SwePub - sökning: AMNE:(ENGINEERING AND TECHNOLO...

Numrering	Referens	Omslagsbild	Hitta
1.	Marklund, Erik, et al. (författare) Stiffness of aligned wood fiber composites: Effects of microstructure and phase properties 2008 Ingår i: Journal of composite materials. - : SAGE Publications. - 0021-9983 .- 1530-793X. ; 42:22, s. 2377-2405 Tidskriftsartikel (refereegranskat)abstract The effect of wood fiber anisotropy and their geometrical features on wood fiber composite stiffness is analyzed. An analytical model for an N-phase composite with orthotropic properties of constituents is developed and used. This model is a straightforward generalization of Hashin's concentric cylinder assembly model and Christensen's generalized self-consistent approach. It was found that most macro-properties are governed by only one property of the cell wall which is very important in attempts to back-calculate the fiber properties. The role of lumen (whether it filled by resin or not) has a very large effect on the composite shear properties. It is shown that several of the unknown anisotropic constants characterizing wood fiber are not affecting the stiffness significantly and rough assumptions regarding their value would suffice. The errors introduced by application of the Hashin's model and neglecting the orthotropic nature of the material behavior in cylindrical axes are evaluated. The effect of geometrical deviations from circular cross-section, representing, for example, collapsed fibers, is analyzed using the finite element method (FEM) and the observed trends are discussed.
2.	Almgren, Karin, et al. (författare) Role of fibre-fibre and fibre-matrix adhesion in stress transfer in composites made from resin-impregnated paper sheets. 2009 Ingår i: International Journal of Adhesion and Adhesives. - : Elsevier BV. - 0143-7496 .- 1879-0127. ; 29:5, s. 551-557 Tidskriftsartikel (refereegranskat)abstract Paper-reinforced plastics are gaining increased interest as packaging materials, where mechanical properties are of great importance. Strength and stress transfer in paper sheets are controlled by fibre-fibre bonds. In paper-reinforced plastics, where the sheet is impregnated with a polymer resin, other stress-transfer mechanisms may be more important. The influence of fibre-fibre bonds on the strength of paper-reinforced plastics was therefore investigated. Paper sheets with different degrees of fibre-fibre bonding were manufactured and used as reinforcement in a polymeric matrix. Image analysis tools were used to verify that the difference in the degree of fibre-fibre bonding had been preserved in the composite materials. Strength and stiffness of the composites were experimentally determined and showed no correlation to the degree of fibre-fibre bonding, in contrast to the behaviour of unimpregnated paper sheets. The degree of fibre-fibre bonding is therefore believed to have little importance in this type of material, where stress is mainly transferred through the fibre-matrix interface.
3.	André, Alann, et al. (författare) Finite element delamination study of a notched composite plate under flexural loads 2009 Ingår i: ICCM-17, 17th International Conference on Composite Materials, Edinburgh, UK. - London : IOM Communications. Konferensbidrag (refereegranskat)abstract The delamination process in notched composite plates under flexural loading has been investigated using finite element analysis. Cohesive elements implemented in the commercial finite element package ABAQUS have been used in the region around the drilled-hole, and positioned between layers where delamination was observed during experiments presented in an accompanying paper. The delamination initiation and subsequent propagation was studied between the layers at the tension side separately and simultaneously. For all FE models, the load displacement curve was in good agreement with the one from experiments. However, the amount of damage reported from the fractography study was more extensive than that predicted by the models. Finally, it was shown that the models with only one cohesive layer show significantly different results to that of the model with four cohesive layers in terms of size of the degradation area.
4.	Edgren, Fredrik, et al. (författare) Failure of NCF Composites Subjected to Combined Compression and Shear Loading 2006 Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 66:15, s. 2865-2877 Tidskriftsartikel (refereegranskat)abstract Earlier studies have shown that formation of kink bands is the mechanism that is likely to govern failure of compression loaded non-crimp fabric (NCF) composite laminates. Because of this, a failure criterion for prediction of failure caused by kinking under multiaxial (axial compression and shear) loading has been adapted to a NCF composite system. The criterion has been validated for compression tests of quasi-isotropic laminates tested in uniaxial compression. By performing compression tests of the laminate at different off-axis angles, it was possible to vary the ratio of compressive axial stress/shear stress in the specimens. The test results proved that the criterion works well for predictions of kinking governed failure for the present material system. Detailed fractographic studies confirmed that formation of kink bands was the mechanism responsible for specimen failure. Kink bands were also found to develop at loads significantly lower than load at specimen failure.
5.	Nyström, Birgitha, et al. (författare) Microstructure and strength of injection molded natural fiber composites 2007 Ingår i: Journal of reinforced plastics and composites (Print). - : SAGE Publications. - 0731-6844 .- 1530-7964. ; 26:6, s. 579-599 Tidskriftsartikel (refereegranskat)abstract Injection molded short natural fiber composites (NFC) are studied in this article. Micromechanical models are employed to investigate effects of fiber length distribution, fiber orientation, and fiber/matrix adhesion on mechanical performance of NFCs. A parametric study showed that length/diameter ratio strongly affects the composite strength, whereas fiber strength has very little influence on strength of the composites when fiber/matrix adhesion is poor. In order to verify conclusions based on theoretical predictions, modeling results are compared with experimental results of wood powder-, flax-, pulp-, and glass fiber composites. Recommendations on improvement of strength of NFC through change of the internal structure of the materials are formulated based on results of this investigation. © 2007 SAGE Publications.
6.	Szpieg, Magdalena, et al. (författare) Reuse of polymer materials and carbon fibres in novel engineering composite materials 2009 Ingår i: Plastics, rubber and composites. - 1465-8011 .- 1743-2898. ; 38:9-10, s. 419-425 Tidskriftsartikel (refereegranskat)abstract The present work was devoted to the development of a technique for manufacture of a novel engineering material from carbon fibres and thermoplastic matrix recyclates. Fibre preforms were manufactured employing a papermaking technique for dispersing the carbon fibres. The polypropylene (PP) matrix recyclate was reprocessed into a film. The carbon fibre preforms and PP films were stacked and composite materials were subsequently manufactured by press forming. The mechanical behaviour of carbon fibre preforms was characterised by a compaction test and compared to the results obtained by consolidation test of the carbon fibres reinforced PP composites. The consolidation experiments were found to follow the trend from compaction tests allowing prediction of the amount of polymer material needed, fibre volume fraction as well as composites thickness. The resulting dispersion of fibres and void content were evaluated by microscopy.
7.	Almgren, Karin M., et al. (författare) Effects of Moisture on Dynamic Mechanical Properties of Wood Fiber Composites Studied by Dynamic FT-IR Spectroscopy 2008 Ingår i: Journal of reinforced plastics and composites (Print). - : SAGE Publications. - 0731-6844 .- 1530-7964. ; 27:16-17, s. 1709-1721 Tidskriftsartikel (refereegranskat)abstract Wood fiber reinforced polylactide is a biodegradable composite where both fibers and matrix are from renewable resources. In the development of such new materials, information on mechanical behavior on the macroscopic and the molecular level is useful. In this study, dynamic Fourier transform infrared (FT-IR) spectroscopy is used to measure losses at the molecular level during cyclic tensile loading for bonds that are characteristic of the cellulosic fibers and the polylactid matrix. This molecular behavior is compared with measured macroscopic hysteresis losses for different moisture levels. The results show that moisture ingress will transfer the load from the fibers to the matrix, and that a more efficient fiber-matrix interface would diminish mechanical losses. Although the dynamic FT-IR spectroscopy method is still qualitative, this investigation shows that it can provide information on the stress transfer of the constituents in wood fiber reinforced plastics.
