Issue 50

A. Marinelli et alii, Frattura ed Integrità Strutturale, 50 (2019) 438-450; DOI: 10.3221/IGF-ESIS.50.37 445 height) is presented in comparison with the corresponding results from 3PB tests for both materials (Fig.12a). The flexural strength becomes less varied for an a/h ratio greater than 0.2 while for insignificant notch length the strength increases, tending to that of the intact specimen. As shown in Fig.12b, intact specimens of Corsehill sandstone have a significantly higher maximum deflection at mid-span just before failure when compared with Portland limestone. With the introduction of a notch though, the maximum de- flection of the Corsehill sandstone specimens drops dramatically, while the maximum deflection of the Portland limestone specimens presents a less pronounced, gradual decrease. It is worth noting however that when the notch length is 20 mm, both Corsehill sandstone and Portland limestone appear to be tending towards a critical deflection at mid-span. Given the maximum load levels achieved by the two materials and as Corsehill sandstone presents generally clearly higher maximum deflections than Portland limestone when un-notched, the former appears to have a less brittle behaviour than the latter. Figure 11: (a) Graph of CMOD against force (Corsehill Sandstone 3PB) (b) Graph of CMOD against force (Portland limestone 3PB). Figure 12: Graphical representation of average values ± STDEV for (a) Flexural strength vs. {notch length/specimen height} and (b) Deflection at mid-span vs. {notch length/specimen height}, for both types of test and materials. During the tests, COD is measured at the notch mouth but for the sake of uniformity and in order to eliminate the dependence from the notch length, a reference length a ref is introduced where the critical COD is defined (Fig.13a). The measured value of CMOD is recalculated as COD according to linear proportion (assuming the flanks remain linear) and then the normalized value δ can be introduced. For the determination of δ crit the selection of a ref =0.1cm as a reference length was made. The parameter δ crit is plotted against the ratio a/h for both materials and types of tests and comparison reveals a similar behaviour characterised by decreasing values. Especially for Portland limestone for which more experimental data exist, a tendency towards a constant δ crit for the presence of a notch of significant length is observed (Fig.13b). Fracture criteria based on COD and f u are alternative expressions of the same natural concepts since there are analytical formulas relating these parameters to each other, especially for brittle materials. For this part of the investigation, the dependence of COD on the specimen height is covered by the study of the behaviour with reference to the ratio a/h. There is also dependence on the specimen thickness but for thick specimens, plane strain conditions are assumed and the COD values become stable. In any case and form, the criterion implied here as a single value (lower limit) based on a limited (a) (b) (a) (b)