1,720,971 research outputs found
Macro- and micro-mechanical perspectives on creep-fatigue interaction in Type 316L stainless steel
No full textCreep-fatigue of Type 316L stainless steel under asymmetric waveforms (specifically slow tension-fast compression, S-F, and fast tension-slow compression, F-S) has been understudied, despite its significant implications as demonstrated in this work. This study bridges macro- and micro-mechanical perspectives through a combined approach, involving high-temperature testing, scanning electron microscopy, X-ray computed tomography, neutron diffraction, and crystal plasticity modelling. Macro-mechanical tests revealed distinct deformation behaviours under S-F and F-S waveforms with and without a 1-hour tensile dwell at 550 °C, with S-F reducing lifespan in both fatigue and creep-fatigue conditions. Post-mortem analyses revealed distinct fracture morphologies induced by tensile dwell, with creep-fatigue S-F specimen exhibiting more pronounced intergranular-dominant fracture and higher internal defect volume. It also exhibited the highest number fraction of medium-sized (10–40 μm) microcracks, which correlates with its shortest fatigue life and more creep damage accumulation. Higher grain-level deformation incompatibility was observed during tensile dwell in the S-F load waveform. Crystal plasticity modelling revealed that the higher tensile stress amplitudes during S-F loading stem from increased dislocation density, with average densities at peak tensile strain during the saturation cycle reaching 186 μm⁻² for S-F and 147 μm⁻² for F-S waveforms. These findings establish a strong link between macroscopic and microscopic behaviours under asymmetric loading, emphasising the potential of S-F waveforms for cost-effective creep-fatigue experiment design. Furthermore, for the asymmetric waveforms studied, creep-fatigue life assessment using the ductility exhaustion method demonstrates greater accuracy than those based on the time fraction method
Characterising and modelling fracture in functional pet foods
Get PDFThe underlying basis of this research is that food oral breakdown heavily depends on the mechanical properties of the material being masticated. So far, researchers have been measuring mechanical properties to establish sensory panels which are useful towards designing food. Although this methodology has proven industrially popular, it displays weakness in addressing complex product development and optimisation tasks. Specifically, mastication involves varying parameters across consumers, such as speed, teeth geometry, bite force, jaw motion, as well as friction. Therefore, this study considers more advanced techniques to help predict how the food will perform during oral processing. A Finite Element (FE) model was developed, where the first bite on a starch based food was simulated, based on digital pet teeth geometry. The food material model was constructed from uniaxial compression and tension data as well as fracture toughness values through the essential work of fracture (EWF) and cutting tests. The main focus is on the development of suitable material constitutive laws including damage, in order to predict deformations and crack patterns in the food item, as experienced during chewing. Emphasis is also given to determining the true material fracture toughness by assessing the validity of the EWF and cutting methods on highly dissipative materials, which leads to the development of a new, more accurate and convenient test method. The composition effect on the mechanical response was also studied by comparing four starch based recipes. The FE jaw force versus displacement results match the experimental data obtained by a physical replicate of the bite model, lending weight to the approach as a powerful tool in understanding food breakdown and in establishing relations between the mechanical properties and sensory attributes. The study also reflects industry needs for time and cost efficient techniques towards product development and optimisation.Open Acces
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
Ratchetting and creep failure in twin-wall turbine blades experiencing severe thermal and centrifugal loading
Get PDFTwin-wall structures can be cooled both externally and internally, raising great potential for use in high-temperature applications. However, their increased geometric complexity imposes a range of potential failure mechanisms for consideration in design. The primary aim of this study is to identify the nature of such mechanisms by constructing Bree type interaction diagrams for idealized double-wall systems under cyclic thermomechanical loading that shows the combination of loading conditions for which cyclic plasticity (leading to fatigue failure)-creep ratchetting occur. Through an extension of the classical Bree analysis, we determine analytical boundaries between different regimes of behavior. We also quantify the effects of wall thickness ratio, temperature field, and yield and creep material properties. Local cyclic plasticity is shown to dominate over structural/global ratchetting when the yield strength reduces with temperature and/or when the temperature gradient through the hot wall thickness dominates over the temperature difference between the walls. Thus, we conclude that global ratchetting is unlikely to occur in the practical loading range of Nickel-based twin-wall turbine blades, but instead these systems suffer from local fatigue at cooling holes and excessive creep deformation. This is verified by 3D cyclic finite element (FE) simulations, demonstrating that the analytical approach provides a powerful, cost-effective strategy for providing physical insight into possible deformation mechanisms in a range of thin-walled components; highlighting the key trade-offs to be considered in design; and directing the use of computer methods toward more detailed calculations
