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Cold dwell fatigue of titanium
Get PDFThere is a long-standing technological problem in which a stress dwell during cyclic loading at room temperature in Ti causes a drastic fatigue life reduction. It is thought that localised time dependent plasticity in ‘soft’ grains oriented for easy plastic slip leads to load shedding and an increase in stress within a neighbouring ‘hard’ grain that is poorly oriented for easy slip. Quantifying this time dependent plasticity process is key to successfully predicting the complex cold dwell fatigue problem. Incorporating the influence of operating temperatures and common alloying elements on cold dwell fatigue will be beneficial for future alloy design to address this problem.
Firstly, stress relaxation tests were performed at four different temperatures on three major alloy systems: commercially pure titanium (two alloys with different oxygen content), Ti-6Al-4V (two microstructures with differing beta phase fractions) and Ti-6Al-2Sn-4Zr-xMo (two alloys with different Mo content x=2 or 6 and portion of beta phase). Key parameters controlling the time dependent plasticity were determined as a function of temperature from the macroscopic stress relaxation results. It was found that the dwell fatigue effect is more significant by oxygen alloying but is suppressed by the addition of Mo. The presence of the beta phase did not strongly affect the dwell fatigue, however, it was suppressed at high temperature due to the low strain rate and strain rate sensitivity.
To understand the mechanism in greater detail, synchrotron X-ray diffraction technique during stress relaxation experiments was utilised to characterise the time dependent plastic behaviour of two commercially pure titanium samples (grade 1 and grade 4) with different oxygen content at 4 different temperatures (room temperature, 75C, 145C and 250C). Such experiments enable direct assessment of lattice strains from grain families with common grain orientations from polycrystal sample. By monitoring the lattice strain response, it is possible to infer the behaviour of different slip systems. Lattice strains were measured by tracking the diffraction rings radii changes from multiple plane families (21 diffraction rings) as a function of their orientation with respect to the loading direction. The critical resolved shear stress, activation energy and activation volume were established for both prismatic and basal slip modes by fitting a crystal plasticity finite element model to the lattice strain relaxation responses measured along the loading axis for five strong reflections.
It was found that cold dwell fatigue highly depends on crystallographic orientation, where accumulation of plastic strain during a stress relaxation period was found to be higher in ‘soft’ grains over ‘hard’ grains, which results from the higher strain rate sensitivity of these ‘soft’ grains. The prism slip parameters correspond to a stronger strain rate sensitivity compared to basal slip. This slip system dependence on strain rate has a significant effect on stress redistribution to ‘hard’ grain orientations during cold dwell fatigue.
Oxygen was found to enhance the cold dwell effect, as plastic accumulation and strain rate sensitivity were both found to be higher in the higher oxygen content CP-Ti grade 4 samples. Among the four temperature assessed, 75C was found to be the worst-case scenario, where the macroscopic plastic strain accumulation was significant during a relaxation cycle due to the greatest activity of both prism and basal slip systems.
Digital image correlation (DIC) and crystal plasticity simulation were utilised to study cold dwell behaviour in a coarse grain Ti-6Al alloy at 3 different temperatures up to 230C. Strains extracted from large volume grains were measured during creep by DIC and were used to calibrate the crystal plasticity model. Stress along paths across the boundaries of two grain pairs, (1) a ‘rogue’ grain pair and (2) a ‘non-rogue’ grain pair, were determined at different temperatures. Load shedding was observed in the ‘rogue’ grain pair, where a stress increment during the creep period was found in the ‘hard’ grain. At elevated temperatures, 120C was found to be the worst-case scenario as the stress difference at the grain boundaries of these two grain pairs were found to be the largest among the three temperatures. Local stress state plays a more important role in the activation of slip systems compared to grain orientations with respect to macroscopic load direction in large polycrystals. As a result, the ‘soft’ and the ‘hard’ grain would dynamically vary depending on the applied stress and operating temperature
The influence of surface oxides on the mechanical response of oxidized grain boundaries
No full textThe mechanical response of oxidized grain boundaries in a nickel alloy used for nuclear applications has been investigated by performing microcantilever bend tests. It was found that whilst failure can proceed along the oxide-metal interface not all oxidized grain boundaries exhibit intergranular failure. The presence of an external surface oxide has been identified as playing a crucial role in influencing the mechanical response. By removing the surface oxide, using a focused ion beam, tests were performed on the same grain boundaries with and without a surface oxide layer, and showed that surface oxides can suppress or delay fracture. Taking into account the effect of the surface oxide on microcantilever tests, it was possible to present the most accurate parameterization of the local stress at failure of oxidized grain boundaries to date and to predict the experimentally observed behavior via realistic cohesive damage finite element simulations, which further underline the experimental observations
