Dependence of frequency and amplitude of the ac current on the GMI properties of Co based amorphous wires

The frequency and the amplitude of the driving ac current dependence of the Giant Magneto-Impedance (GMI) behaviour in (Fe₆Co₉₄)_72.5Si_12.5B₁₅ amorphous wire have been studied. The single and two-peak behaviour in the GMI characteristics are observed which depends on the amplitude and frequency of the ac driving current flowing through the sample. The GMI ratio initially increased and then decreased monotonically with the increase of frequency, f, and the amplitude of the ac current, I_ac. With the increase of amplitude of ac current, the response of the GMI voltage is found non-linear with the generation of higher harmonics. With the increase of higher harmonics, the first harmonic component of the GMI voltage started decreasing. With the increase of the ac current, the field sensitivity of the first harmonic components of the GMI voltage decreases and in higher harmonics it shows an increasing trend. The maximum field sensitivity was observed 7.5% per A/m for I_ac = 2 mA and at frequency 100 kHz. The paper investigates the response of the second and third harmonics with the frequencies and amplitude of the driving current. A voltage equation derived from the rotational model to study the response of the GMI voltage.     

Second Harmonic Generation in a Free Electron Laser

It is pointed out that in a free electron laser higher frequencies, which are very nearly equal to the harmonics, are generated. Second harmonic generation, in stimulated emission in a spatially periodic static magnetic field, is studied using the Feynman diagram technique. The ratio of gain for the second harmonic compared to that of the fundamental harmonic is of the order of 10 ^?9 for the conditions present in the free electron laser of Deacon et al. [1].     

Native tissue imaging at superharmonic frequencies

The second harmonic imaging mode has been adapted to image tissue and shown considerable improvements in image quality in several applications compared to the fundamental mode. The improvements were attributed to the effects of wave distortion due to nonlinear propagation in tissue. However, imaging tissue at the second harmonic frequency only has various drawbacks. Because the energy in the second harmonic frequency band is much lower than that in the fundamental frequency band, there must be excellent sensitivity and dynamic range in the receiving system to achieve an acceptable amount of signal-to-noise ratio. To increase the sensitivity, the spectral overlap between the fundamental and the second harmonic has to be diminished, which in return deteriorates the imaging resolution. Consequently, a trade-off is mandatory between resolution and sensitivity. Using simulations and measurements, we show that, at appropriate scanning acoustic settings, higher harmonics are generated in tissue. The higher harmonics represent additional, relevant information for tissue imaging and characterization. An elegant way to take advantage of the higher harmonics and to bring all the information together is to combine and incorporate all the multiple higher harmonics into a single component that we call the superharmonic component. Using a newly developed array transducer having a wide frequency band, B-mode images of a phantom were made in the superharmonic mode transmitting at 1.2 MHz. These images have exceptionally improved clarity and yield a dramatically cleaner and sharper contrast between the different structures being imaged. In addition to increased signal-to-noise ratio, superharmonic imaging shows better contrast and axial resolution as well as acceptable penetration depth.     

33rd harmonic generation from aluminium plasma

We report the high-order harmonic generation of up to the 33rd harmonic (λ?=?24.1 nm) of Ti?:?sapphire laser radiation from the prepulse-produced aluminium plasma. The plateau pattern for high harmonics was observed, contrary to the steep decrease in harmonic output appearing in the case of carbon plasma under analogous conditions. The harmonic generation appeared to be efficient in the case of singly ionized Al plasma when the high-charged ion concentration was negligible. The conversion efficiency for the 25th harmonic was measured to be 2× 10^?6, which was twenty times higher when comparing to the case for boron plasma. The comparison of the harmonics generating from the Al and Al₂ O₃ plumes showed equal conversion efficiencies and cutoff harmonics.     

