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

Biaxial fatigue tests of notched specimens for AISI 304L stainless steel

High cycle fatigue tests were conducted for stainless steel AISI 304L. The geometry was a thin walled tube with a passing through hole. The tests were axial, torsional and in-phase axial-torsional, all of them under load control with R = ?1. The S-N curves were constructed following the ASTM E739 standard and the fatigues limits were calculated following the method of maximum likelihood proposed by Bettinelli. The crack direction along the surface was analysed, with especial attention to the crack initiation zones. The notch fatigue limits for different hole diameters were compared with the predictions done with a microstructural fracture mechanics model.Ministerio de Educación DPI2014-56904-

Plastic flow equations for the local strain approach in the multiaxial case

This paper presents a system of plastic flow equations which uses and generalizes to the multiaxial case a number of concepts commonly employed in the so-called Local Strain Approach to low cycle fatigue. Everything is built upon the idea of distance between stress points. It is believed that this will ease the generalization to the multiaxial case of the intuitive methods used in low cycle fatigue calculations, based on hysteresis loops, Ramberg?Osgood equations, Neuber or ESED rule, etc. It is proposed that the stress space is endowed with a quadratic metric whose structure is embedded in the yield criterion. Considerations of initial isotropy of the material and of the null influence of the hydrostatic stress upon yielding leads to the realization of the simplest metric, which is associated with the von Mises yield criterion. The use of the strain?hardening hypothesis leads in natural way to a normal flow rule and this establishes a linear relationship between the plastic strain increment and the stress increment.Ministerio de Educación DPI2014-56904-

Desplazamiento de la zona no deslizante del contacto en fretting fatiga con contacto esférico y carga axial

Este artículo analiza las condiciones de contacto y las tensiones producidas durante un ensayo de fretting fatiga con carga axial utilizando un puente de fretting y elementos de contacto esférico. Este análisis tiene en cuenta el desplazamiento de la zona no deslizante producido por la tensión axial a la que se somete la probeta. Describe una aproximación analítica para calcular el tensor de tensiones bajo la zona de contacto incluyendo el efecto de este desplazamiento. Finalmente, se hace una comparación entre las tensiones que se obtienen con esta aproximación y las calculadas con la aproximación de MindlinCattaneo que no tiene en cuenta el efecto de la tensión axial.This paper analyses the contact conditions and stresses induced during fretting fatigue tests of uniaxial specimens using fretting bridges with spherical-tip contact pads. This analysis considers the displacement of the stick zone induced by the variable bulk stress applied to the specimen. lt describes an approach to, analitically, calculate the stress tensor under the contact zone including this stick zone displacement. Finally, the analitically calculated stresses are compared to those obtained using the classical Mindlin-Cattaneo approach which does not consider the bulk stress

Criterios de iniciación en fretting fatiga con contacto esférico

En este artículo se analizan distintos métodos que estiman la vida a fatiga uniendo la iniciación con la propagación en ensayos de fretting fatiga con contacto esférico. En concreto se va a centrar la discusión en cómo tratar la fase de iniciación. Algunos de los métodos presentados definen a priori una longitud de grieta a partir de la cual se consideran y calculan la propagación y utilizan las tensiones existentes cerca de la superficie para estimar la iniciación. Para esto último se pueden promediar las tensiones hasta cierta profundidad o tomar las tensiones a una determinada profundidad. Otro método une la iniciación con la propagación pero en este caso la longitud de grieta a partir de la cual se considera propagación no es fija, se obtiene como resultado de analizar cada caso en concreto. Dichos métodos se aplican a una serie de ensayos de fretting fatiga con contacto esférico realizados sobre la aleación de aluminio 7075 T6 discutiendo sus resultados.In this paper different methods used to estimate fatigue life adding initiation with propagation in fretting fatigue tests with spherical contact are analyzed. In particular, the discusion will be centered in the initiation phase. Some of the methods presented define in advance a crack length after which crack propagation is considered and calculated, afterwards, stresses close to the surface are used to estimate initiation. To do this, either the stresses can be averaged up to a certain depth or the stresses at a certain depth are used. Yet another method combines initiation with propagation but in this case the initiation length is not a fixed quantity, it is obtained as a result of analyzing each specific case. These methods are applied to a series of fretting fatigue tests with spherical contact performed with 7075-T6 aluminum alloy and the results discussed

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

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

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

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