Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

to unexplored plant-relevant aspects, which may

result in non-conservatism.

During the report period, the focus was placed

to the completion of the experimental studies on

mean stress effects with sharply notched fracture

mechanics specimens [9] and on the in-phase (IP)

and out-of-phase (OP) TMF behaviour with tubular

specimens [10]. These investigations showed

that mean stress may have a tremendous effect

on physical fatigue initiation life in high-temperature

water in the fatigue endurance limit range.

The environmental reduction of fatigue life may

be stronger than predicted by the typical mean

stress corrections in air. The TMF tests showed the

expected behaviour based on the known dependencies

from isothermal LCF experiments and no

anomalies were revealed. The TMF life is between

that of the isothermal low-cycle fatigue (LCF) tests

at minimum and maximum temperature. Reasonable

engineering TMF life predictions by the environmental

factor approach of NUREG/CR-6909

and adequate mean temperatures seem to be possible

in high-temperature water.

Within a four months internship [11], a material

science master student performed first metalloand

fracographical post-test evaluations of the

TMF tests with tubular specimens in air and hightemperature

water. These ongoing investigations

will help to identify the mechanism and to better

separate the environmental effects on physical

initiation and on the subsequent short and

long crack growth as a function of applied strain

amplitude. Multiple crack initiation and propagation

at the inner wall of the gauge section of the

tubular specimens was observed in LCF and TMF

tests in high-temperature water and the individual

cracks typically had a semi-elliptical shape (Figure

2a). In air, cracks initiated both on the outer and

inner surface. The corrosion fatigue cracks were

always straight and normal to the stress axis in

high-temperature water. The corrosion fatigue

cracks appear to grow predominantly as Mode I

tensile cracks normal to the stress axis, and small

shear cracks (Mode II) near to the surface were not

observed so far.

The fatigue fracture morphology of crack growth

into the material in air and high-temperature

water did not differ significantly, which suggests

the same underlying crack growth process. Welldefined

fatigue striations were clearly visible on

most parts of the fatigue crack flanks in air and

high-temperature water (Figure 2b). The width or

spacing of the striations was significantly larger in

high-temperature water under otherwise similar

conditions and correlated fairly well with the macroscopic

crack growth and its environmental acceleration

under the given conditions. The presence

of well-defined striations suggests that mechanical

factors are still dominant over dissolution effects

(e.g., slip dissolution mechanism) in the cracking

process.

Close to the inner wall, where the cracks initiated,

the striations were hardly visible or the surface was

even striation-free. This might be related to the

longer oxidation period of the surface closer to the

crack initiation site, to crack closure effects or a different

mechanism for physical crack initiation and

subsequent short crack growth. For both IP- and

OP-TMF, the striation width becomes larger with

increasing strain amplitudes and with increasing

distance from the inner wall, where the cracks initiated,

and thus with increasing local stress intensity

amplitudes. IP-TMF crack growth rates are a factor

of 5 to 10 higher than OP-TMF rates. A similar difference

is observed in isothermal corrosion fatigue

(CF) crack growth rates at the minimum and maximum

temperature in tests with fracture mechanics

specimens. The number of cycles to physical crack

Figure 2:

Multiple crack initiation

(a) and fracture

surface with clear

striations form IP-TMF

in hydrogenated hightemperature

water.

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ENSI Erfahrungs- und Forschungsbericht 2012

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