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Previous HotQC studies of Cu nanovoids undergoing volumetric expansion conducted by the authors have uncovered a quasistatic-to-dynamic transition at a critical strain rate of the order of 10^8 s^(-1). At low strain rates nanovoid expansion takes place under essentially isothermal conditions, whereas at high strain rates it happens under essential adiabatic conditions. In this paper, we present a comparative study concerned with two different scenarios, each representing a variation on the reference case presented in [1]: (i) aluminium (Al) nanovoids undergoing volumetric expansion; and (ii) copper (Cu) nanovoids undergoing uniaxial deformation. Scenario (i) addresses material specificity by replacing Cu by Al in the reference case, whereas scenario (ii) addresses the effect of triaxiality by replacing volumetric expansion by uniaxial expansion in the reference case. We find a distinct quasistatic-to-dynamic transition in both scenarios, which suggests that the transition is indeed universal, i.e. material and strain-triaxiality independent. By contrast, the fine structure of the dislocation mechanisms that mediate void growth are strongly material and loading specific.

To study treatment persistence and mortality using a single-pill, fixed-dose combination tablet compared with a two-pill combination for hypertension.We analyzed Australian Pharmaceutical Benefit Scheme records 2011-2014 in a 10% random sample of concessional patients prescribed concomitant amlodipine and perindopril - either as a single-pill, fixed-dose combination tablet (n = 9340) or as two-pill combination therapy (n = 3093). Main outcome measures were: (a) proportions failing to continue amlodipine + perindopril over time, (b) proportions failing to continue any subsequent calcium channel and angiotensin inhibition therapy over time and (c) proportions dying.After 12 months, 34% of single-pill and 57% of two-pill users discontinued amlodipine + perindopril, median persistence time 42 months versus 7 months; 28% and 47% respectively discontinued any calcium channel-angiotensin inhibition therapy. After 48 months, 8% of single-pill and 18% of two-pill users had died. In a multivariate model adjusted for age, gender, duration and intensity of prior hypertension therapy, initial dose of amlodipine and perindopril, diabetes, hyperlipidemia, and complexity of care, the hazard ratio for risk of discontinuation over 42 months in the two-pill versus single-pill amlodipine + perindopril group was 1.94 (95% CI 1.83-2.06). The hazard ratio for discontinuation in two-pill versus single-pill users of any calcium channel-angiotensin inhibition therapy was 1.86 (1.74-1.99). The adjusted hazard ratio for risk of death over 48 months was 1.83 (1.55-2.16), but the mortality outcome may be an overestimate due to residual confounding.Use of a single-pill, fixed-dose combination in hypertension is associated with superior persistence and reduced mortality compared with use of two pills, suggesting a higher priority for the use of fixed-dose combinations.

We investigate two mechanisms of crystallographic slip in graphene, corresponding to glide and shuffle generalized stacking faults (GSF), and compute their γ-curves using Sandia National Laboratories Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). We find evidence of metastable partial dislocations for the glide GSF only. The computed values of the stable and unstable stacking-fault energies are suggestive of a high stability of full dislocations against dissociation and of dislocation dipoles against annihilation.

This paper presents an anharmonic lattice statics analysis of 180 ° and 90 ° domain walls in tetragonal ferroelectric perovskites. We present all the calculations and numerical examples for the technologically important ferroelectric material PbTiO_3. We use shell potentials that are fitted to quantum mechanics calculations. Our formulation places no restrictions on the range of the interactions. This formulation of lattice statics is inhomogeneous and accounts for the variation of the force constants near defects. The discrete governing equations for perfect domain walls are reduced using symmetry conditions. We solve the linearized discrete governing equations directly using a novel method in the setting of the theory of difference equations. We calculate the fully nonlinear solutions using modified Newton–Raphson iterations.

