490 research outputs found
Weldability of duplex stainless steel
No full textDuplex stainless steels (DSSs) have many advantages due to the unique structural combination of ferrite and austenite grains. The structural change of these materials is very complex during welding, and it deteriorates the functional properties. This research investigates different welding processes such as laser beam, resistance, tungsten inert gas, friction stir, submerged arc, and plasma arc weldings considering the research available in the literature. The welding mechanism, change of material structure, and control parameters have been analyzed for every welding process. This analysis clearly shows that DSS melts in all most all welding processes, but the thermal cycle and maximum heat input are different. This difference affects the resulting structure and functional properties of the weld significantly.A. Pramanik, G. Littlefair, and A. K. Basa
Stress in the interfaces of metal matrix composites (MMCs) in thermal and tensile loading
No full textThe influence of contents, sizes and shapes of reinforcements on the matrix-particle interfaces in terms of (a) von-Mises stress, (b) directions as well as spreading of principal stresses during cooling and under tension were analysed in this investigation through numerical simulation for SiC particle reinforced Al6061 matrix MMCs. The result shows that the rapid variation of von-Mises stress in the interfaces depends on the shape of reinforced particles. The interfaces in MMCs with triangular reinforcements experienced the slimiest stress variation. Nevertheless, the uppermost stress was intensified in the corners of the triangle shaped reinforcements. The shape of the reinforcements affects the stress vectors directions. The range of the von-Mises stress rises through the rise of particle amount and decrease of particles size at a constant number of particles. For nearly all the cases, excluding the MMCs with triangle reinforcements, when the stress rises, with the rise of the applied positive elongation, the stress scatterings are unaffected.A. Pramanik, A. K. Basak, G. Littlefair, A. R. Dixit, S. Chattopadhyay
Prediction of interior daylight under clear sky conditions.
No full textMost available techniques for predicting internal daylight illuminance do not take into
account reflected sunlight, nor the fact that under clear sky conditions the direction of
the illuminance is usually upwards, not downwards from the sky.
The general goal of this study is to investigate the issue of predicting the internal
illuminance from natural light in clear sky conditions. This includes the possibility of
proposing a method based on the concept of the average daylight factor for use in
sunny climates.
This thesis is divided into eight chapters. After introducing the problem in Chapter
One, Chapter Two is a literature review of problems associated with utilising natural
light in sunny regions.
Chapter Three is the statement of the problem and how it will be solved.
Existing equations for finding the average daylight factor have one thing in common:
they assume that the incident light on the window comes directly from an overcast
sky or by external reflection from it. If any of these equations are to be used under
clear sky conditions, or a new method is to be developed based on the same
concepts, the sensitivity of average internal illuminance to the direction of external
light needs to be tested. A study of this is described in Chapter Four.
Chapter Five tests, by numerical simulation, the performance of existing average
daylight factor methods under clear sky conditions. It is concluded that they are not
appropriate for sunny regions. The tests, and the conclusions from Chapter Four,
do, however, suggest a new approach. This has two bases. The first is that it has
been shown to be possible to relate incident light on the window plane to horizontal
sky illuminance, and this sets a minimum condition for window design. In practice
this can be used in conjunction with a limiting maximum window size based on heat
gain and other environmental issues. The second basis is a new formula for relating
average internal illuminance to external window plane illuminance. The overall result
is a formula for predicting internal illuminance as a ratio of external horizontal sky
iluminance.
