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  • 251.
    Farge, L.
    et al.
    Nancy Université-INPL.
    Varna, Janis
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Ayadi, Z.
    Nancy Université-INPL.
    Damage characterization of a cross-ply carbon fiber/epoxy laminate by an optical measurement of the displacement field2010Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 70, nr 1, s. 94-101Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Using Electronic Speckle Pattern Interferometry (ESPI), full-field displacement measurement was performed on the edge of a cracked cross-ply graphite/epoxy laminate subjected to a tensile loading. The displacement jumps corresponding to cracks are clearly visible and can be used to determine the Crack Opening Displacement (COD) values along the cracks. The main objective of this study is to determine if the application of successive loads of increasing magnitude may have modified the existing cracks and thereby changed the COD dependence on the applied stress. Moreover, we have tested the applicability of the assumed linear elastic COD behavior in the presence of very high stress concentration at the crack tips. The profile of the opening along the crack was also studied.

  • 252. Farge, L.
    et al.
    Varna, Janis
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Ayadi, Z.
    Study of the displacement field on the edge of cracked laminated composites2008Inngår i: 13th European Conference on Composite Materials: 2-5 June 2008, Stockholm, Sweden, 2008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Using Electronic Speckle Pattern Interferometry (ESPI), the full-field displacement measurement was obtained on the edge of a cracked laminate subjected to a tensile loading. The displacement jumps corresponding to cracks are clearly visible and can be used to determine the Crack Opening Displacement (COD) values along the cracks. The main objective of this study is to determine if the application of high load may have modified the existing cracks and consequently changed the COD dependence on the applied stress. The profile of the opening was also studied.

  • 253.
    Farge, Laurent
    et al.
    Laboratoire d’Energètique et de Mécanique Théorique et Appliquee, Vandoeuvre Lés Nancy Cedex.
    Varna, Janis
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Ayadi, Zoubir
    Institut Jean Lamour, Nancy Cedex.
    Use of full-field measurements to evaluate analytical models for laminates with intralaminar cracks2012Inngår i: Applied Composite Materials, ISSN 0929-189X, E-ISSN 1573-4897, Vol. 46, nr 21, s. 2739-2752Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Full-field displacement measurements between intralaminar cracks in cross-ply laminates were performed to evaluate the accuracy of shear lag models and Hashin’s model. The dependence of the average crack opening displacement on the crack density was measured and compared with model predictions. It was found that Hashin’s model overestimates the average COD by at least 25%. The value of the shear lag parameter was back-calculated by fitting. With the same goal the strain in the middle between two cracks was measured rendering shear lag parameter which was only 1% different. The found value does not agree with any of the used shear lag models. Measurement shows nonzero intralaminar shear zones in both layers covering a part of the ply thickness.

  • 254. Fernandes, S.C.M.
    et al.
    Freire, C.S.R.
    Silvestre, A.J.D.
    Pascoal Neto, C.
    Gandini, A.
    Berglund, Lars A.
    Salmen, Lennart
    RISE., Innventia.
    Transparent chitosan films reinforced with a high content of nanofibrillated cellulose2010Inngår i: Carbohydrate Polymers, ISSN 0144-8617, E-ISSN 1879-1344, Vol. 81, nr 2, s. 394-401Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper reports the preparation and characterization of nanocomposite films based on different chitosan matrices and nanofibrillated cellulose (NFC) for the purpose of improving strength properties. The nanocomposite films were prepared by a simple procedure of casting a water-based suspension of chitosan and NFC, and were characterized by several techniques: namely SEM, X-ray diffraction, visible spectrophotometry, TGA, tensile and dynamic-mechanical analysis. The films obtained were shown to be highly transparent (transmittance varying between 90 and 20% depending on the type of chitosan and NFC content), flexible, displayed better mechanical properties, with a maximum increment on the Young’s modulus of 78% and 150% for high molecular weight (HCH) and water-soluble high molecular weight (WSHCH) filled chitosans, respectively; and of 200% and 320% for low molecular weight (LCH) and water-soluble filled (WSLCH) chitosans, respectively. The filled films also showed increased thermal stability, with, for example, an increase in the initial degradation temperature (Tdi) from 227 °C in the unfilled LCH film up to 271 °C in filled LCHNFC50% nanocomposite films, and a maximum degradation temperature (Td1) raising from 304 °C to 313 °C for the same materials.

  • 255.
    Fernberg, Patrik
    Luleå tekniska universitet.
    Fiber bridging concepts applied to short fiber composites2000Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Polymer composite materials are in wide-spread use in the transportation industry. In aerospace industry the use these materials are established while in automotive industry the interest is increasing. The attention of automotive industry is to a great deal focused on various kinds of molded composites such as glass mat reinforced thermoplastics (GMT) and sheet molding compound (SMC). Their interest is to a large extent driven by the possibility to manufacture components of complex geometry in a cost- efficient process with these materials. An increasing number of car and truck manufacturers are using SMC for external panels such as trunk covers, hoods, roofs and spoilers. A property of obvious importance for an external car- or truck-panel is its capacity to withstand impact. In this context, improved understanding of crack growth and toughening mechanisms of the material is of great interest. A major part of the work presented in this thesis is driven by an interest to increase the understanding of how material composition and microstructure of short fiber composites influence their overall fracture behaviour. In materials such as metals and unreinforced polymers, linear elastic fracture mechanics (LEFM) is widely used, often with great success, both in design and in development of new materials. Unfortunately, problems arise when LEFM is applied to short fiber composites. This is due to the large process zone that develops ahead of a crack in these materials. The fundamental assumption of LEFM, that the damage zone at the tip of the crack is small compared to crack length, is often violated in experiments. The presented thesis considers a different approach, in which the damage ahead of a crack tip is described by a bridging-law. By considering the bridging-law as the major failure property of the material, a coupling between mechanisms acting on a microscale and the macroscopic failure behaviour can be established. No such information can be obtained using a LEFM approach where the material behaviour is described in terms of a single value, the fracture toughness. Bridging-laws for three different short fiber composites are experimentally determined and presented in the first paper of the thesis. A matter of key importance for future work in this field is that there are methods available for experimental verification of the suggested fiber bridging approach. Optical strain field measurement methods are therefore very useful. The thesis contains a pilot study to evaluate the use of two recently developed optical methods , Stereoscopic Digital Speckle Photography (Stereo-DSP) and combined DSP-DSPI (Digital Speckle Pattern Interferometry), for measurements of fracture behaviour of notched short fiber composites. We found Stereo-DSP to be a versatile technique that can be used when knowledge of overall displacement fields is required. The combined technique can with advantage be used when detailed information about large deformation at small areas is of interest, e.g. the complex fiber bridging interaction at the crack tip of a short fiber composite. The last paper in the thesis presents a study where the influence of fiber surface treatment on transverse cracking in cross-ply laminates was investigated. In the case of tubes and pressure vessels, the formation of transverse cracks ultimately leads to leakage since cracks connect and form a path through the wall. In the presented study, our ambition was to investigate the influence of film former polymer on transverse cracking properties of cross-ply laminates.Both onset of transverse cracking and tendency for multiple crack development were strongly affected by the different film formers. The strong film former effect was proposed to be due to a combination of improved interfacial adhesion and the plasticizing effect from the film former on the interphase region. The thesis is composed by the following papers: Patrik Fernberg, Lars Berglund, Bridging law and toughness characterisation of CSM and SMC composite, to be submitted. Angelica Andersson, Patrik Fernberg, Mikael Sjödahl, Optical methods to study fracture of notched glass mat composites. Proceedings of the International Conference on trends in Optical Nondestructive Testing, Lugano, Switzerland, May 3-6, 2000 (in press). Patrik Fernberg, Lars Berglund, Effects of glass fiber size composition (film former type) on transverse cracking in cross-ply laminates. Accepted for publication in Composites, Part A.

  • 256.
    Fernberg, Patrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Fibre reinforced polyimide composites and structures manufactured with resin transfer moulding: Overview of procedures and properties2016Inngår i: ECCM 2016: Proceeding of the 17th European Conference on Composite Materials, European Conference on Composite Materials , 2016Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In this paper, the major outcomes from a recently completed research program with ambition to develop polyimide carbon fibre composites with temperature ability above 360°C are reported. Data from characterisation of the processing properties such as viscosity and cure behaviour are presented alongside with data on the mechanical properties at room temperature of quasi-isotropic composites based on the developed resin and 8-harness satin weave carbon fibre fabrics. The paper also contains a demonstration of the use the material system in a demonstrator component.

