Analyses of abrasive wear behaviour in pin-on-plate tribo-system for several materials

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Tropical Dry Forest Succession and the Contribution of Lianas to Wood Area Index (WAI)

The transmission and interception of light through the canopy is an important indicator of forest productivity in tropical forest ecosystems, and the amount of light that eventually reaches the forest floor is influenced by its interactions with leaves, branches, fruits, and flowers among many different canopy elements. While most studies of forest canopy light interception focus on leaf area index (LAI), very few studies have examined wood area index (WAI), which may account for a substantial component of light interception in tropical forests. The influence of lianas on the interception of light and their overall contribution to WAI is a potentially important factor, but it is generally overlooked because of its difficulty to assess. In this paper we evaluate the relative contribution that lianas have to the overall WAI and canopy openness as function of successional stage via a latitudinal comparison of sites across the Americas (Mexico, Costa Rica and Brazil). Our results suggest that lianas significantly increase WAI and decreases canopy openness. However, lianas were absent at all of our study sites where canopy openness exceeded 60%. Our data are the first to explicitly document the role of lianas in the estimation of WAI and, overall, they will contribute to better estimations of ecosystem level LAI in tropical environments, where there is a lack of data on WAI

From microlattices to 3d microprinting of multiphase micro-components: Resolution limits and mechanical properties under extreme conditions

Two-photon lithography (TPL) enables the fabrication of metamaterials such as polymer micro-lattices. They are designed to achieve their envisioned mechanical properties through stretching and bending of individual trusses. Several novel approaches are developed here to a) directly print metal microlattices, b) fabricate multiphase composite microlattices and c) shrink the truss diameter below the diffraction limit of light, all with the ultimate goal to enable fabrication of a full dense material with microprinted 3D architecture of different phases. Copper microlattices and micropillars with truss diameters in the few micron range were printed directly via fluid AFM based local electroplating [1]. It was identified that microcrystalline copper micropillars deform in a singleshear like manner exhibiting a weak strain rate dependence at all strain rates. Ultrafine grained (UFG) copper micropillars, however, deform homogenously via barreling and show strong rate-dependence and small activation volumes at strain rates up to ∼ 0.1 s−1, suggesting dislocation nucleation as the deformation mechanism. At higher strain rates, yield stress saturates remarkably, resulting in a decrease of strain rate sensitivity implying a transition in deformation mechanism to collective dislocation nucleation. Finally, the copper microlattices are shown to increase in strength if conformally coated with Nickel with thicknesses in the several 100nm range. Please click Download on the upper right corner to see the full abstract

Conservation and trade of the endangered Hypancistrus zebra (Siluriformes, Loricariidae), one of the most trafficked Brazilian fish

Abstract Hypancistrus zebra, also known as the zebra pleco, is a small sucker-mouth catfish endemic to the Xingu River in Brazil where its survival in the wild is threatened by habitat modification and overfishing for the ornamental fish industry. It is a highly sought-after freshwater ornamental species and one of the most commonly trafficked from Brazil. To date, little is known about its global legal and illicit supply chains within the ornamental fish trade. Through a mixed methods approach (i.e., online survey, key informant interviews and web scraping), we examined the trade and trafficking of this species as well as the awareness of the international aquarist community and local and international stakeholders regarding its conservation. We also establish the historical timeline of zebra pleco keeping and breeding in captivity and assess whether commercial captive breeding can play an important role in the conservation of this species. The retail price of the zebra pleco increased worldwide after an export ban in 2004 but have since decreased to an average of $US 155 (+/-$US 23 based on geographical location) per fish. Fishermen have been consistently paid relatively little ($US 7–60) for each specimen compared to the average wholesale price of$US 100 (+/- $US 94 over time). We conservatively estimate ~100,000 specimens are trafficked out of Brazil annually, of which half or more die in transport, and only a small fraction is seized by law enforcement in Brazil or internationally. The fishes are primarily smuggled from Brazil to Peru and Colombia and then exported internationally with the majority sent to China. The majority of aquarists surveyed (representing 35 countries) were aware the zebra pleco is both endangered and highly endemic. There was less awareness that buying wild caught specimens shipped from Peru, Colombia or elsewhere implies supporting wildlife trafficking. Nevertheless, nearly three quarters of respondents preferred aquarium bred specimens, if available. The zebra pleco is being bred in captivity in high numbers in several countries, yet in Brazil it remains illegal to keep in private aquaria or to commercially breed them. Given the large success of hobby and commercial breeders around the world, H. zebra is well suited for indoor breeding facilities. We argue that implementing regulated local breeding facilities in Brazil to increase the already large numbers reproduced in captivity worldwide, could decrease the demand for trafficked specimens, one of the primary factors threatening its survival. Given its iconic status among freshwater fishes it should be recognized as a flagship species of the Xingu River's conservation

