102,231 research outputs found
Fatigue limit evaluation of a stainless steel using thermal data analysis
No full textFatigue is an irreversible process, accompanied by microstructural changes, localized plastic strains and energy dissipation. The temperature increase of a metallic material undergoing a fatigue test is a manifestation of the thermal energy dissipation and it was experimentally observed that the higher the applied stress amplitude the more pronounced the temperature increase of the material. Moreover, the trend of the observed temperature increase versus the applied stress amplitude presents a more or less abrupt positive change at a certain stress level. This characteristic stress amplitude was associated to the material fatigue limit [1-7]. Since the load-increasing tests are fast and a single specimens can be used to estimate the material fatigue limit, Risitano and co-workers set-up a rapid experimental methodology to determine the fatigue limit based on load-increasing tests monitored with an infrared camera, according to the so-called Risitano method [5].
It is known that due to plasticity, the stress amplitude and the material temperature waves present a phase shift. Therefore, taking advantage of the lock-in thermography, temperature oscillations due to dissipated energy, T2, can be obtained as the component having double frequency of the load signal sine wave by using the classical Fourier analysis, as suggest by Krapez et al [8]. They proposed to use T2, instead of the stabilized material temperature measured during a fatigue test in order to estimate the material fatigue limit. However, they noticed that the applicability of the method depends on the material under analysis. In particular, satisfactory results were found in the case of AISI 1050 steel and AISI 316L stainless steel, while the approach was useless while investigating the fatigue behavior of 7010 aluminum alloy, since temperature signal associated with the dissipated energy was very weak in relation to the available experimental equipment.
Aaki et al [9] proposed to use the dissipated energy, evaluated by using the lock-in thermography, to assess the material fatigue limit. They pointed out that satisfactory results were found in the case of AISI 316L, while in the case of AISI 304 the fatigue limit estimated by dissipated energy measurements provided a conservative value as compared to that evaluated by means of conventional fatigue test.
The phase shift angle was proposed as a suitable damage index for the rapid determination of material fatigue limit by Shiozawa et al [10]. They validated the approach against experimental results obtained from AISI 306L stainless steel specimens and noticed a satisfactory agreement with the fatigue limit determined by the conventional stair-case tests. Recently, Shiozawa et al [11] proposed a new experimental technique for fatigue limit estimation based on the phase 2f lock-in infrared method, where a single fatigue test was performed on AISI 316 steel specimens by increasing the applied stress amplitude, a, and by measuring the in-creasing rate of the dissipated energy, dq/da. By plotting dq/da versus a, an abrupt increase is seen when a exceeds the material fatigue limit.
Meneghetti [12] proposed an experimental technique for the direct evaluation of the heat energy density per cycle dissipated by a material undergoing a fatigue tests (the Q parameter). Q can be easily evaluated by stopping the fatigue test at t = t* after thermal equilibrium has been reached and by measuring the cooling gradient immediately after t*. Originally adopted to rationalise the notch effect in finite-life fatigue of stainless steel specimens [13], it is envisaged that the specific heat loss Q may be used also to estimate the fatigue limit.
This paper analyses all previous thermal methods for the rapid experimental estimation of the fatigue limit. In particular, the rapid engineering techniques proposed by La Rosa and Risitano [5], by Shiozawa et al [10] as well as by using the Q parameter [12] will be applied to estimate the fatigue limit of cold-drawn AISI 304L stainless steel bars and critical issues in practical applications of the analysed techniques will be singled out
Determination of fatigue limit by static thermographic method and classic thermographic method on notched specimens
Get PDFThe aim of the present work is to investigate the possibility to evaluate the fatigue limit of such a component through the so-called Static Thermographic Method (STM). This new approach has been proposed in 2013 by Risitano and Risitano to determinate the fatigue limit of metallic materials through static tensile tests. Investigating the trend of the surface temperature of the material during static tensile test, the method proposes to correlate the first deviation from linearity of the surface temperature to a damage limit that has been proven to have relation with the fatigue limit. Static tensile tests at different stress rate have been carried out on V-notched specimens. Stepwise fatigue tests have been carried out in order to apply the classic Thermographic Method that has been hugely validated to determinate the fatigue limit of metallic components. Static tests at different stress rate showed that the method is dependent on this parameter; anyway, this dependence does not affect the results achieved in terms of fatigue limit. The experimental results showed a good agreement with the energetic methods considered in the present work
Stability of intramolecular DNA quadruplexes: comparison with DNA duplexes
No full textWe have determined the stability of intramolecular quadruplexes that are formed by a variety of G-rich sequences, using oligonucleotides containing appropriately placed fluorophores and quenchers. The stability of these quadruplexes is compared with that of the DNA duplexes that are formed on addition of complementary C-rich oligonucleotides. We find that the linkers joining the G-tracts are not essential for folding and can be replaced with nonnucleosidic moieties, though their sequence composition profoundly affects quadruplex stability. Although the human telomere repeat sequence d[G(3)(TTAG(3))(3)] folds into a quadruplex structure, this forms a duplex in the presence of the complementary C-rich strand at physiological conditions. The Tetrahymena sequence d[G(4)(T(2)G(4))(3)], the sequence d[G(3)(T(2)G(3))(3)], and sequences related to regions of the c-myc promoter d(G(4)AG(4)T)(2) and d(G(4)AG(3)T)(2) preferentially adopt the quadruplex form in potassium-containing buffers, even in the presence of a 50-fold excess of their complementary C-rich strands, though the duplex predominates in the presence of sodium. The HIV integrase inhibitor d[G(3)(TG(3))(3)] forms an extremely stable quadruplex which is not affected by addition of a 50-fold excess of the complementary C-rich strand in both potassium- and sodium-containing buffers. Replacing the TTA loops of the human telomeric repeat with AAA causes a large decrease in quadruplex stability, though a sequence with AAA in the first loop and TTT in the second and third loops is slightly more stable
Influence of loop size on the stability of intramolecular DNA quadruplexes
No full textWe have determined the stability of intramolecular DNA quadruplexes in which the four G(3)-tracts are connected by non-nucleosidic linkers containing propanediol, octanediol or hexaethylene glycol, replacing the TTA loops in the human telomeric repeat sequence. We find that these sequences all fold to form intramolecular complexes, which are stabilized by lithium < sodium < potassium. Quadruplex stability increases in the order propanediol < hexaethylene glycol < octanediol. The shallower shape of the melting profile with propanediol linkers and its lower dependency on potassium concentration suggests that this complex contains fewer stacks of G-quartets. The sequence with octanediol linkers displays a biphasic melting profile, suggesting that it can adopt more than one stable structure. All these complexes display melting temperatures above 310 K in the presence of 10 mM lithium, without added potassium, in contrast to the telomeric repeat sequence. These complexes also fold much faster than the telomeric repeat and there is little or no hysteresis between their melting and annealing profiles. In contrast, the human telomeric repeat sequence and a complex containing two hexaethylene glycol groups in each loop, are less stable and fold more slowly. The melting and annealing profiles for the latter sequence show significant differences, even when heated at 0.2degreesC min(-1). CD spectra for the oligonucleotides containing non-nucleosidic linkers show positive maxima at 264 nm, with negative minima similar to244 nm, which are characteristic of parallel quadruplex structures. These results show that the structure and stability of intramolecular quadruplexes is profoundly influenced by the length and composition of the loops. <br/
Un metodo per la mitigazione delle vibrazioni indotte dal traffico ferroviario tramite barriere assorbenti – sperimentazione in sito
No full textEnergy release as a parameter for fatigue design of additive manufactured metals
Get PDFAdditive manufacturing (AM) is spreading in a wide range of industrial fields. The influence of the printing parameters on the mechanical performance is still an open issue among researchers, particularly when dealing with fatigue loads, which can lead to an unexpected failure. Classical fatigue tests require a large amount of time and materials to be consumed. Compared to the traditional fatigue assessment, the thermographic method (TM) is able to derive in a very rapid way the SN curve and fatigue limit of the material monitoring its energetic release during fatigue tests. In this work, for the first time, the energetic release during fatigue test has been evaluated in specimens made of AISI 316L, obtained by SLM technique. Compared to literature data, the specimens show premature failure, even at low stress levels, with brittle fracture surfaces. The internal microstructure seems to be strictly related to the energetic release of the material
Understanding Green Consumer Behavior in Digital Environment: A Systematic Literature Review
No full textThe interconnection among entrepreneurship, marketing and Innovation: the relevance of value creation
No full textThe GREEN lever: Unraveling the influence of consumption values on brand responses and consumer behaviour
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