Effects of temperature and confining pressure on mixed-mode (I–II) and mode II fracture toughness of Kimachi sandstone

Abstract

1. IntroductionFracture mechanics is becoming increasingly important insolving problems relating to rock strength in mining engineeringand civil engineering. For example, fracture mechanics can be usedfor the design of hydraulic fracturing for geothermal energy, oiland natural gas recovery [1–4], as well as for the analysis of rockmass stability in slopes or those associated with excavations [5,6],and determining rock fragmentation by blasting or in the event ofrock burst [7,8].Therefore, it is desirable to investigate the fracture toughness,which is an important rock property that characterizes its fracturebehavior. In this investigation, fracture toughness was determinedat elevated pressures and temperatures simulating the conditionsprevailing underground. A number of test specimen configurationsand methods have been suggested to determine the fracturetoughness of rock materials. The International Society for RockMechanics (ISRM) has incorporated the chevron-notched bendspecimen [9], the short rod specimen [10] and cracked chevron-notched Brazilian disc (CCNBD) specimen [11] into the standardmethod for measurement of the fracture toughness of rock. Thesemi-circular bend (SCB) specimen proposed in [12,13] is com-plementary to the standard method (Fig. 1).Natural crack propagation occurs as a result of the develop-ment and coalescence of tensile and shear cracks. The geometryand loading are defined as mode I (tensile mode), mode II(in-plane shear), mode III (anti-plane shear) crack propagation,and mixed modes of mode I, II, and III. Previous studies on thefracture toughness of rock were mostly focused on mode I crackpropagation. However, actual crack propagation in rock massoccurs not only in tensile mode but also as part of in-plane shearmode. Mode II fracture toughness and the critical stress intensityfactors in mixed mode (I–II) are crucial to gain understanding intorock fracturing. SCB specimen and Cracked Straight ThroughBrazilian Disc (CSTBD) specimen [14] have been used for evaluat-ing mode II and mixed mode fracture toughness of rock, and thePunch-Through Shear test (PTS) specimen [15] has been developedto evaluate mode II fracture toughness of rock. Furthermore, thePTS specimen is adopted as an ISRM suggested method. Conse-quently, many researchers have investigated the mixed mode (i.e.mode I and mode II) and pure mode II fracture toughness of rock[16–23]. In terms of the ratio of mode II fracture toughness, K