Issue 47

A. Spagnoli et alii, Frattura ed Integrità Strutturale, 47 (2019) 394-400; DOI: 10.3221/IGF-ESIS.47.29 395 and seasonal excursions, through-thickness gradient), mechanical loads (wind, self-weight), chemical attacks (acid rain) and humidity changes. In particular, temperature may induce stresses due to thermal expansion (restraint effects of the anchorage system, non linear temperature fields and non uniform thermal expansion) and thermal fluctuations tend to cause a progressive damage of the material, sometimes accompanied by a curvature of the surface of the slabs [2]. Such bowing phenomenon can progress up to the collapse of the element, with consequences often critical on safety for the users of the buildings where these covering slabs are installed. Bowing is generally accompanied by an overall reduction of strength, which increases with increase in degree of bowing, while at the micro structural level of the material bowing is accompanied by a decohesion of calcite grains. The present paper is devoted to an experimental study on the fracture behaviour of natural stones under both monotonic and cyclic loading, with particular emphasis to white Carrara marble. The effect of progressive damage produced by thermal fluctuations can be investigated through the application of appropriate cyclic mechanical loads. In the experimental tests conducted, some static mechanical properties of marble are characterized by means of three-point bending tests on edge- cracked prismatic specimens for the determination of Young's modulus, tensile strength and fracture energy [3]. Moreover, cyclic three-point bending tests are conducted to determine the propagation rate of nominally Mode-I fatigue cracks [4]. Finally, the fatigue behaviour of the marble is studied through a cohesive crack model, in which the direct tensile strength of the material is determined by a Brazilian test, and the behaviour is calibrated by means of a suitable FE model [5]. The effect of crack path on the fracture resistance of marble is discussed. In particular, it is shown that different level of meandering in the intergranular cracking of marble is observed and correlated with the so-called xenoblastic or homoblastic texture of calcite grains [4]. E XPERIMENTAL TESTING UNDER MONOTONIC LOADING preliminary experimental campaign is carried out to quantify the resistance parameters of a Carrara marble under monotonic loading. Four prismatic specimens were tested under three-point bending. The nominal dimensions of the specimens are as follows: length L =220 mm; span S =180 mm; height W =60 mm; width B =30 mm (Fig. 1). One specimen out of the 4 ones is a smooth specimen. Notched specimens are characterised by a central edge notch machined by means of a water jet technique. Notches with different nominal dimensions are machined, so that the notch depth is between 6.9 mm and 8.1 mm (the notch width is kept equal to 1.5 mm due to some technical constraints). Tests were performed by means of an Instron testing machine. Three LVDT sensors were used to measure the rigid body motion- free mid-span deflection of the beam and a clip-on gauge to measure Crack Mouth Opening Displacement (CMOD). Tests on notched specimens were performed under CMOD control with a of less than 10 -3 mm/min. Digital image correlation technique was adopted to capture full-field two-dimensional displacement maps on specimen surface in the notch vicinity. Figure 1 : Geometrical and testing configuration of notched specimen under three-point bend loading. The rupture load of the smooth specimen is equal to 5.1 kN (corresponding to a nominal bending strength of 12.8 MPa, where the nominal stress  N is equal to 2 6 / (4 ) PS BW ). For the 3 notched specimens, the mean bending stress at failure is equal to 6.7 MPa (corresponding to a mean rupture load of 2.7 kN), with a coefficient of variation equal to 0.13. In Fig. 2, the load against mid-span deflection is shown for two notched specimens, along with the corresponding curves of the nominal stress against the crack mouth opening displacement. A