Issue 40

K. Kaklis et alii, Frattura ed Integrità Strutturale, 40 (2017) 1-17; DOI: 10.3221/IGF-ESIS.40.01 15 Figure 17: The variation of the improved b-value during the CCNBD test of the specimen 2.3 for each of the 4 sensors positioned in the front face (Number of successive events = 90, 50% overlapping, a 1 = a 2 = 1). Figure 18: The variation of the improved b-value during the CCNBD test of the specimen 3.23 for 4 selected sensors (Number of successive events = 50, 50% overlapping, a 1 = a 2 = 1). C ONCLUSIONS number of CCNBD tests were carried out in order to determine the fracture toughness of Nestos marble. Specimens were also instrumented with acoustic sensors in an effort to better determine stress evolution during loading. The available methodologies for calculating the mode I fracture toughness K IC from CCNBD tests were critically evaluated and compared. In addition, the dimensional stress intensity factor of the CCNBD specimen, * Y , is calculated with respect to the dimensionless crack length α * using the dimensionless crack lengths (  0 and  1 ) proposed in the ISRM suggested methods [6]. The calculated minimum SIF value for the CCNBD specimen  * min Y 1.2578 corresponds to the critical crack length   * 0.4915 m and is comparable to published critical length values. In addition, experimental results from the acoustic emissions monitored during the CCNBD tests are evaluated with respect to the load applied to each sample. The localization of the generated events is plotted at different time intervals during the test. Results indicate that during the early loading stages the stress concentration is uniform on both crack edges. As the load increases and the specimen is ready to fail, a rather diffused distribution of the events is observed, while their concentration at the upper tip of the chevron notch is more pronounced. Overall, using acoustic emission signals a A