Search

Fretting fatigue manifests in mechanical systems exposed to contact forces that vary over time, resulting in premature cracks that can lead to component failure. Shot peening treatment is one of the palliatives to mitigate fretting fatigue problems. Recent studies have noted that the fretting fatigue is also extended by exclusively applying SP to the contact pad surface. This suggests that the improvement in the fatigue live produced by SP comes not only from the compressive residual stresses but also due to the characteristic surface roughness from the SP treatment. For this reason, the main goal of this work is to analyse how the shot peened roughness affects the fretting fatigue life. To do so, a confocal microscope was used to measure various profiles of shot peened pads. Using these profiles, and under fretting loads combination, the contact stress and strain field are calculated, followed by the application of a fatigue model. Finally, the estimated fatigue life is contrasted with empirical data derived from a research published by the authors. The study reveals that the fretting fatigue life is influenced by the surface roughness, although to a lesser extent with respect to residual stresses. Besides, the Smith-Watson-Toper parameter and stress intensity factor were analysed, suggesting that the rough surface primarily affects the initiation phase rather than the propagation phase, as it was expected.

In the present work, the fatigue resistance values of Scalmalloy specimens obtained by selective laser melting (SLM) additive manufacturing (AM) are experimentally studied. The fatigue levels of specimens with and without notches after applying different surface treatments, such as sand blasting, shot peening, isotropic finishing, and polishing alone or in combination, are analyzed. To understand the fatigue behavior of this material, the surface finishes and internal residual stresses of the specimens are analyzed. Moreover, the fracture surface of each sample is observed by electron microscopy to determine the failure mechanism. There is a clear separation between two different types of failure: (1) rupture by a crack generated on the surface associated with high applied stress values and (2) failure by a crack generated on an internal defect associated with low applied stress values.

Fretting fatigue is a material damage phenomenon that produces damage on the contact surface, such as wear and surface cracks that can grow through the specimen. It is possible to reproduce the phenomenon in a laboratory using different geometries. In the absence of a common guide, this work studies the effect of different geometries of specimens and contact pairs on the fretting fatigue behaviour. This paper compares the results of a fretting fatigue test campaign made with cylindrical contact pairs made of AL7075-T651 alloy. In addition, 3D numerical models are developed to analyse the stress/strain fields and asses the total life. According to the experimental and numerical results, the differences of total life obtained between the geometries considered are negligible.

The present research work deals with the fatigue assessment of an Al 7075-T651 subjected to fretting fatigue testing characterised by cylindrical contact. A small rolling of the pads is experimentally observed and, consequently, the contact surface stresses are different to the hertzian one. Therefore, the methodology, recently proposed by some of the present authors for the fatigue assessment of fretting-affected metallic components, is here employed for the first time in conjunction with a FE numerical model able to take into account the effect of the rolling of the pads, in order to find out both the crack nucleation orientation and the lifetime for each specimen of the experimental campaign analysed.

In this article a new fretting fatigue life prediction model is presented. The model can be classified as a variable crack initiation length: the crack initiation and crack propagation phases are calculated as a function of the crack initiation length, and among all the feasible crack initiation lengths and orientations, that producing the minimum fatigue life is considered. In this new proposal the crack direction is automatically determined as a function of fatigue parameters in both phases: initiation and propagation. The model is applied to a wide experimental campaign of fretting fatigue tests, and excellent correlation is obtained between experimental and predicted fretting fatigue lives and crack paths.

Fretting fatigue is a material damage phenomenon produced in mechanical contacts. This phenomenon mainly produces surface cracking, and depending on the fretting regime, surface wear. In the bibliography many different types of fretting tests, with different geometries have been proposed. This paper numerically analyses, via FEM models, some fretting tests with cylindrical pads and test specimens having different geometries that are frequently used in the literature. In such cases, non-negligible 3D effects are produced. The differences between the contact stress field obtained with different approaches are analysed and discussed.

Additive manufacturing (AM) of metallic parts is a relatively new manufacturing procedure. Many industry sectors, such as the aerospace or automotive sectors, have started to apply this technology to produce some elements, thus reducing costs and weight. Several metallic alloys have been employed for AM. Due to the high strength-to-density ratio, Ti6Al4V alloy is probably the alloy most used for AM in the aerospace industry. This alloy usually shows good static strength properties. However, the presence of internal defects and the surface roughness result in a fatigue strength that is clearly lower than that of materials produced by traditional pro cesses. Moreover, the scatter of the fatigue results is generally higher than in the case of wrought pieces. Different treatments have been proposed to improve the fatigue behavior by reducing internal defects and roughness or generating a favorable residual stress field. In this work, selected surface treatments were considered to improve the fatigue strength of AM parts, including shot and laser peening as well as a combination of shot peening plus chemical assisted surface enhancement (CASE®). Three groups of specimens, each with one of the surface treatments, were fatigue tested to compare the results produced by these treatments. The residual stresses, roughness and hardness produced by the treatments were analyzed. After testing, the fracture surfaces were also analyzed to better understand the fatigue process of the different groups of specimens. The results indicate that laser peening produced the best results, followed by shot peening plus CASE and shot peening. In all three cases, the fatigue strength was much higher than that of the reference group without surface treatment. It was also observed that all failures initiated from an interior defect in the shot peening plus CASE group, four out of six failures in the laser peened group, but only one failure in the case of shot peened group and none in the referenc

This work proposes a methodology to compute fretting fatigue crack initiation and total life by iterative models calibrated with fatigue stress–strain-life curves. One of the main novelty of such models is the assumption that the crack initiation length is determined by the size of the critical distance. To separate initiation and propa- gation lives, a numerical approach with finite elements is used. The methodology incorporates not only the multiaxial non-proportional characteristic of the stress field in fretting problems, but also the stress gradient effect. Crack initiation path is obtained by means of the SWT parameter, and to validate it, the analysed results are compared with tests considering a cylindrical-flat configuration (Aluminium 7075-T651 alloy). Most of the life initiation estimates, as well as the total life ones, are within a scatter band of 3.

