Nondestructive assessment of local microcracking degree in orthoclase and plagioclase feldspars using spectral analysis of backscattered laser-induced ultrasonic pulses.

The laser-ultrasonic method for nondestructive assessment of a microcracking degree in laboratory specimens of orthoclase and plagioclase feldspars is proposed. The influence of the local concentration of microcracks on the spectral efficiency of backscattering of pulses of longitudinal ultrasonic waves in the studied specimens (the so-called "structural noise power") is studied. A specially designed laser-ultrasonic transducer used in experiments combines laser excitation of probe broadband ultrasonic pulses in a black polyethylene film and piezoelectric detection of both probe pulses and that scattered in the specimen. We study specimens of a potash feldspar and soda-calcium plagioclase with nonuniformly distributed local microcracks. The cracking domains were identified by optical microscopy as well as using the attenuation coefficient of longitudinal ultrasonic waves measured in these domains. The increase in the ultrasonic attenuation coefficient was associated with a higher concentration of microcracks, which efficiently scatter acoustic waves. At the same time, the domains with a higher ultrasonic attenuation exhibited an increased structural noise power. The direct correlation between the growth of the structural noise power and a higher local concentration of microcracks can be used as a basis of a system of nondestructive ultrasonic monitoring of occurrence and evolution of local microcracks in rocks and geomaterials under external loads of different nature.