The paper presents a method for quantifying the porosity of carbon fiber structures based on laser excitation of probing ultrasonic pulses. A method for estimating the porosity of a material from the experimentally measured phase velocity of longitudinal acoustic waves propagating in it is proposed. Using the example of control samples and real structures made of carbon fiber, the possibility of obtaining distributions of local porosity values in the studied section of the structure is demonstrated. It is shown that in the studied structures there are regions with a significant variation in the value of local porosity. The proposed method can be used to control the quality of manufactured composite structures, as well as to study the internal changes in structure during operation.

We compare transverse structure evolution and energy deposition into the medium within focused multifilament arrays created using two different types of diffraction optical elements (DOEs): TEM11 phase plate and a Dammann grating. We show that the employment of the Dammann grating provides a robust way to create regular multifilament arrays, which is far less dependent on laser beam quality than one using the phase plate.

Transformation of laser-induced broadband pulses of longitudinal ultrasonic waves into pulses of shear waves and back into pulses of longitudinal waves (further called as the "double" transformation) in an isotropic solid plate immersed in a liquid is theoretically studied. It is shown that the time profile of the pulse of shear waves strongly depends on the angle of incidence and only at a certain value of this angle the time profiles of the incident longitudinal-wave pulse and induced shear-wave pulse coincide. For various angles of incidence, the broadband pulses of longitudinal waves experimentally obtained after the double transformation in an aluminum and fused silica plane-parallel plates immersed in distilled water correspond to the theoretically calculated profiles except for the increased duration of experimental pulses. Based on the double transformation scheme, the method of broadband acoustic spectroscopy of longitudinal and shear waves for isotropic solid plates in the spectral range of the blue-green glass laser source of ultrasound (1-40 MHz) was proposed and experimentally realized for the first time. The obtained frequency dependences of the attenuation coefficients of longitudinal and shear ultrasonic waves in stainless steel, cast babbit, and brass can be used for appropriate choice of an operating frequency range by ultrasonic nondestructive testing of these materials.

The work demonstrates the use of the laser optoacoustic method for observing the changes of the light extinction coefficient in unstable ferromagnetic fluids over time. The initial stabilized fluids were further diluted with a carrier liquid to violate the stability of the colloidal system. The form of optoacoustic signals excited in a ferromagnetic fluid carries information about its optical properties, which allows one to measure the light extinction coefficient of this fluid and obtain its distribution over the depth of fluid. The effect of dilution of ferromagnetic fluid on its optical properties was demonstrated. It was shown that the light extinction coefficient decreases by about 20% after 7 days after dilution, and after 30 days, its value decreases by half. The observed decrease in the extinction coefficient over time is a result of structural changes occurring in diluted ferromagnetic fluids. The proposed optoacoustic method can be useful for studying the optical properties of ferromagnetic fluids for developing optical devices based on them and also may be used to evaluate the stability of ferromagnetic fluids obtained by new technologies and fluids with a new type of stabilizer or carrier liquid.

In this work, to obtain the frequency dependences of the phase velocity of longitudinal acoustic waves in composite materials, we use the method of broadband laser-ultrasonic spectroscopy, based on laser thermo-optical excitation of broadband acoustic pulses. Unidirectional carbon fiber reinforced plasticsampes with different volume contents of the matrix and fiber were taken as objects of study.For the studied samples, the frequency dependences of the phase velocity were obtained in the spectral range of 0.8 – 10 MHz, and the existence of the phase velocity dispersion in this range was shown. The phase velocity dispersion of longitudinal acoustic waves in the sample was calculated using the phase spectra of the probe ultrasonic pulse and the pulse transmitted through the sample under study. It is also shown that the relative dispersion of the phase velocity characterizes the porosity of the sample, and the higher the porosity, the greater the relative dispersion. Empirical relations obtained for the relationship between porosity and relative dispersion can be used for the operational non-destructive assessment of the porosity of composites. The results obtained can be useful both for quality control of the materials obtained with the aim of modernizing manufacturing technologies and for predicting the behavior of structures and parts of this material under external loads.

