| 1. |
- Olsson, Erik, 1960, et al.
(författare)
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Hippocampal volumes in patients exposed to low-dose radiation to the basal brain. A case--control study in long-term survivors from cancer in the head and neck region.
- 2012
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Ingår i: Radiation oncology (London, England). - 1748-717X. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- ABSTRACT: BACKGROUND: An earlier study from our group of long time survivors of head and neck cancer who had received a low radiation dose to the hypothalamic-pituitary region, with no signs of recurrence or pituitary dysfunction, had their quality of life (QoL) compromised as compared with matched healthy controls. Hippocampal changes have been shown to accompany several psychiatric conditions and the aim of the present study was to test whether the patients' lowered QoL was coupled to a reduction in hippocampal volume. METHODS: Patients (11 men and 4 women, age 31--65) treated for head and neck cancer 4--10 years earlier and with no sign of recurrence or pituitary dysfunction, and 15 matched controls were included. The estimated radiation doses to the basal brain including the hippocampus (1.5 -- 9.3 Gy) had been calculated in the earlier study. The hippocampal volumetry was done on coronal sections from a 1.5 T MRI scanner. Measurements were done by two independent raters, blinded to patients and controls, using a custom method for computer assisted manual segmentation. The volumes were normalized for intracranial volume which was also measured manually. The paired t test and Wilcoxon's signed rank test were used for the main statistical analysis. RESULTS: There was no significant difference with respect to left, right or total hippocampal volume between patients and controls. All mean differences were close to zero, and the two-tailed 95% confidence interval for the difference in total, normalized volume does not include a larger than 8% deficit in the patients. CONCLUSION: The study gives solid evidence against the hypothesis that the patients' lowered quality of life was due to a major reduction of hippocampal volume.
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| 2. |
- Larsson, Ragnar, 1960, et al.
(författare)
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A micromechanically based model for dynamic damage evolution in unidirectional composites
- 2022
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Ingår i: International Journal of Solids and Structures. - : Elsevier Ltd. - 0020-7683 .- 1879-2146. ; 238
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Tidskriftsartikel (refereegranskat)abstract
- This article addresses the micromechanically motivated, quasistatic to dynamic, failure response of fibre reinforced unidirectional composites at finite deformation. The model draws from computational homogenization, with a subscale represented by matrix and fibre constituents. Undamaged matrix response assumes isotropic viscoelasticity–viscoplasticity, whereas the fibre is transversely isotropic hyperelastic. Major novelties involve damage degradation of the matrix response, due to shear in compression based on a rate dependent damage evolution model, and the large deformation homogenization approach. The homogenized quasi-brittle damage induced failure is described by elastically stored isochoric energy and plastic work of the undamaged polymer, driving the evolution of damage. The developed model is implemented in ABAQUS/Explicit. Finite element validation is carried out for a set of off-axis experimental compression tests in the literature. Considering the unidirectional carbon–epoxy (IM7/8552) composite at different strain rates, it appears that the homogenized damage degraded response can represent the expected ductile failure of the composite at compressive loading with different off-axes. Favourable comparisons are made for the strain and fibre rotation distribution involving localized shear and fibre kinking. © 2021 The Authors
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| 3. |
- Larsson, Ragnar, 1960, et al.
(författare)
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A micromechanically based model for strain rate effects in unidirectional composites
- 2020
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Ingår i: Mechanics of materials. - : Elsevier B.V.. - 0167-6636 .- 1872-7743. ; 148
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Tidskriftsartikel (refereegranskat)abstract
- This article addresses dynamic behaviour of fibre reinforced polymer composites in terms of a transversely isotropic viscoelastic-viscoplastic constitutive model established at the unidirectional ply level. The model captures the prelocalized response of the ply in terms of rate dependent elasticity and strength without damage. A major novelty is that the model draws from computational homogenization, with matrix and fibre materials as subscale constituents for a representative volume element of the ply. The micromechanics of the strain rate dependent polymer matrix is represented by an isotropic pressure sensitive viscoelastic-viscoplastic prototype model. For the fibre material, transverse elasticity is assumed. The constituents are homogenized via the fluctuating strain of the subscale, where a simple ansatz is applied to allow for constant stress in the plane transverse to the fibre orientation. Despite the relatively simple modelling assumptions for the constituents, the homogenized model compares favourably to experimental data for an epoxy/carbon fibre based composite, subjected to a variety of challenging uniaxial off-axis tests. The model response clearly reflects observed strain rate dependencies under both tensile and compressive loadings.
