High Stress (high + stress)

Distribution by Scientific Domains

Terms modified by High Stress

  • high stress level

  • Selected Abstracts

    Testing of insulation systems with respect to high-frequency voltage stress

    M. Paede
    Due to the increasing use of power electronics high frequency working voltages are occurring rather frequently. This means a very high stress for any kind of insulatation. This situation is further aggravated by decreasing dimensions due to miniaturization. Therefore this situation has to be taken into account during dimensioning and testing. Different test methods with respect to withstand and partial discharge inception are described. The necessary test equipment and the dimensioning of the test circuits are specified. The aspects of periodic non-sinusoidal voltages are taken into account by harmonic analysis. [source]

    The Role of Dynamic Material Properties in the Performance of Ceramic Armor

    James Lankford Jr.
    The penetration of ceramic armor by a kinetic energy penetrator is complex, involving a continual process of material damage, micro-crack nucleation, growth and coalescence, and multiaxial failure, all under conditions of high loading rate and inertial confinement. It will be shown that at sufficiently high stress, which usually requires either high loading rates or confinement (these conditions are known to prevail in the region just beneath a penetrator dwelling at the surface of an armor), plastic deformation occurs in ceramics. This deformation appears to limit the strength of most ceramics, since micro-cracks are subsequently nucleated at the sites of the deformation defects and these immediately begin to coalesce into fragments. The constraint/rate-limited flow of these fragments is the physical event that permits the penetration of ceramic armor. This paper considers the implications for modeling armor penetration of laboratory experiments involving both intact and fragmented ceramics tested under compressive loading at high strain rates and under confining pressure. [source]

    Experiences of Students in Pediatric Nursing Clinical Courses

    Marilyn H. Oermann
    ISSUES AND PURPOSE. Learning may be inhibited if students experience undue stress in the clinical setting. This study described the stresses, challenges, and emotions experienced by pediatric nursing students. DESIGN AND METHODS. Students (n = 75) completed a modified Pagana Clinical Stress Questionnaire at the end of their pediatric nursing clinical course. The comparison group of students (n = 383) was enrolled in nonpediatric clinical courses in the same nursing programs. RESULTS. The most stressful aspect of clinical practice was giving medications to children. High stress scores were related to more fear and disappointment in clinical practice. Students who experienced high stress were less stimulated by their clinical activities and developed less confidence in practice. PRACTICE IMPLICATIONS. Knowledge of students' perceptions of clinical stress can help educators and clinicians promote a positive and rewarding clinical atmosphere. [source]

    Microfabric of folded quartz veins in metagreywackes: dislocation creep and subgrain rotation at high stress

    Abstract The microfabrics of folded quartz veins in fine-grained high pressure,low temperature metamorphic greywackes of the Franciscan Subduction Complex at Pacheco Pass, California, were investigated by optical microscopy, scanning electron microscopy including electron backscatter diffraction, and transmission electron microscopy. The foliated host metagreywacke is deformed by dissolution,precipitation creep, as indicated by the shape preferred orientation of mica and clastic quartz without any signs of crystal-plastic deformation. The absence of crystal-plastic deformation of clastic quartz suggests that the flow stress in the host metagreywacke remained below a few tens of MPa at temperatures of 250,300 °C. In contrast, the microfabric of the folded quartz veins indicates deformation by dislocation creep accompanied by subgrain rotation recrystallization. For the small recrystallized grain size of ,8 ± 6 ,m, paleopiezometers indicate differential stresses of a few hundred MPa. The stress concentration in the single phase quartz vein is interpreted to be due to its higher effective viscosity compared to the fine-grained host metagreywacke deforming by dissolution,precipitation creep. The fold shape suggests a viscosity contrast of one to two orders of magnitude. Deformation by dissolution,precipitation creep is expected to be a continuous process. The same must hold for folding of the vein and deformation of the vein quartz by dislocation creep. The microfabric suggests dynamic recrystallization predominantly by subgrain rotation and only minor strain-induced grain boundary migration, which requires low contrasts in dislocation density across high-angle grain boundaries to be maintained during climb-controlled creep at high differential stress. The record of quartz in these continuously deformed veins is characteristic and different from the record in metamorphic rocks exhumed in seismically active regions, where high-stress deformation at similar temperatures is episodic and related to the seismic cycle. [source]

