On computational homogenization of microscale crack propagation

The effective response of microstructures undergoing crack propagation is studied by homogenizing the response of statistical volume elements (SVEs). Because conventional boundary conditions (Dirichlet, Neumann and strong periodic) all are inaccurate when cracks intersect the SVE boundary, we herein use first order homogenization to compare the performance of these boundary conditions during the initial stage of crack propagation in the microstructure, prior to macroscopic localization. Using weakly periodic boundary conditions that lead to a mixed formulation with displacements and boundary tractions as unknowns, we can adapt the traction approximation to the problem at hand to obtain better convergence with increasing SVE size. In particular, we show that a piecewise constant traction approximation, which has previously been shown to be efficient for stationary cracks, is more efficient than the conventional boundary conditions in terms of convergence also when crack propagation occurs on the microscale. The performance of the method is demonstrated by examples involving grain boundary crack propagation modelled by conventional cohesive interface elements as well as crack propagation modelled by means of the extended finite element method in combination with the concept of material forces. © 2016 The Authors. International Journal for Numerical Methods in Engineering Published by John Wiley & Sons Ltd.     

A method for identification of geometrical tool changes during machining of titanium alloy Ti6Al4V

There exists an increasing demand for cost and time-efficient cutting tests for describing the performance of different combinations of cutting tools and workpiece materials in the cutting process both in industry and academia. Cutting tools are expected to withstand the heat and the pressure developed during the machining of difficult-to-machine materials such as Ti6Al4V. This article introduces a new test method which may be used in order to analyze both the machinability of a workpiece material as well as the cutting tool behavior. The experiments were performed by using a predefined sequence of feeds, a so-called Stepwise Increased Feed Test. A gradually increased load on the cutting edge was thus applied up to the point where plastic deformation of the cutting edge was obtained. The limit for the initial change in tool geometry was identified through analysis of measured cutting forces.     

Verifying Global Minima for L 2 Minimization Problems in Multiple View Geometry

We consider the least-squares (L2) minimization problems in multiple view geometry for triangulation, homography, camera resectioning and structure-and-motion with known rotation, or known plane. Although optimal algorithms have been given for these problems under an L-infinity cost function, finding optimal least-squares solutions to these problems is difficult, since the cost functions are not convex, and in the worst case may have multiple minima. Iterative methods can be used to find a good solution, but this may be a local minimum. This paper provides a method for verifying whether a local-minimum solution is globally optimal, by providing a simple and rapid test involving the Hessian of the cost function. The basic idea is that by showing that the cost function is convex in a restricted but large enough neighbourhood, a sufficient condition for global optimality is obtained. The method is tested on numerous problem instances of real data sets. In the vast majority of cases we are able to verify that the solutions are optimal, in particular, for small to medium-scale problems.     

The psychophysiology of flow during piano playing.

Expert performance is commonly accompanied by a subjective state of optimal experience called flow. Previous research has shown positive correlations between flow and quality of performance and suggests that flow may function as a reward signal that promotes practice. Here, piano playing was used as a flow-inducing behavior in order to analyze the relationship between subjective flow reports and psychophysiological measures. Professional classical pianists were asked to play a musical piece and then rate state flow. The performance was repeated five times in order to induce a variation in flow, keeping other factors constant, while recording the arterial pulse pressure waveform, respiration, head movements, and activity from the corrugator supercilii and zygomaticus major facial muscles. A significant relation was found between flow and heart period, blood pressure, heart rate variability, activity of the zygomaticus major muscle, and respiratory depth. These findings are discussed in relation to current models of emotion, attention, and expertise, and flow is proposed to be a state of effortless attention, which arises through an interaction between positive affect and high attention. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Current oscillations during chronoamperometric and cyclic voltammetric measurements in alkaline Cu(II)-citrate solutions

It is demonstrated that current oscillations can be observed during chronoamperometric and cyclic voltammetric experiments in solutions containing 0.4 M CuSO₄ and 1.2 M citrate at pH 11 and 50 °C. The oscillations, which are shown to originate from local variations in the pH, result in the deposition of nanostructured Cu and Cu₂O materials. It is concluded that the current oscillations are analogous to the previously described potential oscillations obtained under controlled current conditions in alkaline Cu(II)-lactate, -tartrate and -citrate solutions. Rotating disk electrode results clearly show that the reduction of the Cu(II)-complexes is kinetically controlled and that the rate of the reduction increases with increasing pH and temperature. It is also shown that the presence of a cathodic peak on the anodic scan in the cyclic voltammograms can be used to identify the experimental conditions leading to the spontaneous current (or potential) oscillations. Electrochemical quartz crystal microbalance results indicate that the cathodic peak stems from an increased rate of the reduction of the Cu(II)-citrate complexes due to a rapid increase in the local pH. This causes Cu₂O rather than Cu to be deposited which, however, results in a decrease in the local pH and a decreasing current. In situ ellipsometry data confirm that Cu₂O deposition replaces that of Cu in the potential region of the cathodic peak. The present findings should facilitate syntheses of nanolayered materials based on spontaneous potential or current oscillations.     

