Low-Cycle Fatigue Testing of Tubular Material Using Non-Standard Specimens

Abstract

Coiled tubing life prediction models are based on the use of standard low-cycle fatigue data. However, coiled tubing dimensions and the ultra-low-cycle life regime of interest, combine to complicate “standard” experimental methodology. This paper describes coupon design and testing procedures that alleviate the difficulties inherent to strain-controlled low-cycle fatigue testing of tubular material. Data sets are presented and compared to existing sets generated using smaller specimens. It is shown that the refined approach facilitates the generation of data in the ultra-low-cycle coiled tubing operating regime. The difference between the morphologies of the outer and inner surfaces of the specimens, as manufactured by standard tube rolling techniques, proved to be influential, but could not fully explain the comprehensive tendency for cracks to initiate at the inner surface. The inherently lower fatigue resistance of the inner surface indicates the theoretical existence of a “threshold defect severity” which must be exceeded on the outer surface before it would affect the fatigue life of the tubing.