8.	Wretfors, Christer, et al. (författare) Use of Industrial Hemp Fibers to Reinforce Wheat Gluten Plastics 2009 Ingår i: Journal of Polymers and the Environment. - : Springer Science and Business Media LLC. - 1566-2543 .- 1572-8919 .- 1572-8900. ; 17:4, s. 259-266 Tidskriftsartikel (refereegranskat)abstract The next generation of manufactured products must be sustainable and industrially eco-efficient, making materials derived from plants an alternative of particular interest. Wheat gluten (WG) is an interesting plant material to be used for production of plastic similar materials due to its film-forming properties. For usage of plastics in a wider range of applications, composite materials with improved mechanical properties are demanded. The present study investigates the possibilities of reinforcing WG plastics with hemp fibers. Samples were manufactured using compression molding (130 A degrees C, 1600 bar, 5 min). Variation in fiber length, content (5, 10, 15 and 20 wt%) and quality (poor, standard, good) were evaluated. Mechanical properties and structure of materials were examined using tensile testing, light and scanning electron microscopy. Hemp fiber reinforcement of gluten plastics significantly influenced the mechanical properties of the material. Short hemp fibers processed in a high speed grinder were more homogenously spread in the material than long unprocessed fibers. Fiber content in the material showed a significant positive correlation with tensile strength and Young's modulus, and a negative correlation with fracture strain and strain at maximum stress. Quality of the hemp fibers did not play any significant role for tensile strength and strain, but the Young's modulus was significantly and positively correlated with hemp fiber quality. Despite the use of short hemp fibers, the reinforced gluten material still showed uneven mechanical properties within the material, a result from clustering of the fibers and too poor bonding between fibers and gluten material. Both these problems have to be resolved before reinforcement of gluten plastics by industrial hemp fibers is applicable on an industrial scale.
9.	Sandberg, Dick, 1967-, et al. (författare) Crack formation due to weathering of radial and tangential sections of pine and spruce 2006 Ingår i: Wood Material Science & Engineering. - : Taylor & Francis. - 1748-0272 .- 1748-0280. ; 1:1, s. 12-20 Tidskriftsartikel (refereegranskat)abstract The development of cracks and changes in appearance have been investigated on radial and tangential sections of pine (Pinus sylvestris L.) and spruce (Picea abies Karst.) after exposure outdoors for 61 months. The degradation of the sections has also been studied at the micro-level.The annual ring orientation was the most important factor affecting crack development on weathering. After 61 months of outdoor exposure, the tangential sections of spruce had 1.7–2.2 times greater mean total crack length per area unit than the corresponding radial sections. In pine, the total crack length per area unit on the tangential sections was 2.2–2.6 times greater than that on the radial sections. Tangential and radial sections show the same colour change as a result of weathering.Tangential sections have more and deeper cracks than radial surfaces. The cracks on the tangential sections occur frequently in both earlywood and latewood. On radial sections, cracks occur primarily at the annual ring borders, but to a certain extent also in the earlywood.Decomposition of the cell wall takes place in both radial and tangential cell walls and cracks tend to follow the fibril orientation in the S2-layer of the cell wall. The radial cell wall of the earlywood has a large number of pits which are degraded at an early stage.
10.	Marklund, Erik, et al. (författare) Modeling the hygroexpansion of aligned wood fiber composites 2009 Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 69:7-8, s. 1108-1114 Tidskriftsartikel (refereegranskat)abstract The effect of wood fiber ultrastructure and cell wall hygroelastic properties on wood fiber composite hygroexpansion has been analyzed. An analytical concentric cylinder model extended to include also free hygroexpansion of orthotropic phase materials has been used on several length scales. Using properties of the three main wood polymers, cellulose, hemicellulose and lignin the longitudinal and transverse hygroexpansion coefficients for the microfibril unit cell were obtained and the volume fraction change of the wood polymers in the microfibril unit cell depending on relative humidity was calculated. The fiber cell wall was modeled regarding each individual S1, S2 and S3 layer and the cell wall longitudinal hygroexpansion coefficient was determined depending on microfibril angle in the S2 layer. A homogenization procedure replacing the S1, S2 and S3 layers with one single layer was found not to influence the results significantly for low microfibril angles. Finally the hygroexpansion coefficients of an aligned softwood fiber composite were calculated.
11.	Edgren, Fredrik, et al. (författare) Damage tolerance analysis of NCF composite sandwich panels 2008 Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 68:13, s. 2635-2645 Tidskriftsartikel (refereegranskat)abstract This paper concerns development and validation of impact damage representations in carbon fibre non-crimp fabric reinforced face sheets for damage tolerance analysis of sandwich panels loaded in compression. For this purpose, experimental data accompanied by fractographic observations have been employed to scrutinize numerical predictions by state-of-the-art notch strength models. As a result, equivalent hole representations of visible impact damage (VID) and, more surprisingly, of the subtle barely visible impact damage (BVID) are recommended for reliable damage tolerance prediction of the compression after impact (CAI) load case for the investigated panels. This recommendation relies on the identification of the mechanisms controlling failure resulting in reliable damage tolerance predictions employing a linear cohesive zone model. © 2008 Elsevier Ltd. All rights reserved.
12.	Marklund, Erik, et al. (författare) Modeling the effect of helical fiber structure on wood fiber composite elastic properties 2009 Ingår i: Applied Composite Materials. - : Springer Science and Business Media LLC. - 0929-189X .- 1573-4897. ; 16:4, s. 245-262 Tidskriftsartikel (refereegranskat)abstract The effect of the helical wood fiber structure on in-plane composite properties has been analyzed. The used analytical concentric cylinder model is valid for an arbitrary number of phases with monoclinic material properties in a global coordinate system. The wood fiber was modeled as a three concentric cylinder assembly with lumen in the middle followed by the S3, S2 and S1 layers. Due to its helical structure the fiber tends to rotate upon loading in axial direction. In most studies on the mechanical behavior of wood fiber composites this extension-twist coupling is overlooked since it is assumed that the fiber will be restricted from rotation within the composite. Therefore, two extreme cases, first modeling fiber then modeling composite were examined: (i) free rotation and (ii) no rotation of the cylinder assembly. It was found that longitudinal fiber modulus depending on the microfibril angle in S2 layer is very sensitive with respect to restrictions for fiber rotation. In-plane Poisson’s ratio was also shown to be greatly influenced. The results were compared to a model representing the fiber by its cell wall and using classical laminate theory to model the fiber. It was found that longitudinal fiber modulus correlates quite well with results obtained with the concentric cylinder model, whereas Poisson’s ratio gave unsatisfactory matching. Finally using typical thermoset resin properties the longitudinal modulus and Poisson’s ratio of an aligned softwood fiber composite with varying fiber content were calculated for various microfibril angles in the S2 layer.
