The use of a high speed video system allows one to precisely observe the cracking mechanisms, in particular if shear or tensile fracturing is taking place. The present experimental study on gypsum and marble specimens confirmed that tensile wing cracks (TWCs) are in most cases the first cracks to appear in fracture propagation from existing flaws independent of aperture and material. The study, in addition, has shown that complex additional cracking occurs which depends on orientation of the existing flaws and material type. Either the TWCs or the other tip cracks produce failure. Also important is the formation of a process zone which could be visually observed in some marble experiments, but not in gypsum experiments.
Systematic studies on prismatic pre-cracked molded Hydrocal-B11 gypsum specimens regarding crack propagation and coalescence patterns have been conducted over the past decades at MIT. They reveal that fracturing behavior is significantly influenced by the geometries of the pre-existing flaws (the term flaw is used in this paper to describe a pre-existing crack) [1, 2, 3, 4]. Similar experimental studies were also conducted by others on a variety of materials, including Columbia Resin 39 [5, 6, 7], glass [8, 9], plaster of Paris [10, 11], PMMA [12, 13], molded gypsum [14], sandstonelike molded barite [15, 16], sandstone-like concrete mix [17], and ice [18]. Experimental studies specifically on natural rocks included sandstone [12], granodiorite [19], limestone [19], granite [20], marble [20, 21, 22, 23]. In these studies, varying dimensions of prismatic specimens and cracks were studied. The specimen size ranged from 50mm x 32mm x 5mm [12] to 635mm x 279mm x 203mm [17]. The flaw length varied between 10mm and 50mm; and the flaw width (aperture) varied between 0.1 mm and 2mm.
Due to the differences in the material (rock) types and flaw aperture sizes adopted in various studies, comparison of their results is difficult and was thus seldom attempted. For example, even though the same kind of tensile wing crack (TWC) can be identified in most of the experiments, its properties such as initiation angle and proximity to the flaw tips vary. In addition, several crack types are only reported in some experiments, but not in others. All these studies thus indicate that the influence of individual factors on the eventual fracturing processes and coalescence patterns is very complex and has not been fully understood. In order to better understand the fracture coalescence phenomena in natural rock and rocklike material, a systematic investigation on fundamental fracturing processes in prismatic specimens containing single flaws was conducted. Most importantly, this study relied on high speed videotaping of the fracturing process, which was not done in previous studies.