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
Transient thermo-mechanical response and failure in selected turbine cooling geometries
No full textDecarbonising the aviation industry is imperative, as CO2 accumulates in our atmosphere. Reducing the specific fuel consumption of gas turbine engines is achievable by increasing the turbine entry temperature (TET). A reduction in TET promises reduced emissions and lower costs to operators and consumers. Increasing engine temperatures requires more efficient cooling systems to ensure adequate safety and component life. A double-walled effusion-cooled system promises improved performance. It harmonises several existing technologies to produce favourable aerothermal performance. A growing wealth of literature is focused on the mechanical performance of such a system. Prior analytical and computational modelling of the thermal and stress fields requires confirmation and calibration of experimental results. This work aims to establish a new standardised experimental methodology to assess the aerothermal and mechanical performance of these new systems.
An engine-representative flat-plate test section, or coupon, was designed and modelled alongside a test rig, the Effusion Life and Stress Addition (ELSA). The coupon was designed to promote failure at the film hole wedge, the region created near the film holes’ intersection with the hot-walls’ exterior. This was achieved by removing pedestals from the non-test region and reducing kinematic constraints and the local stress field. Further stress mitigation was achieved by scaling the outermost pedestals and associated fillets, which was shown to reduce local stress by 41%. ELSA employed a combination of vertical-cavity surface-emitting lasers and a compound parabolic concentrator to supply engine-representative heat fluxes of up to 7.4 W/mm2 , and laboratory-temperature cooling air of up to 200 standard litres per minute to achieve engine-representative thermal gradients. Coupons obtained a maximum measured temperature of 1050 ◦C with thermal differences of up to 365 ◦C. The surrounding rig remained below 75 ◦C at all measured locations. The control settings did not exceed 50% of the available laser power, indicating that higher temperatures and thermal ramp rates are possible.
Engine-representative metal temperatures and distributions were evaluated for Inconel 625 and CMSX-4 coupons. The predicted crack initiation location for circular holes was confirmed at film hole wedges for most components. The results showed a negligible dependence on the thermal gradient, normal to the coupon, in determining crack initiation. Instead, a dependence on the peak temperature was displayed. Experiments with varying normal thermal differences for the same maximum temperature show little to no effect on the number of cycles to crack initiation. An increased maximum temperature with an identical thermal gradient led to rapid buckling failure in less than 30 cycles. Furthermore, elliptical holes outperformed circular holes for identical internal geometries. This was with respect to both aerothermal and thermo-mechanical performance. In the ELSA test facility, elliptical holes with identical cross-sectional areas required an increase in laser power and a decrease in coolant to achieve identical thermal distributions. Elliptical holes endured three times more cycles than circular holes before developing cracks. Finally, the single-crystal CMSX-4 coupon was shown to survive eight times longer than a 3D-printed Inconel 625 component for an identical geometry and thermal field.
A numerical model was developed to extract the specific elastic stress state from a transient simulation of the thermal cycling profiles. It estimated an elastic nominal stress state, membrane stresses far from features, as a boundary-value problem. The boundaries were approximated on the basis of known thermal data of the quasi-steady hold region of the testing. The results of the finite elements showed reasonable agreement with the experimental values. A sub-domain approach was used to model the stresses at a critical point: the film-hole exterior wedge. Elasto-plastic models were used to estimate the total strain range. The results were compared to an established empirical Manson-Coffin relation to estimate the number of cycles to crack initiation. All models underestimated the fatigue life of CMSX-4 relative to the observed experimental values. An alternate estimate for the fatigue life, based on crystal plasticity, was then applied. The new estimated fatigue lives straddled the experimental results. A Molski-Glinka model offered a conservative estimate, while a constant-strain approach overestimated fatigue life
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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