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.     

Loading velocity and brittleness of solids

The ductile-brittle transition observed in many solids due to either a decrease in temperature or an increase in the loading velocity can be supplemented with a reversible brittle-ductile transition under loading at a hypervelocity exceeding the velocity of propagation of fracture cracks. This phenomenon occurs in the course of high-velocity penetration of solids into organic and inorganic glasses, as well as into ceramic materials.     

Photoacoustic study of the transmission of wide-band ultrasonic signals through periodic one-dimensional structures

The propagation of wide-band acoustic pulses in one-dimensional periodic structures consisting of alternating plexiglas and water layers is studied theoretically and experimentally. The experiment is carried out with the use of the wide-band photoacoustic spectroscopy based on the laser excitation of ultrasound and a wide-band signal detection. The fact that the transmission spectrum of a periodic structure has alternating pass and stop bands is confirmed experimentally. The width and localization of the stop bands strongly depend on the thickness of the layers and on the phase velocity of ultrasound in them. It is demonstrated that defects of the structure periodicity give rise to one or several local transmission maxima in the stop band and to a modification of the pass band. The amplitude and position of a local maximum in the stop band strongly depend on the position of the defective layer. The experimental data agree well with the results of numerical simulation.     

Kinetics of high-velocity penetration into high-hardness brittle media

A study has been made of the kinetics of penetration of deformable steel and tungsten rods into high-hardness brittle media (ceramics and glass) at impact velocities of 1.3–1.6 km/s. The penetration has been established to occur in two stages. In the first stage, the penetration velocity increases as the ceramic is progressively damaged. The second stage corresponds to quasi-steady-state penetration into a zero-strength medium. It is shown that it is the first stage that determines the high resistance of ceramics to intense impact. The dependence of the resistance to penetration on target geometry and impact pressure has also been established. Fiz. Tverd. Tela (St. Petersburg) 41, 1785–1787 (October 1999) Deceased.     

Detection of nanosecond optoacoustic pulses in steel

The optoacoustic transformation of 5-ns laser pulses was used to excite broadband longitudinal acoustic pulses in a 0.4-mm-thick steel plate. An electromagnetic acoustic transducer that incorporated a flat wire coil with a diameter of 5 mm and an amplifier was proposed as a device for contactless ultrasound detection. A permanent cylindrical magnet and a coil close to the printed-circuit board of an operational amplifier were placed on one side of the sample, thus allowing the detection of the alternating electromagnetic field produced by pressure pulses excited on the opposite side. The frequency spectrum of detected pulses ranged from 5 to 200 MHz.