In the present paper, an experimental campaign performed on shot peened Ti6Al4V specimens under fretting fatigue, available in the literature, is simulated by using the Carpinteri et al. criterion. The experiments examined were carried out using a standard configuration, that is, a cylinder against a flat specimen. The fretting loading is a combination of low-cycle fatigue and high-cycle fatigue, and the corresponding stress field is determined by a FE analysis. A theoretical law is applied to describe the relaxed residual stress produced by shot peening treatment and fatigue loading.

This work performs a detailed analysis of the cracks obtained in fretting fatigue tests with cylindrical contact in the presence of a small oscillatory rolling of the contact pad. To do so, fretting fatigue tests have been carried out. Preliminary observations indicate that the contact area is larger than the theoretical one according to Hertz’s theory. This could mean that, due to the contact geometry and the stiffness of the test setup, rolling of the contact pad is occurring during tests. To reproduce this phenomenon, a 2D numerical model is developed. The results are compared with actual crack profile measurements.

The aim of this work is to perform a detailed analysis of the cracks paths obtained in fretting fatigue tests with cylindrical contact but taking into account the presence of an unavoidable (due to the fretting device’s stiffness) small oscillatory rolling. In order to obtain the crack paths crack surfaces were measured with a confocal microscope after the tests. Besides, the contact area and the surface crack initiation location were obtained by means of an optical microscope. The measurements indicate that the contact area is substantially larger than the theoretical one according to Hertz´s theory, contrary to tests done with only static normal load, in which both theoretical and experimental areas match perfectly. This observation means, that, due to the surface contact pad’s geometry (cylindrical) and the stiffness of the test setup, rolling is occurring during tests when tangential loading is developed. To reproduce this phenomenon, a 2D FEM model is developed. Stress/strain fields along the fretting cycle are analysed, noticing a substantial change of the contact surface hot-spot point and surface contact stress distribution, when compared with the non-rolling case. To predict the initial crack path, a previously developed model, based on a critical plane parameter, is applied using FEM stress/strain results. The results obtained show a better prediction of the surface hot-spot point and initial crack orientation estimation, when compared with the non-rolling case, and considering as a reference the experimental crack paths measured via confocal microscope.

El presente trabajo versa sobre el estudio generalizado de un problema de contacto en el que intervienen tanto rodadura como fuerzas tangenciales oscilantes. Para ello, se ha partido de un modelo en deformación bidimensional plano-cilindro en el que se simulan diferentes relaciones de carga tangencial y rodadura. El objetivo es evaluar las tensiones que se producen en la zona del contacto, con la intención de proyectar un estudio posterior de fretting, problema de fatiga tan presente en la ingeniería actual. Dicho estudio desborda los contenidos que se tratan en este trabajo, por lo que no serán abarcados con tanta profundidad., The present work deals with the generalized study of a contact problem in which both rolling and oscillating tangential forces intervene. To do this, we have started from a plane-cylinder twodimensional deformation model in which different relationships of rolling and tangential load are simulated. The objective is to evaluate the stresses that occur in the contact area, with the intention of projecting a later study of fretting, a fatigue problem so present in current engineering. Said study goes beyond the contents that are dealt with in this work, so it will not be covered in such depth.

Shot peening (SP) is frequently used as a palliative measure against metal fatigue in many engineering components. The performance of this surface treatment depends on different factors including the material, shape and loading conditions of the component, as well as process parameters. Fatigue improvement due to SP depends to a great extent upon the in-depth compressive residual stress profile produced in the specimen. In this work, we study the optimum shape for a residual stress profile in terms of fatigue behaviour. For this task, we assume a residual stress profile that is qualitatively similar to that produced by SP in many metals. Based on this generic profile, we analyse the optimum shape for two simple, but noteworthy, fatigue cases: plain fatigue and notch fatigue. The analysis is conducted in the “fatigue damage tolerance design” framework, in which a certain initial defect is assumed to be present in the component under study.

In the present work a series of fatigue tests on Ti6Al4V SLM parts are analyzed via both SEM and confocal microscopy. On the one hand, fracture surfaces are studied, and a common pattern is found, formed by a series of different textures which show the complex crack front evolution from crack initiation in a particular internal defect to complete failure. On the other hand, fatigue strength is observed to highly depend on the defect where initiation takes place, so experimental observation of that critical entity is carried out. Both defect location within the specimen and shape are studied, considering the crack-like or blunt feature of every defect. Once experimental analysis is complete, numerical simulation is attempted. By making use of critical defect and residual stress measurements obtained experimentally, both fatigue strength and crack front evolution are estimated.

El objetivo principal del proyecto es el diseño de un basculante para el prototipo con el que el equipo USRacing participará en la VI edición de la competición MotoStudent. Para ello, en primer lugar, se realiza una revisión histórica de los sistemas de suspensión traseros de las motocicletas, además de una breve descripción de las características fundamentales de los prototipos diseñados por el equipo en ediciones anteriores. En segundo lugar, se definen los requisitos de diseño que debe cumplir el basculante para que se considere apto para la fabricación y su posterior implementación en el prototipo. Esto incluye la definición de los distintos tipos de ensayos que deben superar los modelos diseñados y el cálculo de las cargas implicadas en los mismos. A continuación, se describe el proceso de diseño del basculante, el cual es iterativo. Consiste en elaborar el modelo con el software específico Catia, para exportarlo posteriormente al programa en el que se ejecutan las simulaciones (ANSYS Workbench) y, obtener unos resultados, a partir de los cuales se va optimizando el diseño. Este procedimiento se repite hasta que se obtiene un modelo que cumple con todos los requisitos. Una vez obtenido el diseño definitivo, este se somete a un estudio para analizar su resistencia a fatiga y, así, poder predecir su vida. De manera previa, se analiza la convergencia de los valores de tensión obtenidos en los ensayos, para garantizar el correcto cálculo de la vida a fatiga. Finalmente, se exponen las conclusiones del proyecto, entre las que se incluyen las principales novedades del mismo. También se comentan las posibles mejoras y líneas de trabajo futuras, que permitirían optimizar el proceso de diseño e incrementar el nivel de fiabilidad de los resultados., The aim of this project is the design of a swingarm projected for the motorbike prototype with which the USRacing team will participate in the 6th edition of the MotoStudent racing competition. For this purpose, the study begins with an historical review focusing on the motorbikes rear suspension systems, together with a brief description of the key characteristics of the early prototypes designed by the team for previous editions. Secondly, this work presents the design requirements that the swingarm must meet in order to be considered suitable for manufacturing and subsequent implementation in the prototype. This section includes the definition of the different types of tests that the designed models have to go through as well as the loads calculation involved in them. At this stage, the swingarm design procedure is then developed. It consists in the elaboration of the model, using the specific software Catia, followed by the subsequent import of this model into the program in which the tests are carried out (ANSYS Workbench) to obtain some results. The design procedure uses these outcomes to follow an optimization process until all the previously imposed requirements are fulfilled. Once the final design is achieved, it is subjected to a fatigue strength study to predict its life cycle. Previously, the convergence of the stress values obtained in the tests is analysed in order to guarantee the correct calculation of the fatigue life. Finally, the conclusions of this study are presented, in parallel with the main novelties contributed by this project. Possible improvements and some future lines of work are also commented on. These possible advances would potentially allow the design process to be optimised and the level of the results reliability to be increased.

