| 1. |
- Araújo, M.C., et al.
(författare)
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Predicting the mechanical behavior of amorphous polymeric materials under strain through multi-scale simulation
- 2014
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332. ; 306, s. 37-46
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Tidskriftsartikel (refereegranskat)abstract
- Polymeric materials have become the reference material for high reliability and performance applications. However, their performance in service conditions is difficult to predict, due in large part to their inherent complex morphology, which leads to non-linear and anisotropic behavior, highly dependent on the thermomechanical environment under which it is processed. In this work, a multiscale approach is proposed to investigate the mechanical properties of polymeric-based material under strain. To achieve a better understanding of phenomena occurring at the smaller scales, the coupling of a finite element method (FEM) and molecular dynamics (MD) modeling, in an iterative procedure, was employed, enabling the prediction of the macroscopic constitutive response. As the mechanical response can be related to the local microstructure, which in turn depends on the nano-scale structure, this multiscale approach computes the stress-strain relationship at every analysis point of the macro-structure by detailed modeling of the underlying micro- and meso-scale deformation phenomena. The proposed multiscale approach can enable prediction of properties at the macroscale while taking into consideration phenomena that occur at the mesoscale, thus offering an increased potential accuracy compared to traditional methods.
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| 2. |
- Atsawawaranunt, Kamolphat, et al.
(författare)
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The SISAL database : a global resource to document oxygen and carbon isotope records from speleothems
- 2018
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Ingår i: Earth System Science Data. - : Copernicus GmbH. - 1866-3508 .- 1866-3516. ; 10:3, s. 1687-1713
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Tidskriftsartikel (refereegranskat)abstract
- Stable isotope records from speleothems provide information on past climate changes, most particularly information that can be used to reconstruct past changes in precipitation and atmospheric circulation. These records are increasingly being used to provide "out-of-sample" evaluations of isotope-enabled climate models. SISAL (Speleothem Isotope Synthesis and Analysis) is an international working group of the Past Global Changes (PAGES) project. The working group aims to provide a comprehensive compilation of speleothem isotope records for climate reconstruction and model evaluation. The SISAL database contains data for individual speleothems, grouped by cave system. Stable isotopes of oxygen and carbon (delta O-18, delta C-13) measurements are referenced by distance from the top or bottom of the speleothem. Additional tables provide information on dating, including information on the dates used to construct the original age model and sufficient information to assess the quality of each data set and to erect a standardized chronology across different speleothems. The metadata table provides location information, information on the full range of measurements carried out on each speleothem and information on the cave system that is relevant to the interpretation of the records, as well as citations for both publications and archived data.
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| 3. |
- Blaschikoff, Ludmilla, et al.
(författare)
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A multidisciplinary study of Iberian Chalcolithic dogs
- 2022
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Ingår i: Journal of Archaeological Science. - : Elsevier BV. - 2352-409X .- 2352-4103. ; 42
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Tidskriftsartikel (refereegranskat)abstract
- Domesticated dogs have been present in the Iberian Peninsula long before other domesticated species, back to the late Palaeolithic period. Their origin is still uncertain, but dogs were already well established during the Chalcolithic period (ca. 5000-4000 BP). This study employed a multidisciplinary approach comprising osteometric, radiographic and palaeogenomic analyses to characterize Chalcolithic Iberian Canis remains. Two Chalcolithic archaeological sites - Leceia, Oeiras, in Portugal, and El Caset ' on de la Era, Villalba de los Alcores, Valladolid, in Spain - were the main focus of this study. Osteometric and odontometric data from eleven other sites in Iberia were also included. Osteometric results show signs of phenotypic variability, likely the result of human-driven selective pressure. Dental radiographic and dental wear analyses allowed age at death estimation for four individuals (two juvenile and two adults). Three Chalcolithic Iberian dogs had their mitogenomes resequenced and the mitochondrial DNA analysis allowed to assign each individual to two of the major known haplogroups - A and C. Molecular sex infered by the chromosomeX/chromosome1 coverage ratio allowed to identify one female and two males. This study unveils some aspects of the Iberian Chalcolithic dogs: these dogs already exhibited various morphotypes whose profiles might be associated to the performance of certain tasks, as well as mitogenomes of two distinct lineages that help tracking the evolutionary paths of Iberian dogs.
