Torsion of perforated cores: Planar formulation

A planar finite element approximate solution for the torsional analysis of symmetric perforated cores is given. The solution involves neglecting the out-of-plane stiffness of the walls of the core. Only one-quarter of the core is analyzed, using super elements assembled from high order plane stress finite elements. The results of the analysis are compared with the results from three-dimensional finite element analysis. It is observed that for almost all realistic designs the planar approximation is justified. The solution is very close with significant reduction in computation time and data preparation effort.     

Pup and object carrying by maternally and nonmaternally behaving female albino rats (Rattus norvegicus).

Pups and toys (rubber dropper-bulbs) were presented to 48 nulliparous and 10 lactating female albino Sprague-Dawley rats either together (choice test) or in separate tests. Regardless of their reproductive state, Ss fell into 3 categories according to their behavior: (a) Postpartum Ss as well as virgins that behaved maternally picked up pups much faster than toys, carried more pups than toys, gathered young into the nest, but scattered toys outside the nest site. (b) Virgins that carried young spontaneously but did not show any other elements of maternal behavior picked up pups and toys after equal latencies, carried pups and toys in equal numbers, and scattered both the pups and the toys about the floor of the cage. (c) Virgins that ignored pups did not carry young, but they did show high levels of toy carrying and dispersed the toys about the cage. It is proposed that the term retrieving should be limited, by its definition, to those cases in which the objects are carried to a specific location. Thus, pup retrieval was seen exclusively in Ss that showed maternal behavior (i.e., crouching, pup licking), because only these Ss carried pups consistently to the nest. On the other hand, pup carrying shown by nonmaternally behaving Ss and toy carrying shown by all Ss are both cases of scattered, nondirected object carrying, rather than retrieving. (13 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Natural and accelerated weathering of PVC and polypropylene waste-water pipes

Plastic pipes and plumbing accessories for waste water are gaining popularity and replace the more conventional metal and asbestos-cement products. The main disadvantage of these pipes is their relative poor durability when exposed to external weathering conditions, particularly to the UV solar radiation. The work presented deals with the weathering behaviour of polyvinylchloride (PVC) and polypropylene (PP) pipes. Commerical pipes were exposed to natural and artificial-accelerated conditions and their mechanical properties tested after various periods of exposure. The results show that both the PVC and PP pipes are damaged by the accelerated and natural weathering within a short time, resulting in a rapid deterioration in the ultimate elongation. The drop in the extensibility of the PVC pipe is more gradual than that of the PP. The tensile strength, in both cases, is hardly affected—as expected—proving that the deterioration is mainly a surface phenomenon and depends on the cracking of the exposed surface.     

Measurement of the complex permittivity of low loss polymer powders in the millimeter-wave range.

An improved measurement method of complex permittivity of low loss polymer powders is suggested. The measurements are done in the mm-wave range using a quasi optical resonator. The 2-D corrugated mode exciter is employed to improve suppression of undesirable higher modes. The model used for reconstructing complex permittivity takes into account ohm losses of metal mesh coupling that provide better accuracy of the reconstructing procedure. An example illustrating this method is reported.     

Fracture toughness of two-dimensional cellular material with periodic microstructure

The brittle fracture behavior of periodic 2D cellular material weakened by a system of non-interacting cracks is investigated. The material is represented as a lattice consisting of rigidly connected Euler beams which can fail when the skin stress approaches some limiting value. The conventional Mode I and Mode II fracture toughness is calculated first and its dependence upon the relative density is examined. To this end the problem of a sufficiently long finite length crack in an infinite lattice produced by several broken beams is considered. It is solved analytically by means of the discrete Fourier transform reducing the initial problem for unbounded domain to the analysis of a finite repetitive module in the transform space. Four different layouts are considered: kagome, triangular, square and hexagon honeycombs. The results are obtained for different crack types dictated by the microstructure symmetry of the specific material. The obtained results allowed to define the directional fracture toughness characterizing the strength of a material with many cracks for the given tensile loading direction. This quantity is presented in the form of polar diagrams. For all considered layouts the diagrams are found to be close to circles thus emphasizing quasi-isotropic fracture behavior. The deviation from isotropy in the case of a square honeycomb is essentially less than for the corresponding published axial stiffness polar diagram.     

The efficiency range of economical cutting conditions and tool replacement under the age replacement strategy

This paper presents models for calculating the optimal cutting feed rate, spindle speed, and age of preventive tool replacement for a standalone cutting machine. The optimal cutting conditions are determined and analysed for three different objective functions: minimum expected cycle time, minimum expected cost per unit, and maximum expected profit-rate, under the Age Replacement Strategy (ARS) and assuming that the tool-life distribution function is Normal. We show that the first two objective functions are separable, and present an efficient, one-dimensional search procedure for the optimization. A definition of the efficiency ranges of feed rate and standard age of tool replacement is suggested for the ARS, which improves the efficiency range defined in the literature for the Failure Replacement Strategy (FRS).     

Model‐based optimization of hydrogen generation by methane steam reforming in autothermal packed‐bed membrane reformer

An autothermal membrane reformer comprising two separated compartments, a methane oxidation catalytic bed and a methane steam reforming bed, which hosts hydrogen separation membranes, is optimized for hydrogen production by steam reforming of methane to power a polymer electrolyte membrane fuel cell (PEMFC) stack. Capitalizing on recent experimental demonstrations of hydrogen production in such a reactor, we develop here an appropriate model, validate it with experimental data and then use it for the hydrogen generation optimization in terms of the reformer efficiency and power output. The optimized reformer, with adequate hydrogen separation area, optimized exothermic‐to‐endothermic feed ratio and reduced heat losses, is shown to be capable to fuel kW‐range PEMFC stacks, with a methane‐to‐hydrogen conversion efficiency of up to 0.8. This is expected to provide an overall methane‐to‐electric power efficiency of a combined reformer‐fuel cell unit of ～0.5. Recycling of steam reforming effluent to the oxidation bed for combustion of unreacted and unseparated compounds is expected to provide an additional efficiency gain. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Solar heating using common building elements as passive systems

The paper reviews the first steps of a study on use of windows as passive solar air collectors, offsetting naturally the excess of heat in the thermal mass of the building itself, and of vertical solar collectors, with air as working fluid, and with storage systems designed as intergral parts of the building, incorporated in the concrete elements. Some examples of architectural solutions combining these principles in a modular design are suggested. Incorporated storage examples using ceilings and partitions with appropriate air transfer through them are proposed. The relevant thermal analysis on the use of windows as passive solar collectors is based on the Total Thermal Time Constant (TTTC) Method, developed by two of the authors. An analysis is also presented for a vertical flat-plate solar collector using air as working fluid and capable of integration in a blank (windowless) part of an external wall. Design and dimensioning of the fin surface are proposed for heat transfer from collector surface to fluid. The final section deals with experimental model based on the above principles and combining a vertical collector and heat storage for use in daytime and at night, respectively. The model was so dimensioned as to represent a 1:1 unit in modular building design. An overall thermal efficiency of about 18 per cent was obtained.