Showing papers by "Northwestern University published in 1983"

Crack band theory for fracture of concrete

Abstract: A fracture theory for a heterogenous aggregate material which exhibits a gradual strain-softening due to microcracking and contains aggregate pieces that are not necessarily small compared to structural dimensions is developed. Only Mode I is considered. The fracture is modeled as a blunt smeard crack band, which is justified by the random nature of the microstructure. Simple triaxial stress-strain relations which model the strain-softening and describe the effect of gradual microcracking in the crack band are derived. It is shown that it is easier to use compliance rather than stiffness matrices and that it suffices to adjust a single diagonal term of the complicance matrix. The limiting case of this matrix for complete (continuous) cracking is shown to be identical to the inverse of the well-known stiffness matrix for a perfectly cracked material. The material fracture properties are characterized by only three parameters—fracture energy, uniaxial strength limit and width of the crack band (fracture process zone), while the strain-softening modulus is a function of these parameters. A method of determining the fracture energy from measured complete stres-strain relations is also given. Triaxial stress effects on fracture can be taken into account. The theory is verified by comparisons with numerous experimental data from the literature. Satisfactory fits of maximum load data as well as resistance curves are achieved and values of the three material parameters involved, namely the fracture energy, the strength, and the width of crack band front, are determined from test data. The optimum value of the latter width is found to be about 3 aggregate sizes, which is also justified as the minimum acceptable for a homogeneous continuum modeling. The method of implementing the theory in a finite element code is also indicated, and rules for achieving objectivity of results with regard to the analyst's choice of element size are given. Finally, a simple formula is derived to predict from the tensile strength and aggregate size the fracture energy, as well as the strain-softening modulus. A statistical analysis of the errors reveals a drastic improvement compared to the linear fracture theory as well as the strength theory. The applicability of fracture mechanics to concrete is thus solidly established.

Schema Theory and ESL Reading Pedagogy.

Abstract: This article discusses the important role of background knowledge in a psycholinguistic model of EFL/ESL reading and demonstrates the relevance of schema-theoretic views of reading to the teaching of reading to EFL/ESL students. According to schema theory, reading comprehension is an interactive process between the text and the reader's prior background knowledge (Adams and Collins 1979, Rumelhart 1980). Reading comprehension involves one's knowledge of the world, which may be culturally based and culturally biased. Classroom implications of the schema-theoretic view of reading for EFL/ESL reading pedagogy are discussed, with techniques suggested for bringing about reader-centered EFL/ESL reading.

Evaluation and treatment of swallowing disorders

Abstract: Evaluation and treatment of swallowing disorders / , Evaluation and treatment of swallowing disorders / , کتابخانه دیجیتال جندی شاپور اهواز

Carcinogenesis by Hepatic Peroxisome Proliferators: Evaluation of the Risk of Hypolipidemic Drugs and Industrial Plasticizers to Humans

Abstract: In this critical review, I would like to provide a brief outline of the morphology, biochemical composition, distribution, and functions of peroxisomes. The induction of peroxisome proliferation and peroxisome-associated enzymes in the rodent liver by two classes of chemicals (hypolipidemic drugs and the industrial plasticizers) will be considered. The role of peroxisomes in lipid metabolism will be discussed. Carcinogenicity studies in rats and mice with these peroxisome proliferators will be evaluated critically. Careful consideration will be given to the hypothesis that "potent hepatic peroxisome proliferators as a class are carcinogenic." The possible mechanism(s) by which peroxisome proliferators induce liver tumors will be outlined. Particular attention will be paid to the possible role of peroxisome proliferation-mediated radical toxicity and generation of endogenous initiators of carcinogenesis.

