PSI - Issue 52

Valery Shlyannikov et al. / Procedia Structural Integrity 52 (2024) 214–223 V.Shlyannikov,A.Sulamanidze,D.Kosov/ Structural Integrity Procedia 00 (2023) 000 – 000

221

8

4. Crack growth rate analysis To establish the behavior of the CGR under isothermal and thermo-mechanical high-temperature tests and computations were implemented for the same specimen thickness using the SENT configuration. The applied maximum load P max and temperature were combined in tests of the pure fatigue, creep – fatigue interaction and TMF conditions, where the value of the initial crack length a 0 was approximately the same for SENT specimens. The experimental relationships between crack length a i and the accumulated number of loading cycles N i contains considerable noise and therefore filtered before da/dN was evaluated. The filtering process included of the smoothing procedure for the test raw data according the following statistical equations:

2

p i

= +  = −

5 N a Na − p p i i

da dN     =  

(5)

2

p i

2

p i

= +  = −

2 p N N − 5

2

i

i

2

p i

where

1 5 = − =  p i p i = +

1 5 = − =  p i p i = +

2

2

and

(6)

N

N

a

a

i

p

i

p

2

2

For analyzed isothermal and thermo-mechanical fatigue conditions all the cyclic fracture diagrams of nickel alloy XH73H are represented in the CGR coordinates da/dN versus K 1 , where the elastic SIF values are determined in previous section. Although the present study is focused on analyzing the behavior of only one nickel alloy, it will be useful to compare the fatigue fracture diagrams for various cyclic deformation conditions with each other. To this end, Fig. 7a shows the fatigue fracture diagrams for the classic harmonic cycle (with constant loading frequency and loading waveform) and trapezoidal cycle, which consists of the dwell time and loading/unloading time, for each loading cycle (Fig.2). It was observed that for the isothermal high temperature test of T= 650˚C the CGR during creep – fatigue interaction increase by approximately one order of magnitude with respect to the harmonic loading conditions at the same applied nominal stress level of σ =80MPa. Lowering the test temperature to T=400 ˚ C leads to a decrease in the crack growth rate both under pure fatigue and creep-fatigue interaction conditions by about two and a half orders of magnitude. At the same time, the CGR differences in the heat-resistant alloy XH73H at ambient temperature T=23°C and moderately elevated temperature T=400°C correspond to the known literature data.

a) c) Fig. 7.Crack growth rate versus elastic SIF for different test program: (a) pure fatigue and creep-fatigue interaction; (b) TMF and creep-fatigue interaction; (c) TMF and pure fatigue. b)