47431. |
- Larsson, Ragnar, 1960, et al.
(författare)
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Atomistic continuum modeling of graphene membranes
- 2011
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Ingår i: Computational Materials Science. - : Elsevier BV. - 0927-0256. ; 50:5, s. 1744-1753
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Tidskriftsartikel (refereegranskat)abstract
- The paper deals with the modeling of thin, monolayer graphene membranes, which have significant electrical and physical properties used for nano- or micro-devices, such as resonators and nanotransistors. The membrane is considered as a homogenized graphene monolayer on the macroscopic scale, and a continuum–atomistic multiscale approach is exploited, focusing the Tersoff–Brenner (TB) potential for the interaction between the carbonic bonds. The associated Representative atomistic Unit Lattice (RUL) is thereby considered as a micro-scale quasi-continuum placed in context of computational homogenization. In this development, the Cauchy–Born rule (CBN) is extended by the atomic fluctuation to allow for relaxation in the RUL. The paper discusses the handling of the TB-potential, both in the context of macro–micro homogenization, and in the context of numerical implementation perspectives. In particular, explicit expressions of the homogenized membrane forces and stiffness are expressed in terms of the first and second gradient of the potential, with due consideration to the involved “non-local” pairwise interaction in the model. In addition, the detailed resulting macroscopic non-linear and linearized finite element response is formulated in terms of the relaxed lattice level atomistic response. Numerical results are provided for the lattice response in terms of the apparent anisotropic behavior induced by the graphene atomic structure. An assessment of the convergence of RULs with respect to different deformation states of the lattice membrane is also carried out. Finally, a validation of an experiment of a circular graphene membrane, using atomic force microscopy (AFM) measurements, is provided based on standard TB-parameters available in the literature.
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47432. |
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47433. |
- Larsson, Ragnar, 1960, et al.
(författare)
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Carbon-fibre composites with graphene filler for enhanced properties
- 2019
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Ingår i: Proceedings of XVIII Brazil MRS Meeting. - 9788563273406 ; , s. 558-
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Konferensbidrag (refereegranskat)abstract
- Carbon fibre reinforced polymer (CFRP) is often used in applications where lightweight design is essential. The material however suffers from inherent drawbacks such as high brittleness, low electrical conductivity. This Swedish-Brazilian collaboration combines experimental studies with modelling and simulation to investigate the improvement of the CFRP material properties after addition of graphene. Graphene exhibits excellent electrical and thermal conductivity, barrier properties, unique strength which makes it an optimal additive. In this industrial-driven project, the electrical percolation threshold is studied and the strength of the reinforced polymer around the threshold value is investigated. The improvement is dependent on dispersion techniques and the grade of exfoliation of graphene. This has been shown both experimentally and from the modeling.
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47434. |
- Larsson, Rasmus, 1994, et al.
(författare)
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Coherent Combining at Ultra-low Optical Signal Powers based on Optically Amplified Error Feedback
- 2022
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Ingår i: 2022 Optical Fiber Communications Conference and Exhibition, OFC 2022 - Proceedings.
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Konferensbidrag (refereegranskat)abstract
- Coherent combining in multi-aperture reception for free-space communication applications requires active phase alignment. Here, using 50/50-fiber-couplers and optically preamplified error feedback, coherent combining of four -80dBm, 10Gbaud QPSK signals is demonstrated with 95.4% efficiency.
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47435. |
- Larsson, Rasmus, 1994, et al.
(författare)
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Coherent combining of low-power optical signals based on optically amplified error feedback
- 2022
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Ingår i: Optics Express. - 1094-4087 .- 1094-4087. ; 30:11, s. 19441-19455
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Tidskriftsartikel (refereegranskat)abstract
- In free-space optical communication links, the combining of optical signals from multiple apertures is a well-known method to collect more power for improved sensitivity or mitigation of atmospheric disturbances. However, for analog optical combining no detailed analysis has been made in cases when the optical signal power is very low (<-60 dBm) as would be the case in very long-haul free-space links. We present a theoretical and experimental study of analog coherent combining of noise-limited signals from multiple independent apertures by applying low frequency optical phase dithering to actively compensate the relative phases. It is experimentally demonstrated that a 97% combining efficiency of four 10 GBaud QPSK signals is possible with a signal power per aperture exceeding -80 dBm, in fair agreement with theory. We also discuss the scaling aspects to many apertures.
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47436. |
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47437. |
- Larsson, Ragnar, 1960, et al.
(författare)
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Compressive failure and kink-band formation modeling
- 2023
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Ingår i: European Journal of Mechanics, A/Solids. - : Elsevier BV. - 0997-7538. ; 99
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Tidskriftsartikel (refereegranskat)abstract
- To increase the use of polymeric structural composites, a major issue is to properly account for intra-laminar failure mechanisms, such as fiber kinking induced under compression. We propose a new continuum damage model that can predict the fiber kinking response at the ply level. The model is based on a previous structure tensor-based model for the response of UD-plies. A novel feature is that the compressive UD-ply response at the macroscale includes the effect of the fiber misalignment shaped as a kink-band that is resolved at the sub-scale. Concepts of computational homogenization are used to include the fiber-shear of the kink-band at the sub-scale. The model calibration is adapted to account for either kink-band formation or shear-splitting depending on the off-axis loading. Finally, the model is validated at the laminate level against experimental data for OHC-tests available in the literature. A good agreement is found for predicted strength values and observed fracture patterns of the laminates. The size effect experienced when different hole sizes are tested is also addressed.
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47438. |
- Larsson, Ragnar, 1960, et al.
(författare)
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Damage growth and strain localization in compressive loaded fiber reinforced composites
- 2018
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Ingår i: Mechanics of materials. - : Elsevier BV. - 0167-6636 .- 1872-7743. ; 127, s. 77-90
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Tidskriftsartikel (refereegranskat)abstract
- To increase the use of polymeric structural composites, a major issue is to properly account for intra-laminar failure mechanisms, such as fiber kinking which is typically induced in compression. We propose a new set of continuum damage models that are able to predict fiber kinking response under compression. A structure tensor based formulation is established at the unidirectional ply level, where the elastic material response is governed by transverse isotropy. To consider geometrical effects in conjunction with fiber kinking instability, a continuum damage formulation at finite strain is developed. The fracture area progression includes a convective and a local damage production involving a finite progression speed. In this framework, two damage evolution models are considered; one non–local model including the gradient damage effect and a local one, without the gradient enhancement. The models are implemented in a FE–code and validated for a compression loaded specimen. The models are computationally robust and can predict the localized nature of fiber kinking. A thorough sensitivity study is presented to show how the different formulations influence the predicted responses.
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47439. |
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47440. |
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