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| 4. |
- Singh, Vivekendra, et al.
(författare)
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A micromechanics based model for rate dependent compression loaded unidirectional composites
- 2023
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Ingår i: Composites Science And Technology. - : Elsevier Ltd. - 0266-3538 .- 1879-1050. ; 232
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Tidskriftsartikel (refereegranskat)abstract
- Strain-rate effects in a unidirectional non-crimp fabric carbon/epoxy composite are addressed. To allow for kink-band formation including strain-rate effects and damage in such composites, the paper advances a recent model focused on compression loading at small off-axis angles. The model is based on computational homogenization with a subscale represented by matrix and fibre constituents at finite deformation. The fibre constituent is assumed to be elastic transversely isotropic and the matrix is viscoelastic–viscoplastic with damage degradation. Novel model improvements of special importance to small off-axis loading relate to the isostress formulation of the homogenized response in transverse shear. In this context, an enhanced homogenized elastic response is proposed based on Halpin–Tsai corrections to account for the nonuniform stress distribution on the microscale. The model captures the strongly rate sensitive kink-band formation due to localized matrix shearing and fibre rotation, confirming the experimentally observed increase in compressive strength for high strain rates. © 2022 The Author(s)
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| 5. |
- Singh, Vivekendra, 1987, et al.
(författare)
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Application of a Rate Dependent Model on a UD NCF Carbon/Epoxy Composite
- 2021
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Ingår i: PROCEEDINGS - 8th ECCOMAS THEMATIC CONFERENCE ON THE MECHANICAL RESPONSE OF COMPOSITES. - : CIMNE.
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Konferensbidrag (refereegranskat)abstract
- To support the modelling of composites under rapid transient loading, e.g. impact, crash, and vibrations, a computational multiscale constitutive model has been developed for the progressive failure of unidirectional carbon fibre composites. The model is computationally efficient and captures anticipated failure modes to an acceptable accuracy. Computational homogenization and micromechanics are utilized in the modelling at the ply scale. A major focus is to predict the strain rate dependent nonlinear constitutive behaviour of unidirectional composite plies. The fibres are assumed transversely isotropic, whereas the polymer is viscoelastic–viscoplastic, including a pressure-dependent strength. Degradation of the polymer matrix is described by a recently developed continuum damage mechanics approach. The model has been successfully implemented as a VUMAT subroutine in Abaqus/Explicit. Figure 1 shows FE simulation of strain localization as compared to experimental results of IM7/8552 in dynamic off-axis compression. Reasonable correlation was found between the measured and numerically predicted results. In the present paper the model is applied to simulate quasi-static and dynamic off-axis tension and compression experiments on composite coupons studied in two of our collaborative projects. The composite is a unidirectional (UD) carbon fibre non-crimp fabric (NCF) uniweave impregnated with LY556 epoxy, manufactured by Resin Transfer Moulding (RTM). The tests have been performed using a high-speed hydraulic test machine and a Split Hopkinson Bar (SHB) setup, involving strain rates of up to about 140 /s in tension and 1100 /s in compression.
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| 6. |
- Singh, Vivekendra, 1987, et al.
(författare)
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Effect of strain rate at compressive and tensile loading of unidirectional plies in structural composites
- 2019
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Ingår i: PROCEEDINGS - 7th ECCOMAS THEMATIC CONFERENCE ON THE MECHANICAL RESPONSE OF COMPOSITES. - : European Community on Computational Methods in Applied Science (ECCOMAS).
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Konferensbidrag (refereegranskat)abstract
- Fibre-reinforced polymer composites are widely used in structural applications due to their high specific stiffness and strength. In some applications the response of dynamically loaded composite components must be analysed. For example, in crash analyses of structural components, where very high loading rates occurs, the composite behaviour is not fully understood. For this, we present a novel transversely isotropic viscoelastic-viscoplastic constitutive model for a unidirectional carbon-epoxy composite. The model is micromechanically motivated so that the matrix and fibre materials of the composite are treated as micromechanical constituents at the ply scale. Based on the Hill-Mandel condition, the phases are homogenized via the macroscopic and fluctuating strain fields. To arrive at a simple but still representative model, a simplistic ansatz is applied to the structure of the fluctuating strains leading to a non-standard homogenized response of the composite. The model is applied to the non-linear rate dependent anisotropic ply behaviour under quasi-static and dynamic loading at different off-axis angles. For a simple viscoelastic-viscoplastic prototype for the rate dependent matrix response, there is a good correlation between measured and model response of the IM7-8552 material system in compression and tension.