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| 4. |
- Comas-Bru, Laia, et al.
(författare)
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Evaluating model outputs using integrated global speleothem records of climate change since the last glacial
- 2019
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 15:4, s. 1557-1579
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Tidskriftsartikel (refereegranskat)abstract
- Although quantitative isotope data from speleothems has been used to evaluate isotope-enabled model simulations, currently no consensus exists regarding the most appropriate methodology through which to achieve this. A number of modelling groups will be running isotope-enabled palaeoclimate simulations in the framework of the Coupled Model Intercomparison Project Phase 6, so it is timely to evaluate different approaches to using the speleothem data for data-model comparisons. Here, we illustrate this using 456 globally distributed speleothem delta O-18 records from an updated version of the Speleothem Isotopes Synthesis and Analysis (SISAL) database and palaeoclimate simulations generated using the ECHAM5-wiso isotope-enabled atmospheric circulation model. We show that the SISAL records reproduce the first-order spatial patterns of isotopic variability in the modern day, strongly supporting the application of this dataset for evaluating model-derived isotope variability into the past. However, the discontinuous nature of many speleothem records complicates the process of procuring large numbers of records if data-model comparisons are made using the traditional approach of comparing anomalies between a control period and a given palaeoclimate experiment. To circumvent this issue, we illustrate techniques through which the absolute isotope values during any time period could be used for model evaluation. Specifically, we show that speleothem isotope records allow an assessment of a model's ability to simulate spatial isotopic trends. Our analyses provide a protocol for using speleothem isotope data for model evaluation, including screening the observations to take into account the impact of speleothem mineralogy on delta O-18 values, the optimum period for the modern observational baseline and the selection of an appropriate time window for creating means of the isotope data for palaeo-time-slices.
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| 5. |
- Correia, A., et al.
(författare)
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Modelling the mechanical behavior of polymer-based nanocomposites
- 2013
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Ingår i: Materials Science Forum. - 1662-9752 .- 0255-5476. ; 730-732, s. 543-548
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Konferensbidrag (refereegranskat)abstract
- Molecular dynamics simulations were employed to analyze the mechanical properties of polymer-based nanocomposites with varying nanofiber network parameters. The study was focused on nanofiber aspect ratio, concentration and initial orientation. The reinforcing phase affects the behavior of the polymeric nanocomposite. Simulations have shown that the fiber concentration has a significant effect on the properties, with higher loadings resulting in higher stress levels and higher stiffness, matching the general behavior from experimental knowledge in this field. The results also indicate that, within the studied range, the observed effect of the aspect ratio and initial orientation is smaller than that of the concentration, and that these two parameters are interrelated.
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| 6. |
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| 7. |
- Ferreira, Bernardo P., et al.
(författare)
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Continuum and Multiscale Modeling of Rubber Toughened Glassy Polymers at Finite Strains
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Over recent years, the modeling of heterogenous multi-phase materials has been a topic of extensive research by the scientific community. Among other approaches, computational homogenizationbased multiscale modeling has emerged as an effective way to relate the macroscopic behaviour of materials with their underlying heterogeneous microstructure by continuous interchange of information between scales. Under the key assumption of the principle of separation of scales, the hierarchically coupled multi-scale finite element method is based on the nested solution of two coupled boundary value problems: (i) at the macroscale, where the material’s macroscopic response is sought, and (ii) at the microscale, where computations are conducted over representative volume elements in order to account for microstructural phenomena in the macroscopic response, through an homogenization procedure. A considerable effort has been made by the scientific community to develop constitutive models that are able to accurately describe the deformation behaviour of polymeric based materials. Concerning their fracture thoughness, it is well known that glassy polymers show brittle behaviour, particulary under specific conditions such as low temperatures and high strain rates. One important and well-known technique to improve their fracture toughness is termed rubber toughnening, which consists in dispersing rubbery particles in the polymeric matrix in order to hinder the propagation of microfractures. Associated with these rubbery particles is the phenomenon of internal cavitation, meaning that they will behave as voids during the deformation of the rubber toughened polymer. In the present contribution, a continuum constitutive model is developed in order to predict the behaviour of porous polymeric materials. This model fully couples the finite strain elastoviscoplastic constitutive model proposed by Mirkhalaf et al. [1] with the yield surface of the wellknown micromechanical void growth model proposed by Gurson [2]. A first order homogenizationbased multiscale model [3] is then employed to critically assess the predictive ability of the developed continuum model, through several numerical comparisons between the continuum approach and the homogenized response of a voided representative volume element.
