Thermal frequency response studies of a hollow cylinder subject to loads of different amplitude and shape

Thermal fatigue is an important degradation mechanism, which must be considered in life management of nuclear plant piping systems. The analysis is very complex due to a number of complicating factors, with the determination of the load as the primary one. There is clearly a need for simplified engineering approaches, such as simulating the spectrum load by a constant frequency thermal load with the nominal temperature difference between mixing fluids. The fatigue life is determined by the frequency that gives the shortest life using fatigue curves for initiation and Paris law for crack propagation. This paper analyses three aspects that affect the conservatism of such an approach: the thermal load shape (sinusoidal or square shaped), the structural boundary conditions at the edges of the modelled pipe (traction free or clamped) and the defect shape (circumferential or elliptic). Furthermore it is shown how the methodology can be used to determine screening criteria, i.e. a lower limit of the temperature difference below which there will be no component failure due to thermal fatigue.