Issue 47

A. Spagnoli et alii, Frattura ed Integrità Strutturale, 47 (2019) 394-400; DOI: 10.3221/IGF-ESIS.47.29 396 From the experimental curves related to notched specimens, fracture toughness of marble is calculated by using two different approaches. The first approach is based on the two-parameter model [6]. Note that a modified version of the two- parameter model has recently been proposed in order to take into account the possible crack deflection during the stable crack propagation [7-9]. Accordingly, the initial crack length a 0 is assumed to grow steadily before the peak load is attained. This nonlinear stable stage terminates when the crack propagates to a critical extent and the SIF K I attains a value K sIC that differs from the nominal K IC (computed on the basis of a 0 ). If the geometric and loading conditions are such that the stress intensity factor is monotonically increasing with the crack length (being the load constant), as occurs in the case of a 3-point bend beam with an edge crack, the critical condition explained before takes place at the peak load. From LEFM formulas and from two compliance experimental measurements, the equivalent crack length and the effective toughness K sIC are worked out. The second approach is based on the work-of-fracture method in Ref. [10] recommended by the RILEM technical committee. For 3-point bending tests on edge-notched beams, the method is based on the experimental determination of the work exerted by the applied mid-span force, which corresponds to the area underneath the complete load–displacement curve. Such a work is assumed to be fully spent to produce a mode I crack through the mid-span ligament of the beam. 0 0.04 0.08 0.12 0.16 Mid-span deflection, [mm] 0 1 2 3 0.5 1.5 2.5 Load, P [kN] (a) Figure 2 : Load vs mid-span deflection curves (a) and nominal stress-CMOD curves (b) for notched specimens under three-point bending. In the graph experimental results for two specimens (black thin lines) are reported along with numerical results (red thick lines). The mean value of fracture toughness according to the two-parameter model is equal to 1.90 MPam 0.5 (coefficient of variation equal to 0.19). According to the work-of-fracture method, the mean fracture toughness is 1.91 MPam 0.5 (coefficient of variation equal to 0.37). For the sake of completeness, a splitting test on four prismatic specimens (nominal dimensions D = 60 mm, L = 30 mm, B = 60 mm) was performed according to ASTM C496. The resulting indirect tensile strength (    2 / ( ) t P DL ) is equal on average to 7.1 MPa (coefficient of variation equal to 0.11). E XPERIMENTAL TESTING UNDER CYCLIC LOADING leven three-point bending tests on notched specimens, with the same geometry as that adopted for monotonic loading and with the initial notch length a 0 in the range 6.6 to 8.5 mm, under nearly pulsating loading were performed. Tests were carried out under load control mode, with blocks of cycles composed by a single square cycle with frequency of 0.1 Hz and by 125 sinusoidal cycles with frequency 2.5 Hz. Square cycles are introduced to take DIC photos at maximum load. Applied cycles have a loading ratio R = 0.1 and a maximum load expressed as a percentage of the mean failure load P u of three-point bend notched specimens under monotonic loading ( P u =2.7 kN). SN-like data are Nominal stress, N [MPa] E