In Chapter Seven this approach is tested. Two methods are used: field measurements
under real sky conditions, and comparison with detailed calculations
Optimizing dimensional accuracy of titanium alloy features produced by wire electrical discharge machining
No full textThis study investigates geometrical errors such as cylindricity, circularity and diametral errors of a feature (a hole) produced from wire electrical discharge machining of Ti6Al4V alloy where tension in wire, pulse on time, and flushing pressure are varied. Pareto analysis of variance (ANOVA), Taguchi design of experiment (DoE), and traditional analysis estimate the influence of variables on errors of holes. It was noted that flushing pressure is the most significant factor with individual contributions of 31.02%, 49.5% and 37.84% to circularity, cylindricity, and diametral errors, respectively. The circularity error of holes decreases as the flushing pressure and tension in wire rise, but decreases with the rise of pulse on time. The cylindricity error decreased with the increase of wire tension, flushing pressure and pulse on time. The absolute diametral error reduced as the pulse on time and tension in wire raised, but it raised with the rise of pulse on time. All these trends are associated with the influence of tension in wire on the flexibility of wire, the dependence of heat generation and dissipation on pulse on time, and ability of the flushing pressure to control the cooling, as well as debris removal from the machining zone.A. Pramanik, M.N. Islam, A.K. Basak, Y. Dong, G. Littlefair, and C. Prakas
A magnetic white dwarf in a detached eclipsing binary
No full textSDSS J030308.35+005444.1 is a close, detached, eclipsing white dwarf plus M dwarf binary which shows a large infrared excess which has been interpreted in terms of a circumbinary dust disc. In this paper, we present optical and near-infrared photometric and spectroscopic data for this system. At optical wavelengths, we observe heated pole caps from the white dwarf caused by accretion of wind material from the main-sequence star on to the white dwarf. At near-infrared wavelengths, we see the eclipse of two poles on the surface of the white dwarf by the main-sequence star indicating that the white dwarf is magnetic. Our spectroscopic observations reveal Zeeman-split emission lines in the hydrogen Balmer series, which we use to measure the magnetic field strength as 8 MG. This measurement indicates that the cyclotron lines are located in the infrared, naturally explaining the infrared excess without the need for a circumbinary dust disc. We also detect magnetically confined material located roughly midway between the two stars. Using measurements of the radial velocity amplitude and rotational broadening of the M star, we constrain the physical parameters of the system, a first for a magnetic white dwarf, and the location of the poles on the surface of the white dwarf. SDSS J030308.35+005444.1 is a pre-cataclysmic variable that will likely evolve into an intermediate polar in ∼1 Gyr
Debate Activity as an Effective Interactive Learning Approach for Civil Engineering Students
No full textThe research team has attempted to improve the learning experience of Civil Engineering students by introducing debate activities in engineering subjects. This could encourage the students to be active learners (against passive learner or lecture receivers) and we consider this process as an activity (what students actually do) and not a task (the work prescribed by the teacher). In this project, a main stream civil engineering subject, namely Soil Behaviour has been targeted. Feedback received from the students has indicated that the developed in-class debate activity has been effective in improving communication and critical-thinking skills of students. Availability of new collaborative theatres and collaborative learning spaces could add great advantages to enhancing the efficiency of collaborative group work, in terms of planning and preparation for the debates. In addition, available online discussion boards and online subject website would provide flexibility for students to access digital resources and prepare their arguments. The professional training sessions, on the other hand, could equip participants with important tips to improve the argument presentation skills while were trained to overcome the panic and fear of making mistakes during the speech. International students, whose English is not their first language, were particularly encouraged to attend the training sessions helping them to obtain profound presentation skills not only for Soil Behaviour debate but also for their future career
Pulsating hot O subdwarfs in ω Centauri: mapping a unique instability strip on the extreme horizontal branch
No full textpeer reviewedWe present the results of an extensive survey for rapid pulsators among Extreme Horizontal Branch (EHB) stars in ω Cen. The observations performed consist of nearly 100 h of time-series photometry for several off-centre fields of the cluster, as well as low-resolution spectroscopy for a partially overlapping sample. We obtained photometry for some 300 EHB stars, for around half of which we are able to recover light curves of sufficient quality to either detect or place meaningful non-detection limits for rapid pulsations. Based on the spectroscopy, we derive reliable values of log g, T[SUB]eff[/SUB] and log N(He) /N(H) for 38 targets, as well as good estimates of the effective temperature for another nine targets, whose spectra are slightly polluted by a close neighbour in the image. The survey uncovered a total of five rapid variables with multi-periodic oscillations between 85 and 125 s. Spectroscopically, they form a homogeneous group of hydrogen-rich subdwarf O stars clustered between 48 000 and 54 000 K. For each of the variables we are able to measure between two and three significant pulsations believed to constitute independent harmonic oscillations. However, the interpretation of the Fourier spectra is not straightforward due to significant fine structure attributed to strong amplitude variations. In addition to the rapid variables, we found an EHB star with an apparently periodic luminosity variation of ~2700 s, which we tentatively suggest may be caused by ellipsoidal variations in a close binary. Using the overlapping photometry and spectroscopy sample we are able to map an empirical ω Cen instability strip in log g - T[SUB]eff[/SUB] space. This can be directly compared to the pulsation driving predicted from the Montréal "second-generation" models regularly used to interpret the pulsations in hot B subdwarfs. Extending the parameter range of these models to higher temperatures, we find that the region where p-mode excitation occurs is in fact bifurcated, and the well-known instability strip between 29 000-36 000 K where the rapid subdwarf B pulsators are found is complemented by a second one above 50 000 K in the models. While significant challenges remain at the quantitative level, we believe that the same κ-mechanism that drives the pulsations in hot B subdwarfs is also responsible for the excitation of the rapid oscillations observed in the ω Cen variables. Intriguingly, the ω Cen variables appear to form a unique class. No direct counterparts have so far been found either in the Galactic field, nor in other globular clusters, despite dedicated searches. Conversely, our survey revealed no ω Cen representatives of the rapidly pulsating hot B subdwarfs found among the field population, though their presence cannot be excluded from the limited sample. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile (proposal IDs 083.D-0833, 386.D-0669, 087.D-0216 and 091.D-0791).The reduced spectra are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/589/A1</A
Cataclysmic variables below the period gap : mass determinations of 14 eclipsing systems
No full textWe present high-speed, three-colour photometry of the eclipsing cataclysmic variables CTCV J1300-3052, CTCV J2354-4700 and SDSS J115207.00+404947.8. These
systems have orbital periods of 128.07, 94.39 and 97.52 minutes respectively, placing
all three systems below the observed “period gap” for cataclysmic variables. For each
system we determine the system parameters by fitting a parameterised model to the
observed eclipse light curve by χ2 minimisation.
