Measurement of Creep Deformation in Weldments

Sakanashi, Yuki (2013). Measurement of Creep Deformation in Weldments. PhD thesis The Open University.



This PhD project has developed a new high temperature strain measurement system using Digital Image Correlation (DIC) in order to investigate spatially varying and time dependent deformation during high temperature creep tests of engineering materials. Several challenges associated with measuring creep deformation at elevated temperature have been overcome including the choice of specimen design, specimen oxidation, furnace temperature uniformity, image distortion caused by thermal currents and sufficient illumination. It is demonstrated that the system created can produce reliable measurement data over a period of several months with a spatial resolution of 0.6 mm for temperatures up to 650°C, but in principle higher resolution and temperatures should be achievable.

The research aim of this project, funded by EDF Energy (formerly British Energy), was to attempt to measure spatially varying creep deformation properties across thick and thin section weldments that operate at high temperatures in UK advanced gas cooled reactor power plant (AGR). The new measurement system is applied to examine the creep behaviour of a thick section multi-pass welded joint made from Type 316H austenitic stainless steel which was supplied by EDF Energy. Specifically the local creep deformation properties across the weldment in the parent material, heat affected zone (HAZ) and multipass weld layers are investigated in medium term creep tests (>2300 hours). This is achieved by cutting samples from three different locations of the thick section joint, that is from top, middle and bottom positions, and subjecting them to tensile and creep testing at a temperature of 545°C. Spatially resolved stress-strain (tensile) and strain-time (creep) results were obtained transversely across the whole section of the multi-pass weldment and across the thickness direction. The DIC in-situ measurements also provided strain information in the transverse to loading direction from which the reduction of area of the specimen and true stress and strain distribution were calculated. The weld metal showed faster creep rates than HAZ and parent materials and this is attributed to the observed introduction of substantial plastic strain in the parent material on initial loading. Locally the creep strain distribution in the weld metal appears to correlate with individual weld passes. The full-field measurement results allowed the development of creep deformation leading to ultimate rupture to be observed.

The high temperature tensile and creep results presented in this thesis demonstrate the capability of the new DIC based system created for full field measurements of displacement and strain at high temperature during creep tests enduring several thousand hours. The system opens a new horizon for studying deformation and rupture behaviour of complex structures at elevated temperature.

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