REQUISITE VARIETY IN THE SYSTEMS AND CONTROL SCIENCES

Trends in the systems and control sciences are discussed in relation to certain results in cybernetics. In particular, optimal control theory is reviewed in the light of practical engineering requirements and servomechanism theory is considered in the light of Ashby's law of requisite variety.     

THE EFFECT OF FREQUENCY AND WAVE FORM ON THE ELECTRICAL CONDUCTIVITY-TEMPERATURE PROFILES OF TURNIP TISSUE1

Electrical conductivity (σ) – temperature (T) curves were determined as a function of frequency and wave form of alternating current. Turnip cylinders were sandwiched between titanium electrodes and heated ohmically to 100C using four frequencies (4, 10, 25 and 60 Hz) and three wave forms (square, sine and sawtooth). The σ-T curve shifted upward at 4 Hz for all wave forms. Curses were not significantly different at 10, 25 and 60 Hz for sawtooth and sine wave forms, while those for square waves were not significantly different at any of the frequencies tested. A comparison of wave forms showed that σ-T curves for square waves were shifted downward significantly with respect to sine and sawtooth waves. Heating rate increased with decreasing frequency. Changes in vegetable tissue cellular structure are believed to be responsible for these results. Further research is necessary to characterize these mechanisms.     

DEVELOPMENT OF A QUANTITATIVE VIDEO-BASED VISUALIZATION METHOD TO CHARACTERIZE THE FLOW BEHAVIOR OF FOOD PARTICULATES IN A MODEL CONTINUOUS ASEPTIC STERILIZER3

The objectives of this study were to develop a quantitative visualization tool for evaluating flow behavior of particles in a compartmented aseptic system or other flow systems. The three dimensional movement of polystyrene balls as influenced by ball diameter (0.95 and 1.90 cm), fluid flow rate (10, 20 and 30 L/min) and conveyor disk design (2 configurations) was recorded in a model heating apparatus and analyzed using motion analysis software. Ball speed and net-to-gross-displacement ratio (NGDR) values were calculated for ball movement in the x;y and x;z planes. As carrier liquid flow rate increased, there was an associated increase in both the mean and standard deviation speed and NGDR values. In general, larger ball sizes yielded lower speed and NGDR values (i.e., less movement). A concave (bowl-like) conveyor disk design as opposed to a 90° flat-edge disk design yielded greater speed and NGDR values when carrier velocity was greater than 20 L/min. Speed and NGDR values having higher standard deviations were interpreted as having more compartmental mixing. Furthermore, speed and NGDR mean and standard deviations were highly correlated. The results of this study demonstrated the potential utility of the flow visualization method for quantitating the flow behavior of particles through tubes. Furthermore, this method should be of value to food process engineers in developing continuous aseptic processes for particulate-containing foods.     

EVIDENCE FOR AN ISOBUTYLAMIDE ASSOCIATED WITH HOST-PLANT RESISTANCE TO WESTERN FLOWER THRIPS, <Emphasis Type="Italic">Frankliniella occidentalis</Emphasis>, IN CHRYSANTHEMUM

Bioassay-directed fractionation of extracts of chrysanthemum leaves using a choice test permitted isolation of a fraction that exhibited repellent activity against the western flower thrips (WFT). Analysis of this fraction from cultivars exhibiting varying degrees of host-plant resistance to WFT by high performance liquid chromatography revealed a distinctive peak, the height of which correlated with the degree of resistance of those cultivars to WFT. The peak was attributed to a novel unsaturated isobutylamide, N-isobutyl-(E, E, E, Z)-2,4,10,12-tetradecatetraen-8-ynamide.     

Melt Growth of High-Critical-Temperature Superconducting Fibers

To determine the intrinsic properties of the newly discovered high-critical-temperature oxide superconductors, their single crystals are necessary. Using the float-zone technique with CO₂ laser heating, coarse-grained polycrystalline superconducting La-Sr-Cu-O fibers have been grown directly from the melt of a sintered compact. Cu-O is lost from the liquid and can be compensated by the addition of an excess of CuO in the compact. Superconducting properties (including T c and Meissner effect), X-ray diffraction, and EDAX indicate that the La/Sr stoichiometry is not maintained from the sintered compact to the melt-grown material. Attempts to grow an analogous fiber in the Y-Ba-Cu-O system were unsuccessful because the very high evaporation rates of the constituents precluded the maintenance of a molten zone. The results also indicate that patterning of circuits on thin films of the Y-Ba-Cu-O superconductor can be achieved using laser irradiation.     

Microstructure-Strength Properties in Ceramics: II, Fatigue Relations

The study of crack-size effects in aluminas and other selected ceramics in Part I is here extended to dynamic fatigue properties. Controlled flaws are used to measure the fatigue response in the large-crack (indentation-controlled) and small-crack (microstructure-controlled) regions. It is demonstrated that the "microstructural driving forces" responsible for the R-curve behavior are readily accommodated into existing indentation fracture theories of fatigue strengths. The modified theory provides well-defined solutions for the strengths in terms of stressing rate and indentation load. Two load-invariant quantities, relating to the exponent and coefficient in an assumed power-law crack velocity function, are sufficient to define the entire data set for a given material, at all stressing rates and loads. This is demonstrated graphically by reducing such data sets onto universal fatigue diagrams. The data for sapphire do not coincide with those for the poly crystalline aluminas, suggesting again that it is the grain-boundary structure which holds the key to the fracture properties in the latter. From the standpoint of reliability, the study emphasizes the need to account for microstructural effects when extrapolating to the domain of naturally occurring flaws. In this context, the adjustable quantities obtained from the dynamic fatigue data fits emerge as appropriate design parameters.     

Measurement of Residual Stresses in Coatings on Brittle Substrati by Indentation Fracture

A method for evaluating stresses in coatings on brittle substrates by indentation is described. The basis for evaluations is fracture mechanics model of the radial crack system in the Vickers geometary, incorpeorating the effects of a thin surface demonstrate the methodology. The crack size on these coated specimens are found to be considerebly small than those on uncoated controls, indicating substantial (∼50 MPa) in-plane expansions observed after applying the coatings to already indented sufaces, are found to make an unexpectedly large contribution to the fracture susceptibility. The procedure offers a simple means for quantifying the mechanical integrity of coating configuration for ceramic components.     

Strength and Fatigue Properties of Optical Glass Fibers Containing Microindentation Flaws

The inert strength and dynamic fatigue properties of fused-silica optical fibers are studied using subthreshold indentation flaws, i.e., flaws without radial cracks. These subthreshold properties differ from those obtained in comparative tests on silica rods containing postthreshold indentation flaws in three major respects: (1) the inert strengths are significantly higher than predicted by extrapolation of the postthreshold data; (2) the slopes of the dynamic fatigue plots are likewise greater, indicating a greater susceptibility of the subthreshold flaws to chemical kinetic effects; and (3) the scatter in strengths is wider. These trends reflect the change in mechanical response reported for optical fibers with "natural" flaw populations in going from ordinary to ultra-high-strength regions. Direct observations of the indentation sites up to the point of failure indicate that the property differences can be interpreted in terms of a transition from propagation-controlled to initiation-controlled fracture instabilities at reduced contact loads. The subthreshold instability condition is modeled qualitatively as a two-step, deformation-fracture process, with strong emphasis on the importance of residual stress fields in parametric evaluations. The relevance of the results to the practical issue of fiber reliability, most notably in connection with the potential dangers of using macroscopic crack velocity data to predict long-lifetime characteristics, is addressed.