Paliativo frente a la fatiga por fretting y análisis del proceso de fatiga en cordones metálicos

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 contacto, dependiendo fuertemente del tipo de configuración del cable ensayado y por tanto del tipo de contacto. Una vez diseñado el equipo se realiza una amplia batería de ensayos con la configuración de cables más básica de las comúnmente empleadas, un cordón de x acero inoxidable de siete hilos. El cordón ha sido sometido a diversas combinaciones de fuerzas axiales y de flexión. Una vez llevados a cabo los ensayos de fatiga, y con el objetivo de analizar las zonas de iniciación de grietas, así como su propagación y determinar el fenómeno que provoca el fallo, las superficies de fractura de los cables ensayados fueron analizadas mediante microscopia electrónica. Debido al tamaño de los hilos que conforman el cordón ensayado, la medición de tensiones y deformaciones in situ es compleja o casi imposible. Por ello y con el objetivo de conocer los campos de tensiones y deformaciones que aparecen en el cordón, se han realizado diversas simulaciones numéricas mediante dos modelos de elementos finitos de formulación implícita. Un primer modelo complejo en el que se ha modelado un cordón completo con el objetivo de reproducir fielmente los ensayos llevados a cabo y un segundo modelo simplificado realizado con el objetivo de una aplicación rápida y práctica. Por último, se han aplicado diversos métodos de predicción de vida a fatiga en función de los resultados experimentales y numéricos obtenidos. En base a ambos resultados se llega a la conclusión que, debido a la geometría de cordón empleada, y por tanto al tipo de contacto que de forma inherente aparece entre hilos, el fallo es producido debido a las tensiones globales aplicadas al cordón y no al fenómeno de fretting, que aunque también se observa en el contacto entre hilos, no es determinante. En base a estos resultados se concluye con un método simple, pero efectivo, para la predicción a fatiga de cordones de acero de siete hilos sometidos a cargas axiales y de flexión.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 various combinations of axial and bending loads. Once the fatigue tests were carried out, and with the aim of analysing the crack initiation zones, as well as their propagation, the fracture surfaces, of the tested strands, were analysed by a scanning electron microscope. xii Due to the size of the wires that shape the strand, the measurement of stresses and strains in situ is complex or almost impossible. For this reason and in order to know the stress/strain fields of the strand, several numerical simulations have been carried out by means of two implicit finite element models. A first complex model in which a complete strand has been modelled in order to faithfully reproduce the tests carried out and a second simplified model developed with a more practical application. Finally, various fatigue life prediction methods have been applied based on the experimental and numerical results obtained. In view of the results, it is concluded that, due to the strand geometry tested, and therefore to the type of contact that inherently appears between wires, the failure is produced due to the global stresses applied to the strand and not because of fretting phenomenon, which although it is also observed in the contact between wires, is not determinant. Based on these results, we conclude with a simple but effective method for the prediction of the fatigue life of 7-wire stainless steel strands subjected to cyclic axial and bending loads

Programa para el cálculo de ciclos de iniciacición de grietas en fatiga

Universidad de Sevilla. Master en Diseño Avanzado en Ingeniería Mecánic

Rolling effect in fretting fatigue test at the crack initiation stage

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.Ministerio de Ciencia, Innovación y Universidades - Agencia Estatal de Investigación (AEI) - Fondo Europeo de Desarrollo Regional (FEDER) RTI2018-09659-B-10

Influence of the rolling of contact pads on crack initiation in fretting fatigue tests

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

A fretting fatigue model based on self-steered cracks

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

Heterogeneidad del material como paliativo en fretting

En la actualidad, la fabricación aditiva con metales es una técnica en auge que permite la fabricación de piezas con formas dificiles de realizar mediante otros métodos de fabricación. El comportamiento a fatiga del material del componente, fabricado con esta técnica, es un campo en desarrollo a día de hoy y del que pocos datos se tiene. En este estudio se plantea analizar el posible efecto que pudiera tener la heterogeneidad del material, intencionadamente introducida, en componentes sometidos a fretting. Para ello, se ha simulado la usual configuración en fretting de un contacto cilindrosemiplano, en el que la heterogeneidad se ha impuesto en el semiplano mediante la introducción de un hueco con forma circular. En este trabajo se han analizado y comparado con respecto al caso de un par de contacto con material homogéneo, los campos de tensiones/deformaciones, además del parámetro de fatiga multiaxial Smith-Watson-Topper en las zonas más susceptibles de sufrir por fretting. Para generalizar los resultados, se han considerado diferentes configuraciones en las que se han ido variando diversos parámetros, como son: radio del hueco, posición del mismo, coeficiente de rozamiento y tamaño de la zona de deslizamiento.Currently, additive manufacturing with metals is a rising technique that allows manufacturing pieces of difficult shapes for other kind of methods. The fatigue behaviour of the material of the component manufactured with this technique, is a field in development. This research tries to analyse the possible effect of using heterogeneous material, intentionally introduced, in components under fretting effects. For that the usual configuration in fretting between a cylinder and a half-plane, in wich it has been introduced a circular hole has been simulated. This work analises and compares respect a case with homogeneous material, the stress and strain fields and Smith-Watson-Topper multiaxial fatigue parameter in the areas sensitives to fretting. Different configurations changing different parameters in relation to contact length like: hole radius, position of the hole, friction coefficient and the size of the slip zone have been considered.Ministerio de Economía y Competitividad DPI2014-59160-

Effect of rolling on fretting fatigue assessment of cylindrical contact in partial slip regime

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/)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

Prediction of initiation and total life in fretting fatigue considering kinked cracks

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

Numerical study on the influence of artificial internal stress relief groove on fretting fatigue in a shrink-fitted assembly

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

Numerical analysis of toroidal voids as stress relievers in shrink-fitted shafts

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