We report on a hybrid optoacoustic method aimed to retrieve the three-dimensional spatial distributions of the plasma electron density and the deposited energy density in the region of plasma formation under optical breakdown in condensed medium. The spatial distribution of the plasma electron density obtained by the proposed method with the accuracy of 3×1018 cm−3 gives the qualitative characterization of the laser pulse propagation in the region of plasma formation. The spatial distribution of the deposited energy density retrieved by the proposed method with the accuracy of 14 J/cm3 provides the quantitative description of the laser impact on the bulk of the medium. The method is based on photoacoustic imaging and shadowgraphy techniques, which have the spatial resolution of 2 μm and 10 μm, respectively. The proposed method can be applied both in the area of technological applications, such as micromachining of transparent materials, and in the field of the fundamental science of laser–matter interaction.

The aim of this work is the development and experimental implementation of the laser-ultrasonic method for CFRPs porosity assessment with one-side access to an investigated object. The scheme with the laser thermooptical generation and backward-mode piezoelectric detection of longitudinal acoustic waves is used. The proposed method is based on the measurement of the acoustic impedance of a composite specimen by using the value of the integral of the ultrasonic pulse reflected from an immersion layer-specimen interface. The existence of the relationship between the porosity content, ultrasound velocity, and material’s density allows one to assess the porosity of CFRPs by the measured acoustic impedance. The proposed method does not require plane parallelism of the input and output surfaces of the studied objects, as well as measuring the thickness of the object. This allows studying CFRP structures with complex shapes. This method may be of use for various types of composite materials, regardless of acoustical waves propagating features related to a periodical material’s structure. CFRP specimens with three reinforcement schemes and different average volume porosity contents were investigated. It is shown that the local porosity distribution in the studied CFRPs is nonuniform along the fiber stacking plane. The porosity content obtained by the laser-ultrasonic method was checked using the X-ray computer tomography. The porosity values, averaged from the laser-ultrasonic measurement results, coincide with the X-ray tomography data within the error limits. The method can be useful for quality control of obtained composite structures with the aim of modernization of technology processes and selecting optimal production conditions and also for the detection of changes in composite structures during their operation or during fatigue testing processes.

In the present work, the method which allows investigating the spatial distribution of light extinction coefficient in colloidal media is experimentally realized. The proposed method is based on the dependence of temporal profiles of excited OA-signals on the absorption and extinction coefficients of the studied medium. Water-based and kerosene-based magnetic fluids with volume content of magnetite particles in the range 0.35–3.5 % were studied as an example of the medium with spatially non-uniform optical properties. The capability of the reconstruction of the one-dimensional spatial distribution for the light extinction coefficient in the investigated media is demonstrated. It was shown that the relative change of the extinction coefficient with depth depends on volume concentration of magnetite particles, type of acoustical boundary and properties of carrier liquids. In the case of acoustical rigid boundary of magnetic fluid, an additional induced anisotropy occurs, which leads to increasing of the light extinction coefficient with depth in comparison with acoustical free boundary of fluid.

An acoustic method for a quantitative assessment of the volume content of a polymeric matrix in CFRPs is proposed and realized experimentally. For this purpose, a laser-ultrasonic method based on the thermo-optical excitation of broadband pulses of longitudinal acoustic waves is used. A formula for calculating the volume content of the polymeric matrix in a CFRP from experimentally measured phase velocities of longitudinal acoustic waves in this material is proposed. Distributions of the volume content of the matrix in a control CFRP specimen and in a CFRP structure made under similar conditions are obtained. It is shown that the laser-ultrasonic method allows one to detect the areas with excess and deficient matrix content in CFRP specimens and areas of CFRP constructions and to quantitatively assess the spread of matrix content. The spread between the maximum and minimum matrix content observed depends on the shape of the investigated area of the structure. This method can be used for the quality control of composite structures before their exploitation.

