Issue 78

S. Vantadori et alii, Fracture and Structural Integrity, 78 (2026) 432-453; DOI: 10.3221/IGF-ESIS.78.25

With reference to Stage III, a marked acceleration of the crack growth rate da dN is observed in Fig. 9(b) exclusively for 0.50 R  , culminating in a final SIF range of approximately 13 14  MPam 0.5 , corresponding to a fracture toughness of 26 28  MPam 0.5 .

Figure 9: Fatigue crack propagation rate curves, da dN vs. K  : (a) results obtained from the three replicate tests performed at R  0.50 and (b) representative curves for the three investigated fatigue ratios ( R  0.10, 0.50, 0.75). The relationships between the characteristic lengths, namely cyclic plastic radius, c r , and striation spacing,  , and the applied SIF range, K  , are also reported. With reference to Stage II, instead, the crack propagation behaviour can be further subdivided into distinct sub-stages, hereafter referred to as Sub-Stages IIa (immediately following Stage I), IIb (intermediate between IIa and IIc), and IIc (immediately preceding Stage III). For  0.10 R and 0.50 , three sub-stages can be identified, each characterized by a

441