Dynamic optimization of track components to minimize rail corrugation

In this paper, the influence on corrugation of the most significant track parameters has been examined. After this parametric study, the optimization of the track parameters to minimize the undulatory wear growth has been achieved. Finally, the influence of the dispersion of the track and contact parameters on corrugation growth has been studied. A method has been developed to obtain an optimal solution of the track parameters which minimizes corrugation growth, thus ensuring that this solution remains optimum despite dispersion of track parameters and wheel–rail contact uncertainties. This work is based on the computer application RACING (RAil Corrugation INitiation and Growth) which has been developed by the authors to predict rail corrugation features.     

Softness of the bacterial cell wall of Streptococcus mitis as probed by microelectrophoresis

Chemical and structural complexity of bacterial cell surfaces complicate accurate quantification of cell surfaces properties. The presence of fibrils, fimbriae or other surface appendages on bacterial cell surfaces largely influence those properties and would therefore play a major function in interfacial phenomena as aggregation and adhesion. The electrophoretic softness and fixed charge density in the polyelectrolyte layer of nine Streptococcus mitis strains, usually carrying long sparsely distributed fibrils, were determined by the soft particle analysis using measured electrophoretic mobilities as a function of the ionic strength. In general, S. mitis cell surfaces are electrophoretically soft (1.0-2.5 nm) with a fixed negative charge density of -1.2 to -4.3 x 10(6) Cm(-3). Further, a comparison with surfaces of other bacterial strains that are reported to be soft indicates that the Ohshima soft layer model does not provide information on the surface morphology causing the softness. The most likely reason is that the electroosmotic flow occurs only in the very outer region of thick extracellular surface layers. Nevertheless, determining the surface softness is essential for proper characterization of the cell surface electrostatics.     

Spatial distribution profiles of magnesium and strontium in speleothems using laser-induced breakdown spectrometry

Laser-induced breakdown spectrometry (LIBS) has been applied to spatially locate several atomic species in speleothems taken from the Nerja’s Cave (Málaga, Spain). Spatial distribution profiles of Mg at 285.21 nm and Sr at 407.77 nm were obtained while the laser was rastered through different paths along the sample. These elements were selected due to their importance as palaeoclimatic indicators. The 532 nm output of a Nd:YAG laser was used to irradiate the samples and generate the plasma that was spectrally analyzed and detected by using an intensified CCD detector. The signals were normalized to the Ca line to minimize pulse-to-pulse fluctuations in the laser source. Several studies were carried out to check for the point-to-point heterogeneity of the natural speleothem. Received: 1 August 1997 / Revised: 23 October 1997 / Accepted: 30 October 1997     

Two-pulse delayed 532-nm laser ionization of metals using collinear sub-threshold beams

Two frequency-doubled Nd:YAG lasers collinearly aligned were used to ionize different metals at specific interpulse delays. The beams were independently operated in order to attain full control over the energy. Each laser beam was always set at fluences below the ionization threshold and an evaluation of the effect that the interpulse delay has on the material ionization and the LIMS signal was performed. The different metallic targets studied (Cu, Si, Al, Ti, Fe, 314 AISI stainless steel) exhibit a characteristic ionization feature consisting of a clear enhancement in the ionization yield at interpulse delays around 60 ns when analyzed as pure foils. In addition, an improvement in the spectral resolution is observed at the specific interpulse delay. Our results indicate that proper control of the energy allows optimization of the different steps in the ionization process and they suggest that the effect of the first laser pulse impinging on the surface enhances the way in which the second pulse interacts with the solid.     

