Attenuation of the dynamic yield point of shocked aluminum using elastodynamic simulations of dislocation dynamics.

When a metal is subjected to extremely rapid compression, a shock wave is launched that generates dislocations as it propagates. The shock wave evolves into a characteristic two-wave structure, with an elastic wave preceding a plastic front. It has been known for more than six decades that the amplitude of the elastic wave decays the farther it travels into the metal: this is known as "the decay of the elastic precursor." The amplitude of the elastic precursor is a dynamic yield point because it marks the transition from elastic to plastic behavior. In this Letter we provide a full explanation of this attenuation using the first method of dislocation dynamics to treat the time dependence of the elastic fields of dislocations explicitly. We show that the decay of the elastic precursor is a result of the interference of the elastic shock wave with elastic waves emanating from dislocations nucleated in the shock front. Our simulations reproduce quantitatively recent experiments on the decay of the elastic precursor in aluminum and its dependence on strain rate.

A genetic algorithm for predicting the structures of interfaces in multicomponent systems.

Recent years have seen great advances in our ability to predict crystal structures from first principles. However, previous algorithms have focused on the prediction of bulk crystal structures, where the global minimum is the target. Here, we present a general atomistic approach to simulate in multicomponent systems the structures and free energies of grain boundaries and heterophase interfaces with fixed stoichiometric and non-stoichiometric compositions. The approach combines a new genetic algorithm using empirical interatomic potentials to explore the configurational phase space of boundaries, and thereafter refining structures and free energies with first-principles electronic structure methods. We introduce a structural order parameter to bias the genetic algorithm search away from the global minimum (which would be bulk crystal), while not favouring any particular structure types, unless they lower the energy. We demonstrate the power and efficiency of the algorithm by considering non-stoichiometric grain boundaries in a ternary oxide, SrTiO(3).

General practitioners' conflicts of interest, the paramountcy principle and safeguarding children: a psychodynamic contribution.

Wainwright and Gallagher propose that when child protection concerns emerge significant difficulties arise for General Practitioners because of conflicts between the individual interests of children and parents who are their patients and the Paramountcy Principle. From a psychodynamic perspective their analysis does not give sufficient weight to the nature of personal as opposed to interpersonal conflict of a conscious or unconscious nature. When issues of major import arise, ordinary parenting inevitably involves parents in putting their children's needs first if competing possibilities occur. It is an over-simplification to present this as a conflict between the interests of children and parents. Parents' own best interests are served by securing their children's safety and welfare. An appreciation of this is crucial in order to implement child protection procedures appropriately. Errors may occur because the complex emotions and relationships involved lead professionals to experience themselves as potential agents of harm rather than benefit.

The role of molybdenum in suppressing cold dwell fatigue in titanium alloys.

We test a hypothesis to explain why Ti-6242 is susceptible to cold dwell fatigue (CDF), whereas Ti-6246 is not. The hypothesis is that, in Ti-6246, substitutional Mo-atoms in α-Ti grains trap vacancies, thereby limiting creep relaxation. In Ti-6242, this creep relaxation enhances the loading of grains unfavourably oriented for slip and they subsequently fracture. Using density functional theory to calculate formation and binding energies between Mo-atoms and vacancies, we find no support for the hypothesis. In the light of this result, and experimental observations of the microstructures in these alloys, we agree with the recent suggestion (Qiu et al. 2014 Metall. Mater. Trans. A45, 6075-6087. (doi:10.1007/s11661-014-2541-5)) that Ti-6246 has a much smaller susceptibility to CDF because it has a smaller grain size and a more homogeneous distribution of grain orientations. We propose that the reduction of the susceptibility to CDF of Ti-6242 at temperatures above about 200°C is due to the activation of 〈c+a〉 slip in 'hard' grains, which reduces the loading of grain boundaries.

Nanocrystalline copper films are never flat.

We used scanning tunneling microscopy to study low-angle grain boundaries at the surface of nearly planar copper nanocrystalline (111) films. The presence of grain boundaries and their emergence at the film surface create valleys composed of dissociated edge dislocations and ridges where partial dislocations have recombined. Geometric analysis and simulations indicated that valleys and ridges were created by an out-of-plane grain rotation driven by reduction of grain boundary energy. These results suggest that in general, it is impossible to form flat two-dimensional nanocrystalline films of copper and other metals exhibiting small stacking fault energies and/or large elastic anisotropy, which induce a large anisotropy in the dislocation-line energy.

