Evaluation of layered composites by laser optoacoustic transducers

An ultrasonic method for evaluation of composite structures by laser-excited wide-band acoustic pulses is proposed. Two types of laser optoacoustic transducers, in transmission and reflection modes, are used in the spectral range of 0.1–10 MHz. In the first case, the frequency dependence of the attenuation factor of ultrasound is measured. In the reflection mode, the optoacoustic transducer allows us to obtain the time-dependent shape and spectrum of the backscattered acoustic signal with one-sided access to a specimen or product. Specimens of a layered graphite-epoxy composite of different porosity (up to 1.2%) are studied. In the frequency range of 1–5 MHz, the attenuation factor of ultrasound measured in the transmission mode is the greater the higher the porosity of the specimen. The noise part of the backscattered acoustic pulse is found to increase with increasing porosity of the specimens. A 1% growth in porosity causes a threefold increase in the noise power. The results obtained confirm the advantages of the laser optoacoustic method for evaluation of composite materials and products during their manufacturing and operation.International Laser Center, Lomonosov Moscow State University, Moscow, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 1, pp. 125–134, January–February, 1999.     

Nondestructive Evaluation of One-Dimensional Periodic Structures by Transmission of Laser-Excited Wide-Band Acoustic Pulses

The propagation of ultrasound in a one-dimensional model and actual periodic structures (PSs) is studied experimentally by the method of optoacoustic spectroscopy based on the laser thermooptical excitation and wide-band piezodetection of short acoustic pulses. It is shown that the ultrasound transmission spectrum of a PS has stop and pass bands, and the greater the number of layers in the PSs, the deeper the stop bands. The case where the thickness, density, and ultrasound velocity of one or several layers in the PS are modified is studied in detail. In this case, a narrow local maximum of ultrasound transmission appears in the stop band, whose location depends considerably on the position of the defective layer in the PS. The experimental data obtained coincide well with the theoretical calculation. The nondestructive evaluation of actual PSs consisting of two epoxy-glued identical aluminum plates is carried out by the optoacoustic method. Such materials are widely used in aircraft industry. It is shown that the ultrasound transmission spectrum for these materials depends considerably on the thickness of the epoxy-glue layer.     

Surface acoustic waves in the testing of layered media. The waves’ sensitivity to variations in the properties of the individual layers

Research on the use of surface acoustic waves for the nondestructive testing of layered media is reviewed. A model to describe horizontally polarized surface acoustic waves in layered anisotropic (monoclinic) media is constructed. A modified transfer-matrix method is developed to obtain a solution. Non-canonical type waves with horizontal transverse polarization are investigated. Dispersion curves are constructed for a multilayer composite in contact with an anisotropic half-space. It is shown that the variation of the physical characteristics and the geometry of any of the internal layers leads to a variation in the dispersion curves. This opens up the possibility of using dispersion analysis for the nondestructive testing of the properties of the individual layers.     

Experimental studies of axial magnetic fields generated in ultrashort-pulse laser-plasma interaction

The quasistatic axial magnetic fields in plasmas produced by ultrashort laser pulses were measured by measuring the Faraday rotation angle of the backscattered emission. The spatial distribution of the axial magnetic field was obtained with a peak value as high as 170 Tesla. Theory suggests that the axial magnetic field is generated by dynamo effect in laser-plasma interaction.     

二维目标声波成像的变分方法

研究二维目标声波成像的新方法.首先根据Green函数理论导出一个关于波数扰动函数和波场的积分方程组;然后通过对该方程组求变分,寻求波数扰动函数微小变化和散射场微小变化之间的关系,得到反演方程;最后对几个典型目标进行模拟识别,考察了方法的收敛速度、收敛精度、抗噪性能及对复杂目标的适应能力.识别结果表明,本文方法对定量无损检测技术具有理论和应用价值.     

The current state of the problem of interaction of acoustic beams with obstacles in a deformable medium

We analyze the state of the problem of the formation of radiated and scattered acoustic beams in application to the development of a methodology for studying the information aspects of hydroacoustics and nondestructive control. We discuss the problems of the selective generation of characteristic vibrations of elastic objects in a deformable medium using sharply directed acoustic impulses. We study the problem of posing and methods of solving a certain class of inverse problems of scattering theory.Translated fromMatematicheskie Metody i Fiziko-Mekhanicheskie Polya, Issue 27, 1988, pp. 56–64.In conclusion we note the papers [7, 74, 100] connected with the traditional method of solving inverse problems-the selection method.     

