Laboratory uniaxial compression tests were conducted on prismatic, molded gypsum and Carrara marble specimens containing two pre-existing artificial straight open flaws. Crack initiation and the sequence of the development of multiple cracks were observed by using a high speed camera. Besides confirming certain coalescence types seen in previous research, additional new coalescence types were observed in the present study.
Crack coalescence, which is the linkage of preexisting fractures by new cracks is common in nature over a wide range of scales from millimeters (e.g. Wang & Kemeny 1992) to meters (e.g. Kim et al. 2003). As an extension of the work presented last year (Wong & Einstein 2006), which studied the fracturing behavior in specimens containing single artificial flaws, the present experimental study investigated the interaction (coalescence) of two artificial flaws in Carrara marble and laboratorymolded gypsum specimens. The use of a high speed video system allows one to precisely observe the cracking mechanisms, in particular if shear or tensile fracturing takes place and to establish the sequence of the development of multiple new cracks. The observed coalescence behavior is very complex regarding the type of coalescence cracks (shear & tensile) and the number of coalescence cracks involved.
2.1 Specimen preparation Molded gypsum has been used in the MIT rock mechanics group for the study of initiation, propagation and coalescence of rock fractures since the late 1960’s (e.g. Einstein et al. 1969, Einstein & Hirschfeld 1973). Systematic investigations have been undertaken in the past 15 years to study the influence of various material and geometric parameters and loading conditions (Table 1) on the coalescence behavior experimentally (Reyes & Einstein 1991, Shen et al. 1995, Bobet & Einstein 1998, Ko et al. 2006, Wong & Einstein 2006).