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Critical Loads (critical + load)
Selected AbstractsDeveloping a critical load approach for national risk assessments of atmospheric metal deposition,ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 3 2006Jane R. Hall Abstract The critical load approach has been proposed for evaluation of the need to reduce atmospheric emissions of metals that lead to transboundary transport and deposition across Europe. The present study demonstrates and evaluates the application of a critical load approach for national-scale risk assessment of metal deposition in the United Kingdom. Critical load maps, calculated using critical limits based on pH-dependent free metal ion activities, are presented. Current concentrations of lead and cadmium in soils are compared with two sets of critical limit values: First, limits based on the reactive soil concentration, and second, a pH-dependent free ion critical limit function, which takes into account variable soil characteristics across the country. The use of these two models leads to different conclusions about which areas of the United Kingdom are at greatest risk, partly because of differences in the range of values of pH and organic matter in soils used in ecotoxicological experiments and in the national database. Critical loads were calculated based on free ion critical limits; the critical loads were lowest in the south and east of the country and were associated with higher soil pH, lower runoff, and lower soil organic matter. [source] Role of substrate material in failure of crown-like layer structures,JOURNAL OF BIOMEDICAL MATERIALS RESEARCH, Issue 2 2007Jae-Won Kim Abstract The role of substrate modulus on critical loads to initiate and propagate radial cracks to failure in curved brittle glass shells on compliant polymeric substrates is investigated. Flat glass disks are used to drive the crack system. This configuration is representative of dental crown structures on dentin support in occlusal contact. Specimens are fabricated by truncating glass tubes and filling with epoxy-based substrate materials, with or without alumina filler for modulus control. Moduli ranging from 3 to 15 GPa are produced in this way. Critical loads for both initiation and propagation to failure increase monotonically with substrate modulus, by a factor of two over the data range. Fracture mechanics relations provide a fit to the data, within the scatter bands. Finite element analysis is used to determine stress distributions pertinent to the observed fracture modes. It is suggested that stiffer substrate materials offer potential for improved crown lifetime in dental practice. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2006 [source] Effect of Flaw State on the Strength of Brittle Coatings on Soft SubstratesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2001Hae-Won Kim A study is made of the role of flaw state on the strength properties of brittle ceramic coating layers bonded to soft polycarbonate substrates. We introduce Vickers radial cracks at prescribed loads into the coating undersurfaces prior to bonding to control the sizes and locations of the starting flaws. A spherical indenter is then loaded on the top bilayer surfaces, directly above the Vickers indentation sites, subjecting the radial cracks to flexural tensile stress. Radial crack responses are monitored in situ, using a camera located below the transparent substrate. Critical loads to cause radial crack instability, and ensuing growth of the arrested cracks, are recorded. Conventional biaxial flexure tests on corresponding monolith coating materials provide a baseline for data comparison. Relative to the monolith flexure specimens, the bilayers show higher strengths, the more so the larger the flaw, indicating enhanced flaw tolerance. A simple fracture mechanics analysis of the radial crack evolution in the concentrated-load field, with due account for distribution of flexural tensile stresses at the coating undersurface, is unable to account completely for the enhanced bilayer strengths for the larger Vickers flaws. It is hypothesized that the epoxy used to bond the bilayer components enters the cracks, causing crack-wall adherence and providing an increased resistance to radial crack instability. The fracture mechanics are nevertheless able to account for the arrest and subsequent stable extension of the radial cracks beyond the critical loads once this extraneous adherence has been overcome. [source] Adhesive, Flexible, and Robust Polysaccharide Nanosheets Integrated for Tissue-Defect RepairADVANCED FUNCTIONAL MATERIALS, Issue 16 2009Toshinori Fujie Abstract Recent developments in nanotechnology have led to a method for producing free-standing polymer nanosheets as a macromolecular organization. Compared with bulk films, the large aspect ratio of such nanosheets leads to unique physical properties, such as transparency, noncovalent adhesion, and high flexibility. Here, a biomedical application of polymer nanosheets consisting of biocompatible and biodegradable polysaccharides is reported. Micro-scratch and bulge tests indicate that the nanosheets with a thickness of tens of nanometers have sufficient physical adhesiveness and mechanical strength for clinical use. A nanosheet of 75,nm thickness, a critical load of 9.1,×,104,N m,1, and an elastic modulus of 9.6,GPa is used for the minimally invasive repair of a visceral pleural defect in beagle dogs without any pleural