Issue 47

Z. Hu et alii, Frattura ed Integrità Strutturale, 47 (2019) 383-393; DOI: 10.3221/IGF-ESIS.47.28 389 be obtained from the post-processing results by applying the value of nominal stress in the FE models. The relationship of the SED, W  , and the fatigue life, N f , can be written as: f B W A N    (6) where A and B are material constants. Keeping constant the material and the nominal load ratio, the fatigue life can be assessed by using Eq. (6) for any geometrical configuration of the notch. T HE SED TO ESTIMATE LIFETIME OF NOTCHED COMPONENTS UNDER VA FATIGUE LOADING n the presence of VA load histories, the value of the SED will change as the amplitudes of the applied load spectrum vary. This clearly implies that, to correctly extend the use of the SED to those situations involving VA loadings, the procedure of the SED has to be redefined coherently according to the specific features of the assessed load spectrum. When the notched sample is subjected to the VA fatigue loading, the VA load spectrum can be counted by the Rain-Flow method [20]. Such a spectrum is formed by j different stress levels that are characterized by a nominal stress amplitude equal to a-i  as shown in Fig. 4. The corresponding number of cycles is equal to n i ( i =1,2,…, j ), where: tot 1 j i i n n    . Figure 4 : Estimation of the fatigue lifetime under VA loading by SED. The different stress levels forming the above load spectrum can be treated as an independent CA loading, which can be applied in the FE model to obtain the value of the SED. Then, the corresponding number of cycles to failure N f, i can directly be determined according to the relation of SED versus the fatigue life N f obtained by Eq. (6). As formalized by Palmgren and Miner [1,2], the fatigue damage content associated with any stress level can be calculated as follows: f, i i i n D N  (7) The resulting total damage is equal to: W  I