Evaluation of Mode I Fracture Toughness Assisted by the Numerical Determination of K-Resistance

Takahiro Funatsu,Norikazu Shimizu,Kikuo Matsui,Mahinda Kuruppu

doi:10.1007/s00603-014-0550-8

Abstract

The fracture toughness of a rock often varies depending on the specimen shape and the loading type used to measure it. To investigate the mode I fracture toughness using semi-circular bend (SCB) specimens, we experimentally studied the fracture toughness using SCB and chevron bend (CB) specimens, the latter being one of the specimens used extensively as an International Society for Rock Mechanics (ISRM) suggested method, for comparison. The mode I fracture toughness measured using SCB specimens is lower than both the level I and level II fracture toughness values measured using CB specimens. A numerical study based on discontinuum mechanics was conducted using a two-dimensional distinct element method (DEM) for evaluating crack propagation in the SCB specimen during loading. The numerical results indicate subcritical crack growth as well as sudden crack propagation when the load reaches the maximum. A K-resistance curve is drawn using the crack extension and the load at the point of evaluation. The fracture toughness evaluated by the K-resistance curve is in agreement with the level II fracture toughness measured using CB specimens. Therefore, the SCB specimen yields an improved value for fracture toughness when the increase of K-resistance with stable crack propagation is considered.

Highlights

In rock engineering problems dealing with the stability of structures, controlling crack initiation and propagation is very important
We investigated the application of the distinct element method (DEM) (Cundall and Strack 1979), which is based on discontinuum mechanics, because crack propagation and microcracking occur in a discontinuous manner
To investigate the mode I fracture toughness using semicircular bend (SCB) specimens, we experimentally studied the fracture toughness using semi-circular bend (SCB) and chevron bend (CB) specimens, the latter being one of the International Society for Rock Mechanics (ISRM) suggested methods, for comparison

Summary

Introduction

In rock engineering problems dealing with the stability of structures, controlling crack initiation and propagation is very important. The International Society for Rock Mechanics (ISRM) has incorporated chevron bend (CB) (ISRM 1988), short rod (SR) (ISRM 1988), and cracked chevron-notched Brazilian disk (CCNBD) (Fowell 1995) specimens into the standard method for the measurement of the fracture toughness of rock materials. The fracture toughness measurement method using the SCB specimen shown in Fig. 1 has been recently approved as an ISRM suggested method (Kuruppu et al 2014). It is a core-based specimen that possesses inherently favorable characteristics such as simplicity, minimal machining requirements, and easy testability through the application of three-point compressive loading using a standard test frame

Methods

Results

Discussion

Conclusion