Thermal stresses in plane elasticity: Numerical solutions based only on harmonic functions

Summary A theoretical treatment is outlined allowing solution of thermal stress problems in plane elasticity by using only numerical methods suited to solving — in 2 D — the Laplace equation. Only one type of element matrix (supposing for the sake of simplicity F.E.M.s are used) and only one mesh would thus be required, both for the determination of the thermal field and of the displacement/stress field. The numerical solutions required in the plane domain of interest entail, consequently, only one variable per node in place of two. Even if numerous unit solutions are required in order to impose arbitrary boundary conditions, this reduction of nodal variables allows to spend less computation time in solving linear systems, at least for problems of a certain extent.

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Sommario Si delinea una tecnica che permette di risolvere i problemi di coazioni termiche in elasticità piana facendo uso solo di metodi numerici atti a risolvere l'equazione di Laplace. Un solo tipo di matrice degli elementi (nel caso si usi una formulazione a E. F.) e una sola reticolazione sono pertanto richiesti tanto per il problema termico come per quello elastico. Ne consegue altresì il vantaggio che le soluzioni numeriche richieste nel dominio di interesse cornportano una sola variabile per nodo, anzichè due; anche tenuto conto che sono richieste numerose soluzioni unitarie per poter imporre condizioni al contorno comunque definite, ne deriva per problemi di una certa ampiezza una riduzione del tempo di calcolo speso nella soluzione di sistemi.

     

The Intramolecular Interaction of Thiophene and Furan with Aromatic and Fluoroaromatic Systems in Some [3.3]Meta(heterocyclo)paracyclophanes: A Combined Computational and NMR Spectroscopic Study

Four thiophene‐ and furan‐containing [3.3]meta(heterocyclo)paracyclophanes were designed and synthesized to study the intramolecular interaction between standard heteroaromatic rings and tetra‐H‐ or tetra‐F‐substituted benzenes. A complete conformational analysis, carried out by DFT calculations and variable‐temperature NMR techniques, showed that, despite their structural similarity, these adducts have different conformational preferences and undergo different types of isomerizations depending on the nature of the heterocycle. The thiophene‐derived adducts adopted a parallel stacked arrangement of the aromatic systems in the ground‐state conformations. Their isomerization pathways involved a thiophene ring‐flip process passing through an edge‐to‐face arranged transition state in which the heterocycle is perpendicular to the benzene platform and its sulfur atom points toward the center of that ring. The threshold energy barrier to the ring‐flip process was higher by 10 kJ mol^?1 in the case of the adduct featuring the perfluorinated benzene. This difference was rationalized by assuming that the ground‐state conformations of the H‐ and F‐substituted compounds have different stability. On the contrary, the furan‐derived adducts were shown by calculations and NMR spectroscopy to adopt, in their ground‐state conformations, a perpendicular edge‐to‐face disposition of the rings with the oxygen atom pointing toward the benzene platform. The adoption of this arrangement was confirmed by X‐ray crystallography. In the case of these compounds, the isomerization process involved distortion of the CH₂SCH₂ bridges connecting the aromatic systems and the adoption of transition‐state geometries for which the rings were arranged in a parallel‐stacked orientation. Once again a very nice agreement was observed between the predicted and the experimentally determined geometries and pathways. In the case of the furan‐containing compounds, the threshold barriers were found to be much lower in energy that those observed for the thiophene derivatives. Remarkably, they were virtually independent of the presence of fluorine atoms on the platform benzene ring.     

Solid-state (15)N CPMAS NMR and computational analysis of ligand hapticity in rhodium(η-diene) poly(pyrazolyl)borate complexes

Novel [Rh(η-diene)Tp(x)] complexes of sterically encumbered Tp(x) ligands (Tp(x) = Tp(4Bo), diene = cod, 1; nbd, 2; Tp(x) = Tp(4Bo,5Me), diene = cod, 3; nbd, 4; Tp(x) = Tp(a,3Me), diene = cod, 5; nbd, 6; Tp(x) = Tp(a*,3Me), diene = cod, 7; nbd, 8) have been prepared by treatment of [Rh(η-diene)(μ-Cl)](2) with TlTp(x) (Tp(x) in general, in detail: Tp(4Bo) = hydrotris(indazol-1-yl)borate, Tp(4Bo,5Me) = hydrotris(5-methyl-indazol-1-yl)borate, Tp(a,3Me) = hydrotris(3-methyl-2H-benz[g]-4,5-dihydroindazol-2-y1)borate, Tp(a*,3Me) = hydrotris(3-methyl-2H-benz[g]indazol-2-yl)borate), and characterized by analytical and spectral data (IR, (1)H, (11)B, and (13)C NMR solution). The structures adopted by [Rh(nbd)Tp(4Bo)] 2, [Rh(cod)Tp(4Bo,5Me)] 3, [Rh(nbd)Tp(a,3Me)] 6, [Rh(nbd)Tp(a*,3Me)] 8, and [Rh(nbd)Tp(a*,3Me*)] 8* (incorporating a borotropomeric ligand), have been investigated. Low steric hindrance between the ligands in 2 and 3 permits κ(3) coordination of the pyrazolylborate while the high steric encumbrance present in 6, 8, and 8* results in κ(2) ligands. The coordination modes of the ligands to the metal have also been established by (15)N CPMAS studies of selected ligands and their corresponding Rh complexes. These spectroscopic data are in agreement with the (15)N chemical shifts obtained by using quantum-chemical methods to assist reliable assignments of the experimental values, affording new insights into the extraction of structural information concerning the hapticity (κ(2) or κ(3)) of the poly(pyrazolyl)borate ligands to the Rh metal.     

