Issue 30

A. Spagnoli et alii, Frattura ed Integrità Strutturale, 30 (2014) 145-152; DOI: 10.3221/IGF-ESIS.30.19 147 parameter can be correlated with the specific surface S s of the grains (i.e. the total surface area of the grains per unit volume of material) as follows for a slab of length L (Fig. 1) 2 s S L n   (3) Consider now an external edge crack of depth a submitted to Mode I (opening) load due to the thermal stress ( , ) z x t  of Eq. 1 ( ( , ) z x t  is the stress acting in the solid slab along the line where the crack is assumed to be located). The Mode I Stress Intensity Factor (SIF) of the crack can be calculated as follows [12] 3 2 0 2 , 2 1 ( , ) ( , ) 1 1 a I z a a G a h K a t a t da a a a h a                             (4) where , a a G a h        is a dimensionless function of the crack geometry ( a  = h–x). At this point, in might be worth considering the influence of calcite grain shape on the mechanical performances of marble (marble microstructures do present sometimes different geometric features ranging from the extreme cases of xenoblastic textures, characterized by wavy contours, and homoblastic textures, characterized by gently curving boundaries). Generally speaking, xenoblastic marbles tend to exhibit a higher resistance to mechanical actions [3]. In order to consider such an aspect, for two-dimensional models it might be appropriate to use a shape parameter like the parameter  equal to area/(perimeter) 2 , which has an upper limit corresponding to the value for a smooth circle ( max 1/ (4 ) 0.080     ) and tends to zero ( min 0   ) for extremely rough boundaries. Typically, homoblastic marbles are characterized by 0.045   and xenoblastic marbles by 0.030   . Figure 1 : Schematics on the intergranular equivalent cracking. The degree of roughness of grain boundaries tends to enhance the interlocking resistance of intergranular cracks. To include such an interlacing effect in the model, it is believed that the driving force can somehow be reduced as a function of fracture surface roughness. The concept of roughness-induced crack closure, largely used for fatigue crack propagation