Investigation on the effect of freeze-thaw on fracture mode classification in marble subjected to multi-level cyclic loads

Abstract

For rock engineering in cold regions, rock is often subjected to coupled fatigue damage caused by freeze-thaw (F-T) and stress disturbance. To investigate the cracking propagation and to quantify the degree of damage and the type of crack classification, multi-level cyclic loading experiments were carried out on marble with F-T treatments of 0, 20, 40 and 60 cycles. Real-time acoustic emission (AE) and post-test computed tomography (CT) scanning technologies were employed to reveal the fracturing evolution and to further classify different crack types to aid in understanding dynamic fracturing. An AE parameter analysis method combined with a b-value analysis is able to identify the cracking event modes and cracking propagation. Results show that the RA (ratio of the rise time to amplitude) and AF (ratio of the AE counts to the duration) pattern is strongly influenced by the previous F-T treatment. The results of the kernel density estimation (KDE) and the k-mean cluster method indicate that a tensile cracking event and shear/mixed cracking event can be classified from an RA and AF data set. The proportion of tensile cracks decreases and shear/mixed cracks increases with the increase of F-T cycles. In addition, b-value analysis reveals the damage propagation at different cyclic levels and a minimum b-value was observed at the last cyclic level. Moreover, post-test CT scanning shows that complex crack network can form for a sample subjected to low freeze-thaw treatment, and the scale of tensile cracks decreases and shear/mixed cracks increases as F-T cycle grows. It is suggested that the deterioration of rock meso-structures impacts the fracturing mode and the associated rock stability in cold regions. The decreasing of the b-value and an increasing of the RA values provide an early warning for rock fracturing and implementation of rock mass stability prediction.