Squeeze-strengthening of magnetorheological fluids (part 1): Effect of geometry and fluid composition

Recent research has shown that magnetorheological fluid can undergo squeeze-strengthening when flow conditions promote filtration. While a Péclet number has been used to predict filtration in non-magnetic two-phase fluids submitted to slow compression, the approach has yet to be adapted to magnetorheological fluid behavior in order to predict the conditions leading to squeeze-strengthening behavior of magnetorheological fluid. In this article, a Péclet number is derived and adapted to the Bingham rheological model. This Péclet number is then compared to the experimental occurrence of squeeze-strengthening behavior obtained from several squeeze geometries and magnetorheological fluid compositions submitted to pure-squeeze conditions. Results show that the Péclet number well predicts the occurrence of squeeze-strengthening behavior in high-concentration magnetorheological fluid made from various particle sizes and using various squeeze geometries. Moreover, it is shown that squeeze-strengthening occurrence is increased when using annulus geometries or by increasing average particle radius. While lowering concentration increases filtration, tested conditions only led to squeeze-strengthening behavior after concentration had increased close to packing limit. Altogether, results suggest that the Péclet number derived in this study can be used to predict the occurrence of squeeze-strengthening for various magnetorheological fluids and squeeze geometries using the well-known rheological properties of magnetorheological fluids.

Quantitative Phase Field Study on the Precipitation Dynamics of γ′ Phase in Ni-Based Superalloys with Different Concentration

A quantitative phase field simulation was performed on the dynamics evolution of γ′ (L12-Ni3X) phase in Ni-based superalloys, the microstructure, the volume fraction and the particle size distribution (PSD) of γ′ phase for Ni-Al alloys aged at 1173K with the Al concentration c=0.178, 0.180 and 0.182 were investigated, and the results were compared with Lifshitz-Slyozov-Wagner (LSW) theory and Brailsford-Wynblatt (BW) theory. As the Al concentration increases the γ′ phase morphology changed from the separated cuboidal shape to the connected rectangle shape, the nucleation and growth of γ′ phase became faster and the volume fraction of the γ′ phase increased. The average particle radius <r> of γ′ phase and the aging time t has a exponent relationship <r> ~ tn at the coarsening stage with the exponents n=0.313, 0.235 and 0.204 for c=0.178, 0.180 and 0.182, respectively. The peaks of the fitted PSDs are less than the predicted value from the LSW theory and the fitted PSDs are wider than that of LSW predicted, while they are similar to that of the BW theory as the Al concentration increases. The peaks appear at a smaller r/<r> than the predictions of the LSW and BW theories.

Characterization of Particle Fracture in Dissimilar Friction Welding Containing Silver Interlayer

The present work discusses about the introduction of silver interlayers in dissimilar friction welding process. The characteristics of silver interlayer influenced friction weld are compared with the silver free dissimilar friction welding process. Particle fracture occurs commonly in welding process. It leads to poor quality of welds and decreases the strength of the weld which creates brittleness. Friction welding process itself reduces the particle fracture but for more precise and reliable welds, silver interlayer can be used. The introduction of silver interlayer not only reduces the particle fracture but also reduces average particle radius and leaves the particle volume fraction unchanged. So the friction welding process with silver interlayer produces more efficient welds. From all the considerations it is concluded that stable strong and friction welds with less particle fracture can be produced by the influence of silver interlayer in dissimilar friction welding.

Superparamagnetism and Microstructural Properties of Carbon Encapsulated Ni nanoparticle Assemblies

ABSTRACTCarbon encapsulated Ni nanoparticles (Ni(C)) were synthesized by modified arc-discharge reactor under methane atmosphere. The presence of carbon encapsulation is confirmed by HR-TEM imaging, and Nano-diffraction. The average particle radius is typically 10.5 nm with spherical shape. The intimate and contiguous carbon fringe around these Ni nanoparticles is good evidence for complete encapsulation by carbon shell layers.Superparamagnetic property studies were performed using SQUID magnetometer for the assemblies of Ni(C) nanoparticles. The blocking temperature (TB) is determined to around 115K at 1000Oe applied field. Above TB, the magnetization M (H, T) can be described by the classical Langevin function L using the relation, M/Ms(T=0) = coth(μH/kT)- kT/μH. The particle radius can be inferred from Langevin fit (particle moment μ) and blocking temperature theory (TB), which values are a little bigger than HR-TEM observations. It is suggested, these assemblies of carbon encapsulated Ni nanoparticles have been showed typical single-domain, field-dependent superparamagnetic relaxation properties.

The Structure of Shear Bands in Idealized Granular Materials

The structure of shear bands in granular materials was investigated by numerically simulating an idealized assembly of two-dimensional particles. Flexible stress-controlled boundaries were used instead of periodic boundaries to avoid constraining the motion of particles within the tested specimen. The particle displacement, particle rotations and rotations of the particle neighborhoods (macro-rotation) were examined within the shear band. The shear band width was found to decrease with axial strain from 18 and 15 times the average particle radius. The particle rotations and macro-rotations were concentrated inside the shear bands. The numerical simulations suggest that the particle rotations are induced by macro-rotations, and support the use of the micropolar theory for examining instable phenomena within granular materials.