Deformation at very high strain rates of Al and ERGAL 7075

Summary Comparative examinations have been carried out on Al and ERGAL specimens deformed by compression either fast or slow. The high-strain-rate compressions were obtained by explosive. The dislocation structures have been examined by transmission electron microscopy and by Fourier analysis of the X-ray diffraction lines. Crystalline textures were also determined. After fast deformation smaller interaction energies appear among the dislocations with comparable total densities because of the accumulation in grain boundaries. The results have been interpreted by assuming that the loads which are reached in the shock and reflected wave fronts give rise to generations or multiplications of the dislocations on levels more differentiated than by slow compression.

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Riassunto Sono stati condotti esami comparativi su campioni di Al e di ERGAL deformati mediante compressione lenta o veloce. Le compressioni ad elevate velocità sono state ottenute per mezzo di esplosive. Le strutture delle dislocazioni sono state osservate con microscopia elettronica in trasmissione e analisi di Fourier delle righe di diffrazione dei raggi X. Sono state inoltre determinate le tessiture cristalline. Con densità totali di dislocazioni comparabili, dopo deformazione rapida appaiono minori energie d'interazione tra le dislocazioni stesse a causa di fenomeni di accumulo nei bordi di grano. I risultati sono stati interpretati ammettendo che i carichi raggiunti in corrispondenza delle onde d'urto, diretta e riflesse, diano luogo a generazione e moltiplicazione di dislocazioni su livelli piú differenziati che con compressione lenta.

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Резюме Проводится сравнительный анализ образцов Al и ERGAL, подвергнутых деформации сжатия либо быстро, либо, медленно. Высокая скорость деформации сжатия получается в результате взрыва. Исследуется структура дислокаций с помощью трансмисионной электронной микроскопии и с помощью Фурье-анализа линий дифракции рентгеновских лучей. Определяются кристаллические структуры. После быстрой деформации возникают небольшие энергии взаимодействия между дислокациями с соизмеримыми полными плотностями, вследствие явления аккумуляции на границах зерен. Проводится интерпретация полученных результатов, предполагая, что нагрузки приводят к образованию или размножению дислокаций на уровнях более дифференцированных, чем в случае медленного сжатия.

     

<Emphasis Type="Italic">C</Emphasis><Superscript>∞</Superscript> regularity of solutions of the Levi equation in<Emphasis Type="Italic">R</Emphasis><Superscript><Emphasis Type="Italic">2n+1</Emphasis></Superscript>

We study the regularity of the solutions of the Levi equation in ?²ⁿ⁺¹. It is a second order quasilinear equation whose characteristic matrix is positive semidefinite and has vanishing determinant at every point and for every functionu∈C ². We show that the operator associated to the equation can be represented as a sum of squares of non linear vector fields. Then, by using a freezing method, we prove theC ^∞ regularity of the solutions.     

Thermal characterization of poly(lactide-co-glycolide) microspheres containing bupivacaine base polymorphs

Solid-state chemistry of bupivacaine base, obtained by precipitation from bupivacaine hydrochloride solutions with ammonia, was investigated. Two bupivacaine base polymorphs (Form I: T _m=105.6±0.0°C; Form II: T _m=97.6±0.2°C) were isolated depending on the precipitation conditions and characterised by thermal analysis and FTIR spectroscopy. No weight loss was evidenced by TG confirming that no solvate formation had occurred. Biodegradable poly(lactide-co-glycolides) microspheres containing bupivacaine base were prepared by spray-drying. In the microspheres bupivacaine base was present as the metastable low melting crystal form independently of the bupivacaine base / poly(lactide-co-glycolides) ratio.     

Short-term evaluation of the thermal endurance characteristics of polymeric materials by TA

Two thermal analysis techniques (isothermal differential calorimetry and isothermal thermogravimetric analysis) are used as analytical methods for short-term thermal endurance characterization of polymeric materials, used for electrical insulation. These techniques are applied to commercial grade ethylene-propylene rubber and polyvinyl chloride. It is shown that thermogravimetric analysis provides very satisfactory results, independently of the degradation reactions which take place in the material. Calorimetry proves to be effective only for polymers (such as polyolefins) in which oxidation is the largely prevailing degradation mechanism in the test and service temperature ranges. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison between CP/MAS 13C-NMR and pyrolysis-GC/MS in the structural characterization of humins and humic acids of soil and sediments

The chemical structure of humins (HUs) and humic acids (HAs) of terrestrial and marine environments was investigated by cross-polarization magic angle spinning ¹³C-nuclear magnetic resonance spectroscopy (CP/ MAS ¹³C-NMR) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Samples of HUs and HAs were obtained from sediments of the Adriatic Sea, the Lagoon of Ravenna (Adriatic Sea) and the Bubano Lake as well as from an agricultural soil. HUs displayed pyrograms and NMR spectra different from those of related HAs. According to NMR spectra HUs were more aliphatic and contained fewer carboxyl groups than HAs, while pyrolysates of HUs were characterized by higher levels of products arising from carbohydrates and lower levels of lignin methoxyphenols with respect to HAs. The relative content of paraffinic carbons determined by NMR was in good agreement with the relative abundance of unbranched aliphatic hydrocarbons released by pyrolysis. Both techniques evidenced the importance of polymethylene structures in HUs. Received: 5 January 1998 / Revised: 24 March 1998 / Accepted: 25 March 1998     

Item selection by latent class-based methods: an application to nursing home evaluation

The evaluation of nursing homes is usually based on the administration of questionnaires made of a large number of polytomous items to their patients. In such a context, the latent class model represents a useful tool for clustering subjects in homogenous groups corresponding to different degrees of impairment of the health conditions. It is known that the performance of model-based clustering and the accuracy of the choice of the number of latent classes may be affected by the presence of irrelevant or noise variables. In this paper, we show the application of an item selection algorithm to a dataset collected within a project, named ULISSE, on the quality-of-life of elderly patients hosted in Italian nursing homes. This algorithm, which is closely related to that proposed by Dean and Raftery in 2010, is aimed at finding the subset of items which provides the best clustering according to the Bayesian Information Criterion. At the same time, it allows us to select the optimal number of latent classes. Given the complexity of the ULISSE study, we perform a validation of the results by means of a sensitivity analysis, with respect to different specifications of the initial subset of items, and of a resampling procedure.