano-diamond nuclei were formed at a relatively high methane concentration, causing a secondary nucleation at the accompanying growth step. Nano-diamond film deposition on pure titanium was always very hard due to the high diffusion coefficient of carbon in Ti, the big difference between thermal expansion coefficients of diamond and Ti, the complex nature of the interlayer created during diamond deposition, and the difficulty in achieving very high nucleation density. A smooth and well-adhered nano-diamond film was successfully obtained on pure Ti substrate. Detailed experimental results on the synthesis, characterization and successful deposition of the nano-diamond film on pure Ti are discussed

  • 276.
    Askari, SJ
    et al.
    Institute of Manufacturing Engineering, PNEC, National University of Sciences and Technology (NUST), Karachi.
    Chen, GC
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Akhtar, Farid
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Lu, FX
    Department of High Tech Thin Films, School of Materials Science and Engineering, University of Science and Technology Beijing.
    Adherent and low friction nano-crystalline diamond film grown on titanium using microwave CVD plasma2008Inngår i: Diamond and related materials, ISSN 0925-9635, E-ISSN 1879-0062, Vol. 17, nr 3, s. 294-299Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The use of titanium alloys for aerospace and biomedical applications could increase if their tribological behavior was improved. The deposition of an adherent diamond coating can resolve this issue. However, due to the different thermal expansion coefficients of the two materials, it is difficult to grow adherent thin diamond layers on Ti and its metallic alloys. In the present work microwave plasma chemical vapor deposition (MWPCVD) was used to deposit smooth nano-crystalline diamond (NCD) film on pure titanium substrate using Ar, CH4 and H2 gases at moderate deposition temperatures. Of particular interest in this study was the exceptional adhesion of approximately 2 μm-thick diamond film to the metal substrate as observed by indentation testing up to 150 kg load. The friction coefficient, which was measured with a cemented carbide ball of 10 mm diameter with 20 N load, was estimated to be around 0.04 in dry air. Morphology, surface roughness, diamond crystal orientation and quality were obtained by characterizing the sample with field emission electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy, respectively.

  • 277.
    Askari, Syed Jawid
    et al.
    University of Science and Technology Beijing, School of Materials Science and Engineering.
    Lv, Fanxiu
    University of Science and Technology Beijing, School of Materials Science and Engineering.
    Akhtar, Farid
    University of Science and Technology Beijing, School of Materials Science and Engineering.
    Wang, Fengying
    University of Science and Technology Beijing, School of Materials Science and Engineering.
    He, Qi
    University of Science and Technology Beijing, School of Materials Science and Engineering.
    Zhou, Zuyuan
    University of Science and Technology Beijing, School of Materials Science and Engineering.
    Nanodiamond films deposited at moderate temperature on pure titanium substrate pretreated by ultrasonic scratching in diamond powder suspension2006Inngår i: Journal of University of Science and Technology Beijing, Mineral, Metallurgy, Material, Vol. 13, nr 6, s. 542-545Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Nanocrystalline diamond (NCD) film deposition on pure titanium and Ti alloys is extraordinarily difficult because of the high diffusion coefficient of carbon in Ti, the large mismatch in their thermal expansion coefficients, the complex nature of the interlayer formed during diamond deposition, and the difficulty to achieve very high nucleation density. In this investigation, NCD films were successfully deposited on pure Ti substrate by using a novel substrate pretreatment of ultrasonic scratching in a diamond powder-ethanol suspension and by a two-step process at moderate temperature. It was shown that by scratching with a 30-μm diamond suspension for 1 h, followed by a 10-h diamond deposition, a continuous NCD film was obtained with an average grain size of about 200 nm. Detailed experimental results on the preparation, characterization, and successful deposition of the NCD films on Ti were discussed