The first-principles prediction of dislocation nucleation in metallic systems subject to realistically sized indenters requires a multiscale approach due to the prohibitive computational expense. The largest empirical atomistic simulations include at most a billion atoms, at the same time requiring the parameterization of new interactions whenever an additional species or crystal structure is added. The multiscale orbital-free density functional theory–local quasicontinuum (OFDFT-LQC) method overcomes these problems by using first-principles OFDFT to capture the atomic interactions while relying upon LQC to evolve the macroscopic system. We use this method to indent the (111) surface of a 2×2×1μm piece of L12 Al3Mg. Using a localization criterion, the first dislocation is predicted to form off-axis on the slip plane in the direction after the indenter has penetrated 70 nm. Other popular dislocation nucleation criteria give different predictions. These results are strikingly similar to those for indentatio...

In the present paper, we analyse several activation configurations of embryonic dislocations nucleated from the tip of a cleavage crack. The activation configurations include nucleation on inclined planes, on oblique planes and on cleavage ledges and are treated within the classical framework of Peierls. A variational boundary integral method with an interplanar tension-shear potential developed earlier is used to solve for the saddle-point configurations of embryonic dislocation loops and their associated energies. Based on the assumption that the brittle-to-ductile transition in cleavage fracture is a nucleation-controlled process (as is expected to be the case in bcc transition metals such as α-Fe) the results of the calculations are used to estimate the brittle-to-ductile transition temperatures. It is concluded that only dislocation nucleation on cleavage ledges furnishes realistic values of the transition temperature. The homogeneous nucleation of dislocations on either inclined or oblique planes requires transition temperatures well above the melting point. This implies that nucleation of dislocations from a crack tip in intrinsically brittle crystals is only possible at local crack front heterogeneities such as cleavage ledges, and that the homogeneous nucleation of dislocations from a straight crack front is not possible. This conclusion is supported by the experimental observation that dislocation nucleation from a crack tip is a rare event which occurs preferentially at heterogeneities.

We develop a method which permits the analysis of problems requiring the simultaneous resolution of continuum and atomistic length scales-and associated deformation processes-in a unified manner. A finite element methodology furnishes a continuum statement of the problem of interest and provides the requisite multiple-scale analysis capability by adaptively refining the mesh near lattice defects and other highly energetic regions. The method differs from conventional finite element analyses in that interatomic interactions are incorporated into the model through a crystal calculation based on the local state of deformation. This procedure endows the model with crucial properties, such as slip invariance, which enable the emergence of dislocations and other lattice defects. We assess the accuracy of the theory in the atomistic limit by way of three examples: a stacking fault on the (111) plane, and edge dislocations residing on (111) and (100) planes of an aluminium single crystal. The method correctly predicts the splitting of the (111) edge dislocation into Shockley partials. The computed separation of these partials is consistent with results obtained by direct atomistic simulations. The method predicts no splitting of the Al Lomer dislocation, in keeping with observation and the results of direct atomistic simulation. In both cases, the core structures are found to be in good agreement with direct lattice statics calculations, which attests to the accuracy of the method at the atomistic scale.

(1996). Dreaming the healer at the end of the world. Psychological Perspectives: Vol. 33, No. 1, pp. 52-59.

The variational boundary integral method of Xu and Ortiz is taken as a basis for studying dislocation nucleation from atomically sharp cracks under combined mode I-mode II loading. The tension-shear potential of Rice et al. is extended to allow for skewness in the shear resistance curve and to account for the surface production resistance which accompanies ledge formation. The calculated unstable equilibrium configurations of the incipient dislocations and the dependence of the associated activation energies on crack tip energy release rate are found to differ from the Rice-Beltz perturbation solution and the Schock-Puschl more approximate solution. Simulations of dislocation nucleation on inclined slip planes reveal that, while tension softening facilitates nucleation, surface production resistance impedes it. The extent to which these two effects influence critical conditions for dislocation nucleation is quantified. The calculations suggest that homogeneous dislocation nucleation on inclined planes is not favoured for materials with all but the lowest of unstable stacking-energy-to-surface-energy ratios. This emphasizes the importance of heterogeneous dislocation nucleation and nucleation on oblique slip planes on which free surface production should play a much weaker role. The implications of these findings on the nucleation-controlled brittle-ductile transition in cleavage fracture are discussed.