A dual frequency ultrasonic probe for medical applications

A dual frequency probe using a multilayer ceramic is proposed for simultaneously obtaining a high resolution B mode and a high sensitivity Doppler mode image. This ceramic consists of two layers in which the poling directions are opposite and the individual thicknesses are different. It is possible to control the values of relative electromechanical coupling factors in the fundamental and the second harmonic by changing the thickness ratio. A thickness ratio of 1:0.7 was decided from computer simulation based on the Mason's model. A sufficient resolution has been shown from the fact that the intima of the carotid artery could be distinguished by an actually fabricated probe with dual frequencies of 3.75 and 7.5 MHz. Also, the sensitivity of this probe in the Doppler mode at 5 cm depth from the surface has been improved as much as 5 dB over that of a conventional one     

Granularity and Linear Flux Dynamics in Sintered LaO0.92F0.08FeAs

The flux dynamics in LaO_0.92F_0.08FeAs polycrystalline samples has been investigated looking at the first and higher harmonics of the AC magnetic susceptibility. The investigation of the real and imaginary part of the first harmonic shows that the critical temperature is reduced when the applied DC field increases. Moreover, the intergrain and intragrain contributions persist up to the highest applied DC field, suggesting that the electromagnetic connection among the superconducting grains is strong in spite of the estimated very short coherence length. Concerning the higher harmonics, a comparison between the absolute value of the real part of the first harmonic and the sum of the higher harmonics clearly indicates the activation of a linear regime below the superconducting critical temperature. Furthermore, the temperature range where this linear regime is activated is smaller than the one found in cuprate perovskite superconductors.      

Harmonic differential quadrature method and applications to analysis of structural components

Summary A harmonic differential quadrature (HDQ) method with application to the analysis of buckling and free vibration of beams and rectangular plates is presented. A new approach is proposed for the determination of the weighting coefficients for differential quadrature. It is found that the HDQ method is more efficient than the ordinary differential quadrature (DQ) method, especially for higher order frequencies and for buckling loads of rectangular plates under a wide range of aspect ratios. Also, some shortcomings existing in theDQ method are removed.     

超声导波混频表征和定位早期局部损伤的研究进展

超声导波具有远距离传输的特性，能够快速、有效地大范围检出薄板中的损伤或缺陷。非线性超声导波相较于传统超声导波，主要研究基波与材料中微观组织演化相互作用而产生的高阶谐波，对尺寸远小于基波波长的损伤或缺陷比较敏感。其中，超声导波的二次谐波相对容易激发，已被用于定量评估早期损伤。但是，超声导波的二次谐波容易受到测量系统非线性的干扰，并且无法定位材料中的局部损伤。超声导波混频在频率、模式、传播方向的选择上具有一定的灵活性，克服了二次谐波的缺点。目前，超声导波混频在理论、模拟和实验上取得了一定的进展，已被用于表征和定位金属材料中处于早期阶段的疲劳、热老化、微裂纹、冲击损伤和局部塑性变形等。高频段超声导波混频、兰姆波相向混频和非共线混频中差频谐波或和频谐波的传播性，以及更多类型损伤的定位和表征仍有待进一步研究。     

Phase matching of high-order harmonic generation in helium- and neon-filled gas cells

Abstract

10 mJ laser pulses are used to study high-order harmonic generation in helium- and neon-filled gas cells of various lengths. Harmonic orders in the range 50–100 are investigated. A semi-infinite cell geometry produces brighter harmonics than subcentimetre cells. In the semi-infinite geometry, the gas occupies the region from the focusing lens to a thin exit foil near the laser focus. Restricting the laser beam with an aperture in front of the focusing lens increases the emission of most harmonic orders observed by as much as an order of magnitude. Counter-propagating light is used to probe the region in the focus where the high harmonics are generated. In neon, the harmonics are generated in the last few millimetres before the exit foil. In helium, the harmonics are produced over a longer distance, indicating good phase-matching conditions.     

