Previsão da vida de fadiga com fretting

Tese de doutoramento em Mecânica dos MateriaisFretting fatigue occurs at the interface between two contacting bodies that are pressed against each other in the presence of cyclic loads that gives rise to a small relative displacement. Fretting fatigue occurs in many machines and engineering structures where vibrating elements are in contact with each other. There are over 50 parameters that influence the fretting fatigue phenomenon out of which the most important are: relative displacement amplitude, normal contact load and tangential contact load. Despite of wide research on variables that affect fretting phenomena, there are still ambiguities and contradictions regarding the contact degradation process, mainly in the fretting fatigue case (partial slip regime) where the contact zone is more complex compared to the gross slip regime. When the materials are subjected to fretting fatigue, the effect of the intrinsic materials properties is more pronounced than under plane fatigue conditions, because there is the effect of fatigue (cracking) plus the effect of fretting (wear). In order to understand the influence of the intrinsic material properties on fretting fatigue phenomena three materials (Al7175, Ck45, and Ti6Al4V) with very different cyclic properties as well as different wear properties, have been tested. All three materials were in contact with conventional 34CrNiMo6 steel. A novel device, capable of reproducing this phenomenon, was specially designed. The results from the fretting fatigue tests highlighted that the damage induced by fretting increased the fatigue damage, resulting in a shorter life of the component (specimen). It was observed that Al7175 and Ti6Al4V alloys are more sensitive to fretting fatigue than Ck45 steel. The reason was mainly attributed to the well known poor tribological characteristics of the aluminum and titanium alloys. It was also confirmed the presence of an asymmetrical fretting scar in fretting fatigue situations (rarely highlighted in technical literature). In this work, and for the first time, an explanation for such behaviour was proposed. The explanation is based on the stress state of the specimen. In fretting fatigue case the specimen stress state, besides the stress due to contact of the pad, changes with the machine load cycle, and it is reasonable to expect that the wear behaviour of materials might be affected by the specimen stress state. The different wear behaviour (due to different stress state) during a fatigue cycle is proposed to be the cause for the asymmetrical fretting fatigue scar. In order to validate this assumption several reciprocating wear tests under an additional elastic stress state were carried out. The results from the reciprocating wear tests were then extrapolated to the fretting fatigue case. It was observed that the additional elastic bulk stress has a substantial influence on the wear behaviour of the materials used in the study. It was concluded, therefore, that it is pertinent to take it into consideration in wear assessment, and consequently in fretting fatigue predictions. In order to take into account the elastic bulk stresses imposed by external loading, a physical model that is a modification of the Archard´s model for the prediction of wear volume was proposed. To study the influence of the additional elastic stress state on the wear behaviour of the materials, a new wear device was designed, built and assembled in a pin-on-plate reciprocating wear machine. In order to develop a method that could be useful to estimate the fretting fatigue life, two strain based fatigue models have been chosen. The first approach is the Smith- Watson-Topper´s model and the second approach is the Morrow´s model. These models were chosen because they are the models that include more intrinsic material properties being then more appropriate to understand either the influence of the material properties as well as to establish a comparison among different materials. The former model was already used in fretting fatigue while the later was tested for the first time in this work. Mean stress effects, due to R ratio and to contact load, were included in both models. The Morrow´s model was modified in order to introduce the local stresses involved in the fretting fatigue process (normal, tangential and axial loads). Furthermore, and in order to obtain good predictions of the fretting fatigue life, as obtained in experimental tests, two modifications to the previous models, were proposed in this work. Both modifications are related to the effect of the contact damaged area in fatigue life. The first modification, herein called the “fretting scar effect”, takes the form of a stress concentration factor, Kt, and is related to the global shape of the scar geometry. The second modification is a surface finishing factor, Ks. This modification is based on the fact that fretting fatigue cracks initiate predominantly at the surface between the two contact zones (slip and stick). In this region, the surface roughness condition seems to be sufficiently important (substantially high roughness values) in order to be considered relevant in fatigue life predictions. The previous two modifications affect only the elastic term component of the models. It was also verified that better predictions could be obtained with a new parameter, which affects the plastic component of the models. This parameter is the so-called ‘Bauschinger effect’, which reflects a material deviation from the ideal elastic-plastic behavior. This effect, never used for fatigue life (initiation) predictions nor incorporated in existing models (SWT and Morrow), was added to the previous models. The predictions, by using the two previous modified models, seemed to be adequate for fretting fatigue life quantifications, depending on the model and material tested. It was concluded that the added parameters, on the elastic and plastic terms, are relevant ones in order to obtain improved fretting fatigue predictions as well as to allow the models to become more universal equations.A fadiga com fretting ocorre na interface entre dois corpos em contacto que se encontram pressionados um contra o outro na presença de cargas cíclicas e de pequenos deslocamentos relativos. A fadiga com fretting ocorre em muitas máquinas e estruturas de engenharia onde elementos, em contacto uns com os