ABSTRACT:

Subcritical crack growth is important for the long-term behavior of rock and can be used to evaluate the longterm stability of underground and surface structures in rock masses. In this study the mode II subcritical crack growth parameters were determined experimentally using the double notch shear test. A series of tests were performed to determine the appropriate geometry for the double notch shear test. The mode II stress intensity factor was determined using finite element analysis using the displacement extrapolation method. The subcritical crack growth parameters in Flagstaff sandstone were determined to be n =28.27 and A =0.00647 m/s.

INTRODUCTION

In the classical fracture mechanics, cracks will propagate when the stress intensity factor reaches or exceeds a critical stress intensity factor, c K . In systems subjected to long-term loading, however, the classical fracture mechanics approach does not work, and crack growth can occur at a value of stress intensity factor that may be substantially lower than the critical value. This phenomenon is known as subcritical crack growth. Subcritical crack growth is important for the long-term behavior of rock and can be used to evaluate the long-term stability of underground and surface structures in rock masses.

A significant amount of research has been conducted to determine the subcritical crack growth parameters in mode I [1-5], but the use of subcritical crack growth parameters in mode II is rare, yet important for many applications. For instance, for the time-dependent stability of underground and surface excavations in rock, the time-dependent growth of shear fractures is often more important than the growth of tensile cracks. Even though the growth of shear cracks is a complex process that usually involves the growth of tensile microcracks, mode I crack growth alone cannot adequately describe the phenomenon of shear crack growth in rocks.

In this research, Flagstaff sandstone has been used to estimate mode II subcritical crack growth parameters using the double notch shear test. Based on work in the fracture mechanics of ceramic materials [6-9], a method for estimating both subcritical crack growth parameters has been developed using the loading rate dependence of tested samples. In particular, the shear strength using the double notch test has been determined using loading rates ranging from 0.01 to 5 / MPa s . The mode II fracture toughness, IIc K , has also been determined for Flagstaff sandstone using the double notch shear test.

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