Разработана экспериментальная установка для лазерно-акустической томографии биологических тканей. Проведены экспериментальные исследования фантомных образцов. Разработана программа для преобразования графических данных лазерно-акустической томографии в стандартный медицинских формат. Ключевые слова: лазерная ультразвуковая томография, формат DICOM, биологическая ткань, цифровая модель, графические данные.

We demonstrate a novel approach to create regular multifilament arrays under additional focusing by use of a Dammann grating. The employment of Dammann grating offers an advantage over the Hermite-Gaussian phase plates in terms of multifilament array robustness and resistance to beam imperfections and fluctuations.

We report a study of the photoacoustic energy conversion efficiency under femtosecond filamentation in water. We characterized the interaction of ultra-short laser pulse with water under different temperatures and filamentation regimes.

The results of an experimental analysis of the effect of the volume content of voids in the range of 0.25–5% on the crack resistance of structural carbon plastics are presented. To obtain a variation of the volume content of voids, samples were manufactured using the vacuum-infusion method with variations in the vacuum strength from –760 to –150 mm Hg. The total volume content of voids was determined by methods of optoacoustic structuroscopy, scanning electron microscopy, and chemical etching to obtain comparative data. The experimental dependences of the characteristics of the interlaminar crack resistance on the volume content of voids under static and cyclic loading of samples in the normal-separation mode were obtained.

We present a novel approach to the characterization of ultrafast laser-matter interaction processes in solids and liquids under extreme conditions of microplasma generation. Through the combination of three-dimensional propagation imaging, absorption measurements, shadowgraphy and photoacoustic imaging we can restore plasma electron density distribution, laser pulse fluence profile and the value of deposited energy density inside the bulk of the material and characterize the regime of the laser pulse propagation. The developped concept is important for understanding the physics of ultrafast laser-matter interactions with strong implications for precision control of laser micromachining, bioprocessing and biotreatment.

A method of measuring the acoustic impedance of carbon fiber plastics based on the laser optoacoustic effect is proposed and experimentally realized. Measurement of the acoustic impedance of the studied composite is made by the value of the antiderivative of ultrasonic pulse reflected from the interface between the immersion liquid and the sample. A method for determining the porosity of a material by the measured value of the acoustic impedance, based on the dependence of the material density and the velocity of propagation of longitudinal acoustic waves in it on its porosity, is presented. Porous samples of crossply reinforced carbon plastics with three types of carbon fiber lay-up schemes were studied. It was found that the studied carbon fiber plastics have a non-uniform distribution of local porosity in the plane of carbon fabric stacking. It is also shown that the variation of the local porosity in the sample depends on the fiber laying scheme. It is shown that the porosity value obtained by X-ray computed tomography coincides with the results of laser-ultrasonic measurements. The advantage of the proposed method is the possibility of rapid diagnosis of porosity with one-way access to the object under study without measuring its dimensions and mass, which can be used for composite structures of complex shape.

A method for analyzing the structural inhomogeneity and optical properties of colloids is experimentally realized. The method uses the dependence of the time profile of the optoacoustic signal excited in the studied medium on the light absorption and extinction coefficients in this medium. As the tested colloid solutions, water- and kerosene-based magnetic fluids with a volume magnetite concentration of 0.35–3.5% were studied. The increase in the extinction coefficient in magnetic fluids by the increase in depth was observed. The relative variation in the extinction coefficient depends on the concentration of magnetite particles, properties of the carrier fluid, and the acoustic boundary type. The relationship between the particle concentration and optical properties of the magnetic fluid allows this method to be used for determining the volume concentration of magnetite in the fluid.

Time-resolved terahertz spectroscopy can be proposed as a method of non-destructive testing of various parts, for example, aircraft composite materials. The theoretical background and the possibility of exciting pulses of electromagnetic waves in the range of terahertz oscillations using picosecond acoustic pulses excited by laser pulses based on the thermoelastic effect are studied.