Short Propagation Research Articles

ON SHORT SURFACE WAVES IN NEMATIC LIQUID CRYSTALS

A question of short gravitational wave propagation on incompressible liquid crystal surface with anysotropic surface tension is discussed. Volume phase is described by Oseen model and surface phase—by Rapini models. Isotropic viscosity is taken into account. The influence of divergent member of free energy on surface stability is analysed.

Postictal language dysfunction in complex partial seizures: effect of contralateral ictal spread.

The authors report postictal language evaluation in patients monitored with bitemporal depth electrodes. Patients whose seizures began in the nondominant temporal lobe and propagated to the contralateral temporal lobe had a prolonged postictal language delay (PILD) with paraphasic errors compared with seizures that did not spread. Shorter propagation time was also associated with a longer PILD. Our study suggests that ictal involvement of the dominant temporal lobe is important in postictal language behavior.

Optical solitons in some SIT type equations

In this paper, we consider the SIT type equations with group velocity dispersion, Kerr nonlinearity, higher order dispersion, self-steepening and pumping effects and analyse the dynamics of ultra short pulse propagation in nonlinear resonant fibre. The nature of the pulse propagation is governed by the nonlinear Schrödinger-Maxwell Bloch (NLS-MB) type equations. Considering the isospectral and non-isospectral eigen value parameter, we generate the soliton solutions through the Bäcklund transformation method.

On the theory of short light pulse propagation in a turbulent medium

We consider the problem of propagation of light pulses in a turbulent medium. Within the framework of the used approximation, we obtain a formula for the average intensity of the pulsed signal. Using this formula, we estimate the time delay of the pulse and its broadening. We find the characteristic time scales determining the light pulse transformation in a turbulent medium. The physical meaning of these time scales is explained qualitatively. The quantitative estimates for the Earth’s atmosphere show that the phenomena in question are significant only for very short, picosecond pulses.

Oscillating edge-flames

It has been known for some years that when a near-limit flame spreads over a liquid pool of fuel, the edge of the flame can oscillate. It is also known that when a near-asphyxiated candle-flame burns in zero gravity, the edge of the (hemispherical) flame can oscillate violently prior to extinction. We propose that these oscillations are nothing more than a manifestation of the large Lewis number instability well known in chemical reactor studies and in combustion studies, one that is exacerbated by heat losses. As evidence of this we examine an edge-flame confined within a fuel-supply boundary and an oxygen-supply boundary, anchored by a discontinuity in data at the fuel-supply boundary. We show that when the Lewis number of the fuel is 2, and the Lewis number of the oxidizer is 1, oscillations of the edge occur when the Damköhler number is reduced below a critical value. During a single oscillation period there is a short premixed propagation stage and a long diffusion stage, behaviour that has been observed in flame spread experiments. Oscillations do not occur when both Lewis numbers are equal to 1.

Fatigue properties and microstructure of Al–Si–Cu system casting alloys

The effect of microstructure on fatigue crack initiation and propagation behaviour of Al–Si–Cu system casting alloys fabricated by varying the Si or Ca contents and the mould temperature was investigated. The fatigue strength was strongly dependent on the size of casting defects that become crack initiation sites. A comparison of the fatigue characteristics of the different alloys was carried out, in which the data of the S–Nf curves were converted into the ΔK0–Nf /A1/2 relationship, where ΔK0 , Nf , and A are, respectively, the initial cyclic stress intensity factor range, number of cycles to fracture, and initial area of casting defects on the fracture surface of the fatigue specimen. The fatigue strength increased with Si content, which was attributed to the small size of casting defects and the reduced crack propagation rate. According to the short crack propagation test results, the crack propagation rate of the alloys with coarse eutectic Si particles increased due to the high stress concentration effect of the particles, although these coarse particles also play a role in decreasing the crack propagation rate, by crack deflection and branching. As a result of these two competitive effects, at least in the present study, the macroscopic crack propagation rate was not influenced by the size of the eutectic Si particles.

Notch and Temperature Effects on Crack Propagation in SS 304 Under LCF Conditions

Crack growth behavior at room temperature and at 600°C of SS 304 plates with a central circular hole is studied using linear elastic and elastic-plastic fracture mechanics concepts. Due to large local plastic zone at the hole edge, short fatigue crack propagation is strongly affected by closure phenomena; and the extension rate of a short crack could not be adequately expressed in terms of stress intensity factor ΔK. However, the parameter ΔK would be appropriate for investigating the behavior of long cracks. Crack acceleration due to temperature increase is discussed and a strain-based normalization factor for ΔK is suggested for long cracks. The J-integral obtained from a finite element analysis is successful in establishing the propagation rate of both short and long cracks; simple power relations exist between da/dN and cyclic J-integral (ΔJf) at room temperature and at 600°C. With the modified J-integral, using the actual flow strain, the temperature effect could be incorporated in the correlation between da/dN and ΔJf.

