Issue 40

K. Kaklis et alii, Frattura ed Integrità Strutturale, 40 (2017) 1-17; DOI: 10.3221/IGF-ESIS.40.01 7 emphasize that the notch width has an important effect on the stress intensity factors and that the crack width should be adequate to ensure crack propagation. At the same time Shetty et al. [5] indicate that they used a machined slot opening of 250 μm without giving any technical details. The indentation depth of the rotary saw on each side of the disc was about 11.72 mm and thus, two V shape chevron notches were introduced in the circular disc. The diamond saw was cooled by water during the chevron notch cutting process. Figure 6: The schematic cutting configuration for preparing CCNBD specimens with some theoretical geometrical parameters. Fig. 7a presents the final configuration of a CCNBD specimen with the theoretical dimensional parameters 0 α and 1 α . In order to obtain consistent test results and to satisfy the plane strain condition, the ISRM [6] prescribes that the range of  1 and  B is restricted as shown in Fig. 7b. It is evident that the selected geometric parameters (2) are located within the valid range. (a) (b) Figure 7: (a) CCNBD specimen. (b) The valid geometrical range for CCNBD specimen. The CCNBD specimens (Fig. 7a) were then placed in the Brazilian testing apparatus and loaded by a diametral compressive force P. For pure mode I loading, the crack direction should be exactly along the loading diameter, while for mixed mode I-II loading the chevron notch orientation lies between 0 and 90 degrees. Load was applied using a stiff 1600 kN MTS hydraulic testing machine and a 500 kN load cell, with a loading rate of 200 N/s under load control mode for all experiments. Monitoring of Acoustic Emissions The AE activity may be represented in time series of detected signals in each sensor (hits), their amplitudes (signal peak in dB), rise time (RT) which is defined as the time between the first threshold crossing (FTC) and the point of peak amplitude, as well as other acoustic parameters. AE signals were acquired and analyzed using the AE WIN software