outros, estão sujeitos a vibração. Há mais de 50 (cinquenta) parâmetros que influenciam o fenómeno da fadiga com fretting dos quais os mais importantes são: a amplitude de deslocamento relativo, as cargas normais e as cargas tangenciais. Apesar da ampla investigação sobre as variáveis que influenciam o fenómeno de fretting, ainda há dúvidas e contradições quanto ao processo de degradação dos materiais, principalmente, no caso da fadiga com fretting, e no regime de deslocamento parcial, onde a zona de contacto é mais complexa em comparação com o regime de deslocamento total. Quando os materiais são submetidos a fadiga com fretting o efeito nas propriedades intrínsecas dos materiais é mais evidente do que quando sujeitos apenas a fadiga sem fretting, porque há o efeito da fadiga (fissura) somado ao efeito do fretting (desgaste). A fim de compreender a influência das propriedades intrínsecas das materiais no fenómeno da fadiga com fretting foram testados três materiais (Al7175, Ck45, e Ti6Al4V) com muito diferentes propriedades de fadiga cíclica assim como diferentes propriedades de desgaste. Um novo dispositivo, capaz de reproduzir esse fenómeno, foi especialmente projectado. Os resultados dos ensaios da fadiga com fretting mostraram que o dano induzido por fretting aumenta o dano por fadiga, reduzindo a durabilidade (vida útil) do componente (provete). Observou-se que as ligas Al7175 e Ti6Al4V são mais sensíveis à fadiga com fretting do que o aço Ck45. Tal fato pode ser justificado pelas conhecidas características tribological do alumínio e titânio que são inferiores em relação às mesmas características do aço Ck45. Confirmou-se, também, que a zona danificada pelo fretting, tem uma forma assimétrica (aspecto raramente destacado na literatura técnica). Neste trabalho, e pela primeira vez, uma explicação para tal fenómeno foi proposta. A explicação é baseada no estado de tensão do provete. No caso da fadiga com fretting, o estado de tensão, para além das tensões de contacto, mudam com o ciclo de carga da máquina de fadiga. É razoável esperar que o comportamento de desgaste dos materiais seja afectado pelo estado de tensão do provete, introduzido pela máquina. O diferente comportamento ao desgaste (devido ao diferente estado de tensão do provete) durante o ciclo da fadiga é proposto como sendo a causa da pista com forma assimétrica, na fadiga com fretting. A fim de validar tal hipótese, vários ensaios de desgaste alternativos sob um estado de tensão adicional do provete, no regime elástico, foram realizados. Observou-se que este estado de tensão adicional, tem uma influência significativa no comportamento de desgaste dos materiais utilizados no estudo. Desta forma, torna-se importante considerar este estado de tensão adicional na avaliação de desgaste e, consequentemente, na previsão da durabilidade à fadiga com fretting. Para casos de desgaste, e a fim de considerar-se o estado de tensão, no regime elástico, imposto por uma carga externa, ou seja, a máquina de fadiga, propõe-se neste trabalho, um modelo físico que é uma modificação do modelo de Archard para a previsão do volume de desgaste. Para estudar-se este fenómeno, influência de um estado de tensão adicional, no regime elástico, no comportamento de desgaste dos materiais, um novo dispositivo pino-sobre-placa foi desenvolvido e associado a uma máquina de desgaste alternativo. A fim de se desenvolver um método que possa ser útil na previsão de vida de fadiga com fretting, foram escolhidos dois modelos de previsão baseados em deformação (strain-life-approach). O primeiro é o modelo de Smith-Watson-Topper e, o segundo é o modelo de Morrow. Estes modelos foram escolhidos por incorporarem muitas propriedades intrínsecas dos materiais, quer propriedades plásticas quer elásticas. Sendo assim seriam os modelos mais apropriados para se compreender tanto a influência das propriedades do material, bem como se modificarem ou ainda para se estabelecerem comparações entre materiais com diferentes propriedades. O primeiro modelo (SWT) já é utilizado no caso da fadiga com fretting, enquanto o segundo (Morrow), foi testado pela primeira vez neste trabalho, na situação de fadiga com fretting. Os efeitos da tensão média, quer devidos à razão de tensões quer devidos ao contacto, foram incluídos nos dois modelos sendo que o foram pela primeira vez no segundo modelo (Morrow). A fim de obter uma estimativa adequada da vida de fadiga com fretting, de acordo com os resultados experimentais, duas modificações aos modelos anteriores, foram propostas neste trabalho. As duas modificações estão relacionadas com o efeito da área danificada na área de contacto, na vida de fadiga. A primeira modificação, aqui chamada de " efeito da pista de fretting", assume a forma de um factor de concentração de tensão, Kt, e está relacionada com a geometria da zona de contacto. A segunda modificação assume a forma de um factor de acabamento superficial Ks. Esta alteração é baseada no facto de que as fendas de fadiga, no caso de fadiga com fretting, iniciarem-se predominantemente na superfície entre as duas zonas de contacto (zona de deslizamento relativo e zona de deformação). Nesta região, as condições da rugosidade da superfície são substanciais e deverão ser consideradas nos modelos de previsão da vida de fadiga com fretting. Estas duas alterações afectam essencialmente a componente de deformação elástica dos modelos. Verificou-se também que melhores previsões podem ser obtidas se se adicionar um novo parâmetro, que afecta a componente plástico dos modelos. Este parâmetro é conhecido como "efeito de Bauschinger”, e reflecte um desvio em relação ao comportamento elasto-plástico ideal do material. Este efeito não é tido em consideração nos modelos testados (SWT e Morrow) nem em qualquer outro modelo de previsão de vida dos materiais, tendo sido proposto como relevante, pela primeira vez neste trabalho. As previsões, usando os dois modelos anteriores, quando modificados, mostraram ser adequadas para a quantificação da vida de fadiga com fretting, dependendo todavia bastante do material testado. Assim, conclui-se que os parâmetros adicionados, quer na componente elástica quer na componente plástica, das equações, são relevantes para se obterem previsões de vida de fadiga com mais rigor assim como para tornar os modelos usados (SWT e Morrow) mais universais.Fundação para a Ciência e a Tecnologia (FCT), Ref. SFRH/BD/19555/200

