Issue 35

O. Plekhov et alii, Frattura ed Integrità Strutturale, 35 (2016) 414-423; DOI: 10.3221/IGF-ESIS.35.47 418 microcrack initiation and dilatation. In current situation we can connect the decrease of Young`s modulus from sample П0 to sample П4 with cracks and voids initiation. We can also observe the decreasing of the decrement  i from sample П0 to sample П3 and high increase of the decrement in fully cracked sample П4. Sample d, mm m, g  , g/cm 3  /  , E, GPa  i, 10 -5 П0 5 4.65252 7.8667 0 186.2 63.0 П1 5 4.68843 7.8654 1.6  10 -4 184.2 52.4 П2 5 4.63733 7.8533 1.7  10 -3 184.4 52.5 П3 5 4.63920 7.8522 1.8  10 -3 182.5 46.5 П4 5 4.50319 7.8522 1.8  10 -3 169.0 890 Table 3 : Physical and geometrical parameters of iron sample under investigation Fig. 3 presents the evolution of Young`s modulus as an exponential function of dilatation. 0,000 0,001 0,002 0,003 0,004 5,05 5,10 5,15 5,20 5,25 ln E, GPa  Figure 5 : The evolution of Young`s modulus versus dilatation caused by the increase of the number of cycles under fatigue test in VHCF regime . In order to study the spatial distribution of dilatation as a function of distance from middle cross section of the sample, the sample П 4 is divided into three parts. The central (working) part had the length of 11 mm and weight 1.62199 g. The magnitude of the dilatation of working part with respect to the end (non operative) parts was 0.1%. Symmetrical decrease of the length of the working part to the length of 8 mm leads to the increase of the magnitude up to 0.2%. This result supports a thesis that the maximum of damage accumulation is located in the middle part of the sample and dilatation is distributed in the volume of the specimen. To study the dilatation distribution along the radius we polished the samples П0-П3 from the radius 5.00 mm to 4.22 mm and measured the density and Young`s modulus after each step of polishing. Tab. 4 summaries the results of this investigation. The density and Young`s modulus of sample П0 are not sensitive to the change of the diameter. The optical microscopy proves the absence of cracks in the samples П0-П2 and existence of cracks in the samples П3, П4 (Fig. 1b). Moreover, the crack opening increases during the polishing. It means that the crack opening is bigger in the volume of the cracked sample. The data presented in Tab. 4 show that the decrease of diameters leads to the increase of the dilation of the samples П1, П2. Fig. 6 presents evolution of the dilatation of the sample П1 as a function of diameter (Fig 6a) and as a function of the length (fig 6b). The same results were obtained for the sample П2. The cracked sample П3, П4 doesn’t exhibit the effect of volume variation of dilatation. It could be caused by the existence of macroscopic crack with the change of the distribution of dilatation.