adhesion caused by wound repair. For the first time, clinical benefits of sheet-type nano-biomaterials based on molecular organization are demonstrated, suggesting that novel therapeutic tools for overlapping tissue wounds will be possible without the need for conventional surgical interventions. [source] A study of the epiphytic communities of Atlantic oak woods along an atmospheric nitrogen deposition gradientJOURNAL OF ECOLOGY, Issue 3 2005R.J. MITCHELL Summary 1Atlantic oak woods are of high conservation value and contain many rare lichens and bryophytes. The effects of nitrogen pollution on these epiphytic communities have not been previously studied. We investigated the composition of Atlantic oak wood epiphytic communities in relation to atmospheric N deposition in order to identify N indicator species and propose a critical load for such communities. 2The epiphytic communities of seven Atlantic oak woods receiving estimated total nitrogen deposition in the range 10,53 kg N ha,1 year,1 were surveyed. Exposure of epiphytes to atmospheric N over 7 months was measured in terms of stemflow flux and concentration and airborne NH3. 3Redundancy analysis (RDA) related the species to stemflow chemistry and bark pH. Different groups of species were found at (i) a coastal low N deposition site (Graphina ruiziana, Lecania cyrtella, Lobaria pulmonaria, Opegrapha atra, Orthotrichum affine, Melanelia fuliginosa ssp. glabratula and Pertusaria hymenea), (ii) inland low N deposition sites (I. myosuroides, F. tamarisci, Plagiochila atlantica, Cladonia chlorophaea, C. squamosa, Hypotrachyna laevigata and Thelotrema lepadinum) and (iii) high N deposition sites (Hypnun andoi, Hypnum cupressiforme, Calicium viride, Chrysothrix candelaris, C. coniocraea, Hypogymnia physodes, Parmelia saxatilis, Phyllospora rosei and Usnea subfloridana). 4RDA analysis showed that bark pH and concentration in the stemflow explained the greatest amount of variation in the species composition among the sites. The sites separated into two groups (low and high concentration) leading to an estimate for the critical load for N deposition for epiphytes in Atlantic oak woods of 11,18 kg N ha,1 year,1. 5Analysis of the occurrence of species against N levels suggested that I. myosuroides and F. tamarisci are indicators of small N inputs, whereas H. andoi, C. coniocraea, P. saxatilis and H. physodes are tolerant of increased N loads. 6Different forms of N (, , NH3) were found to significantly affect the occurrence of different species. Some species were sensitive to total flux while others were sensitive to concentration. [source] Investigation into the traction coefficient in elastohydrodynamic lubricationLUBRICATION SCIENCE, Issue 2 2004Y. S. Wang Abstract The elastohydrodynamic traction coefficients of two Chinese aviation lubricating oils were investigated for various loads, rolling velocities, and lubricant inlet temperatures using a self-made test rig. Traction coefficient versus slide-to-roll ratio curves were generated. The concept of critical load varying with the lubricant temperature is proposed. This paper presents a new empirical formula for the dynamic performance design of high-speed rolling bearings, that relates traction coefficient with normal load, rolling velocity, and lubricant inlet temperature. The coefficients of the formula may be computed by regression analysis of the experimental data. Two example calculations are presented. The predicted results from the formula agree well with experimental observations. [source] Use of Single-Walled Carbon Nanotubes as Reinforcing Fillers in UV-Curable Epoxy SystemsMACROMOLECULAR MATERIALS & ENGINEERING, Issue 8 2008Marco Sangermano Abstract CNT were dispersed in an epoxy matrix and cured by means of UV light. An increase on elastic modulus and Tg values was measured by DMTA analysis and attributed to the constraint effect of CNT on polymer chain mobility. Excellent scratch resistant coatings characterized by high critical load, small cracks and high recovery were obtained in the presence of a very low CNT content (0.025 wt.-%). TEM analysis showed some isolated CNT and some cluster agglomerations of size of about 250 nm. It was shown that it was possible to decrease the surface resistivity of the cured samples by three orders of magnitude in the presence of 0.1 wt.-% of SWCNT content. [source] Nutritional constraints in ombrotrophic Sphagnum plants under increasing atmospheric nitrogen deposition in EuropeNEW PHYTOLOGIST, Issue 3 2004Luca Bragazza Summary ,,We studied the effects of increasing levels of atmospheric nitrogen (N) deposition on nutrient limitation of ombrotrophic Sphagnum plants. ,,Fifteen mires in 11 European countries were selected across a natural gradient of bulk atmospheric N deposition from 0.1 to 2 g/m2 year,1. Nutritional constraints were assessed based on nutrient ratios of N, phosphorus (P), and potassium (K) in Sphagnum plants collected in hummocks (i.e. relatively drier microhabitats) and in lawns (i.e. relatively wetter microhabitats). ,,Nutrient ratios in Sphagnum plants increased steeply at low atmospheric N input, but above a threshold of N deposition of c. 1 g/m2 year,1 the N : P and N : K ratios tended to saturation. Increasing atmospheric N deposition was also accompanied by a reduced retention of Ca and Mg in Sphagnum plants and a decreased stem volumetric density in hummock Sphagnum plants. ,,We suggest a critical load of N deposition in Europe of 1 g/m2 year,1 above which