Modelización multiescala del comportamiento interfacial nanoparticula-matriz en nanocomposites poliméricos de grafeno

El grafeno es un material en auge que está causando un gran impacto en diversos campos de investigación. El grafeno muestra unas excelentes propiedades mecánicas, térmicas, eléctricas y ópticas que hacen del mismo un material muy atractivo para múltiples aplicaciones, una de las cuales son los nanocomposites poliméricos. Desde el punto de vista teórico, la adición de grafeno sobre una matriz polimérica mejora considerablemente las propiedades mecánicas, eléctricas y térmicas. Sin embargo, desde el punto de vista experimental, no se ha conseguido los niveles de mejora esperados. Este fenómeno, se puede explicar por dos causas: una, la debilidad de la interfaz matriz-nanopartícula, y dos, por las desalineaciones o incorrecta dispersión del grafeno. Este trabajo muestra el desarrollo de una herramienta que considera el comportamiento, a nivel nanométrico, de la interfaz matriz-nanopartícula, incluyendo el daño existente en ésta, de modo que se pueden reproducir fenómenos detectados experimentalmente a nivel macrométrico, como la pérdida de rigidez con la deformación o el efecto de la desalineación de las nanopartículas. No existen a fecha de hoy muchos modelos teóricos con esta capacidad y correlación con los experimentos disponibles en la literatura. Se basa en el uso de mapas de daño, que permiten la homogeneización de las propiedades mecánicas en la interfaz, lo cual desbloquea el problema de capacidad de cálculo existente si se desea utilizar el modelo CZM a escala macrométrica

"Engineering Design" course transformación: From a conceive - design towards a complete CDIO approach

“Engineering Design” is a discipline aimed at improving our understanding about the development processes of novel and successful products, processes and systems in general, and at providing engineers with methodical steps for enhancing such processes. It may well be the engineering discipline more linked to the CDIO approach and to the conceive-design-implement-operate process. The benefits of applying “Engineering Design” principles are better appreciated when facing the development of complex systems. In the field of Mechanical Engineering some of the more complex systems an engineer can develop are advanced mechanical systems and machines. In this study we present the transformation process of an “Engineering Design” course, carried out in parallel to the implementation of the new Master’s Degree in Industrial Engineering at ETSII – TU Madrid. In the old Industrial Engineering plan of studies, implemented in 2000, the “Engineering Design” course was taught in the 5th academic year for Industrial Engineering students specializing in Mechanical Engineering and lasted for one semester. In the new Master’s Degree in Industrial Engineering, which started in 2014-2015, the “Engineering Design” course can be chosen by students from all Industrial Engineering specializations. The new subject lasts for two semesters and it is taught, in the 1st academic year of the Master’s Degree, to students having finished a four-year Bachelor’s Degree in Industrial Technologies. When transforming the course, our first aim was to let students live through a complete CDIO process, as having a two-semester structure gave us additional time for reaching the implementation and operation stages. With the old one-semester structure they could just focus on the conceptual and design phases. With the new approach their experience is more complete but several challenges arise, which are systematically analyzed in the following pages. A comparative study, taking account of the opinions of students and teachers is also presented and helps to support the benefits from complete CDIO experiences. Key aspects, including: student motivation, coordination between teachers, supervision of the projects under a tight schedule, rapid prototyping resources for reaching the implementation and operation stages, among others, are discussed and the more relevant lessons learned and proposals for improvement are put forward. To our knowledge it constitutes the first subject following a complete CDIO cycle in the field of Engineering Design applied to machines engineering in our country

Models for predicting friction coefficient and parameters with influence in elastohydrodynamic lubrication

This article shows different friction prediction models applicable to lubricants in point contacts under an elastohydrodynamic regime. The types of models used are two variations of the Newtonian theory, the Limiting Shear Stress model and the one based on Carreau's equation. The article sets out the theoretical calculation procedures and the ensuing equations for calculating the friction coefficient. The aims of the article are to study the effect of the parameters with influence on friction and to compare the model's results with those given by an experimental stage performed on a mini traction machine. This test system allows the measurement of friction coefficient in point contacts (ball–disc) under a wide range of variation of parameters such as temperature, slide-roll ratio, lubricant, material, load, or velocity

Cálculo de la resistencia a la fatiga superficial en superficies con lubricación elastohidrodinámica mediante el criterio de Dang Van

Los fenómenos de fatiga superficial en sus diferentes escalas (micropitting, pitting, spalling), aparecen en multitud de elementos mecánicos en contacto lubricado como pueden ser los engranajes o rodamientos o en elementos con contacto seco como son las ruedas de ferrocarril. Los defectos que aparecen en las superficies en contacto debidos a estos fenómenos empeoran las condiciones de funcionamiento, provocando vibraciones, desgaste prematuro y en los casos más extremos el fallo de la máquina. El procedimiento que se sugiere en este artículo se inicia con el cálculo de los parámetros que definen el contacto elastohidrodinámico y que tienen influencia en la resistencia a fatiga del contacto: espesor de película, distribución de presión y coeficiente de fricción. Una vez se tiene la presión y el coeficiente de fricción en la superficie de contacto, se procede al cálculo de las tensiones en el interior del material obteniendo la distribución de tensiones. Conocidas las tensiones en todo punto interior y el proceso de carga completo se aplica el criterio de fatiga de Dang Van y se verifica si existe, o no, probabilidad de fallo por fatiga. El criterio de Dang Van es aplicable para el cálculo de la vida a fatiga para un alto número de ciclos (donde el tiempo de propagación de grieta es despreciable frente al tiempo de iniciación) y se basa en la aproximación a la escala mesoscópica y la existencia de un plano crítico. Una vez descrita la teoría se propondrá una aplicación para el cálculo de tensiones en contactos puntuales y contactos lineales y el análisis de fatiga superficial en engranaje

