Effects of organic additives on the microstructural,rheological and electrical properties of silver paste for LTCC applications

Silver powders were prepared by a chemical reduction method. Effects of different surfactants during this preparation process were examined. Using these silver powders, silver pastes were prepared for low temperature co-fired ceramics (LTCC) applications. Another batch of different surfactants were added into the silver paste and evaluated. Moreover, various organic thixotropic agents were used to modify the thixotropy behavior of the silver paste. It was found that the surfactants of PVP (Polyvinyl pyrrolidone, for preparation of the silver powder) and Span-85 (for the preparation of silver paste) exhibited the best performance, with an electrical resistivity of 0.11 mΩ mm. The silver paste with the thixotropic agent of hydrogenated castor oil shows the largest thixotropic index of 1.56. Using different organic additives, the microstructural, rheological and electrical properties of the silver paste can be considerably improved, and our results shed light on the optimization of the silver paste for LTCC applications.

     

The effect of phosphate additives on the lubrication of rolling element bearings in a refrigerant environment

Chlorine free replacement refrigerants, HFC (hydrofluorocarbons) and HC (hydrocarbons), have shown less lubricating properties. Phosphate based additives were used to improve the lubricity with refrigerant R-134a, demonstrating positive effects. In the present paper, the ability to form lubricating film and wear of two additives, phosphate ester and acid phosphate, was investigated in a bearing test apparatus.The results show that phosphate additive in polyolester oil, in an R-134a environment, increases the lubricating film and reduce wear. Surface topography during the initial run-in changes to a more favorable profile with lower RMS angle and longer wavelengths that promote load-carrying capacity and film build-up.     

High-temperature anomaly in the yield stress of doped NaCl crystals

The yield stress of NaCl crystals doped with divalent metals was found to rise with temperature in the range 293-600 K, the effect being most pronounced around 573 K. At higher temperatures, the yield stress decreases substantially.     

On the nature of the free edge stress singularity in composite laminated plates

The variational-asymptotic formulation for the edge layer problem is implemented by mixed-hybrid finite elements. This approach is used to analyse the stress singularity at the free edge in composite laminates. Results indicate that the stress singularity (as r → 0) is very closely approximated by log r instead of r^?α.     

Realization of densified microstructure and large piezoelectricity in KNN ceramics via the addition of oxide additives

Journal of Materials Science: Materials in Electronics - Lead-free potassium sodium niobate [(K, Na)NbO3, KNN]-based ceramics have aroused great interest due to their excellent piezoelectricity and...     

Breakdown of stress in the region of the yield plateau in tension of carbon steel

The article presents the results of a study of the process of deformation of specimens of carbon steel (with superfine ferrite grain) in tension on the yield plateau as a function of the structure.Translated from Problemy Prochnosti, No. 1, pp. 10–13, January, 1992.     

The stress intensity factor for a penny-shaped crack in an elastic body under the action of symmetric body forces

A formula is derived for the stress intensity factor at the rim of a penny-shaped crack in an infinite solid in which there is an axisymmetric distributing of body forces acting in a direction normal to the original crack surfaces. An expression for the surface displacement of the crack is also given. The use of these formulae is illustrated by a consideration of the special case in which the solid is deformed by the action of two point forces situated symmetrically with respect to the crack.

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Zusammenfassung Eine Formel für den Spannungsintensitätfaktor am Rande eines pfenninggeformten Risses in einem unendlichen Festkörper ist gewonnen. Ein achsensymmetrisches Verteilen der Körperkräite fand statt, welches in einer Richtung, normal zu der originalen Rissoberfläche wirkt. Es ist auch Ausdruck für den Oberflächenverschiebung des Risses gegeben. Die Benutzung dieser Gleichungen wird verdeutlicht durch die Betrachtung eines Spezialfalles bei dem der Festkörper durch die Wirkung zweier Punktkräfte deformiert wird, die symmetrisch zum Riss angebracht sind.

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Résumé On a établi une formule donnant le facteur de concentration de tension aux extrémités d'une fissure ferrnée disposée dans un solide infini au sein duquel une distribution de forces internes á symétrie axiale agit dans une direction normale par rapport aus surfaces de la fissure. On fournit également une expression du déplacement de ces surfaces. L'utilisation de ces formules est appliquée, à titre d' exemple, au cas spécial d'un solide soumis à l'action de deux forces concentrées symétriques par rapport à la fissure.

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This paper was prepared as a part of the work of the Applied Mathematics Research Group at North Carolina State University through the Grant AF-AFOSR-444-66 and is under the joint sponsorship of AFOSR, ARO, and ONR through the Joint Services Advisory Group.     

Dynamics of moving interface in porous media for power law fluids with yield stress

In this paper the dynamic of moving interface in a porous medium for non-Newtonian fluids of power law with yield stress is investigated. The frontal advance theory, which describes the flow of two immiscible fluids separated by a moving interface, in which the displacing fluid is of power law with yield stress and the displaced one is Newtonian, has been used to illustrate the deviation from Newtonian behaviour in oil displacement mechanism. The limitations of this theory for non-Newtonian fluids have been shown and discussed. A criterion for determining the conditions under which the viscous fingering effect is eliminated and the interface movement may be in a regular manner has also been obtained. An approximate analytical solution for determining the interface position and its velocity at any time is presented.     

