PSI - Issue 2_A

N. Ab Razak et al. / Procedia Structural Integrity 2 (2016) 855–862

860

6

N. Ab Razak et al./ Structural Integrity Procedia 00 (2016) 000–000

1.E+01

T (°C)

f (Hz)

K max (MPa√m)

NSWA PE

1.E+00

CFCG ,CT-FX-1,T=600C, th=600s,f=0.0017 CFCG,CT-A-1,T=625C,th=600s,f=0.0017 CFCG CT-A-2,T=625C,th=600s,f=0.0017 CFCG,CT-M-1,T=620C,th=600s,f=0.0017 CT-B 600 0.0017 25. 2 CT-C1 625 0.0017 22.82 CT-C2 625 0.0017 28.83 CT-A 620 0.0017 25.11

1.E-01

CFCG

CCG Data band (580°C -625°C)

CFCG Ali,T=625,,f=0.001 CFCG Ali,T=625,f=0.01Hz CFCG Mag T=600,th=60 min CCG, Ali, T=625 f=0 CCG,maleki ExPT=600C CCG,Maleki ,ExP3,T=600C CCG,MagT=600C Ref 9 625 0.001 Ref 9 625 0.01 Ref 14 Ref 9 600 Ref 8 600 Ref 8 600 Ref 14 600 CCG .00027

8.1 10.3

1.E-02

da/dt (mm/h)

-

1.E-03

1.E-04

NSWA PS

1.E-05

1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00

C* (MPam/h)

Fig.5. Correlation of creep fatigue crack growth data, creep crack growth data and predictive NSWA model for P91 material . 5.3. Creep-fatigue interaction The frequency dependence of CFCG behaviour can be predicted using Eqn (6) where the constants, determined from static CCG and high frequency FCG testing Webster (1994), are D = 6.5 and φ = 0.7 (Maleki (2015)) and λ =1.5×10-8, p = 3.57, (Mehmanparast et al. (2011)). Figure 6 shows the frequency dependence of crack growth per cycle for P91 steel for Δ K = 30 MPa√m and also literature data for Δ K = 20 MPa√m, which correspond to values of Δ K that fall in the Paris law FCG region for the tests considered. At high frequencies, fatigue is the dominant mechanism and the crack growth per cycle is insensitive to frequency, as shown by the horizontal line. At low frequencies, creep is expected to dominate leading to intergranular fracture. At intermediate frequencies (approx. 0.1 Hz) both fatigue and creep process are significant and mixed intergranular and transgranular fracture is expected. As explained in (Webster, 1994) both types of process are likely to develop intermittently through or around individual grains. Hence, at intermediate frequencies when one mechanism becomes arrested locally, the other may take over to allow cracking to progress at a rate equal to the sum of individual rate (Webster, 1994). It can be seen that at a frequency of 0.0017 Hz, the CFCG data falls within the region that is expected to be creep dominant, leading to intergranular failure.

1.E+00

Intergranular Creep

[3] Δ K = 20 MPa√m [3] Δ K = 30 MPa√m

Ali ,Delta K=20 Ali, Delta 30 da/dN CT-A-1 da/dNCT-A-2 da/dN CT-M-1 da/dN CT-FX-1 CT-C1 CT- 2 CT-A CT-B

Static CCG

1.E-01

Δ K =30 MPa√m

Δ K =30 MPa√m

da/dN ( mm/cycle )

1.E-02

Δ K =20 MPa√m

1.E-03

Transgranular line

1.E-04

1.E-05

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01 1.E+00 1.E+01 1.E+02

Frequenc y (Hz)

Fig.6. Frequency dependence of crack growth per cycle for P91 material