Issue 40

K. Kaklis et alii, Frattura ed Integrità Strutturale, 40 (2017) 1-17; DOI: 10.3221/IGF-ESIS.40.01 10 Furthermore, Zhang [23] correlates the mode I fracture toughness and tensile strength of different rock types through the following equation:    6.88 t IC K (12) For comparison, the average values of mode I fracture toughness obtained for Nestos marble as shown in Tab. 2 were plotted together with Eq. (12). A tensile strength for the Nestos Marble equal to 6.07 MPa (determined experimentally using a traditional Brazilian test) was used for all data points. Experimental results correlate well with the values generated by Eq. (12) except for the value obtained via the ISRM (Fig 10). Rock fracture toughness for pure mode II The orientation of the chevron notch crack of the CCNBD specimen, for the determination of the fracture toughness for pure mode II loading was θ=23 ο from the loading direction. The fracture toughness K IIC values were calculated using Eq. (6a) with the aid of relationship (9) (Tab. 3). Two sets of K IIC values are provided in Tab. 3, calculated for different critical crack lengths. In the first case, K IIC was calculated using a critical crack length calculated by minimizing * min Y while in the second case the critical crack length was set equal to 0.36 [10]. Specimen Thickness (mm) Diameter (mm)  0  1 Fracture load (kN) II N IIC K    1/2 MPa m    * 0.4915 m a IIC K    1/2 MPa m    * 0.36 m a CCNBD 1 20.90 52.30 0.262 0.679 5.10 1.8013 1.64 1.84 CCNBD 2 20.85 52.35 0.263 0.685 4.89 1.58 1.77 CCNBD 3 21.20 52.30 0.229 0.672 5.90 1.87 2.10 CCNBD 6 21.25 52.25 0.270 0.684 5.98 1.89 2.13 CCNBD 7 19.50 52.30 0.350 0.677 4.73 1.63 1.83 CCNBD 8 21.00 52.30 0.281 0.673 4.89 1.57 1.76 CCNBD 9 21.60 52.30 0.242 0.675 4.95 1.54 1.73 Average 5.21 1.67 1.88 St. Dev. 0.51 0.15 0.16 Table 3: Geometrical parameters, fracture loads and fracture toughness of CCNBD specimens under pure mode II loading. Crack growth and mode of fracture The crack growth for mode I specimens was along the chevron notch plane and the tensile splitting was the dominant failure mode. Fig. 11 shows a typical CCNBD specimen fractured under pure mode I loading. For mode II loading where the angle between the applied stress and the chevron node direction is 23 o , a typical fractured specimen is illustrated in Fig. 12. Figure 11: A typical CCNBD specimen fractured under mode I loading.