Experimental uniaxial compression loading tests were conducted on molded gypsum and Carrara marble prismatic specimens to study the cracking and coalescence processes between pre-existing artificial flaws. The study showed that material had an influence on the cracking and coalescence processes (see the companion paper in this issue). As reported in the companion paper, one of the pronounced features as observed in the high-speed video recordings was the development of macroscopic white patches prior to the development of observable cracks in marble, but not in gypsum. This paper (part 2) deals with the microscopic aspects of the study. Specifically, the scanning electron microscope (SEM) and the environmental scanning electron microscope (ESEM) imaging techniques were used to study the microscopic development of white patches and their evolution into macroscopic tensile cracks and shear cracks in marble, and the microscopic initiation of hair-line tensile cracks and their evolution into macroscopic tensile cracks in gypsum. The microscopic imaging study in marble showed that the white patches were associated with extensive microcracking zones (process zones), while the extent of process zone development in gypsum was limited. The comparison of the macroscopic and microscopic results indicates that the different extent of microcracking zone development, related to the material textural properties, is a key factor leading to different macroscopic cracking behavior in gypsum and marble.
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The geometry of a flaw pair is defined by three geometrical parameters as L–β–α, where L is the ligament length (intact rock length between the inner flaw tips), β is the flaw inclination angle (the inclination angle of flaw with the horizontal), and α is the bridging angle (the inclination of a line linking up the inner flaw tips). Please also refer to Fig. 1 of the companion paper for the schematic representation.
Note that, in addition to the coalescence cracks, the sketches contained in Fig. 25 also show the traces of other cracks which also developed from the pre-existing flaws.
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Wong, L.N.Y., Einstein, H.H. Crack Coalescence in Molded Gypsum and Carrara Marble: Part 2—Microscopic Observations and Interpretation. Rock Mech Rock Eng 42, 513–545 (2009). https://doi.org/10.1007/s00603-008-0003-3
- Uniaxial compressive loading test
- Tensile cracks
- Shear cracks
- White patches
- Microcracking zones