基于等效弹性模量的微裂纹-超声波非线性作用多阶段模型

提出了一种基于等效弹性模量的微裂纹-超声波非线性作用多阶段模型.该模型将微裂纹微观层面的界面几何特征和宏观层面的界面相对运动统一为介观单元弹性模量的变化,利用等效弹性模量表征损伤区域的"应力-应变",然后利用分段函数来描述微裂纹-超声波非线性相互作用,最后通过有限元仿真对波动方程进行求解,验证了模型的有效性,获得了超声波在经过微裂纹后传播的非线性波动规律.仿真结果表明本文提出的模型相比于双线性刚度模型、接触面模型,能更好地体现一个谐波周期内超声波经过微裂纹损伤区域时波形会发生畸变.同时,仿真实验还分析了裂纹倾角、裂纹长度和超声波激励幅值对超声波经过微裂纹后产生的二次和三次谐波的幅值的影响.最后,对比分析了该模型的仿真计算结果与实验测试结果,表明本文提出的多阶段模型与实验测试均能较好地体现微裂纹-超声波非线性作用产生的二次谐波信号,且结果基本一致,验证了模型的有效性.该模型为开展超声波非线性效应定量检测微裂纹提供了一种新的仿真手段.     

Influence of nonlinear effects on the results of measuring the ratio of ultrasound absorption in a liquid by the cavity method

We consider the problem of the influence of nonlinear effects on the results of measuring the ratio of ultrasound absorption by using a resonator cavity. The relative amplitudes and the frequency response function of a nonlinear ultrasonic cavity with plane piezoelectric converters were calculated. The ranges of operating frequencies, at which the influence of nonlinear effects is significant, are determined. It is shown that in the case of nonlinear oscillations, the relaxation time of the fundamental harmonic is significantly shorter than that for the linear regime, which leads to overestimation of the measured ultrasound absorption rate.     

Nonlinear propagation in ultrasonic fields: measurements, modelling and harmonic imaging

In high amplitude ultrasonic fields, such as those used in medical ultrasound, nonlinear propagation can result in waveform distortion and the generation of harmonics of the initial frequency. In the nearfield of a transducer this process is complicated by diffraction effects associated with the source. The results of a programme to study the nonlinear propagation in the fields of circular, focused and rectangular transducers are described, and comparisons made with numerical predictions obtained using a finite difference solution to the Khokhlov-Zabolotskaya-Kuznetsov (or KZK) equation. These results are extended to consider nonlinear propagation in tissue-like media and the implications for ultrasonic measurements and ultrasonic heating are discussed. The narrower beamwidths and reduced side-lobe levels of the harmonic beams are illustrated and the use of harmonics to form diagnostic images with improved resolution is described.     

Theoretical prediction of the scattering of spherical bubble clusters under ultrasonic excitation

Due to the nonlinear vibration of ultrasound contrast agent bubbles, a nonlinear scattered sound field will be generated when bubbles are driven by ultrasound. A bubble cluster consists of numerous bubbles gathering in a spherical space. It has been noted that the forward scattering of a bubble cluster is larger than its backscattering, and some studies have experimentally found the angular dependence of a bubble cluster’s scattering signal. In this paper, a theory is proposed to explain the difference of acoustic scattering at different directions of a bubble cluster when it is driven by ultrasound, and predicts the angular distribution of scattered acoustic pressure under different parameters. The theory is proved to be correct under circumstances of small clusters and weak interactions by comparing theoretical results with numerical simulations. This theory not only sheds light on the physics of bubble cluster scattering, but also may contribute to the improvement of ultrasound imaging technology, including ultrasonic harmonic imaging and contrast-enhanced ultrasonography.     

固体粗糙界面与超声的非线性相互作用研究

本文提出了一种随机弹性接触界面与超声的非线性相互作用理论. 首先建立了描述随机弹性接触界面的模型, 然后分别研究了界面间应力-应变关系在线性简化模型、指数模型、高斯模型下与超声的非线性相互作用. 数值仿真和实验研究结果均表明高斯模型更适合描述固体接触界面. 本文在介观结构上解释了固体界面与超声的相互作用, 对工业超声无损检测裂缝、缺陷及损伤有指导意义. 关键词： 超声无损检测 固体粗糙界面 非线性效应     