Inclusion size distribution and endurance limit of a hard steel

High cycle fatigue failure of hard steels is dominated by a long crack initiation phase and a short crack propagation phase. The crack initiation sites are inclusions or surface defects. Due to this, the endurace probability of a hard steel part depends on its crack initiating inclusion size or surface defect size distribution. These consideration lead to the weakest-link concept which allows to calculate local and total endurance probabilities of the cycled parts. From these probabilities, the endurance limit and the probable crack initiation sites can be predicted. Base of the prediction is the knowledge of the endurance limits under three different load conditions. From these data and fracture-mechanic relations for the different inclusion types, the distributions of the crack initiating inclusion sizes can be predicted.

Crack Growth in a Welded Microalloyed Steel under Sulfide Stress Cracking Conditions

In this work, the hydrogen sulfide stress-corrosion cracking (SSC) susceptibility of a welded API X-80 pipeline was investigated. For this purpose, steel welding was carried out normal to the rolling direction using a 60° single V-joint design. After welding, compact modified-wedge opening loading (M-WOL) fracture mechanics specimens were machined and loaded to an applied stress intensity factor, KI, of 27 to 53 MPa√m. This was followed by specimen exposure to H2S saturated synthetic seawater. Each of the M-WOL specimens contained the typical microstructures developed during welding, such as the weld metal (WM), base metal (BM), and heat affected zone (HAZ). No attempt was made to establish a unique KISCC for crack arrest because its significance was not clear. Qualitatively, the experimental outcome indicated that in mode I loading under a KI of 40.3 MPa√m only the base metal region underwent SSC. Apparently, active anodic dissolution of the crack tip started the growth process, but it was followed by a transition to hydrogen induced cracking. At an applied KI of 55 MPa√m and under similar exposure times, crack growth in the base metal was discontinuous and tended to follow the grain boundaries. Moreover, the HAZ exhibited the least SSC susceptibility as inferred from the relatively short crack propagation lengths (0.829 mm). In this case, it was found that the crack path was highly tortuous due to the presence of acicular ferrite and a refined grain structure. The most SSC susceptible condition was found in the weld metal where crack lengths of up to 4.2 mm developed. In this case, the presence of a relatively coarse dendritic structure coupled with interdendritic segregation provided a weak path for crack propagation.

Modelling pulse propagation in semiconductor optical amplifiers using wavelets

A model based on the discrete wavelet transform is presented for the description of short pulse propagation in semiconductor optical amplifiers. The approach is used to analyse amplifier performance taking account of gain saturation, carrier heating, group velocity dispersion and self-phase modulation. Using parameters obtained from fitting to experimental data, simulations are performed that compare well with measured results for semiconductor optical amplifiers and show good agreement with other modelling approaches. The wavelet transform leads naturally to a time-scale representation of the pulse evolution, where the signal is split into a coarse approximation together with a series of local refinements that reveal pulse fine structure which is not apparent in the time domain.

Suivi électrochimique de l’endommagement de matériaux métalliques en fatigue

The damage of metals and alloys under cyclic straining occurs through the progressive development of surface roughness and short crack nucleation and growth before the beginning of fatal crack propagation in the hulk. During the major part of the fatigue lifetime (> 80% N F ), a development of short cracks preceeding the fatal crack propagation takes place. Thus, fatigue lifetime is mainly determined by the resistance against short crack propagation. In aqueous solutions, the reduction of fatigue lifetime results often from complex corrosion-deformation interactions which can lead to the activation of new damage mechanisms which involve generally local electrochemical reaction An experimental method based on the monitoring of current or potential transients during cyclic straining is proposed. Data related to the elementary damage mechanisms and to the damage development during the test can be collected from the shapes and amplitudes of current (potential) peaks within individual cycles and from the evolution of corresponding peak values versus number of cycles. Examples of applications illustrate the possibilities of the method in the field of corrosion fatigue studies of passivating alloys. First, the damage localization in a duplex stainless steel and relationship between strain rate and damage evolution in a single phase c.c stainless steel in a chloride solution are discussed. Then, the application of the method to identify the iodine induced corrosion fatigue mechanisms in Zircaloy-4 is presented. In all cases, the stage of short crack nucleation as well as transitions between different damage mechanisms can be precisely determined.

Performance of capture-division packet access with slow shadowing and power control

A new method for achieving improved spectrum reuse in cellular systems, called capture-division packet access (CDPA), has been introduced. The method uses a single frequency in all cells, but, unlike code-division multiple access (CDMA), it allows each transmitter to exploit the full bandwidth. Practically, the CDPA's way of operation can be seen as a slotted ALOHA scheme among different cells in which the mobile terminals belonging to the same cell transmit using a collision-free mechanism, which is easily obtained due to the very short intracell propagation delay. Parallel transmission in different cells is achieved through the "capture" capability. Packets that are not captured are almost immediately retransmitted, thus assuring that packets are eventually correctly received. We analyze the performance of the CDPA in the presence of slowly varying shadowing. The gain offered by power control techniques is also investigated. Finally, the effect of erroneous signaling information is studied, and a delay analysis is presented.