An optimized finite element method for the analysis of 3D acoustic cavities with impedance boundary conditions

Numerical approaches studying the reduction of dispersion error for acoustic problems so far have focused on the models without impedance. Whereas, the practical acoustic problems usually involve impedance. This situation indicates that it is essential to study the numerical methods by taking into account the influence of impedance. In this work, an optimized finite element method is introduced to solve the three-dimensional steady-state acoustic problems with impedance. This technique resorts to heuristic optimization techniques to determine the integration points locations in elements. It develops a strategy to optimize the integration points locations, and makes use of adaptive genetic algorithm to achieve the best integration points locations for the construction of element matrix. By using the proposed method, a three-dimensional acoustic tube model with impedance is investigated, and the dispersion error, accuracy, convergence and efficiency of solutions are all compared to those of some existing numerical methods and reference solutions. Simultaneously, two practical cavity models are studied to verify the effectiveness and strongpoints of the proposed method as compared to existing numerical methods. Hence, the proposed method can be more widely applied to solve practical acoustic problems, yielding more accurate solutions.     

Short-Lived Large-Amplitude Pulses in the Nonlinear Long-Wave Model Described by the Modified Korteweg–De Vries Equation

The appearance and disappearance of short-lived large-amplitude pulses in a nonlinear long wave model is studied in the framework of the modified Korteweg–de Vries equation. The major mechanism of such wave generation is modulational instability leading to the generation and interaction of the breathers. The properties of breathers are studied both within the modified Korteweg–de Vries equation, and also within the nonlinear Schrödinger equations derived by an asymptotic reduction from the modified Korteweg–de Vries for weakly nonlinear wave packets. The associated spectral problems (AKNS or Zakharov-Shabat) of the inverse-scattering transform technique also are utilized. Wave formation due to this modulational instability is investigated for localized and for periodic disturbances. Nonlinear-dispersive focusing is identified as a possible mechanism for the formation of anomalously large pulses.     

Amplification and compression of Ti:sapphire femtosecond pulses at high-repetition-rate

The experiment of the generation and amplification of femetosecond Ti:sapphire laser pulse at high repetition rate is reported. The laser pulses with minimum pulsewidth 15 fs, maximum spectrum width of 80 nm, average power of 200 mW are generated from a home-built self-mode-locked Ti:sapphire laser. As a seed pulse which is selected from the oscillator, the laser pulse is further amplified by using chirped-pulse-amplification technology in a Ti:sapphire amplifier from which a kind of pulses with single-pulse-energy of 100 uj, pulsewidth after compressing of 50 fs at 5 kHz repetition rate are produced. The system design and experimental results are discussed. Project supported by the National “Climbing Project” of China.     

Modeling of the material removal and heat affected zone formation in CFRP short pulsed laser processing

Short pulsed laser milling is a novel method to process the carbon fiber reinforced plastics (CFRP) which has bad machinability. This paper presents a numerical model studying the material removal mechanism of CFRP laser milling. It is confirmed by both the experiment and the simulation that laser ablation and mechanical erosion caused by the polymer pyrolysis are all involved in the material removal. Because the heating and cooling rate in short pulsed laser milling is high, ablation of two adjacent laser pulses almost has little influence on each other. By conducting the parametric analysis, it was found that the spacing distance under which the matrix between two adjacent laser pulses was completely degraded should be adopted to utilize the mechanical erosion effectively. Laser milling experiments of CFRP laminates were performed using a nanosecond pulsed laser system. The established model could predict the average ablation depth per scanning pass at an optimal spacing distance.     

Some characteristics of the use of the acoustic emission method in the study of the fragmentation of single fibers in a polymer matrix

Conclusions The energy spectra of acoustic signals during stretching of an epoxy specimen reinforced with carbon fiber and of a pure epoxy matrix differ substantially. Results of a comparison of these spectra make it possible to distinguish acoustic emission signals due to failure of the fiber during stretching of a reinforced composite and to plot curves of the average length of failing fiber fragments as a function of strain.Translated from Mekhanika Kompozitnykh Materialov, No. 6, pp. 734–740, November–December, 1993.     

On linear filtering under dependent wide-band noise

Two linear filtering problems with wide-band noise on either the signal or the observations are considered. The optimal filters in these problems are obtained as the Kalman-Bucy type. The case when both the signal and the observations are subjected to the effects of wide-band noise is the synthesis of the above-mentioned cases. The other possible cases are discussed.     

