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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. • The effects of local wall thinning on the cyclic plasticity behaviours of 90° back-to-back pipe bends are investigated. • The pipe bends with three different locations of local wall thinning is considered: intrados, extrados, and crown. • Shakedown and ratchet limit boundaries are analysed using the Linear Matching Method. • Critical loading combinations and the specific wall thinning locations that may lead to adverse effects are identified. [ABSTRACT FROM AUTHOR]

Local wall-thinning of pipe is a common volume defect in application, which would cause pipeline leakage or rupture under complicated pipeline loads. The finite element analysis method was used to analyze the limit load of Pipe Bend with Local Wall-Thinning defect (PB LWT) under internal pressure, bending moment, torsion moment, axial force, and their combinations. The limit load empirical equations of PB LWT under single load and complex loads were fitted. According to the allowable pipeline load on equipment nozzles, the influences of torsion moment and axial force on the load bearing capacity of PB LWT were quantitatively analyzed. It is found that the load bearing capacity is reduced by an average of 1 % for medium and low-pressure equipment. For high-pressure equipment, the load bearing capacity is reduced by 10.07 %∼20.90 % under the influence of torsion moment. Under the effect of axial force, the load bearing capacity is reduced by 2.01 %∼12.40 %. • The limit loads of bends with extrados local wall-thinning defect are analyzed under internal pressure, bending moment, torsion moment, axial force, and their combinations. • Empirical equations for limit load of bend with defects are worked out for complex loads. • Torsion and tension can decrease the load-bearing capacity of bends by up to 10 %∼20 %. [ABSTRACT FROM AUTHOR]