Load measurement and analysis of kick scooters
Get PDFKick scooters are rapidly growing in popularity as a form of transport and are used recreationally. They are also seen as an effective way to combat inner-city pollution. This project is (to the author’s knowledge) the first published work that aims to investigate the loads applied to scooters in real-world use. A data logging system has been built that is able to handle the vibrations associated with stunt scooter riding. The system can measure up to eight channels of strain data and log data up to a frequency of 62.5 kHz. A static force reconstruction model has been made for the scooter to gauge the magnitude of the applied loading to the scooter. Strain data is measured in the deck and the down tube of the scooter. Experimental results match well with finite element simulations. In addition, another force reconstruction model is made that considers dynamic effects. Results for measured strain and reconstructed forces for more than 25 of the following 12 different scooter stunts are given: bunnyhops, j-hops, tailwhips, heelwhips, barspins, opposite barspins, backside 180s, backside half-cabs, frontside 180s, frontside half-cabs, backside 360s and frontside 360s. Results show a significant variation in measured strains both within and between different scooter stunts. Regular barspins gave rise to the highest mean peak strains in the deck and down tube. Some of the peak strains do not appear to follow a normal distribution, as they fail the Shapiro-Wilk test for normality. Finally, a brief sketch is given of how the information could be used to optimise scooter components using topology optimisation. Results from a preliminary topology optimisation study are not successful in reducing the peak stresses in the neck of the scooter deck, highlighting the care needed in optimal scooter design
High resolution characterisation of H on stress corrosion cracking in nuclear materials
Get PDFAustenitic alloys are commonly used in the nuclear industry due to
their high corrosion resistance and excellent mechanical properties. De-
spite boasting an impressive service history, it is possible for critical
structural components made from these alloys to undergo stress cor-
rosion cracking (SCC) under pressurised water reactors (PWR) condi-
tions. As the primary method of material degradation, SCC has been
studied extensively for several decades, and many factors have been
shown to affect SCC susceptibility, these include cold-work, tempera-
ture and water chemistry. To develop a better mechanistic understand
of SCC further research is required. This body of research evaluates the
effect of hydrogen on mechanical deformation in SCC. Localised and
high-resolution testing has been employed successfully to characterise
key processes in SCC crack propagation. For this reason, nanoindenta-
tion, transmission Kikuchi diffraction (TKD), NanoSIMS and Thermal
desorption spectrscopy (TDS) have been used to characterise material
deformation on a length-scale relevant for SCC.
The main objective of this thesis was to correlate the SCC crack growth
rate’s (CGR) dependency on nickel content, first observed by Coriou,
with both the localised mechanical deformation and hydrogen up-take
at PWR temperatures. To satisfy this objective three alloys have been
systematically tested in every chapter, these include SS316L (12%-
Ni), A800 (32%-Ni) and A600 (72%-Ni) as they represent commercially
available alloys with low, intermediate and high nickel contents.
Room temperature micromechanical testing has been used to compare
the mechanical properties of austenitic alloys on a length-scale that is
relevant to SCC. The mechanical properties of all austenitic alloys are
highly comparable at room temperature, and show no deviation with
changing nickel content. High-magnification cross-sectional analysis
shows that the deformation mechanisms in A800 differ from SS316L
and A600 at room temperature. A800 does not undergo Σ3 defor-
mation twinning, but instead deforms by dislocation glide. This is
unexpected as the stacking fault energy of A800 is an intermediate of
the other alloys. Deformation is a main SCC mechanism, and the ob-
served alloy deformation correlates with Arioka’s reported SCC CGR
in PWRs, i.e the slowest CGR undergoes no twinning. As the me-
chanical properties of alloys with a 25-40% nickel content do not differ
significantly from other austenitic alloys, it is thought that their corro-
sion resistance is weighted towards diffusion-based SCC mechanisms.
Such an explanation is consistent with findings at hotter temperatures,
where diffusion-based mechanisms are more active. High temperature
micromechanical testing supports these observations, and can be used
to better understand the individual mechanism weights depending on
alloy composition and temperature.