Cognitive Processes in the Revision of Stereotypic Beliefs

Abstract: Northwestern University Stereotypes have often been assumed to be resistant to change, although some evidence indicates that they are responsive to new information. The present studies examine stereotype change using three models: the bookkeeping model, in which each instance of stereotype-relevant information is used to modify the stereotype gradually; the conversion model, in which stereotypes change radically in response to dramatic or salient instances; and the subtyping model, in which new Stereotypic structures are developed to accommodate instances not easily assimilated by existing stereotypes. The models predict different response patterns as a function of variations in the pattern of stereotype-in consistent evidence and the number of instances encountered. In Experiment 1, subjects were given information about either a small or a large sample of group members in which stereotype-inconsistent evidence was dispersed across many members or concentrated within a few members. Results generally supported the subtyping model when evidence was concentrated and the bookkeeping model when evidence was dispersed. Experiment 3 suggested that the development of subtypes occurs because dramatically inconsistent individuals are seen as unrepresentat ive of the group as a whole. Although the present studies generally favored the subtyping and bookkeeping models, conditions under which the conversion model may operate are considered.

Instabilities of dynamic thermocapillary liquid layers. I Convective instabilities

Abstract: A steady shear flow is set up in a planar liquid layer bounded below by a rigid plate, and above by an interface with a passive gas, by imposing a temperature gradient along the layer and driving the motion by thermocapillarity. The thermal-convective instabilities to which this dynamic state is susceptible include stationary longitudinal rolls involving the classical Marangoni instability, and unsteady hydrothermal waves, whose new instability mechanism derives its energy from the horizontal temperature gradients. The thermal stability characteristics for liquid layers with and without return flow profiles are presently given as functions of the Prandtl number for the liquid and the Biot number for the interface, and comparisons are made with available experimental observations.

Noise in homodyne and heterodyne detection.

Abstract: Quantum-mechanical calculations of the mean-square fluctuation spectra in optical homodyning and heterodyning are made for arbitrary input and local-oscillator quantum states. In addition to the unavoidable quantum fluctuations, it is shown that excess noise from the local oscillator always affects homodyning and, when it is broadband, also heterodyning. Both the quantum and the excess noise of the local oscillator can be eliminated by coherent subtraction of the two outputs of a 50-50 beam splitter. This result also demonstrates the fact that the basic quantum noise in homodyning and heterodyning is signal quantum fluctuation, not local-oscillator shot noise.

Overall moduli of solids with microcracks: Load-induced anisotropy

Abstract: For a linearly elastic brittle solid containing microcracks that may be closed or may undergo frictional sliding, a general method is developed for estimating the overall instantaneous moduli which depend on the loading conditions. When the cracks are all open and when they are randomly distributed, then the overall response is isotropic. The moduli for this case have been obtained by B udiansky and O'C onnell (1976). On the other hand, when some cracks close, and when some closed cracks undergo frictional sliding, then the overall response becomes anisotropic and dependent on the loading conditions, as well as on the loading path. The self-consistent method is used to estimate the overall moduli. The effects of crack closure and loadinduced anisotropy are included. Several illustrative examples are worked out, showing the important influence of the load path on the overall response when crack closure and frictional sliding are involved.

Melatonin is a potent modulator of dopamine release in the retina

Abstract: Melatonin, a hormone originally discovered in the pineal gland1, has also been found in the retina of several vertebrate species2–5. The enzyme system for melatonin synthesis also exists in the retina2,3,6–8, where the activity of one such enzyme, (serotonin N-acetyltransferase) varies with changes in light intensity in a circadian pattern3,8. As the activity of dopamine containing amacrine neurones of the retina is influenced by changes in illumination9–12 it was of interest to determine the effect of melatonin and its precursors, serotonin and N-acetylserotonin, on the release of 3H-dopamine from rabbit retina. I report here that picomolar concentrations of melatonin (IC50 9pM) selectively inhibited the calcium-dependent release of 3H-dopamine from rabbit retina, but not from striatum. Melatonin, was 1,000 times more potent than its precursor N-acetylserotonin in inhibiting the release of 3H-dopamine in retina, while the putative neurotransmitter serotonin13, was inactive. It is suggested that the light-dependent production of melatonin could play a physiological role in modulating the activity of dopamine-containing neurones in the retina.

