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- Ebrahimzadeh, PR, et al.
(författare)
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A model of the dynamic mechanical responses of wood, paper and some polymers to moisture changes
- 1998
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Ingår i: Journal of Materials Science. - SPUIBOULEVARD 50, PO BOX 17, 3300 AA DORDRECHT, NETHERLANDS : KLUWER ACADEMIC PUBL. - 0022-2461 .- 1573-4803. ; 33:5, s. 1201-1209
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Tidskriftsartikel (refereegranskat)abstract
- The dynamic mechanical responses of Scots pine, paper, PA6, cellophane, PVAc and PUR samples subjected to changes in the relative humidity of the surrounding air from 5 to 85% and vice versa have been analysed semiquantitatively on the basis of coupled non-linear rate equations for the moisture concentration in the sample as a function of the time. Important characteristics of the diffusion of moisture into and out of the samples have been studied by measuring the sample weight as a function of time. Moisture sorption results in sample swelling in Scots pine, paper, PA6, cellophane and PUR, for all of which mechanical loss peaks were detected. For PVAc, which does not bind moisture at load-bearing hydrogen bonding sites, no mechanical loss peak could be found. Characteristic of the mechanical loss at low vibration frequencies (0.01 to 1 Hz) is a peak immediately following a change in relative humidity for all studied sample materials except PVAc. This peak is almost certainly due to modulation of the number of load-bearing hydrogen bonds in the material. The relation between the width of the mechanical loss peak and the duration of the moisture sorption and desorption processes is interpreted according to an accepted model of two water molecule binding modes, one in which load-bearing hydrogen bonds are broken unimolecularly by water molecules and one in which further water molecules form clusters on the already unimolecularly bound water. (C) 1998 Chapman & Hall.
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- Ebrahimzadeh, PR, et al.
(författare)
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Mechanosorptive effects in cellophane, polyamide 6 and some other polymers studied by dynamic mechanical analysis
- 1997
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Ingår i: Journal of Materials Science. - 2-6 BOUNDARY ROW, LONDON, ENGLAND SE1 8HN : CHAPMAN HALL LTD. - 0022-2461 .- 1573-4803. ; 32:16, s. 4227-4235
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Tidskriftsartikel (refereegranskat)abstract
- Dynamic mechanical data obtained in the tensile mode with cellophane, polyamide 6, PEG, PVAc and PUR foam (compression) subjected to stepwise humidity changes between 5% and 85% relative humidity are reported. With the exception of PVAc, the loss tangent, tan delta, shows a transient peak every time the relative humidity of the surrounding atmosphere is changed. The strength of the tan delta transients increases when the measuring frequency is lowered. The tan delta transients slowly disappear in response to reduction of the rate of humidity change. High amplitude and prestress with static load reduce the height of the transient peaks. Long-term testing results in relatively small changes in the shape of the loss tangent versus time data. For PA6 a narrowing of the loss tangent peaks with diminishing specimen thickness was observed.
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- Ericson, M.L., et al.
(författare)
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A method of measuring energy dissipation during crack propagation in polymers with an instrumented ultramicrotome
- 1996
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Ingår i: Journal of Materials Science. - 0022-2461 .- 1573-4803. ; 31:3, s. 655-662
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Tidskriftsartikel (refereegranskat)abstract
- In order to characterize very local energy dissipation during crack propagation in polymers, an ultramicrotome was instrumented to measure the energy dissipated during sectioning. The work to section per unit area, W s, was measured for five different amorphous polymers [polymethyl methacrylate (PMMA), polystyerene (PS), polycarbonate (PC) and two epoxy resins] in the glassy state. When the section thickness was varied between 60 and 250 nm, W s varied between 15 and 100 Jm-2, depending on the material and section thickness. The method and the results are compared with other methods used for determining the energy dissipation at a local level as well as at a macroscopic level in polymers. The differences between different polymers were found to be contradictory to macroscopic fracture toughness, G lc, measurements. The material that showed the highest W s had the lowest G lc values reported. Possible mechanisms for energy dissipation during sectioning are also discussed.
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- Gamstedt, E.K., et al.
(författare)
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Fatigue damage mechanisms in unidirectional carbon-fibre-reinforced plastics
- 1999
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Ingår i: Journal of Materials Science. - 0022-2461 .- 1573-4803. ; 34:11, s. 2535-2546
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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- Jarfors, A. E. W.
(författare)
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Influence of carbon on the phases in the copper-titanium system and their precipitation
- 1999
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Ingår i: Journal of Materials Science. - 0022-2461 .- 1573-4803. ; 34:18, s. 4533-4544
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Tidskriftsartikel (refereegranskat)abstract
- The influence of carbon on the phases occurring in the copper-titanium system has been experimentally investigated. Samples were fabricated from elemental copper and titanium. Carbon was added in the form of graphite or as titanium carbide. The samples were encapsulated, heat-treated at 1173 or 1373 K for 24 h and subsequently quenched in brine. The phases present at the treatment temperature were identified, as well as the phases occurring during solidification. The microstructure indicates the presence of a miscibility gap. The composition of the phases were determined using EDS-analysis. The composition of the copper phase was found to show a great variation caused by the precipitation sequence during quenching of the melts. The complex precipitation also caused both TiCu4 and the meta-stable TiCu3 to form. The composition of TiCu was found to show a wider compositional interval than earlier found. The variation of its composition with the nominal composition suggests that TiCu can dissolve carbon. The pure Ti phases also showed larger solubility of copper than earlier found. An outline of the precipitation sequence is made.
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- Jarfors, A. E. W.
(författare)
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Solidification behaviour of AI-7% Si-0.3% Mg during rotary spray forming
- 1998
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Ingår i: Journal of Materials Science. - 0022-2461 .- 1573-4803. ; 33:15, s. 3907-3918
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Tidskriftsartikel (refereegranskat)abstract
- The solidification behaviour of an AI-7% Si-0.3% Mg alloy during rotary spray forming was studied. The ability to form a coating was insensitive to the thermal processing parameters, yielding material exchangess greater than 90%. The level of porosity varied typically between 1.5 and 4.75%. The dendrite arm spacing was evaluated and used to estimate the cooling rates. Typical dendrite secondary arm spacings were of the orderof 3 jam, 12 (am and 25 ÎŒm, corresponding to cooling rates of 4630 K s-1 72 K s-1 and 8 K s -1 respectively. The fraction primary precipitation was experimentally determined and the partition coefficient calculated indirectly using the Scheil equation. The partition coefficient is increased during rotary spray forming. This is explained by the presence of trapped vacancies at the solidification front. The vacancies change the solid's free energy and thus change the phase diagram and the partition coefficient. A simplistic analysis of entrapment and condensation of vacancies and their influence on the partition coefficient is made. © 1998 Kluwer Academic Publishers.
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