  • 257.
    Fernberg, Patrik
    Luleå tekniska universitet.
    Toughness of short fiber composites: an approach based on crack-bridging2002Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The presented work considers how to properly characterize fracture properties of short fiber composites (SFC). Associated with fracture of SFC is the creation of a comparably widespread fracture process zone. This zone develops since a number of inelastic failure mechanisms (e.g. debonding, microcracking, fiber failure and fiber pull-out) take place in the vicinity of an advancing crack. In the present approach, a bridging law (or cohesive zone law) approach is adopted in order to characterize the fracture toughness of the material. Conventional fracture toughness measures, such as KIC were in most cases found not to be applicable. This was because fundamental small-scale yielding geometry requirements could not be fulfilled in experiments. The bridging law approach captures previously mentioned mechanisms in terms of a closure stress (bridging stress). This stress acts between two fictitious crack planes. The relation between crack opening displacement and bridging stress is governed by the bridging law. Parts of the presented work consider determination of bridging laws from experiments (Paper I and Paper III). Different experimental configurations, double cantilever beam (DCB) specimens loaded with pure moments and double edge notched tension (DENT) specimens, were used in the two studies. A main conclusion from Paper I is that the large differences in fracture characteristics between two sheet molding compound (SMC) composites could be explained on the basis of bridging laws and their influence on fracture energy. Similar observations were made in Paper III. In Paper III, it also was evident that the intrinsic non-linearity of bulk SMC material has to be considered separately in the data reduction of experimental results, in order to capture the bridging law. Bilinear approximations of decreasing bridging laws were obtained as a result from the study. A closer investigation on the mechanical behavior of SMC with varied composition was performed in Paper II. Various mechanical tests, including tension, compression, in situ studies, DCB and stiffness degradation measurements through quasi-static cyclic loading-unloading experiments, were employed. The purpose was to characterize and understand observed differences between conventional and toughened SMC with low density additives. The applicability of the proposed bridging law approach is confirmed by the work presented in Paper IV and Paper V. In these papers, the previously measured (Paper I and Paper III) bridging relations are used as a constitutive property in predictions of structural behavior of specimens with varied geometry. Paper IV considers that bridging law parameters can be used to predict and explain the change in notch-sensitivity observed on SMC DENT-specimens with varied geometry. A comparably simple analytical route (neglecting non-linear bulk behavior and shape of bridging law) is employed with satisfactory results. In Paper V, the use of the finite element method (FEM) in conjunction with measured bilinear bridging laws, allows reconstruction of experimentally measured compact tension (CT) specimen load vs. displacement curves with good accuracy. Three different CT specimen geometries are considered. Modeling and experimental results from Paper V also shows that compression failure often is of equal importance as tensile, in real structures and loading conditions.

  • 258.
    Fernberg, Patrik
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A
    Luleå tekniska universitet.
    Bridging law and toughness characterisation of CSM and SMC composites2001Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 61, nr 16, s. 2445-2454Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This work presents an experimental investigation of the fracture properties of three different short-fiber-reinforced composites [one chopped strand mat (CSM) and two sheet molding compound (SMC) materials]. Fracture tests are performed on double-cantilever beam (DCB) specimens loaded with pure bending moments. In this experimental configuration, the bridging law for the material can be derived directly from measurements. No significant dependency on specimen height was observed in our results. The bridging laws determined can, therefore, be considered as material properties. The coupling between microstructure and fracture behaviour is discussed through the measured bridging laws. The beneficial effect (in terms of fracture energy) of increasing tendency for pull-out is confirmed for one SMC, referred to as Flex-SMC, which shows remarkably high fracture energy, Jc=56.0 kJ/m2, compared to a standard SMC, termed Std-SMC, Jc=25.9 kJ/m2. This increasing tendency for pull-out is observed to shift the bridging law towards larger crack openings. On the basis of our observations we find the concept of characterising the failure behaviour in terms of bridging laws attractive since it can be used as a tool for the tailoring of the microstructure towards desired fracture behaviour.

  • 259.
    Fernberg, Patrik
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Effects of glass fiber size composition (film-former type) on transverse cracking in cross-ply laminates2000Inngår i: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 31, nr 10, s. 1083-1090Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Transverse cracking is an important phenomenon in the context of fluid leakage in pipes and pressure vessels. Multiple transverse cracking in [0/90]S glass-fiber-reinforced vinylester and epoxy laminates with six different fiber surface treatments (size) is examined. Film-former composition is the variable since this component can be easily changed also in commercial size formulations. The influence of the film-former polymer on transverse cracking is significant in epoxy laminates and very strong in vinylester laminates. Both onset of transverse cracking and slope of crack density vs. strain are influenced. Remarkably low crack densities were observed in some vinylester laminates. Micromechanisms of cracking are interpreted. Correlation is established between transverse cracking behavior and interfacial shear strength measured by single fragmentation tests. The strong film-former effect is proposed to be due to a combination of improved interfacial adhesion and the plasticizing effect from the film-former on the interphase region.

  • 260.
    Fernberg, Patrik
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap. Swerea SICOMP AB.
    Gong, Guan
    Swerea SICOMP AB.
    Mannberg, Peter
    Swerea SICOMP AB.
    Processing and properties of new polyimide composites with high temperature ability2014Inngår i: 16th European Conference on Composite Materials, ECCM 2014: Seville, Spain, 22 - 26 June 2014, European Conference on Composite Materials, ECCM , 2014Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The communication present results from work on development and evaluation of new polymeric carbon fiber composites with extreme temperature performance: Tg up to 360°C is targeted. The anticipated use of such composites is found in aeroengine-applications. In the work we are exploring a new and tailored phenyl ethynyl terminated imide (PETI) formulation, specially developed for the program. The formulation utilizes crosslinkers of the Nexamide" type (from Nexam Chemical AB, Sweden). The resins are initially evaluated from a processing and property perspective. Both DSC-measurements and rheology characterization are utilized in the development. Suitable RTM-processing schemes are investigated from a viscosity point of view. The schemes are used in the composite sample manufacturing. Besides a processing perspective the study also present the first results on physical behavior of the polymers and their composites.

  • 261.
    Fernberg, Patrik
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Gong, Guan
    Swerea SICOMP AB, Piteå.
    Mannberg, Peter
    Swerea SICOMP AB, Piteå.
    Tsampas, Spyros
    Swerea SICOMP AB, Mölndal.
    Development of novel high Tg polyimide-based composites: Part I: RTM processing properties2018Inngår i: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 52, nr 2, s. 253-260Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this study, an assessment of the composite processing-related properties of a newly developed 6-FDA-based phenylethynyl-terminated polyimide (available under the tradename NEXIMID®MHT-R) is presented. Processing schemes, used for preparing high quality carbon fibre-reinforced composites by the use of conventional resin transfer moulding are developed and presented. The influences of manufacturing parameters on glass transition temperature of the composites are presented. The results confirm that composites with exceptionally high Tg, in the range between 350 and 460℃ can be achieved. A manufacturing scheme that yields in composites with Tg of 370℃ is presented and proposed as a good candidate to serve as baseline for further studies.

