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Materialien/Werkstoffe Poster: Do., 13:00–15:30 D-P408
Assessment of residual stresses in cracked Fracture Mechanics CT specimens.
Robert Charles Wimpory 1 , Kamran Nikbin 2
1 Hahn-Meitner-Institut, Glienicker Straße 100, Berlin – 2 Imperial College London,
SW7 2AZ, UK
A major challenge facing life assessment of components operating at high temperatures
is to understand how residual stresses and prior straining contribute to creep damage
and crack growth failure. The efficiency of conventional steam and gas turbine power
plants can be significantly improved by increasing the operating temperature, leading
to reduced fuel consumption and lower levels of harmful emissions. With this trend
towards higher operating temperatures and the competing need to extend the life
of existing power plants, more accurate and reliable measurements and modelling of
residual stresses are needed to improve predictions of component lifetimes.
Failures in components are most likely to occur at or near weldments by fast fracture,
creep or fatigue cracking. These regions can exhibit microstructural inhomogeneity as
well as the presence of micro-cracks and residual stresses. Cracking normally occurs in
the heat affected zone (HAZ) of these welds and cracks can grow significantly under
service loads. Pilot creep crack growth tests on a type 316H stainless steel weldment
have recently been carried out using compact tension (CT) specimen designs with
specimens machined directly from a non-stress relieved weldment. The pilot creep
crack growth tests showed anomalously short lives, with the crack path sometimes
unexpectedly deviating away from the weld HAZ line at the centre of the specimen.
It is believed that this anomalous behaviour can be related to the presence of residual
stresses resulting from the welding process, which may significantly influence the creep
crack growth rate behaviour in the CT specimen.
In this paper a comparison of the residual strain and stresses and their subsequent
re-distribution in Compact Tension specimens are presented. Results of Neutron Measurements
from a number of EU neutron sources such as HMI, TUM/FRM II, ILL
and ISIS were made following a Round Robin exercise in the collaborative VAMAS
TWA 31 Creep/Fatigue Crack Growth of Welded Components programme. The data
will contribute to quantifying the repeatability of measurements from different sources
as well as identifying the role of residual stress in weldments and understanding their
effects on creep and creep-fatigue crack growth behaviour in components. It is hoped
that the final results and analyses will lead to recommendations for a code of practice
for measuring crack growth in welded specimens.