Elastodynamic image forces on dislocations.

The elastodynamic image forces on edge and screw dislocations in the presence of a planar-free surface are derived. The explicit form of the elastodynamic fields of an injected, quiescent screw dislocation are also derived. The resulting image forces are affected by retardation effects: the dislocations experience no image force for a period of time defined by the arrival and reflection at the free surface of the dislocation fields. For the case of injected, stationary dislocations, it is shown that the elastodynamic image force tends asymptotically to the elastotatic prediction. For the case of injected, moving dislocations, it is shown that the elastodynamic image force on both the edge and the screw dislocations is magnified by inertial effects, and becomes increasingly divergent with time; this additional effect, missing in the elastostatic description, is shown to be substantial even for slow moving dislocations. Finally, it is shown that the elastodynamic image force of an edge dislocation moving towards the surface at the Rayleigh wave speed becomes repulsive, rather than attractive; this is suggestive of instabilities at the core of the dislocation, and likely resonances with the free surface.

Interprofessional education in a midwifery curriculum: the learning through the exploration of the professional task project (LEAPT).

OBJECTIVE: to develop interprofessional education for students of midwifery, nursing and medicine. To foster collaborative working and learning between students of midwifery, nursing and medicine. DESIGN: a quasi-experimental method to evaluate the outcomes of an intervention (a problem-based learning (PBL) scenario) with interprofessional students and facilitators. Data were collected using pre- and post-test questionnaires. SETTING: a University and National Health Service healthcare facilities in the North of England. PARTICIPANTS: 40 students of midwifery, nursing and medicine. FINDINGS: student midwives participating in the PBL scenario in this interprofessional format improved their attitudes to working in this environment. All students enjoyed the opportunity to learn in an interprofessional team, and they felt that the experience provided a safe environment to help prepare for their future roles. The learning opportunity enabled all students to reflect on each other's role and to discuss differing perspectives of care, although the student midwives had mixed feelings about the PBL experience. IMPLICATIONS FOR PRACTICE: interprofessional learning using PBL is a useful environment for students to learn about each others' role, and to prepare for working together as qualified professionals in a collaborative manner. However, time and commitment is required to plan joint working in order to maximise the benefits.

Computational steering in Monte Carlo simulations of thin film polystyrene.

High molecular weight polymer systems show very long relaxation times, of the order of milliseconds or more. This time-scale proves practically inaccessible for atomic-scale dynamical simulation such as molecular dynamics. Even with a Monte Carlo (MC) simulation, the generation of statistically independent configurations is non-trivial. Many moves have been proposed to enhance the efficiency of MC simulation of polymers. Each is described by a proposal density Q(x'; x): the probability of selecting the trial state x' given that the system is in the current state x. This proposal density must be parametrized for a particular chain length, chemistry and temperature. Choosing the correct set of parameters can greatly increase the rate at which the system explores its configuration space. Computational steering (CS) provides a new methodology for a systematic search to optimize the proposal densities for individual moves, and to combine groups of moves to greatly improve the equilibration of a model polymer system. We show that monitoring the correlation time of the system is an ideal single parameter for characterizing the efficiency of a proposal density function, and that this is best evaluated by a distributed network of replicas of the system, with the operator making decisions based on the averages generated over these replicas. We have developed an MC code for simulating an anisotropic atomistic bead model which implements the CS paradigm. We report simulations of thin film polystyrene.

Promoting interdisciplinarity through educational initiative: a qualitative evaluation.

This paper describes a multi-professional teaching innovation that took place at The University of Manchester during October 2001. Ideas for the project were first established in December 2000 when a multi-professional team of educationalists began exploring ways in which three disparate student groups could be brought together as part of an integrated learning activity (). For this reason, a problem-based learning scenario was developed to encourage nursing, midwifery, and medical students to share their previous knowledge and principles of care. In addition, the students were also asked to identify the type(s) of information they felt should be included in a user guide for new parents regarding the uncomplicated labour and puerperium. In total 10-second year nursing, 17-third year midwifery and 13-fourth year medical students volunteered to take part. These were subsequently divided into five integrated problem-based learning groups each supported by two teachers reflecting similar multi-professional backgrounds. In this paper, the discussion will explore a series of themes identified as being of major import by students and tutors during the qualitative phase of the study namely: