Micro- and macrocracking behaviors in granite and molded gypsum containing a single flaw

Abstract

The effect of preexisting defects on fracture behaviors is significant for the stability analysis of rock engineering. To study the effect of preexisting flaws on the micro- and macrocracking behaviors of rocks, photographic monitoring together with strain field analysis, passive acoustic emission (AE) and active ultrasonic testing were carried out on gypsum and granite samples containing a single flaw. Based on coupled analysis of the acousto-optic-mechanical characteristics, it is suggested to identify micro- and macrocrack initiation by AE monitoring rather than stress–strain behaviors, photographic observation and P-wave measurement. The micro- and macrocrack initiation stresses generally increase with the flaw angle. The microcrack initiation stress ranging from 0.22σc to 0.62σc (peak strength) coincides with the onset of AE events, and the macrocrack initiation stress ranging from 0.60σc to 0.85σc coincides well with a dramatic increase in AE energy for flawed granite. The crack types are classified into nine different categories. Macrocrack coalescence is observed before reaching σc, and violent rock failure occurs seconds after σc is achieved for flawed granites, which differs from that process for gypsum samples. The cracking process of flawed granites experiences three typical stages and is highly dependent on the flaw angle. A stress drop accompanied by a significant increase in the AE energy rate appears several times. With an increasing flaw angle, the count of stress drop and the cumulative AE energy decrease, while the maximum value of the AE energy rate and the average hit energy increase gradually. The energy and stress release reduces the violent failure degree of rock. The AE energy correlated with the cracking magnitude changes more violently than the AE event before peak failure. P-wave attenuation increases sharply after macrocrack initiation. A sharp reduction in velocity is always accompanied by a significant increase in the AE rate. Compared with the global cracking damage of rock characterized by AE characteristics, the fracture identified by P-wave velocity is localized.