    Effect of Applied Stress on IR transmission of Spark Plasma-Sintered Alumina

    Dibyendu Chakravarty
    The effect of applied stress on IR transmittance of nanocrystalline alumina prepared by spark plasma sintering was evaluated. Transparent alumina with maximum transmittance >80% was obtained over the entire mid-IR wavelength range of 3,5 ,m by applying a high stress of 275 MPa at 1150°C using specially designed high-strength compound dies. The transmittance observed was similar to previous reports at identical wavelengths, but at a lower sintering temperature. The transparent samples have an average grain size of 0.3 ,m and a hardness of 23 GPa. At lower stresses and sintering temperatures, transmittance reduced drastically due to remnant pores in the matrix as observed from the microstructural analysis. The effect of porosity was found to be critical in developing transparency as even a marginal decrease in porosity led to substantial increase in transmittance. [source]

    Plant traits enabling survival in Mediterranean badlands in northeastern Spain suffering from soil erosion

    Joaquín Guerrero-Campo
    Abstract Question: This study analysed the effect of severe soil erosion on species composition of plant communities by favouring species showing certain growth forms, root-sprouting and clonal growth abilities. Location: The study area was located between the middle Ebro Valley and the Pre-Pyrenees (northeastern Spain). Methods: Root-sprouting and shoot-rooting abilities, clonal reproduction and growth form were assessed for the 123 most common plant species from eroded lands in the study area. We obtained 260 vegetation relevés in three different substrata (gypsum outcrops, Miocene clays and Eocene marls) on areas with different degrees of soil erosion. The frequency of every plant trait in each relevé was estimated according to species presence. The effect of soil erosion on the frequency of plant attributes was assessed by correlation analyses. Results: Bipolar, non-clonal plants and annual species decreased their frequency with increasing soil erosion in the three substrata analyzed, whereas root-sprouters and woody plants (mostly sub-shrubs) increased their frequency in most of the substrata analysed. Conclusions: Woody sub-shrubs, root-sprouters and clonal species are favoured in eroded lands in NE Spain. Bipolar species and annual plants might not be plastic enough to survive the high stress and frequent disturbances prevailing in such eroded areas. [source]

    Abundance and habitat segregation in Mediterranean grassland species: the importance of seed weight

    F.M. Azcárate
    Castroviejo (1986,1999); except taxa yet to be covered which follow Tutin et al. (1964,1980) Abstract. We analysed the relationship between seed traits (weight, shape and dispersal structures) and the abundance and habitat segregation of Mediterranean grassland species. To take into account possible correlations with other plant traits, the study also includes 5 vegetative traits (growth form, plant longevity, clonality, onset of flowering and plant size) of commonly accepted functional importance. Data were recorded for 85 species from dehesa grasslands in central Spain. Species abundance was measured in upper (dry and less productive, high stress) and lower (moist and more productive, low stress) slope zones in the same area. Habitat segregation was estimated using an index based on the relative frequencies of species in upper and lower slope zones. Multiple regression models were fitted using species, as well as phylogenetically independent contrasts, as data points. Annual small-seeded species without specialised dispersal structures are over-represented in dehesa grasslands. Abundance was negatively related to seed weight in upper slope zones. None of the recorded plant traits were related to abundance in the lower slope zones. Habitat segregation was mainly related to seed weight, but also to some vegetative traits. Annual, early flowering and small-seeded species were relatively more abundant in the upper than the lower slope zones. This pattern is independent of phylogeny. Our results suggest that in dry Mediterranean grasslands, abundance of many species is determined by dispersal (production of numerous small seeds) rather than by competitive ability. [source]

    High-stress plasticity and the core structures of dislocations in silicon

    J. Rabier
    Abstract The recent observation of perfect dislocations at high stress and low temperature in silicon [J. Rabier, P. Cordier, J. L. Demenet, and H. Garem, Mater. Sci. Eng. A 309/310, 74 (2001)] in accordance with the calculations of Duesbery and Joos [M. S. Duesbery and B. Joos, Philos. Mag. A 74, 253 (1996)] has solved the apparent paradox regarding the nature of mobile dislocations in silicon under usual deformation conditions. However, although several experiments and calculations are consistent with those perfect dislocations being located in the shuffle set, little is known about the actual core structure of those dislocations which lie along unusual Peierls valleys: ,112,/30°, ,123,/41°. This paper aims to review and discuss recent results related to high-stress plasticity and dislocation core structure in silicon. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Structure and energy of partial dislocations in wurtzite-GaN

