The Effect of Elevated Temperature on the Fretting Fatigue Behavior of Nickel Alloy IN-100

This thesis studied the effect of elevated temperature (600 C) on the fretting fatigue behavior of IN-100. First, fretting and plain fatigue S-N curves were determined over a large range of applied stress at an identical stress ratio of 0.03 at 600 C and for fretting tests, with a constant contact load. The partial slip condition was verified by using hysteresis between shear force and axial stress techniques. The contact width and the crack initiation location were observed for all of the fretting specimens. The contact widths were consistent with expected values and the crack initiation location was at the trailing edge of contact for all fretting specimens. This study compared its high temperature results to room temperature testing accomplished by this study as well as room temperature data from a previous study using IN-100 with an identical microstructure and another room temperature study of IN-100 with a coarser microstructure. It was found that fretting fatigue reduces the cycles to failure compared to plain fatigue at elevated temperature but a more pronounced reduction was noticed than with room temperature. It was also found that the elevated temperature environment increased the both the plain and fretting fatigue life as compared to similar stress levels at room temperature. The study found that elevated temperature does have an effect on crack initiation and crack propagation as compared to room temperature environment. The higher temperature allowed a longer initiation and crack propagation time which strongly suggests the fatigue life increases with temperature up to 600C. The creation of oxide films and temperature induced softening or plasticity were both found to act as crack closure mechanisms in another nickel based superalloy, IN-718, when fatigued in the high temperature environment. It is therefore likely that these oxidation and plasticity crack closure mechanisms are the reasons for the improved performance at elevated temperatures. These findings were different than in titanium alloys, where elevated temperatures allowed a shorter crack initiation time by greater stress relaxation thus hastening crack development

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

Fatigue Tests with Random Flight Simulation Loading

Crack propagation was studied in a full-scale wing structure under different simulated flight conditions. Omission of low-amplitude gust cycles had a small effect on the crack rate. Truncation of the infrequently occurring high-amplitude gust cycles to a lower level had a noticeably accelerating effect on crack growth. The application of fail-safe load (100 percent limit load) effectively stopped subsequent crack growth under resumed flight-simulation loading. In another flight-simulation test series on sheet specimens, the variables studied are the design stress level and the cyclic frequency of the random gust loading. Inflight mean stresses vary from 5.5 to 10.0 kg/sq mm. The effect of the stress level is larger for the 2024 alloy than for the 7075 alloy. Three frequencies were employed: namely, 10 cps, 1 cps, and 0.1 cps. The frequency effect was small. The advantages and limitations of flight-simulation tests are compared with those of alternative test procedures such as constant-amplitude tests, program tests, and random-load tests. Various testing purposes are considered. The variables of flight-simulation tests are listed and their effects are discussed. A proposal is made for performing systematic flight-simulation tests in such a way that the compiled data may be used as a source of reference

Metallurgy: A compilation

A technology utilization program is presented for the dissemination of information on technological developments which have potential utility outside the aerospace and nuclear communities. Discussion is restricted to the effects of hydrogen on a variety of metal alloys, and the mechanical properties of some recently developed alloys. Hydrogen at both low and high pressure is shown to have adverse effects on alloys such as ultrahigh-strength steels, irradiated steels, columbium, inconel alloys, titanium alloys, and certain stainless steels. The mechanical and physical properties of a wide range of alloys, their performance at elevated temperatures, and some of the processes involved in their development are also considered

Effect of a Variable Contact Load on Fretting Fatigue Behavior of Ti-6Al-4V

Effects of a variable contact load on the high cycle fretting fatigue behavior of Ti-6AL-4V were investigated. Experimental tests were performed using a new test setup capable of applying a contact load varying in amplitude, frequency, and phase and independently measuring shear forces on opposite sides of a specimen. Finite element analysis (FEA) of experimental and idealized loading conditions was performed and local mechanistic parameters and the Modified Shear Stress Range (MSSR) fatigue parameter were determined. Correlations between contact width, slip amplitude, fatigue life and a variable contact load were established with variable contact loading shown to have a damaging effect. Predicted values of crack location and orientation using the MSSR parameter were verified experimentally. The MSSR parameter unsuccessfully predicted fatigue life for variable contact loads. A new formulation of the MSSR parameter is proposed

Методика втомних випробувань при аналізі побічних ефектів застосування антикорозійних профілактичних сполук

Робота публікується згідно наказу Ректора НАУ від 27.05.2021 р. №311/од "Про розміщення кваліфікаційних робіт здобувачів вищої освіти в репозиторії університету". Керівник роботи: професор, д.т.н. Карускевич Михайло ВіталійовичThis master's thesis is dedicated to studying the effects of the compounds used on aircraft fatigue when the aircraft is anti-corrosive Computer-aided design CAD/CAE methods have been used, especially CATIA and ABAQUS. The practical value of diploma work is to improve the reliability and service life of aviation structures by studying the side effects of anti-corrosion compounds. The materials for the master's degree and diploma can be used in the education process of aviation industry and aviation major.Ця магістерська робота присвячена вивченню впливу сполук, що використовуються на втому літака, коли літак є антикорозійним. Використовувалися методи автоматизованого проектування CAD/CAE, особливо CATIA та ABAQUS. Практична цінність дипломної роботи полягає у підвищенні надійності та терміну служби авіаційних конструкцій шляхом вивчення побічної дії антикорозійних складів. Матеріали для отримання ступеня магістра та диплома можуть бути використані в процесі навчання авіаційної промисловості та авіаційної спеціальності

Characterization of high cycle fatigue and laser-aided machining and polishing of additively manufactured materials

“Additive manufacturing (AM) and laser-aided machining and polishing (LAMP) of materials are emerging manufacturing processes both for research and industrial sectors. The AM process can manufacture near-net-shape parts with complex geometries. Meanwhile, the LAMP process integrated with an AM system offers a high processing rate, minimum heat-affected zone, and easily adjustable process parameters during machining and polishing. In mechanical properties characterization of AM metals and alloys, fatigue is a vitally important test method to understand the behavior of materials in cycling loading and unloading circumstances since most mechanical failures of structures are due to fatigue. To characterize AM metal fatigue behavior, it is also crucial to understand and analyze how the fabrication process parameters, build orientations, and defect formations affect the ability of materials to resist fatigue failure. This research aims to study the needed fundamental knowledge for a high-speed fatigue testing method with miniature specimens and investigate the effect of build process parameters on the high cycle fatigue performance of AM materials. In this study, the implementation of miniature specimens with increased surface area and uniform stress distribution within gauges captures a large population of surface and subsurface defects, reduces the stress gradient effect, maintains symmetric loading, minimizes material and test equipment costs, and decreases sample preparation and test time. The acquired knowledge from this study helps understand the influence of defects on the fatigue behavior of AM materials and determine the high fatigue strength yielding process parameters. Since the fatigue strength of materials can be improved by machining and polishing part surfaces, the objectives of this research also include developing a multilaser LAMP process and investigating the effect of different process parameters on part surface quality improvements. The research results lead to new knowledge that could benefit a wide range of manufacturing industries”--Abstract, page iv