R.E. Bolmaro

An Eshelby inclusion-based model for the study of stresses and plastic strain localization in metal matrix composites I: General formulation and its application to round particles

Abstract The Eshelby model has been extended to handle incipient plastic deformation following a Prandlt—Reuss plastic law. The current model is a modification of a previous integro-differential model also based in a discretization of the Eshelby one. It has been modified to perform the calculations starting from an integral equation on strain tensor instead of an integro-differential equation on displacements. It allows us, through the use of a novel convergence criterion, to find the solution for the elastoplastic field in the general case of ellipsoidal particles under thermal and mechanical loads. Both modifications greatly improve the ability of the model to handle composites with high ratios between matrix and inclusion shear moduli. The formulation is applied to a model case of Al matrix reinforced with SiC round particles. Both matrix and particles are considered elastic and thermally isotropic. The particle behavior is purely elastic and the matrix flows when it reaches an isotropic yield stress. It is found that the results agree with previous finite element method (FEM) calculations and the method allows us to study local plastic relaxation when mechanical load is below the matrix macroscopic yield stress.

In vivo evaluation of albendazole microspheres for the treatment of Toxocara canis larva migrans.

Albendazole is a benzimidazole derivative with proven efficacy against many parasites such as intestinal helminths. Toxocariasis is one of the important parasitic diseases in humans and animals caused by Toxocara canis. It is well known that T. canis larvae migrate in paratenic hosts, including humans where it may cause visceral larva migrans. Thus, the present research was carried out using in vivo experiments with the aim of finding whether novel albendazole microparticles would be active against migrating larvae of the parasite. Albendazole-chitosan microparticles were prepared by ionotropic gelation with sodium lauryl sulphate or by a liquid-liquid phase separation with sodium hydroxide. Mice were infected with T. canis and then treated with both albendazole-chitosan microparticles. After treatment (28days post-infection), it was examined the anthelmintic effect in mice after oral administration of microparticulate preparations. The number of larvae recovered from mice treated with albendazole formulations were compared with placebo. The results showed that albendazole microparticles were easily prepared in high yield using both aqueous solutions of sodium lauryl sulphate or sodium hydroxide. In vivo evaluation of larva migration showed that albendazole microparticles exhibited a greater anthelmintic effect in the brain (0 larva/mouse). In addition, it was also found that liver and lung showed a significant decrease in the number of larvae. Therefore, these data suggest that albendazole-chitosan microparticles are effective formulations for the treatment of toxocariasis infection by reducing the number of larvae in liver and lung. Particularly, these polymeric preparations were able to totally prevent migration of larvae to the mice brain.

Texture analysis with a time‐of‐flight neutron strain scanner

A time-of-flight (TOF) neutron strain scanner is a white-beam instrument optimized to measure diffractograms at precise locations within bulky specimens, typically along two perpendicular sample orientations. Here, a method is proposed that exploits the spatial resolution (∼1 mm) provided by such an instrument to determine in a nondestructive manner the crystallographic texture at selected locations within a macroscopic object. The method is based on defining the orientation distribution function (ODF) of the crystallites from several incomplete pole figures, and it has been implemented on ENGIN-X, a neutron strain scanner at the ISIS facility in the UK. This method has been applied to determine the texture at different locations of Al alloy plates welded along the rolling direction and to study a Zr2.5%Nb pressure tube produced for a CANDU nuclear power plant. For benchmarking, the results obtained with this instrument for samples of ferritic steel, copper, Al alloys and Zr alloys have been compared with measurements performed using conventional X-ray diffractometers and more established neutron techniques. For cases where pole figure coverage is incomplete, the use of TOF neutron transmission measurements simultaneously performed on the specimens is proposed as a simple and powerful test to validate the resulting ODF.

Crystal spin in two-sites self consistent models: From kinematics to kinetics

Abstract In the current presentation we deal with the interpretation of textures of two-phase co-deformable materials. We use a 2-sites visco plastic self consistent (2S-VPSC) model computing the spin of each phase through an empirical law describing spin sharing between both phases. Cu-Fe and Ag-Ni composite materials represent two kinds of two-phase materials that have been extensively studied in the past. Neutron and X-ray texture measurements of rolled, free compressed and extruded samples are shown and analyzed in light of the model. Many of the particularities shown by both materials are explained and a general discussion of the model is provided.

Finite element method simulations for two-phase material plastic strains

The texture study of two-phase materials requires a previous knowledge of the distribution of strains among both phases. The way they rotate around one another is of special interest. The texture is a phenomenon that manifests itself at medium and high deformations. Therefore the study cannot usually be performed experimentally, particularly when we are dealing with high volume fraction contents. Sharing of strains is also very important in many technological applications such as forming. This paper presents a study, by the finite element technique, of those quantities as a function of volume fraction, geometry, distribution, strain hardening and yield stress ratio between the two phases. Both phases are assumed to be elasto-plastic and isotropic materials. It is shown that strains and internal rotations are highly influenced by the topology of the phase distribution, yield stress ratio and volume fraction. For high yield stress ratio the hardening seems to be of less importance. It is shown that the strain is highly inhomogeneous and that averages of appropriate quantities can give a macroscopic insight on strain and rotation sharing. Among the many strain definitions the equivalent von Mises deformation will be particularly addressed for its importance in hardening, damage and texture. The local variations are not less important, but the focus will be on the calculation of average quantities able to guide to macroscopic constitutive equation development.