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| 8. |
- Householder, John Ethan, et al.
(författare)
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One sixth of Amazonian tree diversity is dependent on river floodplains
- 2024
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Ingår i: NATURE ECOLOGY & EVOLUTION. - 2397-334X.
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Tidskriftsartikel (refereegranskat)abstract
- Amazonia's floodplain system is the largest and most biodiverse on Earth. Although forests are crucial to the ecological integrity of floodplains, our understanding of their species composition and how this may differ from surrounding forest types is still far too limited, particularly as changing inundation regimes begin to reshape floodplain tree communities and the critical ecosystem functions they underpin. Here we address this gap by taking a spatially explicit look at Amazonia-wide patterns of tree-species turnover and ecological specialization of the region's floodplain forests. We show that the majority of Amazonian tree species can inhabit floodplains, and about a sixth of Amazonian tree diversity is ecologically specialized on floodplains. The degree of specialization in floodplain communities is driven by regional flood patterns, with the most compositionally differentiated floodplain forests located centrally within the fluvial network and contingent on the most extraordinary flood magnitudes regionally. Our results provide a spatially explicit view of ecological specialization of floodplain forest communities and expose the need for whole-basin hydrological integrity to protect the Amazon's tree diversity and its function.
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| 9. |
- Luize, Bruno Garcia, et al.
(författare)
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Geography and ecology shape the phylogenetic composition of Amazonian tree communities
- 2024
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Ingår i: JOURNAL OF BIOGEOGRAPHY. - 0305-0270 .- 1365-2699.
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Tidskriftsartikel (refereegranskat)abstract
- Aim: Amazonia hosts more tree species from numerous evolutionary lineages, both young and ancient, than any other biogeographic region. Previous studies have shown that tree lineages colonized multiple edaphic environments and dispersed widely across Amazonia, leading to a hypothesis, which we test, that lineages should not be strongly associated with either geographic regions or edaphic forest types. Location: Amazonia. Taxon: Angiosperms (Magnoliids; Monocots; Eudicots). Methods: Data for the abundance of 5082 tree species in 1989 plots were combined with a mega-phylogeny. We applied evolutionary ordination to assess how phylogenetic composition varies across Amazonia. We used variation partitioning and Moran's eigenvector maps (MEM) to test and quantify the separate and joint contributions of spatial and environmental variables to explain the phylogenetic composition of plots. We tested the indicator value of lineages for geographic regions and edaphic forest types and mapped associations onto the phylogeny. Results: In the terra firme and v & aacute;rzea forest types, the phylogenetic composition varies by geographic region, but the igap & oacute; and white-sand forest types retain a unique evolutionary signature regardless of region. Overall, we find that soil chemistry, climate and topography explain 24% of the variation in phylogenetic composition, with 79% of that variation being spatially structured (R-2 = 19% overall for combined spatial/environmental effects). The phylogenetic composition also shows substantial spatial patterns not related to the environmental variables we quantified (R-2 = 28%). A greater number of lineages were significant indicators of geographic regions than forest types. Main Conclusion: Numerous tree lineages, including some ancient ones (>66 Ma), show strong associations with geographic regions and edaphic forest types of Amazonia. This shows that specialization in specific edaphic environments has played a long-standing role in the evolutionary assembly of Amazonian forests. Furthermore, many lineages, even those that have dispersed across Amazonia, dominate within a specific region, likely because of phylogenetically conserved niches for environmental conditions that are prevalent within regions.
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| 10. |
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| 11. |
- Mirkhalaf, Mohsen, 1982, et al.
