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-

Ciclos de fuerzas y tensiones en fretting fatiga

En este artículo se analiza la evolución de las cargas y tensiones producidas en la zona de contacto en probetas sometidas a cargas axiales mientras tienen aplicado un puente de compresión que produce el efecto de fretting. En particular se analizan las cargas en el caso en que se produce deslizamiento global entre las superficies de contacto. Se analiza también la evolución de las componentes de las tensiones en los puntos próximos a la superficie de contacto en el caso de fretting con contacto puntual. Pueden destacarse dos aspectos fundamentales que diferencian el comportamiento de las tensiones en comparación a otros casos de fatiga. En primer lugar, se ha comprobado que se produce un desfase importante entre las componentes de la tensión durante el proceso de variación de las mismas. Por otro lado, cuando se produce deslizamiento global entre las superficies de contacto y éste es suficientemente grande, la amplitud de varias componentes de la tensión se reduce al incrementar la amplitud del deslizamiento. Estos aspectos pueden tener una influencia apreciable en el comportamiento a fatiga del elemento en cuestión y por tanto deben ser tenidos en cuenta al considerar diferentes criterios de análisis del daño en procesos de fatiga debidos a fretting.This paper analyses the loads and stresses variations during fretting fatigue tests of specimens with fretting bridges. Three different aspects are considered: (i) the relation between the axial load applied to the specimen and the friction forces when the contact surfaces slíde globally; (ii) the non proportionality of sorne of the stresses produced close to the contact surfaces during fretting; (iii) and, in case of global sliding between the surfaces, the reduction of the amplitude of sorne stress components when the relative displacement amplitude increases. These effects may have influence on the fatigue behavior and must be considered in the damage analysis of fretting fatigue processes

On the Use of Maximum Force Criteria to Predict Localised Necking in Metal Sheets under Stretch-Bending

The maximum force criteria and their derivatives, the Swift and Hill criteria, have been extensively used in the past to study sheet formability. Many extensions or modifications of these criteria have been proposed to improve necking predictions under only stretching conditions. This work analyses the maximum force principle under stretch-bending conditions and develops two different approaches to predict necking. The first is a generalisation of classical maximum force criteria to stretch-bending processes. The second approach is an extension of a previous work of the authors based on critical distance concepts, suggesting that necking of the sheet is controlled by the damage of a critical material volume located at the inner side of the sheet. An analytical deformation model is proposed to characterise the stretch-bending process under plane-strain conditions. Different parameters are considered, such as the thickness reduction, the gradient of variables through the sheet thickness, the thickness stress and the anisotropy of the material. The proposed necking models have been successfully applied to predict the failure in different materials, such as steel, brass and aluminiumGobierno español DPI2015-64047-

On the Use of Strain Path Independent Metrics and Critical Distance Rule for Predicting Failure of AA7075-O Stretch-Bend Sheets

The strain-based forming limit curve is the traditional tool to assess the formability of metal sheets. However, its application should be restricted to proportional loading processes under uniform strain conditions. Several works have focused on overcoming this limitation to characterize the safe process windows in industrial stretch-bend forming processes. In this paper, the use of critical distance rule and two path-independent stress-based metrics are explored to numerically predict failure of AA7075-O stretch-bend sheets with 1.6 mm thickness. Formability limits of the material were experimentally obtained by means of a series of Nakazima and stretch-bending tests at di erent thickness-over-radius ratios for inducing controlled non-uniform strain distributions across the sheet thickness. By using a 3D calibrated finite element model, the strain-based forming limit curve was numerically transformed into the path-independent stress and equivalent plastic strain polar spaces. The numerical predictions of necking strains in the stretch-bending simulations using the above approaches were successfully compared and critically discussed with the experimental results for di erent values of the critical distance. It was found that failure was triggered by a critical material volume of around the half thickness, measured from the inner surface, for the both path-independent metrics analyzed.Gobierno de España PGC2018-095508-B-I0

Aplicación de un modelo micromecánico de crecimiento de grieta de dos umbrales a fatiga por fretting

El presente trabajo muestra una estimación de la velocidad de crecimiento de grieta por fatiga debido a cargas producidas por fretting. Esto se realiza utilizando el modelo de crecimiento de grieta de dos umbrales, el cual predice el crecimiento de grieta en función del apilamiento y movimiento de dislocaciones, generando dos zonas plásticas: “zona plástica monotónica” asociada a la tensión máxima y “zona plástica cíclica” asociada a la variación de las tensiones. Los bloqueos de estas zonas plásticas en los bordes de grano u otras barreras y posterior salto de la barrera, determinan la naturaleza oscilante de la velocidad de crecimiento de grieta conforme se superan las sucesivas barreras. Finalmente, a partir de los estados de tensiones producidos por fretting se aplica el modelo a un caso de contacto esfera plano en aluminio Al 7075-T6 como material para las probetas.The present work shows an estimate of the fatigue crack growth rate under fretting conditions. This is done by using a two thresholds crack growth model, which predicts the crack growth depending on the stacking and movement of dislocations, generating two plastic zones: "monotonic plastic zone", associated with the maximum stress; and "cyclic plastic zone", associated with the stress range. The locking of these plastic zones in the grain boundaries or other barriers and subsequent release of the barrier, determines the oscillating nature of crack growth rate under as successive barriers are overcome. Finally, from the stress state produced by fretting, the model is applied to a case of contact between spherical pad and a flat specimen both of 7075-T6 aluminum alloy.Ministerio de Educación y Ciencia BES-2009-02506