  • 278.
    Asnafi, Nader
    Luleå tekniska universitet.
    Aluminiumplåts formbarhet1990Licentiatavhandling, monografi (Annet vitenskapelig)
    Fulltekst (pdf)
    fulltext
  • 279.
    Asnafi, Nader
    Luleå tekniska universitet.
    Formbarhet under dragpressning, sträckpressning och bockning samt egenskaper efter formning av aluminiumplåt1988Rapport (Annet vitenskapelig)
  • 280.
    Asnafi, Nader
    Luleå tekniska universitet.
    Mechanics of sheet metal forming: strength, stiffness, dent resistance, and springback of double-curved autobody panels : fracture and wringling in stretch and shrink flanging by fluid forming1997Doktoravhandling, med artikler (Annet vitenskapelig)
    Fulltekst (pdf)
    FULLTEXT01
  • 281.
    Asnafi, Nader
    Luleå tekniska universitet.
    Återfjädring vid bockning längs krökta linjer1987Rapport (Annet vitenskapelig)
  • 282.
    Asnafi, Nader
    et al.
    Luleå tekniska universitet.
    Magnusson, Claes
    On formability of sheet aluminium1990Inngår i: Sheet metals in forming processes: 16th biennial congress IDDRG, open sessions, Borlänge, June 11 - 13, 1990 ; congress proceedings, reports, posters, Borlänge: Materials Center, HTM , 1990, s. 81-91Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Sheet metal behaves differently for different stress and strain ratios. For instance, the material tolerates less deformation at plane strain compared with uniaxial tension. Hill's yield criteria can be used to describe sheet material's deformation behavior. Hill's original yield criterion for plane stress, assuming that there is no planar anisotropy is given. The equation shows better agreement with experimental results when r > 1 (steel 4010), while there is no agreement with experimental results when r < 1 (Al-5052). Hill proposed, therefore, a new yield criterion, which is given. The value of a can be determined by comparing the experimental work-hardening characteristics for uniaxial tension and balanced biaxial tension; 1 < = a < = 2. For soft Al, having an average r-value of 0.723, the value of a has been found to be 1.8. The parameter a has its greatest effect at balanced biaxial tension. Graphs

  • 283.
    Asp, Leif
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Finite element delamination study of a notched composite plate under flexural loads2010Inngår i: Journal of Materials Science and Engineering. A, ISSN 1934-8959, Vol. 4, nr 8, s. 66-73Artikkel i tidsskrift (Fagfellevurdert)
  • 284.
    Asp, Leif
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Fracture and yield predictions for epoxies in composite-like stress state1994Licentiatavhandling, med artikler (Annet vitenskapelig)
  • 285. Asp, Leif
    Multifunctional composite materials for energy storage in structural load paths2013Inngår i: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 42, nr 4, s. 144-149Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper presents an overview of the research performed to date by a Swedish interdisciplinaryteam of scientists striving to develop multifunctional composite materials for storage of electric energy in mechanical load paths. To realise structural batteries from polymer composites, research pursued on carbon fibres for use as negative electrode in the battery as well as on polymer electrolytes for use as polymer matrix in the composite is reported. The work on carbon fibres comprises characterisation of the electrochemical capacity of commercial carbon fibre grades and how this is affected by mechanical load. Co-polymers are studied for their multifunctional performance with respect to lithium ion conductivity and stiffness. Also, rational processing of these polymer electrolytes and the effect of processing on their properties are addressed

    Fulltekst (pdf)
    FULLTEXT01
  • 286.
    Asp, Leif
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Structural battery materials2012Inngår i: Proceedings of the 15th European Conference on Composite Materials / [ed] Marino Quaresimin; Laszlo Kollar; Leif Asp, Venice, 2012Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Since four years Swerea SICOMP has been leading a team of Swedish researchers developing structural battery materials from polymer composites. The research performed in the Swedish project KOMBATT (Lightweight structural energy storage materials) is funded by the Swedish foundation for strategic research (SSF). The research addresses two technical challenges in particular. Firstly, solid polymer electrolytes that efficiently transfer loads in the composite and simultaneously transports lithium ions, while being electrically insulating, must be developed. Secondly, the ability of the reinforcement, i.e. The carbon fibres, to intercalate lithium ions as part of the chemical redox reactions, while maintaining its mechanical properties must be assured. This paper is the first in a series of papers at this conference from the KOMBATT project team and presents background and overview of the project.

  • 287.
    Asp, Leif
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Transverse failure initiation in polymer composites1995Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Transverse failure is one of the most important failure modes in polymer composites. The phenomenon often causes the first deviations from nonlinear laminate behavior. Also, in pressure vessels and pipes, fluid leakage through a path of transverse cracks is often the limiting design criterion. In the present work, experimental and theoretical studies focused on the micromechanical level have been carried out. The objective was to investigate transverse failure initiation in the matrix. The other major mechanism of failure initiation, fiber/matrix debonding, was not considered. The triaxial nature of the matrix stress state in glass fiber/epoxy was confirmed by finite element analysis. Experimental results for glassy epoxies subjected to composite-like stress states demonstrated large reductions in strain to failure as compared with uniaxial loading. The triaxial stress state is therefore by itself a sufficient explanation for the low transverse strain to failure in polymer composites. Plastic yielding in the matrix was demonstrated not to be the cause of failure initiation. Instead cavity induced cracking was suggested as a failure mechanism. A criterion was proposed based on a critical value for the dilatational energy density. Comparison with experimental results for epoxies subjected to a variety of multiaxial load-cases supported the criterion. Additional support was obtained from comparison with experimental results in the literature for transverse failure of glass fiber/epoxy at different fiber contents. Although the epoxy matrix was different from those in the present study, general trends in data were supported by predictions based on the criterion and finite element analysis. Thermal residual stresses were found to be important for high fiber contents. Based on the criterion, a conservative estimate of composite strain to failure was obtained. This is reasonable since the criterion predicts initiation, not final failure. Based on the model, effects from changes in constituent properties were examined in a parametric finite element analysis. Fiber modulus was found to strongly influence transverse failure. Introduction of a third phase interphase between fiber and matrix was also investigated. Beneficial results on transverse failure strain caused by matrix initiation was observed for thin rubbery interphases.

