Issue 41

A. Mardaliazad et alii, Frattura ed Integrità Strutturale, 41 (2017) 504-523; DOI: 10.3221/IGF-ESIS.41.62 506 distance from the center of the specimen. This symmetrical configuration causes nominally zero shear forces, and accordingly constant bending moment between the two compressive rollers. The normal tensile and compressive stresses appear at the top and bottom of this middle span, respectively. According to the beam theory, the maximum principal stress corresponding to the ultimate loading value can be determined, which is called the flexural strength and gives a rough approximation of the principal tensile strength. The flexural strength tends to overestimate the tensile strength, because the measuring process considers a linear relationship (as the stress-strain behaviour of the material) at the beam critical cross-section. Furthermore, all the materials have a certain amount of anisotropic level in their structure [4]. The flexural strength . flex  [MPa], which is given by the Eq. (1), can be considered as a parameter to validate the numerical models. . 2 3 4 flex WL bd   (1) where, W was measured as the maximum applied force. The experimental tests within this study were performed on five specimens of Pietra Serena sandstone with the same geometries. The span length L , width b and height d of all the specimens were equal to 318, 102 and 32 [mm], respectively. However, the total length of the specimen was measured as 381 [mm]. Two pairs of steel rods were embedded to the testing apparatus (see Fig. 1). The axes of lower rods were fixed to the bed of testing machine while the upper rods were displacement controlled by means of a compressive platen. According to [31] the speed was set to 0.2 [mm/min] in order to apply the load at a uniform stress rate of 4.14 [MPa/min] . Figure 1 : (a) ASTM arrangement of Flexural test [31]; (b) corresponding experimental layout of this research work. Figure 2 : The extensometer set up on the flexural test configuration. (b) (a)