Molecular charge distribution of CO

The difference electron density of CO is studied by comparison of several calculations. It is shown that the Hartree-Fock-Slater and Hartree-Fock methods yield equally good charge-distributions and that the use of minimal basis sets should be avoided

Crystal structure and charge distribution of pyrazine: effects of extinction, thermal diffuse scattering and series termination

The crystal structure and electronic charge distribution of pyrazine (1,4-diazabenzene) has been determined at 184 K by X-ray methods. The structural results of Wheatley [Acta Cryst. (1957), 10, 182-187] have been confirmed. A clear indication of bonding effects is obtained. Neither positional and thermal parameters nor difference-Fourier maps are affected by extinction. The effect of thermal diffuse scattering (TDS) on positional parameters is also negligible. However, after correction for TDS, thermal parameters increase significantly. The difference-Fourier map is influenced by TDS as well as the inclusion of high-order Fourier terms

Note on the relation between the compressive strength of debased alumina and its use as hot-pressing die material

Durability of alumina ceramics when used as die material in compressive loading varies widely. The compressive strength of various alumina ceramics was therefore determined as a function of temperature. The results are discussed in terms of the microstructure and fracture morphology. The (hot-pressing) durability of the materials does not correlate at all with the compressive strength data obtained. Instead, this property seems to be determined entirely by the homogeneity and lack of flaws in the ceramic

Load-depth sensing of isotropic, linear viscoelastic materials using rigid axisymmetric indenters

An indentation experiment involves five variables: indenter shape, material behavior of the substrate, contact size, applied load and indentation depth. Only three variable are known afterwards, namely, indenter shape, plus load and depth as function of time. As the contact size is not measured and the determination of the material properties is the very aim of the test; two equations are needed to obtain a mathematically solvable system. For elastic materials, the contact size can always be eliminated once and for all in favor of the depth; a single relation between load, depth and material properties remains with the latter variable as unknown. For viscoelastic materials where hereditary integrals model the constitutive behavior, the relation between depth and contact size usually depends also on the (time-dependent) properties of the material. Solving the inverse problem, i.e., determining the material properties from the experimental data, therefore needs both equations. Extending Sneddon's analysis of the indentation problem for elastic materials to include viscoelastic materials, the two equations mentioned above are derived. To find the time dependence of the material properties the feasibility of Golden and Graham's method of decomposing hereditary integrals (J.M. Golden and G.A.C. Graham. Boundary value problems in linear viscoelasticity, Springer, 1988) is investigated and applied to a single load-unload process and to sinusoidally driven stationary state indentation processes.Comment: 116 pages, 29 figure

Small specimen compact tension testing of ceramics

The small size compact tensile specimen is useful for measurement of the fracture toughness of ceramics. The tabular data available for the compliance factor are accurately fitted by a polynominal expression. The elimination of residual stress is important for accurate measurement of the fracture toughness. [on SciFinder (R)

Weakest-link failure prediction for ceramics IV : application of mixed-mode fracture criteria for multiaxial loading

Strength data obtained from uniaxial and biaxial bend tests on 2 alumina ceramics were analyzed by a weakest-link model with different mixed-mode fracture criteria and crack shapes. Taking the data from 3- and 4-point bend tests as a ref., strength predictions for the ball-on-ring and ring-on-ring biaxial tests were made, showing large differences in the predictions for the different fracture criteria. The best fitting models for the 2 aluminas cannot be interchanged, thus showing a marked difference in their shear stress sensitivity. Anal. shows that, in general, a combination of tests with a varying degree of stress multiaxiality is required to det. the best applicable mixed-mode fracture criterion. [on SciFinder (R)