Microwave Compressive Strength Estimation of Cement Paste Using Monopole Probes

Abstract

In-situ evaluation of the compressive strength of existing concrete structures using a direct approach is the objective of this investigation. The principle factor affecting the compressive strength of a concrete structure is its water/cement (w/c) ratio. The reflection properties of four cement paste samples with w/c ratios of 0.35, 0.40, 0.50, and 0.55 were measured using several monopole probes at microwave frequencies of 5, 9, and 12 GHz. The effect of several parameters, such as the operating frequency, the probe length (h), and the properties of the cement paste (w/c ratio) on the measurement sensitivity, were studied theoretically as well. For a given monopole probe (fixed diameter), h and the operating frequency can be optimized such that any slight change in the w/c ratio causes a large change in the reflection coefficient. Lower microwave frequencies are shown to be more sensitive in detecting variation in the reflection coefficient of these samples as a function of the w/c ratio. After the reflection coefficient measurements for these samples were conducted, they were tested to measure their compressive strength. Subsequently, a correlation between the compressive strength and the reflection coefficient of these samples was shown. The effect of an air gap around the monopole was investigated as well.     

区间参数平面连续体结构频率非概率可靠性拓扑优化

研究了具有区间参数的平面连续体结构在固有频率非概率基频约束和频率禁区约束下的拓扑优化设计问题。考虑结构弹性模量、质量密度和频率约束限具有区间不确定性,根据SIMP材料插值方法和区间变量运算法则,构建了基于频率非概率可靠性约束的弯曲薄板和平面应力薄板结构的拓扑优化数学模型表达式,并给出了进化优化准则。算例及其结果表明文中模型和方法的有效性。     

Microwave compressive strength estimation of cement paste using monopole probes

In-situ evaluation of the compressive strength of existing concrete structures using a direct approach is the objective of this investigation. The principle factor affecting the compressive strength of a concrete structure is its water/cement (w/c) ratio. The reflection properties of four cement paste samples with w/c ratios of 0.35, 0.40, 0.50, and 0.55 were measured using several monopole probes at microwave frequencies of 5, 9, and 12 GHz. The effect of several parameters, such as the operating frequency, the probe length (h), and the properties of the cement paste (w/c ratio) on the measurement sensitivity, were studied theoretically as well. For a given monopole probe (fixed diameter),h and the operating frequency can be optimized such that any slight change in the w/c ratio causes a large change in the reflection coefficient. Lower microwave frequencies are shown to be more sensitive in detecting variation in the reflection coefficient of these samples as a function of the w/c ratio. After the reflection coefficient measurements for these samples were conducted, they were tested to measure their compressive strength. Subsequently, a correlation between the compressive strength and the reflection coefficient of these samples was shown. The effect of an air gap around the monopole was investigated as well.     

Dynamic characterization of the thickness-tapered laminated plant fiber-reinforced polymer composite plates

Evolution of the laminated woven natural fiber fabric-reinforced polymer composite structures makes a way to the development of the non-uniform laminated composite structures in order to achieve the stiffness variation throughout the structure. An attempt is made in this work to carry out the experimental and numerical investigations on the dynamic characteristics of the thickness-tapered laminated woven jute/epoxy and woven aloe/epoxy composite plates. The governing differential equations of motion for the thickness-tapered laminated composite plate are developed using the h-p version FEM based on higher order shear deformation theory. The validation of the present finite element formulation is carried out by comparing the natural frequencies obtained using the finite element formulation with those natural frequencies determined experimentally. The developed model is further validated with the available literature works on tapered composite plate to confirm the efficiency of h-p version FEM. This work also explores the study of the vibrational characteristics of composite plates under the influence of plant fiber’s transverse isotropic material characteristics and porosity associated with plant fiber composites through the elastic constants evaluated in the author’s previous work. Also the influences of aspect ratios, ply orientations, and taper angles under various end conditions on the natural frequencies of the woven jute/epoxy composite plate are studied using the present finite element formulation. The forced vibration response of the thickness-tapered laminated woven jute/epoxy composite plate under the harmonic force excitation is carried out considering CFCF and CFFF end conditions.     