应用角谱方法研究聚焦声束在层状生物组织中的非线性声传播

应用角谱方法理论研究了聚焦声束在层状生物组织中的非线性传播特性，将声波分解为角谱，可计算垂直于声轴的任意平面的非线性声场。在圆形平面活塞聚焦换能器的焦区中插入多种生物组织样品，数值计算了样品内部及外部的二次谐波声场，并通过实验测量验证了理论方法的有效性。基于快速傅氏变换的角谱方法可直观地描述非线性声传播，对非线性声成像有指导作用。     

液体材料超声处理过程中声场和流场的分布规律研究

针对合金熔体等液体材料的超声处理过程,选取水作为透明模型材料,采用数值模拟计算和示踪粒子实验方法,研究了20和490 kHz两种频率超声作用下水中的声场和流场分布.结果表明,增大变幅杆半径能够提高水中声压水平,扩大空化效应的发生区域.当超声频率为20 kHz时,水中声压最大值出现在超声变幅杆下端面处,且声压沿传播距离的增大而显著减小.如果超声频率增加至490 kHz,水中的声压级相比于20 kHz时明显提高,且声压沿着超声传播方向呈现出周期性振荡特征.两种频率超声作用下水中的流场呈现相似的分布特征,且平均流速均随着变幅杆半径增大表现出先升高后降低的趋势.变幅杆半径相同时,20 kHz频率超声作用下水中的平均流速高于490 kHz频率超声.采用示踪粒子图像测速技术实时观察和测定了水中的流速分布,发现其与计算结果基本一致.     

Ultrasonic study on organic liquid and binary organic liquid mixtures by using Schaaffs＇ collision factor theory

Based on Schaaff's collision factor theory (CFT) in liquids, the equations for nonlinear ultrasonic parameters in both organic liquid and binary organic liquid mixtures are deduced. The nonlinear ultrasonic parameters, including pressure coefficient, temperature coefficients of ultrasonic velocity, and nonlinear acoustic parameter B/A in both organic liquid and binary organic liquid mixtures, are evaluated for comparison with the measured results and data from other sources. The equations show that the coefficient of ultrasonic velocity and nonlinear acoustic parameter B/A are closely related to molecular interactions. These nonlinear ultrasonic parameters reflect some information of internal structure and outside status of the medium or mixtures. From the exponent of repulsive forces of the molecules, several thermodynamic parameters, pressure and temperature of the medium, the nonlinear ultrasonic parameters and ultrasonic nature of the medium can be evaluated. When evaluating and studying nonlinear acoustic parameter B/A of binary organic liquid mixtures, there is no need to know the nonlinear acoustic parameter B/A of the components. Obviously, the equation reveals the connection between the nonlinear ultrasonic nature and internal structure and outside status of the mixtures more directly and distinctly than traditional mixture law for B/A, e.g. Apfel's and Sehgal's laws for liquid binary mixtures.     

Nonlinear ultrasonic phase-conjugate beams and their application in ultrasonic imaging

The latest results of experimental and theoretical research into phase conjugation of nonlinear ultrasonic beams are generalized, and prospects for the development of nonlinear ultrasonic imaging are outlined.     

Synergistic bactericidal effects and mechanisms of low intensity ultrasound and antibiotics against bacteria: a review

Low intensity ultrasonic therapy is always an important research area of ultrasonic medicine. This review concentrates on low intensity ultrasound enhancing bactericidal action of antibiotics against bacteria in vitro and in vivo, including planktonic bacteria, bacterial biofilms, Chlamydia, and bacteria in implants. These literatures show that low intensity ultrasound alone is not effective in killing bacteria, while the combination of low intensity ultrasound and antibiotics is promising. Low intensity ultrasound facilitating antibiotic treatment is still in its infancy, and still requires a great deal of research in order to develop the technology on medical treatment scale.     

Numerical simulations of stable cavitation bubble generation and primary Bjerknes forces in a three-dimensional nonlinear phased array focused ultrasound field

We present a model developed for studying the generation of stable cavitation bubbles and their motion in a three-dimensional volume of liquid with axial symmetry under the effect of finite-amplitude phased array focused ultrasound. The density of bubbles per unit volume is determined by a nonlinear law which is a threshold-dependent function of the negative acoustic pressure reached in the liquid, in which nuclei are initially distributed. The nonlinear mutual interaction of ultrasound and bubble oscillations is modeled by a nonlinear coupled differential system formed by the wave and a Rayleigh-Plesset equations, for which both the pressure and the bubble oscillation variables are unknown. The system, which accounts for nonlinearity, dispersion, and attenuation due to the bubbles, is solved by numerical approximations. The nonlinear acoustic pressure field is then used to evaluate the primary Bjerknes force field and to predict the subsequent motion of bubbles. In order to illustrate the procedure, a medium-high and a low ultrasonic frequency configurations are assumed. Simulation results show where bubbles are generated, the nonlinear effects they have on ultrasound, and where they are relocated. Despite many physical restrictions and thanks to its particularities (two nonlinear coupled fields, bubble generation, bubble motion), the numerical model used in this work gives results that show qualitative coherence with data observed experimentally in the framework of stable cavitation and suggest their usefulness in some application contexts.     

