A multi-scale atomistic-continuum modelling of crack propagation in a two-dimensional macroscopic plate

A novel multi-scale seamless model of brittle-crack propagation is proposed and applied to the simulation of fracture growth in a two-dimensional Ag plate with macroscopic dimensions. The model represents the crack propagation at the macroscopic scale as the drift-diffusion motion of the crack tip alone. The diffusive motion is associated with the crack-tip coordinates in the position space, and reflects the oscillations observed in the crack velocity following its critical value. The model couples the crack dynamics at the macroscales and nanoscales via an intermediate mesoscale continuum. The finite-element method is employed to make the transition from the macroscale to the nanoscale by computing the continuum-based displacements of the atoms at the boundary of an atomic lattice embedded within the plate and surrounding the tip. Molecular dynamics (MD) simulation then drives the crack tip forward, producing the tip critical velocity and its diffusion constant. These are then used in the Ito stochastic calculus to make the reverse transition from the nanoscale back to the macroscale. The MD-level modelling is based on the use of a many-body potential. The model successfully reproduces the crack-velocity oscillations, roughening transitions of the crack surfaces, as well as the macroscopic crack trajectory. The implications for a 3-D modelling are discussed.

Thermo-mechanical modelling of power electronics module structures

Power electronic modules distinguish themselves from other modules by their high power operation. These modules are used extensively in high power application markets such as aerospace, automotive, industrial and traction and drives. This paper discusses typical packaging technologies for power electronics modules. It also discusses the latest results from a UK research project investigating the physics-of-failure approach to reliability analysis and predictions for power modules. An integrated design enviroment for incorporating of affects of uncertainty into the design environment was outlined.

Thermoelectric MHD effects on equiaxed crystal morphology

The effects of a constant uniform magnetic field on a growing equiaxed crystal are investigated using a 3-dimensional enthalpy based numerical model. Two cases are considered: The first case looks at unconstrained growth, where the current density is generated through the thermo-electric effect and the current circulates between the tips and roots of the dendrite, the second represents an imposed potential difference across the domain. A jump in the electrical conductivity between the liquid and solid causes the current density to be non uniform. In both cases the resulting Lorentz force drives fluid flow in the liquid phase, this in turn causes advection of the thermal and solute field altering the free energy close to the interface and changing the morphology of the dendrite. In the first case the flow field is complex comprising of many circulations, the morphological changes are modelled using a 2D model with a quasi 3D approximation. The second case is comparable to classic problems involving a constant velocity boundary.

Assessment of the origin and stability of Nifedipine polymorph IV by differential scanning calorimetry (DSC)

Purpose. To examine the thermal transition(s) between different polymorphic forms of Nifedipine and to define experimental conditions that lead to the generation of polymorph IV. Methods. Experiments were performed using a DSC 823e (Mettler Toledo). Nifedipine exists in four polymorphic forms, as well as an amorphous state. Examination of Nifedipine was conducted using the following method(s): cycle 1: 25ºC to 190ºC, 190ºC to 25ºC (formation of amorphous Nifedipine); cycle 2: 25ºC to X (60,70,80...150ºC), X to 25ºC; cycle 3: 25ºC to 190ºC and holding isothermally for 5 min between cycles (heating/cooling rate of 10ºC/min). Results. The amorphous state Nifedipine can sustain heating up to 90ºC without significant changes in its composition. Cycle 2 of amorphous material heated up to 90ºC shows only the glass transition at ~44ºC. In cycle 3 of the same material, a glass transition has been recorded at ~44ºC, followed by two exotherms (~100 and ~115ºC (crystallisation of polymorph III and II, respectively) and an endotherm (169ºC (melting of polymorphs I/II)). Samples that have been heated to temperatures between 100ºC and 120ºC in the second cycle showed a glass transition at ~44ºC and an additional exotherm at ~95ºC (crystallisation of polymorph III) on cooling a exotherm was observed at ~40ºC (crystallisation of polymorph IV). The same material showed no glass transition in cycle 3 but an endotherm at around 62ºC (melting of polymorph IV) an exotherm (~98ºC) and an endotherm (169ºC) melting of polymorph I/II. Heating the sample to a temperatures greater than 130ºC in cycle two results in a glass transition at ~44ºC, and two exotherms at ~102 and 125ºC (crystallisation of polymorphs III and I, respectively). Conclusions. DSC data suggests that polymorph IV can only be produced from amorphous or polymorph III samples. The presence of polymorph I or II drives the conversion of the less stable polymorphic form IV into the most stable form, I. Although form IV of Nifedipine can easily be created, following defined experimental conditions, it may only coexist with amorphous or polymorph III states. When polymorphs I and II are present in the sample polymorph IV cannot be etected.

Start-up and running forces on bulk solids feeders: experimental findings versus available models

Many different models have been postulated over the years for sizing of feeder drives; these models have different bases, some rationally based and others more rule-of-thumb. Experience of Jenike & Johanson and likewise of The Wolfson Centre in trouble-shooting feeder drives has shown that drive powers are often poorly matched, so there is clearly still some way to go towards establishing a universally-used reliable approach. This paper presents an on-going programme of work designed to measure feeder forces experimentally on a purpose designed testing rig, and to compare these against some of the best known available models, and also against a full size installation. One aspect which is novel is the monitoring of the transition between the “filling stress field” load on the feeder and the “flowing stress field” load.