On the ratcheting of defective low-alloy, high-strength steel pipes (API-5L X80) under cyclic bending: An experimental study

Abstract

Metallic tubes experiences a progressive accumulation of ovalisation, or ratchet, under cyclic inelastic bending. The current paper deals with the ratcheting behaviour of defective small-scale API X80 carbon steel tubes subjected to cyclic bending. The subject appears not to have received due previous attentions but is important in the design and mechanical integrity assessment of the piping systems. Right-angle uniform material loss is used to simulate an external corrosion patch in the test tubes.It is noticed that under inelastic cyclic bending the ovalisation ratchets both with the perfect and defective specimens. The rate of ratcheting increases and the number of cycles to failure decreases in the defective specimens as compared to those in the perfect ones. Both perfect and defective specimens show ratcheting in the circumferential strains, but this quickly ends in shakedown. The local deformation modes in the perfect specimens are characterised by diffusive wrinkling, but by inward kinking in the defective specimens. The perfect specimens demonstrate similar ratcheting rates in the sagging and hogging phases, but the defective specimens not. The modes of local deformation and failure for specimens with identical defects remain almost the same under cyclic and monotonic bending.