Most rocks show brittle behavior when they pass slightly beyond yield stress. Various researchers (e.g., Hucka and Das 1974; Singh 1986; Go¨ktan 1991; Aubertin et al. 1994; Kahraman 2002; Hajiabdolmajid and Kaiser 2003; Gunaydin et al. 2004; Kahraman and Atindag 2004; Yagiz 2004, 2006b, 2009; Go¨ktan and Yilmaz 2005; Tiryaki 2006; Gong and Zhao 2007; Yarali 2007; Yilmaz et al. 2009; Altindag and Guney 2010) have indirectly defined rock brittleness by using different approaches. The brittleness index is often determined using the tensile strength (TS) and uniaxial compressive strength (UCS) of rock in engineering practice. The presence of pores and micro-cracks (porosity) has a considerable effect on the mechanical properties of rock such as UCS and TS.