The Evolution and Development of Mechanical Engineering through large cultural areas

Mechanical Engineering is probably the forerunner of many branches of Engineering and has persistently been their companion up to the present. For this reason, the History of Machines embraces a very broad period of the History of Mankind, and can be studied from many perspectives. This paper attempts to link progress in Mechanical Engineering to the great cultures that have arisen throughout the History of Mankind. This paper begins with the anonymous mechanical developments that appeared in Prehistory and opened up the way to the first civilisations, marked to a large extent, maybe, by Greco-Roman culture in Europe and by China in Asia. After them came the Islamic world, which, in the Middle Ages stimulated society to find new mechanical devices and set the foundations that would lead to the Renaissance. Outstanding in this period was the expansion of Italian, French and German creative and innovative thinking with its “Treatises on Machines”, which, for a short time, coincided with the advance of the Iberian Empire and the development of machinery for the New World. Finally, the Industrial Revolution became the climax of all previous developments and a period of rapid mechanical evolution began that was to be highly interesting from a historical and technological point of view. This was accompanied by a parallel interest in reflecting on and analysing machines, which has led to the appearance of countless “Treatises on Machines”

Analytical model for predicting friction in line contacts

This paper presents the development of an analytical model for the prediction of the friction coefficient in line contacts under thermal elastohydrodynamic lubrication (TEHL). A new theoretical equation is deduced for determining the friction coefficient, taking into account the rheology of common lubricants under TEHL. This approach also considers the heat generated and its penetration into the bulk of the contacting solids. Therefore, the increase in temperature and ensuing variations in the operating conditions are determined. In order to illustrate the use of the new model and verify its accuracy, an experimental stage is performed in a tribological test rig. The predictions of the proposed model are compared with the results obtained in the test rig and other data reported in the literature for diverse lubricants, showing a good agreement in every case

Neural Network and Training Strategy Design for Train Drivers’ Vibration Dose Simulation

Vibration can cause professional illnesses in train drivers, giving also rise to lawsuits to the employer. A possible cause may be the lack of systematic vibration estimation processes, due to operational complexities, subjectivities involved and the cost of dedicated tests. Estimation quality may be improved by using a driver seat model along with cabin floor vibration data acquired during the train dynamic approval tests. However, due to the nonlinearities present, analytical models frequently show inaccurate results

Perpendicular ultrasonic joining of steel to aluminium alloy plates

Standard and high strength steel plates were successfully joined to A2017 and A6061 aluminium plates by applying ultrasonic power perpendicular to the joint interface instead of tangentially, as in other ultrasonic joining methods Using a 2.9 mm diameter tip, cross tensile strengths of 281 N and 460 N were obtained when joining 0.5 mm thick A6061 plates to 0.5 mm thick commercial and high steel plates, respectively. When joining 0.5 mm thick A2017 plates, the cross tensile strengths were 343 N and 437 N, respectively. Temperature measured at less than 1?mm from the joining interface did not exceed 135 °C in any case. The joining mechanisms and resulting joint characteristics are investigated and discussed, alongside parameters affecting the resulting joint performance. Under optimal joining conditions, the cross tensile tests and cross section observations revealed that material fracture resulted in plug failure. No intermetallic layer has been detected with SEM analysis. The joint strengths obtained under those conditions were the maximum attainable for the tip diameter used, provided that they correspond to the stress intensities required to propagate cracks in the aluminium. These results support the conclusion that this ultrasonic joining method, being operationally similar to the method used in resistance spot welding, and providing good performance joints, could be used successfully for high throughput industrial applications taking advantage of the available spot welding tooling and operational procedures, as tangential movement of the whole plate is not required

Analysis of the effect of manufacturing imperfections in the elastic properties of platelet nanocomposites

We have developed and validated a conceptually simple model capable of predicting the macroscale elastic properties of a platelet nanocomposite. The model allows for studying the individual and combined effect of the parameters with influence on those properties, namely nanofiller weight fraction, misalignment, dispersion quality, size distribution and nanofiller-matrix interfacial characteristics. The model shows a very good correlation with experimental results. The interfacial characteristics under different strain states are evaluated at the nanoscale by means of a cohesive model which considers out-of-plane strains and angular distortions, so that the full, strain-dependent elastic tensor can be calculated, allowing for homogenization and subsequent study of the effect of filler orientation, dispersion quality and size distribution on the elastic properties at the macroscale. The use of a low complexity nanoscale model allows us to conceptually and quantitatively explain the causes underlying the divergences between the expected and experimental macroscale material stiffness experimentally found by different researchers

Epoxy toughening through high pressure and shear rate preprocessing

We have successfully conceived and demonstrated a simple, scalable process for improving the fracture energy of epoxy resins. the process is based on the combined application of high pressures (in the order of GPa) and shear rates (in the order of 106 s−1) in the pre-cured polymer, obtaining mechanical forces sufficiently high to increase the reactivity of the monomers due to the scission of the epoxy groups. To achieve these high values of pressure and shear rate, we take advantage of the elastohydrodynamic phenomena occurring at lubricated higher kinematic pairs as, for example, the rolling element – track pair in ball bearings. experimental results show that, under certain combinations of pressure and shear rate, a substantial improvement in fracture toughness is obtained. SeM observations, Raman spectroscopies, nanoindentation and Gpc and nMR measurements show that the process is able to increase the polymer chain length before curing, reducing the number of potential anchor points during the subsequent curing and hence reducing the crosslinking density. the chain lengths obtained are big enough to guarantee adequate stiffness and strength due to increased chain tangling, hence overcoming the drawbacks associated with other toughness promotion methods, such as stiffness and strength reduction