On the Analysis of the Contact Conditions in Temporomandibular Joint Prostheses

Temporomandibular joint replacement (TMJR) is a complex surgical procedure in which the artificial joints available must assure the anatomical reconstruction and guarantee a good range of the natural temporomandibular joint (TMJ) movements. With this aim, different types of TMJ prostheses, including the stock prosthetic system and custom-made prostheses, are being currently implanted. Although temporomandibular joint replacements (TMJRs) are expected to accomplish their function during a number of years, they might actually fail and need to be replaced. This paper analyzes different design factors affecting the contact stress distributions within the TMJ prosthesis interface, which are consequently involved in their deterioration and final failure of the prosthetic device. With this purpose, a numerical model based on finite elements has been carried out in order to evaluate the stress states attained in different prosthesis configurations corresponding to general types of TMJ prostheses. On the other hand, the actual degradation of resected implants has been evaluated via optical microscopy. The linkage between the numerical simulations performed and experimental evidence allowed the authors to establish the different wear and damage mechanisms involved in the failure of stock TMJ prostheses. Indeed, the results obtained show that the contact stresses at the interface between the mandible and the glenoid fossa components play a key role in the failure process of the TMJR devices

On the study of the single-stage hole-flanging process by SPIF

Recent studies show the capability of single-point incremental forming to perform successfully hole-flanging operations using multi-stage strategies. The aim of this work is to investigate the ability of the SPIF process to perform hole-flanges in a single stage, contributing to a better understanding of the formability of the sheet in this demanding situation. To this end, a series of experimental tests in AA7075-O metal sheets are performed in order to evaluate the limiting forming ratio. The physical mechanisms controlling sheet failure during the process are analyzed and discussed. In the test conditions studied this failure is postponed necking followed by ductile fracture in the wall of the flange.Ministerio de Economía y Competitividad DPI2012-3291

Preliminary investigation on homogenization of the thickness distribution in hole-flanging by SPIF

A drawback of the hole-flanging process by single-stage SPIF is the non-uniform thickness obtained along the flange. Multi-stage strategies have been used to improve it, however they increase notably the manufacturing time. This work presents a preliminary study of the tool paths for a hole-flanging process by SPIF in two stages. An intermediate geometry of the piece is proposed from the analysis of the thickness distribution observed in previous single-stage process. A simple optimization procedure is used to automate the intermediate part design, the NC code generation for the tool path and the validation of the optimal forming strategy by means of FEA

Predición del límite de fatiga en fretting con contacto esférico

One of the most relevant events controlling fatigue crack propagation under fretting conditions is the steep stress gradient generated in the material by contact loads. This fact suggests that crack growth process may be analysed using methodologies similar to those used for notched components. The present work assesses the capability of some methods originally developed for notched components to predict the fatigue failure of fretted specimens with spherical contact. The effect of local variation in R-ratio along the crack path is taken into account. The predictions are compared with a number of experimental results obtained using specimens of Al-7075-T6 subjected to fretting fatigue with spherical contact.Uno de los aspectos principales que controlan el crecimiento de una grieta por fatiga bajo condiciones de fretting es el fuerte gradiente generado en el material por las cargas de contacto. Este hecho sugiere que el proceso de fatiga pueda ser analizado usando metodologías similares a las empleadas en componentes entallados. El presente trabajo analiza la capacidad que algunos métodos originariamente empleados en entallas tienen para predecir el fallo por fatiga de elementos sujetos a fretting con contacto esférico. El efecto de la variación de R a lo largo del plano de la grieta es tenido en cuenta y analizado con dichos modelos. Las predicciones se comparan con resultados experimentales obtenidos con probetas de Al-7075-T6 sujetas a fatiga por fretting con contacto esfera-plano.Ministerio de Ciencia y Tecnología PB97-0696-C02-01Ministerio de Ciencia y Tecnología DPI2001-239

Condiciones umbrales de crecimiento de grietas pequeñas por fatiga en entallas. Parte I. Teoría

El presente trabajo propone una descripción micromecánica del proceso de crecimiento grietas por fatiga en entallas. La interacción de la grieta con las barreras al deslizamiento plástico del material (e.g. los bordes de grano) y la influencia del gradiente de tensiones originado por la entalla son tenidos en cuenta intrínsecamente por el modelo desarrollado. Dicho modelo permite obtener las mínimas condiciones de tensión requeridas para que una grieta pueda propagarse en el fondo de la entalla (condiciones umbrales), identificando claramente el límite de iniciación y el límite de propagación hasta el fallo. Así mismo, el modelo propuesto permite explicar la formación de grietas no-propagantes.The present work presents a micromechanical description of the fatigue crack growth process at notches. Crack interaction with the plastic slip barriers of the material (e.g. the grain boundaries) and the influence of the notch stress gradient are intrinsically taken into account in the model. Both the notch fatigue crack initiation limit and the limit for propagation up to failure (i.e. the conventional notch fatigue limit) are clearly identified and calculated. The formation of non-propagating cracks is also explained