A finite element analysis of dynamic fracture initiation by ductile failure mechanisms in a 4340 steel

In some recent dropweight impact experiments [5] with pre-notched bend specimens of 4340 steel, it was observed that considerable crack tunneling occurred in the interior of the specimen prior to gross fracture initiation on the free surfaces. The final failure of the side ligaments happened because of shear lip formation. The tunneled region is characterized by a flat, fibrous fracture surface. In this paper, the experiments of [5] (corresponding to 5 m/s impact speed) are analyzed using a plane strain, dynamic finite element procedure. The Gurson constitutive model that accounts for the ductile failure mechanisms of micro-void nucleation, growth and coalescence is employed. The time at which incipient failure was observed near the notch tip in this computation, and the value of the dynamic J-integral, J d, at this time, compare reasonably well with experiments. This investigation shows that J-controlled stress and deformation fields are established near the notch tip whenever J d , increases with time. Also, it is found that the evolution of micro-mechanical quantities near the notch root can be correlated with the time variation of J d .The strain rate and the adiabatic temperature rise experienced at the notch root are examined. Finally, spatial variations of stresses and deformations are analyzed in detail.

On measurement of top polarization as a probe of t(t)over-bar production mechanisms at the LHC

In this note we demonstrate the use of top polarization in the study of t (t) over bar resonances at the LHC, in the possible case where the dynamics implies a non-zero top polarization. As a probe of top polarization we construct an asymmetry in the decay-lepton azimuthal angle distribution (corresponding to the sign of cos phi(l)) in the laboratory. The asymmetry is non-vanishing even for a symmetric collider like the LHC, where a positive z axis is not uniquely defined. The angular distribution of the leptons has the advantage of being a faithful top-spin analyzer, unaffected by possible anomalous tbW couplings, to linear order. We study, for purposes of demonstration, the case of a Z' as might exist in the little Higgs models. We identify kinematic cuts which ensure that our asymmetry reflects the polarization in sign and magnitude. We investigate possibilities at the LHC with two energy options: root s = 14TeV and root s = 7TeV, as well as at the Tevatron. At the LHC the model predicts net top quark polarization of the order of a few per cent for M-Z' similar or equal to 1200GeV, being as high as 10% for a smaller mass of the Z' of 700GeV and for the largest allowed coupling in the model, the values being higher for the 7TeV option. These polarizations translate to a deviation from the standard-model value of azimuthal asymmetry of up to about 4% (7%) for 14 (7) TeV LHC, whereas for the Tevatron, values as high as 12% are attained. For the 14TeV LHC with an integrated luminosity of 10 fb(-1), these numbers translate into a 3 sigma sensitivity over a large part of the range 500 less than or similar to M-Z' less than or similar to 1500GeV.

A functional style of programming with CSP-like communication mechanisms

This paper introduces CSP-like communication mechanisms into Backus’ Functional Programming (FP) systems extended by nondeterministic constructs. Several new functionals are used to describe nondeterminism and communication in programs. The functionals union and restriction are introduced into FP systems to develop a simple algebra of programs with nondeterminism. The behaviour of other functionals proposed in this paper are characterized by the properties of union and restriction. The axiomatic semantics of communication constructs are presented. Examples show that it is possible to reason about a communicating program by first transforming it into a non-communicating program by using the axioms of communication, and then reasoning about the resulting non-communicating version of the program. It is also shown that communicating programs can be developed from non-communicating programs given as specifications by using a transformational approach.

Dynamic growth of tensile cracks by ductile and brittle fracture mechanisms in a viscoplastic material

In this paper, a finite element analysis of steady-state dynamic crack growth under Mode I, plane strain, small-scale yielding conditions is performed in a rate dependent plastic material characterized by the over-stress model. The main objective of the paper is to obtain theoretically the dependence of dynamic fracture toughness on crack speed. Crack propagation due to a ductile (micro-void) mechanism or a brittle (cleavage) mechanism, as well as transition from one mode to another are considered. The conversion from ductile to brittle has been observed experimentally but has received very little attention using analytical methods. Local fracture criteria based on strains and stresses are used to describe ductile and brittle fracture mechanisms. The results obtained in this paper are in general agreement with micro-structural observations of mode conversion during fracture initiation. Finally, the particular roles played by material rate sensitivity and inertia are examined in some detail.

