Showing papers by "Lehigh University published in 1977"

A Set of Measures of Centrality Based on Betweenness

Abstract: A family of new measures of point and graph centrality based on early intuitions of Bavelas (1948) is introduced. These measures define centrality in terms of the degree to which a point falls on the shortest path between others and there fore has a potential for control of communication. They may be used to index centrality in any large or small network of symmetrical relations, whether connected or unconnected.

Gaseous hydrogen embrittlement of high strength steels

Abstract: The kinetics of sustained-load subcritical crack growth in hydrogen were determined for 18Ni(200) and 18Ni(250) maraging steels over a range of hydrogen pressures and temperatures. Crack growth in each steel was characterized by an apparent threshold stress intensity, a domain where the growth rate increased sharply with stress intensity (K) (Stage I), and a range where the growth rate was independent ofK (Stage II). The rate-limited Stage II crack growth in these steels exhibited three distinct regions of temperature dependency, with a different isothermal pressure dependence in each region. In the low temperature region, Stage II crack growth was thermally activated with δH = 18.2 ±1.7 kj/mol; (δH being independent of hydrogen pressure and yield strength). The growth rates at a givenK were proportional to the square root of hydrogen pressure. In the intermediate temperature region, Stage II growth rates increased at slower rates, passed through a maximum and then decreased with increasing temperature. Within this region, the pressure dependence for crack growth increased from 1/2-power to 2.0-power with increasing temperature. Above a transition temperature, each grade of maraging steel became essentially immune to gaseous hydrogen embrittlement for the hydrogen pressure range considered. The transition temperature was strongly affected by yield strength and hydrogen pressure. Plausible explanations for these phenomenological results are considered.

Water at interfaces: Molecular structure and dynamics

Abstract: Recent advances in molecular mechanics and dynamics of water on inorganic surfaces are reviewed. The structures of hydroxylated surfaces and water-hydroxyl adducts, determined by LEED-Auger studies and infrared spectroscopy, are described to a detail only recently resolved. New mechanistic concepts of nucleation and freezing have ensued from the analysis of time-correlation functions, showing that low-frequency angular perturbations dominate the different behavior of water in bulk phases and at surfaces. While the rotational motion of water molecules in the liquid phase can be interpreted as rotation modulated by making and breaking hydrogen bonds with the neighbors, water adsorbed on nucleating catalysis undergoes an irreversible and complete reorientation in a fraction of the rotational period.

On the generality of discontinuous fatigue crack growth in glassy polymers

Abstract: Fatigue fracture surface characteristics of five commercially available amorphous polymers [poly(methylmethacrylate) (PMMA), polycarbonate (PC), poly(vinyl chloride) (PVC), polystyrene (PS), and polysulphone (PSF)] as well as bulk-polymerized PMMA prepared over a wide range of molecular weights were studied to determine if common mechanisms of fatigue crack propagation prevail among these glassy polymers. In those polymers with viscosity-average molecular weight ¯Mv≲2×105, the macroscopic appearance of the fracture surface showed the presence of a highly reflective mirror-like region which formed at low values of stress intensity and high cyclic test frequencies (∼100 Hz). The microscopic appearance of this region revealed that many parallel bands exist oriented perpendicular to the direction of crack growth and that the bands increase in size with ΔK. In all instances, the crack front advanced discontinuously in increments equal to the band width after remaining stationary for hundreds of fatigue cycles. Electron fractographic studies verified the discontinuous nature of crack extension through a craze which developed continuously with the load fluctuations. By equating the band size to the Dugdale plastic zone dimension ahead of the crack, a relatively constant yield strength was inferred which agreed well with reported craze stress values for each material. At higher stress intensity levels in all polymers and all values of ¯Mv, another series of parallel bands were observed. These were also oriented perpendicular to the direction of crack growth and likewise increased in size with the range in stress intensity factor, ΔK. Each band corresponded to the incremental advance of the crack during one load cycle, indicating these markings to be classical fatigue striations.

Castor-oil-based interpenetrating polymer networks: Synthesis and characterization

Abstract: Castor oil was polymerized and crosslinked with sulfur or diisocyanates to form the vulcanized and urethane derivatives, respectively. Both types were swollen with a plastic-forming monomer plus crosslinker, and a second polymerization was carried out in situ. Polyblends were also made by emulsion polymerization of styrene and methyl methacrylate employing hydrolyzed castor oil as the soap. In all three polymerizations, a wide range of compositions was obtained. The resulting interpenetrating polymer networks were characterized using electron microscopy, modulus–temperature measurements, and stress–strain analysis. The polystyrene phase size of the castor oil–urethane/polystyrene IPN was shown to decrease with increased crosslinking of the castor oil component and with increased polystyrene contents. The modulus–temperature study showed two distinct glass transitions in all cases, with evidence of significant mixing of the two components in many cases. The stress–strain results show that some of the IPN's behave as reinforced, highly extensible elastomers at low polystyrene levels, and a rubber-toughened plastics at high levels of polystyrene or crosslinking.

