Comprehensive numerical study on shakedown and ratchetting behaviours of a deteriorated 90° back-to-back pipe bends

This study aims at investigating the effects of local wall thinning on the cyclic plasticity behaviours of 90° back-to-back pipe bends, which are commonly utilised in flexible piping systems within plant facilities. Over time, the prolonged operation of these pipe bends may lead to wall thinning and a deterioration of their structural integrity. To understand the effects of such wall thinning, a 3D finite element analysis is conducted. Load interaction diagrams are generated using the Linear Matching Method, which considers the cyclic plasticity behaviours of shakedown and ratcheting. The investigation includes pipe bends with three different locations of local wall thinning: intrados, extrados, and crown. The pipe bends are subjected to steady internal pressure and various cyclic bending moments, including in-plane closing, in-plane opening, and out-of-plane bending. Moreover, comprehensive parametric studies are carried out for the pipe bend structures, varying the wall thinning rates. Semi-empirical formulas are derived based on the study results, enabling the prediction of limit load, shakedown limit, and ratchet limit concerning the position of the local wall thinning. Results and discussions have demonstrated the most detrimental combination of wall thinning type and loading on the structural integrity of the pipe bends and have provided valuable insights into the problems associated with deteriorated pipe bends.