Relaxation dynamics of a broadband nanosecond acoustic pulse in a bubbly medium

The first experimental observation of the propagation dynamics of a short broadband acoustic pulse in a resonance medium with gas bubbles is carried out. The probing pulse is generated using the optoacoustic effect. It is shown that the theory of short pulse propagation in media with generalized resonance relaxation adequately and accurately describes the dynamics of short pulse dispersion. A possibility to determine the relaxation and resonance parameters of media by the pulsed testing technique is demonstrated.     

Laser optoacoustic measurement of paper porosity

The propagation of broadband ultrasonic pulses in combined media that consist of printing paper of different porosity saturated with different liquids is studied. The experiments are performed with three types of paper, namely, Zoom Ultra (Stora Enso, Finland) with surface densities of 80 and 100 g/m² and Data Copy (Mo Do, Sweden) with a surface density of 160 g/cm², and with two types of saturating liquids: ethanol and transformer oil. To excite ultrasonic pulses and to detect them with a high time resolution, the laser optoacoustic spectroscopy method is used. For each type of liquid-saturated paper, the phase velocity of ultrasound is measured in the frequency range of 5–35 MHz. The absence of any noticeable frequency dispersion of the phase velocity is revealed. The possibility of measuring the porosity of printing paper on the basis of the theoretical model of a two-phase medium with the use of the corresponding experimental data is demonstrated.     

Direct measurement of the spatial distribution of light intensity in a scattering medium

A direct nonperturbative measurement of the spatial distribution of the light intensity in a strongly scattering medium is performed using an optoacoustic method. It is shown that near a surface the intensity can be five times greater than the incident intensity, and the absolute maximum of the intensity is observed at a depth ℓ(1–R)(1–4.0R) determined by the photon transport mean free path ℓ and the effective light reflection coefficient R of the boundary separating the scattering and external media. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 187–191 (10 August 1999)     

Influence of the porosity on the dispersion of the phase velocity of longitudinal acoustic waves in isotropic metal-matrix composites

The influence of the volumetric porosity of isotropic metal-matrix composite materials, which are reinforced with ceramic microparticles, on the dispersion of the phase velocity of longitudinal acoustic waves is investigated. For this purpose, the method of broadband acoustic spectroscopy with a laser source of ultrasound and piezoelectric detection of nanosecond ultrasonic pulses is used. Composite samples based on a silumin matrix with added silicon carbide (SiC) microparticles in different mass concentrations (3.8–15.5%) were investigated. As the concentration of SiC particles in a sample increases, its porosity that is determined using the hydrostatic-weighing method also increases. The simultaneous increase in the filler concentration and porosity leads to the appearance of a dispersion of the phase velocity of longitudinal acoustic waves in the sample within the frequency range of 3–25 MHz. The obtained empirical relationship between the relative change in the phase velocity and the sample porosity can be used to obtain a proximate quantitative estimate of the bulk porosity of the isotropic metal-matrix composite materials.     

Nondestructive Evaluation of Graphite-Epoxy Composites by the Laser Ultrasonic Method

A laser ultrasonic method for nondestructive evaluation of the structure of composite materials is proposed. Specimens of graphite-epoxy composites with compaction-type defects and air cavities are investigated. The method is based on the laser thermooptical generation of wide-band acoustic pulses - optoacoustic (OA) signals - in the material investigated. The acoustic pulses backscattered by structural ingomogeneities and defects are registered by a wide-band piezotransducer, which makes it possible to detect acoustic pulses in the frequency range from 0.1 to 30 MHz. Since the generation and detection of acoustic pulses takes place on the front surface of the specimen, this method allows us to carry out nondestructive evaluation with one-sided access to the object under study. The spectral and correlation analyses of backscattered OA signals are used for mathematical processing of the experimental data. The method developed makes it possible to determine the type of defects and the depth of their location.     

Determinating the Volume Content of a Polymeric Matrix in CFRP Structures Using a Laser-Ultrasonic Method

Mechanics of Composite Materials - 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 Optoacoustic Method for Quantitative Porosity Assessment of Carbon Fiber Reinforced Plastic Composites Based on Acoustic Impedance Measurement

Acoustical Physics - Abstract—A method for measuring the acoustic impedance for the assessment of the porosity of carbon fiber reinforced plastic composites using laser thermo-optical...     

The influence of porosity on ultrasound attenuation in carbon fiber reinforced plastic composites using the laser-ultrasound spectroscopy

Wideband acoustic spectroscopy with a laser ultrasound source for quantitative analysis of the effect of porosity on the attenuation coefficient of longitudinal acoustic waves in carbon fiber reinforced plastic (CFRP) composite materials was experimentally implemented. The samples under study had different bulk-porosity levels (up to 10%), which were determined using X-ray computer tomography. A resonance ultrasound attenuation peak associated with the one-dimensional periodicity of the layered composite structure was observed for all samples. The absolute value of the resonance-peak maximum and its width depend on the local concentration of microscopic isolated pores and extended delaminations in the sample structure. The obtained empirical relationships between these parameters of the frequency dependence of the ultrasound attenuation coefficient and the type of inhomogeneities and their volume concentration can be used for rapid evaluation of the structural quality of CFRP composites.     

Optoacoustic technique for thickness measurement of submicron metal coatings

The new nondestructive method for thickness measurement of submicron metal coatings on transparent substrate is developed. The method is based on the optoacoustic (OA) transformation in the system, where the coating is covered by an optically transparent liquid. Theoretical treatment of the problem consists of two steps. At the first step laser-induced thermal field in the system is calculated, taking into account the large thermal conductivity of the metal film and partial heat diffusion into the liquid. At the second step the system of wave equations for scalar potential of vibration velocities is solved. Heat sources, determined at the first step, are free form of wave equations. Three chrome coatings of different thickness (approximately 0.2, 0.3, and 0.6 μm) deposited on the quartz substrate are tested experimentally. Two different organic liquids (acetone and ethanol) are used to cover chrome coatings. Nanosecond diode-pumped Nd:YAG laser operated at the main harmonic is used to perform OA transformation (laser pulse duration is τ _L = 12 ns, the laser energy is about 0.2 mJ). Two detection modes are used. In forward mode laser pulse irradiates the film from the side of the substrate and in backward mode—from the side of the liquid. Detection of induced ultrasonic pulses is performed by the wide-band piezoelectric transducer in the liquid in both cases. The thickness of the coatings is determined by the least squares fitting of the theoretical dependencies of spectral transfer functions of OA transformation to experimental data. It is demonstrated, that the developed technique can be used for measurement of metal coatings thickness within the range from 50 nm to 5 μm with the error about 50 nm.     

Determination of uniaxial stresses in steel structures by the laser-ultrasonic method

This paper describes the experimental implementation of the laser-ultrasonic method for diagnosing mechanical compression and tensile stresses in steel structures, based on the acoustoelasticity effect. The special laser-ultrasonic transducer that provides the laser excitation and highly sensitive piezoelectric detection of head (longitudinal subsurface) ultrasonic waves is developed. It is shown on the example of R65 rail samples of various quality that, regardless of the structural phase state of the rail, there is one and the same linear relationship between the relative variation of the velocity of head ultrasonic waves and the absolute value of uniaxial compression and tensile stresses acting in the rail.