A significant number of advanced gas cooled reactor (AGR) plant components with
operating temperatures in the range of 500-650 °C undergo creep-fatigue loading
conditions. Some of these components have previously operated in the creep regime
and subsequently had their operating temperatures reduced. The aims of this project
are to determine the effects of service exposure, thus prior creep damage, on fatigue
crack growth and fracture behaviour of the material and to include these effects in
fatigue and fracture assessments.
In laboratory testing, creep damage can be introduced into the material globally,
where uniform creep damage will form through a section, or locally, where creep
damage is confined to a small region local to the crack tip. The local creep damage
(LCD) approach has been employed in this research and the global creep damage
(GCD) tests will be performed in future work.
The material examined is an ex-service Type 316H stainless steel, which is widely
used in AGR plant components. Creep damage local to the crack tip has been
introduced into the material by interrupting creep crack growth (CCG) tests performed
at 550 °C, which is the typical operating temperature of the plant components. Fatigue
crack growth (FCG) and fracture toughness tests have been performed on these LCD
specimens and the results are compared to those of conducted on the material with no
creep damage.
The yield stress of Type 316H stainless steel is relatively low at 550 °C. In order to
limit the extent of plasticity in CCG tests during loading and unloading, the material has
been uniformly pre-compressed (PC) to 8% plastic strain at room temperature prior to
specimen manufacturing. It has been shown that the pre-compression process
increases the yield stress and reduces the tensile failure strain of the material
compared to the as-received (AR) material’s tensile response. A substantial drop in the
rupture time and creep ductility has also been exhibited in the uniaxial creep behaviour
of the PC material compared to AR, whereas the creep strain rate remains almost
unchanged.
To interrupt CCG tests on LCD specimens at a desired normalised crack length of
around a/W = 0.5, tests have been performed on nominally identical specimens to
calibrate the CCG behaviour of the material. The results from these tests have been
analysed in terms of the creep fracture mechanics parameter C* and compared to the available test data on AR material at 550 °C. It has been shown that the creep crack
growth rate, da/dt, in PC material is around an order of magnitude larger than those of
seen in short term tests on AR material and the data follow the trend of long term tests
on AR material.
FCG and fracture toughness tests have been performed on LCD specimens at room
temperature to investigate the potential effects of prior creep damage on subsequent
fatigue and fracture behaviour of the material, outside the creep regime. The results
from these tests have been compared to those of performed on PC material containing
no creep damage. A considerable drop in the fracture energy of the material has been
observed in LCD specimens in regions close to and away from the creep damage zone,
compared to those of obtained from PC material. Assuming a sharp crack tip in LCD
specimens, though discontinuous, the FCG rate per cycle, da/dN, data have been
correlated with the stress intensity factor range, ΔK. A severe delay in the crack
initiation and also a significant drop in the crack growth rate per cycle in the regions
close to the creep damage zone have been observed in the FCG data on the LCD
specimen compared to the PC material. However, further away from the creep damage
zone the FCG data from LCD specimen converge with the trends attained from the PC
material.
Further FCG and fracture toughness tests on LCD specimens need to be performed
in future work to confirm the observed trends. Also to examine the influence of uniform
creep damage in the absence of micro cracks (which form ahead of the crack tip when
the local creep damage approach is employed) on the subsequent FCG and fracture
behaviour of the material, GCD tests need to be conducted in future work and the
results to be compared to those of obtained from LCD specimens.