Issue 33

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 33 (2015) 215-220; DOI: 10.3221/IGF-ESIS.33.27 216 same defect size, area = 188 µm, where the area denotes the square root of the size of projection of the defect on a plane normal to the loading axis. Fully reversed tension-compression fatigue tests were carried out at a test frequency of 150 Hz. The fatigue limit  FL was determined as the stress amplitude at which the specimen does not fail after 1×10 7 cycles. For every material and notch tilt angle a non-propagating crack was found at the fatigue limit. After performing fatigue tests, and carrying out fatigue crack propagation studies, the following conclusions were obtained: - In JIS-S15C and JIS-S45C, the fatigue limits were nearly independent of the tilting angle, and were found in good agreement with the predicted values by the area parameter model [1-2]. On the other hand, in JIS-SNCM439, the fatigue limit was also in agreement with the prediction at the tilting angle of 0°, but it increased with an increase in the tilting angle. - As shown on Fig. 1, a typical growth behavior of small crack was observed irrespective of the tilt angle; when the stress amplitude was slightly above the fatigue limit, the growth first decelerated to a certain crack length, and then started accelerating with fatigue cycles until the final failure. (a) (b) Figure 1 : Propagating and non-propagating cracks found above and below the fatigue limit in case of JIS S45C steel and 30º notch (a) crack length vs. number of cycles (b) Crack growth rate vs. crack length. - The length of non-propagating cracks varied greatly according to the steel types, i.e. longer non-propagating cracks were observed in softer steel. The crack length measured at the surface was also found to be independent of the tilting angle in the case of JIS-S15C and JIS-S45C, whereas in the case of JIS-SNCM439, the length increased with an increase in the tilting angle. - The observed crack paths were in good agreement with the direction normal to the maximum principle stress near the defect (2D analysis). To complement the information reported above and understand the phenomenon in more detail, it was noted that further investigation was needed to clarify the three dimensional shapes of the non-propagating cracks. As a step forward in this direction, this paper shows 3D experimental characterization of the non-propagating fatigue cracks using high resolution X-ray tomography. E XPERIMENTAL Specimens n order to perform the X-ray analysis of non-propagating fatigue cracks, a smaller specimen with a square cross- section ranging from 0.8 to 1 mm 2 was cut from the larger sample, such that it contained the notch and the non- propagating fatigue crack. As the cross sectional area of the small samples cannot be larger than 1 mm 2 due to experimental restrictions and some of the samples contained cracks measuring up to 700 μm in surface length, extreme care was taken when cutting the sample. The procedure consisted in extracting a larger sample containing the notch plus crack and then applying successive steps of polishing and microscope observation until the final geometry was reached. I