Fretting-Fatigue Analysis of Shot-Peened Al 7075-T651 Test Specimens

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.Junta de Andalucía P12-TEP-263

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

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

Experimental and numerical analysis of fatigue cracks emanating from internal defects in Ti6Al4V SLM

2nd European Conference on the Structural Integrity of Additively Manufactured Materials, ESIAM 2021, Virtual-Online, 5 - 8 September 2021.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

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

Optimal shot peening residual stress profile for fatigue

Article number 103109Shot 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

Medida del desgaste en un ensayo de fretting mediante microscopio confocal

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.Ministerio de Economía y Competitividad DPI2014-59160-

Comparison of two multiaxial fatigue models applied to dental implants

This paper presents two multiaxial fatigue life prediction models applied to a commercial dental implant. One model is called Variable Initiation Length Model and takes into account both the crack initiation and propagation phases. The second model combines the Theory of Critical Distance with a critical plane damage model to characterise the initiation and initial propagation of micro/meso cracks in the material. This paper discusses which material properties are necessary for the implementation of these models and how to obtain them in the laboratory from simple test specimens. It also describes the FE models developed for the stress/strain and stress intensity factor characterisation in the implant. The results of applying both life prediction models are compared with experimental results arising from the application of ISO-14801 standard to a commercial dental implant.Ministerio de Ciencia e Innovación DPI2011-2337

Predicción de vida en componentes con gradientes de tensiones: entallas y fatiga por fretting

La fatiga en componentes con entalla y la fatiga por fretting son dos fenómenos diferentes pero tienen en común la existencia de un gradiente de tensiones. En estos casos la predicción de la vida a fatiga se suele realizar mediante modelos que consideran el proceso como superposición de una fase de iniciación de la grieta y otra de propagación. El problema que se plantea es definir la longitud que tiene la grieta en el instante en que termina una fase y comienza otra. El modelo empleado en este artículo combina las dos fases, sin definir a priori la separación entre ambas. En la fase de propagación se utiliza una ley de Paris modificada con un umbral de crecimiento que depende de la longitud de grieta. En la fase de iniciación se utiliza una curva S-N modificada donde se le ha restado el número de ciclos de propagación. El modelo propuesto se aplica a un conjunto de ensayos tanto de fretting con contacto esférico y cilíndrico como de probetas entalladas para comprobar la validez del modelo a la hora de predecir la vida a fatiga en distintas condiciones.Fatigue in notched specimens and fretting fatigue are two different phenomena but they have in common the existence of a stress gradient. In these cases life prediction is usually done with models that consider the process as a superposition of an initiation and propagation phase. The problem is to define the crack length where one phase finishes and the other begins. The model employed in this paper combines both phases without defining a priori the separation between them. In the propagation phase a crack length dependent growth threshold is introduced in the Paris law. On the other hand, the S-N curve used in the initiation phase is modified by subtracting the propagation cycles. The proposed model is applied to a group of fretting fatigue tests in spherical and cylindrical contact as well as in tests with notched specimens to check the validity of the model in predicting life in different conditions.Ministerio de Educación y Ciencia DPI2007-66995-C03-0

Análisis del proceso de fatiga en entallas mediante la acumulación de daño en el frente de grieta

El proceso de fatiga se divide normalmente para su estudio en dos etapas: iniciación y propagación. La separación entre una y otra es arbitraria, dependiendo de cómo se definan. El modelo presentado en este artículo considera que el proceso de avance de una grieta se debe a un proceso de daño acumulado en sucesivos bloques de material de determinada longitud en la dirección de avance de la grieta. El daño se va acumulando en estos bloques en función del nivel de tensiones y deformaciones, que es función de su distancia al borde de la grieta y cuando el daño llega a un determinado nivel se produce la rotura del bloque, provocando así el avance de la grieta. En el análisis de tensiones necesario se supone que la grieta es roma con un determinado radio de curvatura. La vida de iniciación se estima utilizando la curva ε-N. En este artículo se hace un análisis exhaustivo de los distintos parámetros que afectan a dicho modelo como: el tamaño de los bloques en que se discretiza el material en la dirección de avance de la grieta, el comportamiento del modelo en la fase de grieta pequeña, el lugar y la forma de evaluar las tensiones para estimar la vida de iniciación, las propiedades del material, etc. Este análisis se realiza sobre una serie de ensayos de fatiga de probetas entalladas. Así, aparte de realizar el estudio paramétrico del efecto de las variables del modelo, se puede comprobar la bondad de dicho modelo.To study the fatigue process, it is normally described in two stages: initiation and propagation. The separation between one and other one is arbitrary, depending on how they are defined. The model presented in this article assumes that the process of advance of a crack owes to the damage accumulated in successive blocks of material of certain length in the direction of the crack growth. The damage is accumulated in these blocks depending on the stresses and strains levels produced, which are factors of the distance to the edge of the crack. When the damage comes to a certain level the failure of the block takes place, provoking in this way the advance of the crack. In the necessary stress analysis it is supposed that the crack is blunt with a certain radius of curvature. The life of initiation is estimated using the curve ε-N. In this article an exhaustive analysis of the different parameters that affect the above mentioned model as: the size of the blocks in the discretization of the material in the direction of the crack growth, the behavior of the model in the short crack phase, the place and the way of evaluating the stresses to estimate the life of initiation, the properties of the material, etc. is carried out. This analysis is realized on several cases of fatigue of notched specimens, which also have been tested. This way, apart from parametrically analyzing the effect of the variables of the model it is possible to verify the validity of the above mentioned model