Sphagnum plants change from being N-limited to be K + P colimited, at N : P > 30 and N : K > 3. [source] Physiological and growth responses of the montane bryophyte Racomitrium lanuginosum to atmospheric nitrogen depositionNEW PHYTOLOGIST, Issue 1 2003I. S. K. Pearce Summary ,,The effects of nitrogen (N) deposition on the moss Racomitrium lanuginosum within montane heath in Scotland were investigated over 5 yr. ,,Permanent field plots were sprayed with KNO3 or NH4Cl solutions, at doses equivalent to 10 and 40 kg N ha,1 yr,1, in 3,6 applications each summer. ,,Racomitrium growth and cover were severely reduced by N addition, whilst the proportion of dead shoots greatly increased. N dose decreased inducibility of shoot nitrate reductase activity (NRA), suggesting that N saturation of Racomitrium occurred, and caused an increase in potassium leakage. At high dosage, effects of NH4+ were more detrimental than NO3,. ,,Physiological responses to N indicate that the habitat's critical load (CL) is exceeded by addition of 10 kg N ha,1 yr,1. The differential toxicity of the two forms of N suggests that predominant ion type in deposition should be taken into consideration when CLs are set. In contrast to tissue N, NRA correlated well with shoot growth, and may thus be a useful biological indicator of moss condition. [source] Critical buckling load of paper honeycomb under out-of-plane pressure,PACKAGING TECHNOLOGY AND SCIENCE, Issue 3 2005Li-Xin Lu Abstract Two out-of-plane buckling criteria for paper honeycomb are proposed by analysing the structure properties and the collapse mechanism of paper honeycomb: these are based on the peeling strength and ring crush strength of the chipboard wall. Taking into account the orthotropic, initial deflection and large deflection properties of the chipboard wall, the two new mechanical models and the calculation methods are developed to represent the out-of-plane critical load of paper honeycomb. Theoretical calculations and test results show that the models are suitable for describing the collapse mechanism of paper honeycomb. The peeling strength and ring crush strength determine the critical buckling load of paper honeycomb in different stretch phases. The out-of-plane critical buckling load can be predicted when the two models are integrated. Copyright © 2005 John Wiley & Sons, Ltd. [source] Developing a critical load approach for national risk assessments of atmospheric metal deposition,ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 3 2006Jane R. Hall Abstract The critical load approach has been proposed for evaluation of the need to reduce atmospheric emissions of metals that lead to transboundary transport and deposition across Europe. The present study demonstrates and evaluates the application of a critical load approach for national-scale risk assessment of metal deposition in the United Kingdom. Critical load maps, calculated using critical limits based on pH-dependent free metal ion activities, are presented. Current concentrations of lead and cadmium in soils are compared with two sets of critical limit values: First, limits based on the reactive soil concentration, and second, a pH-dependent free ion critical limit function, which takes into account variable soil characteristics across the country. The use of these two models leads to different conclusions about which areas of the United Kingdom are at greatest risk, partly because of differences in the range of values of pH and organic matter in soils used in ecotoxicological experiments and in the national database. Critical loads were calculated based on free ion critical limits; the critical loads were lowest in the south and east of the country and were associated with higher soil pH, lower runoff, and lower soil organic matter. [source] Fracture behaviour of cracked carbon nanotube-based polymer composites: Experiments and finite element simulationsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 2 2010Y. KURONUMA ABSTRACT This paper studies the fracture behaviour of cracked carbon nanotube (CNT)-based polymer composites by a combined numerical,experimental approach. Tensile tests were conducted on single-edge cracked plate specimens of CNT/polycarbonate composites at room temperature and liquid nitrogen temperature (77 K), and the critical loads for fracture instabilities were determined. Elastic,plastic finite element simulations of the tests were then performed to evaluate the,J -integrals corresponding to the experimentally determined critical loads. Scanning electron microscopy examinations were also made on the specimen fracture surfaces, and the fracture mechanisms of the CNT-based composites were discussed. [source] On evaluation of shape sensitivities of non-linear critical loadsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 6 2003E. Parente Jr. Abstract The present paper focuses on the evaluation of the shape sensitivities of the limit and bifurcation loads of geometrically non-linear structures. The analytical approach is applied for isoparametric elements, leading to exact results for a given mesh. Since this approach is difficult to apply to other element types, the semi-analytical method has been widely used for shape sensitivity computation. This method combines ease of implementation with computational efficiency, but presents severe accuracy problems. Thus, a general procedure to improve the semi-analytical sensitivities of the non-linear critical loads is presented. The numerical