Dynamic steady crack growth in elastic-plastic solids—propagation of strong discontinuities

The near-tip field of a mode I crack growing steadily under plane strain conditions is studied. A key issue is whether strong discontinuities can propagate under dynamic conditions. Theories which impose rather restrictive assumptions on the structure of an admissible deformation path through a dynamically propagating discontinuity have been proposed recently. Asymptotic solutions for dynamic crack growth, based on such theories, do not contain any discontinuities. In the present work a broader family of deformation paths is considered and we show that a discontinuity can propagate dynamically without violating any of the mechanical constitutive relations of the material. The proposed theory for the propagation of strong discontinuities is corroborated by very detailed finite element calculations. The latter shows a plane of strong discontinuity emanating from the crack tip (with its normal pointing in the direction of crack advance) and moving with the tip. Elastic unloading ahead of and/or behind the plane of discontinuity and behind the crack tip have also been observed.The numerical investigation is performed within the framework of a boundary layer formulation whereby the remote loading is fully specified by the first two terms in the asymptotic solution of the elasto-dynamic crack tip field, characterized by K₁, and T. It is shown that the family of near-tip fields, associated with a given crack speed, can be arranged into a one-parameter field based on a characteristic length, L_g, which scales with the smallest dimension of the plastic zone. This extends a previous result for quasi-static crack growth.     

Synthese de materiaux polymers transparents - Partie II Synthese et polymerisation d'acrylates et de methacrylates d'halogenoalkyle

Several fluorinated and chlorofluorinated acrylates and methacrylates were synthesized from the alcohols R_FCH₂OH with R_F=CCl₃, CHClCH₂CCl₃, (CF₂CFCl)_nCl, (C₂F₄)_nH, CF₃, C₇F₁₅ and C₆F₁₃CH₂. The corresponding polymers were also studied for their application as transparent polymeric materials. For this purpose the refractive indices n²⁰_D of the monomers and the polymers were measured as were the Tg values of the polymers. A study of their infrared spectra shows to what extent these products could be utilized taking into consideration their absorption in the long wavelength range between 0.85 and 1.50 m. The refractive indices of these polymers, which are reduced when increasing the number of fluorine atoms in the molecule, and that of the Tg measured between 50 and 70° C for the fluorinated methacrylates, indicate a potential use of these polymers as sheathing materials for optical fibres.     

Electronic absorption spectra of selenocarbonyl and thiocarbonyl compounds

Electronic absorption spectra of a variety of thiocarbonyl and selenocarbonyl compounds have been compared to point out their similarities. Interesting correlations between the absorption maxima and electronegativities of substituents in both the seleno- and thio-carbonyl compounds have been reported.     

New synthesis of 1-triacontanol

Succinic anhydride and behenic acid have been used to attach C₄ and C₂₂ carbon chains on to 2 and 5 positions of thiophene through two alternate acylation sequences. Raney nickel desulphurization to the 2,5-disubstituted thiophene yielded triacontanoic acid esters which on LAH reduction gave 1-triacontanol.     

Elemental analysis of bead samples using a laser-induced plasma at low pressure

An Nd:YAG laser (1064 nm, 8 ns, 30 mJ) was focused on various types of fresh, fossilized white coral and giant shell samples, including samples of imitation shell and marble. Such samples are extremely important as material for preparing prayer beads that are extensively used in the Buddhist faith. The aim of this research was to develop a non-destructive method to distinguish original beads from their imitations by means of spectral measurements of the carbon, hydrogen, sodium and magnesium emission intensities and by measuring the hardness of the sample using the ratio between Ca (II) 396.8 nm and Ca (I) 422.6 nm. Based on these measurements, original fresh coral beads can be distinguished from any imitation made from hard wood. The same technique was also effective in distinguishing beads made of shell from its imitation. A spectral analysis of bead was also performed on a fossilized white coral sample and the result can be used to distinguish to some extent the fossilized white coral beads from any imitation made from marble. It was also found that the plasma plume should be generated at low ambient pressure to significantly improve the hydrogen and carbon emission intensity and also to avoid energy loss inside the crater during laser irradiation at atmospheric pressure. The results of this study confirm that operating the laser-induced plasma spectroscopy at reduced ambient pressure offers distinct advantage for bead analysis over the conventional laser-induced breakdown spectroscopy (LIBS) technique operated at atmospheric pressure.     

Formic acid solubilization of marine biological tissues for multi-element determination by ETAAS and ICP–AES

A simple, fast method is described for the determination of Ag, As, Cd, Cu, Cr, Fe, Ni, and Se in marine biological tissues by electrothermal atomic-absorption spectrometry (ETAAS) and Na, Ca, K, Mg, Fe, Cu, and Mn by inductively coupled plasma–atomic emission spectrometry (ICP–AES). Solubilization of the biological tissue was achieved by using formic acid with vortex mixing followed by heating to 50°C in an ultrasonic bath. Once solubilized, the tissues were diluted to an appropriate volume with water for analysis. Aliquots were sampled into a graphite furnace and ICP–AES using a conventional autosampler. The method was validated by use of biological certified reference materials from NRC, DORM-2, DOLT-2, DOLT-3, LUTS-1, TORT-2, and NIST SRMs 1566b and 2976. Simplicity and reduced sample-preparation time prove to be the major advantages to the technique.     

Electrolyte additive maintains high performance for dendrite-free lithium metal anode

Because of their high capacity and low potential, lithium metal anodes are considered to be promising candidates for next generation electrode materials. However, the safety concerns and limited cycling life associated with uncontrollable dendrite growth hamper practical applications. In this work, the acidified cellulose ester, which is a mixed fiber microporous membrane film, was used as a novel electrolyte additive that effectively improves the cycle stability of the lithium metal anode and inhibits dendrite growth. The focus of this paper is on inhibiting the formation and growth of lithium dendrites. The coulombic efficiency of a Li|Cu battery with this acidified cellulose ester additive remains stable at 99% after 500 cycles under a current density of 1 mA/cm². Symmetric batteries also remain stable after 500 cycles (1000 h) under a current density of 1 mA/cm². These superior properties can be ascribed to the induced nucleation and the uniform distribution of lithium ion flux. This study uncovers an approach for effectively enabling stable cycling of dendrite-free lithium metal anodes.