Tests of fabricated tubular columns.

Abstract: Described herein are tests of 10 long fabricated tubular steel columns, of relatively large diameter, with essentially pine-ended conditions. In addition to the behavior, strength, and the manner of failure of the long columns, a number of stub column tests were also made along with the measurements of residual stresses. These residual stresses are both circumferential, due to formation of a flat plate into a cylindrical shape; and longitudinal, resulting from the welding of longitudinal seam needed to complete the fabricated cylinder. Measurement of these stresses is considered essential to any theoretical analysis of column behavior. The maximum strength of the 10 long columns tested (at diameter-to-thickness ratios of 48 and 70 and with a range of slenderness ratio between 39 to 83) is compared with the CRC column curve and good agreement is generally observed.

The problem of internal and edge cracks in an orthotropic strip

Abstract: The plane elastostatic problem of internal and edge cracks in an infinite orthotropic strip is considered. The problems for the material types I and II are formulated in terms of singular integral equations. For the symmetric case the stress intensity factors are calculated and are compared with the isotropic results. The results show that because of the dependence of the Fredholm kernels on the elastic constants, unlike the crack problem for an infinite plane, in the strip the stress intensity factors are dependent on the elastic constants and are generally different from the corresponding isotropic results.

Fatigue crack propagation in poly(methyl methacrylate): Effect of molecular weight and internal plasticization

Abstract: In spite of the importance of fatigue behavior in engineering plastics, relatively few fundamental studies have been made of the effects of polymer structure, molecular weight, composition, and morphology on fatigue crack propagation (FCP). As, part of a broad program for the study of such effects, the role of molecular weight and internal plasticization has been studied in poly(methyl methacrylate) (PMMA) which had been specially prepared and characterized with respect to molecular weight, dynamic mechanical behavior, and, in some cases, stress-strain response. As expected, values of fracture toughness, Kc, varied considerably as the molecular weight was rai ed, from 0.7 MPa, √m at Mv = 1.0 × 105 to 1.1 at Mv, = 4.8 × 106. However, a specific effect of fatigue was noted: over the same range of Kc, values of FCP rate decreased by two orders of magnitude as molecular weight was; increased. It is proposed that this high sensitivity is due to differences in the degree of chain disentanglement effected by the cyclic loading, with consequent differences in the strength of the craze preceding the crack. With PMMA plasticized internally with a low level (10 percent) of n-butyl acrylate (nBA), the FCP rate and Kc, were similar to those of controls, with very high rates shown. At higher nBA levels (up to 30 percent), the sensitivity of FCP rate to stress intensity factor range decreased considerably, Kc, increased by 30 percent and the pre-exponential constant in the growth rate law increased. Plasticization weakens the polymer but at high degrees leads to enough hysteretic heating to induce local creep and crack blunting.

Determination of the Fe-rich portion of the Fe-Ni-C phase diagram

Abstract: The iron-rich portion of the Fe-Ni-C phase diagram has been determined in the composi-tion range from 0 to 20 wt pct Ni and 0 to 6.67 wt pct C for four temperatures, 773, 873, 923 and 1003 K. Long term heat treatments were used to grow the ferrite plus austenite assemblages, while slow cooling heat treatments (25 K/h) were used to grow the metal plus carbide assemblages. Other types of heat treatments produced metal plus graphite. The two phase tie-lines and three phase tie-triangles were measured using electron mi-croprobe techniques. In samples where bulk equilibration had not been achieved, tie-lines were obtained by using extrapolated interface compositions, on the assumption of local equilibrium at the interface. The tie-lines lie at higher Ni contents than the equilibrium tie-line through the bulk composition. The tie-line shift was required to produce match-ing growth rates of Ni and C for the carbides. The addition of Ni slightly reduces the solubility of carbon in austenite and decreases the stability of the carbide phase. In addi-tion, the carbide is always Ni-poor relative to the coexisting metal phase(s).