examples show that this procedure leads to sensitivities with sufficient accuracy for shape optimization applications. Copyright © 2002 John Wiley & Sons, Ltd. [source] A numerical estimation of the effects of a cylindrical hole and imperfect bonding on stability of a fibre in an elastic matrixINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 6 2001Yuri N. Lapusta Abstract A simple numerical model based on three-dimensional analytical considerations is proposed for an estimation of the local effects of a cylindrical hole in the matrix as well as of fibre,matrix interface imperfections on compressive stability of fibres in fibre-reinforced composites. The geometry of the model includes an inclusion (a fibre) in a matrix with the assumption of an imperfect bonding at the interface and a cylindrical hole in the closest neighbourhood. Then the problem formulation is idealized in two directions, providing a possibility of establishing lower and upper bounds for critical loads for the case of a matrix with a cylindrical hole of a non-circular cross-section as well as for the case of interface imperfections with the possibility of sliding without discontinuity of the displacements normal to the interface. The model takes into account the distinct difference in the properties of the fibre and the matrix and the spatial character of the problem at the microlevel. It is based on individual consideration of the fibre and the matrix with a hole with the necessity of satisfying certain idealized bonding conditions between them. Copyright © 2001 John Wiley & Sons, Ltd. [source] Role of substrate material in failure of crown-like layer structures,JOURNAL OF BIOMEDICAL MATERIALS RESEARCH, Issue 2 2007Jae-Won Kim Abstract The role of substrate modulus on critical loads to initiate and propagate radial cracks to failure in curved brittle glass shells on compliant polymeric substrates is investigated. Flat glass disks are used to drive the crack system. This configuration is representative of dental crown structures on dentin support in occlusal contact. Specimens are fabricated by truncating glass tubes and filling with epoxy-based substrate materials, with or without alumina filler for modulus control. Moduli ranging from 3 to 15 GPa are produced in this way. Critical loads for both initiation and propagation to failure increase monotonically with substrate modulus, by a factor of two over the data range. Fracture mechanics relations provide a fit to the data, within the scatter bands. Finite element analysis is used to determine stress distributions pertinent to the observed fracture modes. It is suggested that stiffer substrate materials offer potential for improved crown lifetime in dental practice. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2006 [source] Impacts of increased nitrogen supply on Norwegian lichen-rich alpine communities: a 10-year experimentJOURNAL OF ECOLOGY, Issue 3 2005ELI FREMSTAD Summary 1Species cover was tested during a 10-year fertilization experiment in the low-alpine Cetrarietum nivalis community and the middle-alpine Phyllodoco-Juncetum trifidi community in the Dovre mountains of south-central Norway. Nitrogen was added at 7, 35 and 70 kg N ha,1 year,1, with the highest dose corresponding to approximately 3.5 times the annual deposition in south-west Norway. 2Both communities are dominated by lichens (Cladonia spp. and Cetraria spp., respectively), have a patchy structure and are ,conservative' as regards species content. 3Lichens, which showed a decrease in cover and size, and after some years developed discoloured thalli, are the best organisms for monitoring changes in alpine vegetation that is exposed to increased nitrogen deposition. The most sensitive species in Cetrarietum nivalis appeared to be Alectoria nigricans and Cetraria ericetorum, but more abundant species (Cladonia mitis, C. stellaris and Cetraria nivalis) are likely to be more reliable indicators. Cetraria delisei seems to be a reliable indicator species for monitoring in Phyllodoco-Juncetum trifidi. 4Fertilization had no significant effect on the vascular plants (dwarf shrubs and a few graminoids) in either community, except for Festuca ovina, the cover of which increased slightly. 5Nitrogen pollution may affect oligotrophic, alpine communities differently, depending on their species composition and horizontal structure (patchiness). 6It is suggested that other factors, such as climate, soil properties and community structure, may be more important than long-range nitrogen pollution for determining species composition and species cover in many of the oligotrophic, alpine communities in southern Norway. However, in lichen-rich communities, critical loads have already been exceeded in the most polluted areas of south-west Norway. [source] Effect of Flaw State on the Strength of Brittle Coatings on Soft SubstratesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2001Hae-Won Kim A study is made of the role of flaw state on the strength properties of brittle ceramic coating layers bonded to soft polycarbonate substrates. We introduce Vickers radial cracks at prescribed loads into the coating undersurfaces prior to bonding to control the sizes and locations of the starting flaws. A spherical indenter is then loaded on the top bilayer surfaces, directly above the Vickers indentation sites, subjecting the radial cracks to flexural tensile stress. Radial crack responses are monitored in situ, using a camera located below the transparent