    Fulltekst (pdf)
    FULLTEXT01
  • 288.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Berglund, Lars A.
    Luleå tekniska universitet.
    A biaxial thermomechanical disk test for glassy polymers1997Inngår i: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 37, nr 1, s. 96-101Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Failure criteria for polymers need to include effects from the stress state. For this reason, biaxial test results are of interest. However, biaxial test methods usually require expensive equipment. In the test method presented here, a disk of epoxy is bonded between a steel ring and a steel disk. The temperature is then lowered until fracture is observed. Experiments were performed on three different glassy epoxy polymers. The biaxial stress state was analyzed by finite element analysis and by an approximate analytical model. Experimental observations support the ability of the method to provide material property data. An approximate analytical model was found sufficiently accurate for stress analysis and determination of the stress state at failure

  • 289.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Gudmundsson, Peter
    Department of Solid Mechanics, Royal Institute of Technology.
    Effects of a composite-like stress state on the fracture of epoxies1995Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 53, nr 1, s. 27-37Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The strain to failure of a transversely loaded composite is much lower than for the pure matrix in uniaxial tension. Several studies of composites suggest the triaxial matrix stress state as one of the explanations. In order to investigate this experimentally, a triaxial tensile test previously used for rubbers (the poker-chip test) was successfully applied to four epoxies in the glassy state. The chosen specimen geometry mimicked the most severe stress state in the matrix as determined by finite element analysis of a transversely loaded glass-fiber/epoxy composite. The poker-chip strains to failure in the primary loading direction were 0.5-0.8%, whereas uniaxial strains to failure were 1.8-7%. The triaxial stress state in composite matrices may therefore by itself be a sufficient explanation for low values of transverse composite strains to failure

  • 290.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Criterion for crack initiation in glassy polymers subjected to a composite-like stress state1996Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 11, s. 1291-1301Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Three epoxy systems of interest as composite matrix materials are examined for their yielding and failure behavior under uniaxial, biaxial and triaxial stress states. Yield criteria applicable to glassy polymers, i.e. accounting for the hydrostatic stress effect on the deviatoric stress to yielding, are assessed. It is found that under stress states resembling those in matrix constrained between fibers, e.g. equibiaxial and equitriaxial tension, yielding is suppressed while brittle failure, presumably caused by crack growth from cavitation, occurs. A criterion for this mode of failure is proposed as the critical dilatational strain energy density. Experimental data are found to support this criterion.

  • 291.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Effects of fiber and interphase on matrix-initiated transverse failure in polymer composites1996Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 45, nr 6, s. 657-665Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Failure initiation in polymer-matrix composites loaded transverse to the fibers is investigated by a numerical parametric study where the effects of constituent properties, interphase properties and thickness are examined. Failure initiation in the matrix only is studied, interfacial debonding not being considered. Two modes of failure - yielding and cavitation-induced brittle failure - are examined. A criterion for the cavitation-induced brittle failure has been proposed previously and failure prediction based on this criterion was found to agree with experimental data for a glass-fiber-reinforced epoxy. The present study shows that the elastic modulus of fibers has a large effect on the stress and strain to failure initiation. A rubbery interphase material is found in most cases to have a beneficial effect. The site at which failure initiates and the governing mode of failure initiation are also affected by the fiber modulus and the interphase properties

  • 292.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Prediction of matrix-initiated transverse failure in polymer composites1996Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 56, nr 9, s. 1089-1097Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A study has been conducted of failure in unidirectionally-reinforced fiber composites loaded in tension normal to the fibers. The case considered is when this failure is governed by failure of the matrix rather than fiber/matrix debonding. Both yielding and cavitation-induced brittle failure of the matrix are considered. The latter mode of failure was suggested previously as the likely mode to occur in epoxies under stress states that are purely or nearly hydrostatic tension. Three fiber packing arrangements (square, hexagonal and square-diagonal) with different fiber volume fractions were studied numerically by a finite element method to determine the local stress states. It is found that cavitation-induced brittle failure occurs much before yielding in all cases. Experimental data taken from the literature support this finding.

