173,436 research outputs found

    Development of a phenomenological constitutive model for fracture resistance degradation of asphalt concrete with damage growth due to repeated loading

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    Discontinuous areas under the asphalt concrete (AC) layer, such as joints and cracks in an underlying layer, induce higher stress concentration than the designed strength. Stress concentration in the vicinity of discontinuities accelerates distress on the AC layer. Repeated traffic and environmental loading applied to the AC layer also induce degradation of the layer’s strength as microcracks grow at stress levels lower than the layer’s designed strength. In addition, this could be magnified when combined with low temperature cracking, one of the main distresses in AC pavements resulting from extreme temperature changes. When loading is applied near the joints or discontinuity, it amplifies the tensile stress at the bottom of the AC layer as well as the shear stress when the Portland cement concrete (PCC) slab or discontinuity moves vertically. Repetitive traffic loading and environmental changes cause continuous damage accumulations which consequently results in the acceleration of movement in the AC layer at the localized area close to the discontinuity region, thus leading to mechanical degradation of the AC materials which become less resistant to fracture. Even a small load can result in fracture failure of AC pavements when the loss of strength in AC pavements progresses significantly through repeated loading. The current approach to determine the critical properties of AC materials is to conduct laboratory testing under monotonic loading and cyclic loading separately. The fatigue testing under cyclic loading can only provide bulk material properties without consideration to any discontinuities, such as cracks in underlying pavement or joints. On the other hand, the current fracture tests conducted under monotonic loading fail to capture the loss of material strength as repeated loading is applied on pavements. For an accurate estimation of pavement life, it is essential to consider the effect of repeated traffic and thermal loading on the fracture resistance of the AC materials. This study investigates the degradation of the fracture resistance of AC materials as a result of the progressive damages caused by repeated loading application. The study develops the phenomenological constitutive model for fracture resistance degradation with damage growth caused by repeated loading. An experimental program was designed to apply monotonic and cyclic loading to the same test geometry and to examine the degradation of fracture properties with damage growth at the crack tip. Fracture and fatigue tests were implemented using semi-circular bending (SCB) test geometry with notched specimens at various temperatures, loading frequencies, and loading amplitudes. It is observed that damage functions and proposed parameters reflect the degradation rate of fracture resistance with respect to damage growth at the notch tip region. A presented constitutive model accurately predicts the remaining service life of existing pavements. It is further observed that the model coefficient distinguishes AC materials in terms of sensitivity to cracking resistance under both monotonic and cyclic loading.U of I OnlyGraduate College 2-year Extension For

    The JCMT BISTRO Survey: Multi-wavelength polarimetry of bright regions in NGC 2071 in the far-infrared/submillimetre range, with POL-2 and HAWC+

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    Polarized dust emission is a key tracer in the study of interstellar medium and of star formation. The observed polarization, however, is a product of magnetic field structure, dust grain properties and grain alignment efficiency, as well as their variations in the line of sight, making it difficult to interpret polarization unambiguously. The comparison of polarimetry at multiple wavelengths is a possible way of mitigating this problem. We use data from HAWC+/SOFIA and from SCUBA-2/POL-2 (from the BISTRO survey) to analyse the NGC 2071 molecular cloud at 154, 214 and 850 μm. The polarization angle changes significantly with wavelength over part of NGC 2071, suggesting a change in magnetic field morphology on the line of sight as each wavelength best traces different dust populations. Other possible explanations are the existence of more than one polarization mechanism in the cloud or scattering from very large grains. The observed change of polarization fraction with wavelength, and the 214-to-154 μm polarization ratio in particular, are difficult to reproduce with current dust models under the assumption of uniform alignment efficiency. We also show that the standard procedure of using monochromatic intensity as a proxy for column density may produce spurious results at HAWC+ wavelengths. Using both long-wavelength (POL-2, 850 μm) and short-wavelength (HAWC+, ≲200μm) polarimetry is key in obtaining these results. This study clearly shows the importance of multi-wavelength polarimetry at submillimeter bands to understand the dust properties of molecular clouds and the relationship between magnetic field and star formation

    Investigation of wear mechanisms in AlSi-polyester abradable - Ti(6Al4V) blade contacts using stroboscopic imaging

