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140

ENSI Erfahrungs- und Forschungsbericht 2011

which, e.g., can be used as engineering criteria to

identify critical component locations and/or oper-

ating conditions [5, 6]. These lab corrosion fatigue

investigations were performed by continuous cy-

clic loading experiments at zero mean stress. In

reality, periods with cyclic loading of PPBC (e.g.,

plant transients) are often interrupted by very long

periods of moderate static stress (stationary pow-

er operation) and it was suggested that these pe-

riods might reduce the resulting environmental ef-

fects. Furthermore, in pressure boundary compo-

nents a positive mean stress (e.g., from internal

pressure and/or residual stress) is superimposed to

the fluctuating secondary thermal stresses in many

cases. The special emphasis during this year was

therefore placed to the effect of load ratio (mean

stress) and very long static load hold periods. The

latter is summarized in [7] and briefly discussed in

the 2010 annual report.

Figure 2 shows the effect of notch-root stress am-

plitude S

a,LEFM

and load ratio on CF initiation in

simulated HWC environment at 288 °C at a strain

rate of 4 × 10

-2

%/s. It seems that environmen-

tal effects disappear at very small strain/stress am-

plitudes and might be maximal in the intermedi-

ate range. It is expected that at very high strain/

stress amplitudes, the environmental effects satu-

rate or even slightly decrease again, but his was

not yet investigated here, since this would require

large specimens to assure small scale yielding con-

ditions.

The stress thresholds seem to decrease with in-

creasing load ratio. The rather limited amount of

data suggest that environmental effects on the fa-

tigue initiation from notched surfaces disappear

below a strain and stress amplitude threshold of

0.3 % and 400 MPa, respectively, which is slight-

ly higher than in strain-controlled LCF experiments

with smooth specimens. A tremendous effect of

load ratio is observed, if the notch stress amplitude

at the low load ratio is below the stress threshold

for environmental effects as shown in Figure 2b.

Here, CF life is reduced by a factor of 50 to 100,

when increasing the external load ratio from 0.05

to 0.5. The corresponding test in air at a load ra-

tio of 0.5 is not yet performed. Based on typical

mean stress corrections and finite element cyclic

plasticity modeling of the notch, a reduction of

less than a factor of 2 is expected in air. Note that

in spite of a positive load ratio of 0.5, the notch

mean stress is only slightly positive and the notch

stress ratio is well below 0. The effect of load ra-

tio (and mean stress) on CF initiation in the endur-

ance limit region seems to be more pronounced

than predicted by typical mean stress corrections

(e.g., Goodman).

Based on these preliminary test results it is thus

strongly recommended to further investigate

mean stress effects in the fatigue endurance lim-

it region. The load ratio only had a moderate ef-

fect on the subsequent short crack growth, which

was similar to that in air or for deep cracks in the

Paris region. More pronounced load ratio effects

may appear close the K-thresholds for fatigue

crack growth.

3.2. Sub-project II – SCC in

Dissimilar Metal Welds

3.2.1. Fabrication of NDT Test Bodies with

Realisitic SCC Cracks

In recent years several SCC cracking incidents oc-

curred in dissimilar metal welds (DMWs) in BWRs

Research Project KORA-II

dε/dt ≈ 4 x 10

-2

%/s

III a

r m

R = 0.05

a,LEFM

[MPa]

Cycle number N

[-]

BWR/HWC, T = 288 °C, 0.15 ppm DH

R 0.68

304 L, A (& 321)

R = 0.5

R = 0.05

ASME

a,LEFM

[MPa]

Cycle number N

[-]

BWR/HWC, T = 288 °C, 0.15 ppm DH

321, C

Figure 2:

Effect of notch stress

amplitude and load ra-

tio on CF initiation life

(a) and effect of load

ratio R on CF initiation

at a notch stress ampli-

tude of 840 MPa (b).

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Caple AUF900 Manual de usuario Pagina 142

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Caple AUF900 Manual de usuario Pagina 142

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