Toward understanding strategic issue diagnosis

Abstract: This paper calls attention to a central but neglected process in strategic decision making, i.e. strategic issue diagnosis (SID). A framework for discussing SID is presented in terms of three critical components: inputs, process characteristics and outputs. The framework is illustrated in the context of PIMS and BCG, two widely recognized strategy models. The major implications highlight the theoretical significance of SID for understanding strategic decision making.

Allergic Aspergillus sinusitis-a newly recognized form of sinusitis-

Abstract: The clinical and pathologic features of seven cases of a newly recognized form of chronic sinusitis are described. Most patients were young adults with a history of asthma, and all had chronic nasal polyps. Radiographically, there was opacification of multiple sinuses. Recurrent sinusitis was common, and several patients underwent numerous surgical drainage procedures. Histologically, a distinct mucinous material containing eosinophils, Charcot-Leyden crystals, and fungal hyphae was found in tissue resected from the sinuses. We believe that these findings constitute a distinct clinicopathologic entity that we term allergic Aspergillus sinusitis. This condition shares similar histopathologic features with allergic bronchopulmonary aspergillosis (ABPA) but affects the paranasal sinuses rather than the lung. Implications for therapy of this form of sinusitis and its possible relationship to allergic lung diseases are discussed.

Discrete choice theory, information theory and the multinomial logit and gravity models

Abstract: The strong “similarity” between “information minimizing” and “utility maximizing” models of spatial interaction has been known for some time (see Anas 1975, Williams, 1977), but the extent of this “similarity” has been underestimated. This paper proves that the two approaches are identical in that the multinomial logit model can be derived and identically estimated by either method. It is also proved that the doubly-constrained gravity model derived by Wilson (1967) is identical to a multinomial logit model of joint origin-destination choice, consistent with stochastic utility maximization. It follows that behaviorally valid “gravity models” can be estimated from disaggregated data on individual choices. In closure, “behavioral demand modeling”, which follows McFadden (1973), and “entropy-maximizing modeling”, which follows Wilson (1967), should be seen as two equivalent views of the same problem. The behavioral content of models estimated by either approach is entirely determined by the model specification and data aggregation beliefs of the analysts, and not by any inherent structural property of the models themselves.

Age dependence of oceanic intraplate seismicity and implications for lithospheric evolution

Abstract: The present investigation is concerned with the determination of the depths for 16 oceanic intraplate earthquakes using body wave modeling. It is found that oceanic intraplate earthquake depths show a clear deepening of the maximum focal depth with increasing lithospheric age. The thickness of the lithosphere in which oceanic intraplate earthquakes occur is approximately equal to or slightly greater than the flexural elastic thickness, but is much less than the seismic thickness determined from surface wave dispersion. Oceanic intraplate earthquakes are found to occur at greater depths and temperatures than has been previously observed for continental crustal events.

Barriers to the Progress of Women and Minority Faculty

Abstract: (1983). Barriers to the Progress of Women and Minority Faculty. The Journal of Higher Education: Vol. 54, No. 2, pp. 123-144.

Binding energy and electronic structure of small copper particles

Abstract: The equilibrium geometry, binding energy, and electronic structure of small metal particles are investigated using self-consistent one-electron local-density theory. Results for ${\mathrm{Cu}}_{2}$, ${\mathrm{Cu}}_{4}$, and fcc ${\mathrm{Cu}}_{13}$ and ${\mathrm{Cu}}_{79}$ clusters show an increasing equilibrium bond length with cluster size, and a stiffening of the ${a}_{1}$ vibrational force constants. The calculated binding energies of 1.05 (${\mathrm{Cu}}_{2}$), 1.26 (${\mathrm{Cu}}_{4}$), 2.19 (${\mathrm{Cu}}_{13}$), and 3.03 (${\mathrm{Cu}}_{79}$) eV/atom compare well with the experimental values of 1.00 (${\mathrm{Cu}}_{2}$) and 3.50 (bulk) eV/atom. For ${\mathrm{Cu}}_{2}$ the theoretical bond length and vibrational frequency are found to be in good agreement with experiment. Densities of states and core-level shifts are analyzed to display cluster-size effects. Charge-density maps are used to display the buildup of metallic bonding charge with increasing particle size.