High stress concentrations, slip and wear appear in shrink-fitted shafts near the contact edge in operational conditions. Due to these high stresses, and time fluctuation nature of the loads supported by shafts, cracks rapidly initiate at this surface location. This combination of events is known as fretting fatigue, which is a damage phenomenon produced in many mechanical joints. Several solutions to increase fretting fatigue life are found in practice, they include the introduction of compressive residual stresses or the reduction of stresses through a geometry modification, e.g., a slot in the shaft outside of the contact, an increase of the shaft's diameter in the contact zone, etc. This paper suggests a new geometry modification based on an increasingly popular technique that allows the manufacturing of pieces of difficult shapes, and nearly impossible to make with other techniques: additive manufacturing (AM). The idea is to introduce an internal void in the shaft, with a toroidal shape and beneath the contact edge. This feature makes the void surrounding zone more flexible, thus, reducing the contact stress field near the contact edge zone. Several geometrical configurations are numerically studied, concluding that, there is an optimal position of the void that produces, in terms of a multiaxial fatigue parameter, a significant decrease of the fatigue damage on the interest zone.

Fretting fatigue failure occurs in shrink-fitted assemblies due to the combination of high stresses and relative displacements near the contact edge. Due to these high stresses, fatigue crack initiates followed by crack propagation until final rupture. Additive manufacturing (AM) is a game changing technology, which enables new component capabilities that cannot be manufactured with conventional techniques. This research work analyses numerically the influence of an artificial internal stress relief toroidal groove inside a shrink-fitted shaft, which could be manufactured using AM technology. Due to the toroidal void, the stress/strain fields are redistributed improving the fretting fatigue crack initiation and propagation lifetimes. To do so, 2D finite element models are created in Abaqus software with and without the internal groove. To estimate the fretting fatigue initiation and propagation lifetime and crack propagation direction, critical plane methods are used. In terms of the crack propagation, eXtended Finite Element Method (XFEM) is used to simulate mixed mode crack advancing in a single mesh structure. Finally, the obtained results with and without void were compared concluding with significant improvements in terms of total fatigue lifetime.

Fatigue failure of cables and strands is a common and complex problem. Failure is typically caused by different combinations of time-variable bending and axial forces. In addition to these loads, contact stresses between wires may play an important role in the fatigue failure of cables. The present work aims to provide deep insight into the fatigue failure of a seven-wire stainless steel strand subjected to a combination of variable axial and bending loads. To avoid side effects in the analysis, fatigue failure of the strand close to the clamps is prevented. Several tests were performed with a new device specifically designed to avoid failure near the clamps. Thus, failure is always produced at the middle length of the specimen. Test simulations were performed by employing the finite element method. The numerical results were validated via comparisons with experimental data. Finally, life prediction curves were obtained.

Se va a estudiar el comportamiento de una probeta ante un ensayo de fretting a alta temperatura. El ensayo se llevará a cabo gracias a dos puentes de fretting situados a cada lado de la probeta. El proyecto consiste en el diseño de un montaje el cual se encarga de conectar la probeta a las mordazas de una máquina hidráulica. El programa utilizado para el diseño de cada uno de los elementos del montaje es Solid Edge. Una vez diseñado cada uno de los elementos que conformen el útil de conexión se procederá a realizar un análisis mediante elementos finitos a través del programa ANSYS Workbench al conjunto completo para estudiar el comportamiento de este ante las cargas cíclicas llevadas a cabo en el ensayo de fretting. Una vez diseñado y estudiado el conjunto que conecta las mordazas de la máquina con la probeta, se analizará el comportamiento de esta en el laboratorio del departamento de ingeniería mecánica y fabricación de la E.T.S.I. El análisis del comportamiento físico tanto de la probeta como de los puentes de fretting será posible gracias a la implantación de una serie de bandas extensiométricas. Este proyecto no incluirá dicho análisis en el laboratorio pero sí un estudio sobre las bandas que se van a utilizar. Además de la máquina hidráulica, se necesitará de un horno el cual se encargue de aportar la temperatura a la que queremos realizar el ensayo (en torno a 650ºC)., The behavior of a specimen will be studied with a high temperature fretting test. The test will be carried out thanks to two fretting bridges located on each side of the specimen. The project consists of the design of an assembly which is responsible for connecting the specimen to the jaws of a hydraulic machine. The program used to design each of the assembly elements is Solid Edge. Once each of the elements that make up the connection tool has been designed, an analysis will be carried out using finite elements through the ANSYS Workbench program for the complete set to study its behavior in the face of the cyclical loads carried out in the fretting test. Once the assembly that connects the machine jaws with the specimen has been designed and studied, its behavior will be analyzed in the laboratory of the mechanical engineering and manufacturing department of the E.T.S.I. The analysis of the physical behavior of both the specimen and the fretting bridges will be possible thanks to the implantation of a series of strain gages. This project will not include such analysis in the laboratory but will include a study on the bands that will be used. In addition to the hydraulic machine, an oven will be needed which is responsible for providing the temperature at which we want to carry out the test (around 650ºC).