Mechanical damage in polymers caused by laser pulses

The damage caused by Q-switched laser pulses and picosecond pulses in specimens of polymethyl methacrylate and polystyrene is described. The damage criterion depends on both characteristics of the laser pulse, i.e., energy and intensity. Nonlinear absorption effects are not observed when the energy threshold is crossed. It is shown that Q-switched and free-running laser pulses react with polymer specimens in essentially the same way. The induction period and rate of crack development have been established. The emission excited in the specimen by the laser pulse is investigated.Institute of Problems of Mechanics, Academy of Sciences of the USSR, Moscow, Physics Institute, Academy of Sciences of the USSR, Moscow, All-Union Scientific-Research Institute of Optico-physical Measurements, Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 436–444, May–June, 1971.     

Effect of synergism in the mechanics of multilayer designs. 2. synthesis of optimal acoustic systems

Conclusion Use of the synergism effect in the mechanics of inhomogeneous systems, and algorithms constructed on the basis of the theory of optimal control makes it possible to synthesize multilayer designs that are optimum in terms of weight capacity, thermal stability, sound conduction, and sound insulation. The proposed approach to solution of the problem of the optimal design of multilayer acoustic systems makes it possible firstly to develop a rather effective method on the basis of this approach; this method reduces to a solution that is optimal in terms of the physical parameters of the layer materials, the layer thicknesses, and the number of layers; and, secondly, to establish a number of qualitative attributes of the parameters of optimal multilayer structures. This approach is also promising for the development of methods for the optimization of multilayer foundations and systems that dampen vibrational loadings, earthquake-resistant designs, and armor protection.Presented at the Fifth All-Union Conference on the Mechanics of Polymer and Composite Materials (Riga October, 1983).See [1] for Report 1.Translated from Mekhanika Kompozitnykh Materialov, No. 4, pp. 686–691, July–August, 1984.     

Synthesis and micro-structural study of one-dimensional nano-materials

Silicon nano-wires (SiNWs) and boron nitride nano-tubules (BN-NTs) were successfully synthesized by excimer laser ablation at high temperature. These one-dimensional nano-materials synthesized by this method have a very high yield, a uniform diameter distribution, and a high purity. Micro-structures of these nano-materials were investigated by transmission electron microscopy (TEM). The SiNWs have a high density of structural defects of microtwin, stacking faults, and low-angle boundary, which are closely related to the formation of SiNWs and the determination of morphology of the nano-wires. BN-NTs are mainly single atomic-layered and the outer surface of tubules is clean without any attachment. The formation of single atomic-layered tubule is attributed to the catalyst effect which makes the axial rate of BN-NTs much higher than the radial growth     

An inverse medium problem for scanning acoustic microscopy

For nondestructive testing of materials, the scanning acoustic microscope is commonly used for the qualitative visualization of elastic properties. However, especially for medical applications, quantitative evaluation of the measured data is of considerable interest. Based on a mathematical model of the process of scanning acoustic microscopy, the problem of recovering elastomechanical parameters of human bone can be cast into the form of identifying the unknown space-dependent speed of sound in the three-dimensional acoustic wave equation. Specific methods have to be devised for this kind of inverse medium problem due to the reduced set of measured data available. Here, an optimal control approach is used. We discuss uniqueness and stability of the reconstruction and present a numerical example. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On evaluating the performance of GSS: Furthering the debate

Following criticism from proponents of workstation group support systems about the lack of validity of ‘wide-band’ group support systems (GSS), a recent article argued that it was inappropriate to attempt to establish the level of experimental validity for such GSS that is demanded by researchers working with 'narrow-band' GSS. It was argued that ‘wide-band’ and workstation GSS are very different and this means that it is unlikely that the same measures of success would be appropriate. This paper agrees with this view but not with its extension, that the overall validation requirement is different; theoretical validity is a necessary feature of a full evaluation but it is not sufficient by itself - experimental validity must also be sought. This paper argues against the view that since experimental validity cannot be obtained, theoretical validity will suffice. A twin-track research approach involving theoretical and experimental validity is proposed that can unite researchers investigating (almost) all forms of GSS, modelled on the PIMS programme and supported by one of the research frameworks that already exist.     

Improving the accuracy of multiple integral evaluation by applying Romberg’s method

Romberg’s method, which is used to improve the accuracy of one-dimensional integral evaluation, is extended to multiple integrals if they are evaluated using the product of composite quadrature formulas. Under certain conditions, the coefficients of the Romberg formula are independent of the integral’s multiplicity, which makes it possible to use a simple evaluation algorithm developed for one-dimensional integrals. As examples, integrals of multiplicity two to six are evaluated by Romberg’s method and the results are compared with other methods.