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    Abradable linings in aero engines have been an area of research interest over the past few decades as small reductions in clearances between stationary and rotating parts can lead to large increases in engine efficiency. The work performed in this article focuses on characterising the blade wear behaviour in contacts between Ti (6Al 4V) blades and AlSi-polyester abradables. This was done by performing three abrasion tests on the new test rig developed at the University of Sheffield. Tests have been performed on the AlSi-polyester abradables of the same nominal hardness over two incursion rates – 0.02μm/pass and 0.2 μm/pass and two blade tip speeds – 85 m/s and 170 m/s. The front-on stroboscopic imaging technique was used for these tests, which allowed capturing images of the entire blade front for a number of blade strikes during a test. It was found that at the incursion rate of 0.02μm/pass, both adhesions to the blade surface and blade wear were observed across the blade width. It was observed that adhesions were more likely to gradually wear off rather than fracture at 0.02μm/pass, and, fracture at 0.2 μm/pass. Tested surface profiles were obtained using an Alicona non-contact measurement system. This allowed the comparison of the blade profile results from the blade images to the surface of the respective tested abradable sample. It was concluded that adhesions that fractured could contribute to the localized gaps between the final blade and the final abradable surface where such adhesions have fractured close to the end of a test. Further testing areas have been identified such as the investigation into the effects of parameters such as incursion rate of a blade into an abradable, blade tip speed and abradable hardness on the results. The developed front-on imaging system also opened a possibility to investigate in-situ the rub performance of blades of varying tip geometry

    Ionic Liquids on Oxide Surfaces

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    Ionic liquids supported on oxide surfaces are being investigated for numerous applications including catalysis, batteries, capacitors, transistors, lubricants, solar cells, corrosion inhibitors, nanoparticle synthesis and biomedical applications. The study of ionic liquids with oxide surfaces presents challenges both experimentally and computationally. The interaction between ionic liquids and oxide surfaces can be rather complex, with defects in the oxide surface playing a key role in the adsorption behaviour and resulting electronic properties. The choice of the cation/anion pair is also important and can influence molecular ordering and electronic properties at the interface. These controllable interfacial behaviours make ionic liquid/oxide systems desirable for a number of different technological applications as well as being utilised for nanoparticle synthesis. This topical review aims to bring together recent experimental and theoretical work on the interaction of ionic liquids with oxide surfaces, including TiO2, ZnO, Al2O3, SnO2 and transition metal oxides. It focusses on the behaviour of ionic liquids at model single crystal surfaces, the interaction between ionic liquids and nanoparticulate oxides, and their performance in prototype devices

    Stratigraphic record of continental breakup, offshore NW Australia

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    Continental breakup involves a transition from rapid, fault-controlled syn-rift subsidence to relatively slow, post-breakup subsidence induced by lithospheric cooling. Yet the stratigraphic record of many rifted margins contain syn-breakup unconformities, indicating that episodes of uplift and erosion interrupt this transition. This uplift has been linked to mantle upwelling, depth-dependent extension and/or isostatic rebound. Deciphering the breakup processes recorded by these unconformities and their related rock record is challenging because uplift-associated erosion commonly removes the strata that help constrain the onset and duration of uplift. We examine three major breakup-related unconformities and the intervening rock record in the Lower Cretaceous succession of the Gascoyne and Cuvier margins, offshore NW Australia, using seismic reflection and borehole data. These data show the breakup unconformities are disconformable (non-erosive) in places and angular (erosive) in others. Our recalibration of palynomorph ages from rocks underlying and overlying the unconformities shows: (i) the lowermost unconformity developed between 134.98–133.74 Ma (Intra-Valanginian), probably during the localisation of magma intrusion within continental crust and consequent formation of continent–ocean transition zones (COTZ); (ii) the middle unconformity formed between ca. 134 and 133 Ma (Top Valanginian), possibly coincident with breakup of continental crust and generation of new magmatic (but not oceanic) crust within the COTZs; and (iii) the uppermost unconformity likely developed between ca. 132.5 and 131 Ma (i.e. Intra-Hauterivian), coincident with full continental lithospheric breakup and the onset of seafloor spreading. During unconformity development, uplift was focussed along the continental rift flanks, likely reflecting flexural bending of the crust and landward flow of lower crust and/or lithospheric mantle from beneath areas of localised extension towards the continent (i.e. depth-dependent extension). Our work supports the growing consensus that the ‘breakup unconformity’ is not always a single stratigraphic surface marking the onset of seafloor spreading; multiple unconformities may form and reflect a complex history of uplift and subsidence during continent–ocean transition