An iterative approach to mechanical properties of MMCs at the onset of plastic deformation

Abstract The current work presents a generalized Eshelby model allowing interaction among reinforcing particles under a Mori-Tanaka like scheme. Different aspect ratios and geometries are studied in the elastic and incipient elasto-plastic regime for a model SiC Al composite. The fibers are taken as purely elastic and the matrix is regarded elastic perfectly plastic responding to a Von Mises yield criterion. The phenomenon of plastic localization in the vicinities of the inclusions is carefully described for different reinforcement volume fractions and thermo-mechanical loading. Equivalent stress, hydrostatic pressure and elastic and plastic strains are depicted as contour levels around a representative inclusion. Effective coefficients of thermal expansion of composites are calculated both under purely elastic composite response and at the onset of plastic localized deformation. The influence of plastic strain over those effective coefficients is shown to be detectable. The simulated stress-strain curves show the influence of interaction stresses over macroscopic yield stress by isolating this phenomenon from matrix hardening. Accumulated elastic energies and plastic work are calculated to show the different nature of purely thermal and mechanical loads.

Texture development in AgNi powder composites

Abstract Both theoretical and experimental studies of texture development in two-phase materials are presented. Finite-element techniques are used in order to characterize the strain and rotation sharing between the two phases. Different volume fractions of AgNi powder composites were simulated in two-dimensional deformation until a von Mises equivalent strain of 1.25. Some average parameters were obtained and used to interpret texture results. 25%AgNi, 50%AgNi and 75%AgNi powder composite samples were deformed in free compression and the texture measured every 20% strain. The texture evolution for Ag shows a saturation due to high internal rotation of this phase. The Ni texture shows a continuous strengthening until it even surpasses the strength of the Ag phase at high deformations. Also some textures for 50%Ag50%Ni composites deformed by rolling are shown. The Ni texture strength is also shown to be higher than the Ag texture at high deformations. This phenomenon is explained by the finite-element simulations and strain path changes. The two phases deform at different paces, which is beneficial for Ag as a softer phase, but they also spin around one another with different rate, which happens to be beneficial for Ni phase strengthening its texture.

An Eshelby inclusion based model for the study of stresses and plastic strain localization in metal matrix composites II. Fiber reinforcement and lamellar inclusions

The extended Eshelby model developed in Part I of the current paper is applied to inclusions with different aspect ratios and the localization of plastic strain is calculated. Its influence on concentration factors is studied for a variety of field variables. The inclusions are considered linear elastic and plastically rigid while the matrix flows when it reaches an isotropic yield stress. Matrix plastic relaxation is found to be a mechanism for composite strengthening by stress homogenization. Fiber-like inclusions provoke the highest reduction of concentration factors with no reduction of its reinforcing ability by a load transferring mechanism. The development of hydrostatic stresses is studied both before and after plastic relaxation of the matrix. New insights are obtained to explain the macroscopic behavior of metal matrix composites and its dependence on inclusion geometry.

On the measurement of Young's modulus of tubes by propagation of longitudinal waves

Abstract The equation that describes the propagation of elastic waves has been solved numerically for longitudinal vibrations of tubes and the results are used to calculate Youngs modulus for Zircaloy-4 fuel sheathings. The values are compared with the results obtained by using the approximate equations proposed in the literature. The differences observed are less than 0.5% for the fundamental frequency. For the harmonics, however, the numerical solution leads to resonant frequencies that are within 1% of the experimental values and the approximate equations give errors higher than 10%.

Chitosan microparticles: influence of the gelation process on the release profile and oral bioavailability of albendazole, a class II compound

Abstract Encapsulation of albendazole, a class II compound, into polymeric microparticles based on chitosan-sodium lauryl sulfate was investigated as a strategy to improve drug dissolution and oral bioavailability. The microparticles were prepared by spray drying technique and further characterized by means of X-ray powder diffractometry, infrared spectroscopy and scanning electron microscopy. The formation of a novel polymeric structure between chitosan and sodium lauryl sulfate, after the internal or external gelation process, was observed by infrared spectroscopy. The efficiency of encapsulation was found to be between 60 and 85% depending on the internal or external gelation process. Almost spherically spray dried microparticles were observed using scanning electron microscopy. In vitro dissolution results indicated that the microparticles prepared by internal gelation released 8% of the drug within 30 min, while the microparticles prepared by external gelation released 67% within 30 min. It was observed that the AUC and Cmax values of ABZ from microparticles were greatly improved, in comparison with the non-encapsulated drug. In conclusion, the release properties and oral bioavailability of albendazole were greatly improved by using spraydried chitosan-sodium lauryl sulphate microparticles.