UTILIZATION OF TOOLS FOR SUCCESSFUL. TROUBLESHOOTING OF STRUCTURAL PROBLEMS

A structural failure problem was solved using an integrated and iterative program of testing and analysis. The steps taken in solving the problem were: analytical calculations; operational testing; qualifications of analytical results; problem identification; design of corrective action; and confirmatory testing.     

A branch-and-cut algorithm for the equicut problem

We describe an algorithm for solving the equicut problem on complete graphs. The core of the algorithm is a cutting-plane procedure that exploits a subset of the linear inequalities defining the convex hull of the incidence vectors of the edge sets that define an equicut. The cuts are generated by several separation procedures that will be described in the paper. Whenever the cutting-plane procedure does not terminate with an optimal solution, the algorithm uses a branch-and-cut strategy. We also describe the implementation of the algorithm and the interface with the LP solver. Finally, we report on computational results on dense instances with sizes up to 100 nodes.     

Suppression of antiferromagnetic interactions through Cu vacancies in Mn-substituted CuInSe2 chalcopyrites

Stoichiometric and Cu-poor Cu(0.95-x)Mn(0.05)InSe(2) (x = 0-0.20) compounds were synthesized by high-temperature, solid-state reactions. The presence of copper vacancies is revealed by Rietveld refinements of combined neutron and x-ray powder diffraction data. The antiferromagnetic interaction is depressed by the copper deficiency, which may be explained as the competition between the antiferromagnetic Mn-Se-Mn superexchange interaction and the hole-mediated ferromagnetic exchange induced by the copper vacancy. The introduction of copper vacancies is proposed to be a viable route to impart carrier-mediated ferromagnetic exchange in the chalcopyrite-based dilute magnetic semiconductors.     

Vortex lattice transitions in YNi2B2C

We have performed extensive small-angle neutron scattering (SANS) diffraction studies of the vortex lattice in single crystal YNi₂B₂C for B‖c. High-resolution SANS, combined with a field-oscillation vortex lattice preparation technique, allows us to separate Bragg scattered intensities from two orthogonal domains and accurately determine the unit cell angle, β. The data suggest that upon increasing field there is a finite transition width where both low- and high-field distorted hexagonal vortex lattice phases, mutually rotated by 45°, coexist. The smooth variation of diffracted intensity from each phase through the transition corresponds to a redistribution of populations between the two types of domains.     

Flow phenomena in floppy tubes

Collapsible tubes, which occur all over the body, have unique properties from the point of view of both physics and physiology. A brief review is attempted of first the basic observable properties, followed by simple theory to explain the steady-flow aspects and an overview of the somewhat more complex theories for unsteady flow, in particular the flow-induced oscillations. The experimental evidence from laboratory studies is reviewed with particular emphasis on the dynamical system aspects. A final section looks at the current position and prospects.     

Magnetism and superconductivity in ErNi2B2C

We have performed a series of neutron diffraction experiments from the magnetic order and the vortex lattice in single crystal ErNi₂B₂C. The incommensurate magnetic structure develops additional even harmonics below the ‘ferromagnetic’ ordering temperature, T _F of 2.3 K. This feature and the existence of rods of diffuse scattering suggest the development of ferromagnetic microdomain walls. The magnetic structure is very sensitive to the application of a magnetic field with changes in modulation vector and harmonic content. Studies of the vortex lattice show the presence of a 45° reorientation transition and a distorted hexagonal to square transition as a function of applied field. Further distortions of the vortex lattice occur at T _N, but no changes are seen at T _F.