Extruded and injection moulded virgin PA 6/6 as abrasion resistant material

Polyamide (PA6/6) is often used as a tribological pair in abrasion prevalent applications such as hinges and sliders. PA6/6 is frequently processed by injection moulding and extrusion process. It is known that these processes influence the polymers mechanical behaviour, but their influence on the polymers wear response has not been studied. Hence the present research attempts to study the influence of different manufacturing processes on tribological behaviour for PA6/6. Wear tests were performed on a pin abrading tester (DIN 50322). Abrasion resistance of both extruded and injection moulded PA6/6 were tested at different loads (20 and 35 N). Single-pass (nonoverlapping mode) and multipass testing (overlapping mode) were used to understand the influence of clogging of wear debris. It is evidenced that with increasing load the specific wear rate decreases; moreover, fine abrasives tend to reduce the wear rate. In multipass testing a transfer layer clogged on the counterface that acted as a protective agent and lowers wear rate. Poor mechanical strength of injection moulded polymers is apparently compensated by microstructural response for having a similar wear behaviour between extruded and injection moulded PA 6/6. Hence a proper balance between microstructural and mechanical characteristics is an absolute must in PA 6/6 for better wear performance

Plastic deformation of microsamples: Intermittent dislocation avalanches and their acoustic emission

On the micrometer scale, deformation properties of metals change profoundly: the smooth and continuous behavior of bulk materials is often replaced by jerky flow due to random strain bursts of various sizes. The reason for this behavior is the complex intermittent redistribution of lattice dislocations due to external loading. This process also leads to the formation of the uneven step-like surface upon deformation. Our highly sensitive micromechanical platform can detect the strain bursts caused by dislocation avalanches in three different ways: (i) by stress and strain measurements using a capacitive displacement sensor measuring the elongation of a spring, (ii) by detection of the emitted acoustic signal using a sensitive piezoelectric transducer and (iii) by visual images using the electron beam of the SEM. In my presentation, I will present two of our recent results obtained with the help of this toolbox. Please click Download on the upper right corner to see the full abstract

In situ 3D mapping of local stress and crystal defect structures during micro-mechanical testing by n3D-XRD-CT

Crystal orientation and local elastic strain/stress evolution were mapped in 3D during in situ loading of single and bi-crystalline metallic micromechanical testpieces. The novel application of the nano-beam 3D-XRD computed tomography technique in micromechanics (~150 nm spatial resolution) at ESRF ID11, enabled the measurement of the 3D arrangement of the geometrically necessary dislocation (GND) density and the local stress states in pure metal and alloy model systems (FCC, BCC & HCP) loaded in both microcompression (Cu, 316L, Mo) and microtension geometries (Mg). Post-mortem TEM analysis provides insights into the total number and real distribution of dislocations in the system, to better understand the meaningfulness of the measured GND arrays, thought to be characteristic of strain transfer at the bicrystal boundaries. 3D-XRD datasets were compared to ex-situ FIB 3D-HR-EBSD slice-and-scan, seeking to learn more about the progressive accumulation of dislocations and useful plastic strain, relative to local stress states during loading

Estimation of the Distribution of Tabebuia guayacan (Bignoniaceae) Using High-Resolution Remote Sensing Imagery