A High-Resolution TEM/EELS Study of the Effect of Doping Elements on the Sliding Mechanisms of Sputtered WS2 Coatings

It has been shown many times that cosputtering low-friction coatings of molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) with other elements can improve the structural, mechanical, and tribological properties. To achieve the lowest friction, MoS₂ or WS₂ should be doped with element(s) improving the hardness and density of the coatings. On the other hand, such elements, or their compounds, should not be present in the outermost molecular layers at the sliding interface. This article suggests that there are important differences between how MoS₂ and WS₂ coatings respond to or react with doping elements, despite the almost identical structure and behavior of the undoped materials. Two systems have been investigated by high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM) electron energy loss spectroscopy (EELS), W-S-C-Cr and W-S-C-Ti, and showed significant amounts of oxides, which typically formed a layer just underneath the crystalline WS₂ top layer. Further, carbon was almost completely absent in the tribofilms, despite the fact that the as-deposited coatings contained as much as 40–50 at% C. An interesting observation here is that WS₂ basal planes surround or embed Fe wear particles, suggesting a relatively strong adhesion or a Fe-S chemical bonding between iron/steel and WS₂. The result of this is that the wear particles become pacified and remain in the contact as low-friction material.     

Distributed representations for extended syntactic transformation

This paper shows how the choice of representation substantially affects the generalization performance of connectionistnetworks. The starting point is Chalmers' simulations involving structure-sensitive processing. Chalmers argued that a connectionist network could handle structure sensitive processing without the use of syntactically structured representations. He trained a connectionist architecture to encode/decode distributed representations for simple sentences. These distributed representations were then holistically transformed such that active sentences were transformed into their passive counterpart. However, he noted that the recursive auto-associative memory (RAAM), which was used to encode and decode distributed representations for the structures, exhibited only a limited ability to generalize when trained to encode/decode a randomly selected sample of the total corpus. When the RAAM was trained to encode/decode all sentences, and a separate transformation network was trained to make some active-passive transformations of the RAAMencoded sentences, the transformation network demonstrated perfect generalization on the remaining test sentences. It is argued here that the main reason for the limited generalization is not the ability of the RAAM architecture per se, but the choice of representation for the tokens used. This paper shows that 100% generalization can be achieved for Chalmers' original set up (i.e. using only 30% of the total corpus for training). The key to this success is to use distributed representations for the tokens (capturing different characteristics for differentclasses of tokens, e.g. verbs or nouns).     

Normalized Cuts Revisited: A Reformulation for Segmentation with Linear Grouping Constraints

Indisputably Normalized Cuts is one of the most popular segmentation algorithms in pattern recognition and computer vision. It has been applied to a wide range of segmentation tasks with great success. A number of extensions to this approach have also been proposed, including ones that can deal with multiple classes or that can incorporate a priori information in the form of grouping constraints. However, what is common for all these methods is that they are noticeably limited in the type of constraints that can be incorporated and can only address segmentation problems on a very specific form. In this paper, we present a reformulation of Normalized Cut segmentation that in a unified way can handle linear equality constraints for an arbitrary number of classes. This is done by restating the problem and showing how linear constraints can be enforced exactly in the optimization scheme through duality. This allows us to add group priors, for example, that certain pixels should belong to a given class. In addition, it provides a principled way to perform multi-class segmentation for tasks like interactive segmentation. The method has been tested on real data showing good performance and improvements compared to standard normalized cuts.     

Experimental study of heat transfer additive influence on the absorption chiller performance

The influence of 2-methyl-1-pentanol (2MP) on the cooling effect of pilot absorption chiller has been studied experimentally. In one experimental series the additive was injected into LiBr solution. The enhancement ratio up to 20% was observed at the optimum additive concentration. In the second experimental series the additive was injected into the refrigerant. The enhancement ratio became 32% at higher additive concentration. Different additive concentrations have been tested in both series. These experimental results clearly showed that the presence of the additive in the vapour phase, even in very small amounts, favours the enhancement more than the additive in the LiBr solution. Also, it has been noticed, that the additive travels around the absorption cycle during long-term operation.