El presente trabajo comprende el análisis numérico y experimental de una serie de probetas concebidas mediante fabricación aditiva y ensayadas a fatiga fuera del alcance de este documento. El análisis experimental se centra en la observación, mediante microscopía óptica, electrónica y confocal de las diferentes regiones apreciables en las superficies de fractura y de los defectos críticos, que son aquellos que dieron lugar a la grieta que propició la fractura final del componente. Por contra, el análisis numérico tiene como objetivo último la simulación del desarrollo de las grietas desde cada uno de los defectos críticos analizados hasta la fractura final, describiendo cada una de las etapas de interés, comparando resultados de dichas simulaciones como la vida a fatiga de cada componente o la evolución del frente de grieta, con la evidencia experimental. El desarrollo de este análisis numérico requiere de la correcta caracterización del perfil completo de tensiones residuales dentro del componente, que se obtiene a partir de una serie de medidas discretas en la zona inmediatamente próxima a la superficie de las probetas; así como de una ley de crecimiento que describa con detalle el comportamiento de la grieta en cada instante, atendiendo a variables como el efecto de grieta pequeña o el desarrollo del cierre de grieta. Por último, una correcta caracterización de los factores de intensidad de tensiones es esencial para afrontar el análisis numérico.

Different alloys can be used for Additive Manufacturing (AM) with good structural strength. Among the titanium alloys, Ti6Al4V is the most used, especially for aerospace applications. There have been many analyses of the mechanical properties of additive manufactured Ti-6Al-4V with very good static strength results in general. However, there are still some difficulties to get fatigue properties close enough to the ones of specimens manufactured using traditional processes Considering the high effect of surface roughness on the fatigue strength of AM specimens, this work deals with the effect produced by some surface treatments on the fatigue properties. Five treatments have been used for comparison. All specimens were annealed previously to reduce residual stresses, as well as sand blasted to reduce the roughness. The treatments considered are: 1) no treatment after annealing and sand blasting; 2) shot peening; 3) shot peening plus Chemical Assisted Surface Enhancement (CASE); 4) laser shock peening, and 5) HIP. After fatigue testing, a comparison of the results has been carried out. It was found that laser peening produced the best results, followed by shot peening plus CASE and shot peening, with the lowest strength produced by HIP as well as just sand blasting after thermal treatment.

Fatigue failure of cables and strands is a common and complex problem. Failure is typically caused by different combinations of time‐variable bending and axial forces. In addition to these loads, contact stresses between wires may play an important role in the fatigue failure of cables. The present work aims to provide deep insight into the fatigue failure of a seven‐wire stainless steel strand subjected to a combination of variable axial and bending loads. To avoid side effects in the analysis, fatigue failure of the strand close to the clamps is prevented. Several tests were performed with a new device specifically designed to avoid failure near the clamps. Thus, failure is always produced at the middle length of the specimen. Test simulations were performed by employing the finite element method. The numerical results were validated via comparisons with experimental data. Finally, life prediction curves were obtained.

La fatiga por fretting es un tipo especial de fatiga que se produce en las superficies de contacto de uniones mecánicas tales como uniones atornilladas, uniones roblonadas, álabes de turbinas, uniones de ejes, cables metálicos trenzados o rotores de generadores. Con la intención de aumentar la vida de estos componentes bajo el efecto de la fatiga por fretting, en la industria es bastante usual aplicar el tratamiento superficial de shot peening sobre estos componentes mecánicos. Esto es debido a que durante la fatiga por fretting se producen unas altas tensiones y deformaciones en las superficies en contacto y que además presentan altos gradientes (variación muy rápida con respecto a la profundidad). Este tratamiento introduce tensiones residuales de compresión y transforma la microestructura del material en las cercanías de la superficie, además de producir un importante aumento de la rugosidad superficial. Los altos valores de tensiones y deformaciones producidos por la fatiga por fretting dan lugar a una pronta nucleación/iniciación de grietas tanto en la superficie como internamente y su posterior propagación hasta la rotura completa de la pieza. Además, se producen daños superficiales tales como el desgaste de la capa superficial y la oxidación del material. En ocasiones estas distribuciones de tensiones llegan a profundidades similares a aquellas en donde se produce la máxima tensión de compresión debida al tratamiento de shot peening. Por este motivo el shot peening es un buen paliativo de la iniciación o propagación de pequeñas grietas e incluso elimina la aparición de estas en algunas condiciones. Así, una pieza con este tratamiento puede alcanzar vidas a fatiga por fretting mucho más altas que sin él, incluso no romper. Además de estas tensiones residuales de compresión, el shot peening, tiene otras consecuencias como la alteración y modificación de las características microestructurales haciendo el que material se endurezca en la zona afectada por el shot, Fretting fatigue is a special type of fatigue that occurs on the contact surfaces of mechanical joints such as bolted connections, riveted joints, turbine blades, axle joints, braided metal cables or generator rotors. With the intention of increasing the life of these components under the effect of fretting fatigue, in the industry it is quite usual to apply the surface treatment of shot peening on these mechanical components. This is due to the fact that during fretting fatigue there are high stresses and strains in the surfaces in contact and that they also have high gradients (very fast variation respect to the depth). This treatment introduces residual stresses of compression and transforms the microstructure of the material in the vicinity of the surface, in addition to producing a significant increase in surface roughness. The high values of stress and strain that fretting fatigue produced give rise to an early nucleation/initiation of cracks both on the surface and internally and its subsequent propagation until the complete breakage of the piece. In addition, surface damage occurs such as the wear of the surface layer and the oxidation of the material. Sometimes these stress distributions reach depths similar to those where the maximum compression stress due to the shot peening treatment occurs. For this reason the shot peening is a good palliative of the initiation or propagation of small cracks and even eliminates the appearance of these in some conditions. Thus, a piece with this treatment can achieve lives to fretting fatigue much higher than without it, not even breaking. In addition to these residual stresses of compression, the shot peening, has other consequences such as the alteration and modification of the microstructural characteristics making the material harden in the area affected by the shot peening. Another consequence is the alteration of the surface roughness of the treated specimens. These aspects, which although greatly influencing the b

