A total of 170 tests (68 tests for monotonic loading, 102 tests for cyclic loading) have been performed to investigate crack initiation, propagation and coalescence. The specimens have two pre-existing flaws which are arranged at different distances and angles. Wing cracks and secondary cracks are observed in both monotonic and cyclic tests. Wing cracks, which are tension cracks, initiate at (or near) the tips of the flaws and propagate parallel to the compressive loading axis. Secondary cracks always appear after wing crack initiation and lead to final failure. Secondary cracks initiate at the tips of the flaws and propagate in the coplanar direction of the flaw or in a horizontal (quasi-coplanar) direction. Six types of coalescence are observed. For coplanar geometry specimens, coalescence occurs due to the internal shear cracks. For non-coplanar geometry specimens, coalescence occurs through combinations of internal shear cracks, internal wing cracks and tension cracks. Contrary to monotonic tests, cyclic tests produce fatigue cracks. Fatigue cracks usually occur when 1) after coalescence, the specimens behave as if they had only one larger crack 2) specimens have been subjected to a particular number of cycles. In these experiments, two different fatigue crack initiation directions are observed: horizontal and coplanar to the flaw.
Rock and rock structures such as bridge abutments, dam and road foundations, and tunnel walls undergo cyclic loading caused by earthquakes, vehicle-induced vibrations, drilling, and blasting. This type of loading often causes rock to fail at a lower stress than its monotonically determined strength.
Under cyclic loading, brittle materials exhibit a failure pattern which differs from that under monotonic loading. Fig. 1 shows the difference of the failure type between monotonic and cyclic loading. Under uniaxial monotonic loading, two typical cones develop in the proximity of the loaded base. Whereas under uniaxial cyclic loading, fracture occurs as a rule along an inclined plane that develops throughout the specimen?s entire height .
It appears, therefore, that crack pattern and coalescence are different for monotonic and cyclic loading but the mechanisms of crack coalescence under cyclic loading have not been sufficiently investigated. The main goal of this study was to increase the understanding of the fracture process in rock material under cyclic loading. Experiments using gypsum as a model material for rock were carried out with the following objectives:
? What is different in crack propagation and coalescence between monotonic and cyclic
? How do pre-existing cracks behave in rock material under cyclic uniaxial compression?
A total of 170 tests (68 tests for monotonic loading, 102 tests for cyclic loading) have been performed to investigate crack initiation, propagation and coalescence.