Range Of Cyclic Frequencies Research Articles

Investigation of energy stealth of radar stations with band antennas from the radio monitoring complex

Using spectral representation of non-harmonic processes and partial characteristics of antennas, energy equations of transmissionreception of broadband signals in the radio channel with scattering on the object and the direct radio channel between the radar and the radio monitoring (RM) complex have been obtained. Energy of received signals is determined by value of product of spectral density of radiation power and partial moment of transmission-reception averaged in the range of cyclic frequencies. The signal-to-noise ratio at the receiver input reaches the highest value when the partial moment of transmission-reception of signals in the radio channel changes according to a linear law. At fixed average radar power, energy of information signals at incoherent accumulation can be increased due to selection of antennas with partial directive gain amplifying wave processes in areas of their spectra concentration. The range of the radar is proportional to the linear dimensions and significantly depends on the shape of the object. The rules of antenna characteristics selection are justified to ensure radar stealth from RM systems while maintaining the required range of target detection and recognition. When using an antenna with a partial directive gain constant in the range of working cyclic frequencies in the radar, and in the RM complex of an antenna system with a frequency-independent effective area, the partial moment of transmission-reception of signals in the forward radio channel is constant. Its value is determined by the product of partial directive gain at the central frequency of the range. Forms of pulses supplied to input of transmitting antenna and radiated into space coincide, and shape of signal at output of receiving antenna has the form of their derivative. Due to application of frequency-independent antennas in radar and RM complex, energy of probing signals increases compared to levels typical for antenna systems with partial directivity indices varying in proportion to current value of cyclic frequency in the first or second degree. This pattern is due to the significant effective area of the receiving antenna in the lower part of the frequency range, which parries the low efficiency of radiation of low-frequency components of the radar signal spectrum. In radio channels with partial moments of transmission-reception of signals varying with frequency increase according to linear law, energy supplied to receiver input increases in comparison with levels achievable at quadratic and cubic dependencies of partial moments on cyclic frequency due to the same type dependencies of partial directive gain antenna of radar and effective area of antenna of RM complex. Stealth of probing radiation increases as dimensions of radar antennas increase due to increase of partial directive gain and decreases as dimensions of antenna systems of RM complexes increase due to increase of their effective areas. The duty cycle of pulses forming regular sequences, established due to the condition of accumulation of energy of reflected signals due to irradiation of the object, which is required to complete the functional tasks of the radar, decreases in proportion to the increase of the partial directive gain antenna at the central cyclic frequency. Energy of information signals increases in proportion to coefficients depending on relative half-width of their spectra, and decreases with increase of central cyclic frequency to the second degree. As pulse repetition frequency increases at maximum permissible average density of radiation power limited by radar stealth requirements, energy density of probing signals decreases proportionally. The average power of the radar at a fixed level of spectral density varies in proportion to the bandwidth occupied by the probing signals due to a reduction in the following period.

Cyclostationary Modeling of Surface Electromyography Signal During Gait Cycles and Its Application for Cerebral Palsy Diagnosis

Cerebral palsy (CP) is a group of permanent movement disorders that appear in early childhood. The electromyography (EMG) signal analysis and the gait analysis are two most commonly used methods in the clinic. In this paper, a cyclostationary model of the EMG signal is proposed. The model can combine the aforementioned two methods. The EMG signal acquired during the gait cycles is assumed to be cyclostationary due to the physiological characteristics of the EMG signal production. Then, the spectral correlation density is used to analyze the cyclic frequency (corresponding to the gait cycles) and spectral frequency (the frequency of EMG signal) in a waterfall representation of the two kinds of frequencies. The experiments show that the asymptomatic (normal) subjects and symptomatic subjects (with CP) can be distinguished from the spectral correlation density in a range of cyclic frequencies.

