Issue 20

H. Jasarevic et alii, Frattura ed Integrità Strutturale, 20 (2012) 32-35; DOI: 10.3221/IGF-ESIS.20.04 33 since the most of reported d values are in the range 1.1-1.3 [5]. As a result, it is necessary to modify w(t) to lower its d . The simulation procedure by applying the operation of fractional integration to realization of w(x) in order to decrease d to desired value is described by Kunin and Gorelik [5] in detail. The realization w(x) with d = 1.5 is first generated as integral of a white noise e(x) written in discrete form as: ( ) ( ) ( )      i i i w x D e (1) where e i is Gaussian random variable with zero mean and unit variance and D is crack diffusion coefficient determined from experimental data. D is statistical parameter reflecting the tendency of crack trajectories to deviate from central axis [5]. Once the realization is generated, it is necessary to modify fractal dimension by applying the fractional integration operation to it as follows:      1 0 1 ( ) x w x w x d            (2) Where  (…) is the Gamma-function, and fractal dimension of w  (x) is d = 1.5-  . A numerical implementation of above described procedure has been coded into the software MatLab® for case study simulations presented later in this text. The algorithm requires 3 input parameters, namely d , D and n (number of trajectories to be generated). Each realization of w(x) with d = 1.5 incorporating parameter D is generated with 1000 time steps (intervals) and fractional integration operation is performed to create final realization w  (x) with d = 1.5-  . The process is repeated in DO loop till n trajectories are created. Due to the above described formalism simplicity and computational performance of modern computers, this whole process is completed in the matter of couple of seconds. However, the major issue is obtaining input parameters ( d, D ) for simulation from experiments (sufficient number of repeated experimental tests is needed). It should be noted that both parameters are strongly scale dependent. Issa at al [6] stated that groups of identically sized concrete specimens prepared with different aggregate sizes have different values of D (one with smaller size aggregate have smaller value of D ). In addition, larger sized specimens with identical mix have lower values of d . Also, Zavarise at al [7] pointed that statistical parameters used in stochastic contact models are scale dependent (being function of profilometer resolution). Figure 1 : Experimentally observed crack trajectories in sandstone discs. Figure 2 : Simulated sandstone crack trajectories with d=1.2 and D=0.01