  • 262.
    Fernberg, Patrik
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Jekabsons, Normunds
    Luleå tekniska universitet.
    Determination of bridging laws for SMC materials from DENT tests2003Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 63, nr 15Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    bridging law (or cohesive zone law) approach is employed to evaluate the fracture of double edge notched tensile (DENT) specimen from two SMC materials (Std-SMC and Flex-SMC) with considerable difference in fracture characteristics. Linear- and non-linear FEM was used to separate volumetric body contributions and true crack opening from measured displacements (measured with extensometer) over the cracked region. We found that extrinsic non-linear material response gave a significant contribution to measured displacements. The paper also considers the influence of specimen dimensions on the characteristics of fracture. This is of great importance since stable crack growth is required in order to allow a complete determination of the bridging law in a single experiment. By applying corrections for volumetric displacements in the data reduction scheme we were able to estimate the bridging laws of the two SMCs respectively. They were found to be of decreasing nature and two distinct regions were identified. For small δ, a steeply decreasing part where debonding and fiber fracture are major micro-scale failure mechanism, is identified. At larger δ, a transition to a less steep bridging law response is observed. Pullout friction governs the bridging law at that stage

  • 263.
    Fernberg, Patrik
    et al.
    Swerea SICOMP AB.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Damage detection in carbon fibre cross-ply laminates by aid of carbon nanotube doped resin2008Inngår i: 13th European Conference on Composite Materials: 2-5 June 2008, Stockholm, Sweden, 2008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The possibility to use resistance variations of carbon nanotube (CNT) doped carbon fibre reinforced plastics (CFRP) to characterise the level of microstructural damage in the form of transverse cracking is experimentally investigated in the current work. The resistance of CNT-doped and unmodified CFRP was measured after they had been subjected to stepwise increased tensile loading. Both increase and decrease resistance was observed which made it difficult to make any firm statement that the changes we observed were due to damage. The conductivity along carbon fibres is dominating electrical conductivity of the studied cross-ply laminate. Any disturbance of conductive CNT based nanocomposite matrix by transverse cracks in 90-layer therefore caused too small changes of resistance to be measured with current experimental setup.

  • 264.
    Fernberg, Patrik
    et al.
    Swerea SICOMP AB.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Edgren, Fredrik
    Volvo Aero Corporation.
    Carbon nanotube treated carbon fibre preforms for improved properties of aeronautical grade composites2012Inngår i: Proceedings of the 15th European Conference on Composite Materials / [ed] Marino Quaresimin; Laszlo Kollar; Leif Asp, Padova: University of Padova , 2012Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The current paper reports work on development of enhanced carbon fibre composites manufactured by resin transfer moulding (RTM). In particular, the possibility to obtain a toughened composite through deposition of carbon nanotubes (CNT) on the fibres is investigated. The hypothesis is that deposition of CNT on the fibre surface has two major advantages: a) filtering problems during manufacturing are eliminated and b) CNT can contribute to a local toughening in the vicinity of fibres, where stress concentrations often are high. Unidirectional and multi-axial composites, with and without CNT-modification, were manufactured using conventional RTM-technique and HexFlow®RTM6 epoxy resin. The composites were characterized by means of mechanical testing (interlaminar shear strength, ILSS) and microscopy. Significant improvements by 13 to37 % in ILSS values were achieved with the addition of CNT. Stiffness degradation tests were also performed. The results from these tests were not completely conclusive. Some treatments promoted stiffness degradation whereas others had no apparent effect.

  • 265.
    Fernberg, Patrik
    et al.
    Swerea SICOMP AB.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Nilsson, Greger
    Swerea SICOMP AB.
    Strain and damage sensing during loading of carbon nanotube doped composites2009Inngår i: ICCM 17, Edinburgh: 17th International Conference on Composite Materials ; 27 Jul 2009 - 31 Jul 2009, Edinburgh International Convention Centre, Edinburgh, UK, London: IOM Communications , 2009Konferansepaper (Fagfellevurdert)
    Abstract [en]

    This work explores the possibility to use resistance changes in a carbon nanotube (CNT) doped composites to monitor deformations and damage. Most other studies in the field are done on simplified uniaxial loading whereas current work also evaluates the possibilities to monitor strain in more complex loading case, such as bending.

  • 266.
    Fernberg, Patrik
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Nilsson, Greger
    SICOMP AB, Swedish Institute of Composites.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Piezoresistive performance of long fibre composites with carbon nanotube doped matrix2007Inngår i: Proceedings of COMP07: 6th International Symposium on Advanced Composites, 16-18 May, 2007, Corfu, Greece, 2007Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The electrical and mechanical properties of carbon nanotube doped epoxy resin and composites based on this matrix were studied. The investigation was carried out on neat nanocomposites and on structural composites i.e. when the nanocomposite is used as matrix in composite materials reinforced with long continuous fibers. Tensile tests showed that carbon nanotube (CNT) doped epoxy exhibited a clear piezoresistive behavior. It was however also shown that geometrical changes of the specimen also contribute significantly to resistance changes during tensile loading. Particular effort was made to establish the relations between transverse cracking in glass fibre cross-ply laminates with nanotube doped matrix and resistance changes. It was shown that resistance changes during tensile loading of composites containing CNT doped matrix gives highly relevant information about the damage state of the material. In an unloaded state the resistance change is proportional to the relative change of stiffness. This work demonstrates that there are three different mechanisms which contribute to resistance changes of a composite specimen subjected to tensile strain. These three mechanisms are: a) geometrical changes of the specimen b) piezoresistive material response and c) accumulation of micro-damage

  • 267.
    Fernberg, Patrik
    et al.
    Swerea SICOMP AB.
    Nilsson, Greger
    Swerea SICOMP AB.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Piezoresistive performance of long-fiber composites with carbon nanotube doped matrix2009Inngår i: Journal of Intelligent Materials Systems and Structures, ISSN 1045-389X, E-ISSN 1530-8138, Vol. 20, nr 9, s. 1017-1023Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The electrical and mechanical properties of carbon nanotube (CNT) doped epoxy resin and composites based on this matrix were studied. The investigation was carried out on neat nanocomposites and on structural composites i.e., when the nanocomposite is used as matrix in composite materials reinforced with long continuous fibers. Tensile tests showed that CNT doped epoxy exhibited clear piezoresistive behavior. It was, however, also shown that geometrical changes of the specimen also contribute significantly to resistance changes during tensile loading. Particular effort was made to establish the relations between transverse cracking in glass fiber cross-ply laminates with nanotube doped matrix and changes of electrical resistance. It was shown that changes of electrical resistance during tensile loading of composites containing CNT doped matrix gives highly relevant information about the damage state of the material. In an unloaded state the resistance change is proportional to the relative change of stiffness. This work demonstrates that there are three different mechanisms, which contribute to changes of electrical resistance of a composite specimen subjected to tensile strain. These three mechanisms are: (a) geometrical changes of the specimen (b) piezoresistive material response, and (c) accumulation of micro-damage.

  • 268.
    Fernberg, S. Patrik
    et al.
    Luleå tekniska universitet, Swerea SICOMP AB, Swerea SICOMP AB, Box 271, 941 26, Piteå.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Tsampas, Spyros
    Swerea SICOMP AB.
    Mannberg, Peter
    Swerea SICOMP AB.
    Influence of post-cure on carbon fibre polyimide composites with glass transition temperatures above 400ºC2015Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The current communication present results from work on polymeric composites with extreme temperature performance. We are studying carbon fibre composites based on a new phenyl ethynyl terminated polyimide formulation NEXIMID® MHT-R (Nexam Chemicals AB, Sweden) based on hexafluoroisopropylidene bisphthalic dianhydride (6-FDA), 4-(Phenylethynyl)Phthalic Anhydride (4-PEPA) and ethynyl bis-phthalic anhydride (EBPA). This study in particular investigates how post-cure conditions such as time, temperature and atmosphere influence Tg of the composites. In addition to this we also trace and analyse the consequences of post-cure on weight loss and occurrence of micro-cracks. We are considering three different post-curing temperatures: 400°C, 420°C and 440°C in the study. Two different atmospheres, air and inert by nitrogen, were also investigated. In summary the results reveal that remarkably high Tg, up to around 460°C, is achieved with only very limited weight loss. It was also observed that some, but limited amounts of, micro-cracks are developed within the laminates due to the inevitable high thermal stresses generated upon cooling from cure temperature.

  • 269. Fernández, A.
    et al.
    Sánchez, M. D.
    Ankerfors, Mikael
    RISE., STFI-Packforsk.
    Lagaron, J. M.
    Effects of ionizing radiation in ethylene-vinyl alcohol copolymers and in composites containing microfibrillated cellulose2008Inngår i: Journal of Applied Polymer Science, ISSN 0021-8995, E-ISSN 1097-4628, Vol. 109, nr 1, s. 126-134Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This study reports on the effect of gamma radiation on morphological, thermal, and water barrier properties of pure ethylene vinyl alcohol copolymers (EVOH29 and EVOH44) and its biocomposites with the nanofiller microfibrillated cellulose (2 wt%). Added microfibrillated cellulose (MFC) preserved the transparency of EVOH films but led to a decrease in water barrier properties. Gamma irradiation at low (30 kGy) and high doses (60 kGy) caused some irreversible changes in the phase morphology of EVOH29 and EVOH44 copolymers that could be associated to crosslinking and other chemical alterations. Additionally, the EVOH copolymers and the EVOH composites reduced the number of hygroscopic hydroxyl functionalities during the irradiation processing and novel carbonyl based chemistry was, in turn, detected. As a result of the above alterations, the water barrier properties of both neat materials and composites irradiated at low doses were notably enhanced, counteracting the detrimental effect on water barrier of adding MFC to the EVOH matrix. © 2008 Wiley Periodicals, Inc.