13.	Nilsson, Sören, et al. (författare) Size effects on strength of notched CFRP laminates loaded in bending 2009 Ingår i: ICCM International Conferences on Composite Materials. - London : IOM Communications. Konferensbidrag (refereegranskat)abstract This paper describes a series of experiments set out to explore the strength of notched monolithic composite panels with different dimensions and notch size loaded in bending. Two laminate thicknesses and two widths are tested. When varying the width, also the notch diameter is varied to retain a constant W/D-ratio. Accompanying tests in compression are performed and reported to allow evaluation of the full effects of bending. A limited fractographic analysis is also performed to study damage distribution after failure initiation. It has been shown that a larger hole results in a lower failure strain although the W/D-ratio is the same. As expected it has also been demonstrated that the far field failure strain is higher in bending than in compression.
14.	Lindström, Tom, et al. (författare) Nanoclay plating of cellulosic fiber surfaces 2008 Ingår i: Journal of Applied Polymer Science. - : Wiley. - 0021-8995 .- 1097-4628. ; 108:2, s. 887-891 Tidskriftsartikel (refereegranskat)abstract A basic problem in making cellulose-reinforced composites is achieving a dispersion of cellulosic fibers in an often olephinic polymer matrix. Drying cellulosic fibers results in the formation of fiber flocs/nodules because of their strong interfiber bonding, and this makes the hydrophilic cellulosic fibers difficult to disperse in a hydrophobic matrix material. One common approach to alleviate floe formation is to adsorb cationic surfactant onto anionically charged cellulose, which reduces the interfiber bonding, decreases floe formation and gives better compatibility with the matrix. In this report, a different approach is taken, namely to adsorb nanoclays onto the cellulosic fibers, and thereby reduce the natural hydrogen-bonding affinity between fibers. In a second report, the same technology will be shown to be advantageous to decrease floe formation in oleophinic composites reinforced with cellulosic fibers. This article summarizes experiments aimed at optimizing the chemistry of deposition of montmorillonite clay onto fiber surfaces. The aim was to optimize the chemical conditions for the heterodeposition of the anionic clay onto cationically charged fluff pulp. The experiments were designed to provide a theoretical framework for the deposition of the nanoclay onto the pulp fibers. High Mw p-DADMAC and an exfoliated clay (achieved by passing the clay through a homogenizer) were used. As expected, a certain degree of charge overcompensation by adding an electrolyte was necessary to bring about deposition. The adsorbed amount of clay could be calculated from the charge balance between the overcompensated charge and the net clay charge, constituting the theoretical framework for nanoclay heterodeposition. As expected, montmorillonite clay greatly destroyed the joint strength between fibers (determined by evaluating the strength of paper made from treated fibers). The surface coverage (determined by ESCA) was shown to be a linear function of the attached amount of clay, and âˆŒ3% clay was required to fully cover the fiber surfaces. Â© 2008 Wiley Periodicals, Inc.
15.	Trey, Stacy. M., et al. (författare) Effects of dual cure and surface treatments on coating adhesion to different SMC substrates 2009 Ingår i: Plastics, rubber and composites. - : Informa UK Limited. - 1465-8011 .- 1743-2898. ; 38:2-4, s. 131-137 Tidskriftsartikel (refereegranskat)abstract Sheet moulded compound (SMC) is a highly filled, glass fibre reinforced, thermoset material used in trim and body panel automotive parts. When SMC substrates are coated with conventional thermally cured paints, inherent porosity and entrapped volatiles of the substrate result in popping ('paint pops') defects. UV curable primers (UVP) provide an order of magnitude reduction of paint defects in SMC coatings, but typically have poor adhesion. The present study investigates a series of UVPs, showing the effect of resin functionality and isocyanate additive content on the adhesion of the coatings to SMC substrates. The SMC formulation is also considered regarding how variables such as surface chemistry, morphology, surface area and degree of post-mould emissions affect UVP adhesion. The present study reveals the crucial factors involved in achieving adhesion including the importance of low post-mould emissions, high surface areas, glass fibre and other oxygen moieties on the surface, and improved wetting of the surface.
16.	Jonsson, Oskar, et al. (författare) CONSUMER PERCEPTIONS AND PREFERENCES ON SOLID WOOD, WOOD-BASED PANELS, AND COMPOSITES : A REPERTORY GRID STUDY 2008 Ingår i: Wood and Fiber Science. - 0735-6161. ; 40:4, s. 663-678 Tidskriftsartikel (refereegranskat)abstract Knowledge about consumer perception and preferences on solid wood, wood-based panels, and wood-based composites is important for product development and marketing. The aim of this study was to identify attributes and associations that people use to describe different types of wood materials and to explore how they relate to preferences. The study involved nine samples that were evaluated with the Kelly's repertory grid technique and content analysis. Based on respondents' answers. 19 core categories reflecting sample attributes were extracted. General preferences for each sample were also recorded. Principal component analysis generated two factors describing 1) naturalness, wood-likeness, softness, unprocessed origin, living, pleasant, and high value; and 2) solid and homogeneous impression. A third, preliminary factor included categories describing irregular pattern,sleekness, and smoothness. The wood samples were most liked, whereas composites and panels were not appreciated. Preferred core categories were naturalness, wood-likeness, smoothness, living impression, and value. The least liked core categories were processed, hard, and high weigh. The implications of the results for product development and marketing are discussed.
17.	Bache, Anja (författare) Ny Beton - Ny Form 2004 Bok (övrigt vetenskapligt/konstnärligt)abstract Präsentation af en ny betonteknologi, CRC, og visioner for dens formverden for kämpe store konstruktioner. Med afsät i et PhD projekt ved Arkitektskolen Århus og med fondsmidler fra Knud Höjgårds Fond og Fonden Realdania, gennemförte jeg et Postdoc på Arkitektskolen Århus med afsögning af CRC´s formpotentialer for byggerier med spändvidder helt op til 500 m. Det blev der denne bog ud af samt en udstilling på Charlottenborgs Forårsudstilling. Tak til Knud Höjgårds Fond, Fonden Realdania og Arkitektskolen Århus også for stötte til at skabe denne bog.
18.	Edgren, FRedrik, et al. (författare) Formation of damage and its effects on non-crimp fabric reinforced composites loaded in tension 2004 Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 64:5, s. 675-692 Tidskriftsartikel (refereegranskat)abstract Non-crimp fabric (NCF) composites, manufactured by resin infusion techniques are one of the most promising next generation composite materials. They offer large potential for application in primary structures as they give excellent performance at low production costs. However, before NCF composites will be efficiently used in design, detailed understanding of governing micro mechanisms must be accumulated and described by predictive models. In the present study, NCF cross-ply laminates have been tested in tension. Intralaminar cracks caused in the 90° fibre bundle layers and their effect on laminate mechanical properties have been monitored. Occurrence of 'novel' type of cracks propagating in the load direction (longitudinal cracks) is explained by a thorough FE analysis using an Representative Volume Element (RVE) approach, revealing stress concentrations caused by 0° fibre bundle waviness. Effects of damage on mechanical properties are modelled using modified micro mechanical models developed for analysis of conventional laminated composites. The analysis reveals mechanical degradation to be ruled by the crack opening displacement (COD). However, unlike traditional composites, transverse cracks do not generally extend through the entire thickness of the 90° layer, but are rather contained in single fibre bundles, limiting the COD. © 2003 Elsevier Ltd. All rights reserved.