NanoSIMS and TDS were used to quantitatively and qualitatively ex-
plore the hydrogen trapping in austenitic alloys exposed to pure heavy
water at PWR temperatures. Hydrogen is found trapped by various
microstructural features, such as carbides, grain boundaries and ox-
ides by NanoSIMS. The total hydrogen content of SS316L and A800
is five-times greater than that of A600, which correlates with the en-
thalpy of diffusion in the alloys. These results show an inverse trend
with the SCC failure reported by Coriou, and a weaker inverse correla-
tion to Arioka’s results. The weaker correlation with Arioka’s results is
due to a change in the corrosive medium from pure water, used in this
study and by Coriou, to simulated primary PWR water. It is suggested
that the inverse relationship could be due to the elastic shielding pro-
vided by the hydrogen enhancing dislocation motion, and preventing
the stress intensity factor reaching a critical values that will initiate
crack propagation
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Electron microscopy and multi-scale modelling of radiation damage recovery in tungsten
Get PDFThe thesis is concerned with understanding the recovery mechanisms of radiation damage in tungsten. Tungsten is a key material in next-generation fusion reactors such as ITER, which will be exposed to intense neutron radiation, plasma and high temperatures, and its durability will determine the reactor’s competitiveness versus other energy sources. The thesis makes a comprehensive, quantitative study of recovery and its depen- dence on temperature, time, and material purity. It employs both experiments (ion-beam irradiation with ex and in situ analysis via transmission electron mi- croscopy) and multi-scale deterministic and stochastic modelling techniques such as molecular dynamics and dislocation dynamics. Annealing experiments on ultra-pure tungsten reveal the presence of several re- covery stages, and an acceleration of dislocation defect recovery and loop coarsening above ∼900°C. They also reveal discrepancies with dislocation climb models in the literature, suggesting the presence of additional recovery pathways leading to coars- ening, most notably conservative self-climb. Presence of helium is found to cause an increase in the equilibrium concentration of defects at a given temperature. Also, very high doses cause morphological changes to the radiation damage structure, from relatively homogeneous defect concentra- tions of dislocations in a narrow size range, to complex dislocations networks with voids. Finally, modelling techniques are able to successfully predict dislocation motion phenomena observed in experiments at high temperature (including quantitatively accurate defect densities), but only after extensive modification of publicly available dislocation dynamics codes to include boundary conditions and stochastic behaviour of crystal defects
Micromechanical testing of oxidized grain boundaries
Get PDFPrimary water stress corrosion cracking (SCC) of metals in pressurized water reactors (PWRs) is known to be one of the most challenging and cost intensive modes of failure in the nuclear industry. Even though it is known that cracking in Ni-base alloys proceeds mainly intergranular (IG), the initiation and propagation of cracks in ductile metals are not yet understood and a much-desired accurate prediction of SCC related failure seems unobtainable.
In this thesis, a combination of microcantilever fracture experiments, scanning electron- (SEM) and transmission electron microscopy (TEM) techniques was employed to study and compare the failure of oxidized grain boundaries of Ni-base Alloy 600 with high and low intergranular carbide coverage and different sample history.
A new technique for lifting-out whole cantilevers after testing and for performing 3D focussed ion beam sequencing (3D FIB-SEM) while preserving a thin central region of the cantilever for further TEM sample preparation was developed and is presented.
In lieu with recent efforts of the main project sponsor Électricité de France (EDF) to build a predictive model for IGSCC based on localized/microscopic information, one of the main objectives was the extraction of the stress at failure of individual oxidized GBs. Supported by finite element simulations, microcantilever fracture tests revealed that surface oxides on top of individual GBs have the capability to alter the mechanical response by delaying/suppressing the onset of failure. An overestimation of the failure stress (> 230 MPa) was observed, proving that the presence of the surface oxide on top of the test structures cannot be neglected. The failure stress on both samples, tested without influence of the surface oxide, was found to cover a range of 300 â 600 MPa, which agreed well with finite element simulations of the tests and further demonstrates the reliability of the obtained data.
The second objective was to gain a better understanding of the observed fracture behaviour and the role of local microstructure. Using the gathered microscopy data, it was found that the crack clearly favours a progression along the IG oxide-metal interface in the presence of carbide precipitates. Electron energy loss spectroscopy (EELS) revealed that the observed crack path can be linked to compositional and density variations of the IG oxide. In the presence of carbides the oxide was layered. An oxide close to the stoichiometry of chromia was located at the original GB and next to the carbides. Next to this Cr-rich oxide, Fe-rich mixed spinel oxides of varying composition and density were found. An explanation for density variations based on the possible formation of defective spinel oxides of the type A2+B3+2O4, due to an unavailability of certain cation species is presented. No clear interface preference was observed in the absence of precipitation, where the IG oxide was found to be thin and often incomplete with Cr-richer oxides preferentially located at the original GB. While these observations were consistent on both samples (high and low carbide coverage), bigger void-like defects were located at the Fe-richer oxide-metal interface of the cold worked sample with high IG carbide precipitation only. These weak spots seemed to be the preferred path for crack propagation on this sample. The sample with low intergranular carbide coverage showed no obvious porosities at this interface but a Cr- depleted region was seen.
Introducing a multi-faceted investigation strategy, supported by finite element simulations, the presented thesis provides the most accurate determination of the failure stress of oxidized GBs on Alloy 600 to date and and adds new valuable insights to our understanding of IGSCC and the future prediction of SCC related failures.</p
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