Delay Fault Coverage Enhancement Using Variable Observation Times

Detection of system timing failures has become a very important problem whenever high speed system operation is required. It has been demonstrated that delay fault coverage loss could be significant if improper propagation paths are used. This occurs when the delay test pair of a target propagation path cannot be effectively generated by an ATPG tool, or when stuck-at test patterns are used as transition (or gate) delay test patterns. In this work, an efficient method is proposed to reduce the amount of fault coverage loss by using variable observation times. The basic idea is to offset the shorter propagation paths (really used) by tightening the observation times. Given a probability distribution of defect sizes and a set of slack differences, this method is able to locate several observation times that result in small fault coverage loss.

COMBINED REFRACTION-DIFFRACTION OF SHORT WAVE PROPAGATION

ABSTRACT Mathematical modelling of combined refraction-diffraction (CRD) equation of short wave propagation provides a rational approach for obtaining wave information in coastal region for engineering purposes. The wave propagation equation is an elliptic equation, which describes all the phenomena involved in the process. However, other forms of the equation such as hyperbolic and parabolic have also been derived depending on the application. Some of the approaches which are in common practice for solving wave propagation problem are reviewed here, with illustrations of practical applications, highlighting their range of applicability and limitations.

Growth of elongated nanostructures

Growth of elongated nanostructures from the gas phase on a masked substrate is modeled using the continuous time Monte Carlo method with the solid-on-solid approximation. We have found that the masked substrate imposes anisotropy on the growth of elongated nanostructures. Thus growth occurs either by nucleation of a single cluster in the short direction and wave propagation in the long direction or by formation of multiple nuclei followed by propagation of waves along the long direction. The growth rate of nanostructures depends significantly on their size. Narrow nanostructures grow about two orders of magnitude slower and there is a size-dependent critical supersaturation for growth to occur. This behavior is explained in terms of the thermal stability of nanostructures.

Short crack growth behavior in a particulate-reinforced aluminum alloy composite

Short and long crack propagation behaviors in a coarse A12O3 particulate-reinforced 6061 aluminum alloy composite (Al2O3/6061 Al) are investigated and compared under different ranges of tensile-compressive cyclic stress. It is found that short cracks up to 400 μm in length propagate in a shear-dominant mode at maximum cyclic stress level below the fatigue limit until they are permanently trapped by the surrounding particles. The microstructure sensitivity of short crack growth in the composite decreases as the short crack length and/or applied stress range increase. The characteristics of short cracks and the mechanisms of short crack trapping by particles in the material are discussed.

Evaluation of resonance contributions to thermal reaction rates using quantum flux correlation functions

We consider the evaluation of thermal rate constants using quantum flux correlation functions for chemical reactions in which metastable states (resonances) play a significant role in the reaction dynamics. The evaluation of rate constants is hindered in this case because of slowly decaying oscillations in the correlation functions but we show that it is possible to remove these oscillations by projecting the resonant states from the wave packets used to calculate flux correlation functions. These projected states do contribute to the reactive flux, but it is not difficult to include for this using the resonance widths. The resulting theory thus uses a combination of short time wave packet propagation for the direct contribution, and bound-state methods for the resonant contribution, thereby achieving a balance between the strengths of time dependent and time independent methods. We illustrate this theory through an application to a simple one-dimensional potential.

Time domain characterization of lossy arbitrary characteristic impedance transmission lines

This paper deals with the characterization of lossy transmission lines. The method developed here delivers the complex propagation constant /spl gamma/ of any arbitrary length and characteristic impedance transmission line, embedded in an arbitrary environment. This approach is based upon time domain analysis of short pulse propagation. Measurements are done with a commercial digital sampling oscilloscope. Only two transmission lines of different lengths are required in order to extract /spl gamma/ and to correct systematic errors of the measurement system. The problem of random errors is also addressed. The method is demonstrated with microstrip lines. A comparison of the developed technique with an other existing time domain approach and a classical frequency domain extraction is also carried out. >

Combined optical–acoustic microscopy for investigating short fatigue crack propagation

Combined optical–acoustic microscopy for investigating short fatigue crack propagation

Ionospheric disturbances produced by powerful explosives

Results of a study of wave-like ionospheric disturbances initiated by powerful explosives are presented and analyzed. Three types of wave processes with differing physical natures which propagate in the upper atmosphere and ionosphere to distances of thousands of kilometers are distinguished. The effect of shock-acoustic waves on indirect short wave radio propagation is considered.