Assessment of cortical bone fatigue using coded nonlinear ultrasound

Bone fatigue accumulation is a factor leading to bone fracture, which is a progressive process of microdamage deteriorating under long-term and repeated stress. Since the microdamage of the early stage in bone is difficult to be investigated by linear ultrasound, the second harmonic generation method in nonlinear ultrasound technique is employed in this paper,which is proved to be more sensitive to microdamage. To solve the deficiency that the second harmonic component is easily submerged by noise in traditional nonlinear measurement, a weighted chirp coded sinusoidal signal was applied as the ultrasonic excitation, while pulse inversion is implemented at the receiving side. The effectiveness of this combination to improve the signal-to-noise ratio has been demonstrated by in vitro experiment. Progressive fatigue loading experiments were conducted on the cortical bone plate in vitro for microdamage generation. There was a significant increase in the slope of the acoustic nonlinearity parameter with the propagation distance(increased by 8% and 24% respectively) when the bone specimen was at a progressive level of microdamage. These results indicate that the coded nonlinear ultrasonic method might have the potential in diagnosing bone fatigue.     

Increased heating by diagnostic ultrasound due to nonlinear propagation

The heating of tissues by the absorption of ultrasound is an important safety consideration in the use of diagnostic ultrasound. This paper shows that models of ultrasonic heating for this situation need to take account of nonlinear propagation. Measurements were made of the temperature rise in a sample of tissue-mimicking gel, caused by the application of 3.6-MHz focused ultrasonic beams for 3 min. The propagation path to the focus was in water, to mimic the situation where the fetus is scanned through the full bladder. The effect of nonlinear propagation was seen by changing the pressure amplitude of the pulse, while altering the pulsing regime to preserve a constant spatial-peak temporal-average intensity of 1 W cm-2. When nonlinear distortion was present, an enhancement in the temperature rise was observed, which correlated with the value of the shock parameter. The enhancement ratio was typically up to a factor of 3, and the maximum temperature rise observed was 2 degrees C. This enhanced heating was seen both at the surface of the tissue-mimicking gel and after propagation through 23 mm of the material. Under conditions of nonlinear propagation, the maximum heating usually occurs in the prefocal region, rather than at the focus.     

Stress-dependent changes in the diffuse ultrasonic backscatter coefficient in steel: experimental results

In this article, the effects of uniaxial compressive loading on the ultrasonic scattering from polycrystalline grains are shown for 10 MHz ultrasound in annealed, 1018 steel. The results show a decreasing value of the stress-dependent backscatter coefficient for normal incident ultrasound when the compression loading is perpendicular to the scattering direction. The change due to scattering is about 2 orders of magnitude greater than changes observed by others using ultrasonic wavespeed measurements. It is anticipated that this research can serve as the basis for many methods associated with nondestructive determination of stress in structural materials.     

Experimental investigation on flow boiling bubble motion under ultrasonic field in vertical minichannel by using bubble tracking algorithm

Bubble dynamics is important in flow boiling of minichannel, and ultrasonic field effects bubble behaviors. However, flow boiling bubble movements in minichannels under ultrasonic field have received little research attention and are still poorly understood. In this paper, the effects of ultrasonic field on bubble dynamics are experimentally studied by capturing the bubble motion behaviors of the flow boiling bubbles. The ultrasonic frequencies are set to 23, 28, 32, and 40 kHz. Bubble tracking algorithm, which studies the growth, trajectories, velocities, and traveled distances for bubbles, is created to qualitatively describe bubble motion behavior of flow boiling in minichannel. It is found that after the application of ultrasound, the detachment frequency, velocity, and travel distance of the bubbles significantly increases, and the growth behavior and trajectory are extremely complex, the two-phase gas-liquid flow is extremely unstable. The bubbles gain kinetic energy as the ultrasound frequency increases. Finally, numerical simulations are used to quantitatively investigate the mechanism of bubble motion in microchannels under ultrasonic fields.     

