Issue 49

M. Bannikov et alii, Frattura ed Integrità Strutturale, 49 (2019) 383-395; DOI: 10.3221/IGF-ESIS.49.38 387 relative 0 /     (5) where β 0 refers to intact material. M EASUREMENT OF NONLINEARITY COEFFICIENT o determine the nonlinearity coefficient, we prologue both parts of the Eqn. (4): 2 1 log( ) log( ) log( ) 2 log( ) k u u     (6) The amplitude of the oscillations is measured in decibels at scales   1 20 1 A log u  and ( 2 20 2) A log u  , then the ratio takes the form: 2 1 20 log( ) 20 log( ) 20 log( ) 40 log( ), k u u     (7) 2 1 20 log( ) 2 K A A     (8) Thus, the relative nonlinearity parameter β can be found from the formula: 0 2 1 2 1 0 20 log( / ) ( 2 ) ( 2 ) A A A A       (10) The authors developed a software and hardware complex which analyzes the signal in real time with a variable duration and performs a fast Fourier transform to search for the amplitudes of the fundamental and second harmonics. When a trend is detected to increase the amplitude of the second harmonic, the software stops the experiment and reports the possible occurrence of a fatigue crack. a) b) Figure 4: The change in the non-linearity factor βrelative (dB) during testing of the sample with an internal crack (a) σ = 138 MPa, N f = 7.51 • 10 8 (Fish-eye) and a sample with a surface crack (b) σ = 140 MPa, N f = 2.72 • 10 8 . The dependences of the non-linearity coefficient βrelative on the loading amplitude during fatigue life for aluminum alloy AMg-6 are given. During the experiment, βrelative increases slightly at the initial stage, which is usually attributed to the effect of heating the sample and, as a result, changing its linear dimensions [17-19], then most of the time of the experiment, the amplitude of the second harmonic remains constant and increases significantly with the formation of a fatigue crack and its growth (Fig. 4, a). The amplitude of the second harmonic is associated with the deformation due to defects. For given loading parameters, the system is macroscopically in the elastic region, and all deformations are T