    G. Savini
    Abstract First-principle calculations have shown that both the partials can be electrically active. In particular we have shown the Ga(g) core partials are a good candidate for the observed absorption peak at 2.4 eV revealed by energy loss spectroscopy measurements. The symmetric and asymmetric reconstructions have relatively close formation energies. Our results have suggested that the asymmetric reconstructions, characterized by strong bonds along the dislocation line are favourable in intrinsic materials. However, in strongly p and n-type materials or in high stress field the symmetric reconstructions can become energetically more stable. These reconstructions are always electrically active with a deep band across the forbidden gap. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Association between psychosocial factors and musculoskeletal symptoms among Iranian nurses

    Ramin Mehrdad MD
    Abstract Background While psychosocial factors have been associated with musculoskeletal symptoms among nurses in some countries, previous studies of Iranian nurses show little association using a demand and control questionnaire. The aim of this study is to assess and evaluate the prevalence of musculoskeletal symptoms and to assess their relationships with psychosocial factors among nurses in Iran. Methods In a cross-sectional study, 347 hospital nurses completed a self-reported questionnaire containing the Standardized Nordic questionnaire for musculoskeletal symptoms and the General Nordic questionnaire for Psychological and Social factors at work (QPS Nordic 34+ Questionnaire). Results Prevalence of low back pain, knee pain, shoulder pain, and neck pain were 73.2%, 68.7%, 48.6%, and 46.3%, respectively. Middle and high stress groups had higher crude and adjusted odds than the low stress group for all body sites. The association for neck, wrist/hand, and upper back and ankle/foot reports (adjusted odds ratio for high stress ranging from 2.4 to 3.0) were statistically significant. Conclusions We observed a high prevalence of self-reported musculoskeletal symptoms at a number of body sites, which were associated with psychosocial factors and specifically stress as defined by the QPS Nordic 34+ Questionnaire. Am. J. Ind. Med. 53:1032,1039, 2010. © 2010 Wiley-Liss, Inc. [source]

    Ankerung im Untertagebau , Entwicklungen in Theorie und Praxis

    Stefan Kainrath-Reumayer Dipl.-Ing.
    Die systematische Ankerung des Gebirges stellt ein wesentliches Verfahren zur Stabilisierung unterirdischer Hohlraumbauten dar. Die geschichtliche Entwicklung verschiedener Ankersysteme wird kurz vorgestellt und Unterteilungsmöglichkeiten werden aufgezeigt. Unterschiedliche Ansätze der Systemankerungen in analytischen und numerischen Berechnungen werden mit der diskreten Modellierung des Gesamtssystems in numerischen Berechnungen mit FLAC3D verglichen. Der Einfluss von Systemankerungen auf das Systemverhalten ist von einer Reihe von Randbedingungen abhängig, der Erfolg der Anwendung oftmals von den verwendeten Ankersystemen. Extreme Spannungszustände im Gebirge stellen hohe Anforderungen an die Methodik der Gebirgsankerung in Untertagebauwerken, die mit den herkömmlichen Ankerungssystemen nur eher unzufriedenstellend zu bewältigen sind. In Abhängigkeit von den Eigenschaften des Gebirges ist unter solch hohen, die Festigkeit des intakten Gebirges meist bereits überschreitenden Spannungszuständen mit großen Verschiebungen, Stabilitätsproblemen sowie Bergschlagphänomenen zu rechnen. Um diesen Anforderungen bestmöglich gerecht zu werden, wird ein neuartiger, energieabsorbierender Ankertyp "Roofex" vorgestellt, der sowohl unter langsam kriechenden, statischen, wie auch abrupten, dynamischen Beanspruchungen einen kontrollierbaren und konstanten Widerstand entlang eines definierten Gleitwegs aufweist. Rock bolting in subsurface constructions , developments in theory and praxis The systematic bolting of rock is a key support element in NATM tunnelling. A short review on the historical development and methods of classification is presented. Different methods of implementation in analytical and numerical calculations are compared to the discrete modelling of the system via FLAC 3D. The influence of the systematic rock bolting to the system behaviour is dependent on many boundary conditions, the success often depends on the used rock bolting techniques. High rock mass stress conditions in underground excavations are serious challenges for rock bolting systems, where conventional bolting systems can only cope with in an rather unsatisfactory way. Depending on the rock mass conditions, high stresses, which mostly exceed already the intact rock mass strength, will lead to serious stability problems, high deformation rates and rockburst phenomena. To provide the mining and tunnelling industry with a rock reinforcement fixture that is better suited to mining with high stress conditions, Atlas Copco GDE designed a new, energy absorbing rock bolt "Roofex" that can accommodate both very large displacements (static yielding) and high energy release (dynamic rupture) in the rock mass by offering an accurate pre-set deformation behaviour. [source]