Species identification and characterization in tropical environments is an emerging field in tropical remote sensing. Significant efforts are currently aimed at the detection of tree species, of levels of forest successional stages, and the extent of liana occurrence at the top of canopies. In this paper we describe our use of high resolution imagery from the Quickbird Satellite to estimate the flowering population of Tabebuia guayacan trees at Barro Colorado Island (BCI), in Panama. The imagery was acquired on 29 April 2002 and 21 March 2004. Spectral Angle Mapping via a One-Class Support Vector machine was used to detect the presence of 422 and 557 flowering tress in the April 2002 and March 2004 imagery. Of these, 273 flowering trees are common to both dates. This study presents a new perspective on the effectiveness of high resolution remote sensing for monitoring a phenological response and its use as a tool for potential conservation and management of natural resources in tropical environments

Orientation, temperature and strain rate effects in deformation twinning of magnesium

Deformation twinning (DT) has a very important role in accommodating plastic deformation in hexagonal close packed (HCP) metals due to their limited number of easy slip systems. Unfortunately, DT is also known to be associated with a lack of ductility and high residual stresses, which can lead to cracking, affecting the industrial applicability of these metals and making them only partially competitive compared to other light metal alloys. Despite decades of research on DT, many questions remains open about their exact nucleation and propagation mechanisms and their associated defects and stresses in the material. Focusing the study on the {1012} twin mode, we performed in situ tensile and compression tests at the micron-scale on suitably oriented single crystal pure magnesium over 7 order of magnitude of strain rate (from 10 -4 to 500 s-1) and at different temperatures (from 293 to 573 K). 3D HR-EBSD is used to characterize the shape of the twins as well as the distribution of the residual stresses and the geometrically necessary dislocation (GND) inside the deformed volume of the material. TEM is used to characterize the activated dislocations and twin plane features for the different deformation conditions. Please click Download on the upper right corner to see the full abstract

Peatland leaf-area index and biomass estimation with ultra-high resolution remote sensing

There is fine-scale spatial heterogeneity in key vegetation properties including leaf-area index (LAI) and biomass in treeless northern peatlands, and hyperspectral drone data with high spatial and spectral resolution could detect the spatial patterns with high accuracy. However, the advantage of hyperspectral drone data has not been tested in a multi-source remote sensing approach (i.e. inclusion of multiple different remote sensing datatypes); and overall, sub-meter-level leaf-area index (LAI) and biomass maps have largely been absent. We evaluated the detectability of LAI and biomass patterns at a northern boreal fen (Halssiaapa) in northern Finland with multi-temporal and multi-source remote sensing data and assessed the benefit of hyperspectral drone data. We measured vascular plant percentage cover and height as well as moss cover in 140 field plots and connected the structural information to measured aboveground vascular LAI and biomass and moss biomass with linear regressions. We predicted both total and plant functional type (PFT) specific LAI and biomass patterns with random forests regressions with predictors including RGB and hyperspectral drone (28 bands in a spectral range of 500-900 nm), aerial and satellite imagery as well as topography and vegetation height information derived from structure-from-motion drone photogrammetry and aerial lidar data. The modeling performance was between moderate and good for total LAI and biomass (mean explained variance between 49.8 and 66.5%) and variable for PFTs (0.3-61.6%). Hyperspectral data increased model performance in most of the regressions, usually relatively little, but in some of the regressions, the inclusion of hyperspectral data even decreased model performance (change in mean explained variance between -14.5 and 9.1%-points). The most important features in regressions included drone topography, vegetation height, hyperspectral and RGB features. The spatial patterns and landscape estimates of LAI and biomass were quite similar in regressions with or without hyperspectral data, in particular for moss and total biomass. The results suggest that the fine-scale spatial patterns of peatland LAI and biomass can be detected with multi-source remote sensing data, vegetation mapping should include both spectral and topographic predictors at sub-meter-level spatial resolution and that hyperspectral imagery gives only slight benefits.Peer reviewe