The fracture of cables is often caused by fatigue phenomena. To analyse the fatigue behaviour of these elements, a device has been designed, manufactured, and validated. This machine can produce different load states for the specimens, combining variable axial loads and variable bending moments. The testing machine has been designed to achieve two goals: to avoid the fracture of the cable in the vicinity of the clamps and to assure that the fracture of the cable is in the middle of the length of the specimen. The type of failure will be due to a combination of global stresses and fretting fatigue phenomena. This work also shows the results of different fatigue tests for a specific strand configuration. Finally, the fracture and contact surfaces of the strand have been analysed with a scanning electron microscope (SEM). The results obtained with this device show that it is adequate for testing cables and strands subjected to different combinations of axial loads and bending moments.

Shot peening is a mechanical treatment that induces several changes in the material: surface roughness, increased hardness close to the surface, and, the most important, compressive residual stresses. This paper analyzes the effect of this treatment on alloy Al 7075-T651 in the case of fretting fatigue with cylindrical contact through the results of 114 fretting fatigue tests. There are three independent loads applied in this type of test: a constant normal load N, pressing the contact pad against the specimen; a cyclic bulk stress σ in the specimen; and a cyclic tangential load Q through the contact. Four specimens at each of 23 different combinations of these three parameters were tested—two specimens without any treatment and two treated with shot peening. The fatigue lives, contact surface, fracture surface, and residual stresses and hardness were studied. Improvement in fatigue life ranged from 3 to 22, depending on fatigue life. The relaxation of residual-stress distribution related to the number of applied cycles was also measured. Finally, another group of specimens treated with shot peening was polished and tested, obtaining similar lives as in the tests with specimens that were shot-peened but not polished.

El objetivo de este trabajo es un estudio de viabilidad del diseño de un banco de ensayo de vibraciones con dos grados de libertad. Para ello, se tomará en consideración un sistema masa-resorte que se ve excitado por la vibración de un motor desequilibrado. Dicho motor tiene las mismas características que uno existente en el Laboratorio de Ingeniería Mecánica de la Escuela Técnica Superior de Ingeniería de la Universidad de Sevilla. En primera instancia se estudiará el comportamiento del sistema con un único grado de libertad (1 GLD) y, posteriormente, con 2 GDL. Dado que el objetivo es estudiar la viabilidad de la construcción del equipo utilizando parte del material ya existente en el Laboratorio de Ingeniería Mecánica y de ciertos componentes comerciales, se realizarán una serie de simulaciones informáticas de los dos sistemas anteriores, tratando de ser lo más fiel al comportamiento real que se espera. Para ello, se ha empleado el software MSC.Adams con el fin de simular el comportamiento dinámico de los sistemas, y para su modelado, se ha hecho uso del software CATIA v5. En cuanto al análisis matemático y postprocesado de los datos obtenidos en la simulación, se ha empleado Matlab. Una vez que se han creado los sistemas, se han realizado pruebas a distintas frecuencias para determinas una serie de parámetros característicos del sistema mecánico, como amortiguamiento, frecuencias naturales, etc. tal y como está pensado que lo hagan los alumnos en las prácticas. Por último, se ha hecho una comparativa de los resultados de las simulaciones con los modelos matemáticos, a fin de verificar la fidelidad de los mismos., This project has as its ultimate object the design of a future equipment to perform hands-on laboratory lessons about mechanical vibrations. In order to reach this goal, two mass-spring systems have been considered, with one and two degree of freedom respectively. Such sistems had been designed to be harmonicly excited by an unbalanced rotor. In this case, the rotor chosen to cause the movement possesses identical characteristics to the one located in the Mehanic Engineering Laboratory of the Higher Engineering School of Sevilla. Since this is a preliminar study of the equipment designment a series of simulations had been made, trying to make them as the most reality-like as possible. For this purpose, it has been used mainly MSC.Adams, in order to simulate the dynamic response of systems. As a complement, it has also been used CATIA v5 for an easier modeling of some elements, and Matlab for mathematic analysis and data postprocessing. Once the systems were created, frecuency scanning has been made to determinate several parameters, such as damping or natural frequences, in the same way the students would do using an actual assembly. Finaly the data obtained in the simulations has been verified, comparing it to the other results obtained through analytic and numeric evaluation.