Assessment on the performance of distinct stress integration algorithms for complex non-quadratic anisotropic yield criteria

In this work, three distinct return mapping algorithms are presented and analyzed in detail: (i) a semi-explicit algorithm that accounts for the sub-incrementation technique, which reduces to (ii) a fully-explicit algorithm and, finally, (iii) a semi-implicit algorithm,. In order to describe the complex anisotropic behaviour of some metals, such as aluminium alloys, two non-quadratic anisotropic yield criteria were implemented: the Yld91 and Yld2004-18p. The performance of the developed algorithms is inferred in a series of sheet metal forming benchmarks and the quality of the results is assessed when compared to experimental results presented in the literature. The numerical simulations show that the semi-implicit algorithm is quite efficient with the von Mises yield criterion. However, when anisotropy is taken into account, the algorithm requires several iterations to return the stresses to the yield surface, particularly when the stresses are located at corner regions of that surface. The semi-explicit algorithm proved to be the most robust and efficient algorithm with anisotropic yield criteria. The good agreement between the experimental data and the obtained numerical results demonstrate the high efficiency of the presented algorithms and the ability of the anisotropic criteria to predict the material’s complex anisotropic behaviour.     

The influence of the temperature dependence of the yield stress on the stress distribution in a thermally assembled elastic-plastic shrink fit

Summary The subject of the investigation is the thermal assembly of an elastic-plastic shrink fit with hollow inner component. Based on Tresca's yield condition and the flow rule associated with it, the transient stress distribution is studied. Special attention is paid to the influence of the reduction of the yield stress at elevated temperatures on the interface pressure. However, it turns out that this influence is not significant.Dedicated to Prof. Dr. Kurt Desoyer on the occasion of the 70^th anniversary of his birthday     

Simulation of concrete flow in V-funnel test and the proper range of viscosity and yield stress for SCC

The present paper highlights the flow simulation of self consolidating concrete (SCC) in V-funnel test that is used to determine the concrete filling ability and its resistance against segregation. Simulations were performed using a two-dimensional smoothed particle hydrodynamic (SPH) method to determine the discharge time where SCC was considered as a homogeneous Bingham fluid. The numerical predictions are lower than experimental data because of the assumptions of two-dimensional and homogeneous flow. Having the SPH method employed, SCCs with different viscosities and yield stresses were simulated to compare the discharge time with the suggested criteria in EFNARC (2002) and (2005) guidelines. Based on simulations results, the appropriate range of viscosities and yield stresses as well as a relation between rheological properties and discharge time for SCC taking into account EFNARC (2002) and (2005) guidelines are suggested. Using the suggested relations, one can assess the proper SCC filling ability without conducting the V-funnel test.     

On inter‐element forces in the FEM‐displacement formulation,and implications for stress recovery

This paper describes a new technique for the determination of the inter‐element forces and tractions, as well as stress state at nodes, as a post‐processing step after the solution of standard FE‐displacement calculation. The work is motivated in the context of a broader development of a procedure to simulate fracture processes using a discrete approach without the need of double‐noded interface elements. The technique, easily implementable, is based on the double minimization of an objective function, representing the error between the inter‐element stress tractions and the projection of the best‐fit stress tensor T along the planes of the interfaces converging at an element corner node. The formulation is illustrated with some basic examples in which the resulting stress tensors and inter‐element forces are compared to theoretical solutions and to the results obtained by using a traditional stress average smoothing method. Copyright © 2005 John Wiley & Sons, Ltd.     

A comparative study of statistical methods used in analyzing the proficiency testing results of yield stress

The proficiency test (PT) is a powerful tool to help a laboratory to demonstrate its competence. The statistical analysis used plays very important role in the PT results. This paper demonstrates two PT numerical examples for tensile test using different statistical methods in their analysis. The study shows that zeta score and the normalized error value (E_n) give representative impression about the consistency of the results with regarding the claimed laboratories uncertainties. The robust z-score value gives opportunity to avoid the effect of the outlier values regardless of the claimed laboratory uncertainty.     

Relation between the compression yield stress and the mechanical loss peak of bisphenol-A-polycarbonate in the β transition range

The compression yield behaviour of polycarbonate, at constant strain-rate, over a wide range of temperatures, is described by the Ree-Eyring theory of non-Newtonian viscosity linked with a treatment which takes into account a distribution function of activation energies.The proposed yield mechanism relies on the assumption that the process considered in the Ree-Eyring theory and the loss peak revealed by oscillatory measurements are related to the same molecular movements.A relation is given between the transition conditions in yield measurements and in damping tests; its validity is checked.The broadness and the shape of the loss peak are correlated with a spectrum of activation energies.The compression yield-stress curve, giving the yield stress versus temperature at constant strain-rate, is computed from the measurements of the loss tangent, as a function of temperature, at the frequency corresponding to the strain-rate and is found to fit the data fairly well.     

Failure phenomena in two-dimensional multi-fibre microcomposites. Part 4: a Raman spectroscopic study on the influence of the matrix yield stress on stress concentrations

Raman spectroscopy was used to study the influence of the shear yield stress of the matrix on the stress situation in carbon/epoxy model composites containing a planar fibre array. The fibre used was a surface-treated high-modulus Tenax® HMS-40 carbon fibre showing good fibre/matrix adhesion. Three matrices were used, all consisting of a common epoxy resin and a mixture of a di-functional and a tri-functional aliphatic amine-based curing agent. By varying the ratio of the di-functional to the tri-functional curing agent, the shear yield stress of the matrix was varied. For all three matrices, it was found that in the area immediately neighbouring a fibre fracture, stress transfer takes place through a locally yielding matrix. More importantly, it was shown that the maximum interfacial shear stress approximately equals the shear yield stress of the bulk matrix. In addition, it was found that an increase in the shear yield stress of the matrix results in a decrease of both the ineffective length and the positively affected length. Further, the experimental results show that the shear yield stress of the matrix does not significantly influence the stress concentration in the fibres adjacent to a broken fibre.