At the stage of the main crack formation, the exponential law N ~ A exp (-cl) (1), which describes multiple fracture patterns at the initial loading stage (Figure 2 a) is replaced by the power law N ~ B l - b (2) (Fig. 2 b). Both laws are characterized by the respective c and b exponents, whose absolute values decrease with load (Figure 2 c) and the specimen thickness. An analysis of the amplitude distributions of the acoustic emission signals evaluated during tension has revealed the regularities similar to those, which were observed during tension; these are the change of the type of cumulative functions describing the number-amplitude distributions of signals and the reduction in the exponents of these functions before fracture. 2.3. Distributions of microcracks by their length at cyclic loading The analysis of the distribution curves of fatigue microcracks in mild steel leads to the conclusion that the change of function describing these curves occurs with increasing a number of cycles (Fig. 3 a) and decreasing a distance from the specimen fracture surface (Fig. 3 b). As in the case of thermal fatigue, the change is accompanied by a reduction of the exponents of these functions. a b Figure 3. The cumulative distributions of fatigue microcracks in mild steel plotted on the data of Suh et al [7] at different relative number of cycles (a): n/nf = 0,17 (1), 0,43 (2), 0,87 (3), 0,95 (4) and on data [5, 6] for microcracks located at a distance of 5.39 (1) 2.31 (2) and 0.79 mm from the fracture surface (b). Solid lines obey the exponential (a-curves 1-3, and b -1, 2) and power- law functions (a- curve 4, b- curve 3) 2.4. Distributions of fragments by their mass at dynamic loading The similar conclusion was also made in studying the dynamic fracture [8, 9] (Fig. 4). The data presented in the Figure 4 show, firstly, that the number - mass distributions of fragments of shells made of brittle and ductile steels may be described by a simple exponential function: ∑N~ NО exp (- m/mo) (3), and, as is seen from Fig. 4 b, the fragment distributions for the ductile material approach the power law N - m dependence. Secondly, the exponents of these distributions (1/mo) are determined by the
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