Abstract:

Laboratory experiments were conducted on prismatic Indiana limestone specimens with a flaw, to examine the effect of uniaxial cyclic loading on crack growth and, most importantly, the effect of opening/closing of the crack on transmitted and reflected elastic waves. All specimens were initially subjected to a load that produced a kink at the flaw tips, and then were subjected to 20 loading-unloading cycles. The maximum applied load at each cycle was the load of kink initiation and the minimum load was such that no detachment between the specimen and loading platens occurred. After cyclic loading, the compressive load was increased until failure. Digital image correlation (DIC) was used to observe the damage evolution around the flaw tips and to monitor the opening/closing of the kink. At the same time, transmission and reflection of compressional and shear waves across the specimen were monitored. The DIC results showed that the kinks that initiated from the flaw tips completely closed during unloading. The normalized amplitude of the transmitted waves showed a progressive decrease with additional cycles of loading until reaching a steady-state. Such decrease in amplitude is thought to be related to additional damage induced to the kink due to the loading-unloading cycles.

Introduction

The mechanical behavior of geomaterials is strongly affected by the presence of micro-cracks. A large number of experiments have been done on crack initiation, propagation, and coalescence in pre-cracked brittle materials and show that a number of cracks initiate from the tips of pre-existing cracks [2, 3, 5, 6, 10, 15, 19-21, 24-26]. There are however two types of cracks that are most common: tensile and shear cracks (Fig. 1). The tensile cracks initiate prior to the shear cracks and are stable, which means that an increase in load is required for propagation. The shear cracks are initially stable and may become unstable as crack coalescence and failure are approached. Two types of shear cracks have been identified, based on their angle of initiation with respect to the flaw: coplanar or quasicoplanar and oblique. Coplanar shear cracks make an angle smaller than 450 with the flaw, while oblique shear cracks make an angle larger than 450 [15]. Prior experimental work was performed almost exclusively using monotonic loading. In addition, most of the observations in the experiments were made based on visual inspection using optical magnification or highspeed cameras. All of these studies have successfully identified the mechanical behavior of rock and cracking phenomena at the laboratory scale. Microscopic characterization of new cracks inside the material and monitoring microcrack development under low amplitude cyclic loading is a very important topic that affects the response of fractured rock mass subjected to repetitive loading such as vibrations, cyclic thermal loading, seismic loading from remote earthquakes, traffic loading, and other ambient noise.

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