School of Civil Engineering
Faculty of Geotechnical Engineering
Engineering Geology - Underground Construction - Rock Mechanics
| Purdue University School of Civil Engineering Faculty of Geotechnical Engineering Engineering Geology - Underground Construction - Rock Mechanics |
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Crack and fracture propagation and coalescence play a very important role in the behavior of any brittle material and on a variety of scales; they are particularly important in the behavior of rock and rock masses. In this research crack coalescence and crack initiation and propagation in brittle materials in uniaxial and in biaxial compression are investigated through a systematic series of experiments and through numerical modeling. Specimens of gypsum, used as a model rock, with two initial cracks or flaws are prepared and tested. Different flaw arrangements are generated by changing the flaw angle, spacing and continuity, and the type of the flaws (open or closed). Crack initiation, propagation, coalescence and crack pattern are studied. Wing cracks, which are tensile cracks, appear only under unconfined or low confining stresses; they initiate at the tips of the flaws at low confining stress, shifting their initiation location to the middle of the flaw for confining stresses below 5.0 MPa; above 5.0 MPa wing cracks cannot initiate. In contrast, secondary cracks appear in all the tests; they initiate as shear cracks and initially propagate in a plane co-planar to the flaw irrespective of the geometry, type of the flaws or the confining stress. For some flaw geometries the internal secondary cracks can become unstable and produce coalescence. Five types of coalescence have been distinguished that depend on the geometry of the flaws. Crack initiation and coalescence stresses increase with flaw angle, ligament length, and confining stress; also, closed flaws present stresses higher than open flaws.
The code FROCK, a DDM (Displacement Discontinuity Method) is used for the numerical modeling of the experiments. Crack pattern, crack initiation, propagation and crack coalescence are satisfactorily predicted with a new stress-based crack initiation an propagation criterion implemented into the DDM. The new initiation criterion requires only three material properties: the critical strength of the material in tension, the critical strength of the material in shear, and the size of the plastic zone. The three parameters can be determined with the results from only one test. The model can also accurately duplicate experimental results in compression and in tension obtained by other researchers.
The observations and conclusions obtained from the extensive testing conducted in this investigation contribute to substantial progress in understanding cracking and crack coalescence. In addition, since the numerical model and the initiation criterion can duplicate the crack pattern and coalescence process quite well it promises to become a valuable tool for future research and for practice.
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