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Low Stress (low + stress)
Selected AbstractsMelt spun thermoresponsive shape memory fibers based on polyurethanes: Effect of drawing and heat-setting on fiber morphology and propertiesJOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2007Jasmeet Kaursoin Abstract Thermoresponsive shape memory (SMP) fibers were prepared by melt spinning from a polyester polyol-based polyurethane shape memory polymer (SMP) and were subjected to different postspinning operations to modify their structure. The effect of drawing and heat-setting operations on the shape memory behavior, mechanical properties, and structure of the fibers was studied. In contrast to the as-spun fibers, which were found to show low stress built up on straining to temporary shape and incomplete recovery to the permanent shape, the drawn and heat-set fibers showed significantly higher stresses and complete recovery. The fibers drawn at a DR of 3.0 and heat-set at 100°C gave stress values that were about 10 times higher than the as-spun fibers at the same strain and showed complete recovery on repeated cycling. This improvement was likely due to the transformation brought about in the morphology of the permanent shape of the SMP fibers from randomly oriented weakly linked regions of hard and soft segments to the well-segregated, oriented and strongly H-bonded regions of hard-segments. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2172,2182, 2007 [source] Polymerization Contraction Stress of Resin Composite Restorations in a Model Class I Cavity Configuration Using Photoelastic AnalysisJOURNAL OF ESTHETIC AND RESTORATIVE DENTISTRY, Issue 6 2000YOSHIFUMI KINOMOTO DDS ABSTRACT Purpose: An important factor that contributes to deterioration of resin composite restorations is contraction stress that occurs during polymerization. The purpose of this article is to familiarize the clinician with the characteristics of contraction stress by visualizing the stresses associated with this invisible and complex phenomenon. Materials and Methods: Internal residual stresses generated during polymerization of resin composite restorations were determined using micro-photoelastic analysis. Butt-joint preparations simulating Class I restorations (2.0 mm ± 5.0 mm, 2.0 mm in depth) were prepared in three types of substrates (bovine teeth, posterior composite resin, and transparent composite resin) and were used to examine contraction stress in and around the preparations. Three types of composite materials (a posterior composite, a self-cured transparent composite, and a light-cured transparent composite) were used as the restorative materials. The self-cured composite is an experimental material, and the others are commercial products. After treatment of the preparation walls with a bonding system, the preparations were bulk-filled with composite. Specimens for photo-elastic analysis, were prepared by cutting sections perpendicular to the long axis of the preparation. Fringe patterns for directions and magnitudes of stresses were obtained using transmitted and reflected polarized light with polarizing microscopes. Then, the photoelastic analysis was performed to examine stresses in and around the preparations. Results: When cavity preparations in bovine teeth were filled with light-cured composite, a gap was formed between the dentinal wall and the composite restorative material, resulting in very low stress within the restoration. When cavity preparations in the posterior composite models were filled with either self-cured or light-cured composite, the stress distribution in the two composites was similar, but the magnitude of the stress was greater in the light-cured material. When preparations in the transparent composite models were filled with posterior composite and light-cured transparent composite material, significant stress was generated in the preparation models simulating tooth structure, owing to the contraction of both restorative materials. CLINICAL SIGNIFICANCE Polymerization contraction stress is an undesirable and inevitable characteristic of adhesive restorations encountered in clinical dentistry that may compromise restoration success. Clinicians must understand the concept of polymerization contraction stress and realize that the quality of composite resin restorations depends on successful management of these stresses. [source] Abundance and habitat segregation in Mediterranean grassland species: the importance of seed weightJOURNAL OF VEGETATION SCIENCE, Issue 2 2002F.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] An Exploration of the Relationship Between Depressive Symptoms and Cortisol Rhythms in Colorado RanchersTHE JOURNAL OF RURAL HEALTH, Issue 1 2009Emily Schulze MA ABSTRACT:,Context: Although the effects of stress on health have been studied in numerous urban-dwelling populations, fewer studies have addressed these effects in rural populations, such as farmers and ranchers. Purpose: The present study focuses on seasonal levels of depressed affect and perceived stress in Western Colorado ranchers, and how those phenomena related to their levels of cortisol. Methods: Twenty-one (21) ranchers, who were permittees on the Colorado Grand Mesa, completed the study. Participants identified 2-week time periods during the year representing relative high, medium, and low stress. During each period, participants took saliva samples, rated stress levels, and completed a daily health diary. In addition, the Beck Depression Inventory (BDI-II), the perceived stress scale (PSS), and a life events scale (LES) were administered. Results: Results showed a strong relationship between BDI-II and PSS scores (r = 0.748, P < .01). The decreased daytime cortisol decline supports the notion that the hypothalamic-pituitary-adrenal (HPA) axis negative feedback loop is disrupted in chronic stress and depression, thus resulting in chronically elevated cortisol levels. Conclusion: This study supports the relationship between stress, depression, and HPA dysregulation in ranchers. [source] Factors modifying stress from adverse effects of immunosuppressive medication in kidney transplant recipientsCLINICAL TRANSPLANTATION, Issue 1 2005Jaroslav Rosenberger Abstract:, Introduction:, The adverse effects of immunosuppression appear in the majority of patients with a negative impact on morbidity, mortality and quality of life. The group of adverse symptoms manifested as changes in appearance, mood and energy are often more stressful than serious metabolic changes because of their direct negative influence on patients' well-being. The aim of this study is to explore the adverse symptoms of immunosuppressive medication which are the most stressful for transplanted patients, and which are the modifying factors. Patients and methods:, A total of 157 adult kidney transplant recipients from two transplant centres in Slovakia with a functioning graft transplanted <7 yr ago were examined. Patients participated in an interview focusing on stress from adverse effects, and their education and social support. Medical records were searched for information about immunosuppression protocols, dialysis treatment before transplantation, type of received organ and period after transplantation. The effect of the selected variables on the total score for stress from adverse effects was tested using ANOVA. The effect of the selected factors on stress from each single adverse effect was explored using t -test and ANOVA. Results:, The most stressful symptoms were pain, weakness, weight gain, facial changes, depression and anxiety. The mean value of the total score for stress from adverse effects was 8.03 ± 6.53 (minimum 0, maximum 30, range: 0,64), indicating low stress. Women and patients with lower education significantly more often felt the adverse effects of immunosuppression as stressful (p < 0.001 and p < 0.05, respectively). Age, social support, dialysis modality before transplantation, time from transplantation and type of immunosuppressive treatment did not affect the total score for stress from adverse effects. However, variables that were not significant in the overall score reached significance in some symptoms. Conclusions:, Women and patients with lower education significantly more often felt the adverse effects of immunosuppression as stressful; in a more detailed analysis the use of new drugs was connected with less stress in some symptoms. The use of these drugs can improve life quality for transplant recipients, decrease non-compliance, and thus prevent graft loss. [source] Ligament creep recruits fibres at low stresses and can lead to modulus-reducing fibre damage at higher creep stresses: a study in rabbit medial collateral ligament modelJOURNAL OF ORTHOPAEDIC RESEARCH, Issue 5 2002G. M. Thornton Ligaments are subjected to a range of loads during different activities in vivo, suggesting that they must resist creep at various stresses. Cyclic and static creep tests of rabbit medial collateral ligament were used as a model to examine creep over a range of stresses in the toe- and linear-regions of the stress,strain curve: 4.1 MPa (n =7), 7.1 MPa (n = 6), 14 MPa (n = 9) and 28 MPa (n = 6). We quantified ligament creep behaviour to determine if, at low stresses, modulus would increase in a cyclic creep test and collagen fibres would be recruited in a static creep test. At higher creep stresses, a decrease in measured modulus was expected to be a potential marker of damage. The increase in modulus during cyclic creep and the increase in strain during static creep were similar between the three toe-region stresses (4.1, 7.1, 14 MPa). However, at the linear-region stress (28 MPa), both these parameters increased significantly compared to the increases at the three toe-region stresses. A concurrent crimp analysis revealed that collagen fibres were recruited during creep, evidenced by decreased area of crimped fibres at the end of the static creep test. Interestingly, a predominance of straightened fibres was observed at the end of the 28 MPa creep test, suggesting a limited potential for fibre recruitment at higher, linear-region stresses. An additional 28 MPa (n = 6) group had mechanically detectable discontinuities in their stress,strain curves during creep that were related to reductions in modulus and suggested fibre damage. These data support the concept that collagen fibre recruitment is a mechanism by which ligaments resist creep at low stresses. At a higher creep stress, which was still only about a third of the failure capacity, damage to some ligaments occurred and was marked by a sudden reduction in modulus. In the cyclic tests, with continued cycling, the modulus increased back to original values obtained before the discontinuity suggesting that other fibres were being recruited to bear load. These results have important implications for our understanding of how fibre recruitment and stress redistribution act in normal ligament to minimize creep and restore modulus after fibre damage. © 2002 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved. [source] High-Temperature Rheology of Calcium Aluminosilicate (Anorthite) Glass-Ceramics under Uniaxial and Triaxial LoadingJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2001Balakrishnan 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 CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2000Rodney 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 NitrideJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2000Kyung 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] | |||||||||||||