Mechanistic aspects of in vitro fatigue-crack growth in dentin

Although the propagation of fatigue cracks has been recognized as a problem of clinical significance in dentin, there have been few fracture mechanics-based studies that have investigated this issue. In the present study, in vitro cyclic fatigue experiments were conducted over a range of cyclic frequencies (1–50Hz) on elephant dentin in order to quantify fatigue-crack growth behavior from the perspective of understanding the mechanism of fatigue in dentin. Specifically, results obtained for crack extension rates along a direction parallel to the dentinal tubules were found to be well described by the stress-intensity range, ΔK, using a simple Paris power-law approach with exponents ranging from 12 to 32. Furthermore, a frequency dependence was observed for the crack-growth rates, with higher growth rates associated with lower frequencies. By using crack-growth experiments involving alternate cyclic and static loading, such fatigue-crack propagation was mechanistically determined to be the result of a “true” cyclic fatigue mechanism, and not simply a succession of static fracture events. Furthermore, based on the observed frequency dependence of fatigue-crack growth in dentin and observations of time-dependent crack blunting, a cyclic fatigue mechanism involving crack-tip blunting and re-sharpening is proposed. These results are deemed to be of importance for an improved understanding of fatigue-related failures in teeth.

Frequency Effects on the Damping Capacity of Superelastic Nitinol

ABSTRACTThe evolution of the stress-strain behavior of a superelastic NiTi alloy over a range of cyclic frequencies up to 10 Hz is presented. Due to the stress-strain hysteresis typical of these alloys work is dissipated as heat during every cycle. Also, the heat release and absorption induced by the cyclic phase transformation responsible for the superelastic effect causes the material to undergo a cyclic temperature history which is superimposed on the average temperature resulting from the dissipative nature of the stress-strain cycle. As the cyclic frequency is increased the imposed strain rate is also necessarily increased. The stress-strain loops at very low frequencies are under isothermal conditions. As the cyclic rate is increased nearly adiabatic conditions are achieved at the higher frequencies. This affects the hysteretic cyclic behavior and the damping capacity of the material.

Frequency Effects in the Elevated Temperature Crack Growth Behavior of Austenitic Stainless Steels—A Design Approach

The fatigue-crack growth behavior of annealed types 304 and 316 stainless steels is determined over a range of cyclic frequencies in an air environment at temperatures of 800°F (427°C), 1000°F (538°C) and 1200°F (649°C). A good portion of the experimental data supports the observation that the fatigue crack growth rates at a given value of ΔK increases with decreasing frequencies. The behavior ot each temperature is normalized relative to the behavior at a “standard” frequency of 40 cpm, and a simple frequency correction factor is derived. This frequency correction factor, more applicable to type 304 than to 316 stainless steels, is further coupled with temperature effects data taken at the “standard” frequency and combined with a stress ratio correction to furnish estimates of the fatigue crack growth behavior of those two steels over a fairly wide range of parameters.

Nitroxyl radical addition to the double bond of methylene quinone controlled by rotational diffusion in a polymeric matrix

The EPR method is used in a kinetic study of the addition of 2,2,6,6-tetramethyl-4-benzoylhydroxypiperidine-1-oxyl ( ▪) to the double bond of α-methyl-2,6-di-tert.butylmethylene quinone in benzene (log ( k 1) = (2·2±0·6)-(29·0±4·0)/2·3 RT) and isotactic polypropylene (log ( k p) = (4·9±0·6)-(46·0±4·0)/2·3 RT). A near linear relationship exists between k p and the rotational microdiffusion constant for the ▪ radical (the latter was varied by adding benzene to polypropylene) in the cyclic frequency range (1–4)×10 8 rad/sec. The factor underlying the latter relationship is that the reaction rate in the polypropylene matrix is controlled by rotational diffusion of reagents owing to its high stereospecificity.

Some Considerations of Frequency and Wave Form Effects in Fatigue of Copper Based on Dislocation Dynamics

The effects of the cyclic frequency and the stress wave form on the fatigue strength have been investigated by many investigators, but the physical meaning of the frequency and wave form dependent fatigue behavior has not been elucidated. In the present work, an analysis of these effects was made employing a modified Johnston-Gilman's formula. The experimantal results, which were obtained from fatigue tests performed under the sinusoidal, the triangular and the rectangular stress waves and under the cyclic frequency range from 0.2 to 60c/s, were well explained by the present analysis. Particularly, the frequency effect on fatigue strength was well interpreted in terms of the dislocation motion over a wide range of cyclic frequencies.