Issue 30

D.S. Paolino et alii, Frattura ed Integrità Strutturale, 30 (2014) 417-423; DOI: 10.3221/IGF-ESIS.30.50 422 Figure 4 : Statistical distribution of the transition life together with the probable fatigue regions. As shown in Fig. 4, the median of the transition life can be usefully considered to discriminate between the two fatigue regions of HCF and VHCF: failures that occur at a number of cycles smaller than the median more probabilistically belong to the HCF region; while failures that occur at a number of cycles larger than the median more probabilistically belong to the VHCF region. For the analyzed case, the median of the transition life is equal to 7.035 , which results in a median transition cycle equal to 7 1.08 10  . As visible in Fig. 5, the median value properly differentiates between the two fatigue regions: each internally nucleated failure is above the median value, while each superficially nucleated failure is below the median value. Figure 5 : Experimental data and probable fatigue regions as discriminated by the median value of the transition life. C ONCLUSIONS procedure for the estimation of the statistical distribution of the transition life in a Duplex S-N curve was shown. The statistical distribution was estimated by numerically solving an equation which correlates the cumulative distribution function to the quantile of the distribution. As shown with a numerical example taken from the literature, the resulting distribution depends on the distance between the HCF and the VHCF regions and on the distribution of the random transition stress. The estimated distribution can be effectively used to predict, with a specified confidence level, the number of cycles for which an internal nucleation may probabilistically occur in a VHCF test and it is also informative for properly choosing the end of HCF tests in terms of number of cycles. A