Post Yield Strain Fatigue Experiments to Validate Low Cycle Methodology for Tubular and Connections

Post yield design methodology using Ductile Failure Damage Indicator (DFDI) for well tubulars was proposed and has been used for tubulars and connections life assessment. The tubular design assessment model incorporates a connection strain localization factor (SLF) to assess the fatigue life of the tubulars. Critical strain, a material-dependent parameter essential for DFDI, is obtained using the uniaxial stress-strain tests (i.e., strained to failure uniaxial tests). Understanding the impact of accumulated cyclic damage on critical strain is essential to the post-yield design approach. This paper aims to validate and evolve the low cycle methodology by 1) quantifying the effect of accumulating cyclic plastic strain on the critical strain through a series of post-yield axial and thermal strain fatigue experiments, and 2) applying the post-yield design approach to assess tubulars and connections. Low cycle fatigue experiments demonstrating the critical strain measurement and its dependency on the thermal and axial-strain cycles will be discussed in the paper. Critical strain (K55 and L80) from monotonic tests is compared to critical strain obtained from cyclically preconditioned samples. Effect of cyclic plasticity on critical strain is established quantitatively. Coupons are also subjected to the post-yield axial and thermal cycles to failure and compared to critical strain-based DFDI design predictions. Since connections are known to be the weakest link in the casing system, the impact of connection thread-forms on the strain localization factor is demonstrated using a series of finite element models and the experimental material responses. Axial strain-controlled loading would be applied on the tubulars and connections to estimate the damage using the DFDI approach. A systematic approach to delineate the dependency of critical strain on cyclic straining validates the effectiveness of DFDI in thermal well design. Further, the quantification of SLF for integrating connection into thermal well design provides a complete solution.