Topographic and Electrochemical Ti6Al4V Alloy Surface Characterization in Dry and Wet Reciprocating Sliding

This present paper shows the behavior of functional integrity of the state Ti6Al4V alloy under reciprocating sliding wear conditions in acomparative way for two different counter materials, steel and ceramicballs in dry and corrosive environment (3.5% NaCl). The surface integrity analysis of the dry reciprocating wear tests was based on the evolution of The roughness parameters with the applied load. In the case of reciprocating wear tests in corrosive environment the surface integrity analysis was based on electrochemical parameters. Comparative analysis of the evolution of the roughness parameters with the applied load shows a higher stability of the Ti6Al4V/Al2O3 contact pair, while from the point of view of the electrochemical parameters the Tribological properties are worse than Ti6Al4V/steel ball contact pair

Multi-material Ti6Al4V & PEEK cellular structures produced by Selective Laser Melting and Hot Pressing: A tribocorrosion study targeting orthopedic applications

This work was supported by FCT through the grants SFRH/BD/128657/2017 and SFRH/BPD/112111/2015, the project PTDC/EMSTEC/5422/2014 and also by project NORTE 01–0145_FEDER-000018. Additionally, this work was supported by FCT with the reference project UID/EEA/04436/2013, by FEDER funds through the COMPETE 2020 – Programa Operacional Competitividade e Internacionalização (POCI) with the reference project POCI-01–0145-FEDER-006941.Ti6Al4V-alloy is commonly used in dental and orthopedic applications where tribochemical reactions occur at material/bone interface. These reactions are one of the main concerns regarding Ti6Al4V implants due to the generation of wear particles, linked to the release of metallic ions in toxic concentration which occurs when TiO2 passive film is destroyed by means of wear and corrosion simultaneously. In the present study, a multi-material Ti6Al4V-PEEK cellular structure is proposed. Selective Laser Melting technique was used to produce Ti6Al4V dense and cellular structured specimens, whilst Hot-Pressing technique was employed to obtain multi-material Ti6Al4V-PEEK structures. This study investigates the tribocorrosion behavior of these materials under reciprocating sliding, comparing them with commercial forged Ti6Al4V. Open-circuit-potential was measured before, during and after sliding while dynamic coefficient of friction was assessed during sliding. The results showed an improved wear resistance and a lower tendency to corrosion for the multi-material Ti6Al4V-PEEK specimens when compared to dense and cellular structures mono-material specimens. This multi-material solution gathering Ti6Al4V and PEEK, besides being able to withstand the loads occurring after implantation on dental and orthopedic applications, is a promising alternative to fully dense metals once it enhances the tribocorrosion performance.info:eu-repo/semantics/publishedVersio