Kramers-Kronig analysis of attenuation and dispersion in trabecular bone

A restricted-bandwidth form of the Kramers-Kronig dispersion relations is applied to in vitro measurements of ultrasonic attenuation and dispersion properties of trabecular bone specimens from bovine tibia. The Kramers-Kronig analysis utilizes only experimentally measured properties and avoids extrapolation of ultrasonic properties beyond the known bandwidth. Compensation for the portions of the Kramers-Kronig integrals over the unknown bandwidth is partially achieved by the method of subtractions, where a subtraction frequency acts as an adjustable parameter. Good agreement is found between experimentally measured and Kramers-Kronig reconstructed dispersions. The restricted-bandwidth approach improves upon other forms of the Kramers-Kronig relations and may provide further insight into how ultrasound interacts with trabecular bone.     

A multiple-frequency hydrophone calibration technique

A method is described for comparing the sensitivity of two hydrophones over the frequency range 1-15 MHz. This technique forms the basis for the dissemination of national ultrasonic standards in the U.K. over this frequency range. A reference hydrophone is placed in an ultrasonic field and then the device being calibrated is substituted and the two output voltages are compared. This substitution method utilizes a broadband ultrasonic field produced by nonlinear propagation. Thus it is possible to cover the whole frequency range with a single measurement on each hydrophone. The overall uncertainty in the intercomparison of two hydrophones increases from +/- 4.2% at 1 MHz to +/- 8.2% at 15 MHz (95% confidence level). The method has been compared with discrete-frequency substitution, time-delay spectrometry, and absolute calibrations using the National Physical Laboratory (NPL) Primary Standard Laser Interferometer. Various designs and sizes of hydrophones were compared, and agreement was within the combined random uncertainties for all the comparisons.     

Friction generated ultrasound from geotechnical materials

Drilling is a process involved with product manufacturing and for civil engineers, site preparation. The usual requirement is for efficient material removal. In this study, the friction pair interaction generated by a drilling process provides ultrasound information related to parameters for the geotechnical material being drilled, where the drill bit has non-degrading ultrasonic characteristics and no essential requirement for material removal. This study has considered monitoring the ultrasonic signal generated by a drilling process, with a view to characterising the parameters of the geotechnical material being drilled and provides a novel method to identify or characterise ground structures. Drilling of geotechnical material systems, typically involve the interaction of a rotating probe and a granular composite medium. The applied load and angular velocity are measured to determine their relevance to the ultrasonic signal. Samples of granular materials have been graded into controlled grain size ranges. Attention has been focused on determining the effects on the ultrasound signal of grain size, bulk density and the water content of the granular material. A comparison between the various granular samples of the different grain sizes, density, water content and the associated ultrasonic signal has been done. The effect of each variable, and existing theory for these effects is commented upon. The broad aim of this research is to evaluate ultrasonic monitoring of drilling and assess its potential for real-time geotechnical ground condition monitoring applications and offer it as an alternative to existing methods.     

列车外圆弹簧疲劳损伤的非线性超声测试

进行了列车外圆弹簧在疲劳载荷作用下所产生损伤的非线性超声测试。对列车外圆弹簧进行疲劳加载,利用专门的非线性超声测试系统激发和接收透过弹簧的超声纵波。在相应的信号处理后,建立了声学非线性系数与弹簧疲劳次数之间的关系。实验结果表明,当疲劳次数小于150万次时,由于弹簧材料内部位错、滑移带等微观缺陷还没有形成足以产生显著非线性的积累,该非线性系数增长不明显;当疲劳次数达到150万次至300万次之间的疲劳响应期时,非线性系数增加明显,表明弹簧材料的内部损伤快速增加;当疲劳次数超过300万次时,弹簧材料的内部损伤积累速度减缓,非线性系数增加放缓。该研究表明列车外圆弹簧的疲劳损伤程度与其声学非线性系数直接相关,利用该方法可以为材料和结构早期力学性能退化的检测和评价提供一种有效的途径。      

Pulsed Sounding of Cracks with the Use of the Modulation of Ultrasound by Vibrations

The possibility of using the effect of the modulation of ultrasound by vibrations due to the presence of cracks for the nonlinear acoustic detection of cracks is demonstrated. The method is based on a pulsed ultrasonic sounding with gating the received signal and simultaneously exciting low-frequency vibrations in the sample. The presence of a crack is characterized by the modulation of the ultrasonic wave reflected from the crack. The visualization of the crack position in a model object (a metal rod) is performed. The possibility of selecting a crack on the background of an intense signal reflected from a cavity is experimentally demonstrated. The manifestation of the nonlinear properties of a crack is studied as a function of the polarization of the flexural vibrations of the rod.