Issue 50

A. Marinelli et alii, Frattura ed Integrità Strutturale, 50 (2019) 438-450; DOI: 10.3221/IGF-ESIS.50.37 444 Average results from 3 compressive tests per material appear in Table 2 as well, with the note that only tests exhibiting a satisfactory failure mode were included (Fig.8). Observing the failure modes of all specimens for 3PB and 4PB tests, it is to be noted that Corsehill sandstone samples failed following a more regular and predictable pattern than the Portland limestone ones (Fig.9). As the machined notch lengths increased, the failure modes became more uniform across the board. The irregularity of the failure modes of the Portland limestone samples can be partly explained by the presence of relatively large mineral build-ups inside the samples, which promote failure planes around them (Fig.10). Once failed, these mineral build-ups resemble a ball and socket joint. Figure 8 : Specimens with uniform failure modes in compression, for (a) Corsehill sandstone and (b) Portland limestone. Figure 9 : 3PB specimens with a 10mm notch, post-testing, for (a) Corsehill sandstone (b) Portland limestone. Figure 10 : Failure around a mineral build-up (Portland limestone). For both 3PB and 4PB tests, the complete series of measurements is available in terms of measured CMOD and deflection at mid-span versus load. Some measurements are inevitably affected by local inhomogeneity characterizing natural building stones, however the trend can be seen: the deeper the notch, the non-linearity is more pronounced for load levels approaching the ultimate load (Fig.11). The two stone types were ultimately compared through key parameters such as flexural strength, displacement at the mid- span and CMOD both at peak load, against the ratio of the test specimen height to the length of the existing machined notch. Flexural strength is defined according to the classical theory of Mechanics as f u = M/w (M the bending moment and w the section modulus). For the average of measured values for each testing category, elaborated data are presented towards the scope of introducing an easy-to-use in practical applications fracture criterion. The variation of the flexural strength (average of ultimate stresses as defined previously) for 4PB tests versus the ratio a/h (notch length/specimen (a) (b) (a) (b)