  • 270.
    Flansbjer, Mathias
    et al.
    RISE., SP – Sveriges Tekniska Forskningsinstitut.
    Honfi, Daniel
    RISE., SP – Sveriges Tekniska Forskningsinstitut.
    Vennetti, Daniel
    RISE., SP – Sveriges Tekniska Forskningsinstitut.
    Williams Portal, Natalie
    RISE., SP – Sveriges Tekniska Forskningsinstitut, CBI Betonginstitutet AB, Betong & Berg.
    Mueller, Urs
    RISE., SP – Sveriges Tekniska Forskningsinstitut, CBI Betonginstitutet AB, Betong & Berg.
    Własak, Lech
    Structural Concept of Novel RPC Sandwich Façade Elements with GFRP Connectors2016Inngår i: IABSE Congress Stockholm, 2016: Challenges in Design and Construction of an Innovative and Sustainable Built Environment. Report, IABSE c/o ETH Hönggerberg , 2016, s. 2172-2186Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The SESBE research project aims to develop novel smart sandwich façade elements with high insulating capabilities while providing a reduced thickness in conjunction with superior mechanical and durability properties. The present paper mainly focuses on the verification of the mechanical performance of the glass fibre reinforced polymer (GFRP) connectors in the façade element composed of reactive powder concrete (RPC) panels with foam concrete insulation between them. Because of the reduced thickness of the large façade elements, the performance of the connectors is critical for the entire structural concept. A description of structural performance and results based on experimental methods and finite element (FE) analysis are presented.

  • 271.
    Forsberg, Sven
    et al.
    Mittuniversitetet, Fakulteten för naturvetenskap, teknik och medier, Avdelningen för naturvetenskap.
    Kumar, Vinay
    Åbo Akademi University.
    Engström, Ann-Christine
    Mittuniversitetet, Fakulteten för naturvetenskap, teknik och medier, Avdelningen för naturvetenskap.
    Maristiina, Nurmi
    Åbo Akademi University.
    Dahlström, Christina
    Mittuniversitetet, Fakulteten för naturvetenskap, teknik och medier, Avdelningen för kemiteknik.
    Martti, Toivakka
    Åbo Akademi University.
    Effect of calendering and coating formulations on conductivity in paper-based electrodes2016Konferansepaper (Fagfellevurdert)
  • 272.
    Friel, R. J.
    et al.
    Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire, United Kingdom.
    Masurtschak, Simona
    Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire, United Kingdom.
    Harris, Russell A.
    Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire, United Kingdom.
    Enabling dissimilar fibre embedding and explicit fibre layout in ultrasonic consolidation2010Inngår i: Proceedings of the 21st International Conference on Adaptive Structures and Technologies 2010, University Park, PA: Curran Associates, Inc., 2010, s. 303-310Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Ultrasonic Consolidation (UC) is a manufacturing technique based on the ultrasonic metal welding of a sequence of metal foils which are bonded to one another in a layer by layer manner. It combines the ability of additive and subtractive manufacturing techniques to create complex three-dimensional shapes. Due to moderate applied pressures and the relatively low temperatures experienced by a sample during manufacture, UC operates as a solid-state process. UC could potentially enable the fabrication of smart structures via integration of sensor, actuator and reinforcement fibres within a single metal matrix. Previous issues with the optimal placement of fibres directly between foils during UC have been identified. Also, different types of integrated fibres require different UC process conditions and thus present complications when integrating them in combination. To truly exploit the full potential of UC for smart structure capabilities it is envisioned that a high volume fraction of dissimilar fibres are required to be integrated together within a single metal matrix structure. Research on a new method to consolidate fibres securely and more accurately during UC is presented. Channels created prior to UC within metal matrix composites are investigated as a method to aid the embedding of high volume fractions of different fibres in unison without damage. Initial research using a 200 W fibre laser as an enabling tool to create channels of specific geometry onto a previously UC processed surface is detailed. The research verifies that controlled channelling on a UC surface is possible and that channel geometry is dependent on: laser traverse speed, laser beam power, and shroud gas flow rate. © (2010) by the International Conference on Adaptive Structures and Technologies (ICAST).

  • 273.
    Fu, Qiliang
    et al.
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Fiber- och polymerteknologi. KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Centra, Wallenberg Wood Science Center.
    Yan, Min
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Jungstedt, Erik
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Fiber- och polymerteknologi. KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Centra, Wallenberg Wood Science Center.
    Yang, Xuan
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Fiber- och polymerteknologi. KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Centra, Wallenberg Wood Science Center.
    Li, Yuanyuan
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Fiber- och polymerteknologi. KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Centra, Wallenberg Wood Science Center.
    Berglund, Lars A.
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Fiber- och polymerteknologi. KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Centra, Wallenberg Wood Science Center.
    Transparent plywood as a load-bearing and luminescent biocomposite2018Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 164, s. 296-303Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Transparent wood (TW) structures in research studies were either thin and highly anisotropic or thick and isotropic but weak. Here, transparent plywood (TPW) laminates are investigated as load-bearing biocomposites with tunable mechanical and optical performances. Structure-property relationships are analyzed. The plies of TPW were laminated with controlled fiber directions and predetermined stacking sequence in order to control the directional dependence of modulus and strength, which would give improved properties in the weakest direction. Also, the angular dependent light scattering intensities were investigated and showed more uniform distribution. Luminescent TPW was prepared by incorporation of quantum dots (QDs) for potential lighting applications. TPW can be designed for large-scale use where multiaxial load-bearing performance is combined with new optical functionalities.

  • 274.
    Fu, Qilin
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion, Maskin- och processteknologi. Plasmatrix Materials AB.
    High dynamic stiffness nano-structured composites for vibration control: A Study of applications in joint interfaces and machining systems2015Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Vibration control requires high dynamic stiffness in mechanical structures for a reliable performance under extreme conditions. Dynamic stiffness composes the parameters of stiffness (K) and damping (η) that are usually in a trade-off relationship. This thesis study aims to break the trade-off relationship.

    After identifying the underlying mechanism of damping in composite materials and joint interfaces, this thesis studies the deposition technique and physical characteristics of nano-structured HDS (high dynamic stiffness) composite thick-layer coatings. The HDS composite were created by enlarging the internal grain boundary surface area through reduced grain size in nano scale (≤ 40 nm). The deposition process utilizes a PECVD (Plasma Enhanced Chemical Vapour Deposition) method combined with the HiPIMS (High Power Impulse Magnetron Sputtering) technology. The HDS composite exhibited significantly higher surface hardness and higher elastic modulus compared to Poly(methyl methacrylate) (PMMA), yet similar damping property. The HDS composites successfully realized vibration control of cutting tools while applied in their clamping interfaces.

    Compression preload at essential joint interfaces was found to play a major role in stability of cutting processes and a method was provided for characterizing joint interface properties directly on assembled structures. The detailed analysis of a build-up structure showed that the vibrational mode energy is shifted by varying the joint interface’s compression preload. In a build-up structure, the location shift of vibration mode’s strain energy affects the dynamic responses together with the stiffness and damping properties of joint interfaces.

    The thesis demonstrates that it is possible to achieve high stiffness and high damping simultaneously in materials and structures. Analysis of the vibrational strain energy distribution was found essential for the success of vibration control.

