Crack initiation in smooth fatigue specimens of austenitic stainless steel in light water reactor environments. Page: 4 of 10
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(PSBs) leads to the formation of extrusions and intrusions. With
continued cycling, microcracks ultimately form in PSBs or at the
edges of slip-band extrusions. At high strain amplitudes,
microcracks form in notches that develop at grain or twin boundaries.
Similarly, microcracks can also develop at phase boundaries, such as
ferrite/pearlite boundaries, or by cracking of second-phase particles,
such as sulfide or oxide inclusions.
Once a microcrack is formed, it continues as a Mode II shear
crack in Stage I growth along its slip plane or along a PSB and grows
at an orientation 450 to the stress axis. At low strain amplitudes, a
Stage I crack may extend across several grain diameters before the
increasing stress intensity of the crack promotes slip on systems other
than the primary slip plane. Once slip is no longer confined to planes
at 45* to the stress axis, the crack begins to propagate as a Mode I
tensile crack normal to the stress axis in Stage II growth. At high
strain amplitudes, the stress intensity is quite large and the crack
propagates entirely by the Stage II process. The Stage II crack
propagation continues until the crack reaches engineering size (3
mm). Stage II fracture in air or mildly corrosive environments is
characterized by fatigue striations.
Fatigue life, or number of allowable cycles, has conventionally
been represented by two stages: initiation, which represents the cycles
Ni that are necessary for the formation of microcracks on the surface;
and propagation, which represents the cycles Np needed for
propagation of the surface cracks to an engineering size. Thus,
fatigue life N is the sum of the two stages, N = Ni + Np. Crack
initiation is considered sensitive to the stress or strain amplitude such
that, at low strain amplitudes, most of the life may be spent in
initiating a crack, whereas, at high strain amplitudes, cracks initiate
An alternative approach to crack initiation and growth considers
fatigue life to be entirely composed of crack propagation (Miller,
1995). A measure of fatigue damage in a material is the current size
of a fatigue crack; damage accumulation is the rate of crack growth.
During fatigue loading of smooth test specimens, cracks form
immediately at surface irregularities or discontinuities that are either
already in existence or are produced by slip bands, grain boundaries,
second-phase particles, etc. (Fig. 1). Growth of these surface cracks
may be divided into three regimes: an initial period that involves
growth of microstructurally small cracks (MSCs) and is characterized
by decelerating crack growth, seen in Region AB of Fig. 1; a final
period of growth that is characterized by accelerating crack growth,
Region CD; and a transition period that is controlled by a
combination of these two regimes, Region BC.
Below a critical crack length, the growth of MCSs is very
sensitive to microstructure (Tokaji et al., 1986; Tokaji and Ogawa,
1992; Tokaji et al., 1988; de los Rios et al., 1992). In the early stage
of growth, the MSCs correspond to Stage I cracks and grow along
slip planes as shear cracks. Microstructural effects are strong because
of the crystallographic nature of Stage I growth, and the growth rates
are markedly decreased by grain boundaries, triple points, and phase
boundaries. Limited data suggest that microstructural effects are
more pronounced at negative stress ratios; the compressive
component of the applied load plays an important role in the
formation of Stage I facets and in acting as a crack driving force
(Tokaji and Ogawa, 1992). Above the critical length of MSCs,
fatigue cracks show little or no influence of microstructure and are
0 0.2 0.4 0.6
Figure 1. Growth of cracks in smooth fatigue specimens
termed mechanically small cracks (Tokaji and Ogawa, 1992); their
growth rates are higher than those of large cracks because they exhibit
less crack closure. These mechanically small cracks correspond to
Stage II or tensile cracks that are characterized by striated crack
growth with a fracture surface that is normal to the maximum principal
stress. For a stress ratio of -1, the transition from an MSC to a
mechanically small crack in several materials has been estimated to be
approximately eight times the microstructural unit size (Tokaji and
At low stress levels (AOv in Fig. 1), the transition from MSC
growth to accelerating crack growth does not occur and results in
nonpropagating cracks. This lack of transition represents the fatigue
limit for the smooth specimen. It is not a stress limit below which
cracks cannot form, but a limit below which microcracks that form on
smooth surfaces can not grow to engineering size. Large cracks that
may preexist in the material or be created by growth of microcracks at
high stresses can grow at stress levels below the fatigue limit.
The reduction in fatigue life in LWR coolant environments may
arise from easier formation of surface microcracks or an increase in
growth rates of cracks during either initial shear crack growth stage or
final tensile crack growth. Specimens tested in water have crystalline
oxides and a thin gray corrosion scale. X-ray diffraction analyses of
these specimens indicate that the corrosion scale consists primarily of
iron and chromium oxides such as FeO4, FeFe,04, y-Fe2,O, and CrO
and in the case of high-dissolved-oxygen (DO) water, a-Fe,03. In
addition to corrosion scales, the specimens tested in water also show
some surface micropitting.
The presence of micropits, which act as stress raisers and provide
preferred sites for the formation of fatigue cracks, has been thought
to contribute to the reduction of fatigue life in high-
temperature water. However, fatigue data for carbon and low-alloy
steels indicate that the presence of micropits alone cannot explain the
large reductions in fatigue lives of these steels in LWR environments
(Chopra and Shack, 1988). Specimens preexposed to high-DO water
and subsequently tested in air do not show a reduction in fatigue life,
as would be expected if micropits were the only contributor, and
Mechanically Small Crack Aoa
_ (Stage I Tensile Crack)
- Small Crack (MSC) A C. > A Q2
(Stage I Shear Crack)
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Chopra, O. K. & Smith, J. L. Crack initiation in smooth fatigue specimens of austenitic stainless steel in light water reactor environments., article, April 8, 1999; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc622842/m1/4/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.