Damage Evolution During Fatigue in Structural Materials

AbstractEarly stages of damage evolution in cyclic loading are described and discussed. The importance of the role of cyclic plastic strain in damage evolution is emphasized and the relation between stress and strain in cyclic straining is clarified. The principal stages of damage evolution in fatigued crystalline structural material are identified. The basic characteristic and theories of fatigue crack initiation are sketched and confronted with experimental observations. Early fatigue crack growth is characterized and quantitatively described. The relation between the growth of short cracks and fatigue life in the form of Manson-Coffin law is established

Accumulation of electricity for a source with unstable production

Tato diplomová práce se zabývá možnostmi akumulace elektrické energie vyrobené obnovitelnými zdroji s nestabilní produkcí. Zadání stanovuje několik cílů. V rámci rešerše jsou na příkladech konkrétních zástupců podrobně představeny aktuální trendy v akumulaci, včetně jejich technických parametrů a ekonomických možností. Dále je pro účely jednotlivých návrhů systémů akumulace definován nestabilní zdroj elektrické energie – větrný park tří elektráren Multibrid 5000, společně s jeho parametry a detaily provozu. Jako první je realizován podrobný návrh pokročilého úložiště energie ve stlačeném vzduchu (A-CAES). Součástí je termodynamický výpočet stanovených komponent, jejich částečné technické řešení, finanční zhodnocení investice a určení teoretické doby návratnosti. Dále je proveden návrh pro akumulátory Tesla Powerpack, který obsahuje výpočet jednotlivých parametrů systému, finanční zhodnocení investice a vymezení teoretické doby návratnosti. Poslední část práce se věnuje srovnání a zhodnocení dosahovaných výsledků obou navržených způsobů akumulace elektrické energie. Na závěr je vybrán perspektivnější z nich a to A-CAES.This diploma thesis deals with possibilities of accumulation of electricity generated by renewable sources with unstable production. Several goals were set by an assignment of this thesis. Within a research, current trends of the accumulation are presented in detail on examples of specific representatives with their technical parameters and economic possibilities. Further, an unstable source of electricity – the wind farm of three power plants Multibrid 5000 with all its parameters and operational details is defined for purposes of individual designs of accumulation systems. Firstly, the detailed design of an advanced compressed air energy storage (A-CAES) is realized. It includes a thermodynamic calculation of chosen components, their partial technical solutions, a financial evaluation of investment and a determination of theoretical payback period. Furthermore, the design for accumulators Tesla Powerpack is realized. It contains a calculation of individual system parameters, a financial evaluation of investment and a determination of theoretical payback period. Last part of the thesis is dedicated to a comparison and an evaluation of the achieved results of both realized methods of the electricity accumulation. In the end, A-CAES is chosen as more perspective variation.

Possibilities for replication of spraying nozzles by means of 3D printing methods

Tato bakalářská práce se zabývá možností použití aditivních technologií (AT) při výrobě tlakových vířivých trysek, za účelem snížení finančních nákladů stejně jako výrobních časů dosahovaných stávajícími metodami výroby. Zadáním stanovuje několik cílů. Konkrétně byly detailně rozebrány jednotlivé metody 3D tisku, představen pro účely práce vybraný typ rozstřikovací trysky společně s požadavky na něj kladenými. Dále byla vyhodnocena použitelnost jednotlivých zástupců AT a také provedeno jejich srovnání. Výsledkem práce jsou dvě potencionálně použitelné metody pro replikaci tlakových vířivých trysek, a to Micro Laser Sintering a Direct Metal Laser Sintering. Z časových důvodů a již tak velkého rozsahu práce, nebyl proveden volitelný 6. úkol a experimentální ověření touto prací stanovených závěrů zůstává otázkou pro další studie.This bachelor thesis deals with the possibilities of using additive manufacturing (AM) in case of replication of pressure swirl atomizers in order to reduce the financial costs as well as the production times achieved by the existing production processes. Several goals were set by an assignment of this thesis. In particular, the individual 3D printing methods were discussed in detail and the specific selected type of spray nozzle was introduced, together with their requirements that have been elaborated in detail. The applicability of the individual representatives of AM was also evaluated, and their comparison was made. The result of this work are two potentially useful methods for replicating pressure swirl nozzles, namely Micro Laser Sintering and Direct Metal Laser Sintering. Due to lack of time and the already large extent of this thesis, an optional 6th task was not carried out and the experimental verification of these set conclusions remains a question for further studies.

