An Investigation, Using Standard Experimental Techniques, to Determine FLCs at Elevated Temperature for Aluminium Alloys

An experimental procedure has been developed for the determination of FLCs at elevated temperatures. The GOM ARGUS system was employed for measuring surface strain based on pre-applied grids (pattern), and limit strains were determined according to the ISO 12004-2:2008 standard. Forming limit curves (FLCs) have been determined for AA5754 under warm forming conditions in an isothermal environment. The tests were carried out at various temperatures up to 300oC and forming speeds ranging from 5 – 300 mm s-1 . Results reveal the significant effect of both temperature and forming speed on FLCs of AA5754. Formability increases with increasing temperature above 200oC. Formability also increases with decreasing speed. The presented FLC results show that the best formability exists at low forming speed and the high temperature end of the warm forming range

Towards a simplified description of thermoelectric materials: Accuracy of approximate density functional theory for phonon dispersions

We calculate the phonon-dispersion relations of several two-dimensional materials and diamond using the density-functional based tight-binding approach (DFTB). Our goal is to verify if this numerically efficient method provides sufficiently accurate phonon frequencies and group velocities to compute reliable thermoelectric properties. To this end, the results are compared to available DFT results and experimental data. To quantify the accuracy for a given band, a descriptor is introduced that summarizes contributions to the lattice conductivity that are available already in the harmonic approximation. We find that the DFTB predictions depend strongly on the employed repulsive pair-potentials, which are an important prerequisite of this method. For carbon-based materials, accurate pair-potentials are identified and lead to errors of the descriptor that are of the same order as differences between different local and semi-local DFT approaches

Cluster growth in the dynamical Erdös-Rényi process with forest fires

We investigate the growth of clusters within the forest fire model of Ráth and Tóth [EJP, vol 14, paper no 45]. The model is a continuous-time Markov process, similar to the dynamical Erdős-Rényi random graph but with the addition of so-called fires. A vertex may catch fire at any moment and, when it does so, causes all edges within its connected cluster to burn, meaning that they instantaneously disappear. Each burned edge may later reappear. We give a precise description of the process CtCt of the size of the cluster of a tagged vertex, in the limit as the number of vertices in the model tends to infinity. We show that CtCt is an explosive branching process with a time-inhomogeneous offspring distribution and instantaneous return to 1 on each explosion. Additionally, we show that the characteristic curves used to analyse the Smoluchowski-type coagulation equations associated to the model have a probabilistic interpretation in terms of the process CtCt

Project Assessment Framework through Design (PAFTD) - A Project Assessment Framework in Support of Strategic Decision Making

Research and development organizations that push the innovation edge of technology frequently encounter challenges when attempting to identify an investment strategy and to accurately forecast the cost and schedule performance of selected projects. Fast moving and complex environments require managers to quickly analyze and diagnose the value of returns on investment versus allocated resources. Our Project Assessment Framework through Design (PAFTD) tool facilitates decision making for NASA senior leadership to enable more strategic and consistent technology development investment analysis, beginning at implementation and continuing through the project life cycle. The framework takes an integrated approach by leveraging design principles of useability, feasibility, and viability and aligns them with methods employed by NASA's Independent Program Assessment Office for project performance assessment. The need exists to periodically revisit the justification and prioritization of technology development investments as changes occur over project life cycles. The framework informs management rapidly and comprehensively about diagnosed internal and external root causes of project performance

New synthetic strategies for xanthene-dye-appended cyclodextrins

Xanthene dyes can be appended to cyclodextrins via an ester or amide bridge in order to switch the fluorescence on or off. This is made possible through the formation of nonfluorescent lactones or lactams as the fluorophore can reversibly cyclize. In this context we report a green approach for the synthesis of switchable xanthene-dye-appended cyclodextrins based on the coupling agent 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM). By using 6-monoamino-β-cyclodextrin and commercially available inexpensive dyes, we prepared rhodamine- and fluorescein-appended cyclodextrins. The compounds were characterized by NMR and IR spectroscopy and MS spectrometry, their UV–vis spectra were recorded at various pH, and their purity was determined by capillary electrophoresis. Two potential models for the supramolecular assembly of the xanthene-dye-appended cyclodextrins were developed based on the set of data collected by the extensive NMR characterization

THE ROLE PLAYED BY DYNAMIC GAMES IN THE SPATIAL-TEMPORAL ORIENTATION IN SPECIAL SCHOOL CHILDREN

Dynamic games can be used both as means of relaxation and as an education and correction method. Spatial-temporal orientation is very well exploited during these games, due to their functions, such as: organizational, adaptive, formative, self-discovery, socialization. This paper aimed to spot the spatial-temporal orientation disorders in special school children and to emphasize the benefits of dynamic games in educating the spatial-temporal orientation

