Issue34

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 34 (2015) 415-421; DOI: 10.3221/IGF-ESIS.34.46 419 Comparing the crack propagation results for R = 0.10 and R = 0.50 (Fig. 5), the influence of the stress ratio R on the constant crack growth rate stage is evident. The decrease of the crack growth rate values correspond to a decrease of the  K range where this phenomenon is evident. 1.0E‐09 1.0E‐08 1.0E‐07 1.0E‐06 1 10 da/dN [m/cycle] ΔK [MPa∙√m] R=0.10 R=0.50 Figure 5 : Comparing fatigue crack propagation behavior for R=0.10 and R=0.50 conditions. The influence of the microstructure modification on fatigue crack propagation micromechanisms is shown in Fig. 6, where fracture surfaces obtained for R=0.10 and R=0.50 are compared (ΔK=10 MPa√m). Corresponding to this ΔK, specimen tested at R=0.10 is in the stage 3: fatigue crack mainly propagates intergranularly, with a “striation-like” morphology, probably due to the need-like microstructure (compare Fig. 6 with Fig.2). For R=0.50 the intergranular morphology is less evident (Fig. 6b), with evident fatigue striations. Difference between “striation-like” morphology and “fatigue striation” can be determined considering the striation spacing (for R = 0.10, “striation spacing” is analogous to the needles thickness). a) b) Figure 6 : Fracture surface SEM analyses: a) Stage 3 at ΔK=10 MPa√m in R=0.10 condition [3], b) Stage 3 at ΔK=10 MPa√m in the R=0.50 condition . Considering higher  K values (e.g., ΔK=12 MPa√m, Fig. 7), striations are more evident and the higher R value (0.75) corresponds to a more fragile morphology, with evident intergranular secondary cracks and cleavage.