  • 275.
    Fu, Qilin
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion, Maskin- och processteknologi.
    Lorite, Gabriela Simone
    Rashid, Md. Masud-Ur
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion, Maskin- och processteknologi.
    Selkala, Tuula
    Uusitalo, Juha
    Toth, Geza
    Kordas, Krisztian
    Österlind, Tomas
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion, Maskin- och processteknologi.
    Nicolescu, Cornel Mihai
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion, Maskin- och processteknologi.
    Suppressing tool chatter with novel multi-layered nanostructures of carbon based composite coatings2015Inngår i: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 223, s. 292-298Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Multi-layered nanostructured Cu and Cu-CNx composites synthesized by plasma-enhanced chemical vapour deposition were applied in the clamping area of a milling tool to suppress regenerative tool chatter. Scanning electron microscopy analysis showed a multi-layered nanostructure with excellent conformality, i.e. coating is not only uniform on planar surfaces but also around corners of the substrate. Cu:CuCNx nanostructured multilayers with thicknesses of approximately 0.5:1.6 mu m were obtained. With a diameter of 20 mm, the milling tool performed slotting processes at an overhang length of 120 mm. Modal analysis showed that a coating, with a thickness of approximately 300 mu m, can add sufficient damping without losing stiffness of the tool, to increase the critical stability limit by 50% or 100% depending on cutting direction.

  • 276.
    Fu, Qilin
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion, Maskin- och processteknologi.
    Lundin, Daniel
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Nicolescu, Cornel Mihai
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion, Maskin- och processteknologi.
    Anti-vibration Engineering in Internal Turning Using a Carbon Nanocomposite Damping Coating Produced by PECVD Process2014Inngår i: Journal of materials engineering and performance (Print), ISSN 1059-9495, E-ISSN 1544-1024, Vol. 23, nr 2, s. 506-517Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Machining dynamic stability has been enhanced through a damping coating based on a novel carbon-based nanocomposite material. The coating was synthesized using a plasma enhanced chemical vapor deposition method, and deposited on to the round-shank boring bar used for internal turning and tested during machining. Comparisons between an uncoated and a coated boring bar were carried out at 0.25 mm and 0.5 mm depth of cut using a five times length to diameter ratio overhang, which are typical conditions known to generate detrimental mechanical vibrations. From sound pressure measurement it was found that the measured absolute sound level during process could be reduced by about 90% when using the tool coated with damping layer. Surface roughness measurements of the processed workpiece showed decreased Ra values from approximately 3-6 mu m to less than 2 mu m (and in 50% of the cases < 1 mu m) when comparing an uncoated standard tool with its coated counterpart. Moreover, it was found that the addition of an anti-vibration coating did not adversely affect other tool properties, such as rigidity and modularity.

  • 277.
    Fu, Qilin
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion, Maskin- och processteknologi. Plasmatrix Materials AB, Sweden.
    Rashid, Md Masud-Ur
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion, Maskin- och processteknologi. Plasmatrix Materials AB, Sweden.
    Nicolescu, Cornel-Mihai
    KTH, Skolan för industriell teknik och management (ITM), Industriell produktion.
    Toth, Geza
    et al.,
    High dynamic stiffness mechanical structures with nanostructured composite coatings deposited by high power impulse magnetron sputtering2016Inngår i: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 98, s. 24-33Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Nanostructured Cu:CuCNx composite coatings with high static and dynamic stiffness were synthesized by means of plasma-enhanced chemical vapor deposition (PECVD) combined with high power impulse magnetron sputtering (HiPIMS). Scanning electron microscope (SEM) images and energy-dispersive X-ray spectroscopy (EDS) mapping from cross-sectioned samples reveals a multi-layered nanostructure enriched in Cu, C, N, and O in different ratios. Mechanical properties of the coatings were investigated by Vickers micro-indention and model tests. It was observed that copper inclusions as well as copper interlayers in the CNx matrix can increase mechanical damping by up to 160%. Mechanical properties such as hardness, elastic modulus and loss factor were significantly improved by increasing the discharge power of the sputtering process. Moreover the coatings loss modulus was evaluated on the basis of indentation creep measurements under room temperature. The coating with optimum properties exhibited loss modulus of 2.6 GPa. The composite with the highest damping loss modulus were applied on the clamping region of a milling machining tool to verify their effect in suppressing regenerative tool chatter. The high dynamic stiffness coatings were found to effectively improve the critical stability limit of a milling tool by at least 300%, suggesting a significant increase of the dynamic stiffness.

  • 278.
    Füssl, Josef
    et al.
    Vienna University of Technology, Austria.
    Bader, Thomas K.
    Vienna University of Technology, Austria.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    Computational Mechanics for Advanced Timber Engineering: from material modeling to structural applications2012Inngår i: IACM Expressions, nr 32, s. 6-11Artikkel i tidsskrift (Annet vitenskapelig)
  • 279.
    Gamstedt, E. Kristofer
    et al.
    Uppsala University.
    Bader, Thomas K.
    Vienna University of Technology, Austria.
    de Borst, Karin
    University of Glasgow, UK.
    Mixed numerical–experimental methods in wood micromechanics2012Inngår i: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 47, nr 1, s. 183-202Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Mixed numerical–experimental methods are increasingly used in various disciplines in materials science, recently also in wood micromechanics. Having a relatively irregular microstructure, direct interpretation of mechanical tests is not always possible since structurally specific properties are quantified rather than general material properties. The advent of combined numerical–experimental methods unlocks possibilities for a more accurate experimental characterization. A number of examples of mixed methods pertaining to both emerging experimental techniques and physical phenomena are presented: nano-indentation, moisture transport, digital-image correlation, dimensional instability and fracture of wood-based materials. Successful examples from other classes of materials are also presented, in an attempt to provide some ideas potentially useful in wood mechanics. Some general pit-falls in parameter estimation from experimental results are also outlined.

  • 280.
    Gamstedt, E.K.
    et al.
    Luleå tekniska universitet.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Influence of interfacial strength on Micro- and macrosopie fatigue behavior of longitudinal glass fiberreinforced polypropylene1997Inngår i: Progress in durability analysis of composite systems: proceedings of the Third International Conference on Progress in Durability Analysis of Composite Systems, Blacksburg, Virginia, USA, 14 - 17 September 1997 / [ed] Kenneth L. Reifsnider; David A. Dillard; Albert H. Cardon, Rotterdam: Balkema Publishers, A.A. / Taylor & Francis The Netherlands , 1997Konferansepaper (Fagfellevurdert)
  • 281. Gamstedt, E.K.
    et al.
    Sandell, R.
    Berthold, F.
    RISE, Innventia.
    Pettersson, T.
    RISE, Innventia.
    Nordgren, N.
    Characterization of interfacial stress transfer ability of particulate cellulose composite materials2011Inngår i: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, nr 11, s. 693-704Artikkel i tidsskrift (Fagfellevurdert)
  • 282.
    Gamstedt, E.K.
    et al.
    Luleå tekniska universitet.
    Sjögren, B.A.
    Luleå tekniska universitet.
    Micromechanisms in tension-compression fatigue of composite laminates containing transverse plies1999Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 49, nr 2, s. 167-178Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    For both monotonic and fatigue loading conditions, debonding is the subcritical micromechanism which leads to transverse cracking and ultimately influences final failure of the composite structure. Previous studies show that tension-compression fatigue is more detrimental than tension-tension fatigue to transverse and multidirectional laminates. By analysing the debonding mechanisms and modelling thereof, the macroscopic fatigue behaviour can be better understood. Also, the dominant crack-propagation mode can be identified which may be of use in selection of constituent material properties to optimise fatigue resistance. In this study, glass-fibre-reinforced vinyl-ester was used. The adverse effect of compressive load excursions was verified by counting the transverse cracks in cross-ply laminates. The mechanisms were studied in low-cycle fatigue of a specimen containing a single transverse fibre. Compressive load cycles led to significantly increased debond growth. In tension, contact zones developed at the crack tips for sufficiently large debonds. Because of the mismatch in elastic properties, an opening zone appeared at the tips of the interfacial crack when the same debond was subjected to a compressive load. Since debond propagation is more susceptible to mode I loading, the sensitivity to tension-compression fatigue is explained by the effective crack-tip opening in compressive loading for sufficiently large debond cracks. This has also been verified by finite-element analysis.

