Issue34

V. Oborin et alii, Frattura ed Integrità Strutturale, 34 (2015) 422-426; DOI: 10.3221/IGF-ESIS.34.47 424 where  is a universal exponent. This form is similar to the equation proposed by Hertzberg for sc l b  , where b is the Burgers vector. M ATERIALS AND EXPERIMENTAL CONDITIONS ynamic preloading of aluminum-magnesium alloy (AlMg6) samples (Fig. 1) was realized using the split Hopkinson pressure bar set-up at the strain rates of ~10 3 s -1 , after which the samples were subject to cyclic loading at room temperature. Then the fractography of fracture surface pattern was studied using the roughness data by interferometer–profiler New-View 5010. Fatigue tests were carried out using the resonance type of testing machine of (Shimadzu USF-2000) that provides the cyclic loading using the generator transforming the frequency of 50 Hz into ultrasonic electrical sinusoidal signal of frequency 20 kHz by piezoelectric transducer, generating longitudinal ultrasonic waves at a frequency 20 kHz and the mechanical stress with maximum amplitude in the gauge length (mid-section) of the sample. Deviation of the frequency by 0.5 kHz was associated with the damage critical stage and considered as failure precursor related to the initiation of a crack with characteristic size of ~2 mm. The level of applied stresses allowed us to investigate fatigue life up to the values associated with 10 10 cycles. The fatigue scenario was studied for the stress amplitude 105, 120, 130 MPa corresponding to a critical number of cycles of about ~10 10 estimated for initially unstressed materials subject to gigacycle fatigue tes ts. Figure 1: Sample geometry (sizes are given in millimeters). The roughness of fracture surfaces was measured by the high-resolution New-View 5010 interferometer-profiler (providing x2000 magnification) and then was analyzed using the assumption concerning fractal geometry of fracture surface profile associated with correlated behavior of multiscale defect structures on the scale of process zone pz L that preceded to the crack growth. The fatigue tests provided the fractured samples of two types. The samples of the first type were broken during the fatigue test. The samples of the second type revealed pronounced variations in the resonance frequency associated with fatigue crack origin. The fracture surfaces of the samples of the first and second type were uncovered by cooling the samples with liquid nitrogen and breaking them in the minimal cross-section of samples. It is assumed that the fatigue fracture surface of the samples subjected to gigacycle loading has already been formed during fatigue tests and occupies the largest part of the fracture surface, which is determined by a change in the resonance frequency. The crack origin in cylindrical samples undergoing high cycle loading in the range 10 6 -10 7 is started from the surface (Fig. 2a). For the samples subject to cyclic loads in the range exceeding 10 8 cycles the crack formation is started in the bulk of the sample. The fracture surface in this case exhibits a fatigue zone known as a “fish-eye” which is a particular feature of such fatigue regimes. The central part of this region comprises a fracture nucleus surrounded by the region of refined (submicrocrystalline) structure (region 2 in Fig. 2b). The New-View scanning was realized over the zones of fatigue crack growth (Fig. 2a) and one-dimensional profiles of the surface relief were made in the radial direction starting from zone 1 to zone. Within each “window” about 12 one- dimensional profiles were analyzed to ensure representativeness of the data along the defect-induced relief with vertical resolution of 0.1 nm and horizontal resolution of ~ 0.5 µm. A minimum (critical) scale l sc that corresponds to beginning of multiscale long-range correlation in defect ensembles is determined by the computing of the Hurst exponent. The function K(r) is calculated for one-dimensional profiles of fracture surface relief according to the formula [5,6]: D