Molecular Dynamics Simulations of Weak Detonations

Detonation of a three-dimensional reactive non-isotropic molecular crystal is modeled using molecular dynamics simulations. The detonation process is initiated by an impulse, followed by the creation of a stable fast reactive shock wave. The terminal shock velocity is independent of the initiation conditions. Further analysis shows supersonic propagation decoupled from the dynamics of the decomposed material left behind the shock front. The dependence of the shock velocity on crystal nonlinear compressibility resembles solitary behavior. These properties categorize the phenomena as a weak detonation. The dependence of the detonation wave on microscopic potential parameters was investigated. An increase in detonation velocity with the reaction exothermicity reaching a saturation value is observed. In all other respects the model crystal exhibits typical properties of a molecular crystal.Comment: 38 pages, 20 figures. Submitted to Physical Review

Influence of the contact–impact force model on the dynamic response of multi-body systems

This work deals with contact–impact force models for both spherical and cylindrical contact surfaces. The incorporation of the friction phenomenon, based on the Coulomb friction law, is also discussed together with an effective computational strategy, which includes the automatic step size selection procedure. Impacts within a revolute clearance joint in a basic slider–crank mechanism are used as an example to compare the different contact force models. The collision is a prominent phenomenon in manymulti-body systems such as mechanisms with intermittent motion, kinematic discontinuities, and clearance joints. As a result of an impact, the values of the system state variables change very fast, eventually looking like discontinuities in the system velocities and accelerations. The impact is characterized by large forces that are applied and removed in a short time period. The knowledge of the peak forces developed in the impact process is very important for the dynamic analysis of multibody systems and it has consequences in the design process. The model for the contact–impact force must consider the material and geometric properties of the colliding surfaces, consider information on the impact velocity, contribute to an efficient integration, and account for some level of energy dissipation. These characteristics are ensured with a continuous contact force model, in which the deformation and contact forces are considered as continuous functions.FEDER - Project POCTI/2001/EME/38281.Fundação para a Ciência e a Tecnologia (FCT)

Student vocational teachers: the significance of individual positions in workplace learning

In most Initial Teacher Preparation (ITP) programmes, learning in teaching placements is considered to be an important component for providing workplace learning experiences to develop the skills of being a teacher. This paper is based on a bigger qualitative study which explored the learning experiences of a group of in-service student vocational teachers prior to and during their one-year ITP programme in Brunei. The study examined these student teachers’ dispositions to learning as revealed through their experiences on different placements during their training. The findings of this paper highlight the importance of the student vocational teachers’ roles and positions relative to their teaching placements. Theoretically, the findings also extend Bourdieu’s thinking, where existing cultural capital in the form of subject knowledge which is valued in one context does not necessarily help the learning of individuals in becoming a vocational teacher in another context. In addition, the paper argues for a need to reconceptualise in-service teacher education, more specifically, the workplace learning aspect. Lastly, it concludes with recommendations to support these student teachers in their placements through creating more expansive learning environments

A study on the response of single and double circular plates subjected to localised blast loading

The response of single and double layered steel plates to localised air-blast loading was examined. Two configurations, both comprising fully clamped circular plates with a 200 mm exposed diameter, were considered: 4mm thick single and (2+2) mm double layered plates. The localised air-blast loading was applied by centrally detonating discs of PE4 plastic explosive. Similar failure modes were evident in the single and double plate configurations, namely, Mode I (large inelastic deformation) and Mode II (capping failure along with deformation) responses. The double plates exhibited larger midpoint deflections than the single plates, and partial tearing of the front plate in the double plates was observed at a lower impulse than in the single plates. However, complete capping of both plates in the double plate configuration occurred at the same charge mass as for the single plates, implying that both configurations offer equivalent protection from capping failure as a result of this type of localised blast loading. A metallographic study of the deformed and torn plate regions did not reveal any phase transformation in the steel. It was also found that the 2 mm thick plates exhibited larger increases in grain size than the 4 mm thick plates

Diversity and pedagogic practice: reflections on the role of an adult educator in higher education

This is a postprint of an article whose final and definitive form has been published in Teaching in Higher Education (c) 2007 Copyright Taylor & Francis; Teaching in Higher Education is available online at http://www.informaworld.comBased on a model developed by Brookfield (1995), a deliberately reflective approach is taken in this paper to the relationship between the author's earlier work in a department of adult education and her current teaching on a course for new university lecturers. As increasing numbers of mature students are being encouraged into universities, she wonders whether the principles and practices of adult education have a place in the pedagogic practices of higher education. She summarises the development of adult education departments in British universities, and draws attention to different pedagogic approaches in adult and higher education. Looking through various 'lenses', the author concludes that there is a need for a new professional agenda in higher education - where commonality and difference provide the starting points for mutual exploration and self-understanding - and that the traditions of adult education have a significant contribution to make to this agenda

A parametric study on the dynamic response of planar multibody systems with multiple clearance joints

A general methodology for dynamic modeling and analysis of multibody systems with multiple clearance joints is presented and discussed in this paper. The joint components that constitute a real joint are modeled as colliding bodies, being their behavior influenced by geometric and physical properties of the contacting surfaces. A continuous contact force model, based on the elastic Hertz theory together with a dissipative term, is used to evaluate the intra-joint contact forces. Furthermore, the incorporation of the friction phenomenon, based on the classical Coulomb’s friction law, is also discussed. The suitable contact-impact force models are embedded into the dynamics of multibody systems methodologies. An elementary mechanical system is used to demonstrate the accuracy and efficiency of the presented approach, and to discuss the main assumptions and procedures adopted. Different test scenarios are considered with the purpose of performing a parametric study for quantifying the influence of the clearance size, input crank speed and number of clearance joints on the dynamic response of multibody systems with multiple clearance joints. Additionally, the total computation time consumed in each simulation is evaluated in order to test the computational accuracy and efficiency of the presented approach. From the main results obtained in this study, it can be drawn that clearance size and the operating conditions play a crucial role in predicting accurately the dynamic responses of multibody systems.Fundação para a Ciência e a Tecnologia (FCT