19.	Fernberg, Patrik, et al. (författare) Piezoresistive performance of long-fiber composites with carbon nanotube doped matrix 2009 Ingår i: Journal of Intelligent Materials Systems and Structures. - 1045-389X .- 1530-8138. ; 20:9, s. 1017-1023 Tidskriftsartikel (refereegranskat)abstract 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.
20.	Katerelos, D.G., et al. (författare) Raman spectroscopy investigation of stiffness change and residual strains due to matrix cracking 2006 Ingår i: Mechanics of composite materials. - : Springer Science and Business Media LLC. - 0191-5665 .- 1573-8922. ; 42:6, s. 535-546 Tidskriftsartikel (refereegranskat)abstract The matrix cracking models developed for cross-ply composite laminates have been poorly extended in the past to more complex geometries used in practice, and they are still under development. In this paper, a new detailed analysis of the effect of matrix cracking on the behaviour of cross-ply and [0/45] s laminates under uniaxial tension is attempted. The model used in this work is applicable both to cross-ply laminates and unbalanced systems. It gives exact closed-form expressions for all thermomechanical properties of a general symmetric laminate with cracks in arbitrary layers. The theoretical approach is backed by experimental data obtained by microscopic strain-state variation measurements within a specimen, with using the technique of laser Raman spectroscopy. Glass-fibre-reinforced epoxy systems were investigated. Embedded aramid fibres-sensors within the 0° ply and near the 0°/θ ° interface were necessary due to the poor Raman signal of glass. Using experimental Raman data, the residual strain and the stiffness reduction are determined as functions of increase in crack density. The stiffness reduction is predicted with a high accuracy, whereas the measured residual strains are larger than predicted. The good results for the reduction in the elastic modulus show that the basic assumption of the model is accurate. The difference is explained by the viscoelastic-viscoplastic behaviour of the off-axis layer in shear, which in creases the "apparent" residual strain. © Springer Science+Business Media, Inc. 2006.
21.	Marklund, Erik, et al. (författare) Micromechanical modelling of wood fibre composites 2009 Ingår i: Plastics, rubber and composites. - 1465-8011 .- 1743-2898. ; 38:2, s. 118-123 Tidskriftsartikel (refereegranskat)abstract A concentric cylinder model for an N-phase composite with orthotropic properties of constituents was previously presented by the authors. With only minor modifications the model allows for including also free hygroexpansion terms in the elastic stress-strain relationship in order to deal with orthotropic phase swelling. Thus the effect of wood fibre ultrastructure and cell wall hygroelastic properties on wood fibre composite hygroexpansion may be analysed. Multiscale modelling was performed to calculate the hygroexpansion coefficients of both the fibre cell wall and the aligned wood fibre composite. Furthermore, the fibre's helical structure leads to an extension-twist coupling and thus a free fibre will deform axially and also rotate upon loading in longitudinal fibre direction. Within the composite, however, the fibre rotation will be restricted. Therefore, the decision was to compare the composite performance in the two extreme cases (i) free rotation (ii) no rotation of the fibre in the composite.
22.	Marklund, Erik, et al. (författare) Nonlinear viscoelastic viscoplastic material model including stiffness degradation for hemp/lignin composites 2008 Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 68:9, s. 2156-2162 Tidskriftsartikel (refereegranskat)abstract In repeating tensile tests with increasing maximum strain for every loading cycle the hemp/lignin composites clearly showed a nonlinear behavior and hysteresis loops in loading and unloading. The explanation for this behavior is the inherent viscoelastic nature for this type of material, but also noticeable stiffness degradation with increasing strain level. Creep tests performed at different stress levels revealed a nonlinear viscoelastic response and after recovery viscoplastic strain was detected for high stress levels. It is demonstrated that Schapery's model is suitable to model nonlinear viscoelasticity whereas viscoplastic strain may be described by a nonlinear functional presented by Zapas and Crissman. In a creep test this functional leads to a power law with respect to time and stress. In order to include stiffness reduction due to damage Schapery's model has been modified by incorporating a maximum strain-state dependent function reflecting the elastic modulus reduction with increasing strain measured in tensile tests. A generalized incremental model of the constitutive equation for viscoelastic case has been used to validate the developed material model in a linear stress controlled loading and unloading ramp. The model successfully describes the main features for the investigated material and shows good agreement with test data within the considered stress range. © 2008 Elsevier Ltd. All rights reserved.
23.	Nilsson, Sören, et al. (författare) Effects of CFRP laminate thickness on bending after impact strength 2009 Ingår i: Plastics, rubber and composites. - 1465-8011 .- 1743-2898. ; 38:2-4, s. 61-66 Tidskriftsartikel (refereegranskat)abstract This paper describes a series of experiments set out to explore the strength of bending loaded impacted monolithic composite panels with different thicknesses. Two laminate thicknesses are tested, both exposed to the same impact event in a drop tower with 35 J using a spherical tup with a radius of 8 mm. Accompanying tests in compression are performed and reported to allow evaluation of the full effects of bending. A limited fractographic analysis is also performed to study damage distribution after impact as well as after mechanical tests. It has been demonstrated that damage distribution through the thickness affects the bending properties of the laminate as the failure load was lower when the side with the largest delamination was loaded in compression. As expected it has also been demonstrated that the far field failure strain is higher at bending compared to compression loading.
24.	Svanberg, J. Magnus (författare) Shape distortion of non-isothermally cured composite angle bracket 2002 Ingår i: Plastics, rubber and composites. - : Informa UK Limited. - 1465-8011 .- 1743-2898. ; 31:9, s. 398-404 Tidskriftsartikel (refereegranskat)abstract A simulation tool for prediction of shape distortion and residual stresses has been developed by implementing a new material model in the general purpose FE package ABAQUS. To validate the simulation tool, three thick wailed angle brackets were manufactured under non-isothermal conditions. Predicted temperatures, degree of cure, and shape distortions were then compared with the experimental values for the thick walled angle brackets. Predicted and experimental values show good agreement, which verifies that the simulation procedure is reliable under non-isothermal curing conditions
25.	Szpieg, Magdalena, et al. (författare) Novel composites from recycled constituents 2009 Ingår i: 24th annual technical conference of the American Society for Composites 2009 and 1st Joint Canadian-American Technical Conference on Composites. - Red Hook, NY : Curran Associates, Inc.. - 9781615676033 ; , s. 2651-2661 Konferensbidrag (refereegranskat)abstract Only a small fraction of the waste generated by use and manufacturing of carbon-based composites is currently reused. However, due to European Union (EU) regulations and limited amount of landfilling, the industry is forced to consider recycling solutions already during the manufacturing process. The following work is devoted to the development of a technique for manufacture of novel engineering materials from recycled carbon fibres and thermoplastic matrix. Fibre preforms are manufactured employing a papermaking technique for dispersing the carbon fibres. The polypropylene (PP) matrix was reprocessed into a film. The carbon fibre preforms and PP films were stacked and composite materials were subsequently manufactured by press forming and evaluated.
26.	Szpieg, Magdalena, et al. (författare) Recycled polypropylene aimed as composites precursor material 2009 Ingår i: Plastics, rubber and composites. - 1465-8011 .- 1743-2898. ; 38, s. 412-418 Tidskriftsartikel (refereegranskat)abstract The aim of this research is to assess the stability and processability of recycled polypropylene (PP) materials for their suitability for use as matrix material in polymer composite. The work comprises development and characterisation of matrix precursors from recycled PP. The reused PP considered comes from two sources: PP from the self-reinforced PP Pure processing scrap and an automotive grade developed to offer an even material quality. To assess the thermal stability of two PP qualities for subsequent composite manufacturing, oxidation induction time (OIT), melting point Tm, decomposition temperature 7d and viscosity μ were investigated. The results indicate, that Pure processing scrap is very well stabilised and therefore suitable for multiple processing, whereas the automotive grade recycled PP cannot be used for this purpose. Fibres for subsequent composite preforming were spun by melt spinning from the Pure scrap processing and its mechanical properties were characterised. © Institute of Materials, Minerals and Mining 2009.
27.	Atashipour, Rasoul, 1983 (författare) Exact elasticity solutions for thick-walled fg spherical pressure vessels with linearly and exponentially varying properties 2009 Ingår i: International Journal of Engineering, Transactions B: Applications. - 1735-9244 .- 1728-144X. ; 22:4, s. 405-416 Tidskriftsartikel (refereegranskat)abstract In this paper, exact closed-form solutions for displacement and stress components of thick-walled functionally graded (FG) spherical pressure vessels are presented. To this aim, linear variation of properties, as an important case of the known power-law function model is used to describe the FG material distribution in thickness direction. Unlike the pervious studies, the vessels can have arbitrary inner to outer stiffness ratio without changing the function variation of FGM. After that, a closed-form solution is presented for displacement and stress components based on exponential model for variation of properties in radial direction. The accuracy of the present analyses is verified with known results. Finally, the effects of non-homogeneity and different values of inner to outer stiffness ratios on the displacement and stress distribution are discussed in detail. It can be seen that for FG vessels subjected to internal pressure, the variation of radial stress in radial direction becomes linear as the inner stiffness becomes five times higher than outer one. When the inner stiffness is half of the outer one, the distribution of the circumferential stress becomes uniform. For the case in which the external pressure is applied, as the inner to outer shear modulus becomes lower than 1/5, the value of the maximum radial stress is higher than external pressure.
28.	Bratkovsky, Alexander, et al. (författare) A metal-wire/quantum-dot composite metamaterial with negative epsilon and compensated optical loss 2008 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 93:19, s. 193106- Tidskriftsartikel (refereegranskat)abstract Numerical simulations of a binary mixture of quantum dots exhibiting gain with silver nanorods are performed, showing the feasibility of lossless negative epsilon operation for realistic material structures and parameters.
29.	Kelly, Gordon, et al. (författare) Bearing strength of carbon fibre/epoxy laminates : effects of bolt-hole clearance 2004 Ingår i: Composites Part B. - : Elsevier BV. - 1359-8368 .- 1879-1069. ; 35:4, s. 331-343 Tidskriftsartikel (refereegranskat)abstract The bearing strength of carbon fibre epoxy laminates manufactured from non-crimp fabric from heavy tow yarn has been investigated. The effects of laminate stacking sequence and geometry on the bearing strength have been determined experimentally together with the effect of initial bolt-hole clearance on the bearing strength at 4% hole deformation and at ultimate load. Significant reduction in bearing strength at 4% hole deformation was found for both pin-loaded and clamped laminates as a result of bolt-hole clearance. It was concluded that the effect of bolt-hole clearance is significant with regard to the design bearing strength of mechanically fastened joints. A three-dimensional non-linear finite element model was developed to investigate the effects of bolt-hole clearance on the stress field in the laminate adjacent to the hole. The magnitude and distribution of stress at the hole was found to be significantly dependent on the level of clearance.
30.	Valyukh, Sergiy, et al. (författare) Spectrophotometric determination of reflective liquid crystal cell parameters 2006 Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550 .- 8750-7587 .- 1522-1601. ; 99, s. 053102-1-053102-6 Tidskriftsartikel (refereegranskat)abstract We introduce a method for determination of the optical retardation, its wavelength dispersion, the cell twist angle, and the orientation of the input director in a reflective liquid crystal (LC) cell. These parameters are found from the extremes of a characteristic function defined as a sum of two spectral reflectivities of the LC cell placed between a pair of linear polarizers. The reflectivities are measured for two cell orientations, one of which is arbitrary and the other one is turned through 45°. Both theoretical analysis and experimental procedures are presented. Excellent agreement between the experiment and our theory has been found. The proposed method can be applied to the measurements of reflective LC cells with small and large cell gaps, as well as with small and large twist angles.
31.	Almgren, Karin M., et al. (författare) Moisture uptake and hygroexpansion of wood fiber composite materials with polylactide and polypropylene matrix materials 2009 Ingår i: Polymer Composites. - : Wiley. - 0272-8397 .- 1548-0569. ; 30:12, s. 1809-1816 Tidskriftsartikel (refereegranskat)abstract Effects of butantetracarboxylic acid (BTCA) modification, choice of matrix, and fiber volume fraction on hygroexpansion of wood fiber composites have been investigated. Untreated reference wood fibers and BTCA-modified fibers were used as reinforcement in composites with matrices composed of polylactic acid (PLA), polypropylene (PP), or a mixture thereof. The crosslinking BTCA modification reduced the out-of-plane hygroexpansion of PLA and PLA/PP composites, under water-immersed and humid conditions, whereas the swelling increased when PP was used as matrix material. This is explained by difficulties for the BTCA-modified fibers to adhere to the PP matrix. Fiber volume fraction was the most important parameter as regards out-of-plane hygroexpansion, with a high-fiber fraction leading to large hygroexpansion. Fiber-matrix wettability during processing and consolidation also showed to have a large impact on the dimensional stability and moisture uptake. POLYM. COMPOS., 30:1809-1816, 2009.
32.	Ståhlberg, Daniel, et al. (författare) Mechanical response of thermoset polymers under high compressive loads, 1 2005 Ingår i: Macromolecular materials and engineering. - : Wiley. - 1438-7492 .- 1439-2054. ; 290, s. 1063-1072 Tidskriftsartikel (refereegranskat)abstract The present Study describes the mechanical response of thermoset polymers under high compressive loads. A well-defined free radically cured vinyl ester resin has been used and studied in six different geometries in order to determine the dependence of apparent mechanical properties on the particular size and shape of a sample. The mechanical response in compression has also been compared to the response in tensile tests. Variation of the film thickness, boundary conditions and loading conditions reveal that there is a significant effect on the mechanical performance (apparent properties) of the polymer. When the thickness-to-width ratio of the sample is reduced in a compression test, the friction between the sample and the compression plates proves to be I of great importance. The yield stress increases dramatically when the thickness of the sample is reduced, whereas it decreases when the friction between sample and the compression plate is reduced. The creep decreases when the thickness of the material is reduced and it decreases even more due to reaction of the material surrounding the compressed part of I! the sample. The described test conditions and observed I phenomena will be subject to simulation in Part 2 of this study.
