Minimum weight limit design under uncertainty

Summary The problem of probabilistic minimum weight limit design is formulated as a stochastic programming problem. The theoretical treatment considers monodimensional structures, geometrical parameters and integer design variables. An algorithm is proposed and illustrated for the case of pin-jointed structures with two random continuous variables. The paper follows previous researches of the Authors in the field of probabilistic approach to structural safety [29 ÷ 33].

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Sommario Il problema della progettazione a rottura per minimo peso su basi probabilistiche viene formulato quale problema di programmazione stocastica. La trattazione teorica si estende alle strutture monodimensionali, comprende variabili di tipo geometrico nonchè variabili intere: l'algoritmo proposto ed illustrato per la risoluzione numerica riguarda le strutture reticolari interessate da due variabili casuali continue. Il lavoro fa seguito a precedenti ricerche degli scriventi in tema di concezione probabilistica della sicurezza [29 ÷ 33].

     

Homogenization limit for electrical conduction in biological tissues in the radio-frequency rangeLimite d'homogénéisation pour la conduction électrique dans les tissus biologiques dans le domaine des radiofréquences

We study an evolutive model for electrical conduction in biological tissues, where the conductive intra-cellular and extracellular spaces are separated by insulating cell membranes. The mathematical scheme is an elliptic problem, with dynamical boundary conditions on the cell membranes. The problem is set in a finely mixed periodic medium. We show that the homogenization limit u₀ of the electric potential, obtained as the period of the microscopic structure approaches zero, solves the equation $\text{?}\text{div}\text{(σ}{}_0\text{?}{}_{\mathrm{x}}\text{u}{}_0\text{+A}{}^0\text{?}{}_{\mathrm{x}}\text{u}{}_0\text{+∫}{}_0{}^{\mathrm{t}}\text{A}{}^1\text{(t?τ)?}{}_{\mathrm{x}}\text{u}{}_0\text{(x,τ)}\text{d}\text{τ?}\text{F}\text{(x,t))=0}$ where σ₀>0 and the matrices A⁰, A¹ depend on geometric and material properties, while the vector function $\text{F}$ keeps trace of the initial data of the original problem. Memory effects explicitly appear here, making this elliptic equation of non standard type. To cite this article: M. Amar et al., C. R. Mecanique 331 (2003).     

New biphasic mono-component composite material obtained by partial oxypropylation of bacterial cellulose

The present work evaluates the partial oxypropylation of dried bacterial cellulose (BC) performed by grafting propylene oxide with potassium hydroxide as the catalyst. Samples were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. Partial transformation of BC was observed, with cellulose fibers being covered by the thermoplastic phase, leading to a new biphasic mono-component composite material. This synthesis can be considered a green chemical process, since it does not require the use of solvents nor the application of any processing operation, given the fact that the final product is ready for further exploitation as it is removed from the reactor. The obtained material can be used in various applications in the polymer field, i.e. composites, reinforced rigid polyurethane foams, and others.     

Analytical pyrolysis-based study on intra-skeletal organic matrices from Mediterranean corals

Off-line analytical pyrolysis combined with gas chromatography–mass spectroscopy (GC–MS), directly or after trimethylsilylation, along with infrared spectroscopy and amino acid analysis was applied for the first time to the characterization of the intra-skeletal organic matrix (OM) extracted from four Mediterranean hard corals. They were diverse in growth form and trophic strategy namely Balanophyllia europaea and Leptopsammia pruvoti—solitary corals, only the first having zooxanthelle—and Cladocora caespitosa and Astroides calycularis—colonial corals, only the first with zooxanthelle. Pyrolysis products evolved from OM could be assigned to lipid (e.g. fatty acids, fatty alcohols, monoacylglicerols), protein (e.g. 2,5-diketopiperazines, DKPs) and polysaccharide (e.g. anhydrosugars) precursors. Their quantitative distribution showed for all the species a low protein content with respect to lipids and polysaccharides. A chemometric approach using principal component analysis (PCA) and clustering analysis was applied on OM mean amino acidic compositions. The small compositional diversity across coral species was tentatively related with coral growth form. The presence of N-acetyl glucosamine markers suggested a functional link with other calcified tissues containing chitin. The protein fraction was further investigated using novel DKP markers tentatively identified from analytical pyrolysis of model polar linear dipeptides. Again, no correlation was observed in relation to coral ecology. These analytical results revealed that the bulk structure and composition of OMs among studied corals are similar, as it is the textural organization of the skeleton mineralized units. Therefore, they suggest that coral’s biomineralization is governed by similar macromolecules, and probably mechanisms, independently from their ecology.     

