Issue34

Y. Li et alii, Frattura ed Integrità Strutturale, 34 (2015) 599-607; DOI: 10.3221/IGF-ESIS.34.66 601 strength is related to fractal dimension and crack rate, according to the design scheme carried out by the direct shear test. One type of the sample’s diameter is 61.8mm and the height is 20mm, another sample’s diameter is 150mm and the height is 50mm. A drying cabinet set at 60°C was used to dehydrate all of these samples for 12h. The wet mode was divided into two methods; artificial humidification (using a watering can to spray distilled water onto the surfaces of the obtained samples for 10 minutes, then putting the samples in cylinder seals for 24h) and vacuum saturation (pumping gas saturation for 1h, then soaking the samples in water for 10h). The specific plan is shown in the Tab. 2. F RACTURE DEVELOPMENT MECHANISM ANALYSIS Fracture characterization analysis his paper selects the artificial humidification using a small cutting ring (sample 1) and a large cutting ring (sample 2) and vacuum saturated (sample 3) for each group, respectively. The article determines fractal dimension and crack ratio of three groups of samples in five dry-wet cycles (see Tab. 3). The crack ratio is determined by the use of Matlab to select and count black and white pixels of the image, then is compared with the amount of black pixels in general pixels by the following formula, fracture area black pixels 100% 100% surface area general pixels      (1) The fractal dimension of the crack is measured by determining the box dimension. The box dimension is defined as coordinates net δ of R n : [m 1 ,(m 1 +1)] ×…× [m n ,(m n +1)], [m i ,(m i +1)] is the side of coordinates net δ, m 1 , … ,m n are integers. Suppose F is a limited non-zero collection on R n , N δ (F) stands for diameter, the maximum δ can cover a minimum set number, the box dimension for F is defined as: δ B δ 0 logN (F) Dim F lim logδ    (2) δ B δ 0 logN (F) Dim F lim logδ    (3) When the dimensions of the upper box and the lower box are equal, F is called the box dimension, indicated as: δ B δ 0 logN (F) Dim F lim logδ    (4) When calculating the box dimension for F, the length of δ intersects F, and the number of the intersectional points is the box dimension N δ (F). When δ approaches zero, this means adding logarithmic speed of N δ (F), or the negative values of the slopes of log δ and logN δ (F) is the box dimension. The box dimension of the red clay crack can be determined as follows: in order to calculate its box dimension, the images of cracks are regard as a set F. We can draw a square grid of coordinate δ in the image to calculate F and grid square intersect number N δ (F), take the value of δ (for example, n=1/ 2 n , n=1, 2,... ), then confirm the different N δ (F). In this region, by using the least square method we can get the regression linear equation:   1 δ gN F = a(lgδ ) b   (5) The slope of the linear can be regarded as approximate value of box dimension F, based on the box-counting dimension, the box dimension of crack image under various δ can be obtained: n n δ B δ 1 n lgN (f) dim f lgδ   (6) T