Issue 48

Y. Yamakazi, Frattura ed Integrità Strutturale, 48 (2019) 26-33; DOI: 10.3221/IGF-ESIS.48.04 29 where ∆ σ is the stress range, ∆ ε e and ∆ ε p are the elastic and plastic strain range, respectively, and a is the half crack length. In this study, the total crack length, 2 a , is defined as the crack length projected on the plane perpendicular to the load axis. Morever n f is the cyclic hardening exponent ( n f = 0.304 for isothermal LCF loading). F and f ( n f ) are given by: ( ) 1.65 1.12 1 1.47 F   = + and ( ) ( ) 3.85 1 f f f f n f n n n  − = + , (2) where λ is the ratio of the crack depth and the surface crack half-length ( λ = 0.84 obtained from the fracture surface). For comparison, the fatigue crack growth curve of physically long crack obtained by using Compact-Tension (CT) and Center- Cracked-Tension (CCT) specimens is also shown in Fig. 3 [5]. The growth rates of naturally initiated short cracks are higher than those of the physically long cracks at a given ∆ J f . This experimental evidence indicates the importance of the investigation of naturally initiated short crack growth behavior since the information on the basis of a physically long crack growth rate provides a dangerous evaluation of the reliability to real components. (a) IP (b) OP (c) CC1 (d) PP Figure 2 : Typical fatigue cracks on the specimen surface: (a) IP, (b) OP, (c) CC1 and (d) PP. IP IPC02 IPC04 CC1 CC2 OP PP Intergranular mode Transgranular mode Table 2 : Propagation modes of fatigue cracks initiated on the specimen surface. Figure 3 : Fatigue crack growth rates under isothermal-LCF conditions correlated with the fatigue J-integral range; the shaded band indicates the fatigue crack growth curve of physically long crack obtained by using CT and CCT specimens in Ref. [5]. 10 -1 10 0 10 1 10 -8 10 -7 10 -6 10 -5 Fatigue J-integral range, Δ J f [kN/m] Crack growth rate, da/dN [m/cycle] : PP : CC1 : CC2 ln y=a + b ln x a=-1.54531359e+01 b=1.23241744e+00 9.34146072e-02 |r|=9.93720155e-01 ln y=a + b ln x a=-1.48552481e+01 b=1.08770312e+00 2.90591114e-01 |r|=9.62104128e-01 Long crack (CT&CCT)* : 550 ℃ : 600 ℃ : 650 ℃ * Ref. [5]