From micro-cantilever testing to deformation patterning in HCP polycrystals

For several years now we have been using micro-scale cantilever bend tests to probe the considerable anisotropy of elastic and plastic deformation behaviour in the hexagonal packed metals Ti and Zr [1-3]. The wider aim of the work has been understanding and modeling the heterogeneous patterns of stress, strain and dislocation density that develop during deformation of HCP polycrystals. Crystal plasticity finite element analysis (CP-FEA) of representative volumes are used to simulate these deformation fields and enable modelling of representative volume elements to aid understanding of in-service component performance. Critical resolved shear stress (CRSS) values for the important slip systems are required inputs for the constitutive laws and populating these has been the aim of our micro-cantilever studies. We follow a well-established route of using a focused ion beam (FIB) to machine micro-cantilevers of triangular cross-section into the sample surface [1]. EBSD is used to identify grains in which cantilevers with suitable orientation can be cut so that the targeted slip systems can be activated individually. The samples are then passed to a nano-indenter with a nano-positioning stage and loaded, with the load point accurately located at the free end of the cantilever using an AFM-like scan with low contact force. Load-displacement data generated from the experiment are compared to CP-FEA simulations of the cantilever bending and the CRSS for each cantilever is varied until a good fit is achieved [1]. The CRSS data show a significant size effect, where smaller cantilevers are apparently stronger. This is very obvious at cantilever widths below ~5 µm but also persists to larger sizes. The size effect is found to be well represented by where is the effective CRSS measured for a cantilever of width , is the CRSS for bulk samples and is a constant representing the strength of the size effect [2]. During bending strains are largest near the built-in end at the top (tensile) and bottom (compressive) regions (twice as large at the bottom due to the orientation of the triangular section). Dislocations tend to be generated in these regions and propagate progressively in towards the neutral axis of the beam where they pile-up. The back-stress from these pile-ups acts against further dislocations being generated and moving to join the pile-up. This effect is seen in discrete dislocation plasticity simulations that inherently capture the size effect [3], but are not present in the length scale independent CP-FEA simulations, where the size effect is manifested instead as an apparent increase in CRSS for smaller cantilever width. This pile-up effect has been confirmed with post-mortem TEM observations of the dislocation pile-ups in Ti alloys [4,5]. Examples of cantilever studies in Ti and Zr alloys will be shown. We will also demonstrate that this approach generates CRSS values which allow the bulk flow stresses of macroscopic polycrystal aggregates to be determined, so enabling micromechanical studies to inform component level performance of industrial alloys [6]

In Vivo Time- Resolved Microtomography Reveals the Mechanics of the Blowfly Flight Motor

Dipteran flies are amongst the smallest and most agile of flying animals. Their wings are driven indirectly by large power muscles, which cause cyclical deformations of the thorax that are amplified through the intricate wing hinge. Asymmetric flight manoeuvres are controlled by 13 pairs of steering muscles acting directly on the wing articulations. Collectively the steering muscles account for <3% of total flight muscle mass, raising the question of how they can modulate the vastly greater output of the power muscles during manoeuvres. Here we present the results of a synchrotron-based study performing micrometre-resolution, time-resolved microtomography on the 145 Hz wingbeat of blowflies. These data represent the first four-dimensional visualizations of an organism's internal movements on sub-millisecond and micrometre scales. This technique allows us to visualize and measure the three-dimensional movements of five of the largest steering muscles, and to place these in the context of the deforming thoracic mechanism that the muscles actuate. Our visualizations show that the steering muscles operate through a diverse range of nonlinear mechanisms, revealing several unexpected features that could not have been identified using any other technique. The tendons of some steering muscles buckle on every wingbeat to accommodate high amplitude movements of the wing hinge. Other steering muscles absorb kinetic energy from an oscillating control linkage, which rotates at low wingbeat amplitude but translates at high wingbeat amplitude. Kinetic energy is distributed differently in these two modes of oscillation, which may play a role in asymmetric power management during flight control. Structural flexibility is known to be important to the aerodynamic efficiency of insect wings, and to the function of their indirect power muscles. We show that it is integral also to the operation of the steering muscles, and so to the functional flexibility of the insect flight motor

Evaluating QBF Solvers: Quantifier Alternations Matter

We present an experimental study of the effects of quantifier alternations on the evaluation of quantified Boolean formula (QBF) solvers. The number of quantifier alternations in a QBF in prenex conjunctive normal form (PCNF) is directly related to the theoretical hardness of the respective QBF satisfiability problem in the polynomial hierarchy. We show empirically that the performance of solvers based on different solving paradigms substantially varies depending on the numbers of alternations in PCNFs. In related theoretical work, quantifier alternations have become the focus of understanding the strengths and weaknesses of various QBF proof systems implemented in solvers. Our results motivate the development of methods to evaluate orthogonal solving paradigms by taking quantifier alternations into account. This is necessary to showcase the broad range of existing QBF solving paradigms for practical QBF applications. Moreover, we highlight the potential of combining different approaches and QBF proof systems in solvers.Comment: preprint of a paper to be published at CP 2018, LNCS, Springer, including appendi