Issue 35

O. Plekhov et alii, Frattura ed Integrità Strutturale, 35 (2016) 414-423; DOI: 10.3221/IGF-ESIS.35.47 415 One of the possible descriptions of defect kinetics is the statistical model of the defect ensemble [15]. This model should take into account the stochastically properties of the defect initiation, their nonlinear interaction and link between micropalsticity and damage accumulation. This work is devoted to the development of such a model in the application to cyclic loading of metals. To verify the theoretical predictions the experimental study of the defect evolution in iron under VHCF regime was carried out. The experiments were carried out on Shimadzu USF-2000 testing machine. To detect the crack initiation moment an original system was assembled based on the monitoring of magnetic field into the specimen. For a set of specimens with different degrees of the life time exhaustion an investigation of mechanical properties (elastic modulus, amplitude-independent damping) and the porosity distribution was carried out. The study of mechanical properties was carried out based on the acoustic resonance method using a piezoelectric vibrator with the longitudinal oscillations at frequencies around 100 kHz. The porosity of the specimens was studied by the method of hydrostatic weighing. As a result, it was shown that the increase of the number of cycles leads to the increase of the porosity of the specimens and to the decrease of their Young's modulus. The porosity of the samples increases from the surface to the axis of the specimen, that confirms the under surface fracture initiation in VHCF regime. M ATERIAL AND EXPERIMENTAL CONDITIONS rmco iron samples were used in our investigation. The mechanical properties and chemical composition of the samples are presented in Tabs. 1 and 2. С % Mn % Si % S % P % Ni % Сr % Mo % 0.004 0.04 0.05 0.005 0.005 0.06 0.038 0.01 Table 1 : Chemical composition of iron. Young's modulus (GPa)  0.2 (MPa)  b (MPa)  (%) 211.4 120-150 180-210 35 Table 2 : Mechanical properties of iron. The geometry of samples is represented in Fig. 1. The structure is presented in Figs. 2. a) b) Figure 1 : Geometry of the samples used in cyclic tests in the VHCF regime (a) , geometry of the samples bars with length 30 mm and diameter 5 mm used in acoustic resonance method (b) . A