Issue 35

O. Plekhov et alii, Frattura ed Integrità Strutturale, 35 (2016) 414-423; DOI: 10.3221/IGF-ESIS.35.47 417 2 2 2 2 1 2 2            n E n I S    where Е – Young`s modulus; ρ – density; l – sample length; S – cross sectional area; μ – Poisson's ratio; I – moment of inertia of the sample relative to its longitudinal axis; n – harmonic number. Eigen frequency of sample can be written as   1 0 2 2 1 0 m m        , where ν 1 – pieze quartz frequency; ν 2 – frequency of the system – piezo quartz and sample; m 0 – weight of the sample; m 1 – weight of the pieze quartz. The amplitude independent decrement δ is   1 0 2 2 1 0 m m        . E XPERIMENTAL RESULTS he results of ultrasonic fatigue tests are presented in Fig. 4. The scatter and lack of experimental points don’t allow us to conclude the duality of Woller-curve and clearly divide the two mechanisms (volumetric and surface) of crack initiation. We can conclude that life time increases with the decrease of the stress amplitude and reaches 10 9 cycles at the stress amplitude equal to 160 MPa. Figure 4 : The stress amplitude versus number of cycles to failure under VHCF test. Five samples with different numbers of loading cycles were chosen for structural investigation: sample 1 (marked П0) is initial state; sample 2 (П1) – 9.01  10 8 cycles of loading at stress amplitude 161 MPa; sample 3 (П2) – 2.1  10 9 cycles of loading at stress amplitude 161 MPa; sample 4 (П3) – 3.1  10 9 cycles of loading at stress amplitude 162 MPa and sample 4 (П4) – cracked (crack is visible to the naked eye) 3.1  10 9 cycles of loading at stress amplitude 162 MPa. Finally, after preliminary polishing a crack was observed in the sample П3 also. This sample was considered as a fractured also. The Tab. 3 presents the geometry, weight, density, Young`s modulus and decrement of an iron at different stage of fatigue experiment. The analysis of data presented in Tab. 3 allows us to conclude that the increase of the number of cycles leads to the decrease of the sample density. It means that the dilatation can be considered as a measure of current damage state of a material. There is a considerable decrease of Young`s modulus with the increase of the number of cycles. Based on the results of previous experiments the decrease of Young`s modulus is linked with the increase of the dislocation density, T