Environmentally assisted fatigue of superelastic NiTi

Superelastic NiTi implants transforming cyclically in body fluids suffer from fatigue failures which are extremely difficult to predict. This clearly points out towards environmental effects promoting surface dominated fatigue degradation. The specialty of phase transforming NiTi shape memory alloy is that either the parent austenite or the product martensite phase exist at the excessively deforming metal/liquid interface covered by the thin TiO2 surface. In order to explore the environmental effects at such mechanically active metal/liquid interface, we have developed dedicated electrochemical apparatus and methods combining electrochemical cell, mechanical tester and thermal chamber. We are able to follow and/or control the mechanically triggered periodical breakdown/passivation process on the metal/liquid interface occurring during cyclic tensile tests on NiTi wires and springs in fluids. In this way we are able to analyze the effect of surface finishing treatments on fatigue performance and/or control it electrochemically. In this talk, we will introduce two in-situ electrochemical methods especially open circuit potential and potentiostatic polarization applied during fatigue testing. We will focus on the problem of non-stationary thermodynamic equilibrium established at the mechanochemically loaded wire surface. Kinetics of the surface reactions encountered during this type of environmental fatigue testing will be revealed. SIMS depth profile analysis and chemical imaging of the surfaces of fatigued wires was employed to prove the assumed electrochemical activity upon cycling, particularly to the hydrogen absorption and growth of passive oxide layer within cracks. Microcracks forming on the surface of fatigued wires were observed by 3D SEM/FIB sectioning method. Based on the results, mechanisms of environmental fatigue degradation of NiTi implants deforming cyclically in body fluids will be proposed

Cyclic Plastic Response and Damage in Materials for High Temperature Applications

In this contribution, the methods allowing to study the sources of cyclic stress and early fatigue damage at room and elevated temperatures are presented and used to reveal details of the mechanisms of cyclic plastic straining and fatigue damage evolution in austenitic steels and nickel superalloys.Анализ кривых гистерезиса показывает, что при различной температуре имеют место характерные изменения зависимости между циклическими пластическими деформациями и напряжениями в нержавеющей стали и суперсплавах. Соответствующее изменение микрорельефа поверхности при мягком нагружении зафиксировано с помощью сканирующей и томографической электронной микроскопии с высоким разрешением. Проанализированы механизмы локализации циклической деформации, образования поверхностного микрорельефа, инициирования усталостных трещин и роста коротких трещин.Аналіз кривих гістерезису показує, що за різних температур відбуваються характерні зміни залежності між циклічними пластичними деформаціями і напруженнями в нержавіючій сталі і суперсплавах. Відповідні зміни мікрорельєфу поверхні за м’якого навантаження зафіксовано за допомогою сканувальної і томографічної електронної мікроскопії з високим розділенням. Проаналізовано механізми локалізації циклічної деформації, утворення поверхневого мікрорельєфу, ініціювання тріщин від утомленості і росту коротких тріщин

Fatigue Life of 7475-T7351 Aluminum After Local Severe Plastic Deformation Caused by Machining

The fatigue properties of thermo-mechanically treated and machined aluminum alloy 7475-T7351 have been studied. The applied advanced machining strategy induced intensive plastic deformation on the machined surface under defined cutting conditions. Therefore, a detailed study of 3D surface topography was performed. Advanced characterization of the material structure and electron back scattered diffraction mapping of selected chemical phases were performed, as well as energy dispersive X-ray analysis of the surface. Advanced mechanical properties of the material were investigated in situ with a scanning electron microscope that was equipped with a unique tensile fixture. The fatigue results confirmed an evident dispersion of the data, but the mechanism of crack nucleation was established. Fracture surface analysis showed that the cracks nucleated at the brittle secondary particles dispersed in the material matrix. The surface topography of samples that had been machined in wide range of cutting/deformation conditions by milling has not proved to be a decisive factor in terms of the fatigue behavior. The incoherent interface and decohesion between the alumina matrix and the brittle secondary phases proved to significantly affect the ultimate strength of the material. Tool engagement also affected the fatigue resistance of the material

