Issue 40

K. Kaklis et alii, Frattura ed Integrità Strutturale, 40 (2017) 1-17; DOI: 10.3221/IGF-ESIS.40.01 6 Intensity Factor * Y . More specifically, the upper curve shown in Fig. 4 represents the dimensionless stress intensity factor of the CCNBD specimen for various crack lengths as obtained from Eq. (5b) in conjunction with Eq. (8) for mode I, while the lower curve corresponds to the dimensionless SIF of the CSTBD specimen as obtained from Eq. (3b) in conjunction with Eq. (8) also for mode I. It is observed that the SIF for the CSTBD specimen monotonically ascends, while the SIF curve for the CCNBD specimen is convex and reaches a minimum value between the initial and final crack length. The minimum SIF value for the CCNBD specimen  * min Y 1.2578 corresponds to the critical crack length   * 0.4915 m . When    * 1 , it is assumed that the SIF for the CCNBD specimen is identical to that of the CSTBD specimen as discussed by [7] and [9]. Figure 4: Dimensionless stress intensity factors for straight through and chevron notches in a CCNBD specimen with   0 0.2637 and   1 0.65 . Specimen preparation and experimental setup Initially, the circular discs of Nestos marble (D = 52 mm) were prepared by coring specimens out of Nestos cubes with their long axis normal to the plane of transverse isotropy (Fig. 5), in order to avoid large variations in the quality of the stone and ensure similar fracture loads. Figure 5: Specimen orientation. The middle plane of the disks is parallel to the plane of transverse isotropy (rift planes). The CCNBD specimens were prepared according to the standard geometrical dimensionless expressions (2) [6]. Two chevron notches (one in each side) were cut in the center of each disc by using a thin rotary diamond saw with a diameter of D s = 38 mm and a thickness of t = 1 mm which results in a crack width of less than 1.5 mm (Fig. 6). This satisfies the requirements suggested by [6,9,10,14] that the machined crack width should not exceed 1.5 mm. Wang et al [19]