Issue 40

K. Kaklis et alii, Frattura ed Integrità Strutturale, 40 (2017) 1-17; DOI: 10.3221/IGF-ESIS.40.01 8 package by Enviroacoustics [20]. Acoustic emission signals were detected through 6-8 miniature piezoelectric sensors (PICO sensors, 200 kHz-1 MHz, MISTRAS Group, SA) mounted on the surface of each specimen and recorded through an integrated multi-channel system of Physical Acoustics Corporation. The positions of the sensors on each side of the CCNBD specimens for the determination of the mode I (θ=0 ο ) and mode II (θ=23 ο ) fracture toughness are shown in Fig. 8 and Fig. 9, respectively. Usage of such sensors was dictated due to the small size of the specimens. Their high sensitivity over a broad bandwidth ensures the accurate detection of the AE signals. It should be noted that in the case of the specimens subjected to mode I loading, the positions of the AE sensors were symmetrically located with respect to the tips, while in the case of the mode II loading the positions were somewhat arbitrary positioned on the surface of the specimens. In both cases, however, the accurate determination of the events’ location is mainly limited by the size of the sensors and to a lesser extent by their positions, as discussed in the next section. The source location of the AE events on a planar projection was determined by using the arrival times of the hits detected by at least three sensors. This procedure is described in details elsewhere [21]. Figure 8: The positions of the sensors for the mode I (θ=0 ο ) loading CCNBD specimens. Figure 9: The positions of the sensors for the mode II (θ=23 ο ) loading CCNBD specimens. E XPERIMENTAL RESULTS ANS DISCUSSION Rock fracture toughness for pure mode I ix CCNBD tests were performed for the determination of mode I fracture toughness (Fig 8). The fracture load was identified using AE records and the specimen fracture toughness was calculated using Eq. (10). The average fracture toughness of these specimens was (0.73±0.08) MPa m 1/2 (Tab. 1). The minimum dimensionless stress intensity factor * min Y , is an important calculation parameter which greatly affects the accuracy of toughness tests. It is calculated using Eq. (11) and the values of parameters u, v, as suggested by the ISRM [6]. In order to improve the fracture toughness tests using CCNBD, recent research [8, 9, 16], based on the slice synthesis method [22] and other numerical methods, presented updated values of the two parameters of the exponential function (Eq. (11)) that cover a wide range of specimen geometries. S