Acoustic emission at the wheel-rail contact with micro-slip and stick-slip

The paper aimsto analyse the occurrence of acoustic emission at the wheel-rail contact during microslip. The experimental model allows the different loads and the low sliding speeds specific to the wheel-rail contact. It is determined experimentally the appearance of the stick-slip phenomenon at the Hertzian contact of cylinder type (fixed-wheel specimen) - plane (mobile with very low speed - rail specimen). The experimental stand simultaneously measures the normal force, the friction force and the acoustic emission at different normal forces, sliding speeds and rigidities of the wheel specimen fixing system. The specimens are made of UIC standard materials used in the driving wheels and rails. The stick-slip phenomenon occurs at low micro-slip speeds and normal bending stiffness. Experimentally, it is found that the jumps specific to the stick-slip phenomenon (friction coefficientCOF) are accompanied by the acoustic emission (AE) at the cylinder-plane interface. The energy emitted by AE (WAE) is correlated with the energy consumed by friction during the stick-slip period (WCOF).Postprint (published version

Stick-slip phenomena and acoustic emission in the Hertzian linear contact

AE detection and analysis usually requires a specific, costly platform due to its particular burst nature and high-frequency content. This experimental study investigates the relationship between low-demand acoustic emission parameters (AE) and the occurrence of stick–slip (SS) at the Hertzian linear contact. Hence, the correlation of basic AE characteristics (amplitude, energy, and evolution in time) with stick–slip characteristics (static and kinetic friction coefficients, amplitude, energy, and evolution in time) is pursued. Tribological tests were conducted on cylinder–plane specimens under dry friction conditions with different loads at different low driving speeds and Hertzian contact pressures at a constant stiffness. The AE, normal, and friction forces were recorded simultaneously on the experimental stand. At the cylinder–plane interface, the jumps specific to the stick–slip phenomenon (friction coefficient—COF) were followed after a few milliseconds by AE jump peaks. The results of the experiments show that the amplitude and energy generated by AE were sensitive to the occurrence of the stick–slip phenomenon, while the AE and COF energies in the stick and slip phases had the same law of variation based on the driving velocities. The results show that the amplitude and energy of the sampled low-frequency AE signals were enough to detect the friction in SS and demonstrate the potential of AE as a tool for detecting and monitoring the tribological behaviour of SS at the linear Hertzian contact.Peer ReviewedPostprint (published version

Fractal evaluation aspects in characterizing the roughness of a driving wheel from a locomotive

For comprehending tribological phenomena such as wear, friction, contact deformation and tightness of contact joints is the essential characterisation of the multiple scale topography of roughness surfaces. Wheel roughness obtained through direct measurement and then a description of wheel roughness has been obtained using the fractal function. The statistical parameters and fractal sizes for drive wheels which ran 2000 km and another ones new wheels have been analysed.By registering the results and programmable automaton of the roughness, the principal statistical characteristics were determined to be stands out the Abbott-Firestone curve and respectively, the fractal character. The appearance of roughness on the surface of the wheel causes the production of vertical vibrations that act on the complex wheel-rail system.In the analysis of the wheel irregularity, the fractal geometry was applied, so that to obtain the fractal parameters Dw, respectively Ltw(opothesylength), the method of the structure function was used [1,2].Therefore, the experimental measurement and obtained results processed presentedPostprint (published version