33.	Al-Zubaidy, M. N., et al. (författare) Properties of orthotropic glass-polypropylene composites manufactured by weaving of prepreg tapes and other routes 2000 Ingår i: Plastics, Rubber and Composites Processing and Applications. - : Informa UK Limited. - 0959-8111. ; 29:10, s. 520-526 Tidskriftsartikel (refereegranskat)abstract This paper reports a study of the melt impregnation and weaving of glass-polypropylene prepreg tapes into sheet for use as a precursor for pressed thermoplastic composite products and a comparison of the properties attainable with those achievable by other comparable routes. Melt impregnation has been used successfully to manufacture well impregnated tapes, with and without internal coupling agent. It appears that weaving could be an economically viable process for converting unidirectional tape into a conformable, press formable prepreg. The properties of glass-polypropylene laminates manufactured by pressing the tape woven product were compared with those of other glass-polypropylene composites, including crossply laminate made from Plytron and samples prepared by film stacking. Quasi-static mechanical properties were found to be comparable with those of Plytron and superior to those of the other materials. In the coupled samples, coupling was somewhat less effective than in Plytron. The impact behaviour of the pressed, tape woven products was impressive and superior to any of the other materials tested. Â© 2000 IoM Communications Ltd.
34.	Fernberg, Patrik, et al. (författare) Strain and damage sensing during loading of carbon nanotube doped composites 2009 Ingår i: ICCM International Conferences on Composite Materials. - London : IOM Communications. Konferensbidrag (refereegranskat)abstract 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.
35.	Oksman, Kristiina, et al. (författare) Mechanical properties and morphology of sisal fiber-epoxy composites 2000 Ingår i: Proceedings from the Third International Symposium on Natural Polymers and Composites and the Workshop on Progress in Production and Processing of Cellulosic Fibres and Natural Polymers. - São Carlos : Embrapa Instrumentacao Agropecuaria. - 8586463078 Konferensbidrag (refereegranskat)
36.	Oksman, Kristiina, et al. (författare) Morphology and mechanical properties of unidirectional sisal-epoxy composites 2002 Ingår i: Journal of Applied Polymer Science. - : Wiley. - 0021-8995 .- 1097-4628. ; 84:13, s. 2358-2365 Tidskriftsartikel (refereegranskat)abstract Plant fibers are of increasing interest for use in composite materials. They are renewable resources and waste management is easier than with glass fibers. In the present study, longitudinal stiffness and strength as well as morphology of unidirectional sisal-epoxy composites manufactured by resin transfer molding (RTM) were studied. Horseshoe-shaped sisal fiber bundles (technical fibers) were nonuniformly distributed in the matrix, In contrast to many wood composites, lumen was not filled by polymer matrix. Technical sisal fibers showed higher effective modulus when included in the composite material than in the technical fiber test (40 GPa as compared with 24 GPa). In contrast, the effective technical fiber strength in the composites was estimated to be around 400 MPa in comparison with a measured technical fiber tensile strength of 550 MPa. Reasons for these phenomena are discussed.
37.	Atashipour, Rasoul, 1983 (författare) An Elasticity solution for static analysis of functionally graded curved beam subjected to a shear force 2009 Ingår i: International Journal of Engineering, Transactions B: Applications. - 1735-9244 .- 1728-144X. ; 23:2, s. 169-178 Tidskriftsartikel (refereegranskat)abstract In this paper, using 2-D theory of elasticity, a closed-form solution is presented for stress distributions and displacements of a FG curved beam under shear force at its free end. The material properties are assumed to vary continuously through the radial direction based on a simple power law model and Poisson's ratio is supposed to be constant. In order to verify the solution, it is shown that all stress and displacement relations are converted to those of a homogenous curved beam when the inhomogeneity constant approaches zero. The effects of inhomogeneity on stress distributions are investigated. It is shown that specified stress distribution profiles can be obtained by changing the variation of volume fraction of constituents. It is observed that for a specific value of inhomogeneity constant, a proper stress distribution along the radial direction is obtained for designing purposes.
38.	Jomehzadeh, E., et al. (författare) An analytical approach for stress analysis of functionally graded annular sector plates 2009 Ingår i: Materials and Design. - : Elsevier BV. - 1873-4197 .- 0264-1275 .- 0261-3069. ; 30:9, s. 3679-3685 Tidskriftsartikel (refereegranskat)abstract In this paper, an exact analytical approach is used for bending analysis of functionally graded (FG) annular sector plates. The governing equilibrium equations are obtained based on the first order shear deformation plate theory. Introducing an analytical method, the coupled governing equilibrium equations are replaced by independent equations in term of transverse deflection and a new function. Using an equivalent flexural rigidity, the solutions of FG annular sector plates can be easily extracted from equation of homogeneous annular plates. Also, it is shown that the present method can provide accurate results. Finally, the effects of power of functionally graded material (FGM), plate thickness, inner to outer radius ratio and boundary conditions on the deflection and stresses of a functionally graded annular sector plate are studied.
39.	Saidi, A.R., et al. (författare) Reformulation of Navier equations for solving three-dimensional elasticity problems with applications to thick plate analysis 2009 Ingår i: Acta Mechanica. - : Springer Science and Business Media LLC. - 1619-6937 .- 0001-5970. ; 208:3-4, s. 227-235 Tidskriftsartikel (refereegranskat)abstract In this paper, a new reformulation of the Navier equations of motion is introduced for solving the known three-dimensional elastostatics and elastodynamics problems. At first, three decoupled equations in terms of displacement components and three decoupled equations in terms of rotation components are obtained. These equations are also invariant with respect to the choice of the coordinate system. In order to solve a three-dimensional elasticity problem based on the presented formulation, one of the three equations in terms of displacement components and the corresponding rotation equation should be solved independently. Using some relations, the other two displacement components can be obtained in terms of the mentioned displacement and rotation component. In order to verify the relations, the closed-form solutions are obtained for deflection and natural frequencies of the thick rectangular plate. The numerical results are compared with available results in the literature and it can be seen that the results of the present study are identical to those of the previous works.
40.	Neagu, Cristian R., et al. (författare) Modelling of effects of ultrastructural morphology on the hygroelastic properties of wood fibres 2007 Ingår i: Journal of Materials Science. - : Springer Science and Business Media LLC. - 0022-2461 .- 1573-4803. ; 42:24, s. 10254-10274 Tidskriftsartikel (refereegranskat)abstract Wood fibres constitute the structural framework of e.g. wood, paper, board and composites, where stiffness and dimensional stability are of importance. An analytical modelling approach has been used for prediction of hygroelastic response, and assessment of the stresses in thick-walled cylinder models of wood fibres. A wood fibre was idealised as a multilayered hollow cylinder made of orthotropic material with helical orientation. The hygroelastic response of the layered assembly due to axisymmetric loading and moisture content changes was obtained by solving the corresponding boundary value problem of elasticity. A simple solution scheme based on the state space approach and the transfer matrix method was employed. This was combined with an analytical ultrastructural homogenisation method, used to link hygroelastic properties of constituent wood polymers to properties of each layer. Predicted hygroelastic response captured experimentally measured behaviour. Fibres that were constrained not to twist showed a stiffer response than fibres allowed twisting under uniaxial loading. It was also shown that the ultrastructure, i.e. the microfibril angle, will control the hygroexpansion in the same way as it controls the compliance of the cell wall. Qualitative failure trends comparable with experimental observations could be established with stress analysis and a simple plane-stress failure criterion.