(författare)
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An elasto-viscoplastic constitutive model for polymers at finite strains: Formulation and computational aspects
- 2016
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Ingår i: Computers and Structures. - : Elsevier BV. - 0045-7949. ; 166, s. 60-74
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes a finite strain elasto-viscoplastic constitutive model to predict the non-linear behaviour of polymeric based materials. The theoretical basis of the material model and the computational treatment are presented. The operator split methodology and the Newton-Raphson method are used to derive the state update algorithm and to obtain the numerical solution. The integration algorithm reduces to the solution of only one scalar non-linear equation. A closed formula for the corresponding consistent tangent operator is presented. Different aspects of the constitutive model and its integration algorithm are investigated by considering a comprehensive set of numerical examples.
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| 12. |
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| 13. |
- Mirkhalaf, Mohsen, 1982, et al.
(författare)
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Determination of the size of the Representative Volume Element (RVE) for the simulation of heterogeneous polymers at finite strains
- 2016
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Ingår i: Finite Elements in Analysis and Design. - : Elsevier BV. - 0168-874X. ; 119, s. 30-44
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Tidskriftsartikel (refereegranskat)abstract
- The definition of the size of the Representative Volume Element (RVE) is extremely important for the mechanics and physics of heterogeneous materials since it should statistically represent the micro-structure of the material. In the present contribution, a methodology based on statistical analysis and numerical experiments is proposed to determine the size of the RVE for heterogeneous amorphous polymers subjected to finite deformations. The approach is applied to Rubber Toughened Polystyrene (RT-PS) composed by a two phase random micro-structure. Different micro-structural samples with two different percentages of rubbery particles, namely 10% and 15%, inside the micro-structure are studied. Periodic boundary conditions (PBC) are enforced to the RVE due to their fast convergence to the theoretical/effective solution when the RVE size increases. The Finite Element Method (FEM) is used in combination with mathematical homogenization to obtain the macro-stress. Two criteria are proposed for the RVE size determination. The proposed statistical-numerical approach is general and easy to use, when compared to the previously proposed approaches, and covers other criteria available in the literature.
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| 14. |
- Mirkhalaf, Mohsen, 1982, et al.
(författare)
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Modelling of the post yield response of amorphous polymers under different stress states
- 2017
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Ingår i: International Journal of Plasticity. - 0749-6419. ; 88, s. 159-187
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Tidskriftsartikel (refereegranskat)abstract
- In this contribution, an elasto-viscoplastic constitutive model based on the single mode EGP (Eindhoven Glassy Polymer) model is proposed to describe the deformation behaviour of solid polymers subjected to finite deformations under different stress states. The polymeric material examined in this work is a specific commercial grade of Bisphenol, a polycarbonate called Makrolon 2607, for which there were experimental results available in the open literature for: uniaxial compression, plane strain compression and tensile test on a dumbbell shape specimen. The material properties of the original model are determined and calibrated from a uniaxial compression-loading test. Then, several numerical examples under different stress states are presented to illustrate the limitations of the single mode EGP model. A more general elasto-viscoplastic model is proposed, which preserves the isotropy of the original model, using the lode angle parameter to distinguish shear-dominated stress states and capture the material post yield response. The numerical treatment of the model, including the state update procedure and also the consistent tangent operator, required for the finite implementation of the model within an implicit finite element scheme, is presented. A comprehensive set of numerical examples is employed to compare the predictions of the original and new models against experimental results and to investigate the effect of the proposed modifications. The numerical results show that the proposed model provides a closer agreement with experimental evidence and opens the possibility for computational simulations of amorphous polymers under different stress states.
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| 15. |
- Moreira, Xoaquín, et al.