Mechanisms of variability and predictability of the tropical coupled ocean-atmosphere system

A conceptual model is proposed to explain the observed aperiodicity in the short term climate fluctuations of the tropical coupled ocean-atmosphere system. This is based on the evidence presented here that the tropical coupled ocean-atmosphere system sustains a low frequency inter-annual mode and a host of higher frequency intra-seasonal unstable modes. At long wavelengths, the low frequency mode is dominant while at short wavelengths, the high frequency modes are dominant resulting in the co-existence of a long wave low frequency mode with some short wave intra-seasonal modes in the tropical coupled system. It is argued that due to its long wavelength, the low frequency mode would behave like a linear oscillator while the higher frequency short wave modes would be nonlinear. The conceptual model envisages that an interaction between the low frequency linear oscillator and the high frequency nonlinear oscillations results in the observed aperiodicity of the tropical coupled system. This is illustrated by representing the higher frequency intra-seasonal oscillations by a nonlinear low order model which is then coupled to a linear oscillator with a periodicity of four years. The physical mechanism resulting in the aperiodicity in the low frequency oscillations and implications of these results on the predictability of the coupled system are discussed.

Texture evolution and operative mechanisms during large-strain deformation of nanocrystalline nickel

The large-strain deformation of nanocrystalline nickel was investigated at room temperature and cryogenic (liquid N-2) temperature. Deformation mechanisms ranging from grain boundary sliding to slip, operate due to a wide distribution of grain sizes. These mechanisms leave their finger print in the deformation texture evolution during rolling of nanocrystalline nickel. The occurrence and severance of different mechanisms is understood by a thorough characterization of the deformed samples using X-ray diffraction, X-ray texture measurements, electron back-scattered diffraction and transmission electron microscopy. Crystal plasticity-based viscoplastic self-consistent simulations were used to further substantiate the experimental observations. Thus, a comprehensive understanding of deformation behavior of nanocrystalline nickel, which is characterized by simultaneous operation of dislocation-dominated and grain boundary-mediated mechanisms, has been developed.

Ball wear mechanisms and control in the grinding of sulfide and phosphate ores

Marked ball grinding tests were carried out in the laboratory using high carbon low alloy steel (cast and forged) and high chrome cast iron balls. Relative ball wear as a function of grinding period and milling conditions was evaluated for the different type of ball materials in the grinding of lead-zinc sulphide and phosphate ores. Results indicated that ball wear increased with time and showed a sharp increase for wet grinding over dry grinding. Ball wear under wet grinding conditions was also influenced by the gaseous atmosphere in the mill. The influence of oxygen on the corrosive wear of grinding balls was increasingly felt in case of sulphide ore grinding. The grinding ball materials could be arranged in the following order with respect to their overall wear resistance:

Origin of magnetism and trend in T-c in Cr-based double perovskites: Interplay of two driving mechanisms

Hamiltonian constructed in a first principles manner, we explored the origin of magnetism and the T-c trend in Cr-based double perovskite series, Sr2CrB'O-6 (B' = W/Re/Os). Our study shows that the apparently puzzling T-c trend in Sr2CrB'O-6 (B' = W/Re/Os) series can be understood in terms of the interplay of the hybridization driven mechanism and the superexchange mechanism.

Synthesis of path generating flexible-link mechanisms

Flexible-link mechanisms are those linkage mechanisms (or structures) which are capable of motion by virtue of elastic deformation of one or more;links. In such mechanisms a single flexible link; can replace several rigid links and joints resulting in fewer links, fewer pin joints, reduced overall weight and reduced mechanical error. In spite of such clear advantages, contributions toward flexible-link mechanisms remain very scarce. The area of flexible-link mechanisms offers much scope for further exploration. This paper attempts to show the potential of flexible-link mechanisms in accomplishing a kinematic task like path generation. Synthesis of a four-bar mechanism with a flexible rocker for circular and straight line path generation is carried out. Displacement analysis of the structure is carried out using finite element method (FEM) and synthesis is formulated and solved as an optimization problem. Several numerical examples are presented for illustration. Based on the results obtained with these examples, the flexible-link mechanism considered shows good promise for-path generation.

Redistribution Mechanisms for Assignment of Heterogeneous Objects

There are p heterogeneous objects to be assigned to n competing agents (n > p) each with unit demand. It is required to design a Groves mechanism for this assignment problem satisfying weak budget balance, individual rationality, and minimizing the budget imbalance. This calls for designing an appropriate rebate function. When the objects are identical, this problem has been solved which we refer as WCO mechanism. We measure the performance of such mechanisms by the redistribution index. We first prove an impossibility theorem which rules out linear rebate functions with non-zero redistribution index in heterogeneous object assignment. Motivated by this theorem,we explore two approaches to get around this impossibility. In the first approach, we show that linear rebate functions with non-zero redistribution index are possible when the valuations for the objects have a certain type of relationship and we design a mechanism with linear rebate function that is worst case optimal. In the second approach, we show that rebate functions with non-zero efficiency are possible if linearity is relaxed. We extend the rebate functions of the WCO mechanism to heterogeneous objects assignment and conjecture them to be worst case optimal.