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| 7. |
- Singh, Vivekendra, 1987, et al.
(författare)
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Rate dependent compressive failure and delamination growth in multidirectional composite laminates
- 2024
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Ingår i: Journal of Composite Materials. - : SAGE Publications Ltd. - 1530-793X .- 0021-9983. ; 58:3, s. 419-431
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Forskningsöversikt (refereegranskat)abstract
- A novel intralaminar model has, for the first time, been applied and validated for the rate-dependent failure of multidirectional carbon/epoxy laminates. Quasi-static compressive failure is evaluated by the growth of intralaminar ratedependent damage combined with the interaction of cohesive zones for interlaminar delamination. A special feature of the intralaminar model is the homogenised ply response, allowing simultaneous damage-degradation of the polymer matrix combined with the fibres. To model the observed quasi-brittle failure response of the plies under finite deformation, we have used a viscoelastic-viscoplastic matrix combined with damage and isotropic hardening behaviour. Elastic transverse isotropy is used to model the fibre reinforcement of the plies. Standard cohesive surfaces are used to model the initiation and propagation of delamination. Numerical simulations using ABAQUS/Explicit are performed to predict the growth and delamination of intralaminar damage under compression in different laminates with 56 plies of IM7/8552 carbon/epoxy. Predictions of stress versus strain and damage growth are shown to agree well with experimental results for a range of strain rates and stacking sequences.
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| 8. |
- Li, Jie, 1967, et al.
(författare)
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0.5 Tbit/s eye-diagram measurement by optical sampling using XPM-induced wavelength shifting in highly nonlinear fiber
- 2003
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Ingår i: IEEE Photonics Technology Letters. - 1041-1135 .- 1941-0174. ; 16:2, s. 566-568
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Tidskriftsartikel (refereegranskat)abstract
- In this letter, we report on a novel all-optical sampling scheme using cross-phase modulation-induced wavelength shifting and optical filtering. Up to 500-Gb/s optical signal eye-diagram measurements have been demonstrated for the first time with a temporal resolution of 0.7 ps. Signal operational wavelength range covering basically the whole erbium-doped fiber amplifier gain range (1535-1569 nm) with temporal resolutions equal to or less than 1 ps was also demonstrated in the experiment. These results show that the sampling system is suitable for directly monitoring and evaluating ultrahigh bit-rate optical time-division multiplexed data at or above 160 Gb/s.
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| 9. |
- Wetterskog, Erik, et al.
(författare)
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Size and property bimodality in magnetic nanoparticle dispersions : single domain particles vs. strongly coupled nanoclusters
- 2017
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Ingår i: Nanoscale. - : ROYAL SOC CHEMISTRY. - 2040-3364 .- 2040-3372. ; 9:12, s. 4227-4235
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Tidskriftsartikel (refereegranskat)abstract
- The widespread use of magnetic nanoparticles in the biotechnical sector puts new demands on fast and quantitative characterization techniques for nanoparticle dispersions. In this work, we report the use of asymmetric flow field-flow fractionation (AF4) and ferromagnetic resonance (FMR) to study the properties of a commercial magnetic nanoparticle dispersion. We demonstrate the effectiveness of both techniques when subjected to a dispersion with a bimodal size/magnetic property distribution: i.e., a small superparamagnetic fraction, and a larger blocked fraction of strongly coupled colloidal nanoclusters. We show that the oriented attachment of primary nanocrystals into colloidal nanoclusters drastically alters their static, dynamic, and magnetic resonance properties. Finally, we show how the FMR spectra are influenced by dynamical effects; agglomeration of the superparamagnetic fraction leads to reversible line-broadening; rotational alignment of the suspended nanoclusters results in shape-dependent resonance shifts. The AF4 and FMR measurements described herein are fast and simple, and therefore suitable for quality control procedures in commercial production of magnetic nanoparticles.
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| 10. |
- Wiberg, Andreas, 1978, et al.
(författare)
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Single Cycle Subcarrier Modulation
- 2009
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Ingår i: 2009 Conference on Optical Fiber Communication, OFC 2009; San Diego, CA; United States; 22 March 2009 through 26 March 2009. - Washington, D.C. : OSA. - 9781557528650 ; , s. OTuE1-
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Konferensbidrag (refereegranskat)abstract
- We report experimental demonstration of a concept of subcarrier modulation with the symbol rate and carrier frequency being identical, and thereby synchronous. The concept is demonstrated with 2.5 Gsymbols/s signals generated with binary digital electronics.
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