  • 293.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Berglund, Lars
    Luleå tekniska universitet.
    Gudmundson, P.
    Mechanical behavior of epoxy resins in uniaxial and triaxial loading1993Inngår i: High temperature ceramic matrix composites: 6th EuropeanConference on Composite Materials, 20 - 24 September 1993, Bordeaux; HT-CMC / [ed] M. Neitzel; J.C. Lambert, Cambridge: Woodhead Publishing Materials , 1993, s. 323-328Konferansepaper (Fagfellevurdert)
  • 294.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Brandt, F.
    Effects of pores and voids on the interlaminar delamination toughness of a carbon/epoxy composite1997Inngår i: Proceedings / Eleventh International Conference on Composite Materials, Gold Coast, Queensland, Australia, 14th - 18th July 1997: ICCM-11 / [ed] Murray L Scott, Melbourne: Woodhead Publishing Materials , 1997, s. 322-331Konferansepaper (Fagfellevurdert)
  • 295.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Edgren, Fredrik
    Sjögren, A.
    Effects of stitch pattern on the mechanical properties of non-crimp fabric composites2004Inngår i: From nano-scale interactions to engineering structures: ECCM 11, 11th European Conference on Composite Materials ; May 31 - June 3, 2004, Rhodes, Greece / [ed] Costas Galiotis, Rhodos: European Society for Composite Materials , 2004Konferansepaper (Fagfellevurdert)
  • 296.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Juntikka, R.
    Swerea SICOMP AB, Mölndal.
    High velocity impact on NCF reinforced composites2006Inngår i: From the science of composites to engineering applications: the dawning future of composites: ECCM 12, 12th European Conference on Composite Materials, Biarritz, 29th August - 1st September 2006 ; [conference proceedings] / [ed] Jacques Lamon, Biarritz, 2006Konferansepaper (Fagfellevurdert)
  • 297.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Marklund, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Varna, Janis
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Modelling stiffness and strength of non-crimp fabric composites: semi-laminar analysis2011Inngår i: Non-crimp fabrics composites: manufacturing, properties and applications, Cambridge: Woodhead Publishing Materials , 2011, s. 402-438Kapittel i bok, del av antologi (Fagfellevurdert)
  • 298.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Marklund, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Varna, Janis
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Olsson, Robin
    Swerea SICOMP AB, Mölndal.
    Multiscale modelling of non-crimp fabric composites2012Inngår i: Proceedings of the ASME International Mechanical Engineering Congress and Exposition--2012: presented at ASME 2012 International Mechanical Engineering Congress and Exposition, November 9-15, 2012 Houston, Texas USA, New York: American Society of Mechanical Engineers , 2012, Vol. 3, s. 581-590Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Damage initiation and evolution in NCF composites leading to final failure includes a multitude of mechanisms and phenomena on several length scales. From an engineering point-of-view a computational scheme where all mechanisms would be explicitly addressed is too complex and time consuming. Hence, methods for macroscopic performance prediction of NCF composites, with limited input regarding micro- And mesoscale details, are requested. In this paper, multi-scale modelling approaches for in-plane transverse strength of NCF composites are outlined and discussed. In addition a simplistic method to predict transverse tensile and compressive strength for textile composites featuring low or no fibre waviness is presented

  • 299.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Nilsson, Karl-Fredrik
    SICOMP AB, Swedish Institute of Composites.
    Delamination criticality in slender compression-loaded composite panels2002Inngår i: Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795, Vol. 221-222, s. 3-16Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, a damage tolerance model based on the assumption of delamination criticality in compression loaded slender composite panels is outlined. In particular, the verification of the model by comparison between numerical predictions and experimental results is reviewed. Growth of shallow delaminations in slender panels is shown to be promoted by the global buckling of the panel. Consequently, care must be taken if structures with delaminations are to be allowed to buckle. In the paper, application of the model for aircraft design is briefly discussed. The overall predicted panel behaviour agrees with observations for test coupons. However, very small geometrical changes are shown to have tremendous effects on the predicted behaviour. Consequently, in structural design one must consider the sensitivity of geometrical conditions on the predicted behaviour. Therefore, reduction of the structural item into a design element is suggested. To generate conservative designs the suggested design element is to represent the worst case

  • 300.
    Asp, Leif
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Sjögren, B.A.
    Berglund, Lars A.
    Prediction of failure initiation in polypropylene with glass beads1997Inngår i: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 18, nr 1, s. 9-15Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The effect of glass bead content and residual stresses on failure initiation in isotactic polypropylene composites has been investigated by finite element analysis for the cases of interfacial debonding, plastic yielding, and cavitation. Residual thermal stresses are demonstrated to have a large effect on global failure initiation stress. Yielding and cavitation occur at higher global stresses than debonding. Modeling results, as well as previous experimental data, support debonding as the initial failure mechanism

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