  • 283.
    Gamstedt, E.K.
    et al.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Fatigue damage mechanisms in unidirectional carbon-fibre-reinforced plastics1999Inngår i: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 34, nr 11, s. 2535-2546Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The fatigue life behaviour and the underlying micromechanisms have been studied in two different Types of unidirectional carbon-fibre-reinforced plastics loaded in tension-tension along the fibre direction. The carbon fibres (AS4) were the same in the two composite systems. One thermoplastic matrix (polyetheretherketone, PEEK) and one thermosetting matrix (epoxy toughened with a thermoplastic additive) were used. The macroscopic fatigue behaviour was characterised by fatigue life diagrams. Surface replicas were taken intermittently during the course of the fatigue tests to monitor the active fatigue damage micromechanisms. The thermoset based composite showed a higher fatigue resistance with few microcracks initiated at distributed fibre breaks growing at a decelerating rate. The thermoplastic composite had a more pronounced fatigue degradation with a steeper fatigue life curve, which was caused by widespread propagating debonds and matrix cracks. The use of a tougher and more ductile matrix results in an inferior fatigue life performance, due to a more widely distributed accumulation of damage that propagates at a higher rate.

  • 284.
    Gamstedt, E.Kristofer
    et al.
    Luleå tekniska universitet.
    Berglund, Lars. A.
    Luleå tekniska universitet.
    Peijs, Ton
    Eindhoven University of Technology.
    Fatigue mechanisms in unidirectional glass-fibre-reinforced polypropylene1999Inngår i: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 59, nr 5, s. 759-768Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Polypropylene (PP) and polypropylene modified with maleic anhydride (MA-PP) reinforced by continuous longitudinal glass fibres have been investigated. The most prominent effect of the modification with maleic anhydride in the composite is a stronger fibre/matrix interface. The effects of interfacial strength on fatigue performance and on the underlying micromechanisms have been studied for these composite systems. Tension-tension fatigue tests (R=0.1) were carried out on 0 glass-fibre/PP and glass-fibre/MA-PP coupons. The macroscopic fatigue behaviour was characterized in terms of stiffness reduction and fatigue-life curves. The results showed that the longitudinal Young's modulus degraded more rapidly for glass-fibre/PP, which was caused by a higher degree of damage growth and accumulation. The improvement in monotonic strength was negligible, but the fatigue life was prolonged by about one decade for the composite with the stronger interface by use of the maleic-anhydride grafted polypropylene matrix. During the fatigue testing, the microscopic mechanisms were monitored intermittently by a surface replication technique. From microscopic observations, it could be concluded that the better fatigue resistance of glass-fibre/MA-PP can be attributed to the greater interfacial strength and the resistance to debond propagation

  • 285.
    Gamstedt, Kristofer
    Luleå tekniska universitet.
    Fatigue damage mechanisms in polymer matrix composites1997Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Polymer matrix composites are finding increased use in structural applications, in particular for aerospace and automotive purposes. Mechanical fatigue is the most common type of failure of structures in service. The relative importance of fatigue has yet to be reflected in design where static conditions still prevail. The fatigue behavior of composite materials is conventionally characterized by a Wöhler or S-N curve. For every new material with a new lay-up, altered constituents or different processing procedure, a whole new set of fatigue life tests has to be repeated for such a characterization. If the active fatigue damage micromechanisms and the influence of the constituent properties and interface were known, it would be possible, at least qualitatively, to predict the macroscopic fatigue behavior. A study of the fatigue damage mechanisms would also give indications of the weakest microstructural element, which is useful information in materials selection for improvement in service properties. In tensile fatigue of a multidirectional laminate, the critical elements are the longitudinal plies which are the last to fail. Although failure of neighboring off-axis plies as well as delamination will influence the fatigue process, an understanding of the behavior of the longitudinal plies forms an important foundation. Effects of plies of other directions may then be interpreted based on this foundation. Fatigue of longitudinal plies is therefore focused on in the present study. The underlying fatigue damage mechanisms were investigated for unidirectional O' carbon fiber reinforced plastics (CFRP) and glass fiber reinforced polypropylene (GF/PP) in tension-tension fatigue. By use of a surface replication technique the evolution of fatigue damage could intermittently be monitored during the course of fatigue testing. In the CFRPS, the matrix was an epoxy resin or polyetheretherketone (PEEK). In the GF/PP system, the matrix was modified with maleic anhydride (MA) to achieve a stronger fibermatrix interface. The macroscopic fatigue behavior was characterized by fatigue life diagrams. A statistical method has been devised to systematically characterize fatigue life data in terms of fatigue life diagrams. On the microscopic level, the CF/epoxy and GF/MA-PP composites have relatively strong interfaces and showed localized and scarce fiber breaks from which matrix cracks propagated perpendicular to the fiber direction. In CF/epoxy, fiber bridged cracks with squeezed fiber tips appeared. Conversely, CF/PEEK and GF/PP have weaker interfaces, and the principal mechanisms were extensive and distributed debonding or longitudinal matrix cracking followed by further fiber breakage. Macroscopically, the weak interface composites showed shorter fatigue lives and more rapid fatigue degradation. This suggests that higher interfacial strengths lead to improved fatigue performance. Modeling studies were undertaken for the two observed mechanisms; debonding from a fiber break, and fiber bridged cracking. The stochastic breakage of fibers next to a growing debond was parametrically investigated with a shear lag model. The stress profile in the surviving fibers becomes attenuated and more distributed as the debonds grow. This results in longer axial distances between fiber breaks, and hence a more jagged and uneven crack propagation. A larger variability in strength along the fibers has basically the same break distributing effect. With a more homogeneous stress distribution caused by long debonds, the variability in fiber stress at failure of the intact fibers decreases. This can explain the experimentally observed lower scatter in fatigue life of composites exhibiting a more homogeneous distribution of damage caused by debonding. Furthermore, the experimental results of fiber bridged cracking was modeled with a fracture mechanics approach. The crack growth curve can be plotted in terms of the effective stress intensity factor where the contribution of the cohesive crack surface forces from the bridging fibers are taken into account. This curve falls somewhat closer to that of the neat matrix material compared to the unbridged crack, but the difference is still considerable. Besides the fiber bridging, there should therefore be other active toughening mechanisms that slows the crack propagation down to account for the fatigue resistant behavior of the tested material. In fatigue of multidirectional laminates, tension-compression loading has shown to be more detrimental than tension-tension loading. The reason for this behavior has not been entirely clarified. The adverse effect of the compressive load excursions is partly caused by the formation of transverse cracks. This was verified by counting transverse cracks in cross-ply laminates. Since debonding is the subcritical mechanism which leads to transverse cracking and eventually influences ultimate failure, the debonding was studied in low cycle fatigue of a single transverse fiber. In tension, contact zones developed at the crack tips for sufficiently large debonds. Due to the inherent geometry and the mismatch in elastic properties of the constituents, an opening zone appeared at the crack tips of the debond in compression. This was also verified by finite element analysis. Since debond propagation is more suseptible to mode I loading, the sensitivity to tension-compression loading is explained by the effective opening zone in compression.

  • 286.
    Gamstedt, Kristofer
    Luleå tekniska universitet.
    Fatigue mechanisms in unidirectional composites1995Licentiatavhandling, monografi (Annet vitenskapelig)
  • 287.
    Gamstedt, Kristofer
    et al.
    Uppsala University.
    Bader, Thomas K.
    Vienna University of Technology, Austria.
    de Borst, Karin
    University of Glasgow, UK.
    Mixed Numerical-Experimental Methods in Wood Micromechanics2012Inngår i: COST Action FP 0802: Experimental and computional micro-characterization techniques in wood mechanics, Edinburgh, UK, 2012, s. 86-87Konferansepaper (Annet vitenskapelig)
  • 288.
    García Vogel, Andrés
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Production of regenerated nanocomposite fibers based on cellulose and their use in all-cellulose composites2017Independent thesis Advanced level (degree of Master (Two Years)), 20 poäng / 30 hpOppgave
    Abstract [en]

    Biobased all-cellulose composites (ACCs), in which the matrix and the reinforcement are made out of the same material, have gained a noticeable increased attention in recent years. Their successful application would solve the commonly faced challenges with natural fiber composites regarding their chemical antipathy between the hydrophilic fiber and the usually hydrophobic polymer matrix, while still keeping the advantages of being environmental friendly. Moreover, the use of man-made continuous regenerated cellulose fibers for this purpose could result in unidirectional all-cellulose composites with excellent mechanical properties. In this study, a new processing technique for unidirectional all-cellulose composites, reinforced with continuous regenerated cellulose nanocomposite fibers, has been developed, where the fibers are wound directly after the coagulation bath and then welded together while still being swelled in order to form all-cellulose composite sheets without the need of adding any additional solvent or chemicals. Scanning electron microscopy and tensile testing were used to investigate and compare the microstructure and mechanical properties, of a reference material without nanoreinforced fibers and two variants reinforced with 2 % cellulose nanocrystals (CNCs) and 2 % halloysite nanotubes (HNTs). Analysis revealed that transparent all-cellulose composites with a high compaction degree and minimal warpage during shrinkage, showing high mechanical properties could be made. However, the addition of nanoreinforcements did not lead to any improvements. 