The effects of grain size, dendritic structure and crystallographic orientation on fatigue crack propagation in IN713C nickel-based superalloy

The polycrystalline IN713C produced via investment casting is one of the widely-used nickel-based superalloy in automotive and aerospace industries. This alloy, however, has an apparent inhomogeneous microstructure generated during casting and contains dendritic structure that gives rise to strain localisation during loading. Yet, the effect of dendritic structure, grain size and shape as well as crystallographic orientation, which directly influence fatigue property and deformation micromechanism in the components, is rarely studied. In the present study, IN713C cast bars are tailored with three different grain structures, i.e., transition, equiaxed and columnar, with substantial grain size variations. The produced bars were tested under strain controlled LCF (Low Cycle Fatigue) and stress controlled HCF (High Cycle Fatigue) conditions at 650 °C. The results showed that most of fatigue cracks initiated from casting pores and fatigue life extended in the microstructure with a small grain size during both HCF and LCF loadings. It is also demonstrated that fatigue striations were mainly observed within dendritic areas during crack propagation, whereas the higher GND (Geometrically Necessary Dislocation) density were predominantly observed in the interdendritic areas. Here, we propose a concept of ‘Crack Propagation Unit (CPU)’ for better description of deformation mechanism at local scale during fatigue loading by combining fracture surface characteristic methodology and dislocation distribution analyses within the dendritic structural unit. Furthermore, this model to understand the deformation micromechanism can provide a new perspective on the interpretation of Hall-Petch relationship in casting materials that contain dendritic structure. This is further demonstrated via direct correlation of the high crack propagation resistance with the crack path divergence instead of the dislocation pile-up at the grain boundary or in-between the γ/γ′ channels. Moreover, by utilising serial sectioning method followed by layered EBSD scanning, quasi-3-D grain orientation mappings were obtained, and crystallographic texture information were directly correlated with the fracture surface observations. This allowed an investigation of the influence of orientation of individual grains and micro/macro texture on crack propagation rate. The critical stage of crack propagation in fatigue life and its correlations with microstructural features is established, offering potential practical applications by controlling the investment casting process parameters

Nízkocyklová únava INCONELu 792-5A při 900 °C

The aim of the present paper is to investigate the low cycle fatigue characteristics of cast nickel base superalloy INCONEL 792-5A in symmetrical total strain cycling at the temperature 900 °C

IN-SITU HIGH TEMPERATURE LOW CYCLE FATIGUE STUDY OF SURFACE TOPOGRAPHY EVOLUTION IN NICKEL SUPERALLOY

In-situ Low Cycle Fatigue test (LCF) at temperature 635 degrees C have been performed in Scanning Electron Microscope (SEM) equipped with Electron Backscatter Diffraction analysis (EBSD) on a small dog-bone-shaped specimen of cast Inconel 713LC superalloy. The aim of the work was to study early stage fatigue damage at high temperature by the observations of the characteristic surface relief evolution and crystallographic characterization changes by EBSD. The detail of slip bands shape was checked by FIB and AFM microscopes. The LCF test was conducted on GATAN stage with pre tilted position and constant stress amplitude of total cycle number of 20. The relief produced in the first cycle determines the other locations of the localized cyclic slip to the primary slip planes (111). The relief was modified in the next cycles but without forming additionally new slip traces in the primary system. Based on EBSD analysis before and after LCF, the orientation of two grains was changed which caused activation of second slip system. The damage mechanism evolution is closely connected with the cyclic strain localization to the persistent slip bands where the fatigue cracks were initiated