41.	Thuvander, Fredrik, et al. (författare) Modeling of cell wall drying stresses in wood 2002 Ingår i: Wood Science and Technology. - : Springer Science and Business Media LLC. - 0043-7719 .- 1432-5225. ; 36:3, s. 241-254 Tidskriftsartikel (refereegranskat)abstract All applications of wood involve drying the material from the green state. The cell wall may be viewed as a laminate consisting of different layers. The layers have different orientations and therefore different moisture expansion characteristics. As a result, stresses will develop in the layers due to drying. Micromechanical models for fibre composite materials were used in combination with a laminate analogy in order to calculate these drying stresses in the cell wall layers S1, S2 and S3. Resulting stresses were very high. In reality viscoelastic effects will significantly reduce stresses at high moisture content. However, at lower moisture content irreversible cell wall damage is likely to form as a result of the stresses computed by the model.
42.	Zhou, Qi, et al. (författare) Nanostructured biocomposites based on bacterial cellulosic nanofibers compartmentalized by a soft hydroxyethylcellulose matrix coating 2009 Ingår i: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-683X .- 1744-6848. ; 5:21, s. 4124-4130 Tidskriftsartikel (refereegranskat)abstract Biomimetic approaches involving environmentally-friendly synthetic pathways provide an opportunity to elaborate novel high-performance biocomposites. Here we have developed a low-energy biosynthetic system for the production of a high-strength composite material consisting of self-assembled and nanostructured cellulosic nanofibers. This biocomposite is analogous to natural composite materials with high strength and hierarchical organization such as wood or tendon. It was generated by growing the bacterium Acetobacter, which naturally produces cellulosic nanofibers, in the presence of hydroxyethylcellulose (HEC). Individual cellulose fibrils were coated by HEC and exhibited a smaller lateral dimension than pure bacterial cellulose (BC) fibrils. They self-assembled to form compartmentalized nanofibers and larger cellulose fibril aggregates compared to pure BC. The tensile strength of nanocomposite films prepared from the compartmentalized cellulosic nanofibers was 20% higher than that of pure BC sheets and wood cellulose nanopapers, and 60% higher than that of conventional BC/HEC blends, while no strain-to-failure decrease was observed. The thin nanoscale coating consisting of hydrated HEC significantly increased the mechanical performance of the nanocomposite films by provoking compartmentalization of individual fibrils.
43.	Emami, Nazanin, et al. (författare) Dynamic mechanical thermal analysis of two light-cured dental composites 2005 Ingår i: Dental Materials. - : Elsevier BV. - 0109-5641 .- 1879-0097. ; 21:10, s. 977-983 Tidskriftsartikel (refereegranskat)abstract ObjectivesClinical observations suggest that some composite resins are more often linked to post-operative sensitivity than others. These differences may relate to differences in modulus of elasticity and polymerization rates among materials. The aim of this study was to identify viscoelastic behavior of two light curable composites and determine whether significant differences in viscoelastic behavior exist between the two materials when light cured at each of three different irradiance values.MethodsTwo composites (Z100 and Z250 by 3M ESPE) were evaluated. Six specimens per composite and irradiance value (250, 500 and 850 mW/cm2) were made. The curing times were chosen to produce a fixed energy value of 30 J/cm2 independent of irradiation value. Dynamic mechanical thermal analysis (DMTA) was performed in single cantilever clamped mode.ResultsThere were significant differences in transition temperatures between the two materials and the three frequencies at their glass transition temperatures, while significant differences did not exist at the lower transitions. The glass transition of Z250 was lower and narrower than that of Z100. Z250 exhibited lower storage modulus values. The irradiance values did not affect any of the transition temperatures significantly.SignificanceThe lower and more distinct Tg of Z250 suggests that Z250 cures more efficiently than Z100. The lower storage modulus of Z250 suggests that Z250 develops less stress in the tooth than Z100 during curing if shrinkage is the same for the two materials. The findings suggest that the material chosen, rather than irradiance, determines the stress level developed during light curing.
44.	Emami, Nazanin (författare) Effect of light power density variation on dental light-cure resin composites 2001 Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract Dental resin based composites are tooth-colored filling materials composed of synthetic resins and particulate ceramic reinforcing filler particles. The resin system also contains molecules that promote and/or modify the polymerisation reaction of the dimethacrylate resin monomers. The filler is bonded to the cured polymer with a film of silane coupling agent covering the filler particles. That silane film is also bonded to the reinforcing filler particles. Dental composites have been used as restorative materials for anterior applications since the 60s. Their tooth matching ability, ability to bond to tooth tissues and their lack of mercury have also promoted them as an alternative to dental amalgam for use in posterior teeth. Favourable results from long-term clinical trails demonstrate that when placed correctly, composites can produce esthetical posterior restorations with acceptable longevity ( el-Mowafy et al., 1994: Taylor et al., 1994 ), although not yet comparable to amalgams (Mjor). Significant problems still remain to be solved and limit their usefulness in the routine practice of dentistry. One of the most significant problems today relates to large material contraction during intra-oral polymerisation of composites. The hardening of composites is the result of polymerisation reactions involving dimethacrylate monomers. A rigid and heavily cross-linked polymer network is produced which surrounds the inert filler particles. The extent of this reaction, the degree of conversion, dictates many of the physical and mechanical properties of the composites. The degree of cure is influenced by many factors, including the light energy used to activate the reaction (Rueggeberg and Jordan, 1993). A reduction in volume, here termed shrinkage, occurs when the monomer polymerises. That shrinkage, which is more than 10-20 times higher in microns than what occurs when an amalgam sets, is caused by a change from van der Waal bonding to covalent bond formation. During that reaction, the monomer molecules rearrange and move closer together (Oleinik, 1986). The magnitude of the shrinkage is dictated by the extent of the reaction, as well as by the nature of the monomers. Research program In the currently ongoing study we are studying the effect of light intensity on polymerisation-induced strain, degree of conversion, volumetric changes and modulus of elasticity of two commercial dental composites. The objective is to test the hypothesis that low light intensity and increased curing time can be used to cure composites with better performance than high intensity cured composites. The benefits with the low intensity long time cure could be improved marginal integrity without loss of mechanical and physical properties. MethodsPolymerisation strain: Small ring shape samples were prepared and cured with three different light intensities (800, 450 and 200 mW/cm2). The polymerisation strain was measured by strain gages. The temperature increase was also measured. The sources of increased temperature are heat generated from the lamp as well as exothermal heat from curing. Volumetric shrinkage: The overall volumetric shrinkage was measured using water and mercury displacement methods. Degree of conversion: The effect of light intensity irradiation time on degree of conversion was measured by spectroscopy (FT- Raman). Modulus of Elasticity: One important factor influencing residual stresses is the stiffness of the dental composite. A miniature tensile machine for small sample size was used to measure the Young's modulus for two materials cured with different light intensities. ResultsA decrease in light intensity decreased the residual strain for the different material systems being evaluated. As long as the lower light intensity was compensated with an increased curing time, degree of conversion, Young's modulus and volumetric shrinkage were compared to high intensity cure for shorter time. The temperature increase, though, was lower for the low intensity cure than for the high intensity cure, even if longer time was used for the low intensity cure. DiscussionThe above results support the proposed hypothesis. A lower light intensity delays gelation, allowing the material to flow more initially. Such flow decreases the induced strain. Another important factor is the lower increase in temperature, which also decreases the thermal shrinkage that occurs during cooling back to room temperature. Differences between the two materials can also be related to differences in molecular structures between the two composites. An important conclusion is that for these materials, the polymerisation reaction is controlled by the total light energy supplied to the dental composite.
