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Creep Relaxation and Crack Growth Predictions in 316H Austenitic Steel
under Combined Primary and Secondary Loading
Hamed Yazdani Nezhad and Noel P. O’Dowd
Materials and Surface Science Institute
Department of Mechanical and Aeronautical Engineering
University of Limerick, Limerick, Ireland
E-mail(s): hamed.yazdani-nezhad@ul.ie; noel.odowd@ul.ie
Abstract
Compact tension 316H specimens, extracted from an
as-received ex-service pressure vessel header, have
been pre-compressed to different load levels in order to
introduce a tensile residual stress (RS) field in the
specimens. Finite element (FE) analysis has been
carried out to predict the required loading level.
Residual stress profiles along the crack path are also
compared with those measured using neutron
diffraction (ND) facilities. The creep relaxation
behaviour has been studied numerically and the results
have been compared with the earlier studies.
1. Introduction
Studies of creep behaviour of crucial structures such
as power generation plant are always important in the
category of structural integrity assessment. Type 316Haustenitic
steel is a typical material used in the steam
headers, working at the elevated temperature (550°C).
Several procedures regarding the structural integrity
assessment have been standardized [1,2] and
successfully used to evaluate the effect of defects on
creep behaviour. The effect of tensile RS in 316H
compact tension, C(T), specimens (25mm thickness)
subjected to pre-compression, has been studied in [3].
In this work, however, the combined stress effects on
creep crack growth and relaxation are studied under
different residual stress.
2. Method
Four C(T) specimens (Fig. 1) were extracted from a
316H header, subjected to pre-compression (53 and 65
kN) and then pre-cracked using electro-discharge
machining (EDM). Neutron diffraction measurements
have been carried out at E3, Helmholtz-Zentrum Berlin
(HZB) to obtain the RS profile ahead of the notch.
Finite element analysis has been carried out and the
results have been compared to the ND data (Fig. 2).
EDM slot
Figure 1. Schematic illustration of the C(T) specimen
169
3. Results
Figure 2. Comparison of residual stress values from the
FE analysis and ND for 53 kN pre-compressed C(T)s
4. Conclusion
The measured data from ND for two C(T) specimens
are in very good agreement and confirms the tests
repeatability. Residual stress profiles measured using
ND, provide a good agreement with the plane stress
results from the FE analysis. The surface and smearing
effects due to the gauge volume size may cause the
disparities near the notch tip that is seen in both free
edges of the specimens. It is seen that the kinematic
hardening material model may improve the agreements
close to the notch root by predicting a lower stress
level. Though not shown here, creep relaxation
behaviour was also studied numerically and compared
with the earlier studies.
5. References
[1] R6, Revision 4, 2009, “Assessment of the Integrity of
Structures Containing Defects'”, British Energy Generation
Ltd., Gloucester, UK.
[2] R5, Revision 3, 2003, “An Assessment Procedure for the
High Temperature Response of Structures”, British Energy
Generation Ltd., Gloucester, UK.
[3] Turski, M., Bouchard, P.J., Steuwer, A., Withers P.J.,
2008, “Residual Stress Driven Creep Cracking in AISI Type
316 Stainless Steel”, J. Acta Materialia, 56 (14), 3598-3612.