ABSTRACT:

The scale effect on subcritical crack growth in rock is important to extrapolate the laboratory test results to the field scale design. A constant stress-rate test has been performed to estimate the mode I and II subcritical crack growth parameters for Coconino sandstone. The experimental results show that the subcritical crack growth parameters of mode I and mode II loading are similar to each other. The subcritical crack growth parameter A values increase with increasing specimen size, whereas the subcritical crack growth parameter n values are independent of specimen size. The subcritical crack growth parameter n was determined to be 35 to 36, and A was determined to be 1.17×102 to 3.02×102 m/s.

1. INTRODUCTION

Subcritical crack growth is a time-dependent phenomenon, where crack growth occurs at a value of stress intensity factor below the critical value. Subcritical crack growth is important and controls the long-term behavior of rocks. Thus, it is necessary to evaluate the subcritical crack growth for the long-term behavior of structures in rocks. Subcritical crack growth under mode I loading can be expressed by the empirical power law relationship [1]: (mathematical equation available in full paper)

where, v1 is the crack velocity under mode I loading, A1 and n1 are mode I subcritical crack growth parameters, and KI is the mode I stress intensity factor and KIC is the mode I fracture toughness. The characteristics of subcritical crack growth are evaluated by determining the subcritical crack growth parameters. The mechanical properties of rocks vary with size [2-4]. For example, it is generally accepted that the uniaxial compressive strength (UCS) of rocks decrease with increasing sample size. Hoek and Brown suggested that the relation between the UCS of a rock specimen with a diameter of d (scd) and the UCS of a 50 mm diameter specimen (sc50) is expressed as [4]: (mathematical equation available in full paper)

The mode I fracture toughness of rock also changes with the specimen size. Many investigations have shown that the mode I fracture toughness of rocks increases with increasing the specimen size [5-7]. Like UCS and the fracture toughness, the size effect also affects subcritical crack growth in rocks. It is necessary to find the relation between the subcritical crack growth parameters and the specimen size when extrapolating laboratory determined values of the subcritical crack growth parameters to field scales. In this research, a constant stress-rate test [8, 9] has been performed to estimate the mode I and II subcritical crack growth parameters for Coconino sandstone. For the mode I subcritical crack growth, compact tension (CT) type specimens were tested and short beam compression (SBC) specimens were employed for the mode II subcritical crack growth [9-11]. The subcritical crack growth parameters were experimentally determined and the variations of the subcritical crack growth parameters with the specimen size were evaluated.

2. CONSTANT STRESS-RATE TEST

The constant stress-rate test uses the relation between the fracture strength and applied stress rate. The basis of the constant stress-rate test is that the strength increases with increasing applied stress rate in a given environment.

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