NONDESTRUCTIVE EVALUATION OF ONE-DIMENSIONAL PERIODIC STRUCTURES BY TRANSMISSION OF LASER-EXCITED WIDE-BAND ACOUSTIC PULSES

摘要

The propagation of ultrasound in one-dimensional model and real periodic structures (PS) is studied experimentally by the method of optoacoustic spectroscopy based on laser thermooptical excitation and wide-band piezodetection of short acoustic pulses. It is shown that the ultrasound transmission spectrum of PS has stop- and pass-bands, and the greater the number of layers in PS the deeper the stop-bands. The case where the thickness, density, and ultrasound velocity of one or several layers in PS are modified is studied in detail. Then, a narrow local maximum of ultrasound transmission appears in the stop-band, whose location depends considerably on the position of the "defect" layer in PS. The experimental data coincide well with the theoretical calculation. Nondestructive evaluation of real 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 an epoxy-glue layer.