  • 289.
    García-García, Daniel
    et al.
    Instituto de Tecnología de Materiales-ITM, Universitat Politècnica de València, Alicante, Spain.
    Lopez-Martínez, Juan
    Instituto de Tecnología de Materiales-ITM, Universitat Politècnica de València, Alicante, Spain.
    Rafael, Balart
    Instituto de Tecnología de Materiales-ITM, Universitat Politècnica de València, Alicante, Spain.
    Strömberg, Emma
    School of Engineering Science in Chemistry, Biotechnology and Health, Department of Fibre and Polymer Technology, KTH-Royal Institute of Technology, Stockholm, Sweden.
    Moriana, Rosana
    Högskolan i Skövde, Institutionen för ingenjörsvetenskap. Högskolan i Skövde, Forskningscentrum för Virtuella system. School of Engineering Science in Chemistry, Biotechnology and Health, Department of Fibre and Polymer Technology, KTH-Royal Institute of Technology, Stockholm, Sweden.
    Reinforcing capability of cellulose nanocrystals obtained from pine cones in a biodegradable poly(3-hydroxybutyrate)/poly(ε-caprolactone) (PHB/PCL) thermoplastic blend2018Inngår i: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945, Vol. 104, s. 10-18Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this work, different loads (3, 5 and 7 wt%) of pine cone cellulose nanocrystals (CNCs) were added to films ofpoly(3-hydroxybutyrate)/poly(ε-caprolactone) (PHB/PCL) blends with a composition of 75 wt% PHB and 25 wt% PCL (PHB75/PCL25). The films were obtained after solvent casting followed by melt compounding in anextruder and finally subjected to a thermocompression process. The influence of different CNCs loadings on themechanical, thermal, optical, wettability and disintegration in controlled compost properties of the PHB75/PCL25blend was discussed. Field emission scanning electron microscopy (FESEM) revealed the best dispersion of CNCson the polymeric matrix was at a load of 3 wt%. Over this loading, CNCs aggregates were formed enhancing thefilms fragilization due to stress concentration phenomena. However, the addition of CNCs improved the opticalproperties of the PHB75/PCL25films by increasing their transparency and accelerated the film disintegration incontrolled soil conditions. In general, the blend with 3 wt% CNCs offers the best balanced properties in terms ofmechanical, thermal, optical and wettability

  • 290.
    Gardner, D. J.
    et al.
    University of Maine.
    Tascioglu, C
    Wålinder, M. E. P.
    KTH, Skolan för arkitektur och samhällsbyggnad (ABE), Byggvetenskap, Byggnadsmaterial.
    Wood composite protection2003Inngår i: Wood Deterioration and Preservation / [ed] B. Goodell, D. Nicholas, and T.P. Schultz,, American Chemical Society , 2003, s. 399-419Kapittel i bok, del av antologi (Fagfellevurdert)
    Abstract [en]

    Wood composites should be protected against microbial and insect attack when used outdoors, especially in construction applications with prolonged exposure to moisture. Preservative systems and treatment processes affect composite properties, especially adhesive/wood bonding and mechanical properties. Several common systems for preservation of composites include, 1) the use of pretreated wood, applicable particularly to some solid lumber laminates; 2) in-process preservative treatments favored for composites made from flakes, particles, and fibers where the preservative treatment is incorporated during the manufacturing process; 3) post-process preservative treatments which are generally favored for wood composites made from lumber and veneer; and 4) the use of recycled treated wood elements in manufacturing or the use of wood species with a high natural resistance against biodegradation. This chapter discusses these four preservative methods and presents a general overview of current research concerning preservation practices and techniques in North America including the effect of preservatives on composite properties, durability issues, and degradation modes.

  • 291.
    Gardner, D. J.
    et al.
    University of Maine.
    Wålinder, Magnus E. P.
    Trätek Swedish Institute for Wood Technology Research.
    Editorial note2006Inngår i: Journal of Adhesion Science and Technology, ISSN 0169-4243, E-ISSN 1568-5616, Vol. 20, nr 8, s. 723-723Artikkel i tidsskrift (Annet vitenskapelig)
  • 292.
    Gardner, D. J.
    et al.
    University of Maine.
    Wålinder, Magnus E. P.Trätek Swedish Institute for Wood Technology Research.
    Special issue on wood adhesion and adhesives2006Collection/Antologi (Fagfellevurdert)
  • 293. Gardner, Douglas J.
    et al.
    Wålinder, Magnus E. P.
    Trätek Swedish Institute for Wood Technology Research.
    Special Issue on Wood Adhesion and Adhesives2006Inngår i: Journal of Adhesion Science and Technology, ISSN 0169-4243, E-ISSN 1568-5616, Vol. 20, nr 8, s. 725-727Artikkel i tidsskrift (Annet vitenskapelig)
  • 294.
    Geng, Shiyu
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Cellulose-based Nanocomposites – The Relationship between Structure and Properties2018Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Nanocellulose materials extracted from various types of biomass have recently attracted significant attention. Due to their remarkable mechanical properties, large surface area and biodegradability, they can be promising reinforcements in nanocomposites. Cellulose-based nanocomposites constitutive of nanocellulose reinforcements and biodegradable polymer matrices have great potential to be used in environmentally friendly applications to replace fossil-based materials. However, the challenge of controlling their nanoscale structure, especially achieving good dispersion of nanocellulose in hydrophobic polymer matrices, still poses significant obstacles to producing high-performance nanocomposites. Therefore, this thesis reports several methods for structural modification of cellulose-based nanocomposites toward the objectives of improving the dispersion of nanocellulose and enhancing the properties of the nanocomposites. The methods include in situ emulsion polymerization in the presence of nanocellulose, crosslinking of polymer matrix, grafting of polymer brushes to nanocellulose and drawing of nanocomposites to obtain aligned structures. The resulting mechanical, thermal and other related properties are investigated, and the relationship between structure and properties of the nanocomposites are discussed.

    To address the challenge of achieving good dispersion of nanocellulose in hydrophobic matrices, in situ emulsion polymerization of vinyl acetate monomer in the presence of cellulose nanocrystals has been developed. Microscopy results show that the in situ method improves the compatibility between nanocellulose and hydrophobic polymers, which consequently improves the dispersion of nanocellulose in the nanocomposites. Compared with direct mixed polymer/nanocellulose composites, the in situ synthesized nanocomposites exhibit higher stiffness and strength arising from their superior interphase volume, which is confirmed theoretically and experimentally. Crosslinking of partially hydrolyzed poly(vinyl acetate) by borate additives under different pH conditions has been studied to further enhance mechanical properties of the nanocomposites. Moreover, the “grafting to” modification method also helps to overcome this challenge. It is revealed that poly(ethylene glycol)-grafted cellulose nanofibers disperse better in poly(lactic acid) matrix than unmodified cellulose nanofibers, which is attributed to the improved compatibility and steric effect provided by the covalently grafted poly(ethylene glycol) brushes.

    To substantially enhance the unidirectional mechanical properties of cellulose-based nanocomposites, a highly aligned structure in the materials is obtained through the drawing process. Drawing conditions including temperature, speed and draw ratio show considerable effects on the mechanical and thermal properties of the nanocomposites. Furthermore, the aligned nanocomposites consisting of poly(lactic acid) matrix and ultra-low weight fraction of poly(ethylene glycol)-grafted cellulose nanofibers demonstrate competitive strength, superb toughness and interesting optical behaviors compared with other aligned nanocellulose-based materials reported in the literature, indicating their potential to be further developed for large-scale environmentally friendly applications.