El fallo de cables de acero es comúnmente producido por el fenómeno de la fatiga. Para analizar el fallo por fatiga en este tipo de elementos, un nuevo equipo de ensayos ha sido diseñado, fabricado y validado. Uno de los principales aspectos de este nuevo útil de ensayos, es la capacidad de aplicar al cable tanto cargas axiales como de flexión simultáneamente. Además, ambas cargas pueden ser estáticas o variables con el tiempo. El primero de los objetivos fue evitar el fallo del cable en la zona de sujeción, y de este modo producir el fallo debido únicamente a las tensiones globales producidas por las fuerzas externas aplicadas y las tensiones inducidas por el contacto entre hilos. Durante el desarrollo de este primer objetivo y tras el análisis numérico de diferentes tipos de elementos de sujeción para evitar fretting en los mismos, surge la idea de un nuevo concepto de paliativo, extensible al fenómeno de la fatiga por fretting en general. La idea básica consiste en realizar orificios debajo de las superficies en contacto y cerca de las zonas en las que se inician las grietas por fretting. Con la introducción de dichos orificios se consigue que la distribución de presiones sea menor en los bordes de contacto y también la tensión axial que provoca la iniciación de grietas. En base a ello se realiza un trabajo complementario, en el que se analiza el efecto producido por estos orificios en un par de contacto básico, como es el contacto cilíndrico. Los análisis numéricos con este tipo de geometrías producen resultados muy satisfactorios que marcan una guía para el uso de un nuevo tipo de paliativo frente al fretting. El segundo de los objetivos, y volviendo al análisis de la rotura en cables de acero, es diseñar un equipo de ensayos versátil, de forma que sea posible ensayar diferentes combinaciones de cargas, así como diferentes configuraciones de cables. El tipo de fallo esperado, será producido por una combinación de tensiones globales (axiales y de flexión) y de c, Metallic cables failure is commonly produced by fatigue phenomena. In order to analyse the fatigue failure of cables, a new device has been designed, manufactured and validated. The main peculiarity of the device is its capacity to apply simultaneously axial and bending loads. In addition, both loads can be static or time variable. The first objective was to avoid the failure of the cable close to the clamping system and thus produce the failure due to global stress produced by external loads and contact stresses prompted between wires. During the development of this first objective and after the numerical analysis of different types of clamping systems, the idea of a new fretting palliative arose. The basic idea is to make holes under the contact surfaces and near the areas where cracks initiate due to fretting. With the introduction of the holes, the pressure distribution is reduced at the contact edge and also direct stress that produce the initiation of cracks. Based on these observations, a complementary work is carried out, in which the effect produced by these holes in a basic contact pair, such as the cylindrical contact one, is analysed. Numerical analysis with this type of geometry produce very satisfactory results that mark a guide for the use of a new type of palliative against fretting The second objective, and returning to the analysis of cable failure, was to design a versatile test equipment. Therefore, with the device designed it is possible to test different combinations of loads, as well as different cable configurations. The expected type of failure is produced by a combination of global (axial and bending) and contact stresses. Nevertheless, the type of failure depends strongly on the type of cable configuration and therefore on the type of contact. Once the device was designed, a wide battery of tests was carried out with the most basic cable configuration of those commonly used, a seven-wire stainless steel strand. The strand was subjected to

La fatiga por fretting es un tipo de fatiga que puede provocar el fallo en multitud de componentes industriales. Este fenómeno de fallo se produce en partes de estos componentes en las que existe una unión mecánica, unión en la que se encuentran en contacto dos o más piezas. Estos contactos poseen una amplia variedad de formas geométricas, sin embargo, en muchas situaciones, por su similitud geométrica y de acción de cargas externas, estos pares de contacto se pueden simplificar a un contaco cilíndrico, es decir, por el contacto entre un cilindro y un plano. El objetivo de este proyecto ha sido la creación de un software para calcular las tensiones y deformaciones y la representación gráfica de las mismas producidas en un contacto cilíndrico. A parte de estos objetivos, este software se ha realizado de forma que sea lo más sencillo posible para que, una vez finalizado, este se pueda usar fácil e intuitivamente por cualquier persona. Para conseguirlo, se ha usado el software Excel por las características que este posee, especialmente por el lenguaje de programación que tiene integrado: VBA. Este lenguaje se ha utilizado para crear dos macros que calculan las tensiones y deformaciones producidas en un contacto cilíndrico. La diferencia entre ambas es que una realiza el cálculo tensiones y deformaciones a lo largo de un segmento y otra muestra los resultados de una matriz de puntos. Los resultados numéricos obtenidos ejecutando cada una de ellas se muestran en hojas de Excel diferentes. En este documento se realiza una introducción a los contactos mecánicos y sus posibles mecanismos de fallos. Posteriormente, se explican las principales características de la fatiga por fretting y las particularidades del contacto cilíndrico, centrándose en las expresiones analíticas que gobiernan este problema y que se usan para calcular tanto las tensiones como deformaciones en el software desarrollado. El siguiente apartado describe las dos hojas del archivo Excel construido, así com, Fretting fatigue is a type of fatigue that can provoke failures in multiple industrial components. This failure phenomenon takes place in parts of those components that have a mechanic union, in which two or more pieces come in contact. This contacts possess a large variety of geometrical shapes, however in several situations, due to their geometrical resemblance and the action of external forces these contacts can be simplified to a cylindrical contact, meaning the contact between a cylinder and a plane. The objective of this project has been the creation of a software to calculate tensions and deformations and the graphical representation of them produced in a cylindrical contact. Leaving these objectives behind, this software has been made as simple as possible so that, once completed, it can be used easily and intuitively by anyone. Excel Software has been used to accomplish this, for the characteristics that it has, and specially for the programing language integrated: VBA. This language has been used to create two macros that calculate the stresses and strains produced in cylindrical contacts. The difference between both is that one calculates the stresses and strains along a segment and the other shows the results of a dot matrix. The results obtained executing each are shown in different excel pages. This document will make an introduction to mechanical contacts and the possible failure mechanism. Afterwards the main characteristics of fretting fatigue and the particularities of cylindrical contact will be explained, focusing in analytic expressions that rule this problem and are used to calculate stresses as well as strains in the developed software. The next section describes the two sheets of the Excel generated, as well as the steps that are needed follow to obtain the numerical results. Finally, the conclusions obtained during the fulfillment of this project are explained and the codes of Visual Basic developed for the macro’s creation annexed.

En este trabajo se caracteriza la aleación de AL7075-T651 ante la fatiga por fretting. En concreto se estudia la influencia sobre la vida a fatiga por fretting del coeficiente de rozamiento, la rugosidad superficial y las tensiones residuales introducidas por un tratamiento de shot-peening. Este tratamiento, además de introducir tensiones residuales de compresión en el material, también modifica la rugosidad superficial debida a la deformación plástica producida por el tratamiento. De las probetas ensayadas, un grupo no ha sido tratado superficialmente. Otro grupo de probetas están tratadas con shot-peening para su comparación con ensayos con las mismas cargas aplicadas. En otro grupo de probetas se induce la rugosidad superficial que causa el tratamiento pero sin la deformación plástica y la consecuente tensión residual de compresión. En un último grupo de probetas tratadas con shot peening se altera la rugosidad superficial sin modificar la tensión residual de compresión. Para analizar cómo afectan a la vida en ensayos de fatiga por fretting las diferentes modificaciones producidas por el shot peening, se ha medido la rugosidad superficial, coeficiente de rozamiento y tensiones residuales para correlacionarlas con la vida a fatiga y lugar de iniciación de las grietas.

