Creep performance of OFP copper for the overpack of repository canisters
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Creep performance of OFP copper for the overpack of repository canisters Pertti Auerkari, Stefan Holmström, Jorma Salonen & Pertti Nenonen VTT Industrial Systems, PO Box 1704, FI-02044 VTT, Espoo, Finland
ABSTRACT To experimentally assess the long term creep performance of oxygen-free phosphorusdoped (OFP) copper for the overpack of repository canisters, the combination of modestly elevated temperature and multi-axial stress state has been applied for accelerated testing. Multiaxiality was induced by using notched compact tension (CT) specimens, with interrupted testing to periodically inspect for creep damage. Uniaxial creep testing was also conducted to support creep analysis of the CT specimens. After about 10 000 h of testing at 150°C/46 MPa (reference stress), the inspected CT specimens showed only marginal creep cavity indications near the notch tip. However, a distinct grain boundary zone with elevated P content was observed to appear and widen during testing, mainly near the notch tip. The significance of the grain boundary zone is not well understood, but indicates stress-enhanced microstructural changes at relatively low temperatures. The predicted isothermal uniaxial creep life at 150°C/46 MPa agreed satisfactorily within a factor of two in time, when obtained independently from converted multi-axial testing results and directly from a creep model based on the available uniaxial data. Although the uncertainties in extended extrapolation remain large, the prediction would suggest safe long term service at least against pure creep failure of intact parent material.
INTRODUCTION Long term integrity of the materials of repository canisters also requires sufficient long term strength of copper overpack, designed to protect the canister against corrosion. Due to the extensive design life of about 100 000 years, effects of creep are of potential interest in spite of relatively modest expected peak temperatures (close to 100ºC). Although the temperature is expected to remain in this peak range only a much shorter time, perhaps up to about 1000 years, this is a long time when compared with the design life of most technical applications. The current creep data for the OFP copper material covers testing times to less than 10 years, so that creep design involves large factors of extrapolation. This extrapolation is not only more extensive than that usually encountered in creep design, but also involves low temperature creep which is not as well covered by the existing theory and experience on life assessment as high temperature creep [1-3]. To support long term creep design by experimental evidence requires in practice test acceleration and creep modelling so that predictions can be made on the creep performance under realistic service conditions. The traditional approach to provide this acceleration is to elevate testing stress or temperature, or both. However, large factors of extrapolation imply relatively large differences between the service and testing conditions, when the maximum testing time is limited to t
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