Some comments on the dispersion relation for periodically layered pre-stressed elastic media

In this paper the dispersion relation associated with harmonic waves propagating in a periodically layered structure is derived and analysed. Specifically, the structure is made up of repeating unit cells, with each layer composed of an incompressible, pre-stressed elastic material, each interface perfectly bonded and the upper and lower surfaces of the structure free of incremental traction. The complexity of the problem is reduced using an approach involving the Cayley-Hamilton theorem. A numerical method is also used which eliminates positive exponential functions, thereby considerably reducing the complexity of solving the dispersion relation numerically. Numerical solutions are presented in respect of both a two-ply and symmetric four-ply unit cell. An interesting feature of these solutions is the grouping together of harmonics as the number of unit cells increases. In the case of n unit cells, n−1 harmonics group together in the moderate wave number region, with an additional harmonic joining the group at a higher wave number.     

Detecting Low Velocity Impact Damage in Composite Plate Using Nonlinear Acoustic/Ultrasound Methods

The objective of this work was to image the presence of impact damage by monitoring the nonlinear response of damaged carbon/epoxy composite samples. The presence of microcracks, debonding, delamination, etc… induce the material to behave in a nonlinear elastic fashion highlighted by the presence and amplitude of harmonics in the spectrum of the received signal when the sample is periodically excited at one of its resonance frequencies. The sensitivity of a second harmonic imaging technique (SEHIT) based on material nonlinear elastic effect known as second harmonic generation (SHG) was investigated. The proposed imaging process was used to detect barely visible impact damage (BVID) due to low velocity impact (<12 J). The results showed that the SEHIT methods appear to be highly accurate in assessing the presence and magnitude of damage with a very promising future for both NDT and possibly structural health monitoring (SHM) applications. Moreover the technique was validated with two conventional NDT techniques: pulse thermography and thermosonics. The first NDT method failed in detecting the damage on the impacted face. The second technique was capable of localising and quantifying the damage on the impacted surface agreeing well with the results obtained using the proposed nonlinear imaging method.     

Distinct Element Model of Energy Dissipation in Vibrated Binary Particulate Mixtures

Distinct element model (DEM) simulations of energy dissipation in vibrated particle beds are compared with experimental results. DMX, a 3-D DEM of polydisperse spheres in an open-top vibrating cylinder, was used. Simulations were conducted for vibrating mono and binary particle systems. Energy dissipation rate per vibration cycle at different frequencies and maximum accelerations was examined. Experimental data from previous publications were compared with the simulations. Reasonable qualitative agreement was achieved on scaled-up (by number of particles) simulation results. These show that DEM can capture the harmonic phenomena, showing resonance in dissipation at several frequencies at low accelerations (<1 g). At high acceleration levels (>1 g) no harmonics are observed. At low frequency levels where the vibration amplitudes are higher, the DEM reproduces experimental energy dissipation levels better than a continuum viscoelastic model. For a larger diameter vessel (fewer layers and decreased wall effects) the resonant dissipation frequency increases. Quantitative agreement between DMX predictions and the experiments is reasonable given the scatter in the experimental results; at high frequency there is at least an order of magnitude difference in the rate of dissipation, which was also observed in viscoelastic model predictions. Results show that even with using only 100 particles the agreement between DMX predictions and the experiments is qualitatively reasonable. This will enable the examination of many more situations and combinations as it can be carried out relatively “fast.”     

Ultra-Sensitive Duffing Behavior of a Microcantilever

We investigate the properties of an electrostatically driven microcantilever exhibiting Duffing-like behavior using Harmonic Detection of Resonance. Its potential use as a highly sensitive sensing platform is discussed. We find high sensitivity of this Duffing system near its bistability point in a gaseous environment. The response of the higher harmonics of the measured charge on the cantilever induced by an ac voltage that drives the counter electrode is investigated. In particular, we follow the Duffing behavior at the higher harmonics (up to the sixth harmonic) as a function of gap distance between the cantilever and counter electrode. To our knowledge, this work represents the first experimental demonstration of sensing a pressure change using the Duffing behavior.      

