Issue34

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 34 (2015) 261 - 270; DOI: 10.3221/IGF-ESIS.34.28 264  K th = -2.60 R + 4.39 : IP series (1)  K th = -2.09 R + 3.82 : Harmonic series (2) Figure 3 : Relationship between crack growth rate and stress intensity range for the (a) IP and (b) Harmonic series. Figure 4 : Relationship between threshold stress intensity range and stress ratio for the IP and Harmonic series. Crack closure The role of stress ratio at near-threshold levels is generally attributed to crack closure [17]. Fig. 5 shows the relationship between d a /d N and  K eff . In each series, the value of  K eff tended to normalize the stress ratio onto a single curve near threshold under R = 0.5. This result indicates that the effect of the stress ratio on the  K th disappears. However, when R is greater than 0.5,  K th value still decreased with increasing stress ratio, R . Same tendency was observed in the previous R = 0.1 R = 0.3 R = 0.5 R = 0.7 R = 0.8 2 3 4 5 6 10 -11 10 -10 10 -9 10 -8 Stress intensity range  K , MPam 1/2 Crack growth rate d a /d N , m/cycle (b) R = 0.1 R = 0.3 R = 0.5 R = 0.7 R = 0.8 2 3 4 5 6 10 -11 10 -10 10 -9 10 -8 Stress intensity range  K , MPam 1/2 Crack growth rate d a /d N , m/cycle (a) 0.0 0.2 0.4 0.6 0.8 1.0 2 3 4 5 Stress ratio R  K th , MPam 1/2 IP series Harmonic series