45.	Emami, Nazanin, et al. (författare) Effect of light power density variations on bulk curing properties of dental composites 2003 Ingår i: Journal of Dentistry. - 0300-5712 .- 1879-176X. ; 31:3, s. 189-196 Tidskriftsartikel (refereegranskat)abstract Objective. The hypothesis that low light intensity and long but sufficient curing time can produce composites with volumetric shrinkage, degree of conversion (DC%) and Young's modulus (E-modulus) comparable to those of high light intensity cured composite was tested, when the contraction strain and heat generation were lower with low light intensity curing. Methods. Dental composites (Z100 and Z250, 3M ESPE) were investigated. Specimens were cured with light intensities of 200, 450 and 800 mW/cm2 for 140, 60 and 35 s from a distance of 7 mm. Strain-gages were used for contraction strain measurements. DC% was measured at the top and the bottom of 4 mm thick samples using FT-Raman spectroscopy. Volumetric polymerization shrinkage was determined using a water displacement method. E-modulus was determined in tension on composite specimens. Results. The results were analyzed using ANOVA and Duncan's multiple range tests and regular t-test. Polymerization stress level decreased significantly (p<0.05) when cured with 200 mW/cm2 rather than with 800 mW/cm2. Temperature rises were significantly different (p<0.05) for different composites and light intensity values. Reduction in light intensity did not decrease the DC% values significantly at the top surfaces. The most dramatic differences existed between top and bottom surfaces (p<0.05) rather than among curing groups. Measured E-modulus and volumetric shrinkage values were not significantly different (p>0.05) between different light intensity groups. Conclusion. DC%, E-modulus and the volumetric shrinkage values in cured composites were not affected by low light intensity, however, the contraction strain and polymerization's exotherm were decreased. Thus our results support the proposed hypothesis.
46.	Emami, Nazanin, et al. (författare) Influence of light-curing procedures and photo-initiator/co-initiator composition on the degree of conversion of light-curing resins 2005 Ingår i: Journal of materials science. Materials in medicine. - : Springer Science and Business Media LLC. - 0957-4530 .- 1573-4838. ; 16:1, s. 47-52 Tidskriftsartikel (refereegranskat)abstract Objective: The hypothesis that the degree and rate of conversion can be modified favourably by using different light-curing procedures and different photo initiator/co-initiator combinations was tested.Method: A photo-initiator (0.02 mM/g resin); either camphorquinone (CQ) or 1-phenyl-1,2-propanedione (PPD), was mixed with bisGMA:TEGDMA (50:50 by weight). In addition, a co-initiator (0.04 mM/g resin); either N,N-dimethyl-p-aminobenzoic acid ethylester (DABE), N,N-cyanoethylmethylaniline (CEMA), or 2-dimethylaminoethyl methacrylate (DMAEMA), was added. These six combinations were subjected to three curing conditions (standard curing, soft-start curing or LED curing). The conversion levels (DC) were determined with differential scanning calorimetry (DSC). The DSC results were analysed using a general linear model (GLM) and Duncans multiple range test and regular t-test.Results: The fastest conversion initially was obtained by standard curing, followed by LED curing and soft-start curing. After 40 s of curing, conventional curing and soft-start curing produced a higher DC than LED curing. However, strong interactions occurred between the different variables (curing method, initiator and co-initiator). Initially, CQ was more efficient than PPD, but after 40 s, this difference was insignificant.Conclusion: By using soft-start curing and an appropriate photo initiator/co-initiator combination it is possible to achieve slow curing and a high DC at within a curing time of 40 s.
47.	Emami, Nazanin (författare) Variables affecting stress development and resin conversion in light-cured dental composites 2004 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract The general aim of this dissertation was to identify and investigate factors that can be used to minimize stress development in light cured dental resins without compromising the conversion level of the polymer. Modulus of elasticity, polymerization contraction strain, degree of conversion and shrinkage of light-cure dental composites were determined after curing with three different light power densities where total irradiated energy (J/cm2) kept constant. FT-Raman spectroscopy was employed to determine the degree of conversion. The cure kinetic of light cured resins was studied by use of photocalorimetry (photo-DSC). Dynamic mechanical thermal (DMTA) analysis was used to investigate how different light curing methods affected glass transition and tangent delta of light curable dental resins when the temperature changed from 0 to 200°C. Optical properties of dental composites were studied. Three different filler types, two different surface treatments and eight different filler fractions per filler type and surface treatment were investigated. Light transmission was measured for the different composite compositions at sample thicknesses of 1 to 5 mm by use of a universal power meter. As long as the total light energy remained the same, the modulus of elasticity remained constant for each composite, even though the power density differed. Composite thickness, irradiance time, composition of the light cure composite and irradiation value had significant impact on degree of conversion. The irradiance value did not significantly affect on the transition temperature value. Initiator, co-initiators and light irradiance value had all significant impact on cure behavior. Different filler types and filler surface treatments had significant effects on light absorption. In general, light absorption increased linearly with filler fraction and sample thickness of the cured composites. Conclusion: Low rather than high light irradiance values decrease stress levels in composites, and comparable conversion levels are reached as long as the total light energy value remains the same for low versus high irradiance. By increasing the composite thickness above 2 mm but not exceeding 6 mm, energy levels exceeding 30 J are needed to achieve acceptable levels of degree of conversion. Different irradiance values do not affect the final Tg of tested composites as long as the total light energy remains the same. By using appropriate photo initiator/co-initiator combination and soft-start curing it is possible to achieve slow curing and high DC within a 40 s. As expected, different filler particle properties have significant effects on light absorption during curing making it important to consider these differences when one tries to develop a general light curing strategy for light curable dental resins.
48.	Andersson, B., et al. (författare) Micro- and meso-level residual stresses in glass-fiber/vinyl-ester composites 2000 Ingår i: Composites Science And Technology. - : Elsevier Science Ltd, Exeter, United Kingdom. - 0266-3538 .- 1879-1050. ; 60:10, s. 2011-2028 Tidskriftsartikel (refereegranskat)abstract Residual stresses in glass-fiber composites were studied on the micro and meso scales by computational and experimental methods. Transmitted polarized light images of thin sections were compared with 3D finite-element solutions of a sample containing 1410 fibers. Calculated point-wise stresses were derived from a linear thermoelastic model with negligibly small numerical errors. Regions with calculated maximum compressive stresses showed good agreement with experimentally observed optical bands. A material with poor interfacial adhesion showed weaker optical effects indicating fiber/matrix debonding. On the basis of these results it stems likely that irreversible matrix deformation and debonding can take place in the curing phase.
49.	Bader, Thomas K., 1980-, et al. (författare) Homogenization and Localization in a Multiscale Microporomechanical Model for Wood Strength 2009 Konferensbidrag (övrigt vetenskapligt/konstnärligt)
50.	Bader, Thomas K., 1980-, et al. (författare) Multiscale Microporomechanics Model for Estimation of Elastic Limit States of Softwood Materials 2009 Konferensbidrag (övrigt vetenskapligt/konstnärligt)

Skapa referenser, mejla, bekava och länka

Länka till träfflistan

Träfflista för sökning "AMNE:(ENGINEERING AND TECHNOLOGY) AMNE:(Materials Engineering) AMNE:(Composite Science and Engineering) srt2:(2000-2009)"

Avgränsa träffmängd

År