Effect of shape-imperfections on fatigue life of elbows under in-plane cyclic loading

Abstract

The elbows play a critical role in industrial and power plant systems. To ensure operational safety, extensive research has been conducted on the fatigue behaviour of pipe bends. This study focuses on evaluating the low-cycle fatigue life of elbows, with particular emphasis on investigating the influence of shape imperfections, namely ovality and thinning. Initially, a validation study is carried out on elbows with circular cross-sections to estimate the fatigue life. Subsequently, a comprehensive set of 81 elbow geometry models is generated, incorporating shape imperfections. The material properties are assumed to exhibit elastic-plastic behaviour, representing the commonly used STPT410 material. Finite element analysis is employed to assess the behavior of elbows subjected to cyclic displacement loading of ±20 mm. The low-cycle fatigue life of the elbows is estimated using revised universal slope method, in conjunction with finite element results. The findings reveal that ovality enhances the fatigue life by 15.26–20.05% for maximum ovality whereas maximum thinning leads to a reduction of fatigue life by 10.81–17.33%. Furthermore, mathematical equations are proposed to quantitatively capture the effects of ovality and thinning.

Keywords

• Elbows
• Low-cycle fatigue
• Ovality
• Thinning
• Finite element analysis
• Revised universal slope method