Con el objetivo de comprender en mayor profundidad el comportamiento a fatiga de metales fabricados mediante AM (Additive Manufacturing), el presente proyecto estudia el efecto que diferentes parámetros tales como la rugosidad, los defectos, las tensiones residuales y los tratamientos superficiales aplicados tras la fabricación, tienen sobre el mismo. Para ello, se dispone de 30 probetas de Ti-6Al-4V procesadas mediante un tipo de fabricación aditiva conocida como Selective Laser Melting. Para analizar el efecto de los tratamientos superficiales, todas las muestras se dividieron en cinco grupos de seis muestras para cada grupo de forma que cada grupo fue sometido a un tratamiento diferente antes del ensayo. Los tratamientos seleccionados para ser aplicados a las muestras en estudio han sido shot peening, shot peening + CASE, laser peening e HIP. Además, a todos los grupos se les ha aplicado previamente un tratamiento térmico y un proceso de sand-blasting. Con el fin de comparar los resultados obtenidos para los distintos tratamientos con el estado original de las muestras, a uno de los cinco grupos no se le fue aplicado ningún tratamiento adicional. Para estudiar la resistencia a fatiga de las piezas, se han realizado ensayos de flexión en cuatro puntos sobre la totalidad de las muestras. A partir de los resultados obtenidos se ha realizado una comparación de las vidas a fatiga de las diferentes probetas en función del tratamiento superficial aplicado a través de las respectivas curvas S-N para cada grupo. Por otra parte, igualmente en función del tratamiento superficial aplicado, se ha realizado una comparación de los resultados obtenidos de rugosidad y tensiones residuales presentes en las diversas muestras. Para la realización de las mediciones de rugosidad se ha usado un rugosímetro proporcionado por el CAM y para la obtención de las tensiones residuales se ha utilizado un equipo de medición de tensiones basado en la técnica de la perforación del orificio propor, The present project studies the effect that different parameters such as the roughness, the defects, the residual stresses and the surface treatments, have on the fatigue behavior of metals AM with the aim of understanding it more in detail. For this, 30 samples of Ti-6Al-4V processed by an additive manufacturing type known as Selective Laser Melting are available. To analyze the effect of the surface treatments, all the samples were divided into five groups of six samples for each group so that each group was subjected to a different treatment before the test. The treatments selected to be applied to the samples under study have been Shot Peening, Shot Peening + CASE, Laser Peening and HIP. In addition, all groups have been previously applied a heat treatment and a sand blasting process. In order to compare the results obtained for the different treatments with the original state of the samples, no additional treatment was applied to one of the five groups. To study the fatigue resistance of the pieces, bending tests have been carried out in four points on all the samples. Based on the results obtained, a comparison of the fatigue lives of the different specimens was made according to the surface treatment applied through the respective S-N curves for each group. On the other hand, also according to the surface treatment applied, a comparison of the obtained results of roughness and residual tensions present in the samples was carried out. To make the roughness measurements was used a rugosimeter provided by the CAM and to obtain the residual stresses was used a stress measurement equipment based on the orifice drilling technique provided by the Mechanical Engineering Laboratory. Finally, a 2D model was carried out at Ansys in order to study the fatigue behavior of the samples taking into account the residual compressive stresses introduced by the surface treatments applied.

There are several procedures used to increase fatigue life of components suffering from fretting, certain of which involve the modification of the surface material properties or the generation of compressive residual stresses near the surfaces in contact. Moreover, relatively little attention has been paid to the situation in which the contact pair stiffness is modified by changing the geometry of the components. The present paper describes an analysis of the possible beneficial effects on the fretting life of introducing voids inside the material of the components under fretting conditions. From a 3D perspective, this manufacturing characteristic is difficult to achieve using conventional techniques, although additive manufacturing could achieve this easily. A numerical analysis of a typical cylindrical contact configuration in fretting is simulated, with the novelty of introducing circular holes beneath the half-plane surface. These holes change the contact pair stiffness, leading to a decrease in the surface contact stress field.

Este trabajo consiste en el estudio del factor de intensidad de tensiones (F.I.T.) en modo I en fretting, para un contacto cilíndrico. Se introducirá brevemente el fenómeno fretting, los efectos que produce en los componentes en que se da, así como una serie de paliativos para combatirlo. Posteriormente, partiendo de la configuración del ensayo de fretting correspondiente para el calculo de ����, y de las tensiones en la superficie de contacto de la probeta dadas por la literatura, se calculará el campo de tensiones interior a ella mediante dos métodos diferentes. Se basarán en las ecuaciones de Flamant y una serie de ecuaciones analíticas que se adaptarán a partir de la literatura para el caso que nos ocupa. Posteriomente, se calculará la distribución de tensiones axiales ������(��) a lo largo de la grieta, la cual permitirá calcular el factor de intensidad de tensiones ����. Finalmente se representarán e interpretarán los resultados obtenidos. Para realizar estos cálculos se emplea una serie de funciones programadas en MATLAB y que se encuentran adjuntas en los anexos de la memoria.

Se ha analizado el desgaste producido en ensayos de fatiga por fretting con contacto esférico realizados con la aleación de aluminio 7075 T651. Las huellas de fretting tienen forma circular, y en estas se distinguen dos zonas: una zona central circular de adhesión y una zona exterior de deslizamiento con forma de anillo. Esta última zona se distingue porque aparece dañada por desgaste y oxidación. El objetivo de este trabajo consiste en distinguir dichas zonas, determinar su tamaño y posición, medir el desgaste y establecer una relación con las tensiones que aparecen en el contacto. Para ello se ha utilizado un microscopio confocal que proporciona un mapa de la superficie: profundidad frente a posición. Dado que las huellas no suelen estar muy limpias, las imágenes y datos obtenidos con el microscopio se someten a una serie de tratamientos digitales de forma que al final se obtienen los parámetros deseados. Finalmente, los datos experimentales se combinan con los cálculos analíticos mediante el modelo de desgaste de Archard.

