Reheat cracking in the heat affected zones (HAZ) of Type 316H welds has been observed in the UK’s Advanced Gas cooled Reactors after 15–50k hours service at 490 to 520°C. These cracks are caused by creep damage from the relaxation of a triaxial residual stress. To validate a model for multiaxial creep damage under triaxial states of stress, notch bar specimens have been tested under displacement control. The test programme included the effects of different notch geometries, test temperatures (475 to 550°C) and both Type 316H parent and HAZ materials. Finite element analysis was used to develop a simplified method to calculate creep damage in the test specimens and this simplified method was then used to analyse the results of the notched bar stress relaxation tests. It was found that the multiaxial creep damage model gave a good prediction of failure in the tests particularly for the levels of triaxiality relevant to welds and at temperatures relevant to service.

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