Tribocorrosion behavior of NiTi biomedical alloy processed by an additive manufacturing laser beam directed energy deposition technique

The purpose of the present study was to experimentally assess the synergistic effects of wear and corrosion on NiTi alloy in comparison with Ti-6Al-4V alloy, the most extensively used titanium alloy in biomedical applications. Both alloys were processed by an additive manufacturing laser beam directed energy deposition (LB-DED) technique, namely laser engineered net shaping (LENS), and analyzed via tribocorrosion tests by using the ball-on-plate configuration. The tests were carried out in phosphate buffered saline solution at 37 °C under open circuit potential (OCP) to simulate the body environment and temperature. The synergistic effect of wear and corrosion was found to result in an improved wear resistance in both materials. It was also observed that, for the process parameters used, the LB-DED NiTi alloy exhibits a lower tendency to corrosion as compared to the LB-DED Ti-6Al-4V alloy. It is expected that, during the service life as an implant, the NiTi alloy is less susceptible to the metallic ions release when compared with the Ti-6Al-4V alloy.This work was partially funded by FCT (Fundação para a Ciência e Tecnologia) through the grant SFRH/BD/112280/2015 and the reference projects UID/EEA/04436/2019, COMPETE 2020 with the code POCI-01-0145-FEDER-006941; NORTE-01-0145-FEDER-000018-HAMaBICo and POCI-01-0145-FEDER-031035_LaserMULTICER. Also, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES-Brazil) and Alexander von Humboldt Foundation are acknowledged for the partial support of this work. This work was also partially supported by the Project “EXPERT”, Contract no. 14PFE/17.10.2018 and Auburn University’s Presidential Award for Interdisciplinary Research (PAIR)

A Test for Rapid Tribological Characterization of Bearing Steels

In this paper is a study the influence of the material and heat treatment on the changes of the fine structure parameters of the superficial layer of triboelements during the first cycles of rolling contact process. The tests were conducted by using a four-roller testing machine. The specimens (rollers) were made of case-hardening, through hardening and high frequency hardening steels. In order to induce different stress distributions in the superficial layer of the specimen specific treatments were applied. Using X-ray diffractometry can be appreciated that the durability of rolling tribosystems can be determined by an optimal relationship between studied parameters. The observed correlation can be used as a fast method to estimate the fatigue life of rolling bearing materials

Tribocorrosion behaviour of hot pressed CoCrMo−Al2O3 composites for biomedical applications

Alumina/alumina wear couple can lower the wear rates and thus metallic ion releasing on load bearing metallic implant materials. However, the low fracture toughness of ceramics is still a major concern. Therefore, the present study aims to process and to triboelectrochemically characterise the 5 and 10 vol.-%Al2O3 reinforced CoCrMo matrix composites. Corrosion and tribocorrosion behaviour of the composites were investigated in 8 g L21 NaCl solution at body temperature. Corroded and worn surfaces were investigated by a field emission gun scanning electron microscope equipped with energy dispersive X-ray spectroscopy. After tribocorrosion experi- ments, wear rates were calculated using a profilometer. Results suggest that Al2O3 particle addition decreased the tendency of CoCrMo alloy to corrosion under both static and tribocorrosion conditions. However, no significant influence on the corrosion and wear rates was observed in composites mainly due to increased porosity and insufficient matrix/ reinforcement bonding.This study was supported by the Portuguese Foundation for Science and Technology (FCT-Portugal), under project no. EXCL/EMS-TEC/0460/2012, and The Calouste Gulbenkian Foundation through 'Programa de Mobilidade Academica para Professores'. The authors also would like to thank Professor A. Ramalho (Universidade de Coimbra) for the provision of profilometry facilities