Currently, additive manufacturing with metals is an increasingly popular technique that allows the manufacturing of pieces of difficult shapes, nearly impossible to make with other techniques. Usually, these shapes try to optimize the solid to have the same strength with a lower weight. The fatigue behavior of the material of the components manufactured with this technique is a field in development. On the other hand, fretting fatigue is a common type of fatigue where a “stress concentration” appears due to the contact between two components. There are some procedures used to increase fatigue life in this situation (shot peening, surface knurling, etc.). This paper tries to analyze the possible beneficial effect on fatigue life of introducing voids inside the material in components under fretting, which is feasible now thanks to additive manufacturing. The problem under study is, for now, a 2D simplification where a cylinder is in contact with a half plane and a normal constant load and a variable tangential load are applied. This geometry has been numerically simulated, introducing a circular hole below the surface. The effect of this hole is to make the contact more elastic, which decreases the stresses near the surface. This work analyses and compares the stress and strain fields and Smith-Watson-Topper multiaxial fatigue parameter in the areas sensitives to fretting with respect to a case with homogeneous material (no internal voids). Various configurations changing different parameters like size and position of the hole, friction coefficient and the size of the slip zone have been considered. The problem analyzed in this paper is two-dimensional, therefore there would be no need to use additive manufacturing in a real situation. However, the results obtained in this paper indicate that it could also work in 3D. Actually, it is in a real three-dimensional problem where the additive manufacturing would be necessary for the introduction of voids inside the materia

Procedimiento de fabricación de una pieza (1) con resistencia aumentada a fatiga por contacto caracterizado porque comprende al menos una etapa de realizar al menos un orificio (2) en el interior de la pieza (1) en una zona próxima a la superficie de cont

En este proyecto se obtiene la función general de peso de carga puntual para grietas semi-elípticas en placas de espesor finito. Se utiliza para calcular el factor de intensidad de tensiones para cualquier punto a lo largo del frente de la grieta con el plano de la grieta bajo distribuciones de tensiones bidimensionales. Se muestra la comparación entre los resultados alcanzados de la función de peso obtenida y los disponibles en la literatura, lo que indica una buena precisión. Con el fin de determinar el patrón de crecimiento de grietas y estimar la vida útil a fatiga, esta función de peso se puede usar fácilmente en placas aplicadas en la industria bajo campos de tensión complicados, como el campo de tensiones residuales debido a la soldadura. En aplicaciones industriales, como es el proceso de soldadura, se aplican complicadas distribuciones de tensiones en el plano de las grietas. Debido a que la soldadura se realiza en los extremos de las placas, la tensión residual varía de forma no lineal a lo largo del espesor y el ancho de las mismas. De hecho, se aplica una distribución de tensiones bidimensional no lineal en el plano de la grieta en placas soldadas. En estas condiciones, los puntos críticos pueden o no ser habitualmente los puntos más profundos y superficiales, por lo que se deben examinar otros puntos. El GPLWF se puede usar fácilmente para calcular el factor de intensidad de tensiones para cualquier punto a lo largo del frente de grieta de las placas soldadas agrietadas bajo distribuciones de tensiones residuales bidimensionales., In this project, a general point load weight function for semi-elliptical crack in finite thickness plates is derived. It is used to calculate stress intensity factors for any point along the crack front with the crack faces under two dimensional stress distributions. The comparison between the results obtained from the derived weight function and those available in literature is presented, indicating a good accuracy. In order to determine the pattern of crack growth, and to estimate fatigue life, this weight function can be easily used in plates as applied in industry under complicated stress fields such as residual stress field due to welding. In industrial applications, such as a welding process, complicated stress distributions are applied on crack faces. Due to butt welding of plates, the residual stress nonlinearly varies along the thickness and width of a plate. In fact, a nonlinear two dimensional stress distribution is applied on the crack face in welded plates. In these conditions, the critical points may or may not be the deepest and surface points as usual, so other points must be examined. The GPLWF can be easily used to calculate stress intensity factors for any point along crack front of cracked butt welded plates under two dimensional residual stress distributions.

El presente trabajo tiene como objetivo último el diseño de un futuro equipo para la realización de prácticas de vibraciones en laboratorio. Para alcanzar este objetivo se han considerado dos sistemas masa-resorte: el primer conjunto corresponde a un sistema de un grado de libertad (1 GDL) y el segundo a uno con 2 GDL. Dichos sistemas han sido diseñados para ser excitados de forma armónica mediante un movimiento de la base. En este caso el movimiento armónico sería provocado por una mesa de vibraciones, en la que se instalaría el equipo vibratorio, y cuyas principales características son idénticas a las de una mesa existente en el Laboratorio de Ingeniería Mecánica de la Escuela Técnica Superior de Ingenieros de la Universidad de Sevilla. Como este es un estudio preliminar sobre el diseño del equipo se han realizado una serie de simulaciones informáticas de los montajes intentando que estas sean lo mas fieles posible al que se produciría en la realidad. Para ello se ha hecho uso principalmente del software MSC.Adams a fin de simular el comportamiento dinámico de los sistemas, y de forma complementaria del software CATIA v5 para un modelado más cómodo de algunos elementos así como Matlab para análisis matemáticos y postprocesado de datos. Una vez creados los sistemas, se han realizado sendos barridos de frecuencia para determinar ciertos parámetros característicos del movimiento vibratorio en sistemas mecánicos, como son el amortiguamiento, las frecuencias naturales etc. al modo en que lo harían los futuribles alumnos al interactuar con un montaje real. Por último se han verificado los datos obtenidos en las simulaciones, comparándolos con otros resultados analíticos y numéricos.