A semiempirical derivation of phase domain size in interpenetrating polymer networks

Abstract: In previous papers, interpenetrating polymer networks were shown to display a cellular structure. The phase domain size of polymer II was shown to depend inversely on the crosslink density of polymer I. The present paper presents a semiempirical derivation of equations which show quantitatively the dependence of the phase domain size of polymer II on the crosslinking density of polymer I, and also on the interfacial energy and the overall composition. If polymer II is linear, the dependence on the molecular weight of polymer II is also included. The values of the phase domain sizes so estimated are compared with experimental results. While theory and experiment yield good agreement, the semiempirical nature of the equations must be borne in mind.

Characteristics of Cr-SiO2-nSi tunnel diodes☆

Abstract: The admittance of Cr-SiO2-nSi tunnel diodes was measured at 195 and 295 K, from which the surface potential ψs(Va) and the energy distribution of the surface states Nss which communicate with the silicon were determined. By employing these data, the I(Va) characteristics measured at 77, 195 and 295 K are interpreted as tunneling current consisting of two components. The first is a net electron tunnel current from the Si-conduction band through the oxide into the metal which dominates at room temperature for forward bias Va greater than 0.4 V. Introducing a simple model of a trapezoidal SiO2 barrier allows us to calculate the band to band current, resulting in typical values of the barrier height ηo = 0.24 eV and barrier width d o = 24 A . The second component is a net recombination current of electrons from the Si-conduction band into surface states which then tunnel through the oxide into the metal; this component dominates for reverse bias and for small forward bias, especially at low temperatures. It is a current via surface states Nsm which are at the Si-SiO2 interface but rapidly communicate with the metal, and it is therefore recombination controlled. Together, these components explain the measured bias and temperature dependence of the d.c. current.

Close Binary Stars

Abstract: Commission 42 convened for two business meetings and two scientific sessions. In the following, the minutes of the business meetings and a summary of the scientific meetings are presented, with the abstracts of some of the papers read.

Castor oil based interpenetrating polymer networks, IV. Mechanical behavior

Abstract: The physical and mechanical properties of interpenetrating polymer networks (IPN's) based on castor oil-urethane and polystyrene are detailed in this paper. Dynamic mechanical spectroscopy showed extensive but incomplete molecular mixing of the two polymers. The glass transition temperatures of the IPN's gradually merged from two distinct transitions into one broad transition at an intermediate temperature as the crosslink level of the castor oil component was increased. At low polystyrene contents, the IPN's yielded stress-strain behavior similar to reinforced elastomers, but at high polystyrene contents, they exhibited increased elongation. For example, the latter materials showed well developed yield points. Stress whitening and necking were also observed, suggesting a possible failure mechanism by crazing. Cold drawing was exhibited by the materials under tension. The tensile strength and Young's modules were enhanced as the polymer II (polystyrene) content was increased at constant crosslink level of polymer I (castor oil-urethane). A similar effect was also observed by maintaining the polystyrene content constant but increasing the crosslink level of polymer I. The impact strength of the materials ranged from 32.1 to 53.4 N · m/m, which is approximately two to three times that of homopolymer polystyrene. The best materials were those with compositions in the range of 40-46 percent castor oil-urethane. The materials prepared by using tolylene diisocyanate as crosslinker for the castor oil phase had the best impact properties, especially at an NCO/OH ratio of 0.95.

Effects of Grain Size and Microcracking on the Thermal Diffusivity of MgTi2O5

Abstract: The laser flash technique was used to study the effect of grain size on the thermal diffusivity of polycrystalline MgTi2O5 from 25° to 800°C. Microcracking decreased the thermal diffusivity by as much as a factor of two with the decrease increasing with increasing grain size. When specimens were heated then cooled, the thermal diffusivity exhibited a horizontal flat figure-eight hysteresis. This hysteresis, which appeared to be a function of the thermal history, was attributed to a balance between crack healing, or closure, at high temperatures and the growth of existing cracks or the formation of new cracks during cooling.

Continuous Emulsion Polymerization

Abstract: The theories presented can be used for prediction of particle size, particle formation, rate of polymerization, and molecular weight for the emulsion polymerization of styrene in continuous stirred-tank reactors. The incorporation of Stockmayer's modification of the Smith-Ewart theory into Gershberg's continuous reactor model has helped to account for the formation of fewer but larger particles when the reactor is operated at long mean residence times. The use of a different particle generation relation by Nomura and coworkers has led to better agreement between theory and experiment for particle number and polymerization rate at low values of mean, residence time. Their model also fits transient data better than Gershberg's. However, the prediction that free-radical movement into particles is independent of particle size is not very satisfactory. Perhaps the better agreement between theory and experiment results because of the introduction of an additional experimentally determined parameter. Da...