Fractal approach to wheel-rail roughness contact theory

Tesi en modalitat de cotutela: Universitat Politècnica de Catalunya i Universitatea Națională de Știință și Tehnologie Politehnica București(English) In this study, the Weierstrass fractal function was used to model the surface roughness of railway tracks and, subsequently, the static coefficient of friction (COF) for Hertzian rail-wheel contact and cylinder-plane interactions. This methodology is based on the assumption that the fractal nature of surface roughness can be effectively captured using fractal parameters, which significantly influence the contact mechanics and frictional behaviour observed in railway systems. Initially, the study focused on a detailed modelling of the railway track surface roughness. To validate the efficacy of this model, a rigorous and meticulous comparison between the theoretical results obtained and the experimental data was conducted. These data were collected through extensive roughness measurements carried out at the Faurei Railway Testing Centre in Romania. For robust validation, 41 roughness parameters derived from the fractal model were compared with those measured experimentally. This comprehensive approach allowed not only for the validation of the fractal model but also for its refinement by identifying which parameters are most critical in representing the real roughness of the tracks, as well as the minimum measuring length for determining acoustic roughness. The experimental determination of the static COF was conducted through laboratory tests involving two cylinder-plane specimens, made from the same material as the railway wheel and track. This configuration was chosen to closely replicate the contact conditions found in actual wheel-rail interactions, thus ensuring that the experimental data collected were representative and reliable. To precisely determine the static COF, the stick-slip (SS) process was meticulously characterized by monitoring acoustic emissions (AE), allowing for convenient identification of each phase. The acoustic emissions provided a non-invasive means not only to detect transitions between stick and slip phases but also to understand the underlying mechanisms driving these transitions. This dual approach of theoretical modelling and experimental validation forms the core of this methodology and offers a comprehensive understanding of the frictional phenomena occurring within railway systems, which essential for designing and maintaining safer and more efficient railway operations. The insights gained from this study are expected to significantly contribute to the development of new standards and practices in railway engineering, particularly in areas of wear reduction, noise control, and overall system reliability. The results of this research pave the way for a more nuanced understanding of the fractal nature of contact surfaces and their implications on tribological performance.(Español) En este estudio, se utilizó la función fractal de Weierstrass para modelar la rugosidad superficial de las vías ferroviarias y, posteriormente, el coeficiente de fricción estática (COF) para el contacto rueda-rail Hertziano y las interacciones cilindro-plano. Esta metodología se basa en la suposición de que la naturaleza fractal de la rugosidad superficial puede capturarse eficazmente utilizando parámetros fractales, los cuales influyen significativamente en la mecánica de contacto y el comportamiento friccional observado en los sistemas ferroviarios. Inicialmente, el estudio se centró en un modelado detallado de la rugosidad superficial de las vías ferroviarias. Para validar la eficacia de este modelo, se llevó a cabo una comparación rigurosa y meticulosa entre los resultados teóricos obtenidos y los datos experimentales. Estos datos se recopilaron a través de mediciones extensivas de rugosidad realizadas en el Centro de Pruebas Ferroviarias de Faurei en Rumanía. Para una validación robusta, se compararon 41 parámetros de rugosidad derivados del modelo fractal con aquellos medidos experimentalmente. Este enfoque integral permitió no solo la validación del modelo fractal sino también su refinamiento al identificar qué parámetros son más críticos para representar la rugosidad real de las vías, así como la longitud mínima de medición para determinar la rugosidad acústica. La determinación experimental del COF estático se realizó mediante pruebas de laboratorio que involucraron dos muestras cilindro-plano, hechas del mismo material que la rueda y la vía ferroviaria. Esta configuración se eligió para replicar de cerca las condiciones de contacto encontradas en las interacciones reales rueda-rail, asegurando así que los datos experimentales recopilados fueran representativos y fiables. Para determinar con precisión el COF estático, se caracterizó meticulosamente el proceso de stick-slip (SS) mediante el monitoreo de emisiones acústicas (AE), lo que permitió la identificación conveniente de cada fase. Las emisiones acústicas proporcionaron un medio no invasivo no solo para detectar transiciones entre las fases de stick y slip sino también para comprender los mecanismos subyacentes que impulsan estas transiciones. Este doble enfoque de modelado teórico y validación experimental forma el núcleo de esta metodología y ofrece una comprensión integral de los fenómenos friccionales que ocurren dentro de los sistemas ferroviarios, lo cual es esencial para diseñar y mantener operaciones ferroviarias más seguras y eficientes. Se espera que las perspectivas obtenidas de este estudio contribuyan significativamente al desarrollo de nuevos estándares y prácticas en ingeniería ferroviaria, particularmente en áreas de reducción de desgaste, control de ruido y fiabilidad general del sistema.DOCTORAT EN ENGINYERIA MECÀNICA, FLUIDS I AERONÀUTICA (Pla 2013

Fractal rigidity of the wheel-rail contact

One of the essential parameters of the roughness with effects on the vibration of the wheel and the rail is the contact stiffness. This rigidity considers the geometry and material of the wheel and rail, which are considered elastic. Also, the micro-geometry of the roughness and the load on the wheel, with elastic, elastoplastic and plastic deformations, depending on the deterministic or random character of the roughness were considered. The dependence of the stiffness on the deformation state of the roughness may explain the different amplitude of the vertical vibrations at the same force on the wheel and the same height of the roughness, but one roughness with different fractal parameters. For the analysis of the dynamic forces in contact, the continuous and underived analytical function that modelled the roughness measured experimentally on the rail was applied.Peer ReviewedPostprint (published version

Fractal evaluation aspects in characterizing the roughness of a driving wheel from a locomotive

For comprehending tribological phenomena such as wear, friction, contact deformation and tightness of contact joints is the essential characterisation of the multiple scale topography of roughness surfaces. Wheel roughness obtained through direct measurement and then a description of wheel roughness has been obtained using the fractal function. The statistical parameters and fractal sizes for drive wheels which ran 2000 km and another ones new wheels have been analysed.By registering the results and programmable automaton of the roughness, the principal statistical characteristics were determined to be stands out the Abbott-Firestone curve and respectively, the fractal character. The appearance of roughness on the surface of the wheel causes the production of vertical vibrations that act on the complex wheel-rail system.In the analysis of the wheel irregularity, the fractal geometry was applied, so that to obtain the fractal parameters Dw, respectively Ltw(opothesylength), the method of the structure function was used [1,2].Therefore, the experimental measurement and obtained results processed presente

Low incidence of SARS-CoV-2, risk factors of mortality and the course of illness in the French national cohort of dialysis patients

International audienceThe aim of this study was to estimate the incidence of COVID-19 disease in the French national population of dialysis patients, their course of illness and to identify the risk factors associated with mortality. Our study included all patients on dialysis recorded in the French REIN Registry in April 2020. Clinical characteristics at last follow-up and the evolution of COVID-19 illness severity over time were recorded for diagnosed cases (either suspicious clinical symptoms, characteristic signs on the chest scan or a positive reverse transcription polymerase chain reaction) for SARS-CoV-2. A total of 1,621 infected patients were reported on the REIN registry from March 16th, 2020 to May 4th, 2020. Of these, 344 died. The prevalence of COVID-19 patients varied from less than 1% to 10% between regions. The probability of being a case was higher in males, patients with diabetes, those in need of assistance for transfer or treated at a self-care unit. Dialysis at home was associated with a lower probability of being infected as was being a smoker, a former smoker, having an active malignancy, or peripheral vascular disease. Mortality in diagnosed cases (21%) was associated with the same causes as in the general population. Higher age, hypoalbuminemia and the presence of an ischemic heart disease were statistically independently associated with a higher risk of death. Being treated at a selfcare unit was associated with a lower risk. Thus, our study showed a relatively low frequency of COVID-19 among dialysis patients contrary to what might have been assumed