    Stress Development During Drying of Aqueous Zirconia Based Tape Casting Slurries Measured by Transparent Substrate Deflection Method

    Christiane Bauer
    During drying of water-based tape casting slurries, high stresses can occur, which lead to crack formation. Under the variety of different techniques to measure the drying stresses, the deflect measurement technique is very common. Due to the nontransparent substrate, this technique has the disadvantage, that a direct observation of the changes in the drying regions during the drying process is not possible. Thus, it cannot be ensured that homogeneous drying occurs, which is the prerequisite for the validity of the Corcoran's equation. Nonuniform drying causes cracks and might lead to errors in data interpretation, if nonuniformity of drying is not visible. To make drying uniformity visible, the deflect measurement technique was improved by the usage of a transparent substrate. The benefit of the new technique is the direct observation of saturation uniformity during drying. The differentiation of nonuniform drying and uniform drying, which is characterized by an optimum permeation of the solvent to the surface, is ensured. The advantage of the method is demonstrated by the characterization of the drying behavior of water-based stabilized zirconia slurries in the presence of dispersants and latex binders. Particularly with regard to slurries containing latex particles as a binder, progress was achieved in understanding drying. Measures to decrease drying stresses are summarized. [source]

    High-Temperature Rheology of Calcium Aluminosilicate (Anorthite) Glass-Ceramics under Uniaxial and Triaxial Loading

    Balakrishnan G. Nair
    The high-temperature creep behavior of two fine-grained (,3 ,m) anorthite-rich glass-ceramics was characterized at ambient pressure and under a confining pressure of ,300 MPa. Experiments were done at differential stresses of 15,200 MPa and temperatures of 1200°,1320°C. Of the two materials, one had a tabular (lathlike) grain structure with finely dispersed second phase of mullite, mostly in the form of ,3,5 ,m grains comparable to that of the primary anorthite phase, whereas the other had an equiaxed grain morphology with fine (,400 nm) mullite precipitates concentrated at the anorthite grain boundaries. The results of creep experiments at ambient pressure showed that the material with the tabular grain structure had strain rates at least an order of magnitude faster than the equiaxed material. Creep in the tabular-grained material at ambient pressure was accompanied by a significant extent of intergranular cavitation: pore-volume analysis before and after creep in this material suggested that >75% of the bulk strain was due to growth of these voids. The equiaxed material, in contrast, showed a smooth transition from Newtonian (n= 1) creep at low stresses to non-Newtonian behavior at high stresses (n > 2). Under the high confining pressure, the microstructures of both materials underwent significant changes. Grain-boundary mullite precipitates in the undeformed, equiaxed-grain material were replaced by fine (,100 nm), intragranular precipitates of silliminate and corundum because of a pressure-induced chemical reaction. This was accompanied by a significant reduction in grain size in both materials. The substantial microstructural changes at high confining pressure resulted in substantially lower viscosities for both materials. The absence of mullite precipitates at the grain boundaries changed the behavior of the equiaxed material to non-Newtonian (n= 2) at a pressure of ,300 MPa, possibly because of a grain-boundary sliding mechanism; the tabular-grained material showed Newtonian diffusional creep under similar conditions. [source]

    Deformation Mechanisms in Compression-Loaded, Stand-Alone Plasma-Sprayed Alumina Coatings

