Issue 74

P. Zuliani et alii, Fracture and Structural Integrity, 74 (2025) 385-414; DOI: 10.3221/IGF-ESIS.74.24

(a)

(b) Figure 20: Correlation between the number of cycles to failure (N) and: (a) the notch fatigue factor (K f ) and (b) the notch sensitivity (q). Experimental data digitized from [11]. The SEM analysis of the fracture surfaces showed that the cracks nucleated from the surface not only for the notched specimens, but also the majority of the smooth specimens had cracks nucleated from the surface. Consequently, it is not possible to attribute the change of slope of the S-N curves to a change of failure mechanism between notched and smooth specimens, as it may happen for other materials. Finally, Gao et al. also defined a weighting function to modify the classical TCD in order to predict the number of cycles to failure of notched specimens in the VHCF fatigue. The method will be discussed in detail in the section of discussion of the present paper, but it is important to say that they obtained a limited scatter within ±5 %. The first study about the notch sensitivity of Additive Manufacturing (AM) materials in the VHCF regime was performed by Tridello et.al. [7]. They studied the fatigue response up to 10 9 cycles of unnotched and notched specimens made of titanium alloy Ti6Al4V ELI using selective laser melting process (SLM). The stress concentration factors were computed by the Authors according to the definition of Paolino et.al. [5]and they are always greater than the static K t values. Moreover, they defined the stress concentration factor with the symbol (K t,d ) to specify that it refers to the stress concentration during a dynamic test at ultrasonic frequency. The unnotched hourglass specimens have a K t,d =1.024, while the notched hourglass

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