[6B2] The detection and evaluation of thermomechanical fatigue in austenitic stainless steel using eddy current arrays
J Watson, J Ramsay, K Cooper, G Trownson and P Gill
Jacobs, UK
Thermo-mechanical fatigue (TMF) is a well-known degradation mechanism in many industrial applications and must be considered when performing assessments of the primary-circuit pressurised water reactors. Currently, such systems are assessed using the ASME Boiler and Pressure Vessel Code Section III Division 1 and NUREG/CR6909 for initiation, as well as ASME Section XI Code Case N809 for crack growth. However, such methods are inherently conservative for certain components when plant loading and other factors are considered. An eddy current array (ECA) was successfully employed to detect, position and evaluate the extent of TMF cracking within thick-walled austenitic stainless steel test-pieces. Subsequent indirect visual testing (VT) and fluorescent dye penetrant (DP) was used to provide additional confidence and assurance prior to the destructive sectioning and microscopy performed. This work was undertaken to develop an understanding of the effects of loading regimes and associated through-wall stress gradients, out-of-phase temperature/strain characteristics and so on, ultimately for a more accurate assessment of fatigue life. This paper outlines the various NDT inspections, evaluation performed and subsequent decision processes made, providing a prime example of how ECA technology can be employed within a materials testing application. These tests provided the go ahead for destructive sectioning and microscopy work, leading to extending the understanding of TMF mechanisms, which are outlined in other published work.
