Issue 40

K. Kaklis et alii, Frattura ed Integrità Strutturale, 40 (2017) 18-31; DOI: 10.3221/IGF-ESIS.40.02 27 that would reduce specimen seating errors would be to conduct Brazilian tests where the disc specimens will be directly loaded by the loading platens of the compression testing frame (as specified by ASTM – D3967) where eccentric loading can be directly alleviated by the spherical seating of the upper compression platen. D [mm] t [mm] Number of tests Number of specimens with strain gages Splitting tensile strength st f [MPa] Intact rock modulus E [MPa] Poisson’s ratio v Shear modulus G [MPa] Average [MPa] St. Dev. [MPa] Average [MPa] St. Dev. [MPa] Average St. Dev. Average [MPa] St. Dev. [MPa] 54 27 8 1 2.88 0.38 19100 - 0.333 - 7163 - 75 37.5 7 3 3.34 0.13 21300 7938 0.293 0.019 8269 3179 100 50 6 3 2.28 0.35 14422 713 0.290 0.033 5588 245 Table 3: The experimental results of the Brazilian tests for Alfas stone specimens loaded diametrically. All specimens, for the Alfas stone, subjected to the Brazilian test, failed as expected by the underlying theory, i.e. by developing an extension fracture along the loaded diametral plane which is assumed to be the result of the induced tensile stress normal to the loaded plane (Fig. 10a). Furthermore, it was observed that in addition to the central primary crack, two symmetrical secondary cracks were developed (Fig. 10b). This behavior is in full agreement with the ideal fracture propagation according to Colback [25]. (a) (b) Figure 10: (a) Center primary fracture plane. (b) Center primary fracture plane with typical secondary fractures. Size effect for indirect tensile tests A total of 21 Brazilian tests were completed in order to investigate the size effect for the Alfas natural building stone under diametral compression. The mean values of rock properties are plotted against the diameter together with their standard deviation values (Fig. 11). The variation of the splitting tensile strength with respect to specimen diameter is not a monotonic function. A clear maximum exists and corresponds to the specimens with D =75 mm (Fig.11a). A similar non-monotonic behavior has been observed by Kourkoulis [12] for the UCS of “Conchyliates” shell-stone, where the maximum value appears for the specimens with diameter D =100 mm. The author attributed this anomaly to the fact that “the specimen starts behaving as a structure rather than as a homogeneous material, since the size of the conches becomes well comparable to the characteristic size of the specimen”. This non monotonic dependence of the UCS and the splitting tensile strength with respect to the diameter of the specimens is also mentioned by Vardoulakis et al. [26, 27] and Kaklis and Vardoulakis [21] for a more homogeneous stone, the Dionysos marble. Although, both the Alfas building stone (investigated in this study) and Dionysos marble (investigated in previous studies) can be considered almost homogeneous materials and their typical grain size cannot be compared to the size of the specimens, nevertheless, they present this non-monotonic behavior. It is clear that additional research needs to be performed to reach definite conclusions regarding this non-monotonic behavior.