Photochemical synthesis of perfluoropolyether (PFPE) nanocomposites containing PFPE oligomer stabilized magnetite nanoparticles

Magnetite nanoparticles were synthesized and their post-synthesis surface modification was carried out with triethoxy terminated perfluoropolyether (PFPE) oligomers. The surface-treated nanoparticles were then dispersed in a UV-curable difunctional methacrylic PFPE oligomer. Thin films prepared from the resulting stable suspensions were photopolymerized. The obtained nanocomposites showed good distribution of the surface-treated magnetite nanoparticles in the polymer matrix. The surface treatment of magnetite nanoparticles with perfluoropolyether oligomers thus was found to be effective in preventing nanoparticle segregation and aggregation, ensuring therefore an increased compatibility with the PFPE matrix.     

Comparison of Different Sorbents in the QuEChERS Method for the Determination of Pesticide Residues in Strawberries by LC–MS/MS

A comparison of sample preparation based on the quick, easy, cheap, effective, rugged and safe (QuEChERS) method for analysis of pesticide residues in strawberries by LC–MS/MS was made using different sorbents in the clean-up by dispersive solid-phase extraction (d-SPE). Some sorbents were laboratory-made, prepared by depositing poly(methyloctadecylsiloxane) (PMODS), poly(methyloctylsiloxane) (PMOS), aminopropyl-terminated poly(dimethylsiloxane) (APPS) and copolymer of (52–48 %)dimethyl-(48–52 %)methylphenyl-siloxane (DMMPS) onto silica supports. The commercial sorbent primary–secondary amine (PSA) and mixtures of two sorbents, primary–secondary amine and poly(methyloctadecylsiloxane), were also used in the experiments. The performances of the sorbents were evaluated by parameters such as color of the final extract, gravimetric measurement, recovery and matrix effect at the fortification level of 100 ng g^?1 of the pesticide mixture in strawberries. The recoveries were in the range 70–120 %, and the RSD values were lower than 20 % for most of the pesticides using the modified QuEChERS method with different sorbents in the clean-up step. The strawberry extracts were cleaned more efficiently with the use of primary–secondary amine sorbent, which has the function of removing sugars, organic acids and especially pigments. The sample preparation method was efficient, and LC–MS/MS determination was optimal because of high selectivity and good detectivity for the multiresidue analyses.     

Stereospecific Couplings of Secondary and Tertiary Boronic Esters

This Minireview highlights advances in the Suzuki–Miyaura cross‐coupling of secondary boron reagents for the creation of C? C bonds with control of stereochemistry. It also includes non‐transition‐metal coupling of secondary and tertiary boronic esters to electron‐rich aromatics.     

Optimizing X‐ray mirror thermal performance using matched profile cooling

To cover a large photon energy range, the length of an X‐ray mirror is often longer than the beam footprint length for much of the applicable energy range. To limit thermal deformation of such a water‐cooled X‐ray mirror, a technique using side cooling with a cooled length shorter than the beam footprint length is proposed. This cooling length can be optimized by using finite‐element analysis. For the Kirkpatrick–Baez (KB) mirrors at LCLS‐II, the thermal deformation can be reduced by a factor of up to 30, compared with full‐length cooling. Furthermore, a second, alternative technique, based on a similar principle is presented: using a long, single‐length cooling block on each side of the mirror and adding electric heaters between the cooling blocks and the mirror substrate. The electric heaters consist of a number of cells, located along the mirror length. The total effective length of the electric heater can then be adjusted by choosing which cells to energize, using electric power supplies. The residual height error can be minimized to 0.02 nm RMS by using optimal heater parameters (length and power density). Compared with a case without heaters, this residual height error is reduced by a factor of up to 45. The residual height error in the LCLS‐II KB mirrors, due to free‐electron laser beam heat load, can be reduced by a factor of ～11 below the requirement. The proposed techniques are also effective in reducing thermal slope errors and are, therefore, applicable to white beam mirrors in synchrotron radiation beamlines.