substrate. Critical loads to cause radial crack instability, and ensuing growth of the arrested cracks, are recorded. Conventional biaxial flexure tests on corresponding monolith coating materials provide a baseline for data comparison. Relative to the monolith flexure specimens, the bilayers show higher strengths, the more so the larger the flaw, indicating enhanced flaw tolerance. A simple fracture mechanics analysis of the radial crack evolution in the concentrated-load field, with due account for distribution of flexural tensile stresses at the coating undersurface, is unable to account completely for the enhanced bilayer strengths for the larger Vickers flaws. It is hypothesized that the epoxy used to bond the bilayer components enters the cracks, causing crack-wall adherence and providing an increased resistance to radial crack instability. The fracture mechanics are nevertheless able to account for the arrest and subsequent stable extension of the radial cracks beyond the critical loads once this extraneous adherence has been overcome. [source] Global and local linear buckling behavior of a chiral cellular structurePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2005A. Spadoni Abstract This paper investigates the flat-wise compression behavior of an innovative cellular structure configuration. The considered layout has a hexagonal chiral geometry featuring cylinders, or nodes, joined by ligaments, or ribs. The resulting assembly is characterized by a number of interesting properties that can be exploited for the design of alternative honeycombs or cellular topologies to be used in sandwich construction. The flat-wise strength of the chiral geometry is investigated through classical analytical formulas for the linear buckling of thin plates and shells and a bifurcation analysis performed on a Finite Element model. The analytical expressions predict the global buckling behavior and the resulting critical loads, and can be directly compared with the results obtained from the Finite Element analysis. In addition, the Finite Element model predicts local buckling modes, which should be considered to evaluate the possible development of localized plasticity. A sensitivity study is performed to evaluate the influence of the geometry of the chiral structure on its buckling strength. The study shows that the considered topology can offer great design flexibility, whereby several parameters can be selected and modified to improve the flat-wise performance. The comparison with traditional, hexagonal centro-symmetric structural configurations concludes the paper and demonstrates the enhanced performance and the potentials of chiral noncentro-symmetric designs. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Mechanical Study of High Resistance Silicon Carbide Based Multi-Nano-Layers Grown by Multifrequency PACVDPLASMA PROCESSES AND POLYMERS, Issue S1 2009Farida Rebib Abstract Titanium alloys are commonly used to lighten aeronautical structures. Unfortunately, their poor tribological properties need to confer onto the surface both high mechanical resistance under load and low friction/low wear against other metals. Based on amorphous silicon carbide (a-SiC:H), periodic multi-nano-coatings have been found to be candidates for this use. Such coatings were deposited on Ti surfaces using multifrequency plasma CVD devices. Scratch-testing measurements have revealed that the critical loads corresponding to cohesive/adhesive cracks are significantly increased using multilayered stacks. Fretting-wear tests also revealed the influence of multilayer architecture and preliminary analysis of wear tracks and material transfers leaded to a first classification of the coatings. [source] Dynamic stability of a porous rectangular platePROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2006Daniel Debowski The study is devoted to a axial compressed porous-cellular rectangular plate. Mechanical properties of the plate vary across is its thickness which is defined by the non-linear function with dimensionless variable and coefficient of porosity. The material model used in the current paper is as described by Magnucki, Stasiewicz papers. The middle plane of the plate is the symmetry plane. First of all, a displacement field of any cross section of the plane was defined. The geometric and physical (according to Hook's law) relationships are linear. Afterwards, the components of strain and stress states in the plate were found. The Hamilton's principle to the problem of dynamic stability is used. This principle was allowed to formulate a system of five differential equations of dynamic stability of the plate satisfying boundary conditions. This basic system of differential equations was approximately solved with the use of Galerkin's method. The forms of unknown functions were assumed and the system of equations was reduced to a single ordinary differential equation of motion. The critical load determined used numerically processed was solved. Results of solution shown in the Figures for a family of isotropic porous-cellular plates. The effect of porosity on the critical loads is presented. In the particular case of a rectangular plate made of an isotropic homogeneous material, the elasticity coefficients do not depend on the coordinate (thickness direction), giving a classical plate. The results obtained for porous plates are compared to a homogeneous isotropic rectangular plate. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||