Buckling of cylindrical shells with stepwise variable wall thickness under uniform external pressure

Cylindrical shells of stepwise variable wall thickness are widely used for cylindrical containment structures, such as vertical-axis tanks and silos. The thickness is changed because the stress resultants are much larger at lower levels. The increase of internal pressure and axial compression in the shell is addressed by increasing the wall thickness. Each shell is built up from a number of individual strakes of constant thickness. The thickness of the wall increases progressively from top to bottom.Whilst the buckling behaviour of a uniform thickness cylinder under external pressure is well defined, that of a stepped wall cylinder is difficult to determine. In the European standard EN 1993-1-6 (2007) and Recommendations ECCS EDR5 (2008), stepped wall cylinders under circumferential compression are transformed, first into a three-stage cylinder and thence into an equivalent uniform thickness cylinder. This two-stage process leads to a complicated calculation that depends on a chart that requires interpolation and is not easy to use, where the mechanics is somewhat hidden, which cannot be programmed into a spreadsheet leading to difficulties in the practical design of silos and tanks.This paper introduces a new “weighted smeared wall method”, which is proposed as a simpler method to deal with stepped-wall cylinders of short or medium length with any thickness variation. Buckling predictions are made for a wide range of geometries of silos and tanks (unanchored and anchored) using the new hand calculation method and compared both with accurate predictions from finite element calculations using ABAQUS and with the current Eurocode rules. The comparison shows that the weighted smeared wall method provides a close approximation to the external buckling strength of stepped wall cylinders for a wide range of short and medium-length shells, is easily programmed into a spreadsheet and is informative to the designer.     

Entropic Ligand Mixing for Engineering 2D Layered Perovskite from Colloidal Monolayer Building Blocks

Layered 2D perovskites are solution-processed quantum-wells. Their effective band-gap is determined via the inorganic perovskite layer thickness and exciton quantum confinement effects. Alternatively, by changing the organic moieties, one can tune the dielectric constant and distance between the monolayers modifying the excitonic interactions. In colloidal perovskites, a dynamic equilibrium exists between the free organic moieties in the solution and the surface of the nanocrystal. Colloidal synthesis is used to make single monolayer L₂PbBr₄ platelets and assemble these into layered 2D stacks. In the experiment, L is an alkylamine surface ligand whose length (4-18 carbons) determines the interlayer distances between the quantum-wells. The dynamic equilibrium of ligand mixtures in solution and perovskite surfaces leads to optimal mixing of the molecules. During the self-assembly of monolayers, the distance between the inorganic layers is thus engineered. The interlayer distance is proportional to the average ligand mixture length. This results in controlled interactions between the 2D-excitons, enabling red-shifted absorption and emission and extended lifetimes for longer alkyl chains. Using entropic mixing of ligands for the engineering of 2D excitonic interactions is therefore demonstrated. Formation of layered 2D perovskites from colloidal building blocks allows intermixing of dissimilar materials opening possibilities for new heterostructures and junctions.     

Finite-Element Modeling of Filling Pressures in a Full-Scale Silo

A detailed nonlinear finite-element analysis is undertaken of the wall pressures exerted by iron ore pellets in a full-scale silo experiment. The ring stiffeners on the exterior of the silo wall cause local small axisymmetric geometric imperfections, and the effect of these on the pressure predictions is explored, together with its sensitivity to the chosen bulk solid constitutive model. This paper focuses on the filling pressures alone, because these are a necessary starting point if discharge pressure predictions are to be made with confidence.     

Cognitive Assessment of Students' Problem Solving and Program Development Skills

A cognitive assessment method for measuring students' problem solving and program development abilities, their skills, knowledge, perception, attitudes and motivation toward problem solving and programming is presented. This method takes into consideration the thinking skills required by students learning how to program, as well as specific knowledge related to program development. The goal is to teach students the skills necessary for formulating the problem; planning and designing the solution; implementing and testing the solution; and monitoring and evaluating the solution's progress. This paper discusses this alternative assessment method and the instruments created to evaluate students' work in an introductory course on problem solving and programming.     

Optical gain and absorption of quantum dots measured using an alternative segmented contact method

An alternative segmented-contact method for accurate measurement of the optical gain and absorption of quantum-dot and quantum-dash active materials with small optical gain is reported. The usual error from unguided spontaneous emission is reduced by subtracting signals acquired from three independently controlled sections as opposed to just two found in the conventional technique. The quantum-dot gain spectra are measured to a precision of less than 0.2 cm/sup -1/ at nominal gain values below 2 cm/sup -1/, and gain spectrum of quantum-dash sample is calculated with an error less than 0.3 cm/sup -1/ at a gain less than 1 cm/sup -1/. These accuracies are checked with a self-calibrating method. The internal optical mode loss measurement is also described.