Transient tertiary structure formation within the ribosome exit port

The exit tunnel of the ribosome is commonly considered to be sufficiently narrow that co-translational folding can begin only when specific segments of nascent chains are fully extruded from the tunnel. Here we show, on the basis of molecular simulations and comparison with experiment, that the long-range contacts essential for initiating protein folding can form within a nascent chain when it reaches the last 20 ? of the exit tunnel. We further show that, in this "exit port", a significant proportion of native and non-native tertiary structure can form without steric overlap with the ribosome itself, and provide a library of structural elements that our simulations predict can form in the exit tunnel and is amenable to experimental testing. Our results show that these elements of folded tertiary structure form only transiently and are at their midpoints of stability at the boundary region between the inside and the outside of the tunnel. These findings provide a framework for interpreting a range of recent experimental studies of ribosome nascent chain complexes and for understanding key aspects of the nature of co-translational folding.     

Marine mammal blubber reference and control materials for use in the determination of halogenated organic compounds and fatty acids

The National Institute of Standards and Technology (NIST) has a diverse collection of control materials derived from marine mammal blubber, fat, and serum. Standard Reference Material (SRM) 1945 Organics in Whale Blubber was recertified for polychlorinated biphenyl (PCB) congeners, organochlorine pesticides, and polybrominated diphenyl ether (PBDE) congeners. SRM 1945 has also been assigned mass fraction values for compounds not frequently determined in marine samples including toxaphene congeners, coplanar PCBs, and methoxylated PBDE congeners which are natural products. NIST also has assigned mass fraction values, as a result of interlaboratory comparison exercises, for PCB congeners, organochlorine pesticides, PBDE congeners, and fatty acids in six homogenate materials produced from marine mammal blubber or serum. The materials are available from NIST upon request; however, the supply is very limited for some of the materials. The materials include those obtained from pilot whale blubber (Homogenates III and IV), Blainville’s beaked whale blubber (Homogenate VII), polar bear fat (Homogenate VI), and California sea lion serum (Marine Mammal Control Material-1 Serum) and blubber (Homogenate V).     

Preparation and characterization of standard reference material 1849 infant/adult nutritional formula

Standard Reference Material (SRM) 1849 Infant/Adult Nutritional Formula has been issued by the National Institute of Standards and Technology (NIST) as a replacement for SRM 1846 Infant Formula, issued in 1996. Extraction characteristics of SRM 1846 have changed over time, as have NIST's analytical capabilities. While certified mass fraction values were provided for five constituents in SRM 1846 (four vitamins plus iodine), certified mass fraction values for 43 constituents are provided in SRM 1849 (fatty acids, elements, and vitamins) and reference mass fraction values are provided for an additional 43 constituents including amino acids and nucleotides, making it the most extensively characterized food-matrix SRM available from NIST.     

Characterization of the volatile organic compounds of Italian 'Fossa' cheese by solid-phase microextraction gas chromatography/mass spectrometry

Fossa cheese is an Italian hard cheese, ripened for up to 3 months in underground pits dug into tuffaceous rock. During this period, the cheese develops a unique flavour and intense and somewhat piquant aroma. Solid-phase microextraction gas chromatography/mass spectrometry (SPME-GC/MS) was utilized to characterize the volatile organic compounds (VOCs) of Fossa cheese. A total of 75 VOCs were separated and identified; in particular, the major class of compounds found in the cheeses ripened in the pits were the esters of fatty acids. Discriminant analysis of volatile profiles allowed us to distinguish between cheeses in different stages of seasoning (60-day-old cheese and cheese ripened an additional 90 days in and out of the pits).     

Acoustic emission localization in complex dissipative anisotropic structures using a one-channel reciprocal time reversal method

This paper presents an imaging method for the localization of the impact point in complex anisotropic structures with diffuse field conditions, using only one passive transducer. The proposed technique is based on the reciprocal time reversal approach (inverse filtering) applied to a number of waveforms stored into a database containing the experimental Green's function of the structure. Unlike most acoustic emission monitoring systems, the present method exploits the benefits of multiple scattering, mode conversion, and boundaries reflections to achieve the focusing of the source with high resolution. Compared to a standard time reversal approach, the optimal refocusing of the back propagated wave field at the impact point is accomplished through a "virtual" imaging process. The robustness of the inverse filtering technique is experimentally demonstrated on a dissipative stiffened composite panel and the source position can be retrieved with a high level of accuracy in any position of the structure. Its very simple configuration and minimal processing requirements make this method a valid alternative to the conventional imaging Structural Health Monitoring systems for the acoustic emission source localization.     

Laboratory measurement of sound absorption of occupied pews and standing audiences

Places of worship, as well as other performing spaces or large arenas are characterized by lightweight pews or seats, with moderate or negligible upholstery, leading to very low absorption coefficients. Consequently, the audience becomes the most important sound absorbing element, capable of playing a fundamental role in determining the acoustic characteristics of the space. Consequently accurate knowledge of its acoustic properties is required for any design purpose. Several studies have been carried out with reference to audiences seated on upholstered theatre seats but there is a considerable lack of information about occupied pews. The well known difficulty of taking into account edge effects during such measurements poses further questions as well as the effect of the density of occupation, and the seasonal variations due to clothing. This paper presents the results of a series of laboratory measurements aimed at clarifying such aspects. The measurements showed that the edge effects are negligible and that total absorption is better related to the number of persons present than to the area they cover. Nonetheless, as the density grows, or when the audience is seated, there is a reduction in absorption which may be explained by the reduction in exposed body surface. Lightweight clothes show a considerable reduction in sound absorption over all the frequency bands, suggesting that significant seasonal fluctuations in reverberation time should be expected in places where the audience is the only sound absorbing surface.