Abstract
Fatigue analyzes are performed to ensure that no damage leading to failure will occur in a component.
In ASME III NB-3222.4(e), a method is provided for analyzing a component’s resistance against cyclic loads. In ASME NB-3228.5 or alternatively NB-3653.6 (c) a simplified elastic-plastic method is given to take plasticity into account. According to NB-3228.4 (c), fatigue evaluation is allowed with plastic analysis where the stress amplitude is determined from the numerically maximum principal total strain range.
This report examines the fatigue life, using a continuum mechanics approach, of a pipe penetration that is subjected to a rapid cooling down followed by a slow heating up. Simplified elastic-plastic analysis and plastic analysis are performed. The analysis also takes into account the FEN-factor according to the method given NUREG/CR6909.
The fatigue life is also determined by a fracture mechanics approach using a R6 based fracture assessment and crack growth analysis.
The results show a large difference in fatigue life depending on the method used.
For the plastic analyzes it is of great importance to the result in which direction the evaluation is made. For the present case the maximum principal total strain range is obtained in the radial direction. The stresses in this direction are small. A crack tangentially to this direction lacks physical significance to a pipe geometry and cannot lead to failure. In this case it is recommended that evaluation is made for the strain range in the axial direction for pipe geometries instead of the maximum principal total strain range.