(författare)
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Impacts of urbanization on insect herbivory and plant defences in oak trees
- 2019
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Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 128:1, s. 113-123
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Tidskriftsartikel (refereegranskat)abstract
- Systematic comparisons of species interactions in urban versus rural environments can improve our understanding of shifts in ecological processes due to urbanization. However, such studies are relatively uncommon and the mechanisms driving urbanization effects on species interactions (e.g. between plants and insect herbivores) remain elusive. Here we investigated the effects of urbanization on leaf herbivory by insect chewers and miners associated with the English oak Quercus robur by sampling trees in rural and urban areas throughout most of the latitudinal distribution of this species. In performing these comparisons, we also controlled for the size of the urban areas (18 cities) and gathered data on CO2 emissions. In addition, we assessed whether urbanization affected leaf chemical defences (phenolic compounds) and nutritional traits (phosphorus and nitrogen), and whether such changes correlated with herbivory levels. Urbanization significantly reduced leaf chewer damage but did not affect leaf miners. In addition, we found that leaves from urban locations had lower levels of chemical defences (condensed and hydrolysable tannins) and higher levels of nutrients (nitrogen and phosphorus) compared to leaves in rural locations. The magnitude of urbanization effects on herbivory and leaf defences was not contingent upon city size. Importantly, while the effects of urbanization on chemical defences were associated with CO2 emissions, changes in leaf chewer damage were not associated with either leaf traits or CO2 levels. These results suggest that effects of urbanization on herbivory occur through mechanisms other than changes in the plant traits measured here. Overall, our simultaneous assessment of insect herbivory, plant traits and abiotic correlates advances our understanding of the main drivers of urbanization effects on plant-herbivore interactions.
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| 16. |
- Pires, Ana Elisabete, et al.
(författare)
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The curious case of the Mesolithic Iberian dogs : An archaeogenetic study
- 2019
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Ingår i: Journal of Archaeological Science. - : Elsevier BV. - 0305-4403 .- 1095-9238. ; 105, s. 116-129
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the genetic composition of six Canis remains from western Iberia, directly radiocarbon dated to 7,903-7,570 years (cal BP). They were identified as dogs via their archaeological and depositional context, osteometry, and a high percentage of aquatic diet shared with humans. For comparison, genetic data were obtained from an additional 37 Iberian dog remains from the Neolithic to Late Antiquity, as well as two Palaeolithic and a Chalcolithic Canis identified as wolves. Previous data indicated that dog mtDNA haplogroup A (HgA) is prevalent in extant European dogs ( > 50%), in the Near East and Asia, but rare or absent ( < 10%) in European Canis older than 3,000 years (cal BP). We found a high frequency (83%) of dog HgA in Mesolithic Iberian dog remains. This is the first report of a high frequency of dog HgA in pre-Neolithic Europe. We show that, contrary to the current view, Canis with HgA did not necessarily arrive in Europe from East-Asia. This phylogeographical difference in HgA frequency demonstrates that genetic differentiation was high prior to, or as a consequence of, domestication which may be linked with pre-Neolithic local processes for Iberian wolf domestication. Our results emphasize that knowledge of both ancient wolves' and early dogs' genetic profiles from the European periphery should improve our understanding of the evolution of the European dog.
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| 17. |
- ter Steege, Hans, et al.
(författare)
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Mapping density, diversity and species-richness of the Amazon tree flora
- 2023
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Ingår i: COMMUNICATIONS BIOLOGY. - 2399-3642. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- Using 2.046 botanically-inventoried tree plots across the largest tropical forest on Earth, we mapped tree species-diversity and tree species-richness at 0.1-degree resolution, and investigated drivers for diversity and richness. Using only location, stratified by forest type, as predictor, our spatial model, to the best of our knowledge, provides the most accurate map of tree diversity in Amazonia to date, explaining approximately 70% of the tree diversity and species-richness. Large soil-forest combinations determine a significant percentage of the variation in tree species-richness and tree alpha-diversity in Amazonian forest-plots. We suggest that the size and fragmentation of these systems drive their large-scale diversity patterns and hence local diversity. A model not using location but cumulative water deficit, tree density, and temperature seasonality explains 47% of the tree species-richness in the terra-firme forest in Amazonia. Over large areas across Amazonia, residuals of this relationship are small and poorly spatially structured, suggesting that much of the residual variation may be local. The Guyana Shield area has consistently negative residuals, showing that this area has lower tree species-richness than expected by our models. We provide extensive plot meta-data, including tree density, tree alpha-diversity and tree species-richness results and gridded maps at 0.1-degree resolution. A study mapping the tree species richness in Amazonian forests shows that soil type exerts a strong effect on species richness, probably caused by the areas of these forest types. Cumulative water deficit, tree density and temperature seasonality affect species richness at a regional scale.
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