Issue 40

K. Kaklis et alii, Frattura ed Integrità Strutturale, 40 (2017) 1-17; DOI: 10.3221/IGF-ESIS.40.01 2 Among many different testing methods for rock fracture toughness, the International Society for Rock Mechanics (ISRM) suggests the Chevron Bend (CB) and the Short Rod (SR) specimens [1]. These tests are relevant to mode I (opening mode) fracture toughness calculations, but are not appropriate for testing the fracture toughness of rock under mode II or mixed mode I-II cases [2, 3]. Considering notched circular specimen geometries, Atkinson et al. in 1982 [4] introduced the Cracked Straight Through Brazilian Disc (CSTBD). Shetty et al., in 1985 [5], based on the CSTBD configuration, presented the Cracked Chevron Notched Brazilian Disc (CCNBD) test, by employing the straight through crack assumption. The introduction of the CSTBD and CCNBD tests, allows the determination of rock fracture toughness under mode I (opening mode), mode II (in plane shear mode) and mixed mode I-II loading cases. Both of these tests configurations show great potential for current and future applications and overcome the disadvantages of the CB and SR specimens. The International Society for Rock Mechanics [6] suggested the cracked chevron notched Brazilian disc (CCNBD) for determining the mode I fracture toughness of rock. There are some unique features characterizing the CCNBD specimen: (a) it is closely related to the Brazilian test which is widely used for tensile strength test for rock and concrete, (b) it can be easily used for mode I, mode II and even mixed mode testing, (c) it can sustain higher critical load than other kinds of specimens with comparable size and (d) it also maintains the merit of convenience for specimen preparation from rock cores. Recent research dealt with the calibration of the minimum (critical) dimensionless stress intensity factor * min Y [7-9] and with measuring the fracture toughness under mode II and mixed mode I-II loading conditions using CCNBD specimens of different rock types [10, 11]. Regarding AE, it is known that a large number of such signals are generated when a rock specimen is loaded to failure. Since AE signals are caused by the formation, expansion and propagation of microcracks, such signals inherently include information related to the structural changes taking place within a rock sample. This study focuses on the correlation between the AE signals and the diametrically applied load during CCNBD tests. The acoustic emission activity was monitored using piezoelectric acoustic emission sensors, and the potential for accurate prediction of the fracture load based on acoustic emission data is investigated. E XPERIMENTS AND ACQUISITION OF ACOUSTIC EMISSIONS Material pecimens consisting of Nestos marble were properly prepared and tested. This marble is quarried by surface mines in northern Greece and is mainly used as a building material. It is composed of 93.4% calcite, 6% dolomite and 0.6% quartz. Its density is 2.67 g/cm 3 and its absorption coefficient by weight is 0.09%. It is of white color with a few thin parallel ash-green colored veins containing locally silver areas due to the existence of dolomite [12]. The geometry of the CCNBD specimen The CCNBD specimen has the same geometry and shape (Fig. 1) as the conventional Brazilian disc used for measuring, indirectly, the tensile strength of rock. Additionally, the CCNBD specimen has a chevron notch. ISRM [6] recommends that the following dimensionless parameters can be used to characterize the geometry of the chevron notch:            * * 0 s 1 0 1 α D α Β α ,  ,  ,  ,  R R R D R s (1) where the diameter D is twice the radius R, B is the thickness of the disc, 0 α is the initial notch length, 1 α is the final notch length, * α is the intermediate crack length, * m α  is the critical crack length, and  0 ,  1 ,  B ,  * ,  * m are the relevant dimensionless expressions. Furthermore, as shown in Fig. 1a, 2b is the crack front width, P is the load applied to the disc and D s is the diameter of the circular cutting blade. Typical standard ratios for a CCNBD specimen suggested by ISRM [6] are given as:         * 0  1  0.2637,  0.65,   0.80,  0.5149 B m (2) S