Issue 49

M. Bannikov et alii, Frattura ed Integrità Strutturale, 49 (2019) 383-395; DOI: 10.3221/IGF-ESIS.49.38 386 a) (b) Figure 3 : (a) Characteristic surface relief of a gigacycle fatigue fracture zone of AlMg6, (b) scaled-up fragment of the «fish-eye» zone obtained by the scanning electron microscope Hitachi S3400 (σ=138 MPa Nf=7.51  10 8 ), the region 1 is the center of crack initiation, the region 2 is FGA zone. N ONLINEAR ACOUSTICS METHOD FOR NON - DESTRUCTIVE TESTING OF FATIGUE FAILURE he method is based on the initiation of a longitudinal finite-amplitude perturbation A 0 with a frequency ω 0 on one side of the sample, while its other end remains free [17-19]. Oscillations of the free end of the sample will contain a number of harmonic components: components with amplitude A 1 at the fundamental frequency ω 0 , amplitude A 2 of the second harmonic with frequency 2ω 0 , and so on. The nonlinearity parameter β e is determined experimentally by measuring the absolute amplitudes of the signals of the first A1 and second A 2 harmonics corresponding to the nonlinear law of elasticity: 2 2 1 1 ... ... 2 3 2 2 2 u u u u e e e e A A A a a a a                                                       (1) where σ is the stress, u is the displacement, a is the spatial coordinate, 2 e A and 3 e A - are the second and third order elastic coefficients, respectively. By introducing the nonlinearity coefficient: 3 2 ( / ) e e e A A    and the wave equation can be represented as: 2 2 2 1 e u u u c a a t                            (2) where u is the component of the displacement vector in the direction, c - is the longitudinal speed of sound, t - is time. Its solution, given that the end oscillation u =u1cos(ωt) , will be: 0 1 2 cos( ) sin 2( ) ... u u u t u t ka        (3) That the amplitude of the second harmonic 2 2 2 1 (1/ 8) e u k u a   , where 0 / k    – wave number, we can derive: 2 2 2 1 8 / e u k u a   (4) In the study of nonlinear phenomena in the gigaclic fatigue regime, the relative parameter is determined by measuring the amplitudes of the main and second harmonics: T