Atomic/molecular depth profiling of nanometric‐metallized polymer thin films by secondary ion mass spectrometry

The capability of secondary ion mass spectrometry (SIMS) to perform atomic and molecular in‐depth analysis in complex nanometric‐metallized thin polymer films used to manufacture capacitors is demonstrated through three different case studies related to failure analysis. The excellent repeatability and sensitivity of the technique allow us to study the degradation process of the nanometric‐metallized layer in the capacitor films and the accurate location of the metal‐polymer interface. The analysis of the sample is challenging due to the extreme difference in conductivity between layers, and the reduced thickness of the metallization grown on top of a rough polymeric base. However, SIMS has provided reliable and reproducible results with relative standard deviation (RSD) values better than 1.5% in the metallic layer thickness estimation. The detailed information of atomic and molecular ion in‐depth distributions provided by SIMS depth profiling has allowed the identification of different factors (demetallization, generation of interstitial oxide regions, and diffusion processes or modification in the metallization thickness) that can be directly related to the origin of the lack of performance of the mounted devices. Copyright © 2010 John Wiley & Sons, Ltd.     

Investigation of metallic interdiffusion in Al x Ga1−x N/GaN/sapphire heterostructures used for microelectronic devices by SEM/EDX and SIMS depth profiling

Secondary ion mass spectrometry (SIMS) depth profiling has been applied to the study of the thermal annealing of ohmic contacts for high electron mobility transistors. The metallic stacks (Ti/Al/Ni/Au) were deposited over the Al_0.28Ga_0.72N/GaN/sapphire heterostructures and subjected to a rapid thermal annealing (850 °C for 30 s under N₂ atmosphere) to improve the contact performance. The surface morphology and the in-depth chemical distribution of the layered contacts were severely modified due to the treatment. These modifications have been analyzed by SIMS depth profiling and scanning electron microscopy–energy-dispersive X-ray microanalysis. The SIMS analysis conditions have been optimized to achieve simultaneously good sensitivity and to avoid ion-induced mixing effects produced by the primary beam sputtering.     

On the interplay between strain rate and strain rate sensitivity on flow localization in the dynamic expansion of ductile rings

In this work a stability analysis on flow localization in the dynamic expansion of ductile rings is conducted. Within a 1-D theoretical framework, the boundary value problem of a radially expanding thin ring is posed. Based on a previous work, the equations governing the stretching process of the expanding ring are derived and solved using a linear perturbation method. Then, three different perfectly plastic material constitutive behaviours are analysed: the rate independent material, the rate dependent material showing constant logarithmic rate sensitivity and the rate dependent material showing non-constant and non-monotonic logarithmic rate sensitivity. The latter allows to investigate the interaction between inertia and strain rate sensitivity on necking formation. The main feature of this work is rationally demonstrate that under certain loading conditions and material behaviours: (1) decreasing rate sensitivity may not lead to more unstable material, (2) increasing loading rate may not lead to more stable material. This finding reveals that the relation between rate sensitivity and loading rate controls the unstable flow growth. Additionally a finite element model of the ring expansion problem is built in ABAQUS/Explicit. The stability analysis properly reflects the results obtained from the numerical simulations. Both procedures, perturbation analysis and numerical simulations, allow for emphasizing the interplay between rate sensitivity and inertia on strain localization.     

A rational fraction polynomials model to study vertical dynamic wheel–rail interaction

This paper presents a model designed to study vertical interactions between wheel and rail when the wheel moves over a rail welding. The model focuses on the spatial domain, and is drawn up in a simple fashion from track receptances. The paper obtains the receptances from a full track model in the frequency domain already developed by the authors, which includes deformation of the rail section and propagation of bending, elongation and torsional waves along an infinite track. Transformation between domains was secured by applying a modified rational fraction polynomials method. This obtains a track model with very few degrees of freedom, and thus with minimum time consumption for integration, with a good match to the original model over a sufficiently broad range of frequencies. Wheel–rail interaction is modelled on a non-linear Hertzian spring, and consideration is given to parametric excitation caused by the wheel moving over a sleeper, since this is a moving wheel model and not a moving irregularity model. The model is used to study the dynamic loads and displacements emerging at the wheel–rail contact passing over a welding defect at different speeds.