198,470 research outputs found
La Cueva de la Olla: Chihuahua
No full textLa información de esta miniguía se basa, a la
fecha de publcación de la misma, en los trabajos de Carl Lumholtz y de los arqueólogos Aurora Montúfar López, Ma. Luisa Reyes Landa y Arturo Guevara Sánchez.Una de las particularidades del Valle de las cuevas es la presencia de un sitio donde se ha detectado una secuencia de ocupación muy larga. En él habitaron grupos humanos que utilizaban un ancestro del maíz que se remonta aproximadamente a 5500 a.C. Esta ocupación, de las más antiguas del estado de Chihuahua y de la república mexicana, se localiza en la cueva de la Golondrina, a sólo 400 m de La Cueva de la Olla.</p
Gli Stati Uniti e i regimi autoritari in epoca bipolare : il dilemma tra stabilità e democrazia
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Cervical false negative cases detected by neural network-based technology. Critical Review of cytologic errors
No full textInfluence of morpho-structural parameters on environmental stress cracking in polyethylene
No full textPolyethylene (PE) is widely utilized in several industries due to its versatility and mechanical strength, yet its long-term performance is often hindered by slow crack growth (SCG) and environmental stress cracking (ESC). This study quantitatively evaluates the influence of key morpho-structural parameters on SCG and ESC resistance in PE, using a linear elastic fracture mechanics (LEFM) approach to assess the effect of different comonomers on fracture toughness. Four PE materials were analyzed: two medium molecular weight, linear low-density polyethylenes (LLDPE) co-polymerized with 1-butene (Material A) and 1-hexene (Material B); a high molecular weight LLDPE copolymerized with 1-hexene (Material C); and a medium molecular weight, high-density polyethylene (HDPE) homopolymer (Material D). The results confirm that molecular weight is a dominant factor in enhancing stress cracking resistance, with the high molecular weight LLDPE (Material C) showing superior performance. Moreover, despite nearly identical structural parameters, Material B exhibited significantly higher SCG and ESC resistance compared to Material A, highlighting the critical role of the commoner type. The research identified three distinct environmental regimes influencing fracture behavior, each dependent on the applied stress intensity factor (K) and material properties. These regimes are: (1) no significant environmental effect at high K values, where fracture is dominated by the material's inherent properties; (2) partial plasticization of craze fibrils at intermediate K values, due to limited diffusion of environmental agents into the crack tip; and (3) full plasticization of craze fibrils at low K values, where extensive diffusion accelerates environmental stress cracking (ESC). By demonstrating how morpho-structural parameters and environmental conditions together influence polyethylene's resistance to SCG and ESC, this study improves our understanding of the underlying mechanisms and underscores the effectiveness of LEFM in evaluating long-term material performance. This knowledge can guide the design with polyethylene materials aimed at improving long-term durability for industrial applications
Non-destructive in-depth composition profile of oxy-hydroxide nanolayers on iron surfaces from ARXPS measurements
No full textIn this work the maximum entropy method (MEM) is applied, for the first time, to angle-resolved X-ray photoelectron spectroscopy (ARXPS) data from oxy-hydroxide films on iron surfaces. This nondestructively derives information on the in-depth distribution of the composition and chemical state. An MEM algorithm was created and first tested on the simulated data. The reconstructed composition depth profiles agreed very well with the theoretical ones up to 5% Gaussian noise added to the data. The same algorithm was then applied to ARXPS data from iron samples to investigate the in-depth variations in the composition and chemical state of the nanosized oxy-hydroxide film naturally grown on the iron surface. The resulting surface film presents a complex multilayer structure with concentration gradients. The effect of air exposure on the structure was also investigated. Copyright (C) 2006 John Wiley & Sons, Ltd
Environmental Stress Cracking of Polymers
No full textSlow crack growth (SCG) is usually the most dominant failure mechanism in polymer products, typically occurring over extended periods under sustained low-stress conditions. This phenomenon is particularly critical in applications requiring long-term durability, such as gas pipes, water distribution systems, and storage tanks.
A phenomenon closely related to SCG is Environmental Stress Cracking (ESC), in which the progression of slow crack growth is accelerated when a polymer is exposed to surface-active agents under stress. These agents do not alter the polymer's chemical structure but drastically reduce the time to failure, by accelerating crack initiation and propagation. In ESC, the same crack growth mechanisms observed in SCG occur, but at a significantly accelerated rate due to environmental factors, making it a critical consideration in the design of polymer products intended for long-term use under stress.
The resistance to ESC is typically evaluated using standard methods, which unfortunately are quite limited in terms of both the amount of information provided (typically a single ranking parameter) and their accuracy, hindering the possibility of establishing clear structure-properties relationships for the materials under investigation. Over the years we developed an alternative approach, based on fracture mechanics (FM), which provides a much richer picture about the active ESC mechanisms and allows the identification of clear structure-properties relationships. These insights enable the optimization of the long-term performance of polymers under various stress conditions, at the same time providing reliable quantitative predictions of product lifetime, which can be of high interest for the industry
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