Issue 47

Z. Hu et alii, Frattura ed Integrità Strutturale, 47 (2019) 383-393; DOI: 10.3221/IGF-ESIS.47.28 384 In order to predict in-service lifetime of notched components under VA loading, it is important to make use of specific methods suitable for taking into account the way fatigue damage accumulates itself. Palmgren [1] and Miner [2] first formulated this concept in a mathematical form as: f , ( / ) i i D n N   (1) where D denotes the cumulative damage sum, and n i and N f, i are the applied cycles and the total number of cycles to failure under the i th constant amplitude stress level, respectively. When engineering metallic materials are damaged by VA load histories, failure is supposed to occur as soon as the critical value of cumulative damage sum D cr is equal to unity. However, experimental evidence under VA loading condition indicates that in situations of practical interest D cr varies in the range 0.02-5 [3, 4]. Clearly, this fact can produce a remarkable discrepancy between estimated and actual lifetime, with this calculation resulting in non-conservative predictions. (a) (b) Figure 1 : (a) Geometries of the tested notched samples (dimensions in millimetres). (b) Fatigue results generated under CA uniaxial fatigue loading [3]. Fatigue life estimation techniques based on local approaches require some detailed information about the stress-strain state around the notch tip [3, 5-9]. Different SED-based approaches to deal with notch fatigue problems have been formulated since the beginning of the last century. Recently, Lazzarin et al. [8, 10, 11] developed a volume-based approach, in which the SED calculations are carried out in a material-related control volume. The averaged SED approach