    Rodney W. Trice
    Cylindrical, stand-alone tubes of plasma-sprayed alumina were tested in compression in the axial direction at room temperature, using strain gauges to monitor axial and circumferential strains. The primary compression-loading profile used was cyclic loading, with monotonically increased peak stresses. Hysteresis was observed in the stress,strain response on unloading, beginning at a peak stress of 50 MPa. The modulus decreased as the maximum applied stress increased. The stress,strain response was only linear at low stresses; the degree of nonlinearity at high stresses scaled with the stress applied. One-hour dwells at constant stress at room temperature revealed a time-dependent strain response. Using transmission electron microscopy and acoustic emission to investigate deformation mechanisms, the stress,strain response was correlated with crack pop-in, growth, and arrest. It is proposed that the numerous defects in plasma-sprayed coatings, including porosity and microcracks, serve as sites for crack nucleation and/or propagation. As these small, nucleated cracks extend under the applied stress, they propagate nearly parallel to the loading direction along interlamellae boundaries. With increasing stress, these cracks ultimately link, resulting in catastrophic failure. [source]

    Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride

    Kyung Jin Yoon
    The creep behavior of a commercial grade of Si3N4 was studied at 1350° and 1400°C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 ± 22.9 kJ/mol, and compression, 489.2 ± 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple-junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ,18% of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution,precipitation of Si3N4. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution,precipitation creep of Si3N4 did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses. [source]

    Tensile creep of a long-fiber glass mat thermoplastic composite.

    POLYMER COMPOSITES, Issue 8 2009

    This work is part of a larger experimental program aimed at developing a semi-empirical constitutive model for predicting creep in random glass mat thermoplastic (GMT) composites. The tensile creep response of a long-fiber GMT material has been characterized for 3- and 6-mm thick material. Tensile tests showed that the variability within and between plaques are comparable with an overall variability of about 6% and 8% for the 3- and 6-mm thick materials, respectively. The thicker material exhibited slightly higher variability and directional dependence due to greater flow during molding of the plaques. Short-term creep tests consisting of 30 min creep and recovery, respectively, were performed over the stress range between 5 and 60 MPa. Three tests for determining the linear viscoelastic region were considered which showed that the 3- and 6-mm thick GMT are linear viscoelastic up to 20 and 25 MPa respectively. The 6-mm thick GMT consisting of a higher fiber weight fraction was linear over wider stress range. Furthermore, it was found that plastic strains were accumulated during creep, which suggests that a nonlinear viscoelastic,viscoplastic model would be more appropriate for long-term creep at relatively high stresses, which will be presented in our companion paper. The magnitude of the plastic strains developed in the creep tests presented here was lower because a single specimen was loaded at multiple stress level over short durations. Hence, a nonlinear viscoelastic constitutive model has been developed for the two thickness materials. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers [source]

    Tensile creep behaviour of polymethylpentene,silica nanocomposites

    Andrea Dorigato
    Abstract For the first time, poly(4-methyl-1-pentene) (PMP) nanocomposites were prepared by melt compounding 2 vol% of fumed silica nanoparticles, in order to study the role of the nanofiller surface area and functionalization on the tensile mechanical response of the material, with particular focus on its creep behaviour. The high optical transparency of the polymer matrix was substantially preserved in the nanocomposites, while the mechanical properties (in particular the creep stability) were improved. Dynamic mechanical thermal analysis showed an improvement of the storage modulus, more evident above the glass transition temperature of the polymer matrix. Uniaxial tensile tests evidenced that the elastic modulus of the material was positively affected by the presence of silica nanoparticles, even if a slight reduction of the strain at break was detected. The reduction of the tensile creep compliance was proportional to the surface area of the nanofiller, being more evident at high stresses and elevated temperatures. Findley's law furnished a satisfactory fitting of the creep behaviour of the composites, even at high temperatures. It clearly emerges that the incorporation of fumed silica nanoparticles in PMP can be an effective way to overcome the problem of the poor creep stability of polyolefins, especially at high temperatures and high stresses. Moreover the possibility of retaining the original transparency of the material is fundamental for the production of completely transparent PMP components. Copyright © 2010 Society of Chemical Industry [source]