    In search of 'The people of La Manche': A comparative study of funerary practices in the Transmanche region during the late Neolithic and Early Bronze Age (250BC-1500BC)

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    This research project sets out to discover whether archaeological evidence dating between 2500 BC - 1500 BC from supposed funerary contexts in Kent, flanders and north-eastern Transmanche France is sufficient to make valid comparisons between social and cultural structures on either side of the short-sea Channel region. Evidence from the beginning of the period primarily comes in the form of the widespread Beaker phenomenon. Chapter 5 shows that this class of data is abundant in Kent but quite sparse in the Continental zones - most probably because it has not survived well. This problem also affects the human depositional evidence catalogued in Chapter 6, particularly in Fanders but also in north-eastern Transmanche France. This constricts comparative analysis, however, the abundant data from Kent means that general trends are still discernible. The quality and volume of data relating to the distribution, location, morphology and use of circular monuments in all three zones is far better - as demonstrated in Chapter 7 -mostly due to extensive aerial surveying over several decades. When the datasets are taken as a whole, it becomes possible to successfully apply various forms of comparative analyses. Most remarkably, this has revealed that some monuments apparently have encoded within them a sophisticated and potentially symbolically charged geometric shape. This, along with other less contentious evidence, demonstrates a level of conformity that strongly suggests a stratum of cultural homogeneity existed throughout the Transmanche region during the period 2500 BC - 1500 BC. The fact that such changes as are apparent seem to have developed simultaneously in each of the zones adds additional weight to the theory that contact throughout the Transmanche region was endemic. Even so, it may not have been continuous; there may actually have been times of relative isolation - the data is simply too course to eliminate such a possibility

    Factors influencing the ground thermal regime in a mid-latitude glacial cirque (Hoyo Empedrado, Cantabrian Mountains, 2006–2020)

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    .Air and near-surface ground temperatures were measured using dataloggers over 14 years (2006–2020) in 10 locations at 2262 to 2471 m.a.s.l. in a glacial cirque of the Cantabrian Mountains. These sites exhibit relevant differences in terms of substrate, solar radiation, orientation, and geomorphology. Basal temperature of snow (BTS) measurements and electrical resistivity tomography of the talus slope were also performed. The mean annual near-surface ground temperatures ranged from 5.1 °C on the sunny slope to 0.2 °C in the rock glacier furrow, while the mean annual air temperature was 2.5 °C. Snow cover was inferred from near-surface ground temperature (GST) data, estimating between 130 and 275 days per year and 0.5 to 7.1 m snow thickness. Temperature and BTS data show that the lowest part of the talus slope and the rock glacier furrow are the coldest places in this cirque, coinciding with a more persistent and thickest snow cover. The highest temperatures coincide with less snow cover, fine-grained soils, and higher solar radiation. Snow cover has a primary role in controlling GST, as the delayed appearance in autumn or delayed disappearance in spring have a cooling effect, but no correlation with mean annual near-surface ground temperatures exists. Heavy rain-over-snow events have an important influence on the GST. In the talus slope, air circulation during the snow-covered period produces a cooling effect in the lower part, especially during the summer. Significant inter-annual GST differences were observed that exhibited BTS limitations. A slight positive temperature trend was detected but without statistically significance and less prominent than nearby reference official meteorological stations, so topoclimatic conditions reduced the more global positive temperature trend. Probable existence of permafrost in the rock glacier furrow and the lowest part of the talus slope is claimed; however, future work is necessary to confirm this aspect.S

    Speciation of organoarsenicals in aqueous solutions by Raman spectrometry and quantum chemical calculations

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    .Knowledge about the existence and stability of different species of organoarsenicals in solution is of the most significant interest for fields so different as chemical, environmental, biological, toxicological and forensic. This work provides a comparative evaluation of the Raman spectra of four organoarsenicals (o-arsanilic acid, p-arsanilic acid, roxarsone and cacodylic acid) in aqueous solutions under acidic, neutral and alkaline conditions. Speciation of some of these organoarsenicals is possible by Raman spectrometry at different selected pHs. Further, we examine the proficiency of computational chemistry to obtain the theoretical Raman spectra of the four organoarsenicals compounds. To this end, we employ a computational protocol that includes explicit water molecules and conformational sampling, finding that the calculated organoarsenicals spectra agree reasonably well with those experimentally obtained in an aqueous solution in the whole pH range covered. Finally, we highlight the effectiveness of quantum chemical calculations to identify organoarsenicals in an aqueous solution.S
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