Fatigue life assessment of welded joints in gas turbine exhaust component

The gas turbine play a central role in power generation. It is a versatile combustion engine where its involvement is paramount in many industries, from aviation to industry applications. The high temperatures and cyclic loading operations are primary drivers of thermo-mechanical fatigue in the exhaust of the gas turbine. Welds can be critical locations in the exhaust system. Estimating the fatigue life of these welds have been challenging without the necessary fatigue data. This thesis project focus on assessing the low-cycle fatigue life of welds in the exhaust system by the implementation of a stress concentration method on the base material for austenitic stainless steel. The implemented stress concentration method chosen in this study was Neuber’s rule which calculates the local plastic stress-strain of a notch based on an elastic global stress that the material is subjected to. Fatigue data is gathered of weld and base tests from literature and incorporated with the notch analysis method to calculate the local plastic stress-strain response from a finite element model of the exhaust. The life was assessed on the exhaust model by the use of the stress concentration factor Kt. The Kt factor was accurate in producing the life of the weld under similar load, acting as an absolute measure between weld and base life. The developed life model shows that the weld life estimation for the base varies due to stress and temperature variations, and can’t be described by a single Kt. However, for certain fatigue data, the Kt was stable over larger stress amplitude intervals when validated in the stress-life curves.