Measuring fracture toughness of crystalline marbles under modes I and II and mixed mode I–II loading conditions using CCNBD and HCCD specimens H. Amrollahi, A. Baghbanan n , H. Hashemolhosseini Department of Mining Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran article info Article history: Received 25 July 2010 Received in revised form 5 February 2011 Accepted 25 June 2011 Available online 23 July 2011 Keywords: Fracture toughness Grain size distribution CCNBD specimen HCCD specimen Fracture criteria abstract Fracture toughness of three crystalline Calcite marbles, which are different only in grain size and distribution, is determined under modes I and II and mixed mode I–II loading conditions using Cracked Chevron-Notched Brazilian Disc (CCNBD) and Hollow Centre Cracked Disc (HCCD) specimens. The results show that mode I fracture toughness (K IC ) is correlated negatively with grain size. For each marble, HCCD yields lower values of fracture toughness, compared with CCNBD. This difference is negligible under mode I loading condition; while it becomes larger as loading condition transits from mode I to mode II. Measured values of P-wave velocity (V P ), Brazilian tensile strength (s tB ) and Schmidt hammer hardness are in direct relation with K IC of the marbles. The obtained results are compared with three fracture criteria, in which the Minimum Strain Energy Density (MSED) criterion has provided better correlations with different critical combinations of modes I and II Stress Intensity Factors. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction According to the applied stress condition on a cracked body, a crack-tip propagates under the three basic modes, namely: mode I (tensile, opening), mode II (shear, in-plane sliding) and mode III (tearing, out-of-plane sliding) [1]. Any combination of these basic modes can be regarded as a mixed mode. Most of the previous studies on rock fracture mechanics have focused on mode I crack propagation. However, existing stress fields near crack-tip practically tend to be more complex rather than pure opening. For example, from an engineering point of view, it is believed that due to arbitrary orientations of pre-existing cracks to the overall applied loads, the cracks in rock masses often experience a combination of modes I and II loading conditions. Thus, studying various combinations of modes I and II rock fracture are also important as well. Fracture mechanics approaches are of growing importance in rock engineering subjects. Rock fracture toughness is known as a fundamental parameter in rock fracture mechanics which repre- sents the potential intrinsic ability of rock to withstand a given stress field at the tip of a crack and to resist against progressive crack extension. This parameter can be assessed experimentally by using different testing specimens and loading configurations. But due to its special part in a wide variety of applications in different fields of rock mechanics, measuring accurate and precise value of this parameter always has been of utmost importance. Consequently, investigations on different measurement techni- ques of rock fracture toughness have been of great concern. In 1995, the International Society for Rock Mechanics (ISRM) suggested the use of Cracked Chevron-Notched Brazilian Disc (CCNBD) specimen in diametrical compression to determine mode I fracture toughness (K IC ) of rocks [2]. So far, this method has proved to have many advantages, and it has been adopted frequently by researchers. However, the ISRM [2] suggested method is limited only to mode I testing. Complexity of chevron notch making is another motivation to seek for alternative methods. Among the proposed alternative specimens, Semi-Cir- cular specimen under three-point Bending (SCB) [3], Flattened Brazilian Disc (FBD) [4], Double Edge Cracked Brazilian Disc (DECBD) [5] and Hollow Centre Cracked Disc (HCCD) [6] are to name a few. These disc-type specimens are core-based and benefit from comparatively simple preparation and testing. However, findings of Khan and Al-Shayea [7] and Chang et al. [8] suggest that due to the inconsistent results of SCB, this specimen may be suitable only for mode I rock fracture toughness tests. Also, SCB has relatively low values of failure load [9]. FBD can be used to measure only mode I rock fracture toughness. Its testing needs precise recording of complete load–displacement curve and its results could not be acceptable unless corresponding criteria are established [4]. To use DECBD for measuring mode II fracture toughness (K IIC ), at least three specimens with different inclined angles of notch are needed [10]. HCCD not only benefits from CCNBD’s merits, but also has a much simpler preparation procedure. Still very scanty research works about this specimen Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ijrmms International Journal of Rock Mechanics & Mining Sciences 1365-1609/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmms.2011.06.015 n Corresponding author. Tel.: þ983113915128; fax: þ983113912776. E-mail address: bagh110@cc.iut.ac.ir (A. Baghbanan). International Journal of Rock Mechanics & Mining Sciences 48 (2011) 1123–1134