Summary: Crack propagation trajectories for rocks under mixed
mode III fracture
Naser A. Al-Shayea *
King Fahd University of Petroleum and Minerals, Civil Engineering Department, Box 368, Dhahran 31261, Saudi Arabia
Received 2 April 2005; received in revised form 23 July 2005; accepted 28 July 2005
Available online 19 August 2005
Propagation of a crack in engineering materials including rocks can cause failure. Knowledge of the stress state under which
a crack can propagate, and the trajectory it may follow during its growth are thus very important for the stability of rock masses/
materials and for the safe design of structures in/on rocks. In this paper, the crack initiation angle and subsequent crack
propagation path are experimentally investigated for limestone rock specimens. This investigation was conducted under various
mixed mode III loading conditions, including pure mode-I and pure mode-II.
This study includes conducting diametrical compression tests on notched Brazilian disk specimens. Moreover, the effect of
confining pressure and temperature on crack initiation and propagation were also studied. The experimental results were
compared with theoretical predictions of crack initiation angle. The results showed that limestone behaves in brittle fashion, and
the effects of confining pressure and temperature on failure trajectories were not significant. Generally, the crack initiation angle
can be predicted by the maximum tangential stress criterion. However, for notched Brazilian disk with high value of crack
orientation with respect to loading direction, crack does not propagate from the tip of the crack. This important observation
indicated that the tensile-strength failure can become more critical than the fracture-toughness failure.
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