86,552 research outputs found
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
L’Omero tragico: luci e ombre nella parodo dell’Edipo Re di Sofocle
No full textAfter a brief excursus about ancient sources helding Sophocles as the ‘Tragic Homer', the paper focuses on some case-studies of Epic vocabulary in the parodos of Sophocles' Oedipus Rex. The study presents various exploitation of Epic lexis by Sophocles such as epitheta ornantia, resemantization, and neologisms. In particular, specific attention is payed to thematic and intertextual links between the chorus and the Epic-Homeric tradition. The parodos of Oedipus Rex is traditionally considered a ‘peanic’ chorus, but within a dark setting. In addition to the ‘peanic’ influences, it is argued that the interference with the Epic genre is also crucial for a better understanding of the chorus as a whole. In fact, some Epic intertextual links help to highlight the two prevalent and conflicting feelings of the parodos: the hope in the help from the gods and a feeling of apprehension and fear. In particular, it is argued that some Epic features involving Apollo contribute to highlighting already from the opening song of the play two major conceptual themes of Oedipus Rex: the ambiguity of the role of the god and the extreme precariousness of human knowledge
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
Influence of morpho-structural parameters on the environmental stress cracking of polyethylene
No full textThe environmental stress cracking resistance (ESCR) of four polyethylenes in active medium (10% Tergitol solution) was investigated.
The four materials were chosen to explore a broad range of performance and applications; they are listed according to increasing expected ESCR:
- an injection moulding, low-molecular weight (MW) HDPE homopolymer
- two rotomoulding LLDPE copolymers with a different comonomer (butene or hexene)
- a blown film extrusion high MW HDPE copolymer (hexene)
The fracture resistance of the hexene LLDPE copolymer was expected to be slightly higher than that of the butene one, because of the longer alkyl group. The two materials were chosen to challenge the ability of the different testing methods to discriminate between similar levels of ESCR.
Several analytical techniques were employed to obtain relevant morpho-structural parameters: density, degree of crystallinity, MW distribution, short chain branch content and average lamellar thickness.
ESCR was evaluated by employing three well-known but widely different approaches:
- the Bell Telephone test (ASTMD1693), performed on notched specimens immersed in the active environment at 50°C
- strain hardening modulus (SHM), obtained from tensile tests performed in air at 80°C
- fracture mechanics (FM) tests on three- and four-point bending notched specimens, performed at varying temperatures both in air and in the active environment; two different loading histories were considered (creep and constant displacement rate)
MW distribution seems to have a greater effect on ESCR behaviour with respect to other morpho-structural parameters. There was instead a strong consistency of the different ESCR testing methods, so that SHM could replace ASTMD1693 as an industrial test aimed at ranking ESCR of polyethylenes. FM, while being more complex to carry out, provides a wealth of additional information which could be used to actually predict the lifetime of products working in an ESC environment
Variations on the Author
No full text“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
An experimental, theoretical and numerical investigation of shape memory polymers
No full textThe present paper deals with the experimental analysis, constitutive modeling and numerical simulation of a class of polymers, exhibiting shape memory effects. We first present and discuss the results of an experimental traction-shrinkage campaign on semi-crystalline shape memory polymers, particularly, on low-density and high-density polyethylene-based polymers. Then, we develop a new one-dimensional phenomenological constitutive model, based on the so-called phase transition approach and formulated in a finite strain framework, in order to reproduce experimental observations. The model is treated through a numerical procedure, consisting in the replacement of the classical set of Kuhn-Tucker inequality conditions by the Fischer-Burmeister complementarity function. Numerical predictions reveal that the model is able to describe qualitative aspects of material behavior, involving both orientation and thermal retraction, as well as to predict experimental orientation processes for semi-crystalline polyethylene-based polymers with different densities
Modulation of soluble receptor for advanced glycation end products isoforms and advanced glycation end products in long-living individuals
No full textBackground: Circulating levels of soluble receptor for advanced glycation end products (sRAGE) and advanced glycation end products (AGEs) correlate with aging/cardiovascular risk, which is delayed in long-living individuals (LLIs). AGEs/sRAGE isoforms (cleaved RAGE [cRAGE] and secretory RAGE [esRAGE]) ratio is a valuable marker for disease risk. Results: We evaluated circulating sRAGE isoforms, and AGEs in LLIs (n = 95; 90-105 years) and controls (n = 94; 11-89 years). cRAGE decreased with age in controls and further declined in LLIs. esRAGE increased in LLIs. AGEs rose with age in controls and decreased in LLIs that were characterized by a lower AGEs/sRAGE ratio. Notably, cRAGE and AGE/esRAGE ratio better discriminated controls from LLIs. Conclusion: circulating cRAGE could be considered a reliable marker of chronological age while esRAGE a protective factor for longevity. Aging is the major risk factor for disease development. Long-living individuals (LLIs) are subjects older than 90 years that represent an invaluable model to study mechanisms underpinning longevity and healthy aging. Circulating levels of soluble receptor for advanced glycation end products (sRAGE) change with aging and can forecast the cardiovascular risk, which is reduced in centenarians. sRAGE is composed of two isoforms, the cleaved RAGE (cRAGE) and the secretory RAGE (esRAGE), that are known to inhibit the oxidative stress and inflammatory activities of their ligands such the advanced glycation end products (AGEs). In this study, we measured the plasmatic levels of both sRAGE isoforms and AGEs in LLIs (90-105 years) and control subjects (11-89 years). We found that cRAGE decreases with age in controls and LLIs. esRAGE increases in LLIs and AGEs increase in controls with age but decrease in LLIs. AGEs/esRAGE ratio and cRAGE were able to discriminate controls from LLIs. Hence, LLIs are characterized by a lower AGEs/sRAGE ratio, due to esRAGE increase and AGEs reduction that may explain their reduced cardiovascular and metabolic risk. Besides, circulating cRAGE could be considered a reliable marker of chronological age, while esRAGE a protective factor associated with longevity
[Newspaper Clipping: Author Claims Evidence of Second JFK Assassin #1]
No full textNewspaper article titled "Author Claims Evidence of Second JFK Assassin." The article states that author Richard J. Whalen concluded "that there is circumstantial evidence to support the theory of a second assassin in the shooting of President John F. Kennedy.
Also By The Same Author: AKTiveAuthor, a Citation Graph Approach to Name Disambiguation
No full textThe desire for definitive data and the semantic web drive for inference over heterogeneous data sources requires co-reference resolution to be performed on those data. In particular, name disambiguation is required to allow accurate publication lists, citation counts and impact measures to be determined. This paper describes a graph-based approach to author disambiguation on large-scale citation networks. Using self-citation, co-authorship and document source analyses, AKTiveAuthor clusters papers, achieving precision of 0.997 and recall of 0.818 over a test group of eight surname clusters
John F. Kennedy telegram to Roosevelt
No full textJersey Homesteads (later the Borough of Roosevelt) was established in the 1930s as an agro-industrial cooperative community. It was established specifically for urban Jewish garment workers, many of whom had emigrated from Europe. President John F. Kennedy sent a telegram to the citizens of Roosevelt, New Jersey, apologizing for not being able to attend the memorial dedication in honor of former President Franklin Delano Roosevelt. (Jersey Homesteads became Roosevelt in 1945 in honor of the president.) President Kennedy expressed his gratitude to the people of Roosevelt for constructing the memorial, and commented that it will serve as a constant reminder of Roosevelt's good works
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