Evaluation of fracture of lafayette street bridge

Abstract: A main girder of the Lafayette Street Bridge over the Mississippi River in St. Paul was discovered to be cracked on May 7, 1975. A detailed examination of the fracture is given in this paper. Fractographic examinations and analyses of crack growth showed that failure crack growth and brittle fracture both contributed to the members' failure. Fatigue cracks were found to originate at large lack of fusion areas in the lateral bracing gusset to transverse stiffener weld. After the crack had nearly penetrated the web thickness, it precipitated a brittle fracture. Subsequent intervals of fatigue crack growth and fracture were also observed. Recommendations are also provided on ways to retrofit the bridge details to prevent reoccurrence at other comparable details. In addition, a suggested detail configuration for this type of connection is recommended for future use. /Author/

Inactivation and Inhibition of Replication of the Enveloped Bacteriophage φ6 by Fatty Acids

Abstract: The enveloped bacteriophage phi6 has been shown to be an interesting model system for the study of chemical agents that might have specific antiviral effects against lipid-containing mammalian viruses In this report, we describe two types of antiviral activity exhibited by several fatty acids against bacteriophage phi6 Oleic acid (18:1) and palmitoleic acid (16:1) were potent inactivators of the virus Treatment with either fatty acid at 50 mug/ml at 25 or 0 degrees C for 30 min reduced the virus titer to about 01% of the initial titer Oleic acid at a concentration as low as 3 mug/ml ( approximately 10(-2) mM) reduced the virus titer to <1% of the initial titer within 30 min Ultracentrifugation analyses of (14)C-amino acid- and (32)P-labeled virus treated with oleic acid indicated that the virion is largely disassembled by the treatment Myristic acid (14:0) and palmitic acid (16:0) did not inactivate phi6 at 50 mug/ml, but nevertheless did prevent phi6 plaque production Single-step virus growth experiments in which fatty acid was added at various times before or after infection indicated that it was an early stage of the phi6 replication cycle that was inhibited by the presence of myristic acid and that the inhibition occurred only if the myristic acid concentration in the extracellular growth medium was greater, similar10 mug/ml phi6 could attach to its host cell in the presence of myristic acid at 50 mug/ml We conclude that the fatty acids that prevent phi6 replication probably do so by interfering with the entry of the viral genome into the host cell

Family Therapy for Disturbed Children: Some Experimental Results in Special Education*

Abstract: The assumption that family attitudes play an important role in motivating children to learn is widely held. It is believed that poor family attitudes can offset the effects of the best educational program. This study employed psychotherapy to influence attitudes of families with a child in a school for the emotionally disturbed. The results indicated that family therapy improved the classroom behavior of the children as well as behavior observed by their parents at home.

Geotactiv behavior of Chlamydomonas.

Abstract: SYNOPSIS The normal negative geotaxis (motility oriented against gravity) of Chlamydomonas is an energy-dependent response that requires coordinated flagellar activity. It is evident from quantitative assays that the rate of geotaxis is steady, and slow relative to the average swimming speed. Geotaxis is inhibited when the horizontal swimming path is less than 200 μm, suggesting that normal geotactic reorientation maneuvers involve long gradual turns. Videomicrographic tracking of cells confirms that such turns are common. In contrast, contact-reorientations generate random cell orientations. When collision frequencies increase, geotaxis in inhibited. The mechanism of normal geotactic orientation, then, depends on long slow reorientation maneuvers (from net downward to net upward vectors) that require hundreds of micrometers of free swimming space. Mechanisms of geotaxis that would require passive reorientation or sedimentation, or rapid active responding, are excluded. Unusually dense populations sediment with atypical rapidity, probably due to formation of functionally aggregated subpopulations. Sodium azide causes an inhibition of orientation behavior that is selective relative to its effects on general motility. Evidence presented suggests that active physiologic mechanisms for geotaxis should be reconsidered.

Aluminum phosphate variants of feldspar

Abstract: The compounds NaAlrPSiO, and KAlrPSiOe have been synthesized hydrothermally at I kbar and temperatures greater than 445"C. Both compounds are isostructural with hightemperature alkali feldspars and involve the coupled substitution of (Al + P) for (Si + Si). Cell parameters for both phases are (1) NaAl,PSiO,: a -- 8.164(6), b : 13.019(17), c : 7.139(3)A,a:94.0 0(11)',A=ll6.6l( 5)",r=89.85(10)' ;and(2)KAI,PSiO' :a:8.621(2), b = 13.084(2), c : 7.203(2)4, 0 : 115.99(2)'.