2,185 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
Influence 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
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
Green-Kubo Formula for Weakly Coupled Systems with Noise
No full textWe study the Green-Kubo (GK) formula κ(ε, ς) for
the heat conductivity of an infinite chain of d-dimensional finite
systems (cells) coupled by a smooth nearest neighbor potential εV .
The uncoupled systems evolve according to Hamiltonian dynamics
perturbed stochastically by an energy conserving noise of strength
ς. Noting that κ(ε, ς) exists and is finite whenever ς > 0, we are
interested in what happens when the strength of the noise ς → 0.
For this, we start in this work by formally expanding κ(ε, ς) in a
power series in ε, κ(ε, ς) = ε
2 P
n≥2
ε
n−2κn(ς) and investigating
the (formal) equations satisfied by κn(ς). We show in particular
that κ2(ς) is well defined when no pinning potential is present,
and coincides formally with the heat conductivity obtained in the
weak coupling (van Hove) limit, where time is rescaled as ε
−2
t,
for the cases where the latter has been established [24, 12]. For
one-dimensional systems, we investigate κ2(ς) as ς → 0 in three
cases: the disordered harmonic chain, the rotor chain and a chain
of strongly anharmonic oscillators. Moreover, we formally identify
κ2(ς) with the conductivity obtained by having the chain between
two reservoirs at
Nyt on lysti olla (4/4 F)
No full textLaulun sanat: Nyt on lysti olla, sun fralialilei, Nyt on lysti olla, sun fralialilei, Kun ei ole heilaa, sun fralialilei, Kun ei ole heilaa, sun fralialilei
Pharmacophore modeling: a continuously evolving tool for computational drug design
No full textIn the latest two or three years progressive
applications of pharmacophore modeling continue to
appear in literature. Pharmacophore based parallel
screening, for instance, has been introduced in 2006.
Moreover, in 2008, a survey discussing the prospective
impact of virtual screening techniques in the
discovery of bioactive natural products has been
published. Finally, virtual screening techniques
from the drug discovery field are beginning to
be used for profiling the bioactivity of chemicals (especially those of potential environmental concern) with the aim of
prioritizing compounds for further testing using more complex systems and
reducing and ultimately replacing the use of animals in regulatory testing.
Pharmacophore modeling might be extremely helpful to allow full
achievement of all the above mentioned goals. In this contribution we report
a couple of case studies where pharmacophore generation and handling
played a pivotal role. In particular, in the first example, the development of a
novel computational pre-screening approach to be used as an in silico
filtering tool for natural products is described, applied to the estrogen
receptor-α subtype. In the second study, differently, the validation of a
preexisting pharmacophore by the prediction of the antifungal activities of
new azole compounds is discussed. In this case, it comes to light the
importance and utility of adding excluded volumes to a pharmacophore, to
increase its predictivity
Kulta mullai olla pitäis (3/2 ja 4/4 F)
No full textLaulun sanat: Kulta mullai olla pitäis, Kulta mullai olla pitäis, Kulta mullai olla pitäis Ja olla oikein soma, Vaikka sitt' yhdessä kerjättäis Ja kerjättäis Ja kerjättäis, Vaikka me yhdessä kerjättäis Ja kuoltais maantien ojaan
Kulta mullai olla pitäis (3/2 ja 4/4 F)
No full textLaulun sanat: Kulta mullai olla pitäis, Kulta mullai olla pitäis, Kulta mullai olla pitäis Ja olla oikein soma, Vaikka sitt' yhdessä kerjättäis Ja kerjättäis Ja kerjättäis, Vaikka me yhdessä kerjättäis Ja kuoltais maantien ojaan
Taidan olla tuhlaajapoika (2/4 F, d)
No full textLaulun sanat: Taidan olla tuhlaajapoika, koska kotoni unhoitin. Kyll' ompi kova aika, vaan täytyy lähteä
Maailmassa on hyvä olla (2/4 F)
No full textLaulun sanat: Maailmassa on hyvä olla, vaikk' on suolaleipä, Kirnun hyrnyn huuhdevettä pannaan palan painoks
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