Vibrations of jammed disk packings with Hertzian interactions

Contact breaking and Hertzian interactions between grains can both give rise to nonlinear vibrational response of static granular packings. We perform molecular dynamics simulations at constant energy in 2D of frictionless bidisperse disks that interact via Hertzian spring potentials as a function of energy and measure directly the vibrational response from the Fourier transform of the velocity autocorrelation function. We compare the measured vibrational response of static packings near jamming onset to that obtained from the eigenvalues of the dynamical matrix to determine the temperature above which the harmonic approximation breaks down. We compare packings that interact via single-sided (purely repulsive) and double-sided Hertzian spring interactions to disentangle the effects of the shape of the potential from contact breaking. Our studies show that while Hertzian interactions lead to weak nonlinearities in the vibrational behavior (e.g. the generation of harmonics of the eigenfrequencies of the dynamical matrix), the vibrational response of static packings with Hertzian contact interactions is dominated near jamming by contact breaking as found for systems with repulsive linear spring interactions.     

Nonlinear viscoelastic properties of tissue assessed by ultrasound

A technique to assess qualitatively the presence of higher-order viscoelastic parameters is presented. Low-frequency, monochromatic elastic waves are emitted into the material via an external vibrator. The resulting steady-state motion is detected in real time via an ultra fast ultrasound system using classical, one-dimensional (1-D) ultrasound speckle correlation for motion estimation. Total data acquisition lasts only for about 250 ms. The spectrum of the temporal displacement data at each image point is used for analysis. The presence of nonlinear effects is detected by inspection of the ratio of the second harmonics amplitude with respect to the total amplitude summed up to the second harmonic. Results from a polyacrylamide-based phantom indicate a linear response (i.e., the absence of higher harmonics) for this type of material at 65 Hz mechanical vibration frequency and about 100 microm amplitude. A lesion, artificially created by injection of glutaraldehyde into a beef specimen, shows the development of higher harmonics at the location of injection as a function of time. The presence of upper harmonics is clearly evident at the location of a malignant lesion within a mastectomy.     

Comments on,Xuan Li,Shanghong Zhao,Zihang Zhu,Bing Gong,Xingchun Chu,Yongjun Li,Jing Zhao and Yun Liu ‘an optical millimeter-wave generation scheme based on two parallel dual-parallel Mach–Zehnder modulators and polarization multiplexing’, Journal of Modern Optics, 2015

Abstract

In the title paper, Li et al. have presented a scheme for filter-less photonic millimetre-wave (mm-wave) generation based on two polarization multiplexed parallel dual-parallel Mach–Zehnder modulators (DP-MZMs). For frequency octo-tupling, all the harmonics are suppressed except those of order 4l, where l is the integer. The carrier is then suppressed by the polarization multiplexing technique, which is the principal innovative step in their design. Frequency 12-tupling and 16-tupling is also described following a similar method. The two DP-MZM are similarly driven and provide identical outputs for the same RF modulation indices. Consequently, a demerit of their design is the requirement to apply two different RF signal modulation indexes in a particular range and set the polarizer to a precise angle which depends on the pair of modulation indices used in order to suppress the unwanted harmonics (e.g. the carrier) without simultaneously suppressing the wanted harmonics. The aim of this comment is to show that, an adjustment of the RF drive phases with a fixed polarizer angle with the design presented by Li, all harmonics can be suppressed except those of order4l, where l is an odd integer. Hence, a filter-less frequency octo-tupling can be generated whose performance is not limited by the careful adjustment of the RF drive signal, rather it can be operated for a wide range of modulation indexes (m ~ 2.5 → 7.5). If the modulation index is adjusted to suppress 4th harmonics, then the design can be used to perform frequency 24-tupling. Since, the carrier is suppressed by design in the modified architecture, the strict requirement to adjust the RF drive (and polarizer angle) can be avoided without any significant change to the circuit complexity.