Quantitative genetics of development: genetic correlations among age-specific trait values and the evolution of ontogeny.

Abstract: It has recently been re-emphasized, by Gould (1977) and others, that evolution of adult forms takes place through the evolution of ontogenies. Direct selection at any age results in a correlated response at all other life history stages. This selection alters growth curves, or causes the evolution of ontogeny. Lande (1982) has presented a quantitative genetic model for the evolution of ontogenetically varying traits which clearly defines the parameters required for an analysis of ontogeny and evolution: (1) the genetic variance/covariance matrix; (2) the phentoypic variance/ covariance matrix; and (3) the vector of selection differentials. Genetic covariances or correlations among age-specific trait values quantitatively describe the genetic link between expressions of the same trait at different points in ontogeny. These genetic links between age-specific trait values are the result of pleiotropy, in this case the effects of one gene on the phenotype as expressed at more than one age, and linkage disequilibrium. The general effects of genetic variation on growth curves may be summarized using two parameters, curve height and curve shape (see Fig. 1). The average height of the curve can be defined as the average value of a trait over the ontogenetic period under consideration or the area under the growth curve divided by the length of the growth period. When there are only height differences between two growthi curves, the two curves will be parallel. Differences in curve shape, or the rate of growth at all ages, can be quantified independent of height, by standardizing the curves so that they have the same average height, or enclose the same area, and then calculating the absolute value of the area remaining between curves and dividing by the length of the growth period (see Fig. 1). This curve shape measure does not specify particular shapes but rather gives a general measure of differences in growth rates to compare with the independent measure of curve height differences. Specific curve shapes are presented without summarization. When heritabilities and phenotypic variances are the same at all ages, one can discern the independent effects of genetic correlations among age-specific trait values on ontogenetic variation and evolution. For example, in Figure 2 the ancestral ontogeny is displayed as a horizontal line (A) for purposes of illustration. Selection on the adult, or any other age, when the genetic correlations between ages are all 1.000, results in the descendant ontogeny, curve B. Note that the average height of the curve has changed but not its shape. When all the genetic correlations are one, the spectral decomposition of the age-specific traits correlation matrix includes only one non-zero eigenvalue equal to 'n,' the number of traits represented in the matrix, and an associated isometric eigenvector with loadings of 1/\Vn for each trait. This vector measures variation in growth curve height independent of variation in curve shape. Any deviation of the correlations

Pedestrian Flow and Level of Service

Abstract: This study analyzed properties and characteristics of pedestrian flow on sidewalks. Data were collected in the central business district of Haifa, Israel, with the aid of a videotape recorder and a digital clock. Walking speeds for men were found to be significantly greater than speeds for women; all speeds were found to be inversely related to densities. One‐ and three‐regime linear speed‐density regression models were calibrated and evaluated, and reasoning is given to support the adoption of the three regime model for speed predictions. Level of service definitions are proposed based on these analyses, and suggestions for their use in planning and design of pedestrian walkways are made.

On Flame Stretch

Abstract: A general invariant expression is derived for the stretch experienced by a flame due either to its motion or to the nonuniform flow of the gas through it. This expression is given in terms of the local fluid velocity and the shape of the flame front. Specific examples in which the flame stretch takes a simplified form are discussed. Some remarks are made regarding the relation between the three distinct properties of flames: stretch, speed and temperature.