  • 295.
    Geng, Shiyu
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    High-strength, High-toughness Aligned Polymer-based Nanocomposite Reinforced with Ultra-low Weight Fraction of Functionalized NanocelluloseManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    Multifunctional lightweight, flexible, yet strong polymer-based nanocomposites are highly desired for specific applications. However, the control of orientation and dispersion of reinforcing nanoparticles and the optimization of the interfacial interaction still pose substantial challenges in nanocellulose-reinforced polymer composites. In this study, poly(ethylene glycol)-grafted nanocellulose fibers (TOCNF-g-PEG) has demonstrated much better dispersion in a poly(lactic acid) (PLA) matrix as compared to unmodified nanocellulose fibers. Through a uniaxial drawing method, aligned PLA/nanocellulose nanocomposites with high strength, high toughness, and unique optical behavior are obtained. With the incorporation of only 0.1 wt% of TOCNF-g-PEG in PLA, the ultimate strength of the nanocomposite reaches 343 MPa, which is significantly higher than that of other aligned PLA-based nanocomposites reported previously. Compared with the aligned nanocomposite reinforced with unmodified nanocellulose, the ultimate strength and toughness are enhanced by 39% and 70%, respectively. Moreover, the aligned nanocomposite film is highly transparent and possesses an anisotropic light scattering effect, revealing its significant potential for optical applications.

  • 296.
    Geng, Shiyu
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Harila, Maria
    Luleå tekniska universitet.
    Yao, Kun
    Division of Glycoscience, School of Biotechnology, Royal Institute of Technology, SE-100 44, Stockholm, Sweden.
    Zhou, Qi
    Division of Glycoscience, School of Biotechnology, Royal Institute of Technology, SE-100 44, Stockholm, Sweden.
    Oksman, Kristiina
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Grafting polyethylene glycol on nanocellulose toward biodegradable polymer nanocomposites2017Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In this paper, we investigated the effects of grafting polyethylene glycol on nanocellulose on microstructure, mechanical properties and thermal behaviors of the polylactic acid/nanocellulose composites.

  • 297.
    Geng, Shiyu
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Wei, Jiayuan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Aitomäki, Yvonne
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Noël, Maxime
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Oksman, Kristiina
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap. Fibre and Particle Engineering, University of Oulu, Oulu, Finland .
    Well-dispersed cellulose nanocrystals in hydrophobic polymers by in situ polymerization for synthesizing highly reinforced bio-nanocomposites2018Inngår i: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 10, nr 25, s. 11797-11807Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In nanocomposites, dispersing hydrophilic nanomaterials in a hydrophobic matrix using simple and environmentally friendly methods remains challenging. Herein, we report a method based on in situ polymerization to synthesize nanocomposites of well-dispersed cellulose nanocrystals (CNCs) and poly(vinyl acetate) (PVAc). We have also shown that by blending this PVAc/CNC nanocomposite with poly(lactic acid) (PLA), a good dispersion of the CNCs can be reached in PLA. The outstanding dispersion of CNCs in both PVAc and PLA/PVAc matrices was shown by different microscopy techniques and was further supported by the mechanical and rheological properties of the composites. The in situ PVAc/CNC nanocomposites exhibit enhanced mechanical properties compared to the materials produced by mechanical mixing, and a theoretical model based on the interphase effect and dispersion that reflects this behavior was developed. Comparison of the rheological and thermal behaviors of the mixed and in situ PVAc/CNC also confirmed the great improvement in the dispersion of nanocellulose in the latter. Furthermore, a synergistic effect was observed with only 0.1 wt% CNCs when the in situ PVAc/CNC was blended with PLA, as demonstrated by significant increases in elastic modulus, yield strength, elongation to break and glass transition temperature compared to the PLA/PVAc only material.

  • 298.
    Geng, Shiyu
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Yao, Kun
    Division of Glycoscience, Department of Chemistry, KTH Royal Institute of Technology.
    Zhou, Qi
    Division of Glycoscience, Department of Chemistry, KTH Royal Institute of Technology.
    Oksman, Kristiina
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap. Fibre and Particle Engineering, University of Oulu.
    High-strength, High-toughness Aligned Polymer-based Nanocomposite Reinforced with Ultra-low Weight Fraction of Functionalized Nanocellulose2018Inngår i: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 19, nr 10, s. 4075-4083Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Multifunctional lightweight, flexible, yet strong polymer-based nanocomposites are highly desired for specific applications. However, the control of orientation and dispersion of reinforcing nanoparticles and the optimization of the interfacial interaction still pose substantial challenges in nanocellulose-reinforced polymer composites. In this study, poly(ethylene glycol)-grafted nanocellulose fibers (TOCNF-g-PEG) has demonstrated much better dispersion in a poly(lactic acid) (PLA) matrix as compared to unmodified nanocellulose fibers. Through a uniaxial drawing method, aligned PLA/nanocellulose nanocomposites with high strength, high toughness, and unique optical behavior are obtained. With the incorporation of only 0.1 wt% of TOCNF-g-PEG in PLA, the ultimate strength of the nanocomposite reaches 343 MPa, which is significantly higher than that of other aligned PLA-based nanocomposites reported previously. Compared with the aligned nanocomposite reinforced with unmodified nanocellulose, the ultimate strength and toughness are enhanced by 39% and 70%, respectively. Moreover, the aligned nanocomposite film is highly transparent and possesses an anisotropic light scattering effect, revealing its significant potential for optical applications.

  • 299. Ghanadpour, M.
    et al.
    Carosio, F.
    Larsson, Per Tomas
    RISE, Innventia.
    Wågberg, L.
    Phosphorylated Cellulose Nanofibrils: A Renewable Nanomaterial for the Preparation of Intrinsically Flame-Retardant Materials2015Inngår i: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 16, nr 10, s. 3399-3410Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Cellulose from wood fibers can be modified for use in flame-retardant composites as an alternative to halogen-based compounds. For this purpose, sulfite dissolving pulp fibers have been chemically modified by phosphorylation, and the resulting material has been used to prepare cellulose nanofibrils (CNF) that have a width of approximately 3 nm. The phosphorylation was achieved using (NH4)2HPO4 in the presence of urea, and the degree of substitution by phosphorus was determined by X-ray photoelectron spectroscopy, conductometric titration, and nuclear magnetic resonance spectroscopy. The presence of phosphate groups in the structure of CNF has been found to noticeably improve the flame retardancy of this material. The nanopaper sheets prepared from phosphorylated CNF showed self-extinguishing properties after consecutive applications of a methane flame for 3 s and did not ignite under a heat flux of 35 kW/m2, as shown by flammability and cone calorimetry measurements, respectively.

  • 300.
    Giannadakis, Konstantinos
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Inelastic behavior of polymer composites2013Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    In the present thesis, the inelastic behavior of polymer composites is investigated. This investigation concerns three fields of study; time dependent behavior during the lifetime of the composite material, influence of micro-damage on its overall mechanical performance and development of chemical shrinkage strains during the curing process. The significance of this work is related to the nature of all composite materials. All polymer composites tend to indicate an inelastic behaviour. This behaviour can be either linear or non-linear. No matter what it is, is very important to be taken into account in the analysis, since it is related to strain rate effects, micro-damage induced to the structure of the composite and/or irreversible plastic strains.The first part of this thesis consists of the time dependent behavior of polymer composites. There are two main assumptions; irreversible strains in a damaged state are higher and that the strains can be decoupled into viscoelastic and viscoplastic response. Each assumption is investigated and a material model that includes all the above is compiled. In order to examine its validity, different material categories have been examined. Pure polymer (paper I), polymer reinforced with short fibres (Paper II), polymer reinforced with continuous fibres (Paper III). As a step further on, the time dependent behavior within a ply level was examined. A [45/-45]s laminate was used and the non-linear shear stress strain response was studied (Paper IV). In the first part of the thesis, damage was only quantified in terms of elastic modulus development after high stress application without going into detail in what is causing it. In Paper V, the effect of damage, in terms of crack density on shear elastic modulus was studied. More accurate expressions for stress calculations in the damaged lamina were suggested, by incorporating shape functions and by checking validity with the principle of minimum complementary energy. Finally, the results from the suggested model are compared with existing models and with results from finite element analysis. A small improvement is observed at all cases. Finally, in Paper VI, the effect of curing parameters and development of chemical strains during the curing process was investigated. A relation between curing time, degree of cure and mechanical performance was drawn. What is more, different procedures for measuring chemical strains were used and a testing methodology is suggested.

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