We also present an updated analysis of all other eclipsing systems previously
analysed by our group. The updated analysis utilises Markov Chain Monte Carlo
techniques which enable us to arrive confidently at the best fits for each system with
more robust determinations of our errors. A new bright spot model is also adopted, that
allows better modelling of bright-spot dominated systems. In addition, we correct a
bug in the old code which resulted in the white dwarf radius being underestimated, and
consequently both the white dwarf and donor mass being overestimated. New donor
masses are generally between 1 and 2σ of those originally published, with the exception
of SDSS 1502 (−2.9σ, Mr = −0.012M⊙) and DV UMa (+6.1σ, Mr = +0.039M⊙).
We note that the donor mass of SDSS 1501 has been revised upwards by 0.024M⊙
(+1.9σ). This system was previously identified as having evolved passed the minimum
orbital period for cataclysmic variables, but the new mass determination suggests
otherwise. Our new analysis confirms that SDSS 1035 and SDSS 1433 have evolved
past the period minimum for cataclysmic variables, corroborating our earlier studies.
We find that the radii of donor stars are oversized when compared to theoretical
models, by approximately 10 percent. We show that this can be explained by invoking
either enhanced angular momentum loss, or by taking into account the effects of star
spots. We are unable to favour one cause over the other, as we lack enough precise
mass determinations for systems with orbital periods between 100 and 130 minutes,
where evolutionary tracks begin to diverge significantly.
We also find a strong tendency towards high white dwarf masses within our sample,
and no evidence for any He-core white dwarfs. The dominance of high mass white
dwarfs implies that erosion of the white dwarf during the nova outburst must be
negligible, or that not all of the mass accreted is ejected during nova cycles, resulting
in the white dwarf growing in mass
The shocking transit of WASP-12b : modelling the observed early ingress in the near ultraviolet
No full text4 pages, 2 figuresNear ultraviolet observations of WASP-12b have revealed an early ingress compared to the optical transit lightcurve. This has been interpreted as due to the presence of a magnetospheric bow shock which forms when the relative velocity of the planetary and stellar material is supersonic. We aim to reproduce this observed early ingress by modelling the stellar wind (or coronal plasma) in order to derive the speed and density of the material at the planetary orbital radius. From this we determine the orientation of the shock and the density of compressed plasma behind it. With this model for the density structure surrounding the planet we perform Monte Carlo radiation transfer simulations of the near UV transits of WASP-12b with and without a bow shock. We find that we can reproduce the transit lightcurves with a wide range of plasma temperatures, shock geometries and optical depths. Our results support the hypothesis that a bow shock could explain the observed early ingress.Peer reviewe
Chip formation characteristics of selective laser melted Ti–6Al–4V
No full textIn this research work, chip formation characteristics of selective laser-melted (SLM) Ti–6Al–4V in both ‘as built’ and ‘heat treated’ conditions are studied and compared with conventional wrought Ti–6Al–4V. Machined chips and partially deformed chips were collected from turning trials and quick stop experiments to study the nature of chip formation characteristics. Chip formation studies reveal that, ‘segmented’ or ‘saw tooth’ chips were produced during machining of SLM Ti–6Al–4V materials. The tendency to form segmented chips was higher in SLM Ti–6Al–4V materials as compared to conventionally produced wrought Ti–6Al–4V. In addition, cracks were found to be a common feature in primary and secondary deformation zones of SLM Ti–6Al–4V chip samples, illustrating that periodic crack initiation is the root cause of ‘saw tooth’ formation during machining. Furthermore, the tendency to form build up edge